Legacy of Suntan
 The Pratt & Whitney Aircraft division of United Aircraft Corporation was operating the first 304 hydrogen-fueled engine in the initial series of Suntan tests at the Florida test center when Sputnik was launched. At that time there was no indication that Suntan was soon to end, but neither this possibility nor Sputnik caught the astute Perry Pratt napping. About two years earlier, he had recruited C. Branson Smith from the Hamilton Standard division of United Aircraft, and one of his first assignments was to study the possibilities for Pratt & Whitney's entry into the rocket field.1 With the Rocketdyne division of North American Aviation and Aerojet-General Corporation the giants in the large liquid-propellant rocket engine business, and Bell Aircraft and the Reaction Motors division of Thiokol smaller but very aggressive, it was obvious that a newcomer would need to do something new and different. As the division began to move fast on the hydrogen-fueled engine for Suntan in early 1956, Smith found his answer. In April 1956, he jotted down in his work log two subjects of potential interest: hydrogen as a high-energy fuel and pentaborane as a storable fuel. Smith's approach to rocket work was methodical: educate the staff, make an evaluation of rockets to establish the best type on which to concentrate, and propose an experimental contract to gain experience and advance the basic technology. In May 1956, Smith briefed Pratt on early results. From this meeting came a decision to summarize the status of hydrogen rocket engine work, to consider rocket engines for aircraft auxiliary  propulsion, and to consult with Wesley Kuhrt on the air turborocket. Smith visited the Los Alamos Scientific Laboratory and learned about their nuclear rocket. Although ammonia was to be the working fluid in early tests, Los Alamos was also interested in using hydrogen; the Livermore Laboratory, also working on nuclear rockets, had definitely planned to use hydrogen. Smith learned a great deal about hydrogen at Los Alamos and at the Bureau of Standards cryogenic laboratory in Boulder. He submitted his report on rockets to Pratt in June and revised it the next month. He concluded that: (1) in the next decade the rocket would become capable of performing most military missions at greater speeds and altitudes than gas turbine powered aircraft, (2) the most immediate application for Pratt & Whitney was an auxiliary thrust rocket to increase aircraft performance, and (3) in the missile field the ultimate fuel was hydrogen. Smith saw the first step for the division as a general educational program; the second step, the development of a small rocket for boosting aircraft in combination with gas turbine engines; and the third step, to propose hydrogen for long-range ballistic missiles. Smith added that the division's present and anticipated experience with hydrogen offered an opportunity to overtake existing rocket competitors.2 Although this is what happened later, Smith's recommendations did not produce immediate action. Pratt & Whitney was fully engrossed in the Suntan program to use hydrogen in a modified J-57 and in developing the 304 engine. By the fall of 1956, Smith was making calculations of hydrogen and oxygen as coolants, and in November he visited NACA's Lewis laboratory to learn about work on hydrogen as a rocket fuel with oxygen and fluorine as oxidizers.
On 4 April 1957, Smith and Pratt made a presentation on hydrogen-fueled rockets at a management meeting of United Aircraft officials.* As a result, Pratt asked Smith to prepare a proposal for the first phase of a hydrogen rocket program. He was to make a cost estimate, note the limited availability of hydrogen, and consider a possible substitute fuel. Apparently, the United Aircraft management was willing to get involved in rocket work, but not wholly convinced that liquid hydrogen was the best fuel. In mid-April Smith was among a group of engineers who met with the Navy on boron fuels. After the meeting, Pratt took Smith to visit Col. Norman C. Appold, the Suntan manager, at the Air Force's Air Research and Development Command headquarters in Baltimore. They proposed that Pratt & Whitney develop a liquid hydrogen-liquid oxygen rocket engine, but Appold was not receptive; he wanted Pratt & Whitney to concentrate on Suntan objectives.
In July 1957, Smith summarized a year's thinking about rockets. In propellant evaluation, he indicated that hydrogen led for vehicles requiring maximum performance, such as ICBMs, satellites, and space ships. Hydrogen was well-suited for long-range missiles and could halve the gross mass of those being developed using kerosene and oxygen. He also saw advantages for hydrogen for shorter-range missiles, particularly stages that must accelerate to very high velocities. The substitution of hydrogen for kerosene in the second stage of an ICBM would increase the payload 50 percent without increasing the gross mass. Alternately, ICBMs then being developed could place payloads into a satellite orbit by using hydrogen in the second stage. He recommended to Pratt that a vigorous effort be mounted to develop a hydrogen-oxygen  rocket engine. Smith also considered boron fuels and proposals to the Navy and Air Force.3
On 25 July 1957, Smith visited the power plant laboratory at Wright Field and made a presentation featuring hydrogen as a rocket fuel for a 267 kilonewton (60 000 lb thrust) second stage for an ICBM. Because of the classification of Suntan, he omitted telling his audience about the considerable experience Pratt &,Whitney had amassed with liquid hydrogen.** The reaction of the Wright Field group to Smith's presentation was that his results appeared reasonable, but hydrogen had not been pursued previously because of its high cost and low availability. Smith was encouraged enough by the reactions to complete a proposal in August, but nothing came of it. It was the same month that the first 304 hydrogen expander engine was assembled at the East Hartford plant for shipment to the Florida test center and two months before Sputnik I.
The Air Force had ample precedent, in previous work at Ohio State University in the 1940s, to be interested in hydrogen for rockets. The fuels and propulsion panel of the Air Force Scientific Advisory Board foresaw the need for a hydrogen-fueled rocket a year before Sputnik I. At the panel's meeting on 14 November 1956, high-energy propellants were considered for upper stages of high-performance rockets. The panel, of which Abe Silverstein was a member, recommended that two rocket engines be developed in the 111 to 222 kilonewton (25 000-50 000 lb thrust) size using high-energy chemical propellants.*** Liquid hydrogen-oxygen was singled out as being a particularly attractive high-energy combination. The panel was aware of the Air Force's plants which had been built to produce liquid hydrogen in quantity for the "airbreathing super fuel program" (i.e., Suntan) and that ample quantities would be available for testing. The ballistic missile program used vast amounts of liquid oxygen, so the panel thought the time for using the hydrogen-oxygen combination had come.4
Although the minutes do not single out the contributions of individual members, the influence of Abe Silverstein is unmistakable. He had been intensely interested in liquid hydrogen as an aircraft fuel since 1955 and was directing a strong research program on it. As early as 1950, he had organized a meeting of government and industry rocket experts on the subject of high-energy rocket propellants for long-range missiles (p.76). At the 1950 meeting, the rocket group at the NACA Lewis laboratory had recommended liquid hydrogen as their first choice for fuel, with hydrazine and ammonia as alternatives. Fluorine was the favored oxidizer, with oxygen as the alternate. At the time of the panel meeting, hydrazine or ammonia with fluorine, and hydrogen with fluorine or oxygen, were the high-energy combinations of greatest interest in the country. Prior to its meeting, the panel had visited the NACA Lewis laboratory, the Air Force power plant laboratory, and Pratt & Whitney.
The Air Force waited over a year (until December 1957) to reply to the board's recommendations. With respect to rocket engines using ammonia-fluorine, the Air Force cited a contract with Bell Aircraft on experiments with a 156 kilonewton (35 000 lb thrust) chamber using ammonia-fluorine. With regard to hydrogen-oxygen, the Air  Force commented that six months previously (about June 1957), Wright Field had prepared a procurement request for a liquid-hydrogen engine, but did not send it to industry because of NACA work with this combination. The November 1957 NACA firing of a liquid hydrogen-fluorine rocket (p.92) was cited. The Air Force intended to follow through when NACA completed its exploratory work-an indication that the Air Force felt no great rush for action, two months after Sputnik I.5 This was consistent with the disappointments Pratt & Whitney was experiencing in trying to sell the Air Force a hydrogen-oxygen engine. The stimulus needed was to come later in a negative way-the demise of Suntan.
During the remainder of 1957 and the early part of 1958, Smith and his group at Pratt & Whitney continued to work on the analysis of hydrogen cycles and the layout of engines with thrusts ranging from 31 to 133 kilonewtons (7000 to 30 000 lb). Exploratory meetings were held with the Air Force, and on 4 March 1958, Perry Pratt sent to the ARDC a preliminary design and proposal for a 68 kilonewton (15 000 lb thrust) advanced rocket engine using liquid hydrogen and liquid oxygen as propellants.6 It was intended for the Air Force's growing astronautics program and specifically for applications being developed by the Missile Systems Division of Lockheed Aircraft Corporation. Lockheed was studying advanced versions of its WS117L reconnaissance satellite, and their work indicated a thrust level of about 31 kilonewtons (7000 lb). Lockheed. however, wanted to use fluorine instead of oxygen with hydrogen.
The hydrogen-fluorine combination produces peak performance using a smaller proportion of hydrogen than the hydrogen-oxygen combination; when this is coupled with the greater density of fluorine over oxygen, the result is a much more compact stage for the same thrust and duration. (Lockheed was also very interested in an even denser combination, hydrazine-fluorine.)
By the spring of 1958, the Suntan management team at ARDC decided that the time had come for a rocket engine using liquid hydrogen to power an upper stage.7 They were aware of the Lockheed studies and the efforts of Krafft Ehricke of ConvairAstronautics to sell ARDC a Mars probe using a hydrogen-oxygen stage on top of the Atlas intercontinental missile.
The Suntan team may have hedged on the selection of oxidizer. Liquid oxygen was the safest choice, but fluorine was also of interest.**** The team coordinated the proposal with Brig. Gen. Marvin C. Demler, deputy commander of ARDC, and it was signed by Lt. Gen. Sam Anderson, the commander. The proposal was addressed to Gen. Thomas C. White, Air Force chief of staff, but the air staff decided to pass it to Richard E. Horner, the assistant secretary of the Air Force for research and development.8 Horner had followed the Suntan work closely and was aware of its coming termination and the desirability of finding an application for the new technology. The Suntan management team arranged a briefing for him and brought in Pratt & Whitney representatives to strengthen their presentation. Horner favored the proposal, but decided it should be  sent to Roy Johnson, director of the Advanced Research Projects Agency which was heading military space coordination. Johnson and his staff were briefed on 13 June 1958 (the day following a briefing of the Air Council on Suntan). Horner also arranged for ARPA staff members to visit the Suntan liquid hydrogen facilities in July 1958. Richard Canright and David Young, both well acquainted with hydrogen, were greatly impressed at seeing liquid hydrogen being pumped through nearly a half kilometer of piping at Pratt & Whitney's Florida test center. The Air Force made a persuasive case for ARPA to choose Pratt & Whitney for the development of a liquid hydrogen rocket, based on the division's experience in building a hydrogen expander engine for Suntan and the ready availability of a large supply of liquid hydrogen.9
While the Suntan team was making a bid within the government to develop a hydrogen-fueled rocket, Smith and others at Pratt & Whitney revised their March 1958 proposal to the Air Force to conform with Lockheed Aircraft desires and resubmitted it on 5 May 1958. In the following weeks, the two companies cooperated in analyses and layouts of five different propulsion systems, all for a proposed advanced reconnaissance satellite. During this period there were many reviews of the work by Air Force and ARPA representatives. At one such meeting on 9 July 1958, Air Force, ARPA, Lockheed, and Pratt & Whitney representatives unanimously agreed to select liquid hydrogen and liquid oxygen as the propellants and a thrust level of 53.5 kilonewtons (12 000 lb). Nine days later, Pratt & Whitney engineers drafted engine specifications and a proposal for company approval before sending them to ARDC. The engine was to be developed in 18 months at a fixed price of $19.8 million.10 Essentially the same engine was in fact developed later as the RL-10.
In August 1958, the ARDC was authorized to proceed with the development of a hydrogen-oxygen engine. Its application was not for a Lockheed-built stage, however, but for a stage proposed by General Dynamics-Astronautics.
* H. M. Horner, L. S. Hobbes, W. P. Gwinn, W. A. Parkins, and B. McNamara.
** Col. Appold, the Air Forceís Suntan manager, had visited the laboratory the previous day and presumably had informed the staff about Pratt & Whitneyís hydrogen experience.
*** Dr. Mark M. Mills was chairman; other memebers attending, besides Silverstein, were Dr. W Duncan Rannie of JPL and C.I.T and Dr. Eward S. Taylor of M.I.T.
**** In addition to
Lockheedís interest in fluoride, three of the Suntan team-Col.
Norman C. Appold, Lt. Col. John D. Seaberg, and Capt. J. R. Brill-had
attended the NACA conference in November 1957 that was devoted
largely to liquid hydrogen as a fuel, including data from the firing
of a liquid hydrogen-fluorine rocket.