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SUNTAN - Hydrogen Engine

United Aircraft Corporation became involved in liquid hydrogen as a propulsion fuel in 1955 on the initiative of the power plant laboratory at Wright Field. Acting on a directive from its headquarters, the laboratory initiated a procurement request in January 1955 to investigate hydrogen as a fuel in turbojet engines. In February, invitations to bid were sent to United Aircraft and three other major engine manufacturers. Proposals were submitted in March; United Aircraft won the competition and was awarded a contract on 15 June.

The contract was not with the corporation's Pratt & Whitney division but with the research department headed by John Lee. The work was exploratory and included cycle analyses, aircraft weight analyses, and some experiments. One of the men involved was Wesley A. Kuhrt, to whom hydrogen was no stranger. When 13 years old, he made hydrogen in his cellar laboratory by adding zinc to hydrochloric acid. Suddenly there was an explosion; glass fragments were imbedded in his chest, but he escaped serious injury. The incident neither cooled his enthusiasm for science nor created a fear of hydrogen.

The Pratt & Whitney division had followed Air Force and NACA interest in hydrogen during 1955 and was also aware of Rae's Rex engines. The Suntan project began for the division with a call from Appold in January 1956; by February, division officials began to believe they would win the contract for the engine. On 17 February, Perry Pratt, chief engineer, summarized what he had learned about hydrogen in jet engines. He cited six companies with experience in pumping hydrogen and described an engine that was somewhat similar to the Rex engine. Pratt had examined the hydrogen supply problem and concluded that conversion of liquid hydrogen to its para form at time of liquefaction was feasible, and this made hydrogen storage, and shipment by truck, rail, or air practical. This optimistic report was written on Friday. The following Monday, Pratt was in California visiting various people knowledgeable about hydrogen, including Kelly Johnson at Lockheed." By this time, it was highly probable that Johnson and Pratt, collaborators in adapting the J-57 engine for the U-2, were aware that they would again be working together on the Suntan project.

Altitude30500 m
SpeedM 2.5
Thrust20000 N (4500 lb)
specific fuel consumption0.076 kg/N . hr (0.75 lb/lb thrust . hr)
Nacelle weight2722-3175 kg
Engine diameter155 cm
William Sens, a Pratt & Whitney engineer, accompanied Pratt on the California trip and while there learned about Rex engines. This excited him, for six weeks earlier he had conceived an idea about hydrogen-fueled engines following a conversation with John Chamberlain, a combustion expert at United Aircraft's research laboratory. Chamberlain had pointed out that heated hydrogen was capable of a large amount of work in a thermodynamic cycle. Sens began thinking of using heated hydrogen to drive a turbine which would power an engine fan or compressor. After passing through the turbine, the hydrogen would be injected and burned in the airstream of the engine. Immediately after returning from California, Sens sent a proposal to Pratt for developing a hydrogen engine. These specifications indicate that Sens was also aware of Johnson's propulsion requirements or those of the CL325-1 prepared by Johnson for Garrett.

Sens described his proposed engine as having a dual cycle, with the basic one resembling a supercharged ramjet: "Air is ... compressed by a low pressure ratio compressor, heated by combustion of hydrogen vapor and discharged through a . . . nozzle. In addition, heat is extracted from the air stream by means of a heat exchanger after part of the combustion of the hydrogen has taken place. This heat is used to vaporize and heat the hydrogen being used in the combustion process. In the secondary cycle the liquid hydrogen fuel is compressed to a high pressure by means of a multi-stage centrifugal type pump. The high pressure hydrogen is then vaporized and heated to a relatively high temperature in the heat exchanger located in the high temperature air stream. The hydrogen is then expanded through a multi-stage axial-flow turbine to a pressure only slightly above that of the fan discharge air. The turbine power output is used to drive the compressor used in the air cycle. Because of the large speed difference between the hydrogen turbine and the air compressor, it is necessary to use a single speed reduction gear between the two components. "

Sens discussed anticipated problems and the applicability of existing Pratt& Whitney experience to solve them. Sens was not the only one in the corporation considering possible hydrogen engines. Wesley Kuhrt in the research department had been working on them for some time, and on 1 March 1956, he conceived three engine systems for which he later filed and was granted patents.21 One was a turbofan engine. Air entering the inlet is compressed by the fan and flows around the centerbody to the aft section, where gaseous hydrogen is injected and burns stoichiometrically. The hot gases expand through the exhaust nozzle to produce thrust. The source of Power for the air fan is a turbine driven by heated hydrogen prior to combustion. Liquid hydrogen flows to the heat exchanger around the exhaust nozzle where it gasifies and is raised to a reasonably high temperature. From the heat exchanger the hot hydrogen drives a multistage turbine which is connected to the air fan through a gear box. After leaving the turbine, the hydrogen is injected in the engine air stream and burned. Kuhrt's engine is similar to Rae's Rex III in that both employ a heat exchanger to heat the hydrogen to drive a turbine, but Kuhrt's concept is much simpler than the Rex III.

For Kuhrt, the beginning of the Suntan work at United Aircraft was a call in early 1956 to come to the office of Wright Parkins. Present were Perry Pratt, Col. Norman Appold, and others. Appold stressed the need to get started quickly on a project to use hydrogen in aircraft engines.

For Richard J. Coar, a rising, brilliant young mechanical engineer hard at work on developing the J-75 turbojet, the Suntan program also began early in 1956 when he was "yanked off his project" and assigned to the hydrogen engine work. His first task was engine analysis and learning all he could about hydrogen. He visited the Bureau of Mines, the Arthur D. Little Company, and a conference at the Bureau of Standards Cryogenic Laboratory at Boulder. Inspection of the liquefaction plant convinced him that production of liquid hydrogen would be a major obstacle to military use of hydrogen. The plant was small and the laboratory techniques required highly skilled personnel. In April, Coar went to Baltimore to negotiate a contract with the Air Force. It was on one page and technical negotiations were completed in a day-a marked contrast to the long and agonizing process that Rae, Garrett, and the Air Force had gone through earlier.

Pratt & Whitney's initial approach to the problem was to analyse the various hydrogen engines that had been proposed, select one, and develop it so as to take the greatest advantage of hydrogen's unique properties. This remained their mainline approach but in a short while, they realized that modification of an existing engine would provide a quicker, though less efficient, engine for early flight experience. They proposed to modify a J-57 for this purpose, the Air Force agreed, and the contract was amended.

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