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4 Sufficiency of F-22 Testing Plans Part of the committee's task was to evaluate the sufficiency of the F-22 test program to meet the requirements of the live fire test law. This chapter contains that evaluation. Discussed first are the F-22 threat environment and its replication by the SPO in the vulnerability assessment program. The vulnerability assessment program is then evaluated. Finally, after some additional observations, the committee presents its conclusions. F-22 THREAT ENVIRONMENT AND ITS REPLICATION The threat environment for the F-22 is derived from its current principal mission of conducting offensive counter-a~r operations. The environment consists of threats the aircraft would expect to face while accomplishing its mission of destroying enemy aircraft over hostile territory. These threats, as characterized by a representative of the Air Combat Command in an unclassified briefing to the committee (Hinton, 1994), are shown in Table 4-~. In addition, the vulnerability specifications (discussed in Chapter 2) reflect a high-power microwave threat and a laser threat. The committee accepts these threats for the courter-air missions. The aircraft has been designed to meet them, and its vulnerability assessment program has been structured accordingly. The following assumptions were made by the SPO in the live fire test program to replicate the threat: . The threat specimen for the F-22 is represented by: (a) two metallic fragments (45 grains and ~ 50 grains), (b) two armor-piercing incendiary (API) rounds (23mm and 30mm), and (c) two high-explosive incendiary (HEN) rounds (23mm and 30mm) (SPO, 199Sa). The effects of the fragments are to be assessed over a range of impact velocities Tom 2,000 to 9,000 feet per second (fps). The APT rounds will be assessed over a range of impact velocities Tom 44
Sufficiency of F-22 Testing Plans TABLE 4-l F-22 Threat Environment 45 Fighters Air-to-Air Missiles Surface-to-Air Missiles Current Mirage 2000 Gripen MiG-29 Fulcrum SU-27 Flanker Future New Fighters IOC by 2004 SU-35 Improved Flanker Rafale Eurofighter 2000 New (Notional) Fighters Available by 2014 Multi-role Fighter Interceptor IOC 2005-2008 Experimental Fighter Interceptor IOC 2010-2015 Reticle IR Seeker with CCM AA-NOB/D AIM-9M AA-7D Active-Radar Seeker AA-X- 12 AIM-120 Multi-element Seeker AA-11 PYTHON 4 MAGIC 2 Imaging IR Seeker AAM (IOCs in 2004) XAAM~ ASRAAM AIM-9X SA-10 SA-12 Source: Hinton, 1994. NOTE: AAM=air-to-airmissile; CCM=counter-countermeasures; IOC=initial operational capability; IR=infrared.
46 Live Fire Testing of the F-22 . . 500 to 4,000 fps, and the HEl rounds wall be assessed for a velocity of 2,500 fps. For each threat that has a range of velocities, the maximum vulnerable area (over the range of velocities) is calculated for each of the 6 cardinal views (top, bottom, front, back, left, and right). In the case of the HE} rounds, the vulnerable areas are calculated for each cardinal view for the single velocity. These maximum vulnerable areas are averaged over the 6 cardinal views to provide the maximum-allowable vulnerable area for each threat. Specifically, 36 numbers (6 for each of the 2 fragments plus 6 for each of the 4 cannon rounds) are averaged to produce 6 numbers (i.e., vulnerable areas) that are incorporated in the F-22 contract as vulnerability specifications. Vulnerabilities for specific encounters with specific weapons can be calculated using the same methodology, but they are not part of any formal requirement. The committee considered the SPO's assumptions. While the threat missiles may well change, the fragments, with the spread in velocity, seem to be a robust representation of the effects of individual fragments from missile warheads. The API and HE] rounds are reasonable representations of air-to-air cannon-fired rounds. The sole use of vulnerable areas for individual fragments to represent missile warheads does imply certain assumptions and limitations: . The fragment data are only intermediate data. The effect of a particular warhead event is a much more complicated affair involving details of the warhead and the end-game geometry (i.e., guidance and control capabilities of the missile and the action and signature of the target). The effects of multiple fragment hits are ignored (consistent with the current state-of-the-art of vulnerability analysis). The usual assumption is made that blast effects are only important for miss distances at which the fragments would certainly kill the target. However, there are intercept geometries for which this assumption is not the case, and blast must be taken into account in the analyses. When anti-air missiles detonate, a spray of fragments, often focused in a specific direction, is propelled from the warhead to He target. In addition, a significant blast wave caused by the detonation of the warhead also propagates toward the aircraft. The fragments and blast wave wait strike the aircraft at different times, sometimes resulting in enhanced kill mechanisms. The committee believes that the two discrete fragment sizes selected by the SPO for analysis and test are representative of the fragments from the spectrum
Sufficiency of F-22 Testing Plans 47 of warheads likely to be encountered. However, vulnerability of the F-22 to these fragments is estimated on a one-fragment-at-a-time basis and therefore, in itself, does not completely represent the vulnerability of the aircraft to missile warheads. Vulnerability of the aircraft against these warheads is assessed in a subsequent analysis that considers three types of kill mecharusms: . . . Blast kill of the structure based on overpressures resulting from detonation of the warhead's high-explosive charge. Impact of multiple fragments with Me fragments spaced far enough apart so that Weir effects on the aircraft are independent; the result effectively is aggregated from independent single fragment assessments. Impact of multiple fragments that are dense enough so that their effects are not independent arid, when taken together, could result in structural kill of the aircraft. This type of kill mechanism is often accounted for by a kinetic energy threshold for structural kill; it is particularly important for annular blast fragmentation or focused blast fragmentation warheads. The first two of these mechanisms are accounted for in the vulnerability analysis currently being conducted for the SPO. The third has not yet been explicitly accounted for. It is safe to ignore this effect for many warheads and encounter geometries because, if such a kill is obtained, a kill from one of the other two mechanisms would also be obtained. However, for some classes of warheads (e.g., the annular or focused blast fragmentation warheads) this kill mechanism may be important and should be considered in future analysis and testing by the SPO. OVERVIEW OF THE AIR FORCE VULNERABILITY ASSESSMENT PROGRAM The committee received extensive briefings on the vulnerability assessment program from representatives of the F-22 SPO during its visit to Wright-Patterson Air Force Base in January 1995. Those briefings and communications with the SPO provided the basic information evaluated in this chapter.) The SPO and its contractors performed a detailed vulnerability analysis of the F-22 using revised versions of star~dardized computer models (see Chapter 51. The outputs of the analysis were estimated vulnerable areas arid overall system ~ The entire briefing is documented here as SPO, 1 995a. Several parts of this briefing are also cited individually in this chapter (e.g., Griffis, 1995a, and Ogg, 19959.
48 Live Fire Testing of the F-22 PK/H (see Chapter 3). The SPO then assessed the analytical results and identified areas of uncertainty. These areas of uncertainty were based on the following specific criteria: Areas currently treated as invulnerable based on analysis for which insufficient or contradicting data exist. Compartments where collateral damage mechanisms cannot be assessed and that represent a potential vulnerability. Components that represent a significant contribution to vulnerable area and have insufficient supporting data. Areas for which the basic material or ballistic data base is inadequate. Areas of uncertainty were next mapped against the F-22 design and the results of the vulnerability assessment to identify test issues, areas of the aircraft that needed to be tested, and specific test hardware requirements. The enumerated F-22 live fire tests and locations of the test areas on the aircraft are shown in Figure 4 I.2 As a part of establishing its test program, the SPO made several assumptions (SPO, 199Sa): . Because the threat scenario is for offensive counter-a~r missions, 60 percent of total usable fuel is assumed to remain after penetration into hostile air space. According to the design filrl-bum sequence, the fuel in certain critical tandcs would have been consumed. This assumption minimizes inlet fuel ingestion kills (see discussion later in this chapter) and reduces vulnerable areas by 30 percent. The flight is straight and level at 500 knots, and the requirements are independent of altitude. The kill category considered is the attrition kill, in which controlled flight is lost within five minutes following the hit. Vulnerable areas used in these calculations are based in part on results of individual tests against electronics modules. This approach may not take flammability of the coolant into account arid may need revision pending the results of the tests on fluid flammability that are recommended later in this chapter. Concern for survivability of the pilot is demonstrated by a double battier between the cockpit arid Me forward fuel tank. There are no 2 Tests 9 and 10 are exceptions; Test 9 is a components-related test and Test 10 is a materials test. Neither of these tests appears in the diagram because they cannot be isolated to a specific location on the aircraft. Also, Test 5 and Test 8 do not appear in the SPO's numbering system because those tests were subsumed by Tests 4 and 7, respectively.
Sufficiency of F-22 Testing Plans Test Test Number Number Location Of Shots 1,2 3 Wing Boxes Full Wing Box 4 Aft Side of Body 4A-C Aft Side of Body 6A Forward Fuselage (Lower) 6B Amad Bay 6C Forward Fuselage (Upper) 6D Win=, Leading Edge 6E Main Wheel Well 6F Wing, Attachment Bay 7 F-1 Fuel Tank 1 1 Aileron Bay 49 3 \ few 6A& 6C 1&2 \ Age \ 6D 1 \,~ ~ 6B & 6E Wk-~;~ (~ 6F 6B 4 &4A-C FIGURE 4-] Locations of the test areas. Source: Griffis, 1995b. . detailed, formal specifications for pilot survivability, but there is a qualitative concern for maximizing the chance that the pilot can eject in case of catastrophe (Griffis, 199Sa). The pilot is included in the assessment of vulnerable area. The specifications ado not include the elects of on-board munitions. The reason given for this omission is that the design values are to be used to assess contractor performance, and the contractor is not responsible for the munitions (SPO, 1995a). However, mission analyses calculating PK,H of the aircraft account for the v uInerable area of on-board munitions. The committee accepts several of these assumptions but has the following comments about the lack of specifications covering on-board ordnance. It is difficult to consider that the system is any less than the sum of the aircraft and its ordnance. The impact of aircraft design on protection of the ordnance and on
so Live Fire Testing of the F-22 possible mitigation of the effects of damage could certainly be important to the survivability of the pilot, even if it were not so important to the survivability of the aircraft. Saying that the ordnance is not the responsibility of the contractor trivializes an important issue. The SPO identified additional areas of uncertainty for which further testing would be beneficial if funding were available (Graves, ~ 995~. These include tests on polyalphaolefin (PAO) coolant fluid flammability, hydraulic fluid flammability, and filet flammability. A second series of tests is proposed for the weapons bay, using a simulator of the weapons bay of representative size and materials. These tests would involve ballistic testing of actual AIM-9 and AIM-120 rocket motors with both protected and unprotected bays to determine the effectiveness of protecting the weapons bay against fires using ablative materials. EVALUATION OF THE VULNERABILITY ASSESSMENT PROGRAM This section evaluates the current test program. The organization is by major F-22 subsystem. Each subsection briefly describes a subsystem and attendant vulnerabilities, the analyses and tests already conducted or planned by the SPO for that subsystem, and the com- mittee's overall assessment. Finally, revisions are suggested to improve He program where the committee believes them to be desirable. Marty live fire tests are planned or have been performed on various components, subsystems, arid sub- assemblies of He F-22. Test articles range from component prototypes to engineering and manufacturing de- velopment (EMIL hardware. The threat kill mechanisms are those discussed above. .... ; I A. ^~ ~ ^~ - ^ - ^= .. . . .
Sz`~iciency of F-22 Testing Plans Structure and Integral Fuel Tanks Description and Attendant Vuinerabilities 51 Airframe outer skins are generally quite thin, highly stressed, and only slightly resistant to penetration from missile warhead fragments and projectiles. However, penetration of the skin and failure of an interior member such as a wing spar or a fuselage frame does not necessarily mean loss of the aircraft. This is particularly true for the F-22, since it is designed with multinIe (redundant) load paths and utilizes structural materials that have a high fracture toughness. The design represents a significant improvement over many past fighter aircraft, which were largely single load path structures and open utilized high-strength aluminum and steed alloys with very low fracture toughness. Figure 4-2 illustrates the structural configuration of the F-22, a largely multiple Toad path construction. For example, the F-22 has multiple wing spars; several wing carry-through fuselage frames and bulkheads; three fin attachment frames; numerous additional fuselage frames; and various stringers, stiffeners, ribs, and other miscellaneous members. The materials are shown In Figure 4-3. A design goal was that the structure be able to sustain the damage from defined threats without loss of the aircraft. The multiple load path design was intended to achieve this goal. The uncertainty in predicting hydraulic ram effects3 in the integral tanks arid Me need for developmental testing of some of these structures were recognized. While the committee agrees that the F-22 structure is predominately of multiple load path design, there are some exceptions. Such exceptions include (a) the horizontal tails, where each tail is supported by a single pivot shaft made of a titanium alloy (see Figure 4-2~; arid (b) the farthest aft fuselage frame (i.e., Frame 6 in Figure 4-2), which carries a significant portion of the horizontal tad] and vertical tail loads across the fuselage. 3 Ball (1985) gives this explanation of hydraulic ram effects: When a pene~ator enters a compartment containing a fluid, a damage process called hydraulic or hydrodynamic ram is generated. Hydraulic ram can be divided into three phases: the early shock phase, the later drag phase, and the final cavity phase .... The hydraulic ram loading on all of the wet walls of the tank can cause large-scale tearing and petalling, with openings very much larger than those made by the actual penetrator. The hydraulic ram loading can also be transmitted through attached lines, causing failure at fittings or other discontinuities in the lines.
52 Live Fire Testing of the F-22 Right Hand Horizontal Tail Right Hand Vertical Tall Left Hand Wing FIGURE 4-2 Structural configuration. Source: Griffis, 1995b. Although the committee believes that the horizontal tad! pivot shafts may be heavy enough to resist complete failure following ballistic impact, these shafts could encounter damage that would degrade their strength and ability to carry operational maneuvering loads. However, if failure should occur and a horizontal tad! were lost, it is not certain that the aircraft would be lost. Although the F-22 prime contractor has indicated that the airplane can be controlled with one tad! missing, it would appear that this capability would depend on flight conditions at the time of loss. Risk of aircraft loss given the loss of a horizontal tad] has not been adequately defined. While Frame 6 is not currently considered to be a vulnerable location, the committee believes that, if He frame fails as a result of a direct hit, loss of the aircraft might result. It appears Hat this potential vulnerability needs further investigation, as discussed below. Also, the aft fuselage booms that support the horizontal tails as well as some of the vertical tad! loads might be considered to be single load path even though Right Hand Aft Boom \~ Of I/ Elm ~ ~/ Pivot Shafts T eft Harld Torizontal Tail
Sufficiency of F-22 Testing Plans Wing Duct Skins Skins - IM7/5250-4 Composite IM7/977-3 Lower Surface Thickness 0.177 to 0.265 inches Upper Surface Thickness 0.212 to O.316 inches \ Aft Fuselage : °^ / TJpp~ Skins - Titaniurn 6-2222 & ~I7/5250-4IHC Cocure / Empennage Skins & Closeouts - IM7/5250-4 Core - Aluminum SO ~ \ / Edges Nicalor~JS2!5250-4 & Nomex Core FIGURE 4-3 Materials applications. Source: Griffis, 1995b. 53 Forward Fuselage Skins & Chine - IM7/5250-4 Composite \ Mid Fuselage Skins: IM7/5250-4 & Titanium 6-4 Composite they consist of several structural members (see Figure 4-4~. These members make up two beams, one on each side of the aircraft, that are subjected to substantial bending, shear, and torsional loads. If either of these boxes fails, it may lead to loss of the aircraft. As discussed below, some live fire testing has been done, and more is planned, for this general area. When assessing the vulnerability of the structure to ballistic threats, it is necessary to consider the effects of other potential damage mechanisms in addition to projectile impacts. These include blast effects, overtemperature due to fire, and hydraulic ram loads in fuel-containing structures. Hydraulic ram can significantly magnify the damage to wing structure, and test results could lead to redesign. It is also the primary mechanism of concern in the fuselage fired tank structures. Of particular concern is the close proximity of the forward fuselage filet tank (designated F-~) to the cockpit. The forward side and top of the tank has a double- walled barrier intended to prevent fuel leakage directly into the cockpit. The
54 Lower Skin Splice Am/ Welded Lower Skin ~ FIGURE 4-4 Aft boom. Source: Griffins, 1995b. Live Fire Testing of the F-22 i/ Access Through Keelson Forward ~ U Rum Outboard ~1 committee is concerned that, if the inner wall were ruptured as a result of hydraulic ram, the second barrier could be penetrated and fuel could leak into the cockpit. Fire could then break out because of the various ignition sources in the cockpit. Also, ram effects could damage the canopy hinge, actuator, support structure, seat rails, or other structures, which could then prevent successful ejection of the pilot. Plannec! Analyses and Tests The F-22 structure has been considered invulnerable because of the multiple load path design. However, there are some uncertainties in analytically predicting the extent of damage that watt be encountered and, to a lesser extent, the residual strength of the remaining structure after it is damaged. The uncertainty in predicting damage is probably greatest for fuel-containing structures (i.e., wings, aft sides of the body, and forward fuel tarmac) because of difficulties in predicting both the pressures and the response of the materials (particularly composite materials) associated with the hydraulic ram phenomena. It was because of this uncertainty that the developmental live fire tests of the wing
Sufficiency of F-22 Testing Plans 55 structure were performed. Subsequently, hydraulic ram live fire tests were also planned for the aft fuselage and forward fuselage fuel-containing structures. The original design damage size for the F-22 wing was estimated to be an 8-inch-diameter hole in the skin plus the loss of a spar. Accordingly, the wing was designed to tolerate this damage without Toss of the aircraft. However, when the live fire tests were performed on a test box containing three composite spars (designated Test lA), it was found that the damage was much more extensive (SPO, 1995a). In fact, the entire skin panel and all composite spars failed. This result led to redesign of the wing and the addition of five new titanium spars, as shown in Figure 4-5. The redesigned configuration was then subjected to live fire testing using four-spar and eight-spar test boxes (designated Tests IB, 2A, and 2C) (SPO, 1995a). In these tests, the damage was largely contained between the titanium spars (the composite skins arid intermediate composite spars were both severely damaged) (Griffis, 1 995a). Based on this result, it has been predicted that the wing can still carry the bending load associated with a 4-g maneuver (Ogg, 1995~. Verification of the final wing design is planned in a full-scale-w~ng live fire, residual-strength test program to be conducted between 1997 and 1998. This testing wall be performed at the Air Force's Wright Laboratory, where the wing will be fueled and have simulated Toads applied and air flowing over the wing during live fire testing. A residual strength test wall then be performed on the damaged wing (SPO, 1995a). The aft fuselage booms consist of a forward boom section, that contains fuel. It is supported by fuselage Frames 2 through 6 (see Figure 4-6) and the cantilevered section of the boom aft of Frame 6, shown in Figure 4-4, which is dry. These booms are fabricated from welded, integrally stiffened titanium (Griffis, 1 995a). The objective of Test 4 is to investigate structural damage to the aft fuselage forward boom area due to impact (SPO, 1993). Hydraulic ram effects (wet bays), blast effects (dry bays), and damage from fragments are investigated. All tests use 30mm HEI projectiles, since these are expected to generate the most severe pressures and fragmentation. As of this writing, three tests have been performed arid one more has been planned. A preliminary live fire test (Test 4A) was perfonned on a small welded box, which contained water, to determine if the impact would cause weld cracking. No cracking of the weld seams occurred, although the box was torn apart (SPO, 19931. This test was not considered to be a good hydraulic ram test because the box was made of nonrepresentative materials that failed during testing, thus relieving the pressure on the weld seams. Test 4B was also performed in 1994 on a welded titanium box, which did not contain water or filial. The purpose of this test was to obtain information on blast and fragment damage that could be expected if a 30mm HE} round
1 56 Live Fire Testing of the F-22 ;~ New Titanium Spars ~/~ 7~d:/ New Rib Line FIGURE 4-S Current wing configuration. Source: Griffins, 199Sb. penetrated the aft dry boom (Griffis, 199Sa). The results indicated that there was less fragment penetration than predicted by analysis. The damage did not appear to be severe enough to jeopardize flight safety should similar damage occur during an operational conflict. No further live fire tests and no residual strength tests are planned for this aft boom structure. The third test (4C) was intended to study the pressures generated by detonation in a watercooled box (Griffis, 1995a). The test articles were stainless steel boxes with the approximate shape and volume of art aft bay. They were filled with water, and an explosive (equivalent to a 30mm HEl) was detonated in the --rip ~ center. Boxes of two different volumes (by IS percent) were tested; the SPO indicated that the peak pressures are independent of these box volumes. This result was used to justify direct application of the test results from scared-down test articles to full-scale test articles. The report for this test was not written at the time of the committee's inquiry. A representative live fire test (Test 4D) was planned for August 1995, when a section of the tank between Frames 5 and 6 was to be filled with water, externally loaded, and hit with a 30mm HE] round (Griffis, 199Sa). As of this writing, analyses are being performed to predict the amount of expected damage and the resulting residual strength. No experimental verification of the residual strength is planned. To evaluate concerns about filer entering the cockpit and about structural damage that could prevent pilot ejection after a hydraulic ram failure of the forward fuselage tank (Fat), the SPO was planning Test 7 to begin in June 199S (Griffis, 199Sa). This test involves firing either 30mm HE] or AP] projectiles into a full-size F-l fuel tank with supporting structure, including the seat back of Fasteners
Sufficiency of F-22 Testing Plans 57 Fuselage Frames F-2 Through .~ FIGURE 4-6 Forward boom A-l file! tanks. Source: Griffin, 1995b. bulkhead. Since this test involves single shots to single points in the F-! tank, the results watt only be significant if they are complemented by a thorough analysis of the problem. It should be noted that no live fire tests are currently planned to determine the damage that would be encountered from a direct 30mm HE] impact on Frame 6, which is common to both the forward and aft sections of the tad] boom structure. As was pointed out above, the committee is concerned that failure of this frame could jeopardize flight safety. Likewise, no live fire tests are planned to assess the damage that would occur from a direct hit on the pivot shafts, which provide only single-Ioad-path support of the horizontal tails. Assessment The basic F-22 structural design appears to derive substantial battle-damage tolerance from the use of materials with high toughness and the incorporation of multiple load paths. Nevertheless, a major uncertainty is the prediction of damage due to hydraulic ram effects from a ballistic hit. The SPO has recognized this uncertainty and constructed a comprehensive live fire test program to uncover weaknesses in the design. Appropriate hydrodynamic modeling (finite element,
58 Live Fire Testing of the F-22 finite difference, etc.) would maximize the information extracted from this test program and allow prediction of response for other geometries, fill ratios, and impact kinematics. In fact, the program has already disclosed a weakness in the wing design and corrective measures have been taken (Griffis, 1995a). Tests 1, 2, 3, 4, and 7 are live fire structural tests, and Test 10 obtains basic penetration data on structural materials. All except Test 4B and material penetration shots in Test 10 involve evaluation of hydraulic ram effects (Griffis, 1995a). Although a hydraulic ram test is planned for the aft boom fuel tank area, the committee is concerned about the lack of a live fire test shot at Frame 6. Also, there has only been one shot to predict damage from a hit in the dry bay areas of the aft boom. The prediction of damage as well as the remaining residual strength of the boom structure, the aft fuselage frame, and the horizontal tad! pivot shafts are all believed to be important. In addition, if it does appear possible that a tall could be lost, it is then important to estimate the risk of aircraft Toss. The committee is concerned that the test specimen plarmed for Test 4D may not be representative of the aft fuel tank. In particular, the concern is that the absence of filet on the other side of Frame 5, and the inaccurate representation of Frame 5, could adversely affect the fidelity of the test. The committee believes that hydrodynamic analyses can be used to establish the credibility of the test specimen and to determine if the specimen can represent only the part of the tank between Frames 5 and 6, or if more of the tank needs to be represented. Suggested Revisions The committee suggests that the following revisions to the vulnerability assessment program be considered: . . . Conduct additional live fire testing to determine Me damage that can be expected from a hit in the Frame 6 aft boom attachment area. The Air Force should detennine the most critical shot lines (i.e., whether or not they should go through the fuel tank area). Expand analyses to predict damage sizes and residual strengths of the aft boom, Frame 6, and horizontal tad! pivot shafts after being hit by 30mm HE] rounds. Also, determine the risk of aircraft Toss should it be found that Toss of a horizontal tail is possible. Conduct further analysis of the aft fuel tank (Am) prior to the conduct of Test 4D. This analysis should be focused on determining the adequacy of the test specimen, with particular emphasis on its ability to simulate accurately the reaction of the entire tank.
Sufficiency of F-22 Testing Plans Fuel System and Associated Dry Bays Description arid Attendant Vuinerabilities 59 The F-22 fuel system consists of integral fuel tanks within the aircraft fuselage and wing structure, as shown in Figure 4-7. The fuel system also includes the pumps, valves, plumbing, and components necessary to supply the required file] flows and pressures to the engines during all flight conditions. Each engine is fed fuel from a common forward and two independent aft feed tank systems. The fuel system is designed to ensure mission completion after one failure and safe aircraft recovery following two failures. All the filed on board the aircraft is available to either engine via a cross-feed manifold, if no more than a single failure has occurred. The fuel system on any aircraft, and particularly a fighter, is the single largest nonstructural subsystem on the aircraft and has a large presented area from any threat aspect. If the fuel system is not protected with vulnerability reduction measures, it is also the largest vulnerable area on anv aircraft The nrimarv kill mechanisms of the filet system are the following (Griffis, 1995a): ~ J ~^ _^ ~-. ~ an_ t~ ^~ ~ ^~$ Fire or explosion inside the fuel tanks caused by ignition of the fuel- air mixture in the ulIage above the filet as a result of APT or HE! projectiles or other ignition sources. Fire in dry bays around fuel tanks caused by projectile or fragment ignition of the fuel spurtback from penetration of the file! tank. Hydraulic ram from projectile or fragment penetration into full or nearly full fuel tanks, which results in fluid shock wave forces that rupture the fuel tank. Fuel depletion from leaking or ruptured fuel tanks or filet lines. Discussion of elements of the fuel system that are primarily related to structural problems is not repeated in this section. The ulIage spaces are vulnerable to threat-induced fire or explosion unless protection is provided. The F-22 design includes an on-board inert gas generating system (OBIGGS), which replaces air in the ulIage spaces with an inert gas (discussed below). In addition. the F-22 has marlv drv have that Allot he nrnt~r.t~1 to prevent fires. The SPO has addressed the dry bay fire problem by providing fire extinguishing in the main wheel wells and the aft wing attachment bays as well as foam on the top and sides of the F-! filet tank. Figure 4-8 depicts these and other vulnerability reduction features. (Fire extinguishing and foam are discussed later in this chapter.) __~_, , -, rat
60 Live Fire Testing of the F-22 _ A-2R Tank , _ it_ A-1R Tank -F-1A Component ~ F-1B Pump Box ~ A-2L Tank 1 1 l O\ ~ A- 1L Tank F-1 Tally oN FIGURE 4-7 Fuel system vulnerability testing. Source: Griffis, 199Sb. A fuel ingestion problem arises when combat damage to a fuel tank allows leakage to be ingested into an engine inlet. For the F-22, this problem has been addressed by having a tailored fuel-burn sequence that wall leave the fuselage fuel tanlcs next to the engine inlets empty when a 60 percent fuel state is reached (Griffis, ~ 995a). This approach assumes that the aircraft wait not see combat until that point in time. Nevertheless, the solution carTies with it a degree of risk, and those who fly the aircraft watt have to decide if the risk is acceptable. At a minimum, mission planners and pilots should be informed of this risk. The F-22 filet transfer lines are located inside Me fuel tanks to reduce the chance of their being hit (SPO, 1995a). The transfer lines are also located in inerted fuel tanks. This combination is a good vulnerability reduction design technique. In addition, with no more than a single failure in the cross-feed manifold and feed tank pump, their redundancy allows for full availability of all We fuel within the aircraft to either engine. This cross-feed capability reduces the vulnerability of the system. Flammable fluids that leak are drained overboard through drain holes located in external surface panels along the bottom of the aircraft.
Sufficiency of F-22 Testing Plans Planned Analyses and Tests 61 The OBIGGS provides the single largest vulnerable area reduction of any vulnerability reduction feature used on the aircraft. No destructive ballistic tests are planned for this system because previous extensive ballistic tests carried out by the Joint Technical Coordinating Group on Aircraft Survivability (]TCG/AS) have shown that, if the fifed tank ulIage is inerted we trogen from the OBIGGS to reduce the oxygen content below 9 percent in the ulIage fuel-air mixture above the liquid fuel level, no fire or explosion wall occur in that area.4 The committee agrees with the facts in this situation and with the decision not to conduct destructive ballistic testing because it is not necessary. Dry bay fire protection tests are covered later in this chapter. Assessment There is a question of whether OBIGGS generates enough nitrogen to keep the fuel tank ulIage spaces inerted at all times. The SPO plans to evaluate this feature by running ground tests in late 1995 on a fuel system simulator to determine inerting performance. This simulator of the complete F-22 fuel system cart simulate filet transfer, slosh, and vibration, arid cart generally test for proper filet system functionality before the aircraft actually flies (SPO, 199Sb). Sensors ~MI] be installed in each file} tank of the simulator. This will verify that the oxygen measurement sensor determines the concentration of nitrogen-enriched air and activates the OBIGGS. EMD aircraft watt have a sensor in each air vent line. Flight test data can then be related to the ground fuel tank simulator data. Suggested Revisions The committee has no suggested revisions to the test program. The lack of testing of a potential fuel ingestion problem and the rationale for it are noted. The committee believes Mat the operational community (e.g., mission planners aIld pilots) should be made fully aware that a fuel ingestion risk to the aircraft exists at a filer state higher than 60 percent. 4 Studies accomplished at the Naval Air Weapons Center in the late 1980s indicated that an oxygen concentration at or below 9 percent provides total fire suppression capability in the fuel tank ullage. These studies agree with tests completed by the Air Force in the 1970s.
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Sufficiency of F-22 Testing Plans Flight Control and Auxiliary Systems Description and Attendant Vuinerabilities 63 The term "flight control and auxiliary systems" is broadly construed to encompass those systems necessary to ensure safe, controlled flight. Included are the flight data systems and avionics that generate signals to control the aircraft, hydraulics that actuate the flight control surfaces, and electrical systems that power the avionics. In addition, an environmental control system provides cooling for mission avionics and the cockpit while an on-board oxygen generating system provides enriched breathing air. These latter two systems are necessary for mission ~ _ completion but not for safe flight. The air data system and control avionics consist of doubly and triply redundant modules and sensors spatially distributed around the cockpit area. The critical units are air cooled and thus not dependent on the closed-Ioop liquid cooling system (which uses the flammable fluid, PAO (polyalphaolefin)) required for much of the other avionics on the vehicle. However, coolant is present in marry of the relevant avionics bays, and there is the potential for a coolant fire to extend to and disable flight-critical components. The principal sources of hydraulic and electric power are the two airframe- mounted auxiliary drives (AMAD). Each driven by a different engine, the AMADs consist of a gearbox-mounted hydraulic pump and two electric generators. Additional electric and hydraulic power can be provided by the auxiliary power unit (a small gas turbine engineer which can be started in flight. Each AMAD c, cat___ ,, _ _ ~ _ . _ . · . . . powers an Independent, multiply branched, isolated hydraulic system equipped with leak detection and automatic shutoff features. The electric power system is similarly redundant and fault tolerant. The design intent is that the aircraft be controllable with a single hydraulic and electric power system. The classic aircraft vulnerability is a single-shot kill of multiple branches of redundant hydraulic, electric, or control systems. Early analysis of the F-22 showed such problems in the electric power distribution centers, the hydraulic system, and the flight control avionics and wiring. Components were relocated and lines rerouted accordingly (Griffis, 199Sa). These systems do, however, continue to present vulnerability concerns. Placement of the AMAI)s is such that both main hydraulic pumps are collocated near the aircraft centerline, close enough for a single-shot kill of both units. The horizontal tail actuator has been of concern in other tactical aircraft with flying tails because actuator failures can result in hard-over actuation, leaving the vehicle uncontrollable (Griffis, 199Sa). This actuator bay does not have fire protection. There is also a fire concern with the aileron actuators and the flight control avionics oaring to the proximity of the flammable cooling fluid.
64 Live Fire Testing of the F-22 The details of fragment interactions in crowded equipment bays are difficult to predict. So are possible synergistic interactions between systems. An example is the reaction of battle-damaged wiring to sprays of combustible liquids such as fated, hydraulic fluid, and coolant. Plannec! Analyses and Tests ~_ The flight control and auxiliary systems have been subjected to extensive analysis. Live fire testing has been conducted or is planned to explore many of these systems. Test 6 will place shots in the forward fuselage Tower and upper landing gear bay avionics bays, AMAD bay, wing leading-edge bay, and main (SPO, 199Sa). The aileron bay was shot in Test ~ ~ (Griffin, 1995a). Test ~ ~ consisted of many shots with 30mm HE} projectiles on a test article representative of the aileron bay. The objectives of the test were to determine (~) the burst radius required to start a fuel-based dry bay fire, and (2) whether the vulnerability analysis overpredicts fires (Holthaus, 1994~. The test article had the same structure, materials, and fuel as the EMD configuration. Mock-ups of representative components were included and contained coolant and electrical lines under operating conditions. The test article was subjected to an air flow of 400 knots during the tests. Eight shots were fired through the dry bay, through the fuel bay, or at the spar in between the bays. In all but one shot, there was no fire (Griffin, 199Sa). The shot closest to the spar (at I.5 inches) ignited a brief fire. Five shots were fired at the electronic warfare box, and 4 of the 5 resulted in fire. The electronic warfare box contained both electrical and PAO lines. Structural predictions were made and compared with results but have not been reported. The results of Test ~ ~ were qualitatively consistent with analysis predictions. Assessment Despite subjecting the flight control and auxiliary systems to extensive analysis and testing, uncertainties remain under the current plans. Chief among them is that the current vulnerability analysis does not properly account for the flammable properties of hydraulic arid cooling fluids. Both test data arid analysis methodology are currently lacking in this area.
Sufficiency of F-22 Testing Plans Suggested Revisions 65 Flammability testing of both the PAO coolant and the hydraulic fluid is needed. The SPO's proposed testing of fluid flammability (discussed earlier) we diminish the uncertainties identified above. Thus, this testing should be undertaken. Test data must then be incorporated into models suitable for vulnerability assessment. Weapons Bay and Ordnance Description and Attendant Vuinerabilities The F-22 weapons carriage system includes internal arid external weapons, missile launchers, and built-in weapons-Ioading equipment. In the low susceptibility configuration, all weapons are carried internally. The primary configuration is four AIM-120 missiles, two AIM-9 missiles, and an M6lA2 20mm gun. While weapon vulnerability is always a major factor in overall vulnerability of the aircraft, its importance is amplified by internal storage. This is especially true for the rocket motors that, if ignited when internally mounted, can cause catastrophic damage. The F-22 design also includes the capability to carry weapons externally. Vulnerabilitv analyses c)f the aircraft to plate have not considered external storage because that configuration is not compatible with the combat missions currently defined for the aircraft. On-board ordnance is vulnerable to fragments impacting the weapon and causing reaction of the high energy components, rocket motor fuel, and high explosives in the warhead. It is also vulnerable to fires in the weapons bay. There have been no special provisions to protect the on-board ordnance from fragments or fire, but some reduction in vulnerability is provided by the surrounding structure of the aircraft, including weapons bay doors. As noted earlier, vulnerable areas for on-board ordnance are not covered by specifications for the aircraft under the assumption that they are out of the control of the aircraft designer. However, the SPO did cover these areas in mission analysis of the survivability of the aircraft. The ammunition for the M6lA2 20mm gun is electronically primed. Tests on the gun system have shown, for this ammunition, that the ignition of any round in the system will not result in the sympathetic ignition of any other round. Thus a hit on the ammunition, while killing the gun itself, will not result in loss of the aircraft. Therefore, the gun ammunition is not included in the vulnerability assessment.
66 Planned Analyses and Tests Live Fire Testing of the F-22 In the analysis of F-22 vulnerability, it is conceded that the burning or explosive reaction of either the rocket motor fuel or the warhead explosive, when carried internally, would result in loss of the aircraft (SPO, 1995a). There are no tests planned of the winerability of on-board ordnance to fragments or projectiles. The lTCG/AS and the Joint Live Fire Test Program will be relied on to provide estimates of the vuinerabilities of the F-22 due to on-board ordnance for purposes of end-game analyses. It is recognized that these estimates are not complete, and new estimates wall have to be included when they are available. This is one case in which the F-22 SPO has not been provided sufficient tools or data to do a completely realistic assessment of the vulnerability of the aircraft (see discussion of tools in Chapter 5~. While there are currently no proposed tests of the ordnance on the F-22, the SPO has suggested that, if additional funding were available, it would consider a series of tests to prove the efficacy of using ablative coatings to protect the internal weapons bays from rocket motor fires (see discussion earlier in this chapter) (Graves, 1995~. Assessment The committee believes that failure to cover the effects of on-board ordnance in the vulnerability specifications is illogical. The SPO could analyze the implications of on-board ordnance in the vulnerability specifications and have the contractor account for them in the aircraft design (e.g., the structure could provide a degree of protection from fragments). Also, the impact of aircraft design on protection of ordnance and on possible mitigation of the effects of damage should be considered important to survivability of the pilot even if it were not so important to survivability of the aircraft. While the committee understands the SPO's decision, the vulnerability contribution of the ordnance has not been given adequate attention. For example, insufficient attention has been given to defensive measures such as sensing the inadvertent ignition of an internally carried rocket motor and ejecting the weapon, using ablative material to protect the aircraft from burning rocket motors, or affording greater physical protection to the internally carried rocket motors, warheads, and gun ammunition. Also, the vulnerability of the ordnance has not been Ally established. The ITCG/AS md Joint Live Fire Test Program require further funding to accomplish this task.
Sufficiency of F-22 Testing Plans Suggested Revisions 67 The committee fully agrees with the SPO's proposal to establish the efficacy of ablative materials in the weapons bays. Further analysis of the tradeoffs associated with additional ordnance protection or defensive measures such as ejecting burning weapons is suggested. The ITCG/AS and the Joint Live Fire Test Program should be funded to assure the completeness of data on the vulnerabilities of on-board ordnance. Engines Description and Attendant Vuinerabilities The F-22 is powered by two FIl9 low bypass ratio, afterbuming turbofan engines adjacently mounted at the rear ofthe aircraft. The FT 19 incorporates many innovations intended to provide superior performance, maintainability, and survivability (SPO, 1995a). Survivability features include the use of nonburning titanium and a robust, adaptable engine control system. It is the first afterburning engine to be equipped with two-dimensional vectoring nc)~:les intePrnter1 into the flight control system. The committee believes that dual-engine aircraft, like the F-22, may have an inherent survivability advantage over single-engine vehicles since, while both engines are needed to complete a mission, only one engine is needed for safe flight. However, there are failure modes of one engine (e.g., a disk burst or uncontained engine fire) that will likely result in Toss of the adjoining engine and structure and thus in loss of the aircraft. Many of the FIl9's technologies have been incorporated into civil or military engines of recent vintage. However, these technologies are not represented in the live fire data base, which mainly contains infonnation derived from tests of older engines. Thus, the accuracy of the empirically based vulnerability analysis of the engine has new uncertainties (e.g., (a) foreign object damage tolerance and fuel ingestion resistance of composite far1 stators, (b) failure dynamics associated with ballistic impact and subsequent containment of hollow fan blades, (c) fracture dynamics of new disk alloys and the integrally bladed rotors fabricated from them s The F119 engine is to be constructed with titanium "alloy C." The committee understands that the engine contractor has tested this alloy under conditions representative of use in this aircraft, and it will not ignite and burn where more common titanium alloys readily do. Fracture mechanics and fabrication problems have inhibited its use previously. The committee did not independently review these data, but it accepts the contractor's judgment.
68 Live Fire Testing of the F-22 in response to ballistic impact, (~) response of the mechanically complex vectoring nodule to impact damage, and (e) vulnerability of an engine actuation system employing fuel as the working fluid). Some other vulnerability concerns are raised by the engines' installation. While there is considerable redundancy in the engine control system, both control units and their associated wiring are located on the bottom of the engine, as are all the engine accessories for reasons of maintainability. Thus, all the accessories and controls are vulnerable to multiple fragment impacts on the bottom of the aircraft. Although the fuel ingestion hazard is considerably reduced once the front tanics are emptied early in flight, fuel remains a concern since there are fuel tanks on either side of the engines. Thus, a projectile shot line from the side that punctures an engine will have first punctured the adjoining fuel tank, admitting filet to the engine bay at the same time the fuel is heated by hot gas from the engine puncture. The engine bay fire protection system is marlually actuated, so prompt action will be required by the pilot in such cases. Planned Analyses and Tests Vulnerability analysis of the F! ~ 9 engines for the F-22 has been carried out with existing models (see Chapter 5) in much the same manner as it has for the other aircraft systems. No live fire testing is currently planned under the F-22 program on the FIl9 engine or its unique components, although the Joint Live Fire Test Program has an unended test program for such engines. Assessment Modern engines are relatively vulnerable systems with little history of full- up, full-scale live fire testing. Rather than depend on such testing, engine vulnerability estimates are derived from analysis based on subcomponent results. Much of the data base used in the FT 19 vulnerability analysis codes stems from the testing of much older engines and components constructed with different materials and design features. Thus, the uncertainty of these calculations must be considered relatively high until they are validated by test data. It should be emphasized that the new technology in the F ~ ~ 9 does not necessarily increase the engine's vulnerability (just the opposite in many cases). It does, however, increase the uncertainty of the vulnerability analysis.
Sufficiency of F-22 Testing Plans Suggested Revisions 69 Although some of the needed component testing is included in the ~ 996 and 1997 plans of the Joint Live Fire Test Program, these tests are not currently funded. The committee believes that these engine-related tests should be pursued, with a focus on F! 19 components. Flight Crew Description and Attendant Vuinerabilities Design of the F-22 includes significant capability for the survivability of the pilot. Three primary systems must be addressed: (~) the pilot, (2) the ejection seat and escape system, and (3) the life support system. In vulnerability analyses, the first two are considered in the context of a kill and the last in the context of mission abort. The pilot is valInerable to fragments, blast, fire, laser damage to the eyes,6 toxic fumes, chemical and biological weapons, spell, ricochet, secondary debris, secondary damage resulting from explosion of fuel in the F-! fuel tank, failure of the ejection system, and so forth. Fire in the cockpit area would certainly, if uncontained, result in loss of the aircraft. However, fire should not prevent crew ejection as long as critical components of the ejection system are not adversely affected (e.g., the seat's ballistic components or components that affect ejection sequence timing). Cockpit fires could result from ignition of PAO fluid in su~rour~ding electronics, from the explosive elements of the ejection system, or Mom Mel leaking from ruptured tanks in the forward area. Vulnerabilities to fire are discussed elsewhere in this chapter. Associated wad fire damage to the a~rcraD are toxic fumes and smoke, which could be drawn into the cockpit from nearby fires or other damage. The life support system helps to protect the pilot from this threat. 6 With respect to laser damage to the pilot's eyes, the pilot's visor is considered to provide adequate protection (Giorlando, 1995~.
70 Planned Analyses and Tests Live Fire Testing of the F-22 Flight crew vulnerability to fragments, including secondary spell and ricochet, is based on standard lTCG/AS tables and determined as a vulnerable area, contributing to the overall vulnerable area of the F-22 (SPO, 1 99Sa). While there is no special protection for the pilot in the design, some shielding is provided by surrounding structure and electronic modules. Modeling is used to assess the vulnerable area in the presence of the surrounding structure and modules. The committee received indications from the SPO (Ogg, 1995) that pilot protection methods had been evaluated. The SPO determined that the protective methods considered (e.g., KevIar) would be only marginally successful. In addition, the added weight of the material (over 100 pounds), while decreasing flight crew vulnerability, had a greater negative effect on overall system survivability than the relatively small positive contribution of the protective material. While pilot vulnerability to blast is not explicitly dealt with, it is implicitly and adequately handled through the blast vulnerability assessment of the aircraft. Vulnerability of the forward fuel tank to fire or explosion, and the potential impact on pilot or escape system, is discussed elsewhere in this chapter. The committee is not aware of arty planned live fire testing of crew or escape system vulnerability. Rather, vulnerability ofthese components is calculated by modeling, and a single fragment hit on these areas is conceded as loss of the aircraft. Assessment Owing to time constraints, the committee was not able to assess the analytical methodologies being applied to flight crew survivability within the vulnerability community. The committee accepts the SPO's belief that it is very difficult to protect flight crews from modem threat systems without severely impacting overall survivability. Nevertheless, the committee believes that efforts to reduce flight crew vulnerability should continue, both by the F-22 SPO and the ITCG/AS, because of the flight crew's contribution to overall F-22 vulnerable area.
Sufficiency of F-22 Testing Plans Suggested Revisions 71 The committee has no suggested revisions to the test program. However, the F-22 SPO and the ITCG/AS should emphasize their continuing efforts to develop improved methodologies for reducing flight crew vulnerability. Fire Protection Systems Description and Attenclant Vuinerabilities The fire protection systems detect, isolate, contain, and extinguish fires and suppress explosions. Advanced optical fire detection sensors are included for rapid fire detection. In addition to active fire detection and extinguishing components, fire safety is enhanced by ventilation and drainage of flammable fluids and fuel tank inerting. Combustibles. Fuel tanks are separated from adjacent compartments by liquid-proof and vapor-proof barriers. Fuel tanks located adjacent to the engine and auxiliary power unit (APU) compartments and the cockpit are separated from these compartments by a second liquid-proof and vapor-proof barrier in addition to the barrier provided by the fuel tank compartment. Environmental control system bleed air ducts are insulated as required to limit the external duct surface temperatures to a maximum of 700°F within the engine compartments and 500°F in dry bays. Steel and titanium plumbing or the equivalent are used in fire zones. When flight-critical flammable fluid lines run through fire zones, the lines are shrouded and the shrouds are vented and drained. Compartments containing flammable fluids or reservoirs, and those adjacent to fuel tanks, are ventilated at to 3 changes per minute to prevent accumulation of flammable vapors (SPO, 995a). Fire Detection. An optical fire detection system provides fire detection capability in the engine and APU compartments. It monitors ultraviolet radiation produced by burning hydrocarbon fuel to sense the presence of a fire. The optical fire detection capability is provided by eight optical sensors located in each engine compartment and four optical sensors in the APU compartment. The fire protection module located in the integrated vehicle subsystem controller monitors and processes the status signals produced by the optical sensors to provide fire and fault information (SPO, 199Sa). A thermal overheat detection system operates along the engine high-pressure bleed ducts and reports overheat conditions. Dual-loop, discrete thermal detector
72 Live Fire Testing of the F-22 units are installed adjacent to the high-pressure bleed ducts in each engine compartment to detect leaks that may impinge on critical structure or component or constitute an ignition source. Coverage is divided into three zones. Both left and right bleed zones provide detection within the respective engine compartments for the sections of duct between the engine arid the bleed air manifold and along the engine ar~ti-icing duct. The center bleed zone provides detection from the bleed air manifold to the primary heat exchanger (SPO, 1995a). Dry Bay Fire Protection. Fire detection and suppression is provided for the left and right main landing gear wells and the left and right wing attachment bays aft of the main landing gear wheel wells. Detection and suppression is provided by a dry bay fire protection unit that combines an infrared optical fire sensor and a pressurized Halon-filled cylinder in an integrated unit (an agent to replace Halon is being sought). The extinguishing agent is automatically discharged when a fire is detected. For fire containment and control, firewalIs separate the engine compartments, engine nozzles, and APU compartment from adjacent compartments. The firewalIs are designed to prevent flame penetration for IS minutes when subjected to a 2000°F flame. Critical components are fire hardened to prevent damage from exposure to flame or fire-generated thermal energy. A fire extinguisher provides suppression capability to either engine or APU compartments. The extinguisher contains approximately five pounds of Halon (a one-shot system) (SPO, 1995b). Dry bay fire remains the largest vulnerability contributor of the file] system. As a general rule, dry bay fire occurs anywhere a fuel tank, component, or line is adjacent to an internal dry bay that is not protected by fire extinguishing. Fire protection is included in 15 separate bays. Foam is located in strategic areas around the F-l fuel tank (behind the pilot) where the maioritY of hit-in~tiated fires could occur (my' llama). These areas were unique in the design because they were also unoccupied by components and could easily be filled with the foam. Fuel System Explosion Suppression. The fite! system is designed who functional redundancy arid component separation so that a single hit will not interrupt fuel to both engines. The current fuel usage schedule buns fuel from selected tanks, which are assumed to be emptied early. The fuel ingestion kill mechanism associated with this assumption was discussed earlier in this chapter. OBIGGS (discussed earlier) protects against ullage explosions. Planned Analyses and Tests Dry Bay Fire Extinguishing. The fire detection sensors are to be tested under laboratory conditions. The fire detection sensor's field-of-view analyses are
Sufficiency of F-22 Testing Plans 73 completed. The plan for Test 6 details the ballistic dry bay fire tests that wall evaluate the dry bay fire protection system (SPO, ~ 995b). The areas of interest for Test 6 are illustrated in Figure 4-9. It is expected that the Halon replacement agent (see below) wall be less efficient. Therefore, containment bottles may need to be resized accordingly web some size and weight increases. Evaluation Plan for Halo n Replacement. The Halon Replacement Program for Aviation covers more than just the F-22 program. An alternate fire suppression agent for Halon has been identified for use in aircraft dry bays and engine cells. The replacement program involves a three-phase effort for both applications (SPO, ~ 995b): . . Phase ~ studied the operational parameters that affect the amount of agent needed for each fire environment. The four most significant parameters were fourth to be surface temperature, air temperature, fire location, and fuel type. Phase IT was art operational comparison of three selected agents. Testing was conducted to screen and compare performance data on alternative agents, arid one agent (known as HFC-125) was selected. Phase Ill is under way now to establish design criteria methodologies. A product of Phase Ill watt be design equations for use in sizing suppression systems that use the new agent. The F-22 program will not need to evaluate the Halon replacement agent. It will test and evaluate the installed F-22 dry bay extinguishing system. Dry Bay Foam. The SPO feels confident that the characteristics of dry bay foam are well understood foam is expected to provide protection only against missile fragments and small caliber API threats. For this reason, no tests are planned to evaluate the material specifically (SPO, 1995a). Synergistic Elects. Synergistic events, such as fires caused by arcing electrical power wires being sprayed by environmental control system coolant or hydraulic fluids, are poorly understood. These events must be recognized as a potential kill mechanism with a high degree of uncertainty. Test 6 will examine the synergistic effects of PAO liquid coolant fluid, reduced flammability hydraulic fluid, aircraft fuel, and electrical power in protected and unprotected, cluttered aircraft dry bays. Test 6 will also examine fire detection and extinguishing capability in protected aircraft dry bays of the F-22.
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Sufficiency of F-22 Testing Plans Assessment The infrared sensors used in the fire detection system are a strength of the system. They are dual-char~nel sen- sors, each with a 90-degree field of view. Their response time is between 5 milliseconds to detect a fast growing fire and 5 seconds to detect a slow growing fire. Their wiring logic incor- porates power interrupt detection. Accessibility for maintenance was a major consideration in design instal- lation. The integrated vehicle subsys- tem controller is designed to provide essential redundancy for cockpit in- dication of any existing fire (SPO, 1995b). Test 6 is a comprehensive and systematic way to evaluate and optimize the effectiveness of dry bay protection. Test article frames are designed to withstand multiple ballistic impacts, can be easily reconfigured after each test, and contain representative bay sizes. Environmental control system airflow wart be used when necessary. Electrical wiring with operating voltage and currents wait be incorporated, significantly adding to the validity of the tests. Test 6 is a valuable subsystem type test Mat cost-effectively uses other-than-flight hardware and allows optimization should problems be uncovered. Test ~ ~ was discussed earlier. In Test 6 and Test ~ ~ together, a large number of shots (approximately 70) have been made or are planned. Even though the test articles do not contain all of the fluid lines and electrical wires, but only the major ones, it seems likely that this number of tests wall allow the development of a robust methodology that can be extrapolated to estimate differences under entirely realistic conditions. Test 6A will examine the synergistic effects of pressurized PAO coolant lines and PAO-cooled avionics modules and the adjacent powered electrical wiring in the F-22 forward fuselage lower avionics bays. The SPO indicated that it may use the prototype air vehicle fuselage in Test 6A and will include all sources of PAO fluid and electrical wiring in this test (Griffis, ~ 995a). Greater realism in this type of test has merit. The test program is quite thorough, but the committee believes that the prototype air vehicle fuselage should be used in Test 6A in order to achieve greater realism. 75 S ., ~erg,lst ~e~=,,.~ !~e ~ ~ . ~t}al ~ag.e to one sub.syste.m.m s It: ::: :-::~:::: :: :::: :::: :::::::::::::::::::::r:: ~ ::::::: '::::;::: ;:; ::::::;:::;:::::::::::::::::::::::::::: .:. In . e3Ie.GIS...t lat. Came ..=tner.. ~ A. .IQ tone . . o.r . more .~er e s ........... , , , ...... , ~; ; ~ , ~ ~ ...... . ~. ~t ~- ., . - ......... ..... . - - - - - . . - . For exempt' ; ....... w. Ir~g ~d . - s. on. A ~ -my fl id . ... condoner. (e.g.~2..~.~u'6 r: .:, , .: , , . ; ... . lme~ Without m~lat~g a f ~frOm Be . ~.~ e~ cot . · . . . . . · . . . : ~]e ~o. ~ g ~ ~ ~ ..................... ~ . I ..... . .. . ........ .. ............... .....severe~ele~rlca. -.ca ~ es, ~ ~ ~ con ~ .msu~ ........... ~,8 ,~. . . . . ... . : . : .. :. . .: . s~.~tlc eats. cowd Q££~.~. ............. : : ' ' """''""'''""""""'""'"'""""'"''"''''' ~..............
76 Suggested Revisions Live Fire Testing of the F-22 The committee urges use of the prototype air vehicle fuselage in Test 6A but has no other suggestions for the planned test program. ADDITIONAL OBSERVATIONS The F-22 SPO has devised and implemented a program consisting of m~lnerahilitv reduction design assessment. and testing that is founded on (a) comprehensive analysts ot the venerates of the aircraft, (b) identification of areas of uncertainty (as discussed in the introduction to this chapter), and (c) tests and assessments required to address these uncertainties. The committee believes that this program is well conceived and sufficiently realistic to support the request , _ ~ ~ ~ O . t ~ ~ · ~ · . , ~ ~ ~_, ~ ~ · ~ , · ,_ , . 1 for the waiver of the live fire test law. If modified as suggested earlier in this chapter, the program watt be strengthened as the F-22 proceeds with EMD and initial production. However, vulnerability assessment is a complicated and difficult problem. Therefore, the committee carefully considered the need for continued testing of the vulnerability of the F-22, beyond the current scope. In the process, the committee held detailed discussions with the vulnerability and lethality assessment communities. At China Lake (Navy and Air Force personnel and a consultant to OSD were present), members of the committee were briefed on recent and planned live fire test programs for Navy aircraft (Tyson and Wise, 1995~. The committee has considered the Navy's assessment methodology in comparison to that of the Air Force and the F-22. While there are some philosophical differences between the Air Force and the Navy, there is a great deal in common where aircraft are concerned. The opinions below are held by the China Lake team and were not disputed in the meetings by the Air Force representative present. . Given a complete aircraft, more information can be extracted by testing its major parts than by testing the entire vehicle at one time. It is possible that additional data could be obtained from such testing of the F-22 that could result in revisions to the aircraft design to further reduce its vulnerability. Similar large assembly testing has been conducted and is planned for variants of the Navy's F-] ~ even though the China Lake team does not expect to discover unanticipated outcomes.
Sufficiency of F-22 Testing Plans 77 The committee grappled with the question of whether to recommend similar testing for the F-22. The paradox seems to be the question of recommending tests that are not expected to yield unanticipated results, especially for such an expensive aircraft. The Navy's rationale seemed to be that ongoing testing makes sense in that something is always learned, vulnerability assessment methodologies are improved, models and other tools are evaluated, and, like the industrial base, the live fire test base cannot be allowed to wither. After much deliberation, the committee agreed that the Navy's approach made sense for the Air Force's F-22. This fighter wall be in the inventory for decades and can be expected to undergo an evolution that includes other missions arid new configurations. That life cycle dictates a continuation of live fire testing. Accordingly, Me committee believes that the Air Force should pearl for expeditious vulnerability assessment testing of the F-22 similar to that being conducted or planned for variants of the Few. In particular, . . As soon as a source of large assemblies can be identified (e.g., from a damaged aircraft or other test hardware that is representative of production aircraft, including assemblies from early production or one of the nine dedicated test aircraft), these assemblies should be provided to the vulnerability assessment community. The committee recognizes that such assets may not become available until after production begins. It is assumed that the most useful information could be derived from the resulting test specimen if it is tested for vulnerability in major subassemblies rather than as a complete system configured for combat. The testing should be directed at (a) verifying predictions derived from the current live fire test program and the models used, and (b) testing the effects on the overall vulnerability assessment brought about by configuration and mission changes through the years. It is recognized that the results of such testing will likely not impact the design of the initial production aircraft of the F-22. The committee believes that this level of risk is acceptable in view of the comprehensive nature of We ongoing program modified to accommodate the revisions suggested by the committee. However, the results of such tests would certainly influence future production blocks or modifications of the F-22 to perform future missions. Techniques for repairing battle damage to the F-22's new composite materials arid new systems could be verified. Vulnerability assessment tools might also be improved.
78 Live Fire Testing of the F-22 CONCLUSIONS Adequacy of F-22 Threat Definition and Replication The committee accepts the threat environment defined for the current mission of the F-22. The assumed AP} and ~} rounds are reasonable replications of air-to-air, cannon-fired threats. The two discrete fragment sizes are representative of the fragments from the spectrum of warheads likely to be encountered. However, for some classes of warheads (e.g., annular or focused blast fragmentation), the kill mechanism that involves dense multiple fragment impacts may be important for the F-22 and should be considered in future analyses and tests. Overall Sufficiency The Air Force and its contractors responsible for the design of the F-22 have incorporated a large number of features in the design of the aircraft that watt reduce its vulnerability. These features include a structural design with largely multiple (redundant) load paths, inerted fuel tanks using OBIGGS, dry bay foam, double-walled barriers between the cockpit and fuel, redundant fuel pumps and cross-feed between tanks, multiply redundant electric power, redundant hydraulics and flight control actuators, separated triple redundant air-cooled mission computers, fault tolerant avionics, dual engines, and engine blade containment. In addition, the Air Force and its contractors have performed a detailed vulnerability analysis to determine the vulnerable area of the F-22 using revised versions of standardized computer models. They assessed the uncertainties in the analysis and then constructed a comprehensive live fire test program to address these uncertainties and validate several of the design features. The committee reviewed this overall assessment program and suggested specific actions by the Air Force and others to alleviate some concerns that the committee has. Given the F-22's current counter-air mission, the program is sufficiently realistic to support the requested waiver. With the committee's suggested actions and Me implementation of possible corrective measures as a result of the program findings, the committee concludes that the Air Force program will be strengthened as the F-22 proceeds with EMD and initial production.
Su~c~ency of F-22 Testing Plans 79 Specific Actions The specific actions suggested by the committee are listed below. For easy cross-referencing, the actions are ordered as in the evaluation section of this chapter. Structure and Integral Fuel Tanks . . . Conduct additional live fire testing to determine the `damage that can be expected from a hit in the Frame 6 aft boom attachment area. The Air Force should determine the most critical shot lines. Expand analyses to predict damage sizes and residual strengths of the aft boom, Frame 6, and horizontal tad] pivot shafts after being hit by 30mm HE] rounds. Also, determine the risk of aircraft loss should it be fourth that loss of a horizontal tad! is possible. Conduct further analysis of the aft filer tank (A-~) prior to the conduct of Test 4D. This analysis should be focused on determining the adequacy of the test specimen, with particular emphasis on its ability to simulate accurately the reaction of the entire tank. Fuel System and Associated Dry Bays Make the operational community fully aware that a fuel ingestion risk to the aircraft exists at a filet state higher than 60 percent. Flight Control ant! Auxiliary Systems . Conduct the tests and analyses proposed by the F-22 SPO on the flammability of coolant and other fluids and We attendant vulnerability of the aircraft. Weapons Bay and Ordnance . Undertake the analysis and test, proposed by the SPO, of ablative materials in the weapons bay. Also, conduct furler analysis of the tradeoffs associated with additional ordnance protection or defensive measures.
80 . Engines . Flight Crew . Live Fire Testing of the F-22 Fund the ITCG/AS and the Joint Live Fire Test Program to assure the completeness of data on the vuInerabilities of on-board ordnance. Fund the proposed Joint Live Fire testing of F! 19 engine components to alleviate the paucity of testing against those components. Emphasize continuing efforts by the F-22 SPO and ITCG/AS to develop improved methodologies for reducing flight crew vulnerability. Fire Protection Systems . Use the prototype air vehicle fuselage in Test 6A in lieu of a mock-up. (The Air Force has considered using this fuselage for Test 6A.) Additional Action As discussed above, the committee believes that continued testing of the vulnerability of the F-22 is desirable. This testing should be conducted expeditiously against representative production hardware in the form of large assemblies when they become available. These tests should be similar to those being conducted or planned for variants of the Navy's F-! 8. The committee recognizes that large assemblies from a damaged aircraft or other production-representative hardware, from any source, may not become available until after production begins, and the subsequent recommended testing will likely not impact the initial F-22 design. The committee considers this arrangement to be acceptable. However, test planning should begin soon. The use of this hardware and the results of these tests would influence fixture production blocks or modifications ofthe F-22 to perform future missions. Additional positive consequences may well include (a) verification of techniques for repairing battle damage to the F-22's new composite materials and systems, and (b) improved vulnerability assessment tools.
Sufficiency of F-22 Testing Plans 81 REFERENCES Ball, R.E. ~ 985. The Fundamentals of Aircraft Combat Survivability Analysis and Design. New York: American Institute of Aeronautics and Astronautics, Inc. GiorIando, ]. ~ 995. High Power Microwave. Presentation to the Committee on the Study of Live Fire Survivability Testing for the F-22 Aircraft, F-22 System Program Office, Wright-Patterson Air Force Base, Ohio, January 19. Graves, I.T. 1995. National Research Council Questions on Live Fire Test. Memorandum from Deputy Director, F-22 System Program Office, to Mike Clarke, Commission on Engineering and Technical Systems, National Research Council, February 14. Griffis, H. 199Sa. Live Fire Test Program. Presentation by to the Committee on the Study of Live Fire Survivability Testing for the F-22 Aircraft, F-22 System Program Office, Wright-Patterson Air Force Base, Ohio, January 19. Griffis, H. 1995b. National Research Council Questions on Live Fire Test. Memorandum to National Research Council, April 28. Hinton, W.S. 1994. Threat, Mission, and Operational Requirements for the F-22. Presentation to the Committee on the Study of Live Fire Survivability Testing for the F-22 Aircraft, National Academy of Sciences, Washington, D.C., December 21. Holthaus, T.M. 1994. Live Fire Test No. ~ I, F-22 Live Fire Test Plan, Live Fire Test of Aileron Dry Bay Fire Test Program. Wright-Patterson Air Force Base, Ohio: Wright Laboratory. Ogg, I. ~ 995. Vuinerability Program Overview. Presentation to the Committee on the Study of Live Fire Survivability Testing for the F-22 Aircraft, F-22 System Program Office, Wright-Patterson Air Force Base, Ohio, January 19. SPO (F-22 System Program Office). ~ 993. Live Fire Test 4A Renort. ~ ~ ~ , ~ ~ ~ ~ r _ · . · r.. _ 1vl~moranuum ro Roland ~ ancey, ~oemg Alrcrart ~omparly. Memo L8932- AL93-059. Wright-Patterson Air Force Base, Ohio: F-22 System Program Office. December 15.
82 Live Fire Testing of the F-22 SPO. ~ 995a. Combat Survivability F-22 Live Fire Test Program. Presentation to the Committee on the Study of Live Fire Survivability Testing of the F-22 Aircraft, F-22 System Program Office, Wright-Patterson Air Force Base, Ohio, January 19. SPO. ~ 995b. National Research Council Questions on Live Fire Test. Memorandum to National Research Council, March 23. Tyson, H., and T. Wise. 1995. Live Fire Test Program for the F/A-18E/F and V-22. Presentation to the Committee on the Study of Live Fire Survivability Testing for the F-22 Aircraft, Naval Air Warfare Center, Weapons Survivability Laboratory, China Lake, California, February 21.
