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Fuel is also used within the engine as a lubricant and as a hydraulic working fluid, though before launch, RJ-1 ramjet fuel is supplied from the ground, it being more suited for this purpose.

https://history.nasa.gov/afj/ap11fj/01launch.html

I can't seem to find any more background on why this was done. What properties made RP-1 unsuitable for ground use?

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What a fascinating question!

Turns out it's less flammable.

Ground Supply Fluid—Because the flash point of RP-1 fuel, which supplies the system in flight, is 110 to 139° F, it is classified as a Class Ill flammable liquid, not suitable for ground operations. A study was made to find substitute fluids with properties similar to RP-1 that could be used in the laboratory and in ground operations.

Such a fluid was found in RJ-1, a ramjet fuel. It has properties very similar to RP-1, but a flashpoint ranging from 190 to 208° F. If purchased with a specified flashpoint exceeding 200° F, it is not classed as a flammable liquid by the National Code and would not have the same stringent handling requirements of RP-1 fuels. Therefore, the RJ-1 fuel can be used in the laboratories or in GSE with the same precautions as taken with MIL-H-5606

GSE = Ground Support Equipment

Source - The December 1963 issue of Hydraulics and Pneumatics magazine

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    $\begingroup$ It seems ironic to use jet fuel because it's the less flammable option! $\endgroup$
    – corsiKa
    Aug 18, 2019 at 4:53
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    $\begingroup$ Only tangentially related, but apparently the US's nuclear ramjet program used the liquid sodium reactor coolant as hydraulic fluid. I can only imagine the lab safety manager's reaction to that one. $\endgroup$
    – 0xDBFB7
    Aug 18, 2019 at 19:19
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    $\begingroup$ That makes sense. So it's roughly: "we use RP-1 as hydraulic fluid & lubricant in flight, because it means we don't need the complexity / weight / etc of having a separate tank / etc; as the system is designed to use RP-1 for lubricant / hydraulic fluid we need something similar on the ground; RJ-1 is 'close enough' while being somewhat less flammable." $\endgroup$
    – TLW
    Aug 18, 2019 at 19:59
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    $\begingroup$ @0xDBFB7 molten radioactive sodium as hydraulic fluid! shudder Of course if that was Project Pluto, that thing was Flying Armageddon anyway. $\endgroup$
    – Organic Marble
    Aug 18, 2019 at 20:10
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    $\begingroup$ @OrganicMarble apps.dtic.mil/dtic/tr/fulltext/u2/839416.pdf one of the greatest table-of-contents I've ever seen in a paper, containing such terrific verbiage as "NaK Leak in Torque Motor after Servovalve Tests " and "Due to the nature of the working fluid it is not practical to have interfaces such as shaft seals directly exposed to an air environment." $\endgroup$
    – 0xDBFB7
    Aug 18, 2019 at 20:45

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