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Saturn V
Long after the last Saturn V lifted off, NASA and industry studied other uses for the F-1 engines that powered its first stage. (credit: NASA)

Apollo Revisited

A mighty thunderous silence: The Saturn F-1 engine after Apollo

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The Saturn V’s F-1 engine is probably the most legendary rocket engine ever built. After a problematic early start that destroyed several test stands, the powerful engine went on to send 12 astronauts to the lunar surface. Later, as NASA planned on retiring the Apollo hardware, astute leaders recognized that they might need it again. This resulted in the F-1 Production Knowledge Retention Program. This was a project at Rocketdyne, the company that built the F-1 engine, to preserve as much technical documentation and knowledge about the engine as possible. According to an inventory of records, the Knowledge Retention Program produced 20 volumes of material on topics such as the engine’s injector ring set, valves, engine assembly, and checkout and thermal insulation and electrical cables, among others.

NASA considered using the F-1 engine in the late 1980s and early 1990s as part of its Space Exploration Initiative (SEI). Even when SEI was politically dead, the agency still continued trade studies that included the F-1.

But the project went beyond simply preserving documentation. Rocketdyne even sought to capture the knowledge inside the heads of the people who designed and manufactured the engines. They conducted tape-recorded interviews with them, asking about parts that were difficult to produce and manufacturing tricks that they had learned in the process of building multiple engines. In addition to all this material, NASA also had several F-1 engines in storage, plus the ones that ended up in museums that could be disassembled and examined. After the Apollo program ended, five engines were in storage at NASA’s Michoud Assembly Facility, with ten others mounted on stages on external display.

Rocketdyne delivered 98 production engines to NASA, of which 65 were launched. A total of 56 equivalent development engines were tested. The company conducted 2,771 production and R&D firing tests of single engines, 1,110 total full duration tests, and accumulated 239,124 seconds—over 66 hours—of engine firing experience. The five-engine cluster used on the Saturn V was fired at the Mississippi and Alabama test facilities 34 times, with 18 full duration tests for a total of 15,534 seconds of engine experience. In 1992, Rocketdyne estimated that the eight-year F-1 engine development program had cost $1.77 billion in fiscal year 1991 dollars.

The F-1 engine was designed to produce 6.7 million newtons of thrust. Rocketdyne uprated the engine and increased the thrust to 8 million newtons at sea level, or about 8.9 million newtons in vacuum. The design changes that increased the engine power were successfully demonstrated on two engines, designated F-1A.

NASA considered using the F-1 engine in the late 1980s and early 1990s as part of its Space Exploration Initiative (SEI). Even when SEI was politically dead, the agency still continued “trade studies,” meaning the trades between costs and capabilities of different choices. The F-1A was considered as part of these trade studies, and a senior study conducted in 1992 recommended that the F-1 be pressed back into service. But the cost of using the F-1A in a heavy-lift launch vehicle was high.

In 1992, Rocketdyne, then a part of Rockwell International, conducted a study of the company’s ability to place the F-1A back into production. The company surveyed its personnel with F-1 engine experience in three areas—engineering, quality, and manufacturing—and identified how many were active and how many were retired and available to work on the program if asked (the totals were 248 and 76, respectively.) The company pointed out that both the Atlas and Delta engines had been out of production for many years and had been restarted. For instance, the Delta engines had been out of production from 1968 until 1989, when the company restarted the RS-27/27A production line, so putting the F-1 back into production was not unthinkable.

But the F-1A would have been expensive. Rocketdyne estimated that activation of the production line would cost $315 million in 1991 dollars. A significant chunk of that money, $100 million, would be required to pay for four test engines and a spare. These costs apparently did not include reactivation of the special test stands that had been used for the F-1.

The per-unit cost for a production engine was difficult to estimate, however, because it depended upon the quantity ordered and the production rate. Rocketdyne estimated that the cost of each engine would be $15 million, assuming an order of 40 or more engines at a rate of 10–12 a year. But with the Space Exploration Initiative dead, further study of restarting F-1 engine production ceased.

In October 2012, the Dynetics-led team was awarded a 30-month, $73.3-million contract to study several aspects of an advanced booster for the SLS. NASA was simultaneously studying the F-1 largely as a training effort to develop engine design skills for younger engineers.

In early January 2013, almost forty years since the last F-1 rumbled, the woods surrounding NASA’s Marshall Space Flight Center were shaken by a thunderous roar. NASA engineers were not testing a rocket engine but only a part of one, something called a gas generator, a device that burns propellant to produce hot exhaust gases to spin a turbine that then spins a pump that pumps fuel and oxidizer into a thrust chamber where it ignites to produce the exhaust that powers a rocket. The gas generator is in many ways similar to a rocket engine, and it has to be fired on a rocket test stand. This gas generator was from a Saturn F-1 engine, the first time one had been fired in over three and a half decades. The F-1 gas generator alone was almost a third of the thrust of a Merlin 1C rocket engine for the Falcon 9 launch vehicle.

The January 2013 test of the F-1 gas generator was followed by several more. It was conducted by NASA, which was taking apart and examining several legacy F-1 engines as part of an engineering training program. But there was also another effort underway to look at adapting the F-1 to a new launch vehicle.

In 2012, NASA had awarded a study contract to a team led by the Huntsville-based company Dynetics and including the original F-1 manufacturer, Rocketdyne. The companies investigated the potential for using the F-1 in boosters attached to an updated version of the Space Launch System. NASA issued several contracts for the Advanced Booster Engineering Demonstration and/or Risk Reduction effort. Dynetics provides engineering solutions for the defense, aerospace, and commercial industries.

In October 2012, the Dynetics-led team was awarded a 30-month, $73.3-million contract to study several aspects of an advanced booster for the SLS, including risk reduction for the engine and the booster structure. This could eventually lead to developing advanced boosters each powered by two F-1 engines. NASA was simultaneously studying the F-1 largely as a training effort to develop engine design skills for younger engineers.

Aerojet Rocketdyne designated the new engine the F-1B. During the early 1970s Rocketdyne apparently used the designations F-1B and even F-1C for possible future versions of the engine. At least one of these rocket engines would have been for a reusable flyback Saturn S-IC first stage. In addition to the proposed early 1990s revival of the engine for the Space Exploration Initiative, by the late 1990s Rocketdyne considered using a version of the engine in a flyback booster for the Space Shuttle, this time designating it the F-1 Block II. More recently, the company evaluated an “F-1C” design for a commercial customer—probably United Launch Alliance—but that project did not go forward and therefore no details have been made public.

The F-1B engine which was studied in 2013 as a possible propulsion system for a booster rocket. (credit: Tim Warchocki)

At the time Dynetics was evaluating the F-1B, the expectation was that if NASA added two advanced boosters each powered by two F-1B engines to the SLS core stage, it would boost the overall SLS Block 2 performance to 150 metric tons, 20 tons over the existing requirement. The team investigating the F-1 approach calculated that if they emphasized performance over cost, they could achieve at least 160 metric tons, albeit at high cost. But Dynetics’ goal was to drive down cost wherever possible, and the F-1B would already provide more performance than NASA mission concepts required.

During their initial evaluation, the Dynetics-led team looked at several booster/tank diameter combinations. Other options included three RD-180 and three RS-84 engines. The RD-180 is the Russian-built engine currently used to power the Atlas V rocket. The RS-84 was a proposed Rocketdyne liquid oxygen/kerosene rocket engine capable of approximately 5.16 million newtons of thrust. The RS-84 was supposed to be reusable, but it was not pursued to actual testing, and development ended in the early 2000s.

Dynetics named their booster Pyrios, after one of the horses that pulled Greek sun god Helios’ chariot. Pyrios would consist of a boattail and aft skirt above two F-1B engines. Above this would be the kerosene fuel tank, an intertank, a liquid oxygen tank, a forward skirt, and a nose cone. Pyrios would have the same attachment points as the SRBs on the Block 1. To reduce cost, the booster tanks and skirts would use aluminum 2219 friction stir welded into structures 5.5 meters in diameter.

The Dynetics-led team looked at a family of booster options, including a Block 1A rocket with their boosters, which would be capable of 103–120 metric tons, and a “single-stick” rocket with the canceled Ares I J-2X-powered upper stage capable of launching 32 tons.

The engine would use the proven elements of the F-1 and the simplifications and performance enhancements of the F-1A engines built and tested late in the Apollo program. The F-1B engine would be capable of 8 million newtons of thrust at sea level, which could be throttled down to 5.8 million newtons, allowing operators to tailor the thrust profile for each flight and vehicle configuration as necessary.

The monster engine does appear to have a certain appeal—and certainly a legendary history—that leads to it being reconsidered whenever substantial thrust is required.

The plan for the F-1B engine included a number of modernized low-cost components. These included a hot-isostatic press bond constructed channel wall main combustion chamber and channel wall nozzle. The original F-1 and F-1A had a large exhaust manifold that curved halfway down the outside of the exhaust nozzle and vented into the nozzle. The exhaust, although hot, was significantly cooler than the hot gasses in the thrust chamber, and the cooler exhaust flowed down the inside side of the nozzle, serving as a heat buffer and preventing the primary exhaust from burning through the nozzle. Two of the highest cost engine components, the turbine exhaust manifold and nozzle extension, would have been eliminated in the F-1B, and the tube wall thrust chamber assembly replaced. Dynetics released concept illustrations and displayed a model showing the new engine with a large exhaust duct running down the side of the exhaust bell rather than curving into the bell like on the F-1A, thereby simplifying construction of the exhaust structure. It looked distinctly different from the F-1A, but would still be massive. Because of the lower pressures in a gas generator engine it could be constructed out of conventional and low-cost materials such as aluminum throughout the primary flow paths.

The original Saturn V F-1 engines were essentially hand-crafted, with many parts being welded together by a skilled welder. Although computer-controlled machines are now capable of taking over many welding jobs, engineers prefer to avoid welding whenever possible, casting a few large parts rather than welding together many smaller ones. Dynetics reduced the parts count on one major section of the engine from 5,600 individual parts to only 40. That would have reduced cost, increased reliability, and simplified many other processes. No more hand-crafted welding would be required.

The original F-1 was built somewhat like a wind-up clock: as one part of the startup sequence occurred it would mechanically or in some other way trigger the next part of the startup sequence and so on in a clever progression. The plan was to replace most of that with modern computer-controlled systems, which would also increase the ability to control and fine-tune the engine.

But NASA discontinued studies of upgrading the SLS, including study of reviving the F-1. Soon the testing of the F-1 gas generator at Marshall Space Flight Center also came to a halt. The F-1 went silent once more.

The last time a Saturn V rocket powered by F-1 engines lifted off was the Skylab Orbital Workshop in 1973. Although that is likely to prove to be the last time that an F-1 ever flies, the monster engine does appear to have a certain appeal—and certainly a legendary history—that leads to it being reconsidered whenever substantial thrust is required.

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