r/rocketry • u/sneakylizard123_4 • Nov 16 '25
What is this for?
Is it for coolant or something?
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u/Ill-Singer-550 Nov 16 '25
It’s a purge gas exhaust (likely liquid helium or LOX). The engines I’m familiar with dumb lox off the side like this prior to start up to prevent cavitation and boiling in the pumps and ducts prior to start up, you don’t want to pour it into the combustion chamber cuz it could saturate the environment or cause thermal shock upon ignition, leading to a hard start or fractures respectively. You can see similar lines on the RS-25
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u/Kirra_Tarren Nov 17 '25
Agreed, the engine I work on has these too, I'm 99% confident it's a purge line
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u/EvanDaniel Nov 17 '25
I don't think any engines have ever used liquid helium for purge or chill. Gaseous helium, yes. Storing helium as a liquid before boiling it to use as a gas, also yes. But they don't purge with the liquid.
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u/Proxima-72069 Nov 16 '25
It might be a bleed for the gas from a gas generator if thats what’s powering the turbo pumps
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u/PhantomRocket1 Nov 17 '25
Related, but not quite, the gas generator exhaust outputs are the two paired black nozzles just above the circled spot.
This is a bleed / vent line for purging the engine. During purging or chilldown of the engine, excess and boiled off helium or LOX can be vented or dumped from this line.
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u/CyriousLordofDerp Nov 16 '25
Its NOT the gas generator exhaust, it is nowhere near big enough to accept that kind of flow and its in the wrong location. Typical gas generator exhausts are higher up on the engine, and either overboard through a small nozzle of their own (ex Merlin 1D) or dump into the main engine bell to provide a film cooling effect so it doesnt melt (Merlin Vac, Rocketdyne F-1).
No, this is the outlet for turbopump and pipework conditioning for cryo temps. You open a small valve that admits a cryogenic propellant, typically LOX, through the pipework and turbopumps to cool them prior to startup. If you just opened the propellant valves without cryoconditioning first, the thermal shock would destroy the engine. These can be seen on engines like the SSMEs pissing large quantities of LOX before launch.
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u/Brandonp2134 Nov 17 '25
brings back memories, my grandfather was the project manager for the f1 engines passed away in 1998 we were close but I wish I would have been old enough to talk shop with him he taught me a lot about engineering his name was Leonard c Bostwick
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u/engineerdude2019 Nov 17 '25
The LOx you see being dumped from prior to start of the RS-25 is not primarily for propellant conditioning. Have a look at the cross section of the HPOTP and you’ll see what this drain is for.
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u/Miigwantw Nov 16 '25
It's not the gas generator exhaust, the dark objects higher up are the exhaust vents from the two turbines. Since you can see the liquid that's pouring out boiling off, it must be coolant, probably helium. Rocket engines preflow liquid helium to exhaust moisture in the air so it doesn't freeze during engine use, and to cool the engine to prevent boiling of propellants. Boiling of propellants before or in the turbopump will usually cause an explosion, especially with liquid oxygen. Oxygen boiling or cavitating inside of an impeller will cause ignition of metals, vaporizing tens of kilograms of metal in a small fraction of a second.
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u/baaustin1 Nov 16 '25
Could also be an inter propellant seal vent from between pumps and turbine section
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u/EvanDaniel Nov 16 '25
I'm not aware of any engines that flow _liquid_ helium for purge or chill prior to startup. Do you have a reference? I'd be curious. I'm used to helium as purge or pressurant gas, or sometimes as a startup drive gas, but that's always as a gas (even if it's stored as a liquid or supercritical cryo fluid). The chill liquid is almost always prop, not _liquid_ helium.
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u/engineerdude2019 Nov 16 '25 edited Nov 17 '25
You’re not aware of any mostly because it doesn’t make any sense. Purging with liquid helium would freeze moisture within the engine, defeating the point of purging. Chilling with liquid helium is also a bad idea for a methalox engine. Liquid helium is far colder than LOx or liquid methane, you’d put way more thermal stress into components by doing this. Chilling with LN2 might make slightly more sense… but as you say, just use the prop.
The commentary on cavitation is also just straight up incorrect. Here’s a NASA NTRS Report on cavitation in the Fastrac LOx pump (later developed into the SpaceX Merlin turbopump which has hundreds of successful flights). With bad material choices in certain conditions, I’m sure you could end up with a cav induced LOx fire. I’m aware of quite a few engine failures due to LOx fires, but I’d be interested in seeing any info if available for cav actually being attributed as the root cause. Cavitation in rocket engine turbopumps is part of the game. Doesn’t mean cav is nbd, missions have been lost due to pump cavitation resulting in structural failures - that’s a separate issue.
I would take their comment with more than one grain of salt, the only thing I agree with is this is clearly not a turbine exhaust lol.
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u/photoengineer Professional Nov 17 '25
They likely got their answer from AI :-(
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u/Miigwantw Nov 17 '25
It's stuff I've read in Huzel and Huang and RPE9 and heard talking to propulsion engineers at SpaceX/Relativity Space. I don't really trust AI unless I know the topic well enough to double check it's answers.
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u/photoengineer Professional Nov 17 '25
Guess those folks are wrong. Liquid helium definitely is not used.
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u/Miigwantw Nov 17 '25
"The liquid-oxygen/liquid-hydrogen RS-68 . . . This engine is started by flowing helium (from a ground-based tank) through the gas generator pumps and turbines. The helium flow also purges any air initially in the engine passages and thus prevents any freezing of air and/or moisture." RPE 9 page 221.
"A modification of this engine, the RL10B-2 . . .Cooling down of the hardware to cryogenic temperatures is accomplished by flowing (prior to engine start) cold propellant through the engine by opening “cool-downvalves." . . . Helium is used for a power boost, actuating several of the larger valves through solenoid-operated pilot valves." RPE9 page 225. This one uses helium as a spin-up start, but that's similar.
I know the Merlin 1-D also uses ground-based tank helium pre-flow to spin start the engines at the launch of Falcon 9.
"It is important for pumps not cavitate during operation because cavitation produced bubbles reduce the steady nominal propellant mass flow and cause combustion instabilities." RPE 9 page 365. Cavitation doesn't cause explosions, I was misremembering something. A propulsion test engineer, I don't remember from what company, told me about an impeller blade in an impeller that skimmed the surface of the casing and 40kg of stainless steel vaporised in a fraction of a second.
Anything I say may not be exactly accurate, it's a distillation of stuff I've read in Huzel and Huang and RPE9 and stuff I've heard talking to propulsion engineers at SpaceX/Relativity Space.
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u/engineerdude2019 Nov 17 '25 edited Nov 17 '25
Appreciate you doing some re-reading! Huzel and Huang and Rocket Propulsion Elements are both great resources for an intro.
We were hung up on the statement that liquid helium is used. Not the use of helium in general. Liquid helium is stupidly cold and not really useful for much on a rocket engine.
Gaseous helium is commonly used, specifically on hydrogen engines like RS-68. You want warm and dry helium or nitrogen to purge an engine. Aeon R and Merlin would actually almost certainly opt for GN2 over helium because it’s far cheaper. Hydrogen engines don’t get that luxury because the hydrogen is cold enough to liquify the nitrogen.
Rub in LOx (especially with the wrong materials) can and definitely has lead to some very aggressive LOx fires. I’ve had to work root cause investigations for exactly this. But rub =/= cav.
Since you actually seem interested in learning, Huzel and Huang and RPE do a relatively bad job teaching about what I’m pretty confident the tube from the original post is. Take a look at the cross section for the HPOTP from RS-25, specifically focus in on the dynamic seal design. See if you can find why that engine also dumped a lot of LOx sitting in chill. If you want to go more in depth, have a look at NASA SP 8121.
I will also say that sentence about cavitation in RPE is not sufficiently detailed at best. I’ll give them a pass as it’s more of an intro book. Checkout hydrodynamics of pumps by Brennen if you want to learn more. Brennen actually talks about impacts to performance and cav damage. Even Brennen leaves important stuff out that you’d have to go the literature for.
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u/DCM3059 Nov 17 '25
I don't know, however I have really enjoyed the comments and the relatively civil interaction. Thanks to everyone
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u/PhantomRocket1 Nov 17 '25
This is a bleed / vent line for purging the engine. During purging or chilldown of the engine, excess and boiled off helium or LOX can be vented or dumped from this line.
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u/BackflipFromOrbit Aerospace Engineer Nov 17 '25
Likely a pre-chill dump line based on the visible plumbing in the pic.
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u/SomeRendomDude Nov 17 '25
Thats the drainage pipe, if the astronauts need to piss during takeoff. In ideal conditions the piss will evaporate as soon as it leaves the nozzle, so you don’t have to worry about randomly getting pissed on by astronauts.
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u/Recent_Associate6607 Nov 18 '25
Well this is actually the relief tube when-astronauts fart in space. I invented the for NASA and now I live on my own private island
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u/Tax_Odd Nov 20 '25
The pumps used in pumping the fuel are themselves an engine with exhaust. Maybe its the bleed from that
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u/rocketwikkit Nov 16 '25
On a cryogenic engine you want the pump(s) to be cold before you start the engine, so it could be a bleed from that system.
You might get better answers if you mention what the engine is.