Ancestors

Written by Winchell Chung ⚛🚀 on 2025-01-11 at 13:14

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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Written by Winchell Chung ⚛🚀 on 2025-01-11 at 14:15

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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Written by Isaac Ji Kuo on 2025-01-11 at 15:41

@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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Written by su_liam on 2025-01-11 at 19:14

@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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Written by Isaac Ji Kuo on 2025-01-11 at 19:24

@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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Toot

Written by su_liam on 2025-01-14 at 00:16

@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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Descendants

Written by Isaac Ji Kuo on 2025-01-14 at 13:37

@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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