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This thing I can not comprehend about rocket engines is how the turbopump manages to hold together. A turbine blade in the SSME about the size of your thumb ma

This thing I can not comprehend about rocket engines is how the turbopump manages to hold together. A turbine blade in the SSME about the size of your thumb makes 600 horsepower. https://www.enginehistory.org/Rockets/SSME/SSME6.pdf https://www.enginehistory.org/Rockets/SSME/SSME6.pdf

bernulli 5y ago
Also try to imagine the thermal stresses when you have cryogenic propellants on the pump side and hot exhaust gases (gas generator or staged combustion) on the turbine side!
- avmich 5y ago
  With "fire wall" of the engine, over the distance of a millimeter you have full temperature of the chamber - that's thousands of Kelvins, not just gas generator temperatures which are lower because otherwise turbine blades will melt - and relatively low temperature of the coolant, and also mechanical stress of high pressure in the cooling channels and lower pressure in the thrust chamber.
  - bernulli 5y ago
    You don't have the full temperature of the chamber b/c of the film cooling discussed in the article. Also, the chamber doesn't spin at tens of thousands of rpm.
    - avmich 5y ago
      Film cooling could be absent, though the temperature distribution is still such that layer next to the wall is cooler than the center of the flow. Chamber doesn't spin, yes, but gas on the turbine blades has mostly dynamic effect - that's how the turbine works - while at the chamber wall the pressure difference is mostly static - relatively low liquid flow speed in the channels and relatively low (tens of meters per second, compared to hundred meters per second at the blades) gas flow speed at the chamber, so the wall has to withstand the difference without moving. Agree that these are rather different stress modes, just wanted to point to another place in the engine with pretty extreme stresses.
      - bernulli 5y ago
        Sure. But another key feature of how the turbine works is -- that it spins. That means that you need bearings and seals that can not only withstand but actually work effectively under those conditions.
jaywalk 5y ago
Well, 63 blades together make 600 horsepower. But as that paper notes, each blade is subject to 50,000 psi which is wild.
- TylerE 5y ago
  It is wild, but not as wild as you might think. The fuel rail in a modern diesel engine is operating at 25-30,000 psi all the time.
  - BenjiWiebe 5y ago
    I know, and that is wild too!
- jhgb 5y ago
  600 horsepower each. The whole turbine has tens of megawatts of mechanical power output.
  - jaywalk 5y ago
    Yep, you are correct. I misread.
HPsquared 5y ago
I the power level of these engines is difficult to comprehend. The fuel pump has thousands of horsepower. Compare this to the fuel pump on a car engine, which is a tiny little electric thing. The combustion power must be in the gigawatt range. Edit: the SSME high pressure fuel pump turbine produces 63000 hp (46 MW). There's also one for the oxygen, and a pair of low pressure pumps as well. Crazy... Edit edit: the fuel pump transfers 155 lb/sec of liquid hydrogen. If fully combusted (142 MJ/kg), that would release 10.0 GW of heat per engine.
- 7952 5y ago
  Saturn V at take off had an equivalent power of 166GW. If that was electricity it would be around 2x the total capacity of all the power stations in the UK.
  - Kye 5y ago
    I wonder if anyone's experimented with using rockets to charge something like a flywheel battery or reservoir for hydro power. A 777's engine puts out tens of megawatts using a less refined fuel than rockets use. What if you hooked a rocket full of RP-1 to a pump to haul water from a river to a dam? This is probably nonsense.
    - adewinter 5y ago
      You should keep in mind that, sure, the rocket produces 166GW of power(which is wild!) but it does so for all of 5 minutes (or less). Whereas "all the power stations in the UK" keep on trucking, day in, day out, year after year.
    - 7952 5y ago
      A 777 engine is actually far more fuel efficient than a rocket engine. Take a look at specific impulse [0] for different engine types. A rocket is optimised to be very light and to use its own oxygen supply. A normal jet engine can be heavier and use the air as an oxidizer. Much more efficient for power production in a CCGT.
throwaway0a5e 5y ago
Don't think of it as "making horsepower", think of it as "resisting the forces upon it". A gear tooth the size of your thumb pulls a semi truck up a mountain.
dekhn 5y ago
It sounded incomprehensible to me too but as I did some more learning about the process of building rocket engines I learned some interesting details. First, remember that people have been making high strength metals through careful processing for thousands of years. Second, the parts of engines are not made as part of large-scale industrialized manufacturing. Almost all the parts are made as few-offs, with far more energy, time, and effort put into making sure that a single instance of something is extremely reliable. Third, we got damn good at materials science in the past 100 year, and metals can be absurdly resistant to deformation under heat.
- dokem 5y ago
  They also only run for a few minutes.
  - yetihehe 5y ago
    Engines in dragsters are also know to have very high power comparing to engine size, but they also make less than 10k rotations at full power before they fail. That is enough to last one drag race which is several seconds.
    - snypher 5y ago
      The top fuel record is around 3.5 seconds and at 9500rpm is around 600 revolutions for the whole race!
  - beerandt 5y ago
    SSMEs are reusable. Designing for thermal cycles and serviceability[0] is at least as difficult a problem as running a hypothetical rocket engine an equal amount of time in one longer, hotter burn. (Such a design isn't needed and wouldn't be practical, but then again multiple aspects of SSMEs being reusable turned out not very practical either, depending on what version of design criterea you evaluate and how the expected vs actual usage changed over the lifetime of the program.) [0]In both the engineering sense, as durability of the various loading cycles (ie lifetime turbine rotations or number of thermal cycles before eol or failure), and as being constructed as able to undergo maintenance and refurbishment between launches.
VBprogrammer 5y ago
If that interests you then this series of videos is definitely worth watching. It details several parts of the German V2 missile, the grand father of all modern liquid propellant rockets. This one is about the turbopump which is one of the most interesting parts. https://youtu.be/EgiMu8A3pi0 https://youtu.be/EgiMu8A3pi0
- mikewarot 5y ago
  I had no idea they individually manufactured those blades on the steam turbine... the number of parts that little factory had to crank out was astounding. I can see why the Russians simplified it to a bent piece of sheet instead. Considering all the manufacturing, precision manufacturing involved the V2 effort, it's astounding they could do it at all.
kunai 5y ago
It is pretty mind-boggling. Makes the average turbofan's turbine assembly look like child's play, and those are also pretty ridiculous in terms of power-to-weight ratio.
- HPsquared 5y ago
  It's mostly because the working fluid is at very high pressure (much denser than the air coming through an aero engine). The turbine side works with hot gas at 100+ bar, and the pump side is dealing with liquid. Therefore small parts can exert a lot of force (large pressure differences) and do a lot of work (high speeds). The most similar technology is a boiler feed pump (used to feed the boilers in a steam turbine system, e.g. in a ship or power station) - these work in a similar way, high pressure steam is bled off from the system to drive a small turbine, which drives a high pressure pump to feed the boiler, which feeds the 'main' steam turbine. Similar pressure (>100 bar), but the turbine on the rocket engine needs to handle much higher temperatures (hot gas rather than steam).
- avmich 5y ago
  Turbofans can be more complex than rocket engines - in the rocket engine thrust chamber you don't have moving parts, but with turbofans the whole engine is like a big turbopump, with air compressor instead of centrifugal pump.
  - bernulli 5y ago
    Sure, and in the gas turbine combustor there are also no moving parts. But there’s lots going on outside.
chasd00 5y ago
The turbo pump is the main barrier to entry when making a large rocket engine. There's so much energy per unit volume that there is no failure mode that does not result in an explosion. Also keep in mind, you have very hot gas on one side and a few inches over cryogenic liquid oxygen. Liquid oxygen turns anything in to fuel so think about how to lubricate that shaft let alone seal it. Another interesting thing to think about, it's the turbopump keeping the combustion from running back up the injectors. So, the turbopump has to outperform the combustion chamber in terms of pressure and flow rate.
- avmich 5y ago
  > Liquid oxygen turns anything in to fuel Not quite literally, in e.g. Soviet engine designs some gas generators are oxygen-rich, they produce oxidized exhaust of high temperature, and the engine still doesn't burn (usually). > So, the turbopump has to outperform the combustion chamber in terms of pressure and flow rate. Yes, the injector pressure drop can't be too small.
- ncmncm 5y ago
  Robert Goddard had terrible difficulties with fuel/oxidizer pumps. It wasn't because he was not smart enough. Most of what was needed hadn't been invented yet.
deleted 5y ago
[deleted]

bernulli 5y ago

Also try to imagine the thermal stresses when you have cryogenic propellants on the pump side and hot exhaust gases (gas generator or staged combustion) on the turbine side!

avmich 5y ago
With "fire wall" of the engine, over the distance of a millimeter you have full temperature of the chamber - that's thousands of Kelvins, not just gas generator temperatures which are lower because otherwise turbine blades will melt - and relatively low temperature of the coolant, and also mechanical stress of high pressure in the cooling channels and lower pressure in the thrust chamber.
- bernulli 5y ago
  You don't have the full temperature of the chamber b/c of the film cooling discussed in the article. Also, the chamber doesn't spin at tens of thousands of rpm.
  - avmich 5y ago
    Film cooling could be absent, though the temperature distribution is still such that layer next to the wall is cooler than the center of the flow. Chamber doesn't spin, yes, but gas on the turbine blades has mostly dynamic effect - that's how the turbine works - while at the chamber wall the pressure difference is mostly static - relatively low liquid flow speed in the channels and relatively low (tens of meters per second, compared to hundred meters per second at the blades) gas flow speed at the chamber, so the wall has to withstand the difference without moving. Agree that these are rather different stress modes, just wanted to point to another place in the engine with pretty extreme stresses.
    - bernulli 5y ago
      Sure. But another key feature of how the turbine works is -- that it spins. That means that you need bearings and seals that can not only withstand but actually work effectively under those conditions.

jaywalk 5y ago

Well, 63 blades together make 600 horsepower. But as that paper notes, each blade is subject to 50,000 psi which is wild.

TylerE 5y ago
It is wild, but not as wild as you might think. The fuel rail in a modern diesel engine is operating at 25-30,000 psi all the time.
- BenjiWiebe 5y ago
  I know, and that is wild too!
jhgb 5y ago
600 horsepower each. The whole turbine has tens of megawatts of mechanical power output.
- jaywalk 5y ago
  Yep, you are correct. I misread.

HPsquared 5y ago

I the power level of these engines is difficult to comprehend. The fuel pump has thousands of horsepower. Compare this to the fuel pump on a car engine, which is a tiny little electric thing. The combustion power must be in the gigawatt range. Edit: the SSME high pressure fuel pump turbine produces 63000 hp (46 MW). There's also one for the oxygen, and a pair of low pressure pumps as well. Crazy... Edit edit: the fuel pump transfers 155 lb/sec of liquid hydrogen. If fully combusted (142 MJ/kg), that would release 10.0 GW of heat per engine.

7952 5y ago
Saturn V at take off had an equivalent power of 166GW. If that was electricity it would be around 2x the total capacity of all the power stations in the UK.
- Kye 5y ago
  I wonder if anyone's experimented with using rockets to charge something like a flywheel battery or reservoir for hydro power. A 777's engine puts out tens of megawatts using a less refined fuel than rockets use. What if you hooked a rocket full of RP-1 to a pump to haul water from a river to a dam? This is probably nonsense.
  - adewinter 5y ago
    You should keep in mind that, sure, the rocket produces 166GW of power(which is wild!) but it does so for all of 5 minutes (or less). Whereas "all the power stations in the UK" keep on trucking, day in, day out, year after year.
  - 7952 5y ago
    A 777 engine is actually far more fuel efficient than a rocket engine. Take a look at specific impulse [0] for different engine types. A rocket is optimised to be very light and to use its own oxygen supply. A normal jet engine can be heavier and use the air as an oxidizer. Much more efficient for power production in a CCGT.

throwaway0a5e 5y ago

Don't think of it as "making horsepower", think of it as "resisting the forces upon it". A gear tooth the size of your thumb pulls a semi truck up a mountain.

dekhn 5y ago

It sounded incomprehensible to me too but as I did some more learning about the process of building rocket engines I learned some interesting details. First, remember that people have been making high strength metals through careful processing for thousands of years. Second, the parts of engines are not made as part of large-scale industrialized manufacturing. Almost all the parts are made as few-offs, with far more energy, time, and effort put into making sure that a single instance of something is extremely reliable. Third, we got damn good at materials science in the past 100 year, and metals can be absurdly resistant to deformation under heat.

dokem 5y ago
They also only run for a few minutes.
- yetihehe 5y ago
  Engines in dragsters are also know to have very high power comparing to engine size, but they also make less than 10k rotations at full power before they fail. That is enough to last one drag race which is several seconds.
  - snypher 5y ago
    The top fuel record is around 3.5 seconds and at 9500rpm is around 600 revolutions for the whole race!
- beerandt 5y ago
  SSMEs are reusable. Designing for thermal cycles and serviceability[0] is at least as difficult a problem as running a hypothetical rocket engine an equal amount of time in one longer, hotter burn. (Such a design isn't needed and wouldn't be practical, but then again multiple aspects of SSMEs being reusable turned out not very practical either, depending on what version of design criterea you evaluate and how the expected vs actual usage changed over the lifetime of the program.) [0]In both the engineering sense, as durability of the various loading cycles (ie lifetime turbine rotations or number of thermal cycles before eol or failure), and as being constructed as able to undergo maintenance and refurbishment between launches.

VBprogrammer 5y ago

If that interests you then this series of videos is definitely worth watching. It details several parts of the German V2 missile, the grand father of all modern liquid propellant rockets. This one is about the turbopump which is one of the most interesting parts. https://youtu.be/EgiMu8A3pi0 https://youtu.be/EgiMu8A3pi0

mikewarot 5y ago
I had no idea they individually manufactured those blades on the steam turbine... the number of parts that little factory had to crank out was astounding. I can see why the Russians simplified it to a bent piece of sheet instead. Considering all the manufacturing, precision manufacturing involved the V2 effort, it's astounding they could do it at all.

kunai 5y ago

It is pretty mind-boggling. Makes the average turbofan's turbine assembly look like child's play, and those are also pretty ridiculous in terms of power-to-weight ratio.

HPsquared 5y ago
It's mostly because the working fluid is at very high pressure (much denser than the air coming through an aero engine). The turbine side works with hot gas at 100+ bar, and the pump side is dealing with liquid. Therefore small parts can exert a lot of force (large pressure differences) and do a lot of work (high speeds). The most similar technology is a boiler feed pump (used to feed the boilers in a steam turbine system, e.g. in a ship or power station) - these work in a similar way, high pressure steam is bled off from the system to drive a small turbine, which drives a high pressure pump to feed the boiler, which feeds the 'main' steam turbine. Similar pressure (>100 bar), but the turbine on the rocket engine needs to handle much higher temperatures (hot gas rather than steam).
avmich 5y ago
Turbofans can be more complex than rocket engines - in the rocket engine thrust chamber you don't have moving parts, but with turbofans the whole engine is like a big turbopump, with air compressor instead of centrifugal pump.
- bernulli 5y ago
  Sure, and in the gas turbine combustor there are also no moving parts. But there’s lots going on outside.

chasd00 5y ago

The turbo pump is the main barrier to entry when making a large rocket engine. There's so much energy per unit volume that there is no failure mode that does not result in an explosion. Also keep in mind, you have very hot gas on one side and a few inches over cryogenic liquid oxygen. Liquid oxygen turns anything in to fuel so think about how to lubricate that shaft let alone seal it. Another interesting thing to think about, it's the turbopump keeping the combustion from running back up the injectors. So, the turbopump has to outperform the combustion chamber in terms of pressure and flow rate.

avmich 5y ago
> Liquid oxygen turns anything in to fuel Not quite literally, in e.g. Soviet engine designs some gas generators are oxygen-rich, they produce oxidized exhaust of high temperature, and the engine still doesn't burn (usually). > So, the turbopump has to outperform the combustion chamber in terms of pressure and flow rate. Yes, the injector pressure drop can't be too small.
ncmncm 5y ago
Robert Goddard had terrible difficulties with fuel/oxidizer pumps. It wasn't because he was not smart enough. Most of what was needed hadn't been invented yet.

deleted 5y ago

[deleted]