5

Hypersonic Facilities

INTRODUCTION

Most hypersonic test facilities in the United States were built in the 1950s and 1960s to support development of intercontinental ballistic missiles, the space program, and research into first-generation hypersonic vehicles such as the X-15. Although many of these facilities have been used during the past two decades to support the space program, several were mothballed or even dismantled in the 1970s. Thus, current U.S. hypersonic test capabilities consist primarily of older test facilities, many of which are in serious need of refurbishment, and a few newer facilities that were built to support the National Aero-Space Plane.

Termination of the National Aero-Space Plane at the end of fiscal year 1994 places the immediate future of hypersonic vehicle development in jeopardy. If the United States wishes to retain its capability to develop future hypersonic aircraft, spacecraft, missiles, and planetary probes, adequate facilities will be needed. The National Research Council noted in a 1989 study that ground test facilities did not exist to support complete testing of scramjet engines (NRC, 1989). Development of such facilities normally takes 10 years, and it is unlikely they could be available if not started before a new development program is underway. The National Aero-Space Plane program encountered this difficulty when it needed additional facilities for testing engines and cryogenic tanks (Richey, 1993).

In summary, significant deficiencies must be addressed if the United States is going to maintain a vigorous hypersonic program into the next century. Areas such as low noise and turbulence levels and real-gas effects need to be accurately simulated for flight greater than Mach 8.

PRIOR STUDIES

As shown in Table 5-1a and Table 5-1b, hypersonic ground test facilities have been examined as part of several major studies during the last few years, and the conclusions of these studies are consistent. They generally agree on the following points:

  1. More-capable hypersonic ground test facilities are needed to adequately support future development programs.

  2. State-of-the-art technology is not adequate to build major new hypersonic facilities that will have the desired capabilities in areas such as model size, run time, pressure, temperature, and velocity.

  3. Near-term efforts should focus on a program of research to select, develop, and demonstrate the most promising hypersonic test facility concepts.

  4. Long-term efforts to build hypersonic development facilities will be contingent on successful completion of the near-term facility research effort and concurrent efforts to validate future requirements for hypersonic vehicles.

THE CHALLENGE

The airframe and propulsion systems of hypersonic vehicles must meet aerodynamic and aerothermal requirements over a wide speed regime, from Mach 5 to Mach 25. Understanding the hypersonic flight environment and how to simulate it in a ground test facility is difficult. Vehicle development requires close simulation of the



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Assessing the National Plan for Aeronautical Ground Test Facilities 5 Hypersonic Facilities INTRODUCTION Most hypersonic test facilities in the United States were built in the 1950s and 1960s to support development of intercontinental ballistic missiles, the space program, and research into first-generation hypersonic vehicles such as the X-15. Although many of these facilities have been used during the past two decades to support the space program, several were mothballed or even dismantled in the 1970s. Thus, current U.S. hypersonic test capabilities consist primarily of older test facilities, many of which are in serious need of refurbishment, and a few newer facilities that were built to support the National Aero-Space Plane. Termination of the National Aero-Space Plane at the end of fiscal year 1994 places the immediate future of hypersonic vehicle development in jeopardy. If the United States wishes to retain its capability to develop future hypersonic aircraft, spacecraft, missiles, and planetary probes, adequate facilities will be needed. The National Research Council noted in a 1989 study that ground test facilities did not exist to support complete testing of scramjet engines (NRC, 1989). Development of such facilities normally takes 10 years, and it is unlikely they could be available if not started before a new development program is underway. The National Aero-Space Plane program encountered this difficulty when it needed additional facilities for testing engines and cryogenic tanks (Richey, 1993). In summary, significant deficiencies must be addressed if the United States is going to maintain a vigorous hypersonic program into the next century. Areas such as low noise and turbulence levels and real-gas effects need to be accurately simulated for flight greater than Mach 8. PRIOR STUDIES As shown in Table 5-1a and Table 5-1b, hypersonic ground test facilities have been examined as part of several major studies during the last few years, and the conclusions of these studies are consistent. They generally agree on the following points: More-capable hypersonic ground test facilities are needed to adequately support future development programs. State-of-the-art technology is not adequate to build major new hypersonic facilities that will have the desired capabilities in areas such as model size, run time, pressure, temperature, and velocity. Near-term efforts should focus on a program of research to select, develop, and demonstrate the most promising hypersonic test facility concepts. Long-term efforts to build hypersonic development facilities will be contingent on successful completion of the near-term facility research effort and concurrent efforts to validate future requirements for hypersonic vehicles. THE CHALLENGE The airframe and propulsion systems of hypersonic vehicles must meet aerodynamic and aerothermal requirements over a wide speed regime, from Mach 5 to Mach 25. Understanding the hypersonic flight environment and how to simulate it in a ground test facility is difficult. Vehicle development requires close simulation of the

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Assessing the National Plan for Aeronautical Ground Test Facilities full conditions of flight. Adequate test-section size and test duration are essential for reliable test results. Numerical simulation is highly dependent upon the accurate modeling of these types of flows. An incomplete understanding of boundary layer transition; combustion; and high temperature, real-gas effects could cause serious computational errors, particularly at high Mach numbers (above Mach 8). Because of these limitations, a full understanding of the performance of hypersonic vehicles is highly unlikely without additional research involving ground tests, flight tests, and computational modeling (USAF Scientific Advisory Board, 1989). Table 5-1a Recent Studies of Hypersonic Ground Test Facilities Future Aerospace Ground Test Facility Requirements for the Arnold Engineering Development Center (NRC, 1992) Focus: Military requirements for new and improved facilities for ground testing and computational modelling at Arnold Engineering Development Center Recommendation (relevant to hypersonic facilities): Develop a road map to institute a two-phase program by building a high quality Mach 8 facility accompanied by a hypersonic facility research effort. Phase Two would build on the results of Phase One by building a major hypersonics development facility. Cost: No estimate provided Hypersonic Test Investment Plan (HTIP Working Group, 1993) Ad hoc committee of personnel from government; industry, and academia; established by DoD and NASA, December 1992 Focus: Civil and military requirements for new and improved facilities for ground testing and computational modelling Recommendation: Institute a long-term program supporting facility research, pilot facilities, instrumentation, test methodologies and computational science. Cost: About $35 million per year during the first 10 years NATIONAL FACILITIES STUDY HYPERSONIC FACILITY REQUIREMENTS The Hypersonics Working Group of the NFS Task Group on Aeronautical R&D Facilities reviewed the historical record relating to hypersonic ground test facilities. It evaluated both U.S. and foreign hypersonic test facilities, many of which were built over two decades ago in support of the space program. These facilities were placed into one of three groups: aerodynamic/aerothermodynamic facilities; aeropropulsion facilities (including internal combustion); and structural/airframe test facilities. As a result of these reviews, the Hypersonics Working Group determined that U.S. national facilities are inadequate for future systems development, particularly for propulsion and real-gas testing above Mach 8. Foreign hypersonic facilities, which are better than domestic facilities in limited areas, also were not considered adequate. As part of its study, the Hypersonics Working Group considered how computational fluid dynamics (CFD) and flight tests help to define ground test facility requirements. Although CFD modeling has made impressive progress, the validity of CFD results are highly dependent upon the accurate modeling of various flow fields, including complex dissociated and combustion-type flows. Since these kinds of flows are still not fully understood, serious computational errors are likely to occur, particularly at high Mach numbers (above Mach 8). Thus, calculations must be validated using ground-based facilities or flight experiments. Because of the limitations associated with CFD verification, complete hypersonic performance evaluation may only

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Assessing the National Plan for Aeronautical Ground Test Facilities be achieved through flight test over the vehicle's entire speed regime. Based on these findings, the NFS recommended starting a two-phased construction and facility refurbishment program. Phase I would focus on facility research to explore new approaches to hypersonic testing, while Phase II would provide the needed systems certification facilities for vehicle development. Three low-risk facilities would be built as part of Phase I to help meet current needs for hypersonic ground testing. In addition, the Phase I effort would include a time-phased development program to study facility concepts for more-complex facilities that are beyond the current state of the art. These facilities would be constructed in Phase II, when the necessary technology is available. The estimated cost to carry out these recommendations is about $220 million to build the initial facilities, plus $20 million per year for the facility research effort. These recommendations are consistent with the results of the other studies summarized in Table 5-1a and Table 5-1b. Finding 5-1: The National Facilities Study report is consistent with prior studies of hypersonic test capabilities. Recommendation 5-1: To address the hypersonic ground test deficiencies identified in previous studies, the ASEB endorses the objectives of the two-phase program recommended in the National Facilities Study report. Table 5-1b Recent Studies of Hypersonic Ground Test Facilities Hypersonic Technology for Military Application (NRC, 1989) Focus: Military applications for hypersonic aircraft and the technologies related to such vehicles Recommendations (relevant to ground test facilities): –Consider building a quiet tunnel for Mach 10 testing at close to full-scale conditions –Consider building a facility for testing components at temperatures above the 1200°C limit of available facilities Cost: No estimate provided Requirements for Hypersonic Test Facilities (USAF Scientific Advisory Board, 1989) Focus: Civil and military requirements for new and improved facilities for computational modelling, ground testing, and flight testing Recommendations (relevant to ground test facilities): –Initiate a research program to support eventual construction of a large national hypersonic ground test facility, then –Proceed with a national facility, if it is justified by a broad-based requirement for a full-scale engineering development capability Cost: $138 million for facility construction and upgrades, plus $20 million per year to support facility research REFERENCES HTIP Working Group . 1993 . Hypersonic Test Investment Plan . Tullahoma, Tennessee : Micro Craft, Inc. NRC (National Research Council) . 1989 . Hypersonic Technology for Military Application . Air Force Studies Board, NRC . Washington, D.C. : National Academy Press . NRC. 1992 Future Aerospace Ground Test Facility Requirements for the Arnold Engineering Development Center . Aeronautics and Space Engineering Board, NRC . Washington, D.C. National Academy Press . Richey, K. 1993 . Hypersonic Facilities Study Review to ASEB . Presentation to the Aeronautics and Space Engineering Board, Committee on National Aeronautical Test Facilities, at West Palm Beach, Florida, September 10, 1993 .

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Assessing the National Plan for Aeronautical Ground Test Facilities USAF Scientific Advisory Board . 1989 . Requirements for Hypersonic Test Facilities . Department of the Air Force . Washington, D.C.