Testing At The Arnold Engineering Development Center

Recent investigations by the AAF Scientific Advisory Group of German engineering and research facilities have revealed that their long range planning of research facilities was more ambitious and forward-looking than our own . . . .

Dr. Frank Wattendorf, June 1945

The appearance of German jet- and rocket-propelled aircraft and missiles over Europe as World War II drew to an end made it clear that the United States was running a poor second to Germany in flight research and development. Although the Germans came up with too little too late,their apparent technological lead so concerned General Henry H. Arnold about the future of American air power that as soon as the war ended he asked Dr. Theodore von Kármán to head a group to investigate German development facilities. These are the investigations referred to by Dr. Wattendorf, a member of that group, in the quote from a memorandum to Dr. von Kármán which he drafted aboard an aircraft on his way home from Germany after the initial investigation.

Some five months later Dr. von Kármán delivered his report, the famous “Toward New Horizons,” to General Arnold. A significant statement in the report was that the substantial German technical progress was “not the result of any superiority in their personnel or engineering competence, but rather was due to the very substantial support enjoyed by their research institutions in obtaining expensive research equipment, such as large supersonic wind tunnels, many years before such facilities were even planned in this country.”

Recommendations in the report included development of wind tunnels capable of generating airflows “up to three times the speed of sound” with test sections large enough to accommodate models of “reasonably large size,” including jet propulsion units, and an ultrasonic wind tunnel “for exploration of the upper frontier of the supersonic speed range.” It also called for ample facilities “for the study of combustion and other characteristics of propulsion systems at very high altitudes.”

For 1945, when many knowledgeable people remained sincerely convinced that flight beyond the speed of sound was impossible, these were indeed ambitious—even visionary—goals. Nevertheless, plans were drawn up according to the recommendations, and construction of what is now the Arnold Engineering Development Center (AEDC) was started in 1950. The first test unit, a small supersonic wind tunnel, went into operation in 1953. Since then, test equipment has been designed to accelerate the development of rocket, turbojet, and ramjet engines, along with aircraft, missiles, satellites, and space systems. The results of this effort have produced test conditions far beyond those envisioned by Dr. von Kármán.

The Center is located at Tullahoma in south central Tennessee, a site selected because of the availability of the large amounts of electrical power and cooling water required to operate test units. Although the Center is an Air Force installation, it also serves the Army, Navy, and National Aeronautics and Space Administration and their contractors, other federal agencies, and educational institutions involved in aerospace research and development. Capital investment in the Center is currently more than $415,000,000, most of which went into the five major test facilities.

The main space chamber of the Aerospace Environmental Facility contains a test area 65.5 feet high and 34 feet in diameter. Its capabilities include real-time trajectory simulation from sea level to a pressure altitude of 15 miles in 82 seconds. Various series of pumps can further reduce pressure to simulate an altitude of 200 miles. Equipment for thermal balance tests includes an energy source to simulate sunshine and the heat radiated by the earth, a cryogenic system for simulating the cold black of space, and a handling system to support and maneuver the test vehicle. Tests of smaller systems and components are run in three other space simulation chambers, one of which can be pumped to a pressure altitude of 1000 miles.

Within the Propulsion Wind Tunnel Facility, the two main tunnels have test sections 40 feet long and 16 by l6 feet in cross section. The transonic tunnel is capable of simulating speeds of mach 0.20 to 1.6 at pressure altitudes from sea level to 103,000 feet. Supersonic tunnel capability runs from mach 1.5 to 6.0 at pressure altitudes between 45,000 and 180,000 feet. Another transonic tunnel has recently been added with a test section 12.5 feet long and four feet square. Its capability is mach 0.20 to 1.5 at pressure altitudes from sea level to 45,000 feet.

The Propulsion Wind Tunnel Facility also has a 5-megawatt heater for tests of re-entry ablative materials. The flow ranges from mach 1.6 to 2.3 at temperatures up to about 11,000 ºF and total pressures between 10 and 100 atmospheres.

The two large tunnels are used for tests involving missile base heating, aerodynamics, and combined aerodynamic inlet and propulsion systems. The supersonic tunnel is also used for aerothermodynamic tests. The smaller transonic tunnel is used primarily for aircraft stores separation tests. Two model tunnels, originally built to obtain data required in the design of the large tunnels, are still used for aerodynamic tests of small models.

The Von Kármán Gas Dynamics Facility contains three conventional, continuous-flow wind tunnels. One has a flexible nozzle permitting mach variation between 1.5 and 6.0 while the tunnel is running. Pressure altitudes in the 50-inch-square test section range from 20,000 to 160,000 feet. The other two tunnels have 50-inch-diameter test sections. One operates at mach 6 and 8 at pressure altitudes between 98,000 and 180,000 feet; the other at mach 8 and 10 and pressure altitudes between 132,000 and 188,000 feet.

One of the two smaller intermittent tunnels operates between mach 1.5 and 5.0; the other at mach 8 at pressure altitudes between sea level and 160,000 feet, and between 100,000 and 170,000 feet, respectively. Flow is generated by releasing air from a pressure bottle, which can be charged up to 4000 pounds per square inch, through the test section and into a vacuum sphere. Run times of up to five minutes are possible, depending on test requirements.

An intermittent tunnel, driven by electric arc, has two test sections—one 54 inches in diameter, the other 108 inches in diameter farther downstream. Flow is generated by discharging a powerful electric arc in a pressure chamber. The sudden increase in temperature and pressure ruptures a diaphragm in the nozzle throat, from which the flow expands and accelerates through the nozzle into the test sections. Test capability is mach 11 to 22 at pressure altitudes from 80,000 to 250,000 feet.

Two closed ranges, one l00 feet long and the other 1000 feet long, are used to test gun launched, free-flying models at velocities up to 30,000 feet per second at pressure altitudes to 299,000 feet. Finally, there are two impact ranges used to study the effects of meteoroid strikes on spacecraft materials at pressure altitudes up to 325,000 feet.

Four of the test cells in the Rocket Test Facility are 12 feet in diameter and range from 16 to 75 feet long. Test conditions for rocket motors generating up to 20,000 pounds of thrust are mach 0 to 3.0 at pressure altitudes to 170,000 feet. There are three other rocket test cells. One is 20 feet in diameter and 69 feet long for testing engines generating up to 60,000 pounds of thrust at a pressure altitude of 120,000 feet. Another is 18 feet in diameter and 32 feet long for testing engines generating up to 20,000 pounds of thrust at a pressure altitude of 350,000 feet. The third cell 18 feet in diameter and 40 feet high for testing engines generating up to 200,000 pounds of thrust at a pressure altitude of 125,000 feet.

There is also a high-altitude test cell for air-breathing engines. It is 16 feet in diameter and 72 feet long and is used for testing turbojets and ramjets in airflows to mach 3.3 and at pressure altitudes up to 80,000 feet.

There are only two test cells in the Large Rocket Facility, but they are the largest at AEDC. One is for testing liquid-propellant rocket engines rated to 500,000 pounds of thrust, at pressure altitude of 100,000 feet. The liquid-propellant test engine is mounted in the 48-foot-diameter capsule at ground level, and it exhausts into a below-grade flame chamber 100 feet in diameter and 250 feet deep, where the gases are cooled in a water spray before being returned to the atmosphere. The other test cell, for solid-propellant engines rated to 100,000 pounds of thrust, is 16 feet in diameter and 50 feet long and tests at pressure altitude of 120,000 feet.

Over the years, tests in these facilities have produced vast amounts of data, all of it vital to aerospace programs. For example, chuffing, or unwanted bursts of low-level thrust after scheduled burnout in solid-propellant rocket motors, was discovered in a high-altitude simulation cell. The phenomenon which had not occurred in sea-level tests, dictated new staging techniques to prevent possible collision after separation.— In-flight failure of a turbojet engine for operational aircraft led to an intensive test program by AEDC. The engine failed during the tests, and a fix was made in the field, based on the test data and results of examination of the failed parts. —Movies of failure in early Atlas E launches indicated the trouble was in the base region, but the precise location could not be determined. Tests at AEDC showed the exhaust gases were recirculating between the clustered nozzles and impinging on the missile base, which led to overheating and failure. The phenomenon was found to be common to all clustered-nozzle configurations.

While these are some of the most dramatic examples of the work done at Arnold Center, there are more recent ones: scale-model tests in support of the C-5A program led to a reduction in drag by 30 counts, each drag point representing 940 pounds in payload. — In simulated high-altitude tests, AEDC found the cause and recommended a fix for the random decreases in the Agena turbopump speed and performance during flights since 1964. Subsequent tests involved 37 successful firings of durations between 75 and 660 seconds and simulated coast times of up to 15 minutes between some pump starts and stops. — Hypervelocity impact tests on materials that could be used for the walls of a spacecraft show that when a simulated meteoroid penetrates the wall, the hot particle and the wall material fragments produce an extremely hazardous condition inside the spacecraft.

Designs for protecting manned spacecraft from damage by meteoroids are being evaluated by AEDC in two test programs in support of the National Aeronautics and Space Administration’s Apollo Applications Program. One is to determine the protection required to make the empty liquid-hydrogen tank of the Saturn IB booster second stage acceptable for use by astronauts as an orbiting workshop for long periods of time. The second is to determine what materials have suitable penetration resistance for construction of an airlock to be used by astronauts in transferring from one space vehicle to another. The tests are being run in an impact range that employs a special launcher to fire projectiles that simulate meteoroids at speeds up to 20,000 miles an hour through a l00-foot long range tube into a chamber containing test material. Air can be pumped out of the 21-foot-long, 6-foot-diameter chamber to simulate altitudes as high as 130 miles.

The ability of the F-l05 and F-4 fighter-bombers to launch or jettison various payloads of rockets, bombs, or pods under combat conditions is being investigated by AEDC. The first test series matched the F-4 with a missile being developed for use against fortified structures, the second paired the F-105 with an air-to-ground guided missile, and the third combined the F-105 with an airborne pod used to dispense a variety of munitions. The studies were conducted with the wind tunnel’s captive trajectory system composed of an aircraft model support in the floor of the four-foot-square test section and equipment suspended from the ceiling that controls movement of the payload model. This arrangement of equipment was arrived at because of the dual use of the tunnel— for study of trajectories followed by payloads upon separation from the parent aircraft, and for the more normal aerodynamic testing. Suspension of the trajectory system from the ceiling simplifies its removal when the tunnel is being used for aerodynamic studies. The model of the parent aircraft is mounted upside down in the tunnel, and the payload, moving along its delivery trajectory, “falls” toward the ceiling of the test section.

Through refinement in testing techniques, research personnel have been able to reduce the time required to plot a trajectory from 45 minutes in their first efforts to as little as 15 minutes in later tests. The two F-105 series averaged a trajectory every 19 minutes of operating time, which included short delays for corrections in tunnel conditions or changes in computer programs,

Key to the rapid operation is closed-loop computer prediction and control system. A plotting operation is started by bringing the payload into contact with the pylon on which it is carried by the aircraft. The forces acting upon the store model are then measured through instruments in the model support. These measurements are examined by the computer and a calculation is made as to the payload’s position after a given time interval. The computer also makes a prediction of what the loads will be at the new position and activates the control system to place the payload in the next predicted position. A new set of measurements is then made, and if these figures agree with the prediction, the computer proceeds to predict a third position on the trajectory. If the figures disagree, the support system is automatically returned half the distance to the payload’s last position for additional measurements. In making its predictions, the computer must take into consideration the speed and attitude of the parent aircraft, the mass and moments of inertia of the payload, and, in the case of powered missiles, the forces generated by the missile motor. The computer can also simulate mathematically a variety of flight conditions not actually created in the tunnel.

There is no question that the AEDC facilities have proven their worth, and they will continue to be valuable in the years ahead. Test facilities needed today and tomorrow to test future systems will be expensive and complex—even more so than some of the systems that will be tested in them. Test facilities must be identified, programmed, and built far enough in advance to serve future needs.

Brigadier General Gustav E. Lundquist (M.S., North Carolina State College) is Commander, Arnold Engineering Development Center, AFSC, Arnold AFS, Tennessee. He enlisted as a flying cadet in 1940 and was an experimental test pilot 1942-48, testing many of the aircraft used in World War II and later the research aircraft X-1. Other assignments have been with the Atomic Energy Commission, 1952-55; as Professor and Head of the Physics Department, Air Force Academy, 1961-63; Deputy for Engineering and Technology, Electronic Systems Division, AFSC, Hanscom Filed, Massachusetts, 1963-65; Deputy Commander, Rome Air Development Center, Griffiss AFB, New York; and Commander, Systems Engineering Group, Wright-Patterson AFB, Ohio, from 1966 until his current assignment in 1967. General Lundquist is a graduate of Army Command and General Staff School and the National War College.

The conclusions and opinions expressed in this document are those of the author cultivated in the freedom of expression, academic environment of Air University. They do not reflect the official position of the U.S. Government, Department of Defense, the United States Air Force or the Air University.