Mastodon spaceflight mass-mind, please come to my aid. My google-fu is failing me.
In the Apollo program, Saturn V’s S-II (second stage) employed four ullage motors situated on the aft interstage skirt. Additionally, the third S-IVB stage incorporated an Auxiliary Propulsion System designed for ullage purposes. These settled the fuel so the engines did not injest vapor and shred.
Question: why didn't the ullage motors need ullage motors?
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Supplemental question:
Some references say that spacecraft can use attitude jets or reaction control systems for ullage. Why don't the attitude jets require ullage themselves? I know that they do not use huge turbines like the main engine, but they are not solid fuel.
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@nyrath Don't hydrazine monopropellant systems usually use reservoirs of non-reactive gas (nitrogen or helium) to maintain pressure in the tank and push the liquid towards the reaction chamber & catalyst?
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@michael_w_busch @nyrath For Apollo on the command module I'm pretty sure the attitude motors were pressurized with... maybe helium?
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@nyrath
Bladders were used for for the Apollo CSM RCS:
"Each tank consisted of an outer titanium-alloy shell, a Teflon bladder, and an aluminum standpipe. Helium pressurant entered the tank between the metal shell and the Teflon bladder, collapsed the bladder, and forced the propellant out of the bladder through the aluminum standpipe."
Source: https://ntrs.nasa.gov/api/citations/19730017174/downloads/19730017174.pdf
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@neffo
Thank you!
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@nyrath Some attitude jets/RCS use gas propellant, such as cold gas thrusters (using, say, compressed oxygen or nitrogen).
In theory, a thruster system could be designed to use propellants that sublimate directly from solid to gas form.
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@isaackuo @nyrath And it doesn’t have to be something that continually sublimates at reasonable temperatures(though you’ll still want a TCS). Using an electric heater to provide gas as needed is a possible solution. It might still be a challenge to find a solid with reasonable sublimation temperature and low molecular weight that doesn’t eat the container and test engineers.
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@su_liam @nyrath Carbon dioxide has an advantage that it's available via orbital atmospheric scooping. At reasonable temperatures and pressures, CO2 directly sublimates to a gas, and it burns well with aluminum/magnesium.
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@isaackuo @nyrath At very reasonable temperatures, CO2 can be stored as a pressurized gas. Al and Mg aren’t gaseous at reasonable temperatures, but handling powder isn’t too hard.
I may need to read this-> https://ntrs.nasa.gov/api/citations/20080002287/downloads/20080002287.pdf
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@su_liam @nyrath Compressed CO2 gas is an order of magnitude less dense than dry ice, and requires 2+ orders of magnitude stronger tank pressure. The bottom line is that you get a lot less CO2 capacity with compressed gas than solid form.
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