THE new 10- by 14-inch hypersonic wind tunnel was put into operation in 1950. It was described by Al Eggers and George Nothwang in TN 3095, and some of the related work on air condensation was reported in TN 2690 by Fred Hansen and George Nothwang.
The unique supersonic free-flight wind tunnel was also put into useful operation in 1950. This tunnel was described by Al Seiff in TR 1222 (ref. B-17) and a description of its unusual instrumentation is presented in RM A52A18 by Messrs. Briggs, Kerwin, and Schmidt of the Research Instrumentation Division.
During the period 1949-1952, the Ames 1- by 3-Foot Tunnel Section put forward numerous proposals for improving the performance-increasing the ranges of Mach and Reynolds numbers-of its No. 1, continuous-flow tunnel. For one reason or another, these proposals were rejected and it was not until 1953 that the latest version was presented to Congress as a part of the NACA fiscal year 1954 appropriations request. The principal change specified in the proposal was a doubling of the power used to drive the tunnel. The power increase was to be obtained by applying forced-draft cooling to the 2500-horsepower motors driving each of the four original compressors and by adding a fifth compressor driven by a 9000-horsepower motor. A further improvement, in the form of downstream injection, was also contemplated. Diverting some of the compressor output to the supersonic diffuser exit would make it possible to reduce the inlet pressure required for starting and thus, also, the starting shock loads on the model. With these modifications, it was expected that the Mach number of the tunnel could be increased to 4.0 and the Reynolds number to about 1.5 million per foot-well Over the RN at which an undesirable phenomenon known as "laminar separation" occurs.
Plans also were made during this period to replace the unsatisfactory electrically operated flexible throats with which the 1- by 3-foot tunnels were  then equipped. Designed for this purpose, by Paul Radach, was a new type of throat having walls flexed and positioned by a set of positive, fast-acting hydraulic jacks. The new throat, with which each of the tunnels was later to be equipped, would allow Mach-number settings to be made in seconds where the old throats had required hours for the same changes.
Ames' work in the fields of low-density aerodynamics and heat transfer had begun on a modest scale. The first facilities were small and, although they were performing usefully, their deficiencies were all too apparent. The low-density tunnel when in operation became nearly filled with boundary layer; moreover, it was designed for a density that was somewhat below the practical range of interest. The 6- by 6-inch heat-transfer tunnel was also too small and too slow, and it lacked any means for adding heat to the airstream. Yet while these deficiencies were being noted, the research fields in which such facilities could potentially contribute were becoming increasingly important. In particular, the problems of the ballistic missile were looming larger; and it was realized that aeronautical scientists, having recently conquered the mighty sonic barrier, were now confronted with an equally formidable obstacle-the heat barrier. New facilities to investigate problems of heat transfer and low-density aerodynamics were thus felt to be necessary.
Proposals for new low-density and heat-transfer wind tunnels were included in the fiscal year budgets for 1950 and 1951 and were approved in 1951. Construction began in 1952. The estimated cost of the two facilities was about $1.3 million. Covered in this estimate were an 8-inch-diameter low-density tunnel, a 10- by 10-inch heat-transfer tunnel, and a new building to house the two facilities. The building would be adjacent to the 12-foot-tunnel auxiliaries building where supplementary air power could conveniently be drawn from the 12-foot compressors. It was amazing how many parasites the 12-foot tunnel had attracted.
The 8-inch low-density tunnel would be of the nonreturn type, powered and evacuated by steam ejection pumps. It would be designed to operate at a density corresponding to an altitude range of from 20 to 40 miles. By removal of boundary layer through porous nozzle walls, a 6-inch-diameter usable jet was believed obtainable at a Mach number of 3.0 and a somewhat smaller diameter jet at the top Mach number of 5.0. The Reynolds number, of course, would be very low-under 10,000. The tunnel was described in TN 4142 authored by Marc Creager.
The 10- by 10-inch heat-transfer tunnel, as planned, would be a continuous-flow tunnel of the return type and would be equipped with a special link-type variable nozzle allowing operation at Mach numbers from 2 to 5. The tunnel would be operated on air from its own compressor augmented by air from the compressors of the 12-foot tunnel. With this power source, stagnation pressures up to 10 atmospheres would be attainable as would also....
...Reynolds numbers of 24 million or more. To extend the possible range of heat-transfer measurements in the tunnel, an electric heater capable of heating the inlet air to 1000° F was provided. Never before at Ames had a wind tunnel been designed to operate at so high an air temperature.
The new low-density and heat-transfer tunnels were still under construction as 1953 ended; but, throughout the 1950-1953 period, research was proceeding in the old facilities, particularly in the old heat-transfer tunnel.
Since the end of the war, aerodynamicists and aircraft designers had been most apprehensive about the problems to be encountered in the transonic range of flight. The dancing shock waves, separated flows, and violent disturbances which seemed inherent in this flow regime were alarming. Fear of the unknown was a psychological aspect of the problem; transonic aerodynamics was not well understood because, for one reason, the transonic range represented a blind spot in the test spectrum of existing wind tunnels. Another reason was that the confused pattern of transonic flow did not lend itself to theoretical treatment. However, in 1947-1948, the work of John Stack and his colleagues at Langley opened the way to the design of transonic tunnels, and soon thereafter light was shed on the little-known phenomena of the transonic range. Theory came a little later in this case, following, rather than leading, experiment.
 Transonic tunnels suddenly became de rigueur for all aeronautical research agencies and the effort and expense of acquiring them may perhaps have exceeded the point of diminishing returns. The transonic range was clearly not a place for an airplane to linger, and it later became common practice to remain just below this range or to push through it so fast that the disturbing effects were of minor consequence. But these practices could not always be followed, and in any case there were many useful things to be done with transonic tunnels. Indeed, the payoff for the construction of the first transonic tunnel at Langley was immediate and huge. In 1951 the unique capabilities of this tunnel allowed Richard Whitcomb to confirm, and thus bring into early use, an important design principle. This principle, dubbed "Area Rule," made possible large reductions in the "brick wall" of transonic drag that heretofore had prevented most airplanes from reaching supersonic speeds. The Area Rule principle had existed in theory prior to Whitcomb's work but, as is frequently the case, the significance of the theory remained largely unrecognized until revealed by experiment.1
At Ames the first tunnel modified for transonic operation was the 1 by-3 1/2, but the modification of the 1-by-3 1/2 was only a small part of the transonic-wind-tunnel program undertaken at the Laboratory. Proposals to convert the 16-foot tunnel to a 14- by 14-foot transonic tunnel were put forward by the Laboratory in 1947, 1948, and 1949. Also, in 1950, plans were laid to build an 11- by 11-foot transonic tunnel as a part of the Ames Unitary Plan facility. As a pilot model for the 11- by 11-foot tunnel, a 2- by 2-foot tunnel was built in 1951. Finally, in 1953, plans were made to modify the 6- by 6-foot supersonic tunnel in a way that would allow it to cover the transonic range. The total amount of time and money involved in these plans was rather large.
There was quite a little resistance both in Washington and at Ames to the proposal for converting the 16-foot tunnel. The $2 million originally invested in the tunnel had been amply repaid by its contributions, but the conversion was to cost a whopping $9 million. An important justification offered for the conversion was that a large transonic tunnel was needed to fill the gap between the test ranges of the 16-foot and the 6- by 6-foot tunnels. The justification seemed valid at the time, but was later weakened when the 6-by-6 itself was converted for transonic operation. And since power was already available in a supersonic tunnel, there was a practical question as to whether it was not more feasible to convert a supersonic, rather than a subsonic, tunnel to transonic operation.
Some people at Ames felt the conversion of the 16 foot represented a very poor investment. "A damned waste of money," said Carl Bioletti, who argued strongly against the undertaking. The transonic problem, he felt, would be solved by simpler means long before such grandiose facilities as  the 14- by 14-foot tunnel were completed. The arguments pro and con seemed to hinge on whether transonic problems were of a transient or a long-range nature. And there was the further consideration that it was always easier to get money from Congress for a modification than for a new tunnel. In congressional eyes, a new facility meant additional staff, a larger activity, and a permanent increase in annual operating costs. The modification of the 16-foot tunnel could perhaps thus be regarded as a relatively easy, if not a wholly efficient, method of augmenting the Ames test facilities.
Those who favored the 16-foot conversion prevailed. Congress approved the expenditure and design work got under way. The modifications to the 16 foot would be extensive-essentially a new tunnel. It would still operate at one atmosphere stagnation pressure but would have both a transonic throat and a flexible nozzle and would be powered by a new axial-flow compressor driven by motors totaling 110,000 horsepower. Construction began in 1951-1952 and was still under way at the end of 1953.
Actually Ames did not launch into the modification of the 16 foot or the construction of the 11- by 11-foot transonic leg of the Unitary Plan facility without a great deal of preparatory work at small scale. This work led to a variation of the transonic throat design originally developed and first used at Langley. Both laboratories recognized the purposes of a transonic throat: (1) to eliminate choking; (2) to allow speed changes to progress smoothly from subsonic to supersonic while maintaining a uniform velocity distribution in the test section; and (3) to absorb shock waves by the tunnel walls thus preventing reflection of the waves back onto the test model.
Langley felt that these objectives could be accomplished reasonably well, and most simply and quickly, by a fixed-geometry ventilated throat. Harvey Allen and others at Ames felt that a useful improvement could be made to the Langley version by adding a simple flexible nozzle just ahead of the fixed ventilated throat. The flexible nozzle would relieve the ventilated throat of the task of providing flow uniformity, thus allowing more freedom in the design of the throat as a wave trap to prevent reflected shock waves. This improvement would add complexity and cost but it was thought would be worthwhile. Moreover, the simple single-jack nozzle which Allen designed for the purpose was not required to be very precise in its operation, for any wave disturbance that it produced in the flow would be removed by the wave-trap action of the ventilated walls. Allen's theories of transonic throat design were first tried out in an experimental 5- by 5-inch nozzle which the Laboratory built and also in the 1- by 3 1/2-foot tunnel. They were then incorporated in a new 2- by 2-foot transonic tunnel which, during 1951, was built inside the loop of the 40-by-80. Although the 2-by-2 was considered a pilot model for the 11-by-11, it also served as a very useful research facility and was the first wind tunnel at Ames that was built from the start with transonic capabilities.
The 2- by 2-foot transonic tunnel was designed to operate through ranges of Mach number from 0.60 to 1.4, of Reynolds number from 1 to 8.7 million per foot of model length, and of static pressures from 0.16 to 2.33 atmospheres. It was quite a versatile facility and was the first Ames tunnel to use the new color schlieren technique. Color added definition to schlieren pictures and compensated for errors arising from flexibilities or other inaccuracies in the mountings of the optical components of the schlieren system It was thus particularly valuable in transonic wind tunnels where windows were broken into segments by the ventilating slots. Joseph Spiegel and  Frank Lawrence, who contributed much to the design and use of the 2-by-2, described the tunnel in RM A55I21.
During this period of transonic-tunnel excitement, Ames put forward a proposal to boost the power of the 12-foot tunnel by 50,000 horsepower to a total of 60,000. The idea was not, at least not immediately, to make the tunnel transonic but rather to increase the operating pressures, thus Reynolds numbers, at high subsonic speeds. As the proposed power augmentation involved many costly modifications to the tunnel, the proposal was rejected by higher authorities. Thus the operating characteristics of the 12-foot tunnel remained unchanged except that, near the end of the period, the maximum operating pressure was, for reasons of safety, reduced to 5 atmospheres from its original value of 6.
The amount ($75 million) finally appropriated for NACA Unitary Plan facilities was considerably less than had been authorized and expected. The building program thus had to be greatly curtailed and every effort made to economize. The funds appropriated would provide one major facility at each laboratory. Of the Unitary Plan facilities, an 8-foot, Mach 0.7 to 3.5 tunnel planned for Ames had been assigned highest priority and its construction was now scheduled to proceed, although at a reduced budget of a little over $27 million. A slightly lower priority had been given to a 4- by 4-foot Mach 1.5 to 5.0 missile-development tunnel at Langley, and to a large supersonic propulsion tunnel at Lewis.2 The construction of all, however, was to proceed with minimum delay.
Design work on the Ames Unitary Plan wind tunnel began late in 1949 and it was soon discovered that the 8-foot dimension originally adopted for the tunnel was impractical. Instead, by clever design, it was found possible within the limited budget to have three tunnels. To achieve this worthy objective, the three tunnels were to be in the form of a Siamese triplet, if the term will be allowed, in that they were to have certain important and expensive elements in common. This arrangement provided for an 11- by 11-foot transonic tunnel (Mach 0.7 to 1.4), a 9- by 7-foot supersonic tunnel (Mach 1.55 to 2.5), and an 8- by 7-foot supersonic tunnel (Mach 2.45 to 3.45), all capable of operating at stagnation pressures ranging from 0.1 to 2.0 atmospheres.
The major common element of the tunnel complex was an enormous electric powerplant consisting of four intercoupled motors capable of generating a total of 240,000 horsepower continuously. With but one powerplant, only one tunnel, or leg, could operate at a time; however, this limita-....
....-tion was felt to offer no serious handicap, as supersonic tunnels seldom operated more than a third of the time. Other common elements of the two supersonic legs were a portion of tunnel tube and an 11-stage axial-flow compressor. The compressor, made of steel the better to resist heat, was a massive machine. Its 22-foot-diameter rotor contained 1,122 solid-chrome-steel blades and weighed 445 tons. The momentum of the rotor was, indeed, so great that the rotor, while operating at low air density, would have taken up to 2 hours to come to rest after power was shut off had not electrodynamic braking been provided. The difficult problem of matching the output of tile compressor to the widely varying air volume and pressure requirements of the two tunnels was solved by providing a variable-speed drive for the motors as well as means for bypassing air around the 8- by 7-foot nozzle.
The use of a common portion of tunnel tube, for the two supersonic legs, was necessitated by the fact that the common compressor was contained in this portion of the tube. So that the common portion of the tube might be used in either of the supersonic tunnels, it was necessary to install a flow diversion valve at each end of that portion of the tube. These valves were of unique design, 20 feet or more in diameter, and weighed over 250 tons-the largest airtight valves of this type ever built. Designed by Paul Radach  of the Ames staff, the valves were but another unusual feature of a most unusual wind-tunnel facility.
The 11- by 11-foot transonic tunnel had its own, separate, air circuit and its own compressor-a huge three-stage unit with aluminum blades. But it used the same powerplant as the other tunnels. The powerplant was equipped with clutches by means of which its four motors could be connected to either the 11-stage or the 3-stage compressor.
The transonic throat of the 11- by 11-foot tunnel incorporated a single-jack flexible nozzle and a slotted section-the same system that had been developed for the 2- by 2-foot tunnel. The 8- by 7-foot tunnel was equipped with a symmetrical, flexible-wall throat, the sidewalls of which were positioned by a system of jacks operated by hydraulic motors. The 9- by 7-foot tunnel, on the other hand, had an asymmetric sliding-block-type nozzle. It also had a flexible upper plate by means of which any minor flow corrections that later seemed necessary could be made.
The shell of the Ames Unitary Plan facility was constructed of welded steel plates from 1 to 2 1/2 inches thick. Its supporting structure was designed with unusual care to allow for thermal expansion and to resist 0.2-g seismic side loads. The shell was designed as a pressure vessel to operate at static pressures ranging from 0.1 to 2.0 atmospheres; the critical loads were found....
 ....to occur in the low-pressure range of operation. Hydraulic tests to confirm the integrity of the shell were considered unnecessary.
Construction of the Ames Unitary Plan facility began in 1950-1951 and was still under way at the end of 1953. The facility was certainly a landmark in the development of more or less conventional supersonic wind tunnels. In it, such wind tunnels had probably reached the ultimate in size, complexity, and refinement. The facility was the largest, most complex, supersonic wind-tunnel system ever built by NACA and certainly the most costly ever erected at the Ames laboratory. Moreover, it appeared that, in view of trends developing in aeronautical research, the Ames facility represented perhaps the end of the line in large, continuous-flow wind-tunnel construction. The extreme conditions of flight now becoming of interest to aeronautical engineers would doubtlessly require smaller, specialized facilities having short operating periods.
The Ames Unitary Plan facility spoke well of the design skill of NACA engineers. Among those of the Ames staff who were most responsible for its design and construction were Jack Parsons, Ralph Huntsberger, Gerald McCormack, Lloyd Jones, Adrien Anderson, Paul Radach, Edward Wasson, Joseph Spiegel, and Norman Martin. Others, such as Loren Bright, Ed Perkins, and Alun Jones, made important contributions in getting the new facility into operation. A paper describing the Ames Unitary Plan wind tunnel was prepared by Lloyd Jones and Ralph Huntsberger, and the Ames philosophy of large wind-tunnel design is given in a paper (ref. B-18) entitled "The Design of Large High-Speed Wind Tunnels," by Ralph F. Huntsberger and John F. Parsons.
It now appeared that for simulating the flight of ballistic missiles, which travel at very high speeds in the upper atmosphere, the usefulness of continuous-flow tunnels was rapidly diminishing. The thermal conditions encountered by such missiles were so extreme that neither the model nor the wind tunnel, nor for that matter the missile itself, could withstand them for any length of time.
In view of the limitations of continuous-flow tunnels, Ames engineers turned their attention to the design and development of special research devices in which the extreme conditions of missile flight could be produced for brief periods of time. Such facilities would, of course, greatly increase the importance and cost of individual data points; and the development of instrumentation to recover data on so fleeting a basis would certainly tax the ingenuity of the Laboratory's designers. Moreover, the transient flow conditions would greatly complicate the analysis of such data as were obtained The disadvantages of transient-flow test facilities were all too apparent, but  how else were the extreme conditions of flight to be investigated in an earthbound laboratory?
At Ames the supersonic free-flight tunnel represented the first important step taken by the Laboratory in the development of transient-flow test devices The next step was taken with the arrival, in January 1952, of Dr. Alex Charters. Alex came from the Aberdeen Proving Ground where he had acquired considerable experience with ballistic ranges, in which bullets fired down the range are observed in flight by means of special optical instrumentation Alex was assigned to the SSFF tunnel, reporting to Alvin Seiff, who was then in charge of that facility. As his first assignment, Alex was encouraged by Seiff to undertake the design and development of a special model-launching gun similar to, but hopefully better than, one that had been developed for range work by the New Mexico School of Mines. When developed, the Ames gun, it was expected, would be used to shoot simple models down an instrumented test range. Such a test arrangement had, indeed, already been demonstrated in the SSFF tunnel by launchings made with zero tunnel airspeed.
The fastest ordinary rifles had muzzle velocities of about 4000 feet per second, and those that had been modified for use in the SSFF achieved velocities of perhaps 6000 feet per second. What Charters was attempting to do, however, was to develop a gun that would shoot simple bulletlike models at speeds of 10,000 feet per second or more. The gun would derive energy from exploding powder and use rifle barrels for accelerating tubes, but would otherwise bear little resemblance to a conventional rifle. In the new gun, exploding powder would drive a metal piston down a barrel (driving tube) compressing, just ahead of it, a charge of helium gas. The hot compressed gas would finally burst through a sealing diaphragm into a second barrel (launching tube) in which the test model would be located. The compressed helium gas would then propel the model out of the second barrel at terrific speed. The reaction would be taken by ejecting a second piston out of the driving tube in the opposite direction. The reaction piston was to be caught in a special "catcher." Since the gun in effect shot bullets in two, opposite, directions, it was named Janus and the range in which it was first used was called the Janus range. Inasmuch as the driving gas in the gun was helium, used because its small mass absorbed little accelerating energy, the gun was known by the generic name of "light-gas gun." It was on the development of such a device that Charters and his colleagues were working as 1953 ended.
Meanwhile A1 Eggers had conceived the idea that a gun something like the one being developed by Alex Charters could be used to power a supersonic wind tunnel. In this case, the gas compressed by the piston-it did not have to be helium-would exit through a supersonic nozzle in which a test model could be rigidly mounted. Such a device, he felt, would generate a  high-temperature, high-Mach number gas flow for a sufficient length of time to allow data to be taken. A rather small-scale version of the gun tunnel, made with a barrel of 20-mm bore, was built in 1952 and actually used for certain heat-transfer measurements, which were reported by Eggers and Charters at the Conference on the Aerodynamics of High Speed Aircraft held at Ames in 1953. The gun tunnel produced flows at speeds up to 8200 feet per second for periods of about 1 second; but, as a result of flow unsteadiness arising from oscillations of the piston, it was not a very satisfactory research instrument. Although the gun tunnel, as built, was little used, it nevertheless represented a pioneering effort in the design of transient-flow wind tunnels and led to more useful developments.
The gun tunnel, it now appeared, might possibly be improved if the shock front of the exploding gas, rather than a metal body, were used as the driving means. This principle had, of course, been employed in shock tubes used elsewhere. Jack Stalder, and later Warren Ahtye, experimented with small shock tubes at the Laboratory, but Ames management considered it impracticable to compete with certain other groups, such as AVCO, which were further advanced in the design and use of such equipment. On the other hand, a shock-driven tunnel, producing a flow of relatively long duration, was regarded by Ames engineers as being a feasible development project as was also a shock-driven light-gas gun, for the launching of test models. In one case, the device would shoot air, or any chosen gas, past a fixed model; and, in the other, it would shoot a model into a stationary body of gas, as in a range. By applying the SSFF tunnel counterflow principle, two such devices could be put in opposition, one firing a model into an oncoming gas stream produced by the second. The relative velocities thus developed would be tremendous. These certainly were good ideas to work on, but associated development problems were of shocking complexity.
In wind-tunnel simulations, air had commonly, indeed almost universally, been used as the working fluid. Air seemed a logical choice for the simulation of flight in the earth's atmosphere but for the simulation of flight at very high speeds, where compressibility became an important factor, a case could be made for the use of some other gas mixture or gaseous element. For example, the troublesome liquefaction problem could be solved through the use of helium; and other benefits, such as the reduction of tunnel size, power, and cost, could conceivably be achieved through the use of various gas mixtures. During the period 1950-1953, the subject of the use of gas mixtures in wind tunnels was rather thoroughly and competently explored by Dean Chapman. The results of Chapman's work in this field appeared in TR 1259.
Helium was first used at Ames by Jack Stalder in the low-density tunnel and, for several years in succession, Ames had proposed the construction of a 1- by 1-foot helium tunnel for the simulation of very high Mach number flow. Each year and again in 1950, the proposal was turned down by Headquarters. Even at Ames there was some question about the use of helium for, although it did overcome the liquefaction problem, it nevertheless could not take the place of air in simulations of certain important conditions of flight.
The first electronic computing machine used at Ames was a Reeves Analog Computer (REAC) acquired in 1949. It was used by the Flight Research Section for control simulation work and, as this kind of work expanded, so also did the Laboratory's supply of analog-computer elements.
Ames in 1950 gave its first serious consideration to the use of electronic digital computers and the next year leased a Card Program Calculator (CPC) from IBM for wind-tunnel data reduction. Shortly, a second CPC was procured and a portion of the time of this machine was given over to the theoretical studies. However, in the 1951-1952 period, some of the more....
 ....extensive theoretical calculations at the Laboratory were performed under contract by the U.S. Bureau of Standards Computing Center in Los Angeles.
In 1952 the computing-machine work at Ames had reached a level such as to justify the formation of an Electronic Computing Machine Section in the Theoretical and Applied Research Division. This Section, which was to deal only with digital computers, was established in May of that year with Dr. William Mersman as its head. Harold Harrison and Marcelline Chartz joined the computing section at that time. Harrison, who shortly became Assistant Section Head, concerned himself with the application of computers to the working up of wind-tunnel data. Mersman, on the other hand, devoted much of his time to the use of computers in theoretical research.
1 The concept of Area Rule is further discussed in the next chapter.
2 Priority was established by representatives of NACA, military, industry, and universities. Stated in minutes of meeting of NACA Panel on Research Facilities in Washington, D.C., Dec. 16, 1949.