SUNTAN - Shamrock
In the spring of 1956, Suntan engine activities at Pratt & Whitney were in full swing. Coar selected Richard C. Mulready, a bright young engineer, as his assistant. Liquid hydrogen handling tests began immediately with hydrogen obtained from the Cambridge Corporation in dewars. Associated with this activity were preparations for component and engine testing, including obtaining a supply of liquid hydrogen. With the help of Capt. Jay Brill, a hydrogen liquefier of 227-kilogram-per-day capacity wits purchased from Herrick L. Johnston and installed in the engine test area behind the  East Hartford plant. The test area was called the "Klondike" because of the cold Connecticut winters and well-ventilated test stands that were designed to prevent the accumulation of hydrogen. Coarand Mulready also began to round up all the gaseous hydrogen tube trailers they could find to supply the liquefier.
The second activity, code named "Shamrock," began in April to convert a J-57 to burn hydrogen. The design was completed in May; thereafter, component testing and engine modifications ran concurrently. The hydrogen liquefier was ready in September, engine testing began in October. The test engineers were agreeably surprised by the ease of engine operation. They ran it at full power and throttled back so far that the air fan was revolving so slowly the individual blades could be counted. Under this latter condition, the throttle could be opened and the engine would quickly and smoothly accelerate to full power. They found that the temperature distribution was good and there were no major problems. Such satisfactory results came only after careful design studies, modifications, and component testing. Among these precursory activities were the development of a heat exchanger using air bled from the compressor to gasify the hydrogen, modifications to the J-57 electronic fuel control system, and development of an oil-lubricated, liquid-hydrogen pump.
By the fall of 1957, the J-57 experiments demonstrated beyond question that a conventional turbojet could be readily adapted to use hydrogen. Such engines could have been used to meet Kelly Johnson's tight airplane development schedule, but modifying an existing turbojet could not optimize the advantages of hydrogen. The Pratt & Whitney engineers had realized this early in their studies, as had their counterparts in the Rex division of Garrett and the Air Force. The mainline Pratt & Whitney effort from the start focused on a design of a special hydrogen engine, and its design started in April 1956 with the first contract.
By mid-August 1956, Pratt & Whitney engineers had designed the new engine to use hydrogen. It was designated the "304," taken from the division's engine order number 703040, 16 April 1956. It was essentially the one proposed earlier by Sens and Kuhrt and is shown schematically by figure 40. Liquid hydrogen was pumped at high pressure through a heat exchanger in the aft section of the engine. The heated hydrogen drove a multistage turbine which, through a reduction gear. powered a multistage air fan. The fan compressed incoming air, the primary working fluid of the engine. Part of the hydrogen discharged from the turbine was injected and burned in the air-stream behind the fan. The amount of hydrogen injected and burned was controlled to limit the temperature of the combustion gases which furnished the heat for the heat exchanger downstream. The remaining hydrogen was injected and burned in the after-burner section bevond the heat exchanger, and the hot gases and air expanded through the nozzle to produce propulsive thrust.
The engine was similar to the Rex III but much simpler, as only one heat exchanger was used. The maximum diameter of the 304 engine was 203 centimeters, as compared to the 150 centimeters proposed by Garrett for Rex III. Nacelle length was 10.7 meters, weight 2722 kilograms, thrust at 30 500 meters altitude, 21.4 kilonewtons (4800 lb). and specific fuel consumption 0.082 kilogram/ newton hour (0.8 lb/ lb.hr). These are close to the specifications in Sens's draft of 24 February 1956.
Pratt & Whitney engineers were well experienced in all the components of the 304 engine except the liquid-hydrogen pump and the hot-gas heat-exchanger. They purchased a liquid-hydrogen pump for study, but became dissatisfied with it and proceeded to make a better one.27 They saw two critical problems: an impeller that would handle liquid hydrogen without cavitation, and adequate sealing between the high-pressure liquid hydrogen at 20 K and the oil-lubricated bearing. Apparently they were not familiar with the work at Ohio State University on oil-free ball bearings operating in liquid hydrogen. They designed a two-stage centrifugal pump with a seal protecting conventional bearing lubrication. Figure 41 is a photograph of the pump rotor. The pump worked well and a total of 25 hours test time was accumulated in 75 tests over two years.
The hot-gas-to-hydrogen heat exchanger was the most unusual and interesting component of the 304 engine. With an outside diameter of 182 centimeters, the unit consisted of banks of 48-millimeter stainless steel tubing in an involute pattern to ensure uniform air flow. An enormous amount of tubing was used-enough to stretch over 8 kilometers; 2240 tube joints were furnace-brazed. The hydrogen passing through the heat exchanger was heated from 20 K to 1000 K, and the entering combustion gas temperature was 1500 K. The rate of heat transfer was 21 000 kilowatts (72 million Btu/hr), enough to heat 700 six-room houses.
Pratt & Whitney engineers, experts in designing gas turbines, built the 304 hydrogen turbine with 18 stages, the largest of which was 45 centimeters in diameter. Operating temperature was 1000 K and power output was 8950 kilowatts (12 000 hp). The turbines were tested for a total of 64 hours over a two year period. The first model 304 engine was assembled in East Hartford, Connecticut, by 18 August 1957-sixteen months after go-ahead.