Rocketdyne H-1

• 1. Introduction

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  The Rocketdyne H-1 burned liquid oxygen and a refined kerosene to lift the first stages of America's Saturn rockets. These engines, arranged in a ring at the base of each stage, pushed each Saturn I and Saturn IB off its launch pad. After the Apollo program ended, the H-1's descendants kept flying on smaller rockets for decades. But the H-1's origins reach much further back than the Space Race. How did a fuel injector that no one had ever successfully deployed become the heart of America's most capable early rocket engine? And how did an unexpected $10 million surplus at a summer budget meeting end up shaping the design of the Saturn I?

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• 2. From V-2 to Kerosene

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  North American Aviation was handed several V-2 engines at the end of the war, along with a wealth of German technical documentation. Each engine produced 264.9 kilonewtons of thrust. Converting the metric specifications into SAE measurements required a dedicated internal team, which NAA called the Propulsion Division. That division would eventually become Rocketdyne.

  Inside the German documents was a plan for an injector the Germans called the 'waterfall' design. It worked by directing burning fuel through many small nozzles arranged to spray downward into the combustion chamber like a curtain of water. German engineers had designed it but never gotten it to function reliably, and it never went into service. NAA's propulsion team took up the problem and found solutions where the Germans had not. That success raised the engine's thrust first to 75,000 pounds and then to 78,000 pounds, the figure used on the Redstone missile.

  NAA was simultaneously developing the SM-64 Navaho, a cruise missile whose ramjet engines could only ignite once it was moving fast enough. A booster rocket pushed the Navaho to that speed. As the Air Force demanded higher performance from the system, the missiles grew larger and the boosters had to keep pace. By the early 1950s, this pressure had pushed the basic engine design to 120,000 pounds of thrust.

  All of these engines burned ethanol, as the original V-2 had, but the engineers tested other fuels alongside. Kerosene, diesel oil, paint thinner, JP-4, and JP-5 jet fuel all went into test chambers at various points.

  In January 1953, Rocketdyne launched a program called REAP to develop a purpose-built kerosene specifically engineered for rocket use. The resulting fuel, designated RP-1, was formally defined in Military Specification MIL-R-25576 in 1954.

  By 1955, the Air Force had chosen a JP-4 burning version to power the Atlas missile. The US Army requested 150,000 pounds of thrust for the Jupiter missile; the Air Force adopted a similar engine for the Thor. Rocketdyne designated this engine the S-3D, also known as the LR-79.

  The S-3D carried the waterfall injector into service alongside a turbopump starting system that required secondary fuel tanks. Those tanks fed the gas generator and the combustors during startup while the main pumps came up to pressure. A network of electropneumatic valves sequenced the fuel flows through a complex startup procedure.

  Rocketdyne's engineers were already working to replace that complexity with something far more elegant, and they called it the X-1.

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• 3. The X-1

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  Originally designated the S-3X, the engine that would become the H-1 replaced the complex valve network of its predecessor with a simpler principle. The new valves operated on fuel pressure alone, with no external sensors or electronics controlling the startup sequence. This made the process self-regulating, driven entirely by the propellant flow itself.

  A small solid-fuel rocket replaced the start tank system in the X-1. Its exhaust was routed through the gas generator to spin the turbopumps into motion. This change simplified the plumbing considerably, but it carried a consequence: the X-1 could be started only once. Earlier engines could theoretically be restarted in flight, but a single starter cartridge ruled that out for the X-1.

  Triethylaluminum replaced the mechanical ignitors of earlier designs. Older ignitors had to be inserted physically through holes in the combustion chamber wall before each firing. The new system held triethylaluminum inside a cube fitted with pressure-sensitive diaphragms. When fuel flow in the injector reached a set threshold, the diaphragms burst and the compound ignited on contact with the propellants.

  RP-1 fuel also took on a new role in the X-1. A small amount of additive mixed into the fuel was fed under pressure to the turbopump bearings. The additive lubricated the bearings and carried heat away from them as the fuel flowed through.

  Although the X-1 was still experimental, it was already within the thrust range a Saturn application would require.

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• 4. Saturn's First Stage

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  Saturn began as a US Department of Defense requirement for a heavy-lift vehicle. It had to lift between 10,000 and 40,000 pounds to low Earth orbit, or accelerate between 6,000 and 12,000 pounds to escape velocity. In April 1957, Wernher von Braun handed the preliminary design work to Heinz-Hermann Koelle.

  Koelle proposed clustering fuel tanks from Redstone and Jupiter missiles on a single thrust plate, with engines mounted below. His calculations showed the combination would need roughly one million pounds of total thrust. That figure severely limited the engine choices. Koelle learned of the E-1 from Rocketdyne's George Sutton. Rocketdyne was developing the E-1 as a backup for the Titan missile, rated at 400,000 pounds of thrust. It had been designed to be as straightforward to develop as possible, in case Aerojet General's LR-87 did not pan out. It was the largest engine approaching readiness within the timeframe ARPA had given the program.

  Sputnik launched that October, and American space policy changed quickly. The government chose to separate its civilian and military rocketry programs, creating the agency that would become NASA. Von Braun's ABMA team agreed to transfer to NASA, where they would form the Marshall Space Flight Center. The handover was set for 1960.

  In July 1958, ARPA arrived at ABMA carrying $10 million they needed to commit before the transfer. Von Braun showed them a model of the vehicle, still called the Juno V at the time. The ARPA visitors noted that the E-1 would not be ready by 1960.

  The group settled on an alternative: boost the S-3D from 175,000 to 200,000 pounds of thrust. Eight upgraded engines would replace the four E-1s in the original layout. When Koelle brought this plan to Rocketdyne, the company presented the X-1 instead. A contract for the new engine was signed on the 15th of August 1958.

  By early 1959, the vehicle had been renamed Saturn, taking the name of the planet that follows Jupiter. Jupiter was also the name of ABMA's preceding missile design, giving the progression an intentional logic.

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• 5. Starting and Firing

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  The H-1's combustion chamber was formed from 292 stainless steel tubes, all brazed together in a furnace. RP-1 fuel was pumped through those tubes before reaching the combustors. As it flowed, the fuel absorbed heat from the chamber walls and carried it away, protecting the structure from the temperatures of combustion.

  To fire the engine, a 500-volt AC signal was applied to the Solid Propellant Gas Generator. The SPGG was a compact solid-fuel rocket; its combustion produced hot gas that built pressure inside the system. When that pressure reached between 600 and 700 pounds per square inch, a bursting diaphragm released the gas into the turbine. The turbine spun the pumps that drove RP-1 and liquid oxygen into the gas generator and the combustion chamber. Once those propellants began burning in the gas generator, their exhaust took over the work of sustaining the turbopumps for the duration of the burn.

  The SPGG had to be replaced after every firing, which meant the H-1 carried only one start in flight. Engines were typically subjected to two or more static test firings on the ground before each mission to confirm they were flight-ready. Each full burn lasted 155 seconds before shutdown.

  The mixture ratio of liquid oxygen to RP-1 was set at 2.23, held within a tolerance of plus or minus two percent. Nominal chamber pressure was 633 pounds per square inch. The oxidizer pump moved liquid oxygen at 3,330 US gallons per minute; the fuel pump moved RP-1 at 2,092 US gallons per minute.

  Early production engines, flown through Saturn mission SA-205, were rated at 200,000 pounds of sea-level thrust. From SA-206 onward, a revised production standard raised that rating to 205,000 pounds.

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• 6. A Second Life

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  Surplus H-1 engines from the Saturn program were reworked and renamed the RS-27 after Apollo. Their first flight under the new designation came in 1974, on the Delta 2000 series rocket.

  The RS-27 went on powering successive Delta rocket series until 1992, when the Delta 6000, the first version of the Delta II, was retired. A follow-on variant, the RS-27A, brought modestly improved performance. It flew on later versions of the Delta II and on the Delta III as well. The Delta II kept flying with the RS-27A until 2018.

  That 2018 retirement came sixty-five years after Rocketdyne first formalized RP-1 in the REAP program in January 1953.

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