LIQUID HYDROGEN AS A PROPULSION FUEL,1945-1959

 

Part II : 1950-1957

7. New Initiatives in High-Altitude Aircraft

 

 

Rae's Rex I Proposal

 

[117] There was nothing unusual about the visitor who came to Wright Field on the chilly, overcast day of 24 March 1954. He was one of dozens who were processed through the large visitor's center adjoining the security fence to go to one of the many buildings of the huge Wright Air Development Center. Typical also was the reason for his visit. He was bringing an idea, neatly packaged in a brochure, and seeking a contract. The Center receives hundreds of unsolicited proposals annually and is geared to evaluate them. As with most such proposals, this one was destined to be rejected. What was [118] unusual, however, was the novel solution proposed for a difficult problem, the sensitive nature of the subject, and the timing. The proposal triggered waves of interest within the government, and there followed a series of events involving hydrogen that extend to this day-events that shuttled the proposer to the sidelines and left him bewildered and embittered. His name is Randolph Samuel Rae (1914- ).

 

Randy Rae is a quiet, soft-spoken man with the imagination and creativeness that mark the practical innovator and inventor. He received his engineering education at a Swiss technical school and began his career in electronics and underwater detection systems for locating submarines. He worked for the British Admiralty from 1939 to 1948, serving" in four research and development groups in underwater acoustics, aerodynamics, thermodynamics, and propulsion, rising to the position of a principal scientific officer. He came to the United States in 1948 and worked in the Applied Physics Laboratory of Johns Hopkins University for four years. He started in aerodynamics and developed a supersonic diffuser for ramjet engines and later was placed in charge of the development of a complete guided missile system. More at home with technical details than overall project management, Rae soon was immersed in a difficult missile stability and control problem and devised a solution involving a gyro with a mechanical feedback. The system was put out for bid and a small company, Summers Gyroscope, won the contract. Rae met a kindred soul in dynamic, innovative Thomas Summers.12

 

The missile development that Rae was managing used a ramjet engine for propulsion. A ramjet operates at high altitudes and speeds, but as with all air-breathing engines, it is altitude-limited. The ramjet's altitude-speed limitations set Rae to thinking about other solutions to the problem in April 1953. Was there a way to operate at very high altitudes but at lower speeds, specifically in the subsonic speed range? The rocket was not the answer, for although it operates independent of the atmosphere, it is very inefficient at low speeds. Could he combine the altitudeindependent feature of the rocket engine with a propulsion system efficient at low speeds? The most efficient means for aircraft propulsion at low speeds is the propeller, but it is, of course, altitude-limited. Rae conceived of using a rocket as a gas generator to drive a turbine which, through suitable gearing, would drive a large propeller. Such a propulsion system had no place in the high-speed, high-altitude operating regime of the Navy's work at the Applied Physics Laboratory. Rae became so intrigued with his concept that he left APL/JHU to work full-time on the new propulsion system. He soon learned the handicaps a lone inventor faces. He needed not only monetary support but a corporate identity as well. He turned to his friend, Thomas Summers, who very generously offered both, although propulsion was a far cry from gyroscopes and instruments.

 

Rae joined Summers in September 1953 and began analysis of what he called the Rex engine. The week before Christmas, Summers engaged Homer J. Wood, a mechanical engineering consultant. Wood had left the Garrett Corporation, makers of small gas turbine engines and other aircraft components, in October after ten years service during which he became assistant chief engineer in charge of turbomachinery. Wood assisted in the analysis and design of Rae's new engine.13

 

By March 1954, Rae was ready to present his idea to the government. He visited the headquarters of the Air Force Air Research and Development Command (ARDC), [119] then located in Baltimore, and discussed his idea with Col. Donald Heaton, chief of the aeronautics and propulsion division, and Lt. Col. Langdon F. Ayers, who headed the propulsion branch. The two were engaged in planning research and development to increase the altitude capability of aircraft. and Rae's idea caught their interest. They suggested that he visit Wright Field and discuss the proposal with the specialists there.14 This was what brought Rae to Wright Field on 24 March 1954, with brochures describing the proposal.

 

Rae presented his proposal to a group in the new developments office ofWADC and passed out copies of his brochure. It bore the date of February 1954 and the title, "REX-1, A New Aircraft System" (fig. 27). Rae described it as "a lightly loaded low speed plane having an exceptional L/ D (lift/ drag) characteristic." By lightly, loaded, he meant a low weight per unit area of wing; the aircraft resembled a low-powered glider. The speed of about 800 kilometers per hour would make a military airplane quite vulnerable were it not for the very high operational altitude that Rae proposedover 24 000 meters, which was well above the capability of other aircraft and hopefully beyond the range of antiaircraft weapons. What stirred the interest of the Wright Field audience was the novel engine that Rae proposed: a three-stage turbine engine using liquid hydrogen and liquid oxygen (fig. 28). Ahead of each turbine was a small combustion chamber. All of the hydrogen and part of the oxygen were fed to the first combustion chamber. This partial combustion of the hydrogen produced a gas temperature of about 1100 K, the then practical limit for turbine materials. After....

 


top, front and side sketch of high-altitude airplane

Fig. 27. Sketches of REX-1, a low-speed. high-altitude airplane using liquid hydrogen, proposed to the Air Force by R. S. Rae in Mar. 1954. Gross mass, 32 660 kg; empty mass, 16 330 kg; wingarea, 434 m2; power, 1790 kW (2400 hp); take-off speed, 113 km/hr; cruising speed, 640-800 km/hr at 26000 m altitude; range, 10 000 km. When empty, it could glide an additional 1000 km. From brochure "REX-1. A New Aircraft System," by R. S. Rae, Summers Gyroscope Co., Feb. 1954.


Rex 1 engine component diagram

[120] Fig. 28. Schematic of Rex I engine. Liquid hydrogen and oxygen, gasified by passing through heat exchangers, flow to three small combustion chambers. The hot gases drive three turbines connected to a common shaft. The gases for the second and third turbines are a mixture ofthe exhaust from the previous turbine and combustion gases. After the third turbine, the exhaust gases supply heat for the heat exchangers and then discharge. From the brochure "REX-1, A New Aircraft System," by R. S. Rae, Summers Gyroscope Co., Feb. 1954.

 

....leaving the first turbine, the gases were reheated by adding additional oxygen and burning. The process was repeated for the third turbine. After leaving the thir d turbine, the gases passed through heat exchangers to heat the incoming liquid hydrogen and liquid oxygen.* Rae was attracted to hydrogen by its high specific heat, relatively low combustion temperature, and high energy content. 15

 

The three high-speed turbines, on a single shaft, were geared down to drive a propeller. The conceptual engine was very compact (fig. 29). With both liquid hydrogen and liquid oxygen on board the aircraft, the turbine engine was independent of altitude. Rae proposed to use the turbine engine to drive a large propeller which provided the propulsive thrust by accelerating atmospheric air. The propeller, obviously, depended very much on altitude; the size of the propeller needed for thrust at high altitude later became an issue in evaluating the proposal. After pointing out the military advantages of a high-altitude aircraft, the brochure ended with a low-keyed request: "The Summers Gryoscope Company is desirous of obtaining a Government contract to develop the revolutionary REX-I aircraft system."

 

As is usual in such cases, Rae left that day wondering how his proposal would be received, after the noncommittal attitude of the Wright Field listeners. In fact, his proposal caught the attention and interest of many in the Air Force and several....

 


cross-sectional diagram of a Rex 1 engine

[121] Fig. 29. Drawing of Rex I engine showing the heat exchanger, hydrogen-oxygen combustors, and three turbines. In the foreground is a reduction gear train transmitting power from the high-speed turbines to the engine application which, in the first proposal, was a propeller. From the brochure "REX-1, A New Aircraft System," by R. S. Rae, Summers Gyroscope Co., Feb. 1954.

 

...analyses were started immediately. In response to a request for more information, Rae sent considerable detail about the proposed engine with a cost analysis. The cost was estimated to be on the order of $3 million a year for three years.16

 

In an analysis completed in May, Weldon Worth, R.E. Roy, and R.P. Carmichael examined propulsion aspects of the proposal and concluded: "There are numerous examples of optimism in the proposal but nevertheless, if the development does not bog down under adverse problems that result from impractical features, the small engine size and weight, the reasonably low fuel consumption, the high altitude combustion capability of hydrogen, and the surprising aircraft performance present a stimulating approach to a high attitude performance regime well beyond present aircraft capabilities." They added that there were other possible ways of achieving the same flight regime and discussed adverse technical factors that were based on hydrogen's characteristics and the possibility, from preliminary estimates of the propeller laboratory, that a much larger propeller than proposed might be necessary. Large, insulated lightweight tanks and a circulating gas-heat exchanger system were considered major development problems; these and a larger propeller or fan could substantially increase rize and mass of tankage, engine, and gearing between engine and propeller.17

 

In Rae's opinion, Wright Field's principal objections to his proposal centered on mass estimates and propeller size. He was kidded that his airplane would need a runway with trenches ori each side of the wheels to accommodate propellers 12 meters in diameter. Rae believes he was vindicated later on both these points, but at the time he felt that the brickbats were coming at him thick and fast.18

 


* Rae used an initial pressure of 69.7 atm, and a heat exchanger efficiency of 90%. He quoted an aachievable specific fuel consumption of 1 lb/hp (0.61 kg/kW . hr) and gave data indicating this could be attained with a four-stage turbine system with a turbine efficiency of 50%. He had analyzed both three-and-four-stage turbines; by specific fuel consumption, he apparently meant both hydrogen and oxygen.

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