Ground testing

Gaps/Challenges

(1)Similarly, the inverse of q, the amount of rework normally performed, is also process driven. For most aerospace systems in development, q is approximately 0.25, resulting in 4 to 10 rework cycles. The incremental increase in program costs is proportional to (1/q)-1, indicating the potential to easily double development costs through late defects and rework.

(1)CSE can be an in valuable tool to ensure better use of ground-test facilities to preclude design defects from finding their way into the flight-test program. Use of CSE to account for Reynolds number scaling effects and potential bias errors such as wind tunnel wall interference is well understood and effectively applied. An area where scaling effects are not well understood and CSE may have the potential for producing new insights is simulation of military tactical aircraft at high-angle maneuvering conditions. In these conditions, the flow is dominated by vortex structures and flow separation. Surprisingly, a large number of tactical military aircraft have required a significant modification to control surface size or structure even after a comprehensive wind tunnel campaign. Changes of this magnitude during the flight-test program can have a profound impact on program cost and schedule. Coupled effects on manufacturing costs can also become significant during this phase.

1. (3)On average, the committee classifies the facilities and equipment observed in the NASA laboratories as marginally adequate, with some clearly being totally inadequate and others being very adequate. The trend in quality appears to have been downward in recent years. NASA is not providing sufficient laboratory equipment and support services to address immediate or long-term research needs and is increasingly relying on the contract technician workforce to support the laboratories and facilities.

2. (3)The facilities that house fundamental research activities at NASA are typically old and require more maintenance than funding permits. As a result, research laboratories are crowded and often lack the modern layouts and utilities that improve operational efficiency. The lack of timely maintenance can lead to safety issues, particularly with large, high-powered equipment.

3. (3)Over the past 5 years or more, the funding of fundamental research at NASA, including the funding of facilities and equipment, has declined dramatically such that unless corrective action is taken soon, the fundamental research community at NASA will be unable to support the Agency’s long-term goals. For example, if funding continues to decline, NASA may not be able to claim aeronautics technology leadership from an international and in some areas even a national perspective.

4. (3)Based on the experience and expertise of its members, the committee believes that the equipment and facilities at NASA’s basic research laboratories are inferior to those at comparable DOE laboratories, top-tier U.S. universities, and corporate research laboratories and are about the same as those at basic research laboratories of DOD.

(5)Current CFD technology cannot mitigate this risk. The required accuracy and throughput capabilities are simply not there, and the test data that are required to validate the CFD do not exist. Current wind-tunnel instrumentation cannot provide sufficiently detailed flow diagnostics at cryogenic conditions.

(5)For example, Federal Aviation Administration regulations place limits on the nonlinearity of the pitching-moment characteristics of the aircraft. Modern control systems can compensate for many of these nonlinearities, provided that they can be properly identified and that control sensors and actuators are capable of providing the required compensation. Small changes in the separated-flow patterns determine whether or not these nonlinearities and the resulting handling characteristics are acceptable. Derivatives in the linear range are adequately predicted, but most critical conditions lie in the nonlinear range. The inability to capture these differences adequately is a problem for both CFD and typical low-Reynolds-number wind-tunnel testing [22]. This might also be a problem for high-Reynolds number (flight) wind-tunnel testing, but facility limitations have prevented the exploration of this area in detail. The result is that today many of these handling quality issues must be identified and resolved in flight tests.

(5)Among these scaling parameters, Reynolds-number differences, which are caused by the model-scale factor as modified by various tunnel pressurization/cryogenic mitigation approaches, constitute an often major and long-standing flight-to-wind-tunnel scaling issue. Other major scaling issues include the presence of wind-tunnel walls, aeroelastic distortion differences (flight to ground), model mounting effects, and influences of installed propulsion, among others. (See table 2.) These scaling issues can in turn be influenced by

other flight-to-wind-tunnel differences, such as the effects of stream disturbances on boundary-layer transition and, hence, on Reynolds-number scaling.

(5)Table 2. Wind-Tunnel-to-Flight Scaling Issues1. Wind tunnel walls: solid, porous/slotted, adaptive, open jet.2. Aeroelastic distortion differences: specific wind-tunnel/model conditions

versus flight.3. Sufficient Reynolds number scaling: especially critical for transonic flows,

longitudinal vortices, large transitional flow influences (separation, hypersonics).

4. Stream disturbance fields: vorticity dynamics, acoustics, entropy spottiness, particulates, and, especially, influence(s) on transition.

5. Model mounting influences: sting, strut, wire (e.g., rear, side).6. Stream gross unsteadiness, of special concern for buffet. 7. Installed propulsion influences or lack thereof: various propulsion

simulators/effects. 8. Geometric fidelity: potential criticality of even minor differences in flight to

ground, including curvatures and second derivatives, difficulties in scaling small features, boundary-layer tripping, and trip drag.

9. Stream mean distortions/inhomogeneities. 10.Leakage/spillage/efflux differences.11.Wall-to-total-temperature ratio, humidity.12.Differences flight to ground in instrumentation details (i.e., nature, locations,

and accuracy), including variability of the various/multitudinous transition detection schemes and approaches, and data-reduction errors.

13.High-energy, high-density effects for hypersonics.

(5)Industry has developed rigorous testing techniques and complementary computational tools to account for many of the scaling effects, including Reynolds-number effects, wall-interference effects, support effects, and so on. However, in some cases, accounting for certain effects, such as the influence of free-stream disturbances through their effect on transition, is not possible. The ground-to-flight scaling details are often considered to be “sensitive information” by various industrial players; therefore, the available published details are often sparse or even absent.

(8)Defects discovered late during the development process not only increase cycle time but also can impact manufacturing costs if significant tooling and production have already occurred. Since concurrent engineering is routinely used to reduce procurement cycle time, almost always tooling and initial production are in progress by the time flight testing occurs

(8)The challenge to reducing late defect discovery is to determine the root cause for reoccurring late defects. A prime example for the need to better understand the root cause for late defects is the frequency of structural failures discovered during flight testing, even after numerous hours of analysis and wind tunnel testing were used to design the aircraft structure. On average, 10 structural failures are uncovered during flight testing irrespective of the type of aircraft. In addition, many flight systems resize control surfaces after discovering inadequate control authority during flight testing. Working control surface sizing and structural issues this late in development can lead to significant delays in completion and considerable cost increases

(9b)Low-Use Strategically Important Capabilities RiskLoss under Closure or Mothballing• Low-use facilities can be closed for long periods, but cost savings may be lower than expected and capabilities will degrade quickly

– Closures can reduce contractor labor and variable center costs– Cost of any infrastructure and some civil servant staff shared with other

open facilities may not be reduced when a single facility is closed– Possibly higher testing costs, travel, models, etc., for programs– Facility hardware and equipment may degrade quickly without a level of

mothballed maintenance• Mothballing a strategically important facility is preferred to closure, but mothballing still involves risk

– Harder to reconstitute workforce expertise required to safely and effectively operate the facility as time goes on

– Hardware will still likely have some degradation (depending on what is done in mothball preservation)

• An alternative is to provide strategic financial support for periodic use of the capabilities to exercise staff and equipment to maintain knowledge, skills, and equipment

– Example: Funding a few academic research tests per year– Will want to make sure that these capabilities are still strategically

important if such strategic resources are to be obtained and applied

(10)However, this does not imply that focused research associated with the mobility goals and objectives alone is sufficient. Foundational research provides the “building blocks” of a technology base to successfully address the stated goals and objectives.Hence, complementary foundational aeronautical research efforts are also required in areas such as guidance, navigation, and control; fluid

mechanics; advanced structures and materials; combustion chemistry; airframe/propulsion system integration; and advanced mathematics, statistics, computational science, and optimization techniques.

(10)A number of fundamental challenges are barriers to technical progress, as well as opportunities for advancement through sustained aeronautics R&D: • Improved aerodynamics and innovative airframe structural concepts for high-efficiency fixed- and rotary-wing aircraft would provide greater aircraft range, endurance, survivability, and payload capability.• Quiet, efficient rotorcraft would be more operationally effective, more

survivable, and less expensive to operate.• Highly efficient propulsion systems would enable greater range and endurance

and could provide greater mission flexibility. • Integrated power and thermal management on aircraft is becoming increasingly

important as power requirements and heat loads increase. • High-speed and hypersonic flight offers advantages for national security in

terms of global reach, responsiveness, and survivability.• Finally, airspace integration and deconfliction, especially as UAS become ubiquitous to aviation operations, are growing issues affecting not only military operations, but civil operations as well.

(10)FUNDAMENTAL SAFETY CHALLENGES TO OVERCOME Shortfalls associated with the state of the art discussed above will have to be overcome to continually improve safety in the decades ahead. The following are the major challenges: • Enhancing the probability that passengers and crew will survive and escape safely when accidents do occur. [Structures/survivability]

(11)The gap in subsonic acoustics is the inability of the existing test facilities to provide a quiet environment that can distinguish between test facility and research hardware noise. Acoustic and turbulence levels in existing large-scale facilities are too high to achieve the aeronautics goals in the 2010 National Aeronautics R&D Plan. Unsteady turbulence and flow angularity levels are 3 to 10 times too great to provide accurate data for complex dynamic models.

(11)The gap in materials testing is the inability to duplicate the flight envelope between current facilities that provide high-pressure and low-power simulation (for intercontinental ballistic missile testing) and those that provide low-pressure and high-power simulation (for shuttle-like and other planetary reentry vehicles).This limitation of current facilities contributes to the expense of highly conservative material designs for hypersonic vehicle airframe thermal protection

systems and propulsion system inlet leading edges. This shortfall exists in the near, mid, and far terms.

(11)Demonstration of sustained, controlled, air-breathing hypersonic flight above Mach 5 requires an infrastructure capable of testing scramjet propulsion systems. The current infrastructure fails to meet required capabilities because of three limitations. The first is the inability to test full-scale propulsion systems because of limited test cell size and limited mass flow capability. The second limitation is the inability to test scramjets at Mach numbers greater than 5 in clean air and greater than 7 in vitiated air because current high Mach number facilities are limited in mass flow, flow quality, and run time. The third limitation is the inability to vary the wind speed during a test (time-variant Mach number). While the test article is in the test section of a hypersonic propulsion facility, it is highly desirable to test the operation of the propulsion system over a variable Mach number range. These limitations exist in the near, mid, and far terms, and impact not only scramjet propulsion systems, but potential turbine-based combined cycle systems as well.

(11)A greater understanding of the impact that icing conditions have on turbine engine operations is needed to develop enhanced design and operations technologies that help prevent accidents. No facility is now available to conduct research by testing turbine engines at altitude for icing conditions that include ice particles. This shortfall exists in the near term.

[Note – PSL and C2 can do this]

(11)Current combustor component test facilities offer lower flow rate capability than required and thus limit component testing to individual fuel injector concepts and sectors of annular combustors. Full annular testing allows researchers to quantify the interaction between the individual fuel injectors as opposed to extrapolating the data from sector test rigs. This shortfall exists in the near, mid, and far terms.

Flight test

(11)There is a gap in the capability to flight test on-board avionics systems as part of the Next Generation Air Transportation System (NextGen) and the increased safety-of-flight goals. Specifically, there are no transport-category research aircraft dedicated to performing this testing. . . . To integrate and test these emerging technologies, flight test aircraft are required for all aircraft categories [general aviation, regional, transport, and unmanned aerial systems (UAS)], but a clear shortfall currently exists in the transport aircraft category.This category of

aircraft is critical to best emulate the real world environment for many of the goals. This aircraft will test and validate new systems in upset recovery due to damage, degradation, and failures. This aircraft will be modified with the specific systems under test, as well as the associated instrumentation, flight deck displays, caution/warning systems, and risk mitigation systems to ensure safe flight testing. Specific ground and airspace assets will also be required to safely facilitate testing. The majority of work in this area is in support of the Mobility and Aviation Safety Goals to meet mid-term and far-term objectives.

(11)There is a shortfall in the capability to flight test hypersonic vehicles overland and limitations on the ability to test over the ocean in response to our National Security and Homeland Defense goals. Overland testing will require developing new hypersonic test corridors that extend beyond current test ranges and test routes and the operational procedures to use them. The long distances and execution times will strain our abilities in tracking, telemetry, flight termination, and control systems over both land and ocean. Range support aircraft that have traditionally supported these capabilities will require improvements or new tech-nologies to replace them.This shortfall exists in the near, mid, and far terms

(11)Comprehensive interagency management policies for aeronautics infrastructure do not yet exist. It is difficult to prioritize national RDT&E needs across D&A boundaries, particularly given the existence of different budget processes and agency goals that are often reviewed by separate Congressional committees. Challenges that may impede interagency cooperation include:

Competing authorization and appropriations legislation among the various D&As, which may present legal and procedural barriers to the sharing of resources;

Lack of imperative or incentive to prioritize and ensure the availability of facilities that are inconsistently or intermittently used but that remain critical in those instances when they are needed Lack of consistent cost accounting and usage policies driven by individual D&A budgeting and accounting practices that hinder sharing of agency resources and raise an access barrier for non-Federal users;

High costs for infrastructure construction, maintenance, and upgrading, which may create institutional barriers when considering the allocation of infrastructure resources to priorities outside of the owning agency’s mission

(11)Table of Relationships between Shortfalls and Goals and Objectives Shortfall Goals Impacted Objectives Impacted

Subsonic Acoustic Measurement and Low Turbulence Flow Test Facilities

NSD-2 N-22, N-25, M-24, M-27, F-21 ENE-2 N-51, M-53, F-42

ENE-3 N-53, N-54, N-55, N-58, M-55, M-59, M-62, F-46, F-47, F-50

Hypersonic Materials Test Facilities NSD-5 N-31, N-32, M-32, M-33, M-34, M-35, F-25, F-26

AVS-1 N-36, N-38, M-38, M-40, F-29, F-31

Hypersonic Engine (Scramjet) Development Propulsion Test Facilities

NSD-5 N-31, N-32, M-32, M-33, M-34, M-35, F-25, F-26

Turbine Engine Icing Test Facilities AVS-1 N-36, N-38, M-38, F-29, F-30 AVS-2 N-43

Turbine Engine Combustion Facilities NSD-3 N-26, N-27, M-28, M-29, F-22 ENE-1 N-47, M-49, M-50, F-39

ENE-2 N-51, M-53, F-42

ENE-3 N-53, N-54, N-55, M-55, M-56, M-57, M-58, M-59, M-60, F-45, F-47, F-48

Full-Scale Rotorcraft Test Rig NSD-2 N-22, N-25, M-24, M-25, N-23, N-24, M-26, M-27, F-19, F-20, F-21

AVS-1 N-36, N-37, M-38, M-39, F-29, F-30

ENE-3 N-58, M-62, F-50

Transport Category Flight Test Aircraft MOB-1 N-3, N-4, M-3, F-1, F-2 MOB-2 F-4

MOB-3 M-8, F-9

MOB-4 N-12, M-12, F-10

AVS-1 N-37, M-38, M-39, F-29, F-30

AVS-2 N-40, N-41, M-42, M-43, F-33, F-35

ENE-3 N-53, M-55, F-44

(11)Table of Relationships between Shortfalls and Goals and Objectives—continued Shortfall Goals Impacted Objectives Impacted

Hypersonic Test Range Capabilities NSD-5 N-31, M-34, F-25 AVS-1 M-38

Airborne Icing Capability AVS-1 N-37, M-39, F-30 Flight Simulators Representative of Aircraft in Service

MOB-1 M-1, M-2, M-3, M-4, F-1, F-2, F-3 MOB-2 M-5, M-6, F-4, F-5, F-6

MOB-4 M-12, M-14, M-15, F-10, F-11, F-12

MOB-5 M-16, M-17, M-18, M-19, M-20, F-13, F-14, F-15, F-16

AVS-1 M-38

AVS-2 M-41, M-42, M-43, F-32, F-33

ENE-2 M-52, F-41

ENE-3 M-55, M-60, F-44, F-47, F-48

Four Dimensional Trajectory Simulation Capability

MOB-1 N-4, M-1, M-2, M-3, M-4, F-1, F-2, F-3 MOB-2 N-7, M-6, F-4, F-5, F-6

MOB-4 N-12, N-13, N-14, M-12, M-14, M-15, F-10, F-11, F-12

MOB-5 N-15, N-16, N-17, N-18, M-16, M-17, M-18, M-19, M-20, F-13, F-14, F-15, F-16

AVS-1 N-37, M-38, M-39, F-30

AVS-2 N-39, N-40, N-41, M-41, M-42, M-43, F-32, F-33

ENE-2 N-49, N-50, M-52, F-41

ENE-3 N-53, N-55, M-55, M-60, F-48

High-End Computing Capacity All goals Nearly all objectives

(13)Timely and cost-effective acquisition of military systems has deteriorated significantly over the last three decades. Despite numerous attempts at acquisition reform, the number of acquisition programs behind schedule and over costs continues to escalate

I. Technology Development Plana. [Address the gaps – include anything already identified in the

references.]

(13)The mathematics of the DOE methodology helps assure the optimum data set is taken. The alpha and beta (or power coefficients) of the DOE process can be used to address how much further variance can be reduced on the response surface by an additional calculation, wind tunnel test, or flight test. There is a point at which doing another CFD solution will not reduce uncertainty further; hence, one needs to move on to wind tunnel testing. Likewise, there is a point of diminishing return for doing another wind tunnel test and the program needs to move on to flight testing. Thus, unnecessary modeling and/or testing can be minimized. The beta coefficient also provides some insight into the probability that a defect is being passed downstream to the next development step.

(14)The challenge for developing an integrated roadmap for HS/H engineering development and test and evaluation (T&E) capability requirements is to identify the capability requirements for each discipline – M&S, ground test, and flight test – and then leverage the individual capabilities to provide a comprehensive, but cost-effective, approach.

(14)The challenges for applying M&S to hypersonic flight conditions encompass deficiencies in modeling turbulence, flow separation, aerothermochemistry, plasma interactions, and conjugate heat transfer. These deficiencies are intensified by a lack of sufficiently detailed measurements at hypersonic conditions to permit either improved fidelity in the models themselves or validation of the models. This lack of needed data is largely the result of shortcomings in existing test facilities and partly a result of the lack of a comprehensive approach to capturing needed validation data.

(14)The flight condition simulation capabilities required for the transformational HS/H systems envisioned far exceed current ground-test capabilities with respect to:

Sufficient run times to achieve necessary thermal equilibrium conditions Sufficiently high enthalpy conditions to properly simulate propulsion and/or

heat-transfer phenomena Flow medium and flow quality to provide a sufficiently accurate simulation

for scale-to-flight conditions Test facilities with sufficient scale to simulate a fully integrated

airframe/propulsion system

(14)Ideally, hypersonic systems would be developed with the same testing rationale as today’s supersonic systems (e.g., development of systems technologies and major components in ground-test facilities that closely simulate actual flight conditions, with adequate testing time to evaluate performance, durability, operability, and component integration prior to flight). However, the higher the system performance Mach number, the larger the gap in the ability to test the system using today’s facilities. For the higher Mach numbers, it will likely be technically impossible to design and build ground-test facilities that will concurrently provide both adequate simulation of flight conditions and extended run time. Thus, it is clear that complementary ground-test facilities (none which fully satisfy simulation requirements) must be available to provide the total data set needed to produce system design and to reduce system risk. Consequently, M&S and flight testing assume an even more demanding role.

(14)In Fig. 4, the High-Speed Aircraft (Mach 2 to 4) shows some green and red. The large engine test facilities are limited to Mach 3.2, yet turbine engine R&D programs predict future engine performance to Mach 4 and above. Therefore, development of new, high-performance turbine engines will incur considerable increased risk without testing to these speeds. All of the three large, high-speed aircraft propose use of turbine engines as either primary or first-stage propulsion.

The Access-to-Space vehicle could possibly do the same. Clearly, this test facility performance gap must be addressed.

(14)The primary propulsion test capability that exists above Mach 8 currently is relatively small impulse (shock) tunnels where the test time is of the order of milliseconds and the test medium is usually not clean air. The air is usually in a thermal and chemical nonequilibrium state and may contain metallic contamination from facility nozzles. Electrical arc-heated facilities, which usually produce contaminated air, have also been used for propulsion research testing on a limited basis.

(14)The effects and limitations of combustion-vitiated testing, development of clean air test facilities below Mach 8, definition of needed facility run times, and definition/development of test capabilities above Mach 8 are all test facility technology gaps that must be addressed.

(14)Several of the off-ramp vehicles change propulsive mode as they accelerate or decelerate. Consider that a transfer from turbine (or rocket) to ramjet to scramjet will usually involve a flow-path change while maintaining thrust. It is not likely that the vehicle can carry three independent propulsion systems, so there will necessarily be engine components and fuel systems that have multiple uses. It is anticipated that development of such a combined-mode propulsion system will require many hours in an engine test facility with a free-jet nozzle and perhaps a clean airflow. This propulsion concept is a requirement for all hypersonic vehicles that operate throughout the flight envelope from takeoff to cruise.

(14)Most of the off-ramp vehicles will require a ramjet or scramjet engine to change thrust output as the vehicle accelerates or decelerates. This will require a test facility with a variable Mach number test capability. Development of a programmed accelerating and decelerating hypersonic propulsive system is considered to be a challenge for both the engine designer and the test facility designer. Conceptual approaches to multimode propulsion systems need to be defined to assist test facility designers.

(14)The aerodynamic test capabilities for the near-term NAI off-ramp systems are generally adequate, as also illustrated in Fig. 4. However, improvements in inlet airframe integration and nozzle after-body test capabilities are needed, especially for large-scale (aircraft size) testing where geometric fidelity is needed and can only be achieved at reasonably large scale. A minimum of 25-percent scale is recommended for inlet airframe integration. This is also true for the mid-term and far-term aircraft and Access-to-Space systems.

(14)Whenever these dissociation, ionization, and abnormal recombination effects occur, the effects on the aerodynamic data must be taken into account because the “real gas” effects of flight usually are not simulated. These dissociation and recombination phenomena are also important for aerothermal testingThus, there is a need for real-gas flight simulation test capability, and/or analytical procedures are needed to correct data taken from perfect gas and nonreal-gas wind tunnels to predict flight results. It may be impossible or at best very difficult to build the desired real-gas test facility, so M&S data corrections are expected to be necessary.

(14)Hypersonic wind tunnel productivity is an issue, especially above Mach 10. For production development testing, thousands of data points are typically needed to support aircraft design. Existing blow-down wind tunnels provide seconds to minutes of test time. Impulse tunnels operate for milliseconds, and only a limited number of these facilities exist. One set of data points per run is generally available in these cases. Productivity must be a major consideration when selecting a solution.

(14)The types of aero-optic testing needed are listed in Fig. 6. The need exists to evaluate the optical distortion induced by the flow field over the sensor window and thermal effects on the window itself. At present, aero-optic testing is limited to cold-flow (perfect gas) and impulse wind tunnels. The impulse wind tunnels are the only high-enthalpy (energy) wind tunnels available for this type of testing. As with the case of aerodynamic testing, test time is an issue and a gap for any testing above Mach 10, which is needed for the Mach 8 to 12 Hypersonic Interceptor/Attack Missile.

(14)Developers of missiles favor a fullscale test in highly productive facilities and will acquiesce to flight duplication enthalpies that can be provided only by very short run time facilities (shock/impulse wind tunnels). This, of course, provides a limited database with flight duplication enthalpies without the benefit of testing for transient effects. A facility with flight enthalpy duplication capabilities and run times of the order of seconds does not presently exist, but such a facility would provide added transient testing capability at flight enthalpies. The need for highly productive cold-flow facilities will continue to exist as before.

(14)There is no significant gap in test capability to obtain first-order heat transfer data. However, there is a gap in providing the correct flow chemistry, which may be addressed with M&S. There is also a gap in providing sufficient scale for

testing. If gaps are filled for air-breathing propulsion, the aerothermal gaps will also be closed.

(14)The vehicle’s cooling system must accommodate the very high aircraft leading-edge temperatures and the propulsion system. These temperature extremes will defy analytical solution in the complex structures. It will be necessary to test major full-scale aircraft components in nonflow ground-test facilities that produce both external and internal loading. Such a facility, which was being planned in the NASP program prior to program cancellation, is critical for production of manned long-range hypersonic aircraft.

1. Gap challenges/issues1. Page 4

i. (14)Some of the desired test facilities must themselves be supported by research to investigate new approaches for energy addition and materials/cooling techniques to permit containment of the required high-pressure, high-temperature test gases.

ii. (14)Reasonably good hypersonic perfect gas aerodynamic wind tunnels exist today, but none simulate the real-gas and aerothermal effects encountered in flight at Mach numbers above 8. Aerothermal test capability is currently limited to perfect gas wind tunnels and nonequilibrium flow shock tunnels capable of gathering heat-transfer data.

2. Page 8i. (14)The research flights, though successful in providing scramjet

operation at hypersonic speeds, have shown deficiencies in areas such as micro-instrumentation development and the difficulty in calibrating various instrumentation components to the vehicle’s operating speed.

ii. (14)Improved monitoring capability is needed. Sensors are a critical component not only for assessing health and status, but also for understanding a vehicle’s performance characteristics. Several drawbacks exist in today’s sensor systems. First, they are generally intrusive. Second, they are less reliable than the hardware that is being monitored. Third, most need manual calibration. Fourth, they are unable to detect when the output is degraded or has failed. Finally, they cannot detect off-nominal reading caused by the effects of failures in other parts of the system.

iii. (14)Gaps between current capabilities and required capabilities were the basis for determining a HS/H T&E capability requirement roadmap.

3. Page 13i. (14)The large engine test facilities are limited to Mach 3.2, yet

turbine engine R&D programs predict future engine performance to Mach 4 and above.

ii. (14)Although short run times may be useful for evaluating performance at discrete design points, seconds, minutes, or tens of minutes of run time will be required for operability and durability testing.

iii. (14)This type of vitiated air test capability is limited to about Mach 8 simulated flight total temperature.

iv. (14)The effects and limitations of combustion-vitiated testing, development of clean air test facilities below Mach 8, definition of needed facility runs times, and definition/development of test capabilities above Mach 8 are all test facility technology gaps that must be addressed.

4. Page 14i. (14)Development of a programmed accelerating and decelerating

hypersonic propulsion system is considered to be a challenge for both the engine designer and the test facility designer. Conceptual approaches to multimode propulsion systems need to be defined to assist test facility designers.

ii. (14)…improvements in inlet airframe integration and nozzle after-body test capabilities are needed, especially for large-scale (aircraft size) testing where geometric fidelity is needed and can only be achieved at reasonably large scale.

iii. (14)Thus, there is a need for real-gas flight simulation test capability, and/or analytical procedures are needed to correct data taken from perfect gas and nonreal-gas wind tunnels to predict flight results.

5. Page 15i. (14)Hypersonic wind tunnel productivity is an issue, especially

above Mach 10. For production development testing, thousands of data points are typically needed to support aircraft design. Existing blow-down wind tunnels provide seconds to minutes of test time. Impulse tunnels operate for milliseconds, and only a limited number of these facilities exist. One set of data points per run is generally available in these cases. Productivity must be a major consideration when selecting a solution.

ii. (14)As with the case of aerodynamic testing, test time is an issue and a gap for any testing above Mach 10.

iii. (14)Test article thrown weight, velocity, and fidelity are inadequate for the projected weapon systems. Sled tracks are limited in velocity, while gas guns are limited in weight and scale.

6. Page 16i. (14)…there is a gap in providing the correct flow chemistry, which

may be addressed with M&S. There is also a gap in providing sufficient scale for testing. If gaps are filled for air-breathing propulsion, the aerothermal gaps will also be closed.

ii. (14)For advanced hypersonic vehicle demonstrators, some shortfalls exist in our range infrastructure to adequately validate their applicable technologies.

7. Page 17i. (14)There is a gap in the ability to cost-effectively recover

expendable hypersonic vehicles for data analysis of flight vehicle and propulsion system materials for vehicles launched over the water.

ii. (14)A proposed solution would still look at developing a land/sea range capability for hypersonic air vehicles, as well as developing viable recovery system (e.g., chute) options – a gap that still exists.

8. Page 21i. (14)A flight-test gap exists in the capability to air-launch the

heavier hypersonic missile systems, support technology demonstrations, and provide flight-test support for hypersonic and Access-to-Space vehicles.

ii. (14)A major cost-reduction gap lies in advanced ground support systems for hypersonic vehicles.

iii. (14)There is currently a gap in the capabilities for testing hardware in the loop, including needs to have communication links to the launch and landing sites, control room, and range for checkout and validation of communication links prior to launch.

9. Pagei. (14)Although short run times may be useful for evaluating

performance at discrete design points, seconds, minutes, or tens of minutes of run time will be required for operability and durability testing.

ii. (14)The primary propulsion test capability that exists above Mach 8 currently is relatively small impulse (shock) tunnels where the test time is of the order of milliseconds and the test medium is usually not clean air.

(15)Similarly, the inverse of q, the amount of rework normally performed, is also process driven. For most aerospace systems in development, q is approximately 0.25, resulting in 4 to 10 rework cycles. The incremental increase in program costs is proportional to (1/q)-1, indicating the potential to easily double development costs through late defects and rework.

(15)CSE can be an in valuable tool to ensure better use of ground-test facilities to preclude design defects from finding their way into the flight-test program. Use of CSE to account for Reynolds number scaling effects and potential bias errors such as wind tunnel wall interference is well understood and effectively applied. An area where scaling effects are not well understood and CSE may have the potential for producing new insights is simulation of military tactical aircraft at high-angle maneuvering conditions. In these conditions, the flow is dominated by vortex structures and flow separation. Surprisingly, a large number of tactical military aircraft have required a significant modification to control surface size or structure even after a comprehensive wind tunnel campaign. Changes of this magnitude during the flight-test program can have a profound impact on program cost and schedule. Coupled effects on manufacturing costs can also become significant during this phase.

a. (15)Grand Challenge Problems

Single process owner

Low observables

Weapons integration

Buried inlet/engine

Holistic advance in integration of people, processes, and tools

Billion plus mesh grid points

Hypersonics, real-gas chemistry

Shock-shock interactions

Uncertainty estimate for CFD computations

(16)Several areas that need extensive reengineering include:1. (16)Early integrated use of high-fidelity physics modeling with ground testing

to determine system integration issues (e.g., airframe/structure, airframe/propulsion) as early as possible in the development cycle. Integration issues are a common cause of late defect discovery.

2. (16)A complete reexamination of flight system, aeropropulsion system, and space & missile system ground-test approaches to try to reduce significantly the traditional number of hours used.

3. (16)A comprehensive understanding of the impact of ground testing on late defect discovery and a corresponding development of improved testing methodologies and processes for scaling of ground data to flight conditions robust enough to avoid late defect discovery.

(16)Under the “Fostering Technical Collaboration” section the author suggests three areas that could be focused on:

An integrated use of advanced computational fluid dynamics (CFD) codes, modern design of experiments (MDOE), and "fly the mission" testing techniques to reduce the overall test hours in a wind tunnel “campaign”. Today it takes approximately 2.5 million data points in a wind tunnel test program to develop the stability and control (S&C) and performance laws database for a SUT. This amount of wind tunnel testing drives cycle time for system development, which is one of the key D&SWS effectiveness measures. A basic theoretical/experimental/computational study of scaling principles for vortex-dominated or massively separated flows. Current wind tunnel scaling principles are based on circa 1975 studies of Reynolds Number scaling of attached flows on transport configurations. For military aircraft, high angle of attack (alpha) is a more common state and can have a strong influence on structures, control response, and other design parameters (e.g., vortex breakdown on vertical tails, control surface effectiveness, control system gains). Concomitant with the high-alpha study above, the same collaborative effort could help develop a database for validation of the next generation of CFD codes.

a. Grand Challenge Problems(16)Optimum integration of the Science and Technology(S&T) and Test and Evaluation(T&E)

1. (16)Develop people2. (16)Create an Environment Conductive to Emphasizing, Recognizing,

and Promoting Technical Improvement3. (16)Streamline Process4. (16)Sustain/Improve Infrastructure

(17)Grand Challenge Problems:

Databasing of massive amounts of data

Inlet/engine compatibility

Pressure sensitive paint

Computing power

Data storage

Integrated test and evaluation

(18) See figure 8 “Technologies”

(18) Improved accuracy and surface finish, less prone to curing and temperature-induced warpage of the model.

(18) Required cost and perceived value leads a strategy of critical wind tunnels and critical capabilities with Knowledgeable test workforce

(18) Finding ways to create W/T models faster and cheaper. With current CAD and CFD capabilities vehicle design concepts can be evaluated and discarded in 1/3 the time it takes to build a W/T model.

[Note: Rapid prototyping, 3D printing. Longer to make, but lots of data after. Additive manufacturing]

(18) In 2008 the GTTC made recommendations for the future of W/T testing but so far no champion has stepped up to take on the challenges, except maybe for the efforts being done by this working group.

(18) As wind tunnel facilities and capabilities decrease and test costs almost certainly increase, programs become forced to choose between accepting increased vehicle development risk through limited testing, utilizing foreign test assets, or forgoing testing altogether and committing to full dependency on computational modeling. These alternate scenarios present potential risks of significant design issues and flight failures that can be costly and time consuming to rectify.

(18) Wind tunnels/test facilities being regarded as valuable by budget minded owners

(18) Inlet/engine compatibility

(18) A key challenge has been integrating computational simulation and experimental efforts. Because of the highly specialized nature of each approach, practitioners have usually either been experimentalists or computationalists. The well-meaning questions posed in a 1980’s article, “Will computers replace the wind tunnel?” probably did more to polarize and set back efforts to truly integrate the tools.

(18) Both tools, wind tunnels or computational fluid dynamics, are perfectly capable of producing “garbage” if not properly used. Expertise and experience are still overriding factors in producing results that are meaningful. Therefore, it is the integration of these tools, in the hands of knowledgeable experts that ultimately will produce the improvements required.

(18) Compounding the problem, existing tunnels are experiencing a steady decline in overall usage, forcing cutbacks that impact facility up-keep and improvements. At present, the repertoire of tunnels still open is adequate to support existing research and development programs. However, many of these tunnels are in need of maintenance and upgrade to meet the needs of future programs. With average facility ages nearing fifty years (illustrated in Fig. 6), maintenance and upgrades are an ever increasing and largely unfunded issue with the current tunnel suite.

(18) In the near future, many existing programs will have progressed past their ground test phase and will be in flight test or are in final certification. As previously discussed, programs currently in the conceptual phase are not utilizing the tunnels to a high degree, which results in putting many of the test facilities in a low-use state. Therefore, for those tunnel facilities that are seen to have low utilization will be in danger of closure, further degrading the nation’s capability to maintain our global leadership in aerospace.

(18) The challenges of maintaining this infrastructure center on required costs and perceived value. The cost of owning and operating these facilities is substantial, and the burden is heavier when the facility is not fully used. Their value, at the national level, is our ability to effectively develop and field leading-edge technologies both for commercial and military aeronautical systems. This value, in our current wind tunnel business model, is not reflected in the operating budgets that sustain our existing capabilities. This is analogous to the national highway system which does not generate income directly but without which we would not have a viable economy. The continued decline in our wind tunnel infrastructure is similar to closing several national interstate highways each year; soon there will be no way to effectively move our industry forward. The aerospace industry (both government and private sector) must adapt to a strategy of maintaining and operating key and critical wind tunnels as essential assets which insures the nation’s leadership in the aerospace field.

(18) [Point of discussion: Age of facility versus condition of facility. Age is a factor in maintenance costs, but upgrades can keep it current to meet needs. At some point, recapitalization is required. The article notes age; suggest we discuss age, condition, investments. At some point, decide to pay a lot to fix and update and buy a new one.]

II. Technology Development Plana. [Address the gaps – include anything already identified in the references.]

(18) Throughout, most national infrastructure forums have made a number of key recommendations for the future of wind tunnel testing. Highly representative of those recommendations, the AIAA Ground Test Technical Committee (GTTC) made the following recommendations9:

1) (18) “Development of a knowledgeable test workforce is critical for the national infrastructure.”

2) (18) “Improved test technology is crucial to enabling future system development.”

3) (18) “Maintenance and improvement of key test assets is a vital component of enabling future test capabilities.”

4) (18) “Divestment of redundant and nonessential test infrastructure is required to focus limited resources on critical capabilities and new infrastructure requirements.”

5) (18) “New high-speed test infrastructure is required to meet anticipated requirements for future systems.”

(18) [There’s a really good list of recommendations at the end of the paper – figure out how toi best utilize these.]

(19) (GT) The strengths of ground testing (GT) are that its measurements are finite and its physics are well defined and irrefutable, except that it’s in a wind tunnel.

o Wind tunnels, flights, and computations are physics based but not necessarily physics-perfect.

o We estimate in each, make approximations and only replicate some physics in all three methods.

o The difficulty is in extrapolating these results to actual flight conditions.

(19) (GT) We are losing our edge in test capabilities.o We need to maintain the ability to do the fundamental set of things

in ground test facilities that we do well now.o We’re having to teach things we used to do well to a new generation

of wind tunnel testers.o Skill loss due to both facility under-utilization and personnel

retirement (19) (GT) Improvements are needed in wind tunnels:

o Efficiencyo Rapid prototypingo New measurement techniques

Unsteady flow measurements Skin-friction measurements Non-intrusive: Pressure sensitive paint and particle image

velocimetry (19) (GT) Over the years wind tunnel infrastructure has decayed and the

workforce knowledge base has dwindled. o 16S is a prime example of loss of capability. Who’s next?

(19) (GT) Running wind tunnel tests is the quickest way to fill in a database, especially if you want to capture control (effector) power. You can get a lot of data quickly in a tunnel. Modeling all the different flap settings takes forever and rerunning solutions for each setting can take more computer time than just running a few cases to fill in a polar.

(19) (GT) We need [wind tunnel] models to enable us to treat flows inside scramjet engines as close to reality as possible.

(19) (GT) We are running the risk of losing people with experimental expertise because we are doing less testing

Long-term Issues/Gaps

(19) (CFD) Turbulence modeling is a pacing item, and is still not well understood.

o We are far from automatic grid generation.o It is very case-dependent

(19) (CFD) A major weakness of CFD is in turbulence modeling – you get a different answer depending on the flow conditions.

o Yet, some test facilities cannot replicate the turbulence found in nature as well; or answer such questions as when does the flow separate or when does it transition?

o While these situations are improving, turbulence modeling is a pacing item, and is still not well understood; but, again, this is very case dependent.