29
$\begingroup$

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?

$\endgroup$

1 Answer 1

38
$\begingroup$

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

$\endgroup$
6
  • 16
    $\begingroup$ It seems ironic to use jet fuel because it's the less flammable option! $\endgroup$
    – corsiKa
    Aug 18, 2019 at 4:53
  • 5
    $\begingroup$ 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
  • 6
    $\begingroup$ 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
  • 1
    $\begingroup$ @0xDBFB7 molten radioactive sodium as hydraulic fluid! shudder Of course if that was Project Pluto, that thing was Flying Armageddon anyway. $\endgroup$ Aug 18, 2019 at 20:10
  • 2
    $\begingroup$ @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

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.