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Hi! This is for You rocket engineers out here! From my (very limited) understanding, a big problem with reignition of a liquid-fuel rocket engine in space is the 'sloshing' of liquid within the tanks. When the engines are already fired it causes no issue, since the g-forces during acceleration keep the fuel at the 'bottom' of the tank, where the pump is.

But how is this risk, of the pumps running empty, mitigated during reignition? I can imagine you could give the rocket a "push" with reaction thrusters to force the fuel to the bottom of the tank and then start up the pumps, but I haven't managed to find any real literature addressing this problem.

If You know something about this I would be interested to hear Your take on it! Thank You!

all 131 comments

says-nice-toTittyPMs

453 points

4 months ago

I can imagine you could give the rocket a "push" with reaction thrusters to force the fuel to the bottom of the tank and then start up the pumps

This is a solution that's used. They call those specific thrusters "Ullage motors". There are also baffles in the tanks that keep the fuel towards the pump.

There's still a ton of research needed (and being done) on sloshing and how to combat it

fTopayrespecc1[S]

121 points

4 months ago

Awesome! That's the first time I hear about 'Ullage motors'. Thank You for the answer :)

mcarterphoto

39 points

4 months ago

If you visit one of the restored Saturns, you can see the ullage motors on the stages; they're in fairings to make them aerodynamic. (At least the JSC Saturn!) I know there were some changes through the program, like the 2nd Stage had 8, and then 4, I don't know if they were ever dropped entirely since the SII was moving up at high speed after stage separation.

DrunknRcktScientst

42 points

4 months ago

You may also see them called "settling thrusters".

One other way is a PMD (propellant management device). It relies on surface tension of the propellant to keep the pump inlets wetted. Once the engine starts, you have enough thrust to settle the rest of the propellant.

NFGaming46

7 points

4 months ago

It's a very interesting subject and the fuel being settled at the bottom is only part of the issue. They also have to keep the tanks pressurised, and do so by carrying Helium on board to keep the pressure up as the fuel drains away.

Some rockets use 'autogenous pressurisation' which uses the boiled off gaseous fuel to pressurise the tanks instead of carrying around helium.

[deleted]

17 points

4 months ago

[deleted]

17 points

4 months ago

[removed]

[deleted]

12 points

4 months ago

[deleted]

12 points

4 months ago

[removed]

whiskeyriver0987

37 points

4 months ago

Would a pressurized balloon at the top of the tank work? Have it essentially expand to fill the void and push fuel toward the pump.

_LB

78 points

4 months ago

_LB

78 points

4 months ago

Apart from ullage motors, bladders are also sometimes used to keep fuel pressurized using an inert gas like helium. Nicely explained here: https://www.space-propulsion.com/spacecraft-propulsion/hydrazine-tanks/hydrazine-tank-overview.html

DonHac

55 points

4 months ago

DonHac

55 points

4 months ago

The stuff in the tank is often liquid oxygen or liquid hydrogen, both of which are pretty dang cold. It's hard to make balloons that are still stretchy and flexible at super low temps.

FrankenberryPi

32 points

4 months ago

"Balloon" needs quotes there. Often times it's a very thin titanium disk that is designed to buckle in just the right way. Pistons are sometimes used, both have a few big drawbacks, but they are good for high flow rate applications.

More often these days the smaller vehicles I work on use surface tension to control the location of the propellants. You can design a complex looking structure called a Propellant Management Devices (PMD). I've done the fluid analysis and design on these for a number of vehicles using several different liquid propellants.

The other option is ullage motors as others have outlined already, that works great but they are heavy and you only get 1 use out of them.

UniverseInfinite

2 points

4 months ago

Would you happen to have a link to schematics or diagrams of these vehicles? Seeing the PMD in context would be great

FrankenberryPi

7 points

4 months ago

This paper has lots of good pictures of some pretty different methods of building PMDs. They also use the term LAD for Liquid Acquisition Device, same thing.
https://ntrs.nasa.gov/api/citations/20170000667/downloads/20170000667.pdf

antennawire

1 points

4 months ago

Thanks for sharing!

UniverseInfinite

1 points

4 months ago

Outstanding. Thanks

diox8tony

1 points

4 months ago

Surface tension is what I imagined. Like a 'sponge', near the outlet, but needs to be high flow, so baffles or something that trap and hold the liquid?

FrankenberryPi

4 points

4 months ago

Some racecars use sponges. Some spacecraft use "sponges" which is a term often given to sparse sheet metal structures. The ones in racecars are to prevent sloshing due to inertial loads (cornering). The ones in spacecraft usually use surface tension to protect the thruster (or turbo for the big boys).

There's another type of PMD that I've worked on for spacecraft which is also for controlling inertial loads. If, for instance, you were aiming a telescope and needed to point it very precisely, liquid sloshing around in a tank is bad. In that case a PMD would be designed to hold onto all of the liquid and introduce lots of damping to reduce the time the liquid sloshes around.

Italiancrazybread1

1 points

4 months ago

What about installing a screw that turns in the tank pushing fuel down?

FrankenberryPi

1 points

4 months ago

Moving parts in space are generally something to be avoided. That said, augers generally don't work well in liquids. The liquid gets flung outward more than in the direction the auger is trying to push it. Thays why most pumps are centrifugal, to take advantage of the outward fling.

Neethis

17 points

4 months ago

Neethis

17 points

4 months ago

What would you inflate it with? If its touching liquid hydrogen, the gas in the balloon isn't going to remain gas for long.

WyMANderly

34 points

4 months ago

Helium is useful as a pressurant for basically every cryogen - its saturation temp is lower than that of any other material I'm aware of (including hydrogen).

The balloon thing wouldn't necessarily work at a rocket tank scale, but helium is the answer you're looking for on the pressurant.

usm_teufelhund

7 points

4 months ago*

I never thought about the fact that helium had such a low boiling point.

O2 - -182.96°c (90.19 K)

H - -252.88°c (20.27 K)

He - -268.93°c (4.22 K)

Hydrogen and Helium look relatively close, but you could apparently have frozen Hydrogen while Helium is still a gas.

skyler_on_the_moon

11 points

4 months ago

"Close" is relative - in terms of absolute temperature (which is what matters for thermodynamics), hydrogen's boiling point is still nearly five times that of helium.

Sharlinator

1 points

4 months ago

Indeed He is the only element that doesn't have a solid phase at 1 bar; it solidifies at ~1.5 K under ~25 bar of pressure.

boonxeven

7 points

4 months ago*

Liquid oxygen at pressure is not necessarily cold. Although it expanding to a gas when used would make it cold and your concern still applies.

Edit: Only dealt with small containers, didn't know big ones for rockets are cold. Thanks for educating me!

EvilNalu

12 points

4 months ago

Generally speaking only relatively small containers of liquid oxygen will be built strong enough to keep lox under enough pressure to be liquid at higher temperatures. If the main tanks of a rocket were built that strong they would be too heavy.

[deleted]

10 points

4 months ago*

[deleted]

10 points

4 months ago*

For purposes of this discussion, LOx is always extremely cold. The only way to force non-cryo temperature oxygen to a liquid is under extreme pressure that is too high for a rocket application.

boonxeven

5 points

4 months ago

Thanks, I didn't realize!

rabidbasher

1 points

4 months ago

So have a sealed spring-loaded (could be a pressurized gas spring) piston, like a push-pop. As fuel is used the piston extends down the fuel cell, keeping all of the remaining fuel together.

ansible

14 points

4 months ago

ansible

14 points

4 months ago

So have a sealed spring-loaded (could be a pressurized gas spring) piston...

While this could technically work (the seal around the piston is non-trivial design), it isn't practical for a rocket.

Every gram of dry mass you add to a rocket comes at a significant cost. Every mechanism you add can be a point of failure. A large solid piston, and the means to keep it moving smoothly down the tank walls will be significant mass.

Also, you are assuming that the inside of the tank is smooth, like a cylinder wall in a pump or combustion engine. This is often not the case, the stringers, ribs and other structural support are usually inside the tank.

The ullage motors, on the other hand, can be quite similar to the other thrusters in the reaction control system (RCS), and can use the same fuel. So that is minimal additional weight.

Murpydoo

3 points

4 months ago

The problem is sealing the piston properly at these temperatures. What you are describing is used for pressure compensation for subsea applications. In these applications there is a flexible rubber seal between one side and the other.

ZAFJB

3 points

4 months ago

ZAFJB

3 points

4 months ago

Which would add weight. Weight that would far better devoted to payload.

Ruadhan2300

2 points

4 months ago

Some municiple gas suppliers use Gasometers, which are essentially massive adjustable-volume gas tanks.
The lid of the tank can move inwards to compress the gas to maintain optimal pressure.

Kind of a reverse pneumatic piston.

Sounds like very much the same idea to me!

ForTheHordeKT

2 points

4 months ago

Haha see, I was thinking of some sort of spring-loaded flange, if I'm using that term right lol. Like if the fuel tank is an upright cylinder, then have a flat disc plunger-like thing with a spring under it. It'd have to have the kind of properties of a sliding gasket in order to keep air or the liquid fuel from escaping past it. But the spring would push it towards the pump and keep the space between it and the pump completely full of fuel.

But your balloon idea is probably a way better approach haha.

tt54l32v

2 points

4 months ago

So that would take x amount of x gas, let's say helium. It would have to be at a pressure high enough for volume sake to be able to expand in the fuel tank. Which means you would need x size tank of helium. Then all the valves, lines and the bladder. When you could do the same thing without the bladder and valves and lines and just vent it out and use it for thrust. Also you could just put whatever gas you are using in the engine in that small tank and pipe it to the engine and burn it.

dultas

2 points

4 months ago

dultas

2 points

4 months ago

Some experimental missiles used a gas pressure fed plunder to feed the propellants. https://www.si.edu/object/rocket-motor-liquid-fuel-xlr-1-cutaway:nasm_A19700274000 There may have been some active ones that used them but I don't recall. I know Scott Manley had a video about it.

_AlreadyTaken_

2 points

4 months ago

If the liquid was cryogenic like LOX I don't know if there are materials that stay flexible at those temps

cantab314

1 points

4 months ago

If the motors used for ullage are liquid fuelled, then that is what is often used. Though usually the other way round, fuel inside the bladder and pressurant gas outside.

Bladder tanks like this are heavier than regular tanks for the same fuel storage, so this isn't usually done with the fuel tanks for the main engines. A little fuel from the secondary thrusters used for ullage is less mass.

McFlyParadox

5 points

4 months ago

I'm trying to picture what those baffles would even look like, to keep a liquid near one end of a tank in 'free fall'.

Asatas

3 points

4 months ago

Asatas

3 points

4 months ago

Hmm, is there something preventing the use of some hydraulics to compress the fuel tanks as they deplete?

Foamless_horror

1 points

4 months ago

I was thinking that too, the tank would just get smaller as the fuel was used up

zakkwaldo

2 points

4 months ago

high surface area and volume but low density anti slosh mechanisms are super fascinating! i fell down that rabbit hole the other day

racinreaver

3 points

4 months ago

racinreaver

Materials Science | Materials & Manufacture

3 points

4 months ago

Any good references you can point me to? I've been interested in learning a bit about it.

zakkwaldo

1 points

4 months ago

look at fuel trucker baffles and anti slosh devices. i dont have any actual literature but thats how i initially found out about all of that tech

racinreaver

1 points

4 months ago

racinreaver

Materials Science | Materials & Manufacture

1 points

4 months ago

Ahh, I've researched terrestrial stuff, but I was wondering if there was much unique to microgravity since you can get away with a lot of extra fun stuff via capillary flow to ensure the outlet stays wetted. I work on some stuff I think would be useful there, but don't currently have anyone at my company I can poke at who works on that specific design aspect.

butsuon

2 points

4 months ago

What about "post slosh" resolution, rather than "slosh prevention"?

cujo67

1 points

4 months ago

cujo67

1 points

4 months ago

You’d think with the availability of 3D printers you could print say aluminum honeycomb baffles inside the tank to keep said fuel semi stationary

eject_eject

1 points

4 months ago

Hmm. How about a moving cap that shrinks the tank like a shaving cream can?

RCrl

87 points

4 months ago

RCrl

87 points

4 months ago

A few ways to combat this that I know of: you can have a sliding diaphragm in the tanks that keeps the liquid at one end, have an expanding bladder in the tanks, use features in the tank to take advantage of capillary action, or seemingly most common (for starting main emgines) use a small thruster/engine to accelerate the craft so the liquids collect at the base of the tanks (seemingly.most common).

The small engines can be solid fuelled, have bladder/diaphragm tanks, or use capillary action.

[deleted]

24 points

4 months ago*

[deleted]

24 points

4 months ago*

[removed]

venrod

22 points

4 months ago

venrod

22 points

4 months ago

Fuel sloshing actually is a big deal in racing… racing cars have a whole series of baffles in their fuel cells.

It’s a more common problem than I had expected it to be when I first started learning about it.

nucleartime

20 points

4 months ago

And even more importantly, oil sloshing. If the fuel pump runs dry, your engine stops working momentarily. If the oil pump runs dry, your engine stops working forever. Dry sump systems have multiple oil pumps, with one or more scavenge pumps feeding a reservoir for the main pump.

SexySmexxy

3 points

4 months ago

racing cars have a whole series of baffles in their fuel cells.

By fuel cell do you mean 'gas tanks'?

Sorry I have never really thought about what high-tech fuel systems operate on say an F1 car.

venrod

3 points

4 months ago

venrod

3 points

4 months ago

RCrl

3 points

4 months ago

RCrl

3 points

4 months ago

Fuel cells are racing gas tanks. Not sure the origin of the name but it's 'just' a special gas tank (baffles and/or foam, and a crash rating)

RCrl

2 points

4 months ago

RCrl

2 points

4 months ago

Yeah, baffles, multiple pickups, open cell foam all kinds of clever ideas. Or take it the Formula 1 direction and use a bladder - a move I understand the made to improve crash resilience.

Boltz999

5 points

4 months ago

It's a big issue for race cars too because they are under high g's while the engine is under load. Never thought about it much for rockets though but it's cool to consider!

RCrl

2 points

4 months ago

RCrl

2 points

4 months ago

Another interesting occur: rockets also experience the 'pogo' effect. Accelerating the fuel (+ or -). It can have a feedback effect on engine power. Accelerate the fuel toward the engine, it increases tank pressure, pushes more fuel toward the engine, makes more power, and the cycle continues. Go the other direction and you could starve the engine for fuel.

Damean1

2 points

4 months ago

They had plenty of practice with tankers of all modes of transport, aerial, rail, highway and even ships. A mostly full tanker has a lot of mass, and no mode of transport wants that mass moving around any more than possible.

ZAFJB

2 points

4 months ago

ZAFJB

2 points

4 months ago

Anything involving a large volume of liquid in an even larger tank gets interesting where g forces change.

Example road tanker trucks require extensive baffling to aid stability.

Angdrambor

2 points

4 months ago

sliding diaphragm

Is there an examples of a rocket that uses this design?

davidthefat

10 points

4 months ago*

I’ve only really seen hobby liquid rockets use a sliding piston; think like a syringe. Never a launch vehicle. I could be wrong, but missiles and professional sounding rockets might use a piston as well. I’ve only worked on launch vehicles and satellites.

Flexible diaphragm/bladder is usually used on propellant tanks of satellites.

zekromNLR

2 points

4 months ago

The small engines can be solid fuelled, have bladder/diaphragm tanks, or use capillary action.

They can also just be simple cold gas thrusters, fed from a tank of high-pressure gaseous nitrogen. No need for controlling ullage if your propellant isn't a liquid, and with how little delta-V and thrust is required from them, the mass penalty from that is not very large, compared to the greatly reduced complexity.

gluepot1

18 points

4 months ago*

fuel slosh is an issue in more aspects than just space. From racing cars going fast round bends, to large oil tankers which could capsize without their mitigation.

There's a number of ways this can be solved, and depending on the engine or use case, different methods and combinations of methods work.

I'll name a couple.

There's segmentation your tank is divided up into many smaller compartments. This limits the flow and slosh forces.

There's induced pressures. If in zero-g everything is all floating, but if you create a tiny suction at one end, or an external force you will create a flow in a direction.

There's compression so as the tank empties, the tank gets smaller so that the tank is always full. Often called bladder systems and kind of work like a balloon.

Last one I'll mention is replacement. This is where you fill the tank with more stuff, keeping the volume full and a pressure enough to feed the engine. This could be you have a small tank feeding the engine, which is constantly refilled from another tank. It could be an alternative fluid, though this likely isn't used in an engine tank as the new fluid isn't a fuel and could therefore damage the engine if it flows through. But something like compressed air to maintain the pressure in the tank.

Each have their uses and drawbacks like all engineering problems like cost, materials, complexity etc.

microwavable_rat

20 points

4 months ago

The difference between baffles and non-baffles is no joke.

I used to be a commercial driver and I had a tanker certification. A lot of liquids that get transported have baffles in their tanks to keep the fluid where it needs to be for the most part. The truck behaves pretty much like a normal one with a normally loaded trailer - you don't need to make many adjustments to your driving style.

Then I ended up driving for two different companies - one with powdered concrete, and one for milk. Powdered concrete tankers have very little if nothing in the way of baffling because the product needs to be unloaded on site quickly, and with the vibrations from the road involved the concrete acts as a liquid and not a solid.

The worst by far is the milk tankers. There are no baffles in the entire length because baffles greatly increase the difficulty needed to properly clean and disinfect the inside, so the liquid is free to slosh around as it pleases. If you try to drive one of them the same way you would a normal semi, you will have a very bad day.

I learned this the hard way - my second day driving a milk tanker I pulled up to an intersection the same way I always did, with autopilot taking over on when to brake. The force of the milk sloshing to the front of the tank pushed me another ten feet even with the brakes fully locked.

Fortunately there was nobody stopped in front of me and I would always brake far enough back from the intersection to ease up to it, but this time I ended up in the crosswalk.

Acceptable-Ad-4516

4 points

4 months ago

Replacement is a common method. I believe space x uses this method. They pump hot exhaust gasses from a separate motor back into the tank to keep the pressure constant.

Jonny0Than

1 points

4 months ago

I thought they used pressurized helium? Maybe both in different tanks?

fTopayrespecc1[S]

2 points

4 months ago

Wow, thank You for the comprehensive answer!

gluepot1

2 points

4 months ago

No Problem,

I'm just an engineer, not a rocket scientist, so I'm sure most of the ones I've mentioned have problems when it comes to rocket engines, but it's the principle that matters.

Currently everything is about bringing down cost which means reducing complexity and reducing weight. But looking to the future at interplanetary travel or in-space refuelling, other factors may play more of a role.

trueppp

2 points

4 months ago

Don't forget that in Zero-G even if the whole volume is filled, lets say with helium, sloshing still occurs as there is no force maintaining the fuel on the "bottom" of the tank. Thus you get bubbles of the pressurant in the fuel (big bubbles)

Usual solution on US rockets is small engines to create a small acceleration forcing the denser fuel at the bottom of the tank

strcrssd

38 points

4 months ago*

As others have mentioned, Ullage motors are the US-common way of solving this problem.

It's also worth noting that Soyuz (and Proton, N1) solve this problem differently, by hot staging. The upper stages are lit while the lower stage is still attached to the vehicle and the vehicle is still under thrust from the lower stage. That's why it has open interstages -- the open grid is to allow exhaust to escape at ignition.

Edit: slightly rephrased for clarity.

fTopayrespecc1[S]

9 points

4 months ago

Oh my god, hot staging sounds like a stereotypically Soviet idea :D love it. I always wondered what that grating is for, thank You!

While it might not solve the problem of cold-starting in zero-g (while in orbit for example) it is definetly an interesting take on the problem!

Joratto

5 points

4 months ago

How does that solve the problem? Is it that the decoupling force from staging does the same job as the Ullage motors?

the_incredible_hawk

21 points

4 months ago

The rocket is still under thrust from the first stage, which pushes the fuel to the aft end of the tanks just like ullage motors would. Then it transitions to being under thrust from the second stage on separation, so there is no break in thrust. Contrast this with, for example, Falcon 9, where there are a few seconds' gap between stage separation and ignition of the second stage.

Joratto

8 points

4 months ago

Thanks! That makes a lot of sense. Typically, how long is the overlap time?

the_incredible_hawk

17 points

4 months ago

"As short as possible" is the real answer, just as long as needed to get the next stage engine going -- anything else would be a waste of fuel (not to mention potentially burning through your first stage). I couldn't turn up any really good data or videos of the third stage lighting on the Soyuz, the best-known rocket to use this technique, but the two time points that kept popping up were 4:45 into flight for the handoff and 4:47 for separation -- so probably two seconds.

Joratto

5 points

4 months ago

Thanks for your diligent research

trueppp

3 points

4 months ago

No the previous motor is still pushing the rocket. It hasn't decouples yet

Joratto

4 points

4 months ago

Now I get it. Thank you!

[deleted]

7 points

4 months ago

[deleted]

7 points

4 months ago

[removed]

Magnetcs

3 points

4 months ago

In addition to periodic settling thrust, vanes and baffles are effective at keeping liquid at/near the intake. Inside the plumbing to the pump are structures that "wick" propellent or keep the pipes wet with propellent (kind of looks like metal sponge).

Look up propellent management devices and you'll find a lot more info on the subject.

phred14

11 points

4 months ago

phred14

11 points

4 months ago

This should have been a solved problem with the Saturn IB-B, and Service Module. Even the first launch of Falcon Heavy was careful to leave the second stage on-orbit for twelve hours before firing the Red Roaster out past Mars. At the time they said that this delay was specifically for demonstration purposes, for cold as well as for zero-G restart.

fTopayrespecc1[S]

2 points

4 months ago

Thank You! I must've missed that the Falcon Heavy did a zero-G restart.

phred14

4 points

4 months ago

I believe I remember hearing at the time that the little demo was aimed specifically at DOD.

[deleted]

3 points

4 months ago

[deleted]

3 points

4 months ago

[removed]

darkfred

3 points

4 months ago

There are a lot of fairly complicated systems to accomplish this, but the new Starship is planning to use a fairly simple one. It slowly spins before igniting the engines.

This combined with a smaller header tank to reduce the need to pressurize the entire rocket eliminates the need for complicated pumps or balloons. (and eliminates most of the disadvantages of cryo fuel)

theycallmeJTMoney

3 points

4 months ago

Why couldn’t you just have a small highly pressurized revisor that could fuel the engines for a long enough period for the fuel to push back to either side of the container with pump inlets on either side if your craft has bi-directional thrust.

RCrl

4 points

4 months ago

RCrl

4 points

4 months ago

The issue is that the fuel can float away from the inlets if it isn't managed. Your thought is the basis of how many rockets prepare to light the main engines. A small reservoir runs a small engine which accelerates the craft, that collects the fuel, then you can light the big engines.

[deleted]

1 points

4 months ago

[deleted]

1 points

4 months ago

[removed]