A question about RCS systems.

[Leszek]

Ok so, I understand that with the main tanks in a rocket stage, the fuel needs to be pooled at the bottom instead of floating around if you want to light the engine. Some rockets, (Titan, Soyuz) do this by lighting the next stage while the first is still burning. Some stages such as Saturn V stages use ullage motors, solid rocket fuel motors that keep the stage in a state of acceleration while the main engine lights. Ullage motors are solid and the fuel doesn't move so zero G is not an issue. Some stages such as Centaur (As far as I know anyway) use RCS to push the stage and function as ullage motors just before the stage lights.

So far so good.

How is the RCS fuel kept from floating around the tanks they are in? My guess is that inside the tank the fuel itself is in a bladder and that you can pressurize the inside the tank but outside the bladder so that as the fuel is used it can't float around. But I am not sure such a system works in the cold. My next guess is that if the line is large enough, the fuel in the line can be used to bootstrap the rest of the system as needed. If this were the case I would expect RCS systems would need to sometimes do and then undo pointless maneuvers to keep the lines pressurized. I haven't heard of any such maneuvers and it also seems wasteful.

So I Googled it, and then Youtubed it. Actually I did that in reverse order.... Anyway, my Googlefoo isn't good enough apparently, or I found the answer and just don't see it. (Even after looking at this text for example, I find that it is a lot of dry text for a simple answer that may not be in there: http://science.ksc.nasa.gov/shuttle/technology/sts-newsref/sts-rcs.html)

Then I figured there is probably someone here who knows. So, why don't RCS tanks need ullage motors or equivalents of their own?

_______Update________

So in the same text page I linked I found this: "surface-tension, propellant acquisition device"

Which doesn't sound like any of my two guesses. I have read more about it from the text but I am having a hard time visualizing the text. It would be nice if there was a video but "surface-tension, propellant acquisition device" and similar doesn't find it. Any ideas?

Edited April 16, 2015 by Leszek

[Bill Phil]

RCS is pressure fed to my knowledge. And typically hypergolic. Just open the valves and you get thrust.

[A_name]

RCS is pressure fed to my knowledge. And typically hypergolic. Just open the valves and you get thrust.

I have the same understanding. The pressurization makes the RCS work regardless of Zero G.

[DaveofDefeat]

Well in the case of the space shuttle, the fuel was pressurized using helium.

http://science.ksc.nasa.gov/shuttle/technology/sts-newsref/sts-rcs.html

"Pressurized helium gas is the propellent transferring agent."

http://history.nasa.gov/afj/aoh/aoh-v1-2-05-rcs.pdf

Edited April 16, 2015 by DaveofDefeat

[Kibble]

Yes, if the tanks are pressurized, you don't need ullage or a turbopump to get fuel to the rocket engine.

[architeuthis]

I have the same understanding. The pressurization makes the RCS work regardless of Zero G.

Not generally. Many RCS systems are cold-gas, in which case there is no liquid that requires settling force. But with a liquid monopropellant you need avoid stratified two-phase flow at all costs. One way of doing this is with positive expulsion devices, such as bladders, but more commonly capillary channels are used to wick propellant into a collection sump. These usually go by the acronym PMD, for 'propellant management devices'. Here is a schematic diagram of what I'm talking about:

http://patentimages.storage.googleapis.com/US20120241459A1/US20120241459A1-20120927-D00000.png

Edited April 16, 2015 by architeuthis

[Leszek]

Pressurization.

A non-answer.

If you are a blob of fuel floating in the middle of a tank, and you double the pressure of that tank, you are still in the middle of that tank. Triple the pressure, raise an order of magnitude, another... Eventually you will burst the tank but the blob of fuel will still be in the middle of it until then. Once the gas is at the intake the pressure helps push it through, but you would still need an ullage motor or equivalent to get it there. That is why I guessed a bladder above, the bladder would hold the fuel to the pickup while the pressure squeezes the outside of the bladder.

Architeuthis, I see. So the surface tension part is the mechanism that holds the fuel to the PMD?

Edited April 16, 2015 by Leszek

[DaveofDefeat]

It doesn't matter if the fuel is in the middle of the tank. If its pressurized and you open a valve, the helium, along with that fuel will follow the path of least resistance and go into that valve.

[sgt_flyer]

It's still at least partial vacuum outside a spacecraft the simple pressure difference will make whatever's inside the tank want to rush outside without the need of a pump

Afterwards, there's several variations - cold gases use pressurized gases (like the nitrogen RCS on falcon 9) those only use pressurized gases, which produce thrust when they are released and expands through a nozzle.

For hypergolic monopropellant, there's a first nozzle oriented so the fuel will hit a catalyst, triggering the fuel's exothermic decomposition. The resulting gas expansion exit through the main nozzle, producing thrust.

Then there's bipropellant hypergolic, where both the fuel & oxydizer are fed inside a combustion chamber at the same time. As they mix, they start burning - and resulting hot gases are released through the nozzle.

[Leszek]

In the case of pressure difference, how does one avoid a sputtering and inconsistent engine?

[xcorps]

The link daveofdefeat provided tells you.

[architeuthis]

From daveofdefeats link (underlines are mine):

Each tank is pressurized with helium, which expels the propellant into an internally mounted, surface-tension, propellant acquisition device that acquires and delivers the propellant to the RCS thrusters on demand. The propellant acquisition device is required because of the orbiter's orientation during boost, on orbit, and during entry and because of the omnidirectional acceleration spectrum, which ranges from very high during boost, entry or abort to very low during orbital operation. The forward RCS propellant tanks have propellant acquisition devices designed to operate primarily in a low-gravity environment, whereas the aft RCS propellant tanks are designed to operate in both high and low gravity, ensuring propellant and pressurant separation during tank operation.

[OhioBob]

Pressurization.

A non-answer.

If you are a blob of fuel floating in the middle of a tank, and you double the pressure of that tank, you are still in the middle of that tank. Triple the pressure, raise an order of magnitude, another... Eventually you will burst the tank but the blob of fuel will still be in the middle of it until then. Once the gas is at the intake the pressure helps push it through, but you would still need an ullage motor or equivalent to get it there. That is why I guessed a bladder above, the bladder would hold the fuel to the pickup while the pressure squeezes the outside of the bladder.

It's my understanding that they use a pressurized bladder system; however, I'm not exactly sure how it is configured. You surmise that the fuel is inside the bladder with the pressure squeezing the outside of it. I envision a system that would have a gas pressurized bladder expanding into the tank as the liquid is drawn down.

(edit)

Here you go...

http://www.space1.com/Artifacts/Apollo_Artifacts/SM_RCS_Tank/sm_rcs_tank.html

Quote: "Helium pressurizing gas pushed against flexible bladders in the fuel and oxidizer tanks to force the weightless propellants out of the tanks."

The link also included a diagram of the tank construction. As I guessed, the gas inflates the bladder and it expands into the tank.

Edited April 17, 2015 by OhioBob

[Reactordrone]

In addition to pressurised bladder tanks there's also surface tension tanks, that use the surface tension of the fuel between the tank wall and a conical collector to keep fuel at the bottom of the tank, and centrifuge tanks that use spin.

http://www.newscientist.com/article/dn11254-new-spacecraft-fuel-gauge-may-reduce-space-junk.html#.VTCBMpPQOVI

[Leszek]

Thanks guys. I have read the documents and the posts and I understand.

Reading back, it might have sounded like I was a little ungrateful in previous posts. This is not the case, I was just thinking too much about the problem and not enough about how my posts might be interpreted. I do that sometimes.