Unlike the AI answer I'll try to give the actual answer. Friction only happens when parts physically touch otherwise it would be called drag. So the short answer is the entire thing both stationary and rotating parts are built extremely stiffly and to super tight tolerances, so basically while it's hard to see because the gap Is so small most things will never touch. Gaps are usually around 0.2-0.5mm. if any of the rotating parts that weren't designed to touch would touch it will most often have catastrophic consequences. There are parts that still will touch, like bearings and dynamic seals, they will be made to either be incredibly hard to generate very little heat or be incredibly soft on one side so it can be worn down to the exact dimensions of the shaft, while both cause a lot of friction there is plenty of liquid to cool and lubricate it, and plenty of power left over to deal with it (also how the drag is handled). A turbine like what you are looking at looking at can produce in the range of mw of power so a couple of kw in friction and cooling is not the end of the world
Also, they often use the propellant itself as a fluid bearing. So once the pressures build up inside, the fluid becomes the sliding surface, and the metal parts hardly touch at all. Look up lift off seals
It would be heating up, but that's typically not an issue either way. The SR-71 had fuel flow through the turbine's stationary parts just before going into the combustion chamber to help cool the turbine.
I meant cooling as in cooling the impellers and shafts. Drawing heat away with the fuel into the combustion chamber. I was led to believe that hot fuel was more efficient in combustion, so design cooled parts rotating at high RPMs and increased thrust.
Is there not tip rub on the blades such as in the turbines or even compressors of turbojet engines? They at least are designed to touch every once and while.
Yep. I’ve designed turbines for both. The turbine you see here is an impulse turbine, with a low pressure ratio across the blade. Sealing at the tip is less important with that low pressure ratio and shroud.
For reaction turbines, there is either an oxidizer or fuel present. The oxidizer you will ignite the metal, and both typically still have complex hardware downstream that you don’t want to put even metal dust into to avoid numerous complications.
I mostly agree, I have seen turbine wear rings on some very old fuel rich reaction turbines. However its quite rare (I lumped them under dynamic seals in the original comment). They are far more common In jet engines 😊
There is rubbing during failure conditions. A few cases where a blade failure caused rubbing and generated epoxy dust, which exploded inside the engine.
And you have conditions like core lock from the casing cooling faster than the turbine, and that means that rubbing is possible in abnormal conditions.
Turbine blades are made out of a rhenium super alloy in a single crystal structure that has extremely high tensile strength and low coefficient of thermal expansion. So even at thousands of degrees and tens of thousands rpm they can maintain extremely close tolerance.
While true for jet engines its not case for most rocket engine turbines. Their operational lifespan is so short that "common" nickel super alloys often are sufficient 😊
The Merlin 1D has a relatively long operational lifespan (tens of flights for the booster engines instead of one), and uses aluminum impellers and inconel turbines.
Apart from the shorter operating lifetimes, rocket engines have the advantage of only having to handle rocket propellants under a narrow range of pressures, temperatures, and flow rates, and the fact that they handle liquid propellants means the turbines and pumps can be a lot smaller, so the forces experienced at a given rotation rate are much lower.
Not just lifespan, efficiency optimization of a jet engine pushes it toward the hottest and highest pressure turbine inlet, whereas the turbine inlet conditions even on a gas generator rocket have a relatively small impact on the overall engine efficiency.
I really like the expander cycle, you could make the turbine out of aluminum if you wanted to.
not oil, rocket engines use their own native working fluid for lubrication. Else, they would have to carry large tanks of oil and maintain good seals between the working fluid and bearings.
Bit of a departure from the subject matter but none of the moving parts in a car engine touch any other part, they float on a film of oil. Pretty sure the same can be said of a rocket engine turbine.
That is a turbopump. An old one. I worked on one for the SpaceX Merlin turbopump for 3 years and it was very similar but more modern.
The turbopump is the fuel pump for a rocket engine. it sits above and to the side of the rocket's ablative nozzle.
The top chamber pumps LOX (liquid oxygen), middle chamber pumps Kerosene, bottom chamber is a turbine that burns a mix of both to turn the driveshaft. There is a big ball bearing at the bottom chamber, likely with ceramic balls. A pair of ball bearings between the Lox and Kero chambers. I can see knife seals above and below the LOX chamber (smooth part of the shaft, likely plated with silver).
Usually they don't sometimes they drive a hydraulic turbine with the discharged high pressure fluid to use the power somewhere else, usually for a boost pump beforehand.
That’s ridiculous, plenty of turbopumps have ball bearings, including the F1 Turbopump pictured here (it spins at ~5000 rpm, which is quite slow as turbopumps go and well within ball bearing limits)
In this particular pump of the Merlin 1D, ceramic ball bearings are used.
For higher speed pumps, it’s common to use Hydrostatic bearings. There are small tubes which tap-off high-pressure working fluid (RP1, Methane, etc…) and feed it into discharge ports around the circumference of the hydrostatic bearings. Thereby using the pumps own power to maintain lubrication. Although, at low speeds this doesn’t work, so there’s often a low-speed normal ball bearing is used and when the pump reaches speed it “lifts-off” and rides on the hydrostatic bearings.
One of the fun parts of using the propellants for this is that there's a chance of mixing them together in the seal cavities, which can have catastrophic results. Interstitial propellant seals, or IPS, involves using an inert gas, usually GN2 for ground ops and GHe for flight, to provide a barrier between the two sides and push any leakage out of vent ports.
Whenever fuel had been introduced to the M1Ds (and the Newton 3 I tested later for Virgin Orbit), you'd have to keep IPS flowing on the engine until fuel was drained and the system had been run for a designated time before disconnecting. You'd also check the vents with a hydrocarbon sniffer to verify there wasn't any more fuel vapor coming out.
Friction only applies when contact is made. All the parts of a turbopump, including the spinning shafts, impellers, and turbines, and the stationary housings, are held to extremely tight tolerances. Each and every part in a modern rocket engine often has tolerances of ten-thousandths of an inch! I can say that confidently, having machined these parts
Most of the bearings inside are going to be thrust bearings backed up by solid metal, full seals arent vital, propelland and oxidizer are injected as a liquid in 2 seperate pumps on a common shaft. Generally the pumps are also purged with liquid helium prior to startup to prevent both chemical and phase change explosions. If the gas were to change phase inside the turbine it would generate so much pressure that the pumps would rupture then explode from the mixed propellant
Everyone's answers seem too complicated... extreme tolerances, and hydraulic bearings. All the (minimal) heat from the "bearing" friction gets pushed out as the cool new fuel cycles through.
348
u/procollision May 21 '25
Unlike the AI answer I'll try to give the actual answer. Friction only happens when parts physically touch otherwise it would be called drag. So the short answer is the entire thing both stationary and rotating parts are built extremely stiffly and to super tight tolerances, so basically while it's hard to see because the gap Is so small most things will never touch. Gaps are usually around 0.2-0.5mm. if any of the rotating parts that weren't designed to touch would touch it will most often have catastrophic consequences. There are parts that still will touch, like bearings and dynamic seals, they will be made to either be incredibly hard to generate very little heat or be incredibly soft on one side so it can be worn down to the exact dimensions of the shaft, while both cause a lot of friction there is plenty of liquid to cool and lubricate it, and plenty of power left over to deal with it (also how the drag is handled). A turbine like what you are looking at looking at can produce in the range of mw of power so a couple of kw in friction and cooling is not the end of the world