Appendix C
A Framework for Comprehensive Analysis

In addition to asking for an initial analysis to estimate speed and observability trade-offs for the next-generation long-range strike system (LRSS), Task 2 of the statement of task requested that the committee outline a framework for a more comprehensive analysis that might subsequently be performed by the U.S. Air Force. This appendix outlines one such framework, not only to meet the statement of task, but also to serve as a guide-post for the analysis performed by the committee (described in Chapter 4). The appendix begins with a description of the complexity of the analysis problem: that is, the identification of a good balance between speed and observability for a future air vehicle. Then the committee presents a methodology (via a sequence of analytic tasks) for a comprehensive utility analysis that addresses the full complexity of speed and observability trade-offs.

COMPLEXITY OF THE ANALYSIS PROBLEM

Speed and observability both affect the operational utility of air vehicles, but any analysis of their trade-offs is confounded by many other design and operational variables that interact with them and also affect the operational utility of the air vehicle. Some of these design variables are listed below and defined in the Glossary in Appendix D:1

1

The reader is referred to Robert E. Ball, The Fundamentals of Aircraft Combat Survivability Analysis and Design, Second Edition, American Institute of Aeronautics and



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Future Air Force Needs for Survivability Appendix C A Framework for Comprehensive Analysis In addition to asking for an initial analysis to estimate speed and observability trade-offs for the next-generation long-range strike system (LRSS), Task 2 of the statement of task requested that the committee outline a framework for a more comprehensive analysis that might subsequently be performed by the U.S. Air Force. This appendix outlines one such framework, not only to meet the statement of task, but also to serve as a guide-post for the analysis performed by the committee (described in Chapter 4). The appendix begins with a description of the complexity of the analysis problem: that is, the identification of a good balance between speed and observability for a future air vehicle. Then the committee presents a methodology (via a sequence of analytic tasks) for a comprehensive utility analysis that addresses the full complexity of speed and observability trade-offs. COMPLEXITY OF THE ANALYSIS PROBLEM Speed and observability both affect the operational utility of air vehicles, but any analysis of their trade-offs is confounded by many other design and operational variables that interact with them and also affect the operational utility of the air vehicle. Some of these design variables are listed below and defined in the Glossary in Appendix D:1 1 The reader is referred to Robert E. Ball, The Fundamentals of Aircraft Combat Survivability Analysis and Design, Second Edition, American Institute of Aeronautics and

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Future Air Force Needs for Survivability Speed, Signature and/or observability, Countermeasures, Situation awareness, Range, Persistence, Altitude, Maneuverability, Payload, Weapon lethality, and Tactics and rules of engagement. The utility of an air vehicle depends on the operational missions that it may be called on to perform in the future. From today’s planning perspective, these are uncertain dimensions. The three focus missions for this study are (1) Global Strike, (2) Global Persistent Attack, and (3) Space and Command, Control, Communications, and Computers, Intelligence, Surveillance, and Reconnaissance (C4ISR), as discussed in Chapter 2. For analysis, air vehicle missions can be evaluated in 1-on-1, 1-on-N, M-on-N (where M and N represent the number of attacking and defending air vehicle systems in a particular engagement, respectively), or campaign contexts. Many threat dimensions affect the operational utility of air vehicles while they are performing a mission. These include the following: Threat type (airborne interceptor/air-to-air missile, anti-aircraft artillery, surface-to-air missile, directed energy, and so on); Threat ordnance (missiles, kinetic energy rounds, high explosive rounds, and so on); Threat sensors (radio frequency, infrared, acoustic, visual); Command and control (single site, networked sites, integrated air defense systems); and Degree of cueing available. The utility of an air vehicle also depends on the target type specified for the mission (hard and deeply buried target, weapon of mass destruction Astronautics Education Series, 2003, for a more complete listing and discussion of relevant aircraft design variables.

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Future Air Force Needs for Survivability [WMD], others), the time-sensitivity of the targets (mobile, moving), and the environmental conditions, such as the weather, daylight, and terrain. It is convenient for subsequent discussions to refer at times to the set of threat dimensions, target types, environmental conditions, and missions as the operational situation (Ops Sit) within which the utility of an air vehicle is to be evaluated and analyzed. These dimensions are uncertain as the Air Force plans for a future air vehicle, and most are uncontrollable by U.S. planners and operational commanders. Although not part of the utility analysis, the following dimensions need to be considered in assessing the overall value of an air vehicle: Costs (science and technology, R&D, procurement, operating and supportability); and Risks (technological and/or performance risks, time to field, cost). Given the multidimensional nature of the problem, the analysis of speed-and-observability trade-offs on the bases of their contribution to the utility of air vehicles is a complex activity. Fortunately, there are methodological approaches used in the Department of Defense (DOD) that can reasonably and appropriately consider this level of complexity in assessing weapons system design trade-offs on the basis of the forecasted operational utility of the system. One such approach is outlined in the following section. SUGGESTED FRAMEWORK FOR COMPREHENSIVE ANALYSIS The framework is presented by summarizing a sequence of major tasks in the approach. Task 1: Define Proxies for the Utility of an Air Vehicle The utility of any system in general, and the LRSS in particular, is its raison d’etre—the objective that it was designed to achieve. In conducting analysis, it is difficult to determine the objectives and appropriate proxy metrics (that indicate the degree to which objectives are achieved) from the customer because these objectives are rather subjective. They vary depending on whether the customer is a component designer, an aircraft designer, a planner, or an operating commander, and they vary with the level of the individual within the organization—for example, flight leader, wing com-

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Future Air Force Needs for Survivability mander, or Air Force commander. Under this first task, appropriate utility metrics need to be defined. As a general rule, utility metrics need to be defined at a level higher than all the decision variables in a decision problem in order to capture all of the interactions and interdependencies among the decision variables. In the air vehicle problem, the decision variables are the aircraft’s design variables, with particular emphasis on speed and observability. A number of utility proxies have been used in—or proposed for—analysis of aircraft strike systems. Aircraft susceptibility is a commonly used utility proxy that is affected by a number of design variables. It is defined as the “inability of the aircraft to avoid being hit by threat ordnance along a mission flight path.” Some proposed metrics of susceptibility include these: The time required to fly the mission flight path: presumably more time results in more hits, but this metric ignores other design variables that can reduce susceptibility (e.g., increased speed, low observability, countermeasures, and so on); The expected number of shots fired at the air vehicle: ignores design variables that can prevent hits by fired shots (e.g., countermeasures, maneuverability); and The probability of being hit by at least one round during the mission flight path: only this metric logically considers all of the design variables that affect susceptibility and can be computed in 1 on 1, 1 on N, and M on N mission profiles. Susceptibility is deficient as an air vehicle utility measure because it does not consider design variable interactions, design effects, and threat dimensions that affect the vulnerability of the aircraft, which is defined as “the likelihood that the aircraft will be killed if hit by threat ordnance.”2 As an example of design variable interaction, significant increases in speed will likely require an increase in aircraft size with current engine technologies, which will require more fuel to maintain range or persistency. The additional fuel could make the air vehicle more vulnerable. Many studies inappropriately use susceptibility as a measure of an air vehicle’s survivability. 2 Ball (see footnote 1) describes methods and models for calculating vulnerability metrics such as vulnerable area and the probability that the aircraft is killed when it is hit—Pr [aircraft kill hit].

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Future Air Force Needs for Survivability Survivability, as a proxy of air vehicle utility, is defined as “the ability of the air vehicle to avoid being killed as it transits a complete mission flight path.” It incorporates decision and design variables that affect both susceptibility and vulnerability metrics. Some appropriate survivability metrics used in studies include the following:3 Pr [an aircraft survives one mission | Ops Sit]. Pr [an aircraft survives N missions | Ops Sit]. E [number of sorties the aircraft can fly | Ops Sit]. Sortie loss rate. Although purportedly used in many aircraft-planning studies, survivability too is deficient as a utility proxy, in that it does not consider design and operational decision variables that affect accomplishing the mission objectives (e.g., destroying a WMD site), such as sensor capabilities, number of weapons, lethality of the ordnance, and employment tactics. A number of these design variables have second- and third-order interactions with speed and signature. Operational mission effectiveness is an appropriate utility proxy that is defined as “the ability of the air vehicle to accomplish the mission objectives and avoid being killed while doing so.” This metric includes both the ability of the aircraft to defend itself against the threat (“survivability”) and the “offensive (end game) ability” to accomplish the mission objectives (e.g., destroy a WMD site or hold a threat force at risk). Some example mission effectiveness metrics include the following: Pr [Destroy target and survive | Ops Sit]. E [Number of missions air vehicle can successfully perform | Ops Sit]. E [Number of critical targets destroyed by air vehicle in a campaign | Ops Sit]. Force exchange ratio in an air-land campaign involving significant numbers of the air vehicles. Air vehicle effectiveness measures need to be defined consistent with the level of operational missions used in the study: 1 on 1, 1 on N, M on N, air-land campaign, and so on. 3 Here Pr stands for probability, and E stands for effectiveness.

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Future Air Force Needs for Survivability Obviously, the two components of effectiveness (survivability and offensive capability) are interdependent phenomena and are often traded off during operations (e.g., deliver ordnance on targets at nonoptimal altitudes to enhance survivability). Perhaps more importantly for the air vehicle case, they are both affected in the design process when trade-offs are made among design variables that affect susceptibility, vulnerability, and lethality of delivery means. These trade-offs should be made consciously and explicitly in the comprehensive study. Task 2: Identify Criteria and Transformation of Models to Conduct the Study Conducting the study will require the use of simulation models and analytic methods that can be used efficiently to relate air vehicle design capabilities (speed, observability, countermeasures, sensor suite, weapons lethality, and so on) to the selected effectiveness and survivability measures for different relevant missions or vignettes and operational situations (threats, targets, environments). Said symbolically, analysts need to identify the function (1) where fi represents the simulation models and methods that describe the ith mission and relevant utility measures are effectiveness and/or survivability. For a given set of Ops Sit descriptors and mission, the study will search over the air vehicle design capabilities to identify (within a cost constraint) air vehicle designs that maximize mission effectiveness, trading off survivability with the offensive (end game) part of the mission. Given the criticality of aircraft survivability, a more reasonable criterion would be to search for air vehicle designs that maximize effectiveness subject to a constraint on achieving a desired level of survivability, for example, Pr[survivability ≥ .98]. Designing an air vehicle to maximize mission effectiveness subject to cost and survivability constraints for a fixed operational situation is the classic cost-effectiveness criterion most frequently used to develop systems and forces in the DOD. If uncertainty regarding future missions and operational situations is large, one alternative criterion that might be considered

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Future Air Force Needs for Survivability is to identify air vehicle design capabilities that maximize the versatility4 of an air vehicle across missions and Ops Sits, subject to effectiveness, survivability, and cost constraints. Symbolically, it develops the inverse of equation (1): (2) Designing an air vehicle using this criterion maximizes the number of missions it can perform at the desired level of effectiveness and provides commanders and pilots with a flexible response capability in downstream operations. The versatility planning approach has been used in studies for the Supreme Allied Commander Europe5 and the Commander in Chief, U.S. Army Europe.6 Some available models and methods that might be used to conduct the comprehensive study are noted below:7 Extended Air Defense Simulation (EADSIM) (mission level), Suppressor (mission level), Thunder, Storm (campaign level), Radar Directed Gun Simulation (RADGUNS) (engagement level), and Brawler (engagement level). Note, however, that in the committee’s view, these models require improvements in their ability to handle track fusion and the human decision-making process. 4 For a description of versatility planning and some of its applications, see S. Bonder, Versatility Planning: An Idea Whose Time Has Come—Again!, Steinhardt Lecture presented at the Institute for Operations Research and Management Science Conference, Salt Lake City, Utah, May 9, 2000. 5 Vector Research, Multinational Forces in NATO, Quick Response Analysis of Rapid Reaction Forces, Final Report, VRI-G-91-25, Ann Arbor, Mich., 1991. 6 G. Miller and C. Johnston, Analysis of Alternative Structures for U.S. Army Forces in Europe, Proc. of the 33rd U.S. Army Oper. Res. Symposium, Ft. Lee, Va., November 7-9, 1994. 7 An extensive discussion of available models is contained in Ball (see footnote 1), p. 141.

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Future Air Force Needs for Survivability Task 3: Develop Experimental Designs for Simulation Runs Conceptually, one would like to examine the effectiveness and survivability impacts for all combinations of air vehicle design variables over their feasible ranges (with consideration given to their engineering interactions) to identify designs that maximize effectiveness and survivability in important missions and operation situations. Even for a “comprehensive study,” the combinatorics make this infeasible. Instead, the committee suggests the following process: Task 3a: Based on previous study results and experience, select a set of air vehicle design capabilities as “primary variables” for the experiments. This should likely include the following: Aircraft speed, Observability, Countermeasures (active and passive), Situation awareness, Payload, and Weapon lethality. These variables should be varied over ranges based on the development feasibility analyses and their engineering interactions. Task 3b: Set the remaining relevant air vehicle design variables as “parameters” for the analysis. These should be fixed for all the runs except where engineering and operational interactions suggest that they change with changes in primary variables. For example, altitude as a parameter would likely change with speed for operational efficiency. Persistence and vulnerability parameters might change owing to engineering interactions as speed is varied. Task 3c: Select relevant missions or vignettes and Ops Sits (threats, targets, and so on) for the analysis. The committee suggests that these be “stressful” so that insights regarding less stressful missions would be gained by interpolation rather than extrapolation. Task 3d: Develop appropriate experimental designs for efficient conduct of the simulation-based experiments and specify how the

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Future Air Force Needs for Survivability simulated outputs would be analyzed. In addition to all the combinations of primary variables, the design should include runs of Current systems versus current threats, and Current systems versus future threats. These can be used as baselines to compare with runs involving simulated future air vehicles. Task 4: Set Up, Conduct, and Analyze Initial Simulation Runs Although many design variables impact on the survivability and effectiveness of potential air vehicles, a major focus is to identify a good balance between speed and observability capabilities for different levels of other design parameters, missions, and Ops Sits. Toward this end, analysis of simulation results should try to identify functional input-output relationships such as: (3) (4) for each of i missions. These relationships could be portrayed graphically through the use of scatter plots, some examples of which are shown in Figures C-1 and C-2. Task 5: Some Analysis Thoughts In reviewing the simulation outputs, it is important to look for knees in the effectiveness and survivability functions to identify good speed and observability pairs. For “robust” air vehicle capabilities, one should design away from inflection points so that the operating point is out along flat parts of the curve.

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Future Air Force Needs for Survivability FIGURE C-1 Conceptual simulation results showing speed and observability combinations that yield iso-effectiveness or constant effectiveness (E) of the air vehicle at high, medium, and low levels, given a particular mission and specified probability of survival. Bulleted variables are treated as parameters in the simulation. FIGURE C-2 Conceptual simulation results showing speed and observability combinations that yield iso-survivability or constant survivability (S) of the air vehicle at high, medium, and low levels, given a particular mission and specified probability of mission success. Bulleted variables are treated as parameters in the simulation.

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Future Air Force Needs for Survivability One should look for changes in design parameters and/or tactics that might provide equal survivability and effectiveness levels with less costly or risky speed and observability pairs. One should look for design parameter and/or tactics changes that might enhance effectiveness and survivability for the same speed and observability pair. It is important to conduct sensitivity analyses on operational situation dimensions (threat, targets, environment) to assess the impact of (1) uncertainties in these dimensions and (2) threat countermeasures on effectiveness and survivability and the speed and observability balance. One should develop and analyze potential design and/or operational counters to threat countermeasures. One should continually assess the engineering “feasibility” associated with high-utility (effectiveness and survivability) pairs.