Outer space offers no appreciable resistance to the flight of spacecraft. It is only during the launch, reentry, and landing phases of space missions that NASA's wind tunnels contribute to space vehicle design. It is impossible to relate all the wind tunnel experiments that preceded the hundreds of U. S. spacecraft and launch vehicles. The Viking soft landing mission to Mars has been selected to portray the aerodynamic problems of terrestrial launch and entry into a thin but palpable alien atmosphere. The Space Shuttle illustrates the manifold tests required to operate successfully at the fringes of the atmosphere and return to a landing on Earth.
Even on the launch pad, the wind tunnel plays a role. The winds, which in Florida can be of hurricane force, exert forces on the launch vehicle that must be well understood before the rocket and its payload are designed. After launch, high winds in the upper atmosphere tend to make the ascending vehicle pitch and yaw. Viking, with its unusual hammerhead shape at launch, was particularly sensitive to the over turning moments created by winds and to the heavy buffeting induced by local shock waves. These had to be evaluated early in the design cycle. During the Viking Program the 10 x 10-foot supersonic wind tunnel at Lewis was called on to investigate aerodynamic heating, the level of buffeting, and shock wave interference in the Mach 2 to Mach 3.5 range. The mundane but critical design problem of deciding when to jettison the huge 14-foot Viking nose shroud was solved in the Lewis 8 x 6-foot supersonic tunnel. (Note that these tunnels were not built with the Viking application in mind at all.)
The density of the Martian atmosphere, which is composed mostly of carbon dioxide, is only a few percent that of the Earth. Far from simplifying the mission-less aerodynamic heating, and so on-the very thin atmosphere could not provide the braking forces needed to slow the Viking Lander down to near zero velocity for a soft landing. Three separate stages of deceleration had to be used: a high drag aeroshell, a large parachute, and retrorockets for the final delicate touchdown.
As the entry vehicle hurtled toward the planet at 14 600 feet per second, the first concerns were the possible disturbance of the trajectory by the Martian atmosphere and aerodynamic heating of the aeroshell. Drag data at entry speed came from the high speed ballistic ranges at Ames where true flight....
....velocities, the CO2 atmosphere, and actual Reynolds numbers could all be duplicated. Aerodynamic heating became serious in the hypersonic range, where the CO2 piled up in front of the aeroshell, creating an incandescent shock wave. NASA, Air Force, and commercial wind tunnels all pitched in to simulate the wide span of speeds, pressures, and CO2 densities. A half dozen wind tunnels at Langley alone were called on to provide aerodynamic design data for the aeroshell up to the moment of parachute deployment.
The aeroshell slowed the entry vehicle down to 1230 feet per second; then the parachute reduced the velocity to about 200 feet per second. Naturally, the tenuous atmosphere permitted parachute operation at much higher speeds than in the terrestrial atmosphere. Would the parachute work properly in the wake of the large, flattish aeroshell? The Langley transonic dynamics tunnel was the lead facility here. It helped define the chute size, the canopy shape, the length of the shroud lines, and the pressures on the lander as it descended.
Once the Viking Lander settled on to the Martian surface, the role of the wind tunnel would seem to be ended. Not so! The tenuous Martian atmosphere is sometimes whipped by 100 mph winds. The transonic dynamics tunnel was again pressed into service. A model of the Viking Lander, mounted on a turntable on the tunnel floor, answered many not so obvious questions. For example, the tunnel tests indicated that the extendable lander instrument boom had to be at least 10 feet long to reach beyond the distorting flow field set up by the blast from the lander's retrorockets. Also, the winds could overcool the radioisotope power generators unless they were protected by wind screens. All these seemingly small details had to be checked out, for overconfidence in terrestrial engineering techniques could well spell disaster on a planet as alien as Mars. Wind tunnels had to recreate Mars on Earth.
The Space Shuttle is a reusable launch vehicle that can orbit 65 000 pounds plus a crew of four to seven.
It is launched vertically with the help of two recoverable solid rocket boosters plus an expendable liquid propellant tank. The Shuttle can deorbit itself, reenter, and fly and land much like a normal airplane. Except for orbital maneuvering, the most critical Space Shuttle operations occur in the sensible atmosphere. Wind tunnels played an important part in the design and development scenario. Since the Shuttle is a hybrid spacecraft/aircraft of unusual shape operating under extreme flight conditions, theory alone could not handle all the complex flow conditions encountered from launch to orbit to landing.
The Space Shuttle wind tunnel support involved every major NASA facility as well as help from the Air Force and industry. At least 50 different wind tunnels participated in this national effort. Upwards of 100000 hours (almost 25 years) of wind tunnel time testifies to the scope of the program. Aerodynamic loads had to be defined, as did structural heating, stability and control parameters, flutter/buffet boundaries, propulsion system integration, and the intricate factors controlling rocket and fuel tank separation. The Space Shuttle was one of the biggest challenges to the NASA wind tunnel complex.
As with Viking, the Langley transonic dynamics tunnel was assigned to check out the effects of high wind loads on the Space Shuttle and its ground service tower. The next phase of the mission, as seen through the eyes of NASA wind tunnels, ranged from launch (Mach 0) to Mach 5, the point at which the solid rocket boosters were cut loose. Tunnels at Ames and Langley were used to explore this speed range, while the Lewis 10 x 10-foot supersonic wind tunnel explored the effects of base heating on the launch vehicle. One of the most effective wind tunnels during this phase of the investigation was a small blowdown facility at Marshall Space Flight Center-the 14 x 14-inch Trisonic Wind Tunnel. Too small to...
...be called a major facility, it nonetheless provided 10000 hours of Space Shuttle tests, mainly in the area of launch vehicle design and rocket and fuel tank separation. This small, intermittent tunnel may well have been the largest single contributor of time to the Space Shuttle effort.
The overriding concern during the atmospheric reentry of the Space Shuttle is aerodynamic heating. Engineers relied almost completely on wind tunnel data because theory was deficient at the high angle of attack presented by the vehicle plowing into the Earth's atmosphere. The phenomena of vortex flow and the separation of flow on the lee sides of the fuselage and wings are not well understood. Contrary to expectations, lee side heating at some angles was found to be higher than that for zero angle of attack. The heat transfer studies for the Space Shuttle were carried out in various tunnels at the NASA centers at Ames, Langley, Houston, and the Air Force Arnold Engineering Development Center. As discussed earlier, the Langley 8 foot high temperature structures tunnel was large enough to test complete arrays of full sized tiles used for Shuttle thermal protection.
A hallmark of the Space Shuttle is its maneuverability. Before its aircraft type controls (elevators, rudder, etc.) become useful in the lower atmosphere, the Shuttle depends on reaction controls, that is, small jets that orient the vehicle. In case of an emergency, reaction controls must be able to maneuver the craft as much as 1000 miles cross range. But how do these reaction jets perform as they interact with the thin but high velocity flow of air past the body of the Shuttle? This is one more instance in which wind tunnels were invaluable in detailing what would happen in a hard to calculate situation.
The Space Shuttle approaches its chosen airport and lands without power. During the approach it has an extremely high sink rate of about 75 feet per second. The feasibility of this phase of Shuttle flight, which would have to be repeated routinely many....
....times during future operations, had to be tested exhaustively. To do this, the Shuttle was released from a Boeing 747 and made dead stick landings in 1977. Preceding this seemingly straightforward demonstration were long series of wind tunnel runs that had drastic effects on the final 747 Shuttle configuration. First, a large afterbody fairing had to be added to the Shuttle itself to reduce drag and heavy buffeting on the 747 vertical tail. Six tunnels from NASA and more from industry and universities worked on the fairing problem. In addition, two small vertical fins were found necessary on the 747 to provide more directional stability while it was carrying the Shuttle piggyback. The wind tunnel work paid off, for the unpowered landing tests confirmed the performance predictions for the mated vehicles and the crucial separation event. Without thorough testing with models beforehand, two large, expensive craft and their crews would have been in jeopardy from the undefined aerodynamic interference. This sparing of men and machines through preflight testing is no different in the Space Age than it was for the Wright brothers and their successors. Aerospace vehicles not yet conceived will doubtless "fly" in NASA's wind tunnels before they embark for Alpha Centaur I and beyond.