Breaking the Wind-Tunnel ‘Barrier’

Dec. 1, 1962

Extended hypersonic flight is today’s great tech­nical challenge in high-performance vehicle design. A massive preliminary effort is in progress to replace the ballistic capsule and the ICBM with more effi­cient vehicles for transporting men, equipment, and weapons. Two-stage and one-stage recoverable boosters, lifting reentry vehicles and highly maneu­verable anti-ICBM weapons are among a variety of hypersonic-cruise vehicles in various stages of de­sign and study.

One important requirement is holding up the successful development of these advanced hypersonic vehicles that are more complex than ICBMs and bal­listic reentry capsules. A large wind tunnel which can truly simulate hypersonic flight must be avail­able if detailed engineering data is to be gathered on the interactions of wings, tails, engine air inlets, high-temperature structure, etc., from Mach 6 to orbital speeds. Without such detailed engineering information the design of any vehicle becomes con­jectural, and history has shown that its develop­ment is erratic.

A new hypersonic test facility, the Cornell Aeronautical Laboratory’s Wave Superheater appears to be able to overcome the hypersonic facility “bar­rier.” Recent tests have shown that the Wave Super­heater can produce the same type of detailed aero­dynamic information now supplied by subsonic and supersonic wind tunnels, plus the high-temperature ablation and structural data so vital at hypersonic speeds. The Wave Superheater is a Gatling-gun-­type multiple shock-tube apparatus that discharges the equivalent of about 14,000 shock tubes a second to provide a large-volume hypersonic flow in a wind tunnel.

Aeronautical progress has been impeded on sev­eral occasions due to the lack of proper test facili­ties on the ground. A major instance occurred during the embryonic years of supersonic flight when it was impossible to simulate transonic flight in a wind tunnel. Once the perforated-wall transonic tunnel overcame this problem, the development of super­sonic aircraft proceeded rapidly.

Many aeronautical engineers believe that the Wave Superheater can perform the same service for hypersonic flight of sophisticated vehicles.

Prospects for the early development of hy­personic-cruise airplanes, recoverable boosters, ground-launched anti-ICBMs and other key high-speed vehicles have brightened considerably.

The Cornell Aeronautical Laboratory, Buffalo, N. Y., has announced the first successful tests of its Wave Superheater, a unique type of hypersonic wind tunnel that is the closest thing to a conventional wind tunnel yet devised for testing above Mach 5. It is the first hypersonic test facility that will accommodate very-large-scale models of aerospace vehicles, such as Dyna­-Soar and Apollo, subjecting them to the same heating and air chemistry conditions the actual vehicles encounter during most of their atmospheric flight.

The Wave Superheater is expected to provide virtually all of the detailed airflow data needed to permit the design of hypersonic airplanes. For the first time it appears possible to study the complete interactivity between structural heating, boundary-layer stability, the performance of engine air inlets, vehicle stability and control, and other vital transient-flow conditions from Mach 6 to 15. With the Wave Superheater researchers can now get down to the fine points of hypersonic airplane design and make it a more precise act.

Exhaustive wind-tunnel tests, investigating small but significant configuration changes, can now be run on hypersonic airplanes and winged boosters.

The Superheater was developed by the Cornell Aeronautical Laboratory on a cost-sharing basis with the Defense Department’s Advanced Research Projects Agency, with USAF’s Arnold Engineering Develop­ment Center serving as the technical monitor. Feasi­bility was determined through the testing of a small-scale model of the facility, underwritten by the USAF’s old Office of Scientific Research.

Total cost is approximately $5 million. This sum would pay for only about ten flights of large-scale, rocket-propelled models of a size that can be tested in the Wave Superheater. The rocket models can cover the entire flight regime while the Superheater can’t, but these short-duration flights are too costly to be used for configuration improvement work.

Until now the aerodynamic and aerostructural prob­lems of very-high-speed vehicles have been investi­gated in bits and pieces. No single test facility has been able to completely simulate a hypersonic airstream so that a relatively large-scale model could be studied in detail at close range, under precisely controlled con­ditions. Researchers have had to laboriously piece to­gether bits of isolated data to form a “grossly correct” picture of hypersonic flight. The Wave Superheater now seems to provide a means of getting much more detailed information.

Hypersonic test facilities currently in operation fall into the following general classes:

• “Cold” hypersonic shock tunnels, in which a high-pressure gas drives a shock wave through a mona­tomic gas such as helium, creating a hypersonic flow condition over a model for a few thousandths of a second. Very high Mach numbers of 20 to 25 can be achieved in these tunnels. They are valuable in funda­mental studies involving phenomena such as the inter­actions between the boundary layer and the shock waves on a vehicle. Their great limitation is that they do not produce the proper “real gas” effects, because their atmosphere is monatomic, with only one atom per molecule. Air, the gas in which we are practically interested, is diatomic, with its molecule shaped like a dumbbell. When it is heated, the air molecule not only moves about rapidly, but also absorbs considerable energy by spinning. At some point enough energy is absorbed to break up the molecule into separate atoms. Finally the atoms become ionized when an electron is broken away. These “real gas” effects, which alter the heat transfer characteristics of the airflow and cause chemical reactions (erosion) between the air and a vehicle’s surface, are the primary difference between hypersonic and the slower-speed flows.

• Heated nitrogen tunnels, which simulate the flow of air more closely than does the helium tunnel. These facilities operate with a stagnation gas temper lure of around 4,000 degrees Fahrenheit.

• “Hot” hypersonic shock tunnels, in which air is the working atmosphere and “real gas” effects a produced. The maximum test Mach number of most of these facilities is 10 to 12, but some reach Mach 17 so. Testing time in this type of tunnel is measured thousandths of a second, as it is in the “cold” tunnel but this is long enough to record very accurate fort and moment data, dynamic stability information, at heat-transfer measurements. Fast response pressure transducers, thermocouples, and other sophisticated instrumentation have been in operation long enough build up an unassailable performance record. The larger “hot” shock tunnels have test sections around four feet in diameter, and they will accommodate fairly large models. The stagnation temperature of the air in these tunnels is 9,000 degrees F or more. The great limitation of these facilities is their short test time, which does not allow a study of air chemistry structural ablation, and the effect these have on thickening the boundary layer, altering the heat-transfer characteristics, or otherwise disturbing the flow around a vehicle.

• Arc-jet tunnels, which can produce a very-big energy, high-temperature flow for many seconds at time. They are widely used to study structural ablation, heating effects in large segments of actual structure, and the properties of materials in high-temperature airstreams. These facilities will accommodate large models and will reproduce the temperature and heat-transfer conditions which will be encountered by many types of reentry vehicles over a large portion of their reentry flight paths. The limitation of the arc-jet tunnels is that their gas streams are contaminated when their electrodes burn away. The exact chemistry of their flow is not known at any given point at any Oven time. Therefore, there is always a question as to whether the “real gas” chemistry is being simulated

The Cornell Aeronautical Laboratory’s Wave Super­heater overcomes the limitations of all of these devices and will duplicate the heat flux and actual airflow chemistry over a wide band of speeds and altitudes which will be traveled by lifting reentry vehicles, re­coverable boosters, and hypersonic-cruise vehicles. The Wave Superheater combines all of the virtues of the ‘hot” hypersonic shock tunnel in producing uncon­taminated “real air” with the long testing time advan­tages of the arc-jet.

The central element in the Wave Superheater is a large five-foot-diameter, 12,000-pound drum with 288 shock tubes cut into its periphery. During operation, the drum rotates at speeds up to 2,700 rpm, and the shock tubes are discharged one at a time in Gatling­-gun fashion to produce a high-temperature, high-velocity flow that fills the tunnel. Tests have shown that the Superheater flow is smooth and stable and does not fluctuate due to the Gatling-gun action, as predicted by some wind-tunnel experts.

From mechanical engineering and machine design standpoints the Wave Superheater advances technol­ogy in several respects. For instance, the rotor, which was formed by U.S. Steel, is the largest precipitation-hardening stainless steel forging ever made. Cutting 288 5.5-foot-long shock tubes in this forging is one of the most difficult deep drilling and broaching jobs ever undertaken. To accomplish it the Twentieth Century Machine Co., of Utica, Mich., had to cut the rotor in three sections, drill each section, and put them back together again with an alignment tolerance on each shock tube of less than 0.002 inch. The job took more than a year.

Actually the Wave Superheater consists of four pressurized, blowdown wind tunnels which must be operated in a precise, split-second sequence or the facility will not fire. One of the blowdown systems forces a helium cooling charge through the shock tubes during a portion of each turn of the rotor. Another system then supplies high-pressure helium at 700 degrees F to purge the coolant gas. As the rotor continues to revolve, a third system forces air into the shock tube at stagnation temperatures up to 9,000 degrees F. When this high-temperature air charge fills the tube the rotor has turned so that it lines up with the nozzle of the fourth blowdown system. This ore releases a charge of helium driver gas heated to about 1,400 degrees F and under about 2,000 psi pressure. The helium-driver gas slams into the hot air charge and creates a strong shock wave which moves rapidly down the tube. There is little or no mixing of the two gases. As the shock wave travels through the relatively still charge of air a pocket of “hypersonic” air grows behind it. The composition, temperature, velocity, pressure, and heat content of the air in the pocket can be closely controlled by varying the conditions of the charge and driver gases. Since the Wave Superheater rotor discharges a pocket of air from a shock tube nearly 14,000 times a second, a smooth and uninter­rupted flow is created in the test section (see drawing).

One of the main design problems has concerned the collection and purification system for the helium gas which is used in the operation of the rotor at the rate of about seventy pounds per second. Loss of helium at this high rate is impractical for any purpose because of the nation’s limited supply of this precious gas. Therefore, all of the gas in the rotor is scavenged, puri­fied, and repressurized after each running of the tunnel. This recycling process requires about four hours so that a maximum of six runs of up to fifteen seconds each are possible on any day. In terms of con­ventional hypersonic testing this will provide an extremely large amount of data.

Construction of the collector nozzles which channel the high-temperature flow from the rotor shock tubes into the tunnel was especially critical. It is possible to use a stainless-steel collector nozzle for flows up to 3,000 degrees F depending upon the time of operation and the test pressure. Above these conditions, to 4,000 or 5,000 degrees F at moderate pressures, a water-cooled copper nozzle can be used. At the higher tem­peratures and pressures needed for Mach 15 simulation a specially designed nozzle has been prepared. It is the only classified portion of the Superheater apparatus.

Cornell’s successful operation of the large Wave Superheater is being watched in several quarters out­side of aviation. The multishock tube rotor originated in Europe where its possibilities as an efficient air compressor and temperature multiplication device have been of industrial interest. Some US chemical firms are known to have sponsored some of Cornell’s first work with the device in the early 1950s. They were interested in the Superheater as a possible ele­ment in continuous-flow production processes, such as the conversion of methane to acetylene. Presumably an interest still exists but the chemical industry oper­ates within a shroud of secrecy that the Kremlin would envy, and little is known of recent activity.

Another possible use of the Wave Superheater in­volves the gaseous-core nuclear rocket, which has a very high theoretical efficiency and ultimately is ex­pected to replace the solid core nuclear engines now being developed by NASA in Project NERVA. Ap­parently rapid progress is being made in proving the feasibility of the gaseous-core reactor and engineers are beginning to seriously attack the big problem of containing the expensive gaseous nuclear fuel within the engine and preventing it from escaping out of the nozzle with the heated propulsion gas. Some authori­ties regard the Wave Superheater concept as a prime contender for this job. They believe that nuclear fuel can be used to heat the propulsive gas and then can be scavenged and reused in much the same way that helium is recycled in the Superheater. Flightweight rapid recycling systems appear to be possible.

The main objective now, however, is to get the Wave Superheater tunnel into active vehicle development work. This is getting under way, and four major projects are scheduled for the near future. One Project Trailblazer, a reentry physics research vehicle managed by the Lincoln Laboratory of Massachusetts Institute of Technology.

The rest of the schedule currently is classified, but the Superheater would provide a fine check on extensive data already assembled on the Dyna-Soar in what has been described officially as the largest wind tunnel program ever undertaken on a single aircraft. The Superheater can also perform vital air inlet tests that can’t be duplicated elsewhere, for the development of hypersonic air-breathing engines—the key to the aerospace plane and economical recoverable boosters. The problems of ground-launched anti-ICBM missiles and other high-acceleration, high-heating-rate vehicles can be studied to greater advantage in the Cornell Aeronautical Laboratory facility than in any other now in service.

The next year of operational activity will tell the tale on the true value of the Wave Superheater. However, enough is known now for many aeronautical engineers to predict that it will have as great an effect on aircraft design as the transonic wind tunnel did in the late 1940s and the early 1950s. Until the perforated wall-test-section transonic tunnels provided an accurate simulation of transonic flight, it was impossible to get large amounts of engineering data on Century-series aircraft. Consequently their design was plagued by uncertainties, and their development was slowed because of doubtful operational usefulness.

John Stack and his associates at the Langley Laboratory of the old National Advisory Committee Aeronautics received a Collier Trophy for designing the first transonic tunnel and eliminating this road block. If the Wave Superheater lives up to expectations and opens up hypersonic design in the same manner it undoubtedly will be in contention for similar honors.

Even though the Wave Superheater sets new standards for accurate simulation of high-speed flight over a wide Mach number and altitude range for long periods, it does not obsolete the many hypersonic test facilities now in use. Short-duration shock tunnels, arc-jets, and other partial-simulation devices are extremely valuable. These facilities have provided the experimental data that underlies present understanding of hypersonic flight. In the future they will even more valuable sources of information. Flight experience will reveal what corrections are needed to adjust their data to actual conditions, and their gem economy of operation will always make them valuable test tools. Flight tests undoubtedly will reveal that some corrections are needed for Superheater data, it is probable that this facility will be able to provide an early and accurate check on the performance many of the partial simulation facilities, as well as provide a new type of data of its own.