7 ms·

This is an excellent article. Just curious, how much more efficient is the full-flow engine? And, what exactly is the deal with the seals the article is talkin

This is an excellent article. Just curious, how much more efficient is the full-flow engine? And, what exactly is the deal with the seals the article is talking about? Anyone have more info?

CarVac 8y ago
A full-flow staged combustion engine has more efficiency due to both propellants being fully vaporized before they mix, rather than a liquid-liquid mixture (as in a gas generator engine) or a liquid-gas mixture (as in a normal staged combustion engine). The seals are between the turbine side (hot gas) and compressor side (liquid fuel) of the turbopump. In a full-flow staged combustion engine, you don't have to worry about accidentally getting a little of the hot gas into the liquid propellant, because that would be mixing fuel with fuel-rich gas or oxygen with oxygen-rich gas. But in a normal staged combustion engine, you have to carefully seal off the oxygen from the fuel-rich gas (in a fuel-rich staged combustion engine) or the oxygen-rich gas from the fuel (in an oxygen-rich staged combustion engine.
- ballooney 8y ago
  No, this is all just wrong. The efficiency is because you dont throw a load of enthalpy overboard as in a gas generator. The phase is _exremely_ second order by comparison. You’re all over this thread being wrong and confused. Source: am a rocket engine designer.
  - buboard 8y ago
    does the mass of the exhaust fuels count as well? whats the percentage of thrust being lost in a gas generator?
    - ballooney 8y ago
      It definitely counts. The effect it has on exhaust velocity is maybe 5-10%, which is a big deal when you consider that has an exponential effect on your mass fraction, i.e the percentage of total lift-off mass that is fuel vs machinery and payload.
  - erobbins 8y ago
    I'm not clear what the efficiency gains are here at all over a SSME style staged combustion cycle. In both designs all propellants go through the chamber. Comparing to a gas generator cycle there are gains, sure. It seems like the major win here is running oxygen rich on the oxygen side and fuel rich on the fuel side to reduce the chance of a leak destroying the whole vehicle. Am I missing something?
    - avmich 8y ago
      > I'm not clear what the efficiency gains are here at all over a SSME style staged combustion cycle. You have higher pressure in the chamber with full-flow approach. Since the mass flow is the same, and Isp is higher, thanks to higher pressure, you're winning. And you have higher pressure because you supply more power to pumps. Reducing risks to destroy the vehicle due to leaks is, I agree, very secondary benefit. May be even tertiary - how about possibility to optimize turbine frequencies for both propellant flows without losses on gears?
      - erobbins 8y ago
        But... you don't. The turbines in any kind of staged or full flow system run at a pressure higher than the chamber is at... I don't remember the numbers offhand but I want to say the SSME's preburners were at 8000ish psi, with a drop to 4000ish through the turbines, and finally to 3000 in the main combustion chamber. The SSME was only partially preburned, and so the flow through the turbine has to expand over a higher pressure ratio, but I still don't see a fundamental difference.
        avmich 8y ago
        Let me explain. The principal advantage of full flow system over regular staged combustion is higher Isp of the engine. Higher Isp of the engine is achieved because the pressure in the chamber of full flow engine is higher, than chamber pressure of staged combustion engine. The pressure in the chamber is higher because pumps produce higher pressure. Pumps can produce higher pressure, because with full flow pumps have more power driving them than with staged combustion. Power here is roughly equal to volume flow (m^3 / s) multiplied by pressure drop (N / m^2). Full flow scheme gives you more volume flow, since all the propellant is used to turn turbines. So even though all the propellant in both cases is burned in the chamber, Isp is higher with full flow scheme. Rocket engine maximizes momentum of the exhaust, not the energy, which is conserved.
  - CarVac 8y ago
    I was under the impression that the question was regarding FFSC versus non-FF staged combustion. You say the main benefit is just increased pressure?
twic 8y ago
I think this is about safety. You need two pumps, one for fuel, one for oxygen. The compressor side of the pump will be full of whatever it is you're pumping. But if you use a single type of preburner, then the turbines of both pumps are full of either hot fuel gas, or hot oxygen gas. That means you have one pump where you have hot fuel or oxygen right next to cold oxygen or fuel. If the seals leak, the pump blows up.
- usrusr 8y ago
  In short: you accept a higher failure rate (twice as many pumps) to get a lower explosive failure rate (much less explosive pumps). A very good tradeoff when building a rocket with a high number of engines and some redundancy which does not help against explosions. It gets even better when the pumps turn out to be more reliable individually due to relative simplicity.
CabSauce 8y ago
Scott Manley has done a few videos (youtube) recently around rocket architecture; including several videos about the Raptor engine. Well-worth checking out. (I don't have a link at the moment, sorry.)
- buboard 8y ago
  Scott Manley's channel : https://www.youtube.com/watch?v=Sdwy9fzQzl4 https://www.youtube.com/watch?v=Sdwy9fzQzl4 This guy has some simple short videos on rocketry. this one is on combustion cycles https://www.youtube.com/watch?v=jheMusS0JwA&t=20s https://www.youtube.com/watch?v=jheMusS0JwA&t=20s
buboard 8y ago
Wikipedia has some stats comparing their current engines (probably speculative for the raptor) Raptor: Thrust 1900 KN / 330 specific impulse Merlin 1D: 480KN / 275 s.i. i can't find the weight of the Raptor. Merlin is 630Kg
- Eridrus 8y ago
  So, for the folks like me who don't know anything about rockets, it seems like "specific impulse" is the measure of how much "impulse" is generated per unit of fuel. So 330/275 = 20% better.
  - Tuna-Fish 8y ago
    Most of that is actually because of the difference in fuel -- Raptor runs on Methane, which produces more H2O and less CO2 than RP-1, and H2O is a smaller molecule and therefore more efficient. Yep. The actual physically relevant number is exhaust velocity, which is the mean velocity of the particles in the rocket exhaust. Isp = Specific Impulse = exhaust velocity/(9.81ms^-2). 9.81ms^-2 there is not any actual acceleration, just an agreed conversion factor. The unit of Isp has no physical significance, the convention just arose because early rocketry was a collaboration of people who all used different units, and they wanted to be able to compare numbers. Since the unit they all shared was a second, and they all agreed (roughly) on the real value of g, they just divided exhaust velocity by their g to get a number that could be compared. The convention stuck.
    - deleted 8y ago
      [deleted]
    - Gibbon1 8y ago
      > 9.81ms^-2 there is not any actual acceleration, just an agreed conversion factor. I'd look it up but my textbooks[1] are in boxes, but the units everyone uses are wrong. Actually inappropriately simplified. [1] I have one and exactly one textbook that has the full units for gc.
Symmetry 8y ago
A staged combustion engine is intrinsically a lot more efficient than an engine where you dump some of your fuel to power the pumps, like in a tapoff or gas-generator engine. Compared to a oxygen-rich full flow you can reach a higher pressure while having better safety margins which will allow you to re-use the engine. The higher your chamber pressure the closer the rocket's thrust at sea level is to its maximum thrust in vacuum. Also higher pressure tends to correlate with higher thrust to weight ratios. Also, having both inputs already be gasses mean they mix better. This will have a tiny benefit in them burning more completely. But more importantly they'll burn better across a wider range of thrusts, which could be important for throttling down the engine for landing.
- SECProto 8y ago
  > Also, having both inputs already be gasses mean they mix better. This will have a tiny benefit in them burning more completely. But more importantly they'll burn better across a wider range of thrusts, which could be important for throttling down the engine for landing. Also, and importantly for the design of the engine, it's much much easier* to computationally model the mixing of two gases, to ensure complete mixing and no combustion instability (especially when the reaction components are limited to ch4, o2, and partial- and complete-reaction products like c02, c0, h2o, oh-, ...). * by much easier, i mean less impossible
  - Symmetry 8y ago
    Have you seen SpaceX's talk at some NVidia conference about their homegrown Computational Fluid Dynamics simulator? It was really interesting. https://www.nextplatform.com/2015/03/27/rockets-shake-and-rattle-so-spacex-rolls-homegrown-cfd/ https://www.nextplatform.com/2015/03/27/rockets-shake-and-ra...
    - SECProto 8y ago
      I hadnt seen that exact article, but i recall reading (or listening to a lecture) that mentions how one advantage of methane is the possibility to use CFD in design, while that was still outside the realm of possibility with RP-1 due to complexity of reactants (differing phases and many compounds)
mabbo 8y ago
Scott Manley has a fantastic video explaining all of this: https://www.youtube.com/watch?v=4QXZ2RzN_Oo https://www.youtube.com/watch?v=4QXZ2RzN_Oo In terms of efficiency, it's worth mentioning that even tiny gains have big impacts because of the rocket equation. Having a 1% boost the entire time the rocket is running can have an enormous impact in the overall velocity, or reduce the overall amount of fuel needed for the same targets.

This is an excellent article. Just curious, how much more efficient is the full-flow engine? And, what exactly is the deal with the seals the article is talking about? Anyone have more info?

CarVac 8y ago

A full-flow staged combustion engine has more efficiency due to both propellants being fully vaporized before they mix, rather than a liquid-liquid mixture (as in a gas generator engine) or a liquid-gas mixture (as in a normal staged combustion engine). The seals are between the turbine side (hot gas) and compressor side (liquid fuel) of the turbopump. In a full-flow staged combustion engine, you don't have to worry about accidentally getting a little of the hot gas into the liquid propellant, because that would be mixing fuel with fuel-rich gas or oxygen with oxygen-rich gas. But in a normal staged combustion engine, you have to carefully seal off the oxygen from the fuel-rich gas (in a fuel-rich staged combustion engine) or the oxygen-rich gas from the fuel (in an oxygen-rich staged combustion engine.

ballooney 8y ago
No, this is all just wrong. The efficiency is because you dont throw a load of enthalpy overboard as in a gas generator. The phase is _exremely_ second order by comparison. You’re all over this thread being wrong and confused. Source: am a rocket engine designer.
- buboard 8y ago
  does the mass of the exhaust fuels count as well? whats the percentage of thrust being lost in a gas generator?
  - ballooney 8y ago
    It definitely counts. The effect it has on exhaust velocity is maybe 5-10%, which is a big deal when you consider that has an exponential effect on your mass fraction, i.e the percentage of total lift-off mass that is fuel vs machinery and payload.
- erobbins 8y ago
  I'm not clear what the efficiency gains are here at all over a SSME style staged combustion cycle. In both designs all propellants go through the chamber. Comparing to a gas generator cycle there are gains, sure. It seems like the major win here is running oxygen rich on the oxygen side and fuel rich on the fuel side to reduce the chance of a leak destroying the whole vehicle. Am I missing something?
  - avmich 8y ago
    > I'm not clear what the efficiency gains are here at all over a SSME style staged combustion cycle. You have higher pressure in the chamber with full-flow approach. Since the mass flow is the same, and Isp is higher, thanks to higher pressure, you're winning. And you have higher pressure because you supply more power to pumps. Reducing risks to destroy the vehicle due to leaks is, I agree, very secondary benefit. May be even tertiary - how about possibility to optimize turbine frequencies for both propellant flows without losses on gears?
    - erobbins 8y ago
      But... you don't. The turbines in any kind of staged or full flow system run at a pressure higher than the chamber is at... I don't remember the numbers offhand but I want to say the SSME's preburners were at 8000ish psi, with a drop to 4000ish through the turbines, and finally to 3000 in the main combustion chamber. The SSME was only partially preburned, and so the flow through the turbine has to expand over a higher pressure ratio, but I still don't see a fundamental difference.
      - avmich 8y ago
        Let me explain. The principal advantage of full flow system over regular staged combustion is higher Isp of the engine. Higher Isp of the engine is achieved because the pressure in the chamber of full flow engine is higher, than chamber pressure of staged combustion engine. The pressure in the chamber is higher because pumps produce higher pressure. Pumps can produce higher pressure, because with full flow pumps have more power driving them than with staged combustion. Power here is roughly equal to volume flow (m^3 / s) multiplied by pressure drop (N / m^2). Full flow scheme gives you more volume flow, since all the propellant is used to turn turbines. So even though all the propellant in both cases is burned in the chamber, Isp is higher with full flow scheme. Rocket engine maximizes momentum of the exhaust, not the energy, which is conserved.
- CarVac 8y ago
  I was under the impression that the question was regarding FFSC versus non-FF staged combustion. You say the main benefit is just increased pressure?

twic 8y ago

I think this is about safety. You need two pumps, one for fuel, one for oxygen. The compressor side of the pump will be full of whatever it is you're pumping. But if you use a single type of preburner, then the turbines of both pumps are full of either hot fuel gas, or hot oxygen gas. That means you have one pump where you have hot fuel or oxygen right next to cold oxygen or fuel. If the seals leak, the pump blows up.

usrusr 8y ago
In short: you accept a higher failure rate (twice as many pumps) to get a lower explosive failure rate (much less explosive pumps). A very good tradeoff when building a rocket with a high number of engines and some redundancy which does not help against explosions. It gets even better when the pumps turn out to be more reliable individually due to relative simplicity.

CabSauce 8y ago

Scott Manley has done a few videos (youtube) recently around rocket architecture; including several videos about the Raptor engine. Well-worth checking out. (I don't have a link at the moment, sorry.)

buboard 8y ago
Scott Manley's channel : https://www.youtube.com/watch?v=Sdwy9fzQzl4 https://www.youtube.com/watch?v=Sdwy9fzQzl4 This guy has some simple short videos on rocketry. this one is on combustion cycles https://www.youtube.com/watch?v=jheMusS0JwA&t=20s https://www.youtube.com/watch?v=jheMusS0JwA&t=20s

buboard 8y ago

Wikipedia has some stats comparing their current engines (probably speculative for the raptor) Raptor: Thrust 1900 KN / 330 specific impulse Merlin 1D: 480KN / 275 s.i. i can't find the weight of the Raptor. Merlin is 630Kg

Eridrus 8y ago
So, for the folks like me who don't know anything about rockets, it seems like "specific impulse" is the measure of how much "impulse" is generated per unit of fuel. So 330/275 = 20% better.
- Tuna-Fish 8y ago
  Most of that is actually because of the difference in fuel -- Raptor runs on Methane, which produces more H2O and less CO2 than RP-1, and H2O is a smaller molecule and therefore more efficient. Yep. The actual physically relevant number is exhaust velocity, which is the mean velocity of the particles in the rocket exhaust. Isp = Specific Impulse = exhaust velocity/(9.81ms^-2). 9.81ms^-2 there is not any actual acceleration, just an agreed conversion factor. The unit of Isp has no physical significance, the convention just arose because early rocketry was a collaboration of people who all used different units, and they wanted to be able to compare numbers. Since the unit they all shared was a second, and they all agreed (roughly) on the real value of g, they just divided exhaust velocity by their g to get a number that could be compared. The convention stuck.
  - deleted 8y ago
    [deleted]
  - Gibbon1 8y ago
    > 9.81ms^-2 there is not any actual acceleration, just an agreed conversion factor. I'd look it up but my textbooks[1] are in boxes, but the units everyone uses are wrong. Actually inappropriately simplified. [1] I have one and exactly one textbook that has the full units for gc.

Symmetry 8y ago

A staged combustion engine is intrinsically a lot more efficient than an engine where you dump some of your fuel to power the pumps, like in a tapoff or gas-generator engine. Compared to a oxygen-rich full flow you can reach a higher pressure while having better safety margins which will allow you to re-use the engine. The higher your chamber pressure the closer the rocket's thrust at sea level is to its maximum thrust in vacuum. Also higher pressure tends to correlate with higher thrust to weight ratios. Also, having both inputs already be gasses mean they mix better. This will have a tiny benefit in them burning more completely. But more importantly they'll burn better across a wider range of thrusts, which could be important for throttling down the engine for landing.

SECProto 8y ago
> Also, having both inputs already be gasses mean they mix better. This will have a tiny benefit in them burning more completely. But more importantly they'll burn better across a wider range of thrusts, which could be important for throttling down the engine for landing. Also, and importantly for the design of the engine, it's much much easier* to computationally model the mixing of two gases, to ensure complete mixing and no combustion instability (especially when the reaction components are limited to ch4, o2, and partial- and complete-reaction products like c02, c0, h2o, oh-, ...). * by much easier, i mean less impossible
- Symmetry 8y ago
  Have you seen SpaceX's talk at some NVidia conference about their homegrown Computational Fluid Dynamics simulator? It was really interesting. https://www.nextplatform.com/2015/03/27/rockets-shake-and-rattle-so-spacex-rolls-homegrown-cfd/ https://www.nextplatform.com/2015/03/27/rockets-shake-and-ra...
  - SECProto 8y ago
    I hadnt seen that exact article, but i recall reading (or listening to a lecture) that mentions how one advantage of methane is the possibility to use CFD in design, while that was still outside the realm of possibility with RP-1 due to complexity of reactants (differing phases and many compounds)

mabbo 8y ago

Scott Manley has a fantastic video explaining all of this: https://www.youtube.com/watch?v=4QXZ2RzN_Oo https://www.youtube.com/watch?v=4QXZ2RzN_Oo In terms of efficiency, it's worth mentioning that even tiny gains have big impacts because of the rocket equation. Having a 1% boost the entire time the rocket is running can have an enormous impact in the overall velocity, or reduce the overall amount of fuel needed for the same targets.