Sea Basing: Ensuring Joint Force Access from the Sea (2005)

As discussed in Chapter 1, the Navy and Marine Corps are exploring the issues involved in developing a new capability to deploy from a sea base amphibious forces that would move from the sea base directly to objectives inland. This new tactical concept for conducting amphibious operations is called Ship-to-Objective Maneuver (STOM). The logistics connections required for STOM from a sea base are illustrated in Figure 2.1.

If amphibious forces of the future operate according to the vision for Sea Basing, highly mobile Marine units will be widely dispersed well inland when needed. They will be focused on key objectives with high military or political value. They will not be clearing and securing areas through which they pass en route to their objectives.

In some conflicts of relatively low intensity, it may be possible to establish traditional supply lines for moving supplies and equipment and to evacuate the wounded by truck convoy. However, in more typical situations such as in the ongoing war in Iraq, traditional supply lines will be vulnerable to attack from bypassed enemy units, suicide bombers, land mines, and so on. Army and Marine units pushing toward Baghdad, for example, were held up by the need to bring supplies overland through a gauntlet of bypassed Iraqi forces.

FIGURE 2.1 Maneuver from a sea base will depend on heavy-lift, vertical-takeoff-and-landing (VTOL) airlift. NOTE: A list of acronyms is provided in Appendix C. SOURCE: Constructed from data supplied by Maj Scott Kish, USMC, N703M, “Analyses of Sea Basing Connectors,” presentation to the committee, September 9, 2004, Washington, D.C.

The ability to resupply combat forces over long distances without depending on truck convoys is fundamental to the operational concepts of Sea Basing. This means that Marine Corps ground forces and early-entry Army forces will be more dependent on air transport than they ever have been in the past.

The basic requirement for a Marine Expeditionary Brigade (MEB) is the capability to lift one battalion by air and one battalion by surface assets from the sea base within the 8 hour period of darkness. Army requirements for a Brigade Combat Team (BCT) are not available for such a scenario. The minimum threshold value of the payload is 40,000 lb.¹ The objective value is 50,000 lb, based on

the weight of the Stryker Interim Combat Vehicle, which is representative of a future light tank. (It is assumed that if heavier armored combat vehicles, such as the M-1 tank, are needed, they would be landed from the sea.) Ongoing studies by the Center for Naval Analyses and the Marine Corps indicate that the distance requirement is an operational radius of 150 to 300 nautical miles (nmi). Greater distances are certainly desirable, perhaps out to 500 nmi, reaching beyond the littoral. The radius and payload capabilities and requirements of the heavy-lift aircraft are illustrated in Figure 2.2 and listed in Table 2.1.

In many respects, the heavy-lift aircraft will be a design driver for major ships of the sea base, and because of the time that it could take to develop a suitable aircraft, that aircraft could be a pacing item in the overall sea base design. It is therefore worth examining the technical aspects of the design of such an aircraft in some further detail. The following discussion thus examines in more depth the technical details and problems of several options for heavy-lift aircraft.

FIGURE 2.2 Heavy-lift, vertical-takeoff-and-landing capabilities: radius and payload requirements to support Sea Basing. NOTES: NAVAIR, Naval Air Systems Command; Ops, Operations; “Stop-Gap,” temporary; IPT, Integrated Product Team; Ambient TBD, ambient temperature and altitude are to be determined; COD, carrier onboard delivery; STOM, Ship-to-Objective Maneuver; lb, weight in pounds; nmi, distance in nautical miles; NATO, North Atlantic Treaty Organization.

The payload and range performance of current aircraft is summarized in Table 2.2. “Range” is the total distance that can be flown on one tank of fuel, whereas “radius” is the distance that can be flown out and back on one tank of fuel. The radius is generally half the range, unless the aircraft must land and take off again at midmission, in which case the radius can be significantly less than half the range.

With its maximum payload of two Humvees, the operational radius of a CH-53E helicopter is about 100 miles. At a speed of about 110 knots, the CH-53 could transport the assault echelons of an MEB a distance of about 110 nmi in one 8 hour period of darkness. The operational radius of an MV-22, with its maximum external load equivalent to one Humvee, is also about 100 miles.²

However, these aircraft are not capable of transporting a light assault vehicle (LAV), each of which weighs about 30,000 lb, over that distance (the Stryker combat vehicle will weigh even more). In addition, an operational radius of 100 miles is not great enough to support the needs of STOM. The maximum payload of the CH-53E and MV-22 decreases with distance, especially with external loads. These two aircraft will not be able to meet all of the needs of Marine Expeditionary Forces over the distances envisioned for Operational Maneuver From the Sea (OMFTS).

Both the Navy and the Marine Corps have considered a notional helicopter designated as the CH-53X with improved performance. However, there are some recognized limitations in the performance that ultimately may be achievable. These limitations are related to the diameter, loading limitations, and tip speeds of the rotor blades. Because of these limitations, the CH-53X has been discussed but is not included in acquisition programs of record or supported by Navy science and technology funding. The Technology Readiness Levels (TRLs) of a notional CH-53X have not been established, but the members of the committee

estimate the TRL of the CH-53X concept to be at about TRL 4 (TRL 4 is defined as component and/or breadboard validation in a laboratory environment). The committee estimates that approximately 3 years of research and development (R&D) would be required before design could begin. Optimistically, an initial operational capability (IOC) might be achieved 7 years subsequent to the initiation of procurement. If the CH-53X procurement were to be initiated in Fiscal Year (FY) 2006, an IOC might be achieved by 2015.

The anticipated requirement to carry more than 44,000 lb as far as 300 nmi is similar to the current capabilities of the Russian Mi-26 helicopter, so that the capability to meet the range and payload requirements of STOM has been demonstrated. The existence of the Mi-26 suggests that the technology for such an aircraft is already beyond TRL 6. However, it is not clear that such an aircraft could support the need to deploy the assault echelons of an MEB over 300 nmi in 8 hours, owing to its time en route. Each heavy-lift rotorcraft would be able to make just one round-trip, so that the number of aircraft required would be prohibitve. However, this mission has not yet been analyzed and should be studied.

The committee agrees with the assessment of the Office of the Secretary of Defense³ that a more modern version of the XCH-62 Tandem Rotor aircraft is the best alternative for a new heavy-lift replacement rotorcraft, because this reduces technical risk compared with that involved in developing a single-rotor aircraft. However, the requirement to transport supplies and heavy equipment over long distances also includes very real operational and survivability considerations. Helicopters are inherently complicated and relatively fragile, requiring up to 50 maintenance hours per flight hour. At altitudes below 8,000 ft, helicopter operations are hampered by weather, dust, and terrain. In addition, ground threats are being keyed toward antihelicopter operations. Enemy interdiction of air routes from the ground will make it difficult to rely on helicopters for support.

For example, the assault on the Medina Division of the Republican Guard near Karbala, Iraq, on March 24, 2003, by AH-64D Apache Longbows revealed the vulnerability of even armored helicopter gunships to small-arms fire. Alerted to the Apaches’ approach by lookouts along their route, Iraqi civilians armed only with AK-47s and soldiers with unguided artillery fired barrage-style in a crude but effective ambush. After the failed raid, 27 of the 34 Apaches on the mission were no longer flightworthy. Rotorcraft will have limitations in meeting the air transport requirements envisioned for OMFTS.

The development of compound rotorcraft (which utilize wings to develop a significant fraction of their lift in flight), such as the experimental AH-56 Cheyenne, or Gyrocopters and Gyrodynes, would be a step in the right direction. They may be a somewhat faster than helicopters, as demonstrated by the Cheyenne compound helicopter (although similar speeds have not actually been achieved by any Gyrocopter or Gyrodyne). However, these speeds represent only an incremental improvement over those of helicopters. Compound rotorcraft cannot deploy the assault echelons of an MEB over the distances required (150 to 300 nmi) for Sea Basing in an 8 hour period.

In addition, conversion of the Gyrodyne from rotorborne flight to wingborne flight is dangerous because of the need to cut power to the cruise propulsors in order to power up the rotor. During this conversion, the aircraft is falling along a 1 in 4 glide path. Due to the weight and design of such an aircraft, autorotation will not be effective in retarding the fall in the event of failure to start the rotor. It is important to recall that no tipjet-powered helicopters were ever put in service, despite several development programs.

The Air Force Future Air Mobility Command (AMC) is directed toward the development and eventual acquisition of a super-short-takeoff-and-landing (SSTOL) transport aircraft, capable of operating with a balanced field length of 2,000 ft. Even such super-short-takeoff-and-landing distances are too long for the envisioned sea base. The sea base would have to be longer than this to allow for dispersion on landing and braking under the worst conditions—a wet deck with the ship entering the trough of a wave. The wingspan of the SSTOL transport aircraft would be comparable to that of the C-130 (probably more than 130 ft), which would require a flight deck without an island and would prohibit parking another SSTOL transport on the flight deck. Each such aircraft would require that the entire deck be cleared for it to take off and land, limiting the sea base to cycling one aircraft at a time.

There will be other problems involved in the operation of SSTOL aircraft from the sea base. While these aircraft are transferring cargo from the ship to the forces ashore, the cargo on the sea base must be replenished by sealift arriving from an advanced base or from the continental United States (CONUS), as shown in Figure 2.1. The transfer of cargo to the sea base will require the sea base ship to be heading in a direction that protects the transfer from adverse wind and wave effects. On the other hand, launch and recovery of the aircraft will require the sea base ship to be headed into the wind at significant speed. The intricate choreography of heading and speed changes necessary to transfer cargo from the resupply ships, through the sea base, to SSTOL delivery aircraft will make it difficult to support the requirements of STOM in a timely manner. A seaplane is a possibility for resupplying the sea base from CONUS or the intermediate base, but a seaplane would not be suitable for landing at the advanced base.

An extreme STOL aircraft designed to take off and land on the sea base in 500 to 1,000 ft would also be capable of taking off vertically with virtually the same payload as that of the SSTOL. The thrust required to accelerate an aircraft that large to flying speed in those short distances would be sufficient to enable the aircraft to make a vertical takeoff. Similarly, the slow liftoff and approach speeds of such an extreme STOL aircraft (around 75 knots) would require the use of the same reaction control jets that are required for hover. Even with the reduced fuel at midmission, taking off or landing at a remote site in 500 to 1,000 ft and clearing a 50 ft obstacle would probably require the same thrust as taking off vertically from the sea base.

For all of the reasons cited above, the committee believes that the requirements of Sea Basing could lead to the need for a fixed-wing aircraft utilizing some form of powered lift for performance off the sea base ship’s flight deck—certainly for SSTOL or STOVL performance, and possibly leading to full vertical-takeoff-and-landing (VTOL) performance, depending on how the aircraft design emerges from detailed design studies.

The committee emphasizes the need to pursue advanced technology to develop a ship-capable, fixed-wing, VTOL transport aircraft to meet the requirements of Sea Basing. Efficiency during the 10 to 15 seconds of hover time required for VTOL is not the most important requirement. Cruise efficiency and speed in loading and off-loading cargo are more important. In order to provide for the resupplying and reinforcing of highly mobile Marine Corps, Army, and Special Operations Force (SOF) assault forces operating far inland, the committee recommends the development of the technologies for a ship-capable, fixed-wing, VTOL transport aircraft. As described below, some possibilities include a stowed-rotor aircraft, an aircraft with lift fans in the wings, or an aircraft with thrust-augmenting ejectors in the wings.

A stowed-rotor aircraft would be similar to the stowed-rotor concepts demonstrated in the past, but would carry the concept to the next step by stopping and stowing the rotor in order to achieve significant improvements in speed. The rotor would be slowed, then stopped and stowed in a compartment on top of the aircraft’s fuselage, similar to the payload bay of the space shuttle. The folding mechanism would be similar to that developed for the V-22. Stopping and folding a rotor in this way was demonstrated in the National Aeronautics and Space Administration’s (NASA’s) Ames full-scale wind tunnels more than 30 years ago. The critical technologies are as follows:

• The heavy-lift rotor and transmission system, and

• Integration of the folding mechanism.

Vertical-takeoff-and-landing operation requires thrust-to-weight ratios greater than that required for cruise. Significant increases in the static thrust of turbofan engines can be obtained by increasing the bypass ratio of the cruise engine for vertical takeoff and landing. The effective bypass ratio can be increased by using the energy in the cruise engine exhaust jet to power a lift fan installed in the wing of the aircraft. The system needs to have the lift fans in the wing, as the fuselage will be taken up with cargo. Similar aircraft, such as the Ryan XV-5, have been capable of taking off conventionally if the lift fans are damaged. Integrating the lift fans with the wing to produce a lift-and-propulsion system would provide a VTOL aircraft that also has good STOL capabilities and increased range and payload performance if they can utilize even a short runway.

The wing lift fans can be driven either by shaft power (as in the X-35B) or by hot gas tip drive (as in the XV-5A). The fans in both concepts would be large in diameter with low fan pressure ratios (FPRs) of 1.08 to 1.20. Previous studies have indicated thrust augmentation (fan lift/shaft horsepower (SHP) or fan lift/ thrust) in the range of 2.2 to 2.8 lb per SHP for the shaft-driven fan-in-wing and 2.0 to 2.8 lb per lb for the gas-driven fan-in-wing. The fans would be located at the longitudinal center of gravity with pitch control from fore and aft jets.

Both concepts would have at least two independent engines (one for each side), with cross shafting and cross ducting for one-engine-out capability. Good design practice would have more than two engines to lessen the impact of one engine out and to provide better thrust matching between VTOL and subsonic cruise or loiter. During cruise or loiter, the engines could be powered back or even shut down to match the power or thrust to the cruise or loiter drag.

Using augmentation in the range of 2.2 to 2.8 lb per SHP for the shaft-driven concept gives a power requirement of 25,000 to 32,000 SHP per side. The critical technologies are as follows:

• A lightweight gearbox and clutch to absorb ~25,000 to 32,000 SHP (the current limit is the gearbox for the JSF-35B at 15,000 SHP);

• Turboshaft engines rated at ~10,000 SHP (current limits are the Rolls Royce T 406 and Tyne engines at about 6,200 SHP); and

• Louvers, covers, and structure for the large-diameter wing fan.

Using augmentation in the range of 2.0 to 2.8 lb per lb for the gas-driven concept gives a static thrust requirement of 25,000 to 35,000 lb. The critical technologies are these:

• Louvers, covers, and structure for the large-diameter wing fan;

• In-flight inlet closure;

• Tip-driven turbine seals; and

• Plumbing the equivalent of more than 35,000 lb of thrust in hot, high-pressure gas to the wing-mounted fans.

Significant increases in the static thrust of turbofan engines can also be obtained by diverting the engine exhaust jet through an ejector, which is a pneumatic device that uses entrainment by the engine exhaust jet to pump a larger mass of air drawn from the atmosphere. A simple ejector consists of a nozzle that directs a jet through a duct. The thrust of the engine is increased by the suction forces that the entrained flow develops on the inlet of the duct. In effect, the ejector functions like a ducted fan and is a mechanically simpler alternative to the lift-fan system. Since ejectors can be used to augment the engine thrust, the additional thrust necessary to give an aircraft VTOL capabilities can be developed from a smaller engine that provides more efficient cruise. Integrating the ejector with the wing to produce a lift-and-propulsion system also provides an aircraft with good STOL performance.

Thrust-augmenting ejectors can also provide the advantage of lower disk loading and a more benign footprint compared with that of a lift-fan system. Mixing of the engine exhaust jet and the entrained air within the ejector duct reduces the velocity, temperature, and noise of the lift jets. The low-temperature and -pressure footprint of this mixed flow would enable an aircraft to operate from ships other than aircraft carriers, and in unprepared, constrained, tactical landing zones ashore. The critical technologies are these:

• Ejector design,

• Enhancement of turbulent mixing, and

• Noise abatement.

There may be even more innovative approaches than these described above. The requirements to fly long distances with a heavy payload and to take off and land vertically are almost mutually exclusive. Long-range aircraft must be large in order to carry the necessary fuel, but it is difficult for large aircraft to hover. This is a consequence of the square-cube law, which implies that as the size of an aircraft increases, its weight increases faster than its thrust can be increased. As an aircraft is made larger, its vertical thrust increases only with engine cross-sectional area (L²), while its weight increases with volume (L³); therefore, its thrust-to-weight ratio for hovering must decrease. At some size, the propulsion

system becomes too heavy for the aircraft to lift. It is for similar reasons that a flea can jump 100 times its own height, while an elephant cannot get all four feet off the ground at the same time.

However, the actual requirement is not to take off and land a large aircraft vertically, but rather to deliver and recover a 40,000 to 50,000 lb payload vertically. Therefore, an alternative approach might be a compound aircraft system consisting of two or more aircraft flying wingtip to wingtip in order to take advantage of the induced drag reductions available from flying in close formation. An aircraft’s best lift to drag (L/D) ratio occurs at the speed at which its profile drag equals its induced drag. Because the profile drag of an aircraft is proportional to the ratio of its wetted surface area to its wing area, S_w/S, and its induced drag depends on the aspect ratio of its wing b²/S, the best L/D ratio depends on its wetted aspect ratio b²/S_w. The expression is L/D = (π e /4 C_f) ^1/2b²/Sw.

This equation is plotted in Figure 2.3, in which the best L/D ratios of some representative aircraft are spotted on the curve to validate the relationship.

If two aircraft are joined at the wingtips, both the span and surface area are doubled, so that the wetted aspect ratio is also doubled. From Figure 2.3, this can be seen to increase the L/D ratio and range of the paired aircraft by about 60 percent. Joining three aircraft further increases the L/D ratio and range. As long as no aircraft flies in the downwash behind another, all of the aircraft in the formation benefit from formation flying.

FIGURE 2.3 Formation flight increases lift-to-drag ratio. SOURCE: Paul Bevilaqua, Lockheed Martin Aeronautics Company.

FIGURE 2.4 Project Tom Tom demonstrated drag savings from formation flight. SOURCE: Courtesy of the National Museum of the U.S. Air Force. Available online at <www.wpafb.af.mil/museum/history/postwwii/tomtom.htm>. Accessed June 2005.

In the early 1950s, the U.S. Air Force actually attached and flew a pair of F-84s on the wingtips of a B-36, as part of the Fighter Conveyer program. The exercise was called Project Tom Tom. The test aircraft are shown in Figure 2.4. The hook-up mechanism was similar to the Navy’s probe and drogue system. The drag of the system of three aircraft was shown to be the same as that of the B-36 alone, so that the F-84s were ferried with no increase in the fuel required. In terms of the expression in Figure 2.3, the increase in “span²” that results from attaching the fighters to the bomber was equal to the increase in surface area, so that the L/D ratio did not increase. Air-to-air refueling was selected as a better alternative—it did not seem as dangerous, and fuel was cheap. However, that was the era of bell crank and cables for flight control. With today’s fly-by-wire technology, this method of extending aircraft range should be reconsidered.

More recently, the NASA Dryden Flight Research center has validated the performance benefits of formation flight, having measured a 20 percent drag reduction for one F-18 flying behind another, and up to a 60 percent savings for an F-18 flying behind a KC-135. Greater increases in range could be achieved by using one of the aircraft to carry fuel for the others. Since the VTOL delivery aircraft would not have to carry its own fuel, it could be made even smaller. By flying behind and outboard of the tanker aircraft, the VTOL delivery aircraft could realize significant drag reductions, thus increasing range or payload.

Operating from the sea base, two or more smaller VTOL aircraft would take off at the same time and join in formation flight. This could double the range of any of the aircraft flying alone. Greater increases in range could be achieved by using one of the aircraft to carry fuel for the others. Since the VTOL delivery aircraft would not have to carry its own fuel, it could be made even smaller. By flying behind and outboard of the tanker aircraft, the VTOL delivery aircraft could realize significant drag reductions and increase its range. The VTOL delivery aircraft would detach from the tanker at midmission to deliver the payload and reattach for the return flight.

This hitchhiker concept would reduce the technical risk and cost associated with developing heavy-lift VTOL aircraft to support Sea Basing, by increasing the range of smaller aircraft that can more easily make vertical takeoffs and landings. The critical technologies are these:

• The VTOL lift system, and

• The software for automatic formation flight.

Concepts for all of the heavy-lift VTOL transport aircraft discussed in the preceding section are at TRL 2. Technology for heavy-lift aircraft would be matured to TRL 6 in four phases of increasing scope and complexity. The first phase would consist of analytical predictions of lift system and airplane performance in hover and transition, together with small-scale, wind tunnel model testing of airplane configurations in hover and transition. This work would bring the concepts to TRL 3.

The second would consist of a test of a full-size propulsion system powered by an aircraft engine in order to provide data at the full-scale lift jet Mach number and Reynolds number. These tests would be conducted on a whirl rig to investigate transition to wingborne flight. This work would bring the concepts to TRL 4.

In the third phase, a large-scale airplane model would be built and suspended from a gantry for hover testing. It would be mounted in the NASA Ames 80 ft × 120 ft wind tunnel to show transition characteristics. This work would bring the concepts to TRL 5.

In the fourth phase, to bring the concepts to TRL 6, it might be possible to replace the wing on an existing transport aircraft, such as the C-130 Hercules or C-27 Spartan, with a new wing. The aircraft would be flown from a hover through transition to wingborne flight in order to demonstrate successful development of the control laws, the ability to generate adequate control power, and the VTOL performance of a heavy-lift transport aircraft.

A very rough cost and schedule estimate for such a program is shown in Figure 2.5. The costs shown are not cumulative; each TRL level has been esti-

FIGURE 2.5 Very rough schedule for enhanced heavy-lift aircraft development, in years.

mated separately, but the estimate depends on the earlier levels having been accomplished. These estimates reflect the level of effort expended on similar development programs in the past. However, the very rough scope and phasing of these tasks should be considered useful only to begin discussion of the priorities and goals of Sea Basing.

The committee concludes the following:

• Given a heavy-lift replacement for the CH-53E helicopter and acquisition of the V-22 tilt-rotor aircraft, the Marine Corps will have the capability to move substantial quantities of materiel by air. However, these two aircraft do not have the range, payload, and speed performance to meet all of the needs of the assault echelons of a Marine Expeditionary Brigade over the 150 to 300 nmi distances and 8 hour deployment times envisioned for Operational Maneuver From the Sea, nor to meet the requirements for an Army brigade-level force of some strength, to be determined, for operation from the joint sea base. Therefore, the committee believes that the requirements of Sea Basing could lead to the need for a fixed-wing aircraft utilizing powered lift for VTOL performance.

• Powered-lift system concepts for such a heavy-lift VTOL transport aircraft are currently at TRL 2. Maturing the technology for each preferred

powered-lift system to TRL 6 could be accomplished in four phases of increasing scope and complexity. This will require up to 8 years and at least $150 million for each concept. The required time could be reduced with a corresponding increase in risk.

Recommendation: The use of advanced technology should be pursued to develop a ship-capable, fixed-wing aircraft having powered lift of some as-yet-undetermined configuration with approximately the payload capacity of a C-130J, and that the aircraft will be able to operate in super-short-takeoff-and-landing (SSTOL) or short-takeoff-and-vertical-landing (STOVL) mode, and possibly in full vertical-takeoff-and-landing (VTOL) mode. Such a transport aircraft should have the capability to carry a standard International Organization for Standardization (ISO) 20 ft container or the Stryker combat vehicle to an operational radius of 150 to 300 nmi at high speed and altitudes in order to meet the requirements of Sea Basing. The development of such an aircraft should be undertaken as a joint Service program in collaboration among the Air Force, Army, Navy, and Marine Corps.

Recommendation: The Joint Heavy-lift Aircraft Exploration of Concepts being coordinated within the Office of the Secretary of Defense should involve the U.S. Transportation Command in the process. This effort should be transferred to the Joint Sea Base Planning Office when this office is created.