Hypersonic and Hypervelocity Ground Test Facilities: A Brief Informal Summary

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1 Hypersonic and Hypervelocity Ground Test Facilities: A Brief Informal Summary Steven P. Schneider Professor School of Aeronautical and Astronautical Engineering Purdue University November 9, General Issues No single ground-test facility can fully simulate all aspects of hypervelocity flight. Flow duration, velocity, gas chemistry effects, Mach number, altitude or Reynolds number, model surface temperature, ablation effects, and the quality of the freestream flow cannot be controlled simultaneously in any single facility, if at all. Nearly all tunnels suffer from noise levels much higher than flight, and all tunnels simulating gas chemistry effects also have freestream chemistry contaminants. Ground test simulations are an exercise in the art of combining partial simulations each having different advantages and disadvantages. 2 Approximate Facility Catalog Most of the major international hypersonic flow facilities were recently reviewed in Ref. [1]; however, this book does not review or catalog all of the major facilities. For North American facilities, the most recent such public-release catalog known to this author appeared as Ref. [6] in This review is limited to major facilities with test sections greater than 1 foot in diameter. The number of such facilities has decreased dramatically over the past 40 years, from a high of 82 during a 1963 review to 75 in a 1971 review and 24 in the 1987 review. The helium tunnels have now all been closed, so the tunnels listed in Table 2 all use air or nitrogen, except for the CF4 tunnel at Langley. Since the 1987 tabulation, this author counts 14 more tunnels that have been closed or mothballed or are otherwise no longer in regular operation. The table has not been checked in detail through communications with the various operators, so it is for preliminary use only. A general discussion of hypersonic tunnels can be found in Refs. [2] and [3], although both these books are somewhat dated. Figure 1, taken from Ref. [1], places some of the major facilities on a Mach number vs. altitude map. Several of the facilities shown there have been closed (Tunnel 8) or modified (Tunnel 9). Many of these are cold facilities, which achieve high Mach numbers due to very 1

2 Designation Test Mach Reynolds Max. Stagnation Section No. No. per ft. Pressure Temp. Size (millions) (psia) (deg. R) Continuous Flow AEDC Tunnel B 50-in. dia. 6, AEDC Tunnel C 50-in. dia Intermittent LaRC 8 ft. High Temp. 96-in. dia AEDC Tunnel 9, N2 60-in. dia LaRC 31-in. Mach in. sq LaRC 20-inch Mach 6 20x20.5 in Sandia 18-in. dia. 5,8, LaRC CF4 20-in. dia GRC (Plumbrook) 42-in. dia. 5,6, Shock Tunnels CUBRC 48-in 48-in. dia ?? CUBRC LENS I in. dia ?? CUBRC LENS II 66-in. dia ?? GASL HyPulse 26-in. dia.? Table 1: Summary of Current United States Hypersonic Tunnels (Adapted from Ref. [6]) low sound speeds at very low freestream temperatures, just above condensation. Fig. 2 shows a map of velocity vs. altitude, emphasizing air chemistry effects, which come in due to high enthalpies at high velocities. There are also arc-jet facilities (Fig. 3), which blow very hot air of lesser flow quality, mainly for the purpose of long-duration tests of thermal-protection system materials. Ballistic ranges fire small-scale models at up to orbital velocities, down long tubes of controlled air, and can reach high enthalpies with air chemistry effects. The 8-ft High Temperature Tunnel at Langley uses vitiated air (combustion is carried out in the air stream to provide the high temperature, with the oxygen lost being made up with later injection). The LENS and HyPulse shock tunnels can provide high enthalpies with air chemistry effects. The Plumbrook facility uses nitrogen to which oxygen is added after the airstream is heated. 3 Summary and Discussion Facility usage generally follows fairly well established patterns. NASA programs commonly use NASA tunnels, which are also smaller and less expensive than AEDC tunnels. DoD programs commonly use AEDC tunnels. After the early 1990 s, when the Cold War ended, strategic reentry vehicle work declined, and the NASP program ended, hypersonic facilities were short of work and therefore many were closed. The new hypersonic programs that started ca are working from a much smaller facilities base in which many of the skilled workers have retired and capabilities have eroded. For a decade the community was in a defensive position, trying to keep important facilities from being scrapped. University 2

3 Figure 1: Hypersonic Tunnel Map Figure 2: Hypervelocity Tunnel Map expertise has also decayed with retirements and the funding cuts at the end of the cold war. During this period Europe and Japan built new facilities with capabilities that in many cases exceed those found in the USA. However, these foreign facilities will not be discussed here. Perfect-gas tunnels are less expensive, have higher flow quality, and are easier to measure in. These are used for general aerothermodynamic effects not requiring high enthalpy. The freestream flow is cold, near the condensation point of nitrogen, and the sonic speed is reduced, so the Mach number is high but the gas velocity is moderate and air chemistry effects are mostly not present. The continuous-flow tunnels at AEDC (Tunnels B and C) are useful for force-and-moment tests, since a very large number of points can be acquired at various model attitudes during a single continuous-operation day. Tunnel B and the 20-inch Mach-6 tunnel at LaRC are often used for transition measurements, in part because they have a long history of such measurements enabling comparisons to earlier programs. The flow quality of Tunnel B is excellent and very well documented. Tunnel 9 provides very high Reynolds number and higher Mach number. The NASA Langley tunnels are less expensive 3

4 Figure 3: Arc-Jet Tunnel Map but smaller. The 31-inch Mach-10 tunnel at LaRC is used for evaluating Mach number effects, as compared to the 20-inch Mach 6. The Sandia tunnels have not been used much recently, and they are smaller. High temperature facilities have also been developed for tests that do not require simulation of full flight velocities. The 8-foot high temperature tunnel at Langley uses vitiated air to enable reasonable tests of scramjet-engine vehicles like the Hyper-X. Vitiated air tunnels combust fuel in the air stream to provide high temperature, and then add oxygen to make up the oxygen lost by combustion. The air stream thus contains combustion byproducts such as water, which must be accounted for. The NASA Glenn tunnel at Plumbrook uses a graphite heater to heat nitrogen (like in Tunnel 9), and then adds oxygen downstream to make up an air-like mix. It was also developed to test scramjet engines. Hypervelocity facilities that better simulate hypervelocity freestream flows also tend to be more difficult and expensive to operate, and more difficult to make measurements in. These facilities attempt to match gas velocity or enthalpy, simulating the dissociation and other chemistry effects present in very high velocity flow. In the USA, the only large facilities presently capable of obtaining hypervelocity effects are the LENS shock tunnels at CUBRC and the GASL complex. The LENS tunnels follow a 4-decade heritage, as shock tunnels were originally developed at Cornell in the late 1950 s. A continuous effort has been maintained there, due in large part to the efforts of Mike Holden, who developed and operated the new LENS tunnels with funds from the missile-interceptor community. These tunnels are also capable of running at low density to obtain the only rarefied-flow capability presently operational in the USA. Their runtime is short, but the major limitation appears to be the flow quality, which is not yet well documented in the available literature; the freestream chemistry always includes facility-related effects and never fully duplicates flight. The CUBRC facilities have well-developed instrumentation capabilities that appear to trace continuously 4

5 back to the 1960 s. The GASL tunnels were developed from an expansion tube surplused from NASA Langley. They have been used primarily for scramjet testing, but should be able to provide hypervelocity aerothermodynamic effects also. The T5 free-piston shock tunnel at Caltech also offers hypervelocity flow but in a fairly small 12-inch dia. nozzle. Larger free-piston tunnels are in operation in Germany and Japan. The quality of the freestream flow in hypervelocity tunnels is a critical issue, since the chemistry is difficult, so all these facilities must be studied very carefully. The CF4 tunnel at Langley was built to emulate hypervelocity effects in a perfect-gas facility by providing a lower value of the specific-heat ratio, and although this emulation is very incomplete the facility remains useful. Arc-jet tunnels were originally developed for aerodynamics work, but the flow quality was not good enough. Hot-shot tunnels have all been shut down, for the same reason. The arcjet tunnels remain in use for testing thermal-protection system materials. They are able to maintain very high enthalpies long enough to perform heat-soak tests on thermal protection systems. For example, reentry vehicle nosetips can be ablated to determine ablated shape, roughness, and survivability. Quiet tunnels maintain laminar boundary layers on the nozzle walls to obtain noise levels comparable to flight. These are important for phenomena such as laminar-turbulent transition that are sensitive to tunnel noise, which is otherwise typically 1% [4]. Although NASA Langley had a 7-inch Mach-6 perfect-gas quiet tunnel, it was shut down. Purdue has developed a 9.5-inch Mach-6 quiet tunnel, which is now the only hypersonic tunnel which is quiet at high Reynolds numbers [5]. Ballistic ranges fire a small projectile down a long tube of controlled gas. These are able to reach speeds to 7 km/sec. for 1 kg projectiles. In the USA, ranges remain at AEDC and Eglin AFB. NASA Ames also has a range which can fire counterflow into a shock-tunnel airstream to get very high flow velocities relative to the model; however, this NASA Ames range has suffered from limited funding for decades. These ranges are the only ground-test facilities capable of obtaining true air chemistry effects, and in principle they are free of the noise which is nearly always radiated from wind tunnel walls. However, these facilities are also very limited. The flight times are too short for substantial ablation or other soak effects and the models are very small, usually uninstrumented, and at angles of attack that are difficult to control. Measurements are usually limited to photographs with Schlieren or infrared cameras. Thus, each facility has advantages and disadvantages, and none can fully simulate hypervelocity flow. Hypersonic vehicle development must combine computational and theoretical efforts with appropriately designed experimental programs that use the best features of each facility. A skilled program manager uses available resources to carry out a design and development program that most effectively improves performance and reduces risk. 4 References [1] Frank Lu and Dan Marren, editors. Advanced Hypersonic Test Facilities. AIAA, Volume 198 in the Progress in Astronautics and Aeronautics series. 5

6 [2] J. Lukasiewicz. Experimental Methods of Hypersonics. Marcel-Dekker, [3] A. Pope and K. Goin. High-Speed Wind Tunnel Testing. Wiley, New York, [4] Steven P. Schneider. Effects of high-speed tunnel noise on laminar-turbulent transition. Journal of Spacecraft and Rockets, 38(3): , May June [5] Steven P. Schneider. The development of hypersonic quiet tunnels. Paper , AIAA, June [6] C.E. Witliff. A survey of existing hypersonic ground test facilities North America. In Aerodynamics of hypersonic lifting vehicles, pages 1 1 to 1 17, AGARD CP