4
Assessing the Operational Utility of Speed and Observability Trade-offs

Determining a “good balance” between the speed and observability of a future air vehicle logically requires assessing their contributions to the operational effectiveness1 of the air vehicle. This is a complex analysis process that is confounded by the many other design and operational variables that interact with speed and observability and also affect the vehicle’s operational effectiveness. The complexity of this analysis process is described in Appendix C, along with a framework for a comprehensive, simulation-based analysis methodology that considers this level of complexity. Although the approach is logically sound,2 its employment as described in Appendix C requires significantly more time and resources than were available to the committee. In the short time available to it, the committee used a historical and experienced-judgment approach for its analysis and focused on aircraft survivability as its utility metric rather than mission effectiveness.

In this chapter, the committee combines the insights that it drew from its analysis of recently published reports in the field, the information provided during the briefings that it received, and its own expertise to arrive at some conclusions regarding the utility of speed, stealth, and other key variables in air vehicle survivability in various operational situations.

1

Mission effectiveness and a number of other utility measures are defined in Appendix C, “A Framework for Comprehensive Analysis.”

2

The methodology considers multiple missions, many interdependent design variables, many operational threats, and mission effectiveness and survivability metrics.



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Future Air Force Needs for Survivability 4 Assessing the Operational Utility of Speed and Observability Trade-offs Determining a “good balance” between the speed and observability of a future air vehicle logically requires assessing their contributions to the operational effectiveness1 of the air vehicle. This is a complex analysis process that is confounded by the many other design and operational variables that interact with speed and observability and also affect the vehicle’s operational effectiveness. The complexity of this analysis process is described in Appendix C, along with a framework for a comprehensive, simulation-based analysis methodology that considers this level of complexity. Although the approach is logically sound,2 its employment as described in Appendix C requires significantly more time and resources than were available to the committee. In the short time available to it, the committee used a historical and experienced-judgment approach for its analysis and focused on aircraft survivability as its utility metric rather than mission effectiveness. In this chapter, the committee combines the insights that it drew from its analysis of recently published reports in the field, the information provided during the briefings that it received, and its own expertise to arrive at some conclusions regarding the utility of speed, stealth, and other key variables in air vehicle survivability in various operational situations. 1 Mission effectiveness and a number of other utility measures are defined in Appendix C, “A Framework for Comprehensive Analysis.” 2 The methodology considers multiple missions, many interdependent design variables, many operational threats, and mission effectiveness and survivability metrics.

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Future Air Force Needs for Survivability The committee used this approach to define a range of speed and stealth combinations that can provide equivalent survivability. It then overlaid the technical analysis of Chapter 3 on these results to draw conclusions about the feasibility of achieving these combinations in the 2018 initial operational capability (IOC) time frame, with a view to identifying the recommendations for research and development investment presented in the next chapter. COMMITTEE ANALYSIS OF PREVIOUS STUDIES A number of studies have been performed over the past few years by industry, federally funded research and development centers, and government agencies to examine various aspects of the speed and observability (and related variables) trade-offs for various existing and notional aircraft. The committee reviewed a number of studies that provided useful insights.3 A general summary of these follows. Each of these studies required a large number of inputs and presented many output tables and graphics relating some design variables to various output utility metrics. From these studies the committee has with confidence and clarity shown that, as a function of speed and stealth in terms of radar cross section (RCS), there exist regions of very high survivability. This was done for surface-to-air missiles (SAMs) and airborne interceptors (AIs) separately and in combination for a specific foreign, moderate capability. The committee assessed aircraft survivability as a function of speed and signature for a current widespread SAM threat, the current stressing SAM threat, and a projected advanced responsive threat. As the examples above suggest, the studies examined by the committee varied significantly in design variables, parameters, mission vignettes, specific threats, assumptions, and utility metrics considered. In order to assess the commonality of the studies, each was abstracted in terms of its inputs (design variables, design parameters, missions and threats considered, and so on) and outputs (utility metrics, utility results), as well as study observations regarding speed and observability. Examples of the inputs for the seven studies are presented in Table 4-1. Analysis by experienced committee members suggested a number of qualitative and quantitative observations and conclusions, presented in the next section. 3 A listing of these studies is given in the classified report.

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Future Air Force Needs for Survivability TABLE 4-1 Summary of Inputs of Relevant Aircraft Studies Inputs Study 1 Study 2 Study 3 Study 4 Study 5 Study 6 Study 7 Design variables Speed Vulnerability Maneuver Speed Stealth Countermeasures (CM) Altitude Situation awareness (SA) Speed Stealth CM SA Speed Stealth CM Weapons Payload Speed (subsonic to hypersonic) Stealth Speed Stealth Speed Stealth Design parameters Stealth Altitude Persistency Platform size and weight Procurement costs Altitude Altitude Weight Range Cost Manned/unmanned Weapons Endurance CM Altitude Size/weight due to speed Affordability Shape impacts on signature Technology readiness levels Range Altitude Systems evaluated Notional missile, with speed (subsonic to hypersonic) Parametric long-range strike system Speed (subsonic to hypersonic) Full attack suite Unknown Notional vehicles (3) Speed (subsonic to hypersonic) F-22 FA-18G Cruise missiles Speed (subsonic approaching hypersonic) Stealth (appropriate for systems) Parametric aircraft F-22A B-1B B-2 F-117 SOURCE: Committee-generated abstract of earlier studies.

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Future Air Force Needs for Survivability COMMITTEE OBSERVATIONS AND CONCLUSIONS Qualitative Observations and Conclusions Speed and Stealth The committee’s review shows that speed and low signature are both primary factors in determining survivability. When these factors are analyzed independently, it is clear from the committee’s review of recent reports that aircraft with hypersonic speed can survive against highly capable current threats because air defenses are not expected to have time to engage them successfully at their achievable observability; it is also clear that very stealthy vehicles can survive because air defenses cannot find them in time to track them and engage them. In the region between these extremes, balanced speed and stealth solutions can also be robust against the threats posed by air defenses. Improvements in stealth reduce the threat’s defended area and have a greater utility in defeating SAMs than do increases in speed. Increases in speed and stealth both reduce the exposure time in the threat’s defended area; increases in speed have a greater utility in defeating airborne interceptors than do just improvements in stealth alone. However, the committee believes that the analyses available on the impact of speed and stealth on AI threats were quite limited. Furthermore, there was a lack of analysis on the implications of high stealth levels and speed for integrated air defense system (IADS) cueing capabilities. As discussed in Chapter 3, the long-range strike system might consist of a standoff aircraft armed with fast hypersonic missiles. Missiles traveling at this speed would be capable of striking targets quickly and would likely have a high probability of survival against air defense threats, regardless of the signature level achieved. Situation Awareness The level of situation awareness (SA) can significantly impact the use of signature, speed, countermeasures, and tactics for increasing survivability. In addition to the use of sophisticated cyber and information operations techniques, the ability to decrease a weapon system’s susceptibility in an adversary’s airspace is a result of achieving the right balance of speed and signature reduction, both of which are synergistically enhanced by SA.

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Future Air Force Needs for Survivability Therefore, accurate, current knowledge of threat location and type is important. From mission planning through mission execution, gaining and maintaining SA will provide the weapon system operator an enormous advantage in understanding the battlespace and, therefore, in selecting the appropriate plan of attack and use of tactics. High-stealth vehicles will be less dependent on SA than will supersonic higher-signature designs, and therefore the former will be more robust on missions where SA is denied. Countermeasures Countermeasures (CM) can enhance the survivability of aircraft, and they offer a significant improvement in survivability in the face of future threat advances. While the utility of specific CM was not assessed in this study, the studies reviewed suggest that CM offers an increase in survivability equivalent to a significant reduction in RCS, and CM appears particularly valuable for aircraft with higher signatures. It also appears that a high level of SA enhances the performance of CM by optimizing their selection and timing. However, poor SA can reduce the effectiveness of CM to zero. The traditional philosophy has been that CM is designed-in as a kind of “insurance” to maintain the aircraft’s level of survivability against uncertainties or improvements in threat capabilities, and this design philosophy continues today. An attractive feature of CM is that they can be adapted or changed to respond to changing threats, such as redesigned IADS electronics, new radar waveforms, and so on. More analysis of the effectiveness of radio-frequency stealth and countermeasures against SAMs would be desirable. Similarly, more analysis is needed on potential electro-optical/infrared (EO/IR) and visual threats that might have to be countered, especially from AIs. Some Additional Observations In general, the studies examined by the committee did not consider the interactions among the aircraft design variables. For example, increasing the sustained speed of an air vehicle requires an increase in its size to maintain range and payload, and this in turn affects its signature, not to mention its affordability. It will be important for future comprehensive studies of the type described in Appendix C to take account of these interactions. The committee is concerned with the level of USAF and industry investigation of the relative merits of speed, stealth, electronic CM, and

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Future Air Force Needs for Survivability SA. There is both a lack of emphasis in past efforts as well as shortfalls in the analysis tools used. Specifically, before choosing a design point on the speed/ stealth performance curve, the USAF needs to conduct rigorous analysis and trade-off studies as a basis for that decision. These studies need to go beyond susceptibility and survivability metrics and consider higher-level effectiveness measures4 to assess the impacts of—and trade-offs among—all relevant design variables and their interactions. However, the committee found that the analytical tools possessed by and used by the community for this task can be improved. Specifically, the tools’ fidelity for assessing the effects of aspect and radar frequency upon the ability of an IADS to detect, track, and provide cueing for AI can be improved. Also, the effects of speed are difficult to analyze effectively because models lack fidelity in dealing with the human decision process, command and control, and reaction times. Finally, tools are needed that can deal with the potential EO/IR and visual threat impacts. Low signature does not guarantee survivability. The survivability of extremely low signature aircraft is sensitive to the number of SAMs fired in an engagement as well as to the SAMs’ location areas. The survivability of aircraft also drops steeply with loitering time in the defended area. For an aircraft with IOC in 2018, system definition and design must be initiated around the 2009 time frame when critical technologies must have achieved TRL 6. Specific near-term changes in emphasis that can be made are discussed at the end of Chapter 3. Speed and Stealth Requirements for Survivability The committee has determined with confidence the speed-stealth combinations (at the primary frequency of interest) that would be expected to yield an equivalent high level of survivability against threats of primary concern in the 2018 time frame. The findings represent a composite view of the several studies presented to the committee, combined with a consensus summary of the committee’s expert judgment. The results provide the means for the Air Force to carry out trade-off studies to establish a family of baseline configurations for systems that could achieve IOC in 2018. Additional analyses extending the studies presented to the committee will enable the refinement of those baseline configurations. 4 These measures are defined in Appendix C.

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Future Air Force Needs for Survivability The committee assessed the variation in results from the wide range of relevant studies that have been carried out and found that the uncertainty band is reasonable. The committee observes that there is no sharp breakpoint (or “knee” in the curve) in the speed-versus-signature relationship, and a trade-off exists over the range of conditions investigated. This data compilation shows a continuous relationship of speed versus stealth in achieving constant survivability. Speed and Stealth Requirements Versus Feasibility The achievable levels of stealth discussed in Chapter 3 were compared with the requirements discussed here that were derived from the reports presented. The results show a significant degree of agreement in achievable signature versus speed, with a reasonable spread over most of the speed range from subsonic to approaching hypersonic. Further analyses and configuration refinement will reduce the variation in technology assessment. The assessment discussed above mainly dealt with fire-control radars. A significant aspect of survivability concerns the detection capability of search radars. While the committee determined the achievable capability of very-high-frequency systems, additional work is needed to determine the requirements. The committee assessment of achievable signatures leads to the conclusion that, for constant survivability, technologies are available to support multiple solutions at differing speed and signature combinations. In particular, solutions for these combinations exist at both subsonic and supersonic conditions. With multiple solutions potentially available, selection of the “best” option will involve other considerations including costs, schedule risks, robustness to potential countermeasures, and so on. While one desired outcome of this study was the definition of a “sweet spot” in the range of feasible solutions, the committee did not find this to exist, possibly owing to the spread in requirements derived from existing studies and to variations in available estimates of achievable signature levels. Shortfalls in the analyses and analytic tools identified earlier will need to be resolved in order to further refine the speed-versus-signature assessment. The finding regarding these shortfalls, along with the committee’s other overarching findings and recommendations, are presented in the next chapter.