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The F-1 engine used for the first stage of the Saturn V is still the most powerful liquid fuel rocket engine. The development of the F-1 was started very early in 1955 by the US Air Force. In 1958 the Air Force eventually halted development of the F-1 because of a lack of requirement for such a large engine, but the recently created NASA decided to continue the development. Without the F-1 engine a manned Moon landing would have been impossible.

What was the planned usage for the F-1 engine at the start of development in 1955?

Some thrust numbers used for early manned spacecrafts:

F-1 engine thrust 6,770 kN used for Apollo to the Moon (5 engines)
Vostok 4,795 kN used for first orbital manned flight.
Mercury-Redstone 350 kN used for suborbital manned flight.
Mercury-Atlas 1,880 kN used for orbital manned flight.
Titan II 1,900 kN used for orbital manned flight with two astronauts.
(For the rockets total thrust of all engines running at launch.)

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It seems it was, essentially, a hedge. The best history of the F-1 I know, "The Saturn V F-1 Engine" by Young, states

Although the Air Force had no immediate requirement for such an engine, this forward-thinking branch of the military felt there might be a need for an engine with 1 million pounds of thrust in the near future.

(page 43)

Note that this 1955 contract was a feasibility study only.

It was not until June 1958 that a contract was let for preliminary hardware development. Further contracts would be dependent "on successful demonstration tests."

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  • $\begingroup$ But the feelings seemed to have changed from 1955 to 1958. $\endgroup$ – Uwe Jun 7 '18 at 19:24
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    $\begingroup$ A little thing called "Sputnik" happened between then :) $\endgroup$ – Organic Marble Jun 7 '18 at 19:24
  • $\begingroup$ So after Sputnik they thought a rocket like PGM-19 Jupiter using a single engine with only 667 kN (1/10 of F-1) thrust will do? A medium range ballisitc missile, not an intercontinental one like the R-7 used for Sputnik? $\endgroup$ – Uwe Jun 7 '18 at 20:29
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    $\begingroup$ Ah, to be in an era where an enormous rocket engine could be developed just because it might be useful in the future... $\endgroup$ – Sean Jun 10 '18 at 15:02
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    $\begingroup$ The fifties, the era with the belief that small nuclear reactors to power a passenger car used on normal streets will be possible. $\endgroup$ – Uwe Jun 11 '18 at 20:29
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There was no specific rocket design associated with the Air Force's original F-1 project, but the primary application was almost certainly going to be an engine for an ICBM.

The 1955 Air Force specification for the F-1 engine called for a million-pound (~4500kN) thrust engine; it wasn't until NASA took over the contract that it grew to the 6700kN engine used on the Saturn V. A single 4500kN engine would be very comparable to the total thrust of the Soviet R-7 ICBM which begat Vostok and the Soyuz family.

Used for a simple single-stage ICBM, it would be only a little more capable than the Atlas at three times the launch mass -- not particularly attractive given that the Atlas was already well in development in 1955. With a kerosene-fueled upper stage, however, it could put two tons into orbit, or send a nuclear warhead almost anywhere.

There were other applications considered for large boosters. In 1955, the Air Force was also interested in rocket planes; the X-15 project was in proposal mode, and orbital bombers were being contemplated; million-pound engines would definitely be needed for the latter.

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