How do they do that? [Liquid rocket fuel]

[Radical Yellow Duck]

I have been thinking lately of some of the engineering problems associated with spaceflight. the one that is currently causing me to scratch my head deals with cryogenic fuels and oxidators.

I understand that the fuel and oxidizer tanks are essentially large vaccum bottles in order to contain the cryogenic fuel. While on the surface of the earth, earths gravity will cause the liquids to collect at the bottom of the tanks, and so fuel pumps at the bottom of the tanks can transfer the liquids to the combustion chamber of the rocket motor. I understand that during acceration the same effect can happen, i.e. the liquids collecting at the rear or bottom of the tank, with respect to the direction of motion.

When a liquid fueled rocket shuts down in microgravity, the liquids should rebound and spread throughout the interior of the tank. Unless confined by a bladder of some type there should be voids that would cause the pump to fail or "lose prime". I am not aware of any substance that would remain flexable enough to be a bladder in a cryogenic environment. I have not seen any diagrams indicating such a bladder or a pressurization system for compressing such a blader in the spacecraft I have studied.

I know that this issue has been solved, at least since the saturn S-IV-B, and also with the OMS pods for the shuttle. But for me it remains something of an engineering mystery.

[Astaro]

The wikipedia article on slosh dynamics may be of interest to you.

My understanding is that specialist 'turbo-pumps' are used in liquid fuel rockets, I suspect they are self-priming.

and lastly, I imagine there is sufficient gaseous or vaporised fuel and oxidezer emited to sustain combustion even when the bulk of the liquid fuel is not being pushed against the fuel delivery opening of the tank.

[danscope]

Hi, To the best of my knowledge, most of the vehicles in free fall and transient vector to luna employ hydrazene for thrust vectoring, docking
and attitude correction.
Best regards,
Dan

[dwnielsen]

Hi, I don't have anything of importance to add, but I did see a lecture once by an old coworker which was using a little condenser plate to gather vapor and then pulling off the liquid with capillary action through some fiber tubes.

[JMV]

See ullage motors. They provide the acceleration needed to settle the propellants at the bottom of the tanks.

S-IVB 500-series had two 3,390 lbf solid rocket ullage motors for staging and two 70 lbf liquid fuel ullage motors in the Auxiliary Propulsion System modules for use at MECO and J-2 engine restart. Burn duration for the solids was 4 seconds and the motor assemblies were jettisoned about 9 seconds after J-2 ignition. The liquid type used hypergolic propellants and employed expulsion bellows to force the propellants out. They were first activated at J-2 engine cutoff and fired for approximately 50 seconds after which the LH2 Continuous Propulsive Vent System was activated. During parking orbit coast hydrogen fuel was slowly vented through two aft-facing nozzles located in the forward skirt to provide a thrust that could be varied from 7 lbf to 45 lbf. This maintained proper propellant positioning throughout the parking orbit phase. Fuel boil-off ensured pressurization for this. APS ullage motors were fired again for 77 seconds before J-2 engine restart.

OMS pods in the space shuttle don't use cryogenic propellants but hypergolics. The tanks in the OMS have more complex system to prevent gas ingestion at ignition, something called propellant acquisition and retention assembly. Here's more on that: http://science.ksc.nasa.gov/shuttle/...f/sts-oms.html
Read the section PROPELLANT STORAGE AND DISTRIBUTION.

[PraedSt]

Good question. I'd wondered about this myself.

[NEOWatcher]

... Here's more on that: http://science.ksc.nasa.gov/shuttle/...f/sts-oms.html
Read the section PROPELLANT STORAGE AND DISTRIBUTION.

That gave me a headache, so I searched for the Apollo Service Propulsion system thinking that they might have a simpler process and not an application that can use the ullage motors.

I pulled up the Block II operations manual (HUGE PDF, so I patiently did a lot of other work while it loaded)
Page 176 basically describes the two tank system and standpipe that provides ullage when there is sufficient fuel in the system.

Fuel storage (pressurized with helium) -> sump tank (fuel only) -> standpipe.

From what I can gather, the ullage from the standpipe is when the sump is full and some in the storage tank. Otherwise the RCS system provides the ullage.

Fuel quantity is only measured during thrusting.

[Radical Yellow Duck]

Thank you all for your research.

The answer appears to be ullage motors. A small solid fuel or hypergolic motor provides thrust sufficient to cause the liquid fuel to settle. This can also be done by allowing some of the fuel to vent to provide a small thrust.

I remember seeing the films of the S-IV-B seperation. What I thought were motors just providing for the seperation from the second stage were also providing a thrust sufficient to allow the fuel to reposition. How clever.

[BetaDust]

I pulled up the Block II operations manual (HUGE PDF, so I patiently did a lot of other work while it loaded)

Quite large PDF indeed, but a great great link! Thank you, NEOWatcher.

--Dennis

[JMV]

Fuel storage (pressurized with helium) -> sump tank (fuel only) -> standpipe.

From what I can gather, the ullage from the standpipe is when the sump is full and some in the storage tank. Otherwise the RCS system provides the ullage.

From what I gather from the diagram on page 177 is that the the propellant flow goes more like this:

storage tank -> transfer line -> standpipe -> sump tank -> retention reservoir -> outlet

My impression is that standpipe is the vertical pipe inside the sump tank that guides propellant from the transfer line to the top of the sump tank. Notice how the transfer line goes from the bottom of the storage tank and attaches to the bottom of the sump tank. I think standpipe is the continuation of the transfer line that goes inside the sump tank and has an opening high up in the top part of the sump tank. From this I assume that the standpipe is needed to trap propellant inside the sump tank. In other words, it prevents propellant backflow from the sump tank to the storage tank when under RCS ullage acceleration. If there were no standpipe and the sump tank was full while the storage tank was empty, and if you were to do a RCS ullage burn, about half of the propellant in the sump tank would flow back into the storage tank to equalize surface levels of the liquid.

Propellant retention reservoir at the exit end of the sump tank seems to be the important part in ensuring that propellant remains at the outlet. The document doesn't explain it's construction very well, but I gather there are some sort of mesh screens that retain liquid by adhesion and by forming a surface tension barrier that prevents helium bubbles from getting into the outlet. In principle it seems somewhat similar to the propellant acquisition and retention assembly in OMS tanks, though I had hard time visualizing what the stub galleries and collection manifolds might look like.


I have to pick a nit from my previous post too. What I called MECO was properly called SECO at the time for Single/S-IVB/Sustainer Engine Cutoff. MECO means Main Engine Cutoff and is used or shuttle flights.

[mugaliens]

HUGE PDF, so I patiently did a lot of other work while it loaded).

You must be on dial-up as mine downloaded in less than 3 min.

Thanks for the resource!

The term "ullage" has some interesting history, from Middle English meaning to fill a partially empty casks. Essentially, it's the amount that a container lacks being full, meaning how much it would take to top it off.

Thus, the next time you get full service in Oregon, impress those sharp youngsters when they ask you how much you want by answering, "She's got an ullage of 7 gallons."

That'll learn 'em.