Hypersonic Wind Tunnel

Schlieren picture of a rocket model in the tunnel which is operating at len times the speed of sound. Shock from. the nose of the model are hard to see because of the low density of the air in the tunnel test section. The Hypersonie Wind Tunnel In this new Caltech-Army Ordnance tunnel men can study the inevitable intercontinental missiles of the future. A NEW "HYPERSONIC" wind tumel which has just been completed in the Guggenheim Laboratory of Aeronautics has already attained a speed of more than ten times the speed of sound. This not only surpasses all other highspeed wind tunnels now in operation; it opens up a whole new, unexplored range of air speeds to the designers of rockets, planes, and projectiles. kor the first time now man has an apparatus that will allow hìm to make tests at speeds well above those of our most advanced rockets and missiles. The top speed for a rocket officially reported to date (E & S. March "49) is 5,200 miles an hour, or a little more than seven times the speed of sound. This record was established by the two-stage rocket-a WAG Cor poral launched from the nose of a German V-2 in fight fired by the Ordnance Department at White Sands, New Mexico, on. February 24, 1949. A guided missile designed with the aid of the new hypersonic tunnel should be able to travel at speeds above 6,500 miles an hour, It could take of from san Francisco and land in Sydney, Australia (7,600 miles) in about an hour, Or it could span the Atlantic, from New York to London, in less than half an hour. The new tunnel was built for the Army Ordnance De-partment, whose ballistie experts foresaw the need for tunnels capable of extremely high speeds in which to study the inevitable intercontinental missiles of the future. It will be operated under Ordnance Department contract and. as the pioneer tunnel in the hypersonic range, it will be used to obtain basic information about the design, performance, and instrumentation of hyper sonic tunnels. Basic experimental data will also be ob tained on shock waves, boundary layers, and the low past models at hypersonic speedsmmalong with other information essential to the design and construction of future rockets and missiles. In all wind tunnels a uniform stream of air is blown through a test chamber in which a scale modal of an airplane, rocket or projectile is mounted. The test see. tion of the hypersonic tunnel is only five inches high by five inches wide, though the entire working section stretches to an overall length of four feet. The air enters this working section through a slot. or throat, only a few thousandths of an inch high (the exact height de. pending on the air speed desired). Thus, at Mach 10* * A "Mach number" expresses the ratío of the speed of a moving body to the speed of When a body moves through the air at & very low speed the disturbance it makes is propagated to all parts of the air so rapidly as body is concerned--that the air is effectively in-compressible. However, as the speed of the bod becomes higher, relative to the speed of sound the rate at which the disturbance is sent through the air may affect the appearance of the pattern around the body. Thus the important criterion of speed is the ratio of the speed of the body to the speed of sound (750 mph at sea level and normal atmospheric temperature): This ratio is known as the "Mach number" after the Austrian scientist who made some fun- 19th century. When the Mach number is less than I, the flow is said to be subsonic; greater than 1, supersonic; in the region of 1, transonie. Aero. dynamicists nos say little about actual speed in miles per hour, hut talk only of Mach number. The limnel requires intricate instrumentation and ihree nen to operale and record date when it is running. 8 NOVEMBER 1949 NO PAYIL Of LIGHY FROM LIGHT SUURCE Air enters tunnel through nozzle throat, goes through region of acceleration into test section, then to diffuser section: which means ten times the speed of sound, air at tremendous pressure blasts through the tiny slot. As the air expands into the test section, the temperature drops to about 430 degrees below zero Fahrenheit. Stationary air at this temperature would turn to liquid, but here the air apparently moves so fast through the cold zone that it hasn't time to freeze. Along with the drop in temperature the pressure in the test section drops to about one millimeter of mercury. or 1/1,000 ol normal atmospheric pressure, Consequent ly, the an is so thin-like that of the atmosphere dozens of miles above the earth that it is extremely difficole to observe its flow patterns. A schlieren optical system is used to photograple these patterns, made by the fast-moving air as it speeds past the models in the test sec tion. Shock waves in the air stream act as miniature lenses to disturb a parallel light beam, and the disturbance is recorded by a specially designed camera. camera can photograph an eight-inch circular area at a time, and the whole system can be moved along the tunnel on an overbead track to get a complete picture of what happens ahead of, and behind, the model. The tunnel is housed in a building specially designed for it in the Guggenheim Laboratory. The building has two basements, one for the tunnel under it for the enormous power plant which is needed to operate the tunnel. It takes no less than 15 pressors to supply air for the tunnel. And this aur flows first into a large supply reservoir lank, 12 feet long " feet in diameter, and weighing approximately sis tons. From the standpoint of the speed ranges it explores the hypersonic tunnel is "the fastest in the world." It has already been operated at higher than Mach 10. fastest speed ever reported in any tunnel before is approximately Mach 7, and that could be mintained for only a few seconds in an intermittent-type tunnel. The hypersonic tunnel operates continuously and air veloci ties of Mach 10 can be maintained for any length time. In raising the Mach number to 10, the new tunnel makes it possible for the aerodynamicists to move on to the design of rockets, planes and missiles with speeds far greater than any yet attempted. Test section of lunnel, where models are placed, measures 5% * 5"- bur it lokes 15 compressors lo force air in. Robert M. James, Project Engineer, and Dr. Henry T Negumatsu, Director of the tunnel, check test chamber. NOVEMBER 1949m9