Ullage Motors for Pressure-fed Engines

[palioxis1248]

Do pressure-fed engines require ullage? Because to the best of my knowledge most RCS systems are pressure-fed, and can fire without ullage, yet shouldn't they face the same ullage problems as turbopump-fed engines?

[cantab]

I don't know exactly how different rockets do things, but one option would be a bladder tank, with the propellant inside a flexible container and the pressurising gas outside it.

[cpast]

I'd imagine it's because RCS systems are hypergolic. In non-hypergolic systems (like lots of lower rocket stages), the combustion has to be kept going, and so fuel flow needs to be pretty consistent. On the other hand, hypergolics ignite spontaneously when the two propellants come into contact (or when the one propellant comes in contact with a catalyst, depending on the system), so it's not as big a deal if non-fuel enters the combustion chamber.

[palioxis1248]

I have a few problems with that explanation. I imagine that no matter what kind of fuel you're using, ingesting gas bubbles is going to play merry havoc on combustion stability, which could very well lead to problems even with hypergolic fuels. Wouldn't want your R4D thrusters undergoing pogo oscillations, would you?

Of course that's just my imagination. I might be wrong.

[Nuke]

you usually have the fuel, say hydrazine, in a flexible bladder inside of a tank, which is pressurized with an inert gas. the pressurant never touches the fuel. all you have to do is activate a solenoid valve between the tank and the engine, and because of the pressure differential fuel will get squeezed out of the badder through the piping and into the combustion chamber. it hits the catalyst in there and goes boom. you want to turn it off, close the valve. its stupid simple.

[K^2]

Yup. As Nuke says, bladders. The advantage of non-cryogenic fuels is that you can keep them in a bladder without adding much weight. If RCS required ullage, they'd be quite useless, as you could just use ullage engine for maneuvering.

[sgt_flyer]

For the lem, it's RCS fuel tanks used bladders, but the main pressure fed engines fuel tanks did not. (Bladders are not easily upscalable)

They used their RCS as ullage to settle the fuel in the main tanks before sending the helium.

[cpast]

Ah, thanks for the better explanations.

Is there a reason bladders wouldn't work for non-cryogenic non-hypergolic fuels? Or do those not really exist, and all non-hypergolics use liquid oxygen?

[shynung]

Ah, thanks for the better explanations.

Is there a reason bladders wouldn't work for non-cryogenic non-hypergolic fuels? Or do those not really exist, and all non-hypergolics use liquid oxygen?

You could make a similar system for non-hypergols, but these need an external igniter. Simply opening the valve wouldn't fire the engine; usually, a small amount of hypergol is injected into the chamber along with the first few drops of fuel-oxidizer mix to ignite it. Problem is, bladders are not easily scalable, so there is an upper limit to the biggest pressure-bladder tanks that can be built. Though, there are non-hypergol engines designed to run off pressurized tankage systems, usually used as upper stages. One example would be SpaceX's Kestrel engine.

Non-hypergolic non-cryogenic fuel/oxidizer mixes are rare. Most use LOX because of its proven performance. There are a few rockets using N₂O (which are non-cryogenic, and non-hypergolic) as oxidizer, mostly for amateur use for their non-toxic properties.

[palioxis1248]

Thanks for all the great answers, guys!

[rdfox]

For the record, the Apollo LM *did* demonstrate the need for good ullage in hypergolic engines. It used hypergolic engines throughout (for reliability), and during the first manned test flight (Apollo 9), the initial burn of the throttleable descent engine was started at 20% thrust. The crew reported a very rough ride and shut down the engine, but on a hunch, they tried again at 40% thrust for startup and found that it was very smooth, and they could reduce the throttle to the minimum setting without gaining roughness after starting at the higher thrust setting. Their verdict was that, most likely, the lower thrust setting at startup didn't provide sufficient ullage thrust to prevent the engine from swallowing some helium, resulting in pogo-style oscillations and a rough ride. From then on, the standard procedure was to start the LM DE at 40% thrust or higher, then reduce thrust to the desired level, since this would stratify the propellants from the pressurization helium enough to provide a smooth burn. (The RCS ullage, apparently, was just enough to get the engine started, but not enough to ensure there was no helium swallowed by the engine. This wasn't an issue for the CSM's SPS engine or the LM Ascent Engine, since both were non-throttleable designs that were sure to generate sufficient ullage for stratification, and, in all but the landing abort contingency mode, the LMAE had the benefit of lunar gravity to provide zero-charge ullage...)

[shynung]

Seems like the LM descent engines didn't use bladder tanks. Was it not available back then?

[Nuke]

they may have been omitted to save weight or reduce complexity. neither of which you want on a mission critical engine.

Edited June 13, 2014 by Nuke

[xcorps]

For the record, the Apollo LM *did* demonstrate ...)

Could you provide a source please? Appreciated.

[sgt_flyer]

The LEMs hypergolic tanks were too big for easy bladder integration. (Bladders don't scale well) - the Lem's RCS monopropellant used a bladder system - basically, the bladders were only useful to them from a 0g start for the RCS. They then used those RCS to provide enough ullage to use the non bladder tanks.

[rdfox]

Could you provide a source please? Appreciated.

As it turns out, I wasn't quite right on the exact sequence of events, as I initially got this from "From the Earth to the Moon," but I've since found details in the Apollo 9 Mission Report, found here: http://www.hq.nasa.gov/alsj/a410/A09_MissionReport.pdf

It's briefly mentioned on page 5-8, as rough operation at 27% throttle and "throttle changes were terminated until smooth operation was achieved." It's also mentioned on page 9-60 as one of the anomalies in descent propulsion system performance: "c. The crew experienced a rough engine condition while throttling from the 10- to the 37-percent setting during the second firing", and again a bit later on the page and onto page 9-61: "Engine roughness was reported by the crew when the engine was throttled from the 10 to the 37 percent setting during the second descent engine system firing. The onset of roughness occurred as the throttle setting reached approximately 27 percent, at which time the setting was held constant until the roughness ceased. This roughness is typical of that experienced with helium ingestion into the combustion chamber. The roughness lasted approximately 2.5 seconds (see figure 9.8-2) and the remaining portion of the second firing and all of the third firing appeared nominal." A detailed summary of the anomaly is found in section 17.2.11, starting halfway down page 17-35 and ending about a third of the way down page 17-36. Apparently, testing showed that there was no detrimental effect from suffering helium ingestion, and therefore, no corrective action was taken. I'm sure some discussion of it can also be found in the mission transcripts, but I am *not* gonna go trawling through THOSE right now...

[xcorps]

Thanks very much, I'll be doing some reading woo-hoo :-)

By the way, if you have read Apollo by Charles Murray and Catherine Bly Cox how you compare them?

Edited June 15, 2014 by xcorps