Early engines

The H-1 is one of a series of engines developed from the wartime V-2 ballistic missile. During the war, North American Aviation (NAA) was given several 59,600 lbf (264.9 kN)^[3] V-2 engines to examine and convert from metric to SAE measurements. They formed their "Propulsion Division" to handle this work, later becoming Rocketdyne.

NAA had also been given a wealth of technical documentation on the engine. Engineers studying them came across plans to improve the V-2 engine using a new "waterfall" fuel injector. The Germans were unable to get the design to work and it never went into service. NAA engineers decided to attack this problem and quickly came up with solutions. This allowed them to raise the thrust of the design to 75,000 lbf (330 kN), and then 78,000 lbf (350 kN) for the Redstone missile.

NAA had also been working on the SM-64 Navaho cruise missile project, which used the same engine as a booster to get the missile up to speed so its ramjet engines could light. The Air Force continually demanded higher performance from the Navaho, which forced NAA to build larger missiles, and larger boosters to launch it. By the early 1950s, the basic engine design had been enlarged to produce 120,000 lbf (530 kN).

All of these designs, like the V-2 that spawned them, burned ethanol, but other fuels had also been experimented with, including kerosene, diesel oil, paint thinner, JP-4, and JP-5 jet fuel. In January 1953 Rocketdyne started their "REAP" program to convert these engines to a specific and well-engineered kerosene fuel specifically for rocket engines, which became RP-1, officially specified in Military Specification MIL-R-25576 in 1954.

In 1955, the Air Force selected a JP-4 burning version of the engine to power their Atlas missile. The US Army requested a further boost to 150,000 lbf (670 kN) for their Jupiter missile, and the Air Force used the same version for their Thor, producing the Rocketdyne S-3D (or LR-79).

All of these engines were based on a similar design concept, featuring a "waterfall injector", where many small fuel injectors were used to spray burning fuel into the main combustion chamber. They also shared a complex system for starting the turbopumps, using a set of secondary fuel tanks and plumbing that fed the gas generator and main combustors while the pumps were still bringing the main fuel lines up to pressure. A complex series of electropneumatic valves operated the various fuel flows until the engine was fully started.

X-1

With the successful running of the S-3D for the Thor and Jupiter, the company turned their attention to a radically updated version, originally known as the S-3X, but later becoming the X-1. This engine replaced the complex valve system and all of its attended sensors and electronics with new valves that operated on the pressure of the fuel itself. This meant that the complex start-up procedure was entirely automated and driven off the fuel flow itself.

Additionally, the X-1 removed the entire start tank system and replaced it with a small solid fuel rocket engine that fed its exhaust through the gas generator to spin the turbopumps. This change dramatically simplified engine plumbing, at the cost of making the design a single-shot device. Earlier engines could, in theory, be restarted in flight, but with a single starter cartridge, the X-1 could be started once only.

Another change was to introduce an ignitor using a pyrophoric fuel in place of the solid fuel versions of earlier designs. The earlier engines required the ignitors to be inserted through holes in the engine into the combustion chamber, but the new system allowed the fuel to be sprayed into the main injector. The fuel, triethylaluminum, was delivered in a cube with diaphragms that burst when the fuel flow in the injector reached a set threshold.

Finally, the X-1 introduced a new lubrication system that added a small amount of additive to the RP-1 fuel as it flowed through the various components. This was fed under pressure into the various bearings in the turbopump system, both lubricating it and carrying away heat.

Saturn and H-1

Saturn started as a paper project to meet a new US Department of Defense requirement for a heavy-lift vehicle able to lift 10,000 to 40,000 pounds into low Earth orbit (LEO), or accelerating 6,000 to 12,000 lb to escape velocity. Existing launchers might be extended to reach 10,000 lb to LEO, below the requirements. A new and larger design was needed, and in April 1957, Wernher von Braun handed the preliminary design task to Heinz-Hermann Koelle.^[4]

Koelle's solution to reducing the development time was to use a cluster of fuel tanks from Redstone and Jupiter missiles, sit them on top of a single thrust plate, and then attach the required engines to the bottom of the plate. Calculations demonstrated that a total thrust of about 1 million pounds would be needed, greatly limiting their engine selection. Looking for suitable designs, Koelle learned of the E-1 from Rocketdyne's George Sutton.^[5] Rocketdyne was developing this 400,000 lbf (1,800 kN) engine for the Titan missile, and it was the largest engine nearing introduction within the time frame that ARPA gave Wernher von Braun to develop what was then known as the "Juno V".^[6] The E-1 had originally been developed as a backup engine for the Titan missile, designed specifically to be as simple to develop as possible, in case the Aerojet General LR-87 did not pan out.^[7]

The launch of Sputnik that October led to rapid changes in the US rocketry establishment. In order to demonstrate peaceful intent, the US decided to spin out its various non-military rocketry programs to a new agency, which would evolve as NASA. As the Army had lost interest in large rockets, they agreed to turn over von Braun's ABMA team to NASA, becoming the Marshall Space Flight Center.^[8] The handover would take place in 1960.^[8]

Shortly after these plans were made, in July 1958 ARPA visited ABMA and told von Braun that they still had $10 million in their budget to spend before the handover, and asked if there was any way to effectively use the money. Von Braun called in Koelle and showed them a model of the Juno V, but the ARPA visitors noted that the E-1 engine wouldn't be ready by 1960.^[9] Brainstorming, they decided that the best approach was to make a minor upgrade to Rocketdyne's existing S-3D engines to boost them from 175,000 lbf (780 kN) to 200,000 lbf (890 kN), and use eight of these engines instead of four E-1s.^[9]

When Koelle returned to Rocketdyne looking for an upgraded version of the S-3D, they instead presented the X-1 and suggested it be used in place of a further upgrade to the S-3. Although experimental, the X-1 was already in the right thrust range and ready for full development. A contract for development was tendered on 15 August 1958,^[10] and by early 1959 the name had changed from Juno to Saturn, referring to the succession as the planet after Jupiter, the Jupiter missile being the previous ABMA design.^[11]