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The Tech Behind SpaceX’s New Engine

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Re: The Tech Behind SpaceX’s New Engine

#71

Post by burfog »

Looking at the list of cycles on Wikipedia, I'm surprised that nobody seems to have used preburners to pressurize the tanks. Use a fuel-rich preburner to pressurize the fuel tank, and an oxygen-rich preburner to pressurize the oxygen tank.

Mixing should be limited even if nothing special is done, due to the temperature and phase of matter and short timeframe. One could of course pay the weight penalty of a piston (need not have a perfect seal) or collapsing bag.

Doing a heat exchanger (to boil and thus pressurize) is another option, but then you're back to needing a place for the exhaust. It would let you do a sort of full-flow engine without turbopumps however, which is great. All those issues with cavitation and lubrication and stress cracking just go away.

Re: The Tech Behind SpaceX’s New Engine

#72

Post by Tuna-Fish »

avmich wrote: post #61
Tuna-Fish wrote: post #36

Earlier quoted context omitted.

> TL; DR Full flow lowers turbine temperatures at the expense of parts complexity. Ehhh. I'd argue it also lowers part complexity, because it eliminates interpropellant seals. > The pay-off isn’t so much efficiency It improves achievable chamber pressure (assuming similar maximum pressures and temperatures at the turbines), which improves both thrust and Isp. > (This also explains why full flow hasn’t been a priority…

> ...it also lowers part complexity, because it eliminates interpropellant seals. But it adds the whole second gas generator and turbine. No, it doesn't lower part complexity as a whole :) . > It improves achievable chamber pressure Yes, I'd even argue that's the penultimate goal of full-flow scheme. 1) For a rocket engine you usually (almost always) want as high Isp (which is approximately the speed of gasses flowin…

> But it adds the whole second gas generator and turbine. No, it doesn't lower part complexity as a whole :) .

Because the fuel side and oxidizer side typically have a very different flow rates, many engines have separate pumps and turbines for them on separate shafts even if both sides use same fluids in the preburner. For example, see the SSME.

With FFSC, each of the turbines have no sealing requirements. This means, among other things, that bearings become easier and safer, as you can afford larger margins in your design. In many ways, FFSC allows the individual parts of the engine to be simpler.

The main cost of doing it over gas generators (other than oxidation-resistant superalloys...) is that all parts of the engine sort of circularly depend on each other -- when the Merlin engine was in very early development, they drove into the desert and ran the gas generator and fuel pump on it's own with basically no support infrastructure. Testing Raptor in a similar way is just not possible. It would have never gotten built without the NASA Stennis facility that can provide all the intermediate fluids at the pressures required.

I fully agree with the rest.

Re: The Tech Behind SpaceX’s New Engine

#73

Post by davedx »

zackmorris wrote: post #67
buboard wrote: post #21

Earlier quoted context omitted.

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

To put this in perspective, the F-1 engine of the Saturn V (one of the biggest rocket engines ever made) only put out about 3.5 times more thrust (6.7 MN or 1.5 Mlbf) than the Raptor (1.9 MN or 0.43 Mlbf). Also some people might not realize how much liquid is being pumped by rocket engines (the Saturn V F-1 could drain a 30,000 gallon swimming pool in 10 seconds): Each second, a single F-1 burned 5,683 pounds (2,578…

Methane also means they can refuel in-situ on Mars, provided the infrastructure is there.

Re: The Tech Behind SpaceX’s New Engine

#74

Post by Tuna-Fish »

zackmorris wrote: post #67
buboard wrote: post #21

Earlier quoted context omitted.

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

To put this in perspective, the F-1 engine of the Saturn V (one of the biggest rocket engines ever made) only put out about 3.5 times more thrust (6.7 MN or 1.5 Mlbf) than the Raptor (1.9 MN or 0.43 Mlbf). Also some people might not realize how much liquid is being pumped by rocket engines (the Saturn V F-1 could drain a 30,000 gallon swimming pool in 10 seconds): Each second, a single F-1 burned 5,683 pounds (2,578…

Another tradeoff is size. You can pack about 4 raptors in the same area you can fit a single SSME. (Or, 8 in the space you can fit a single F-1). This is relevant because to lift the shuttle on 3 SSMEs, or the SLS on 4, they need to pack in massive amount of thrust in SRBs.

Instead of doing that with all it's cost and safety implications, SpaceX intends to put 31 raptors on the bottom of their rocket. You could not fit the same thrust in other major US engines without making the rocket comically large.

Re: The Tech Behind SpaceX’s New Engine

#75

Post by JumpCrisscross »

burfog wrote: post #71

Looking at the list of cycles on Wikipedia, I'm surprised that nobody seems to have used preburners to pressurize the tanks. Use a fuel-rich preburner to pressurize the fuel tank, and an oxygen-rich preburner to pressurize the oxygen tank. Mixing should be limited even if nothing special is done, due to the temperature and phase of matter and short timeframe. One could of course pay the weight penalty of a piston (ne…

> I'm surprised that nobody seems to have used preburners to pressurize the tanks

This combines the worst of both worlds. You get the weight and complexity of turbopumps. And you get the weight, explosion risk and leakiness of pressure vessels.

Re: The Tech Behind SpaceX’s New Engine

#76

Post by burfog »

JumpCrisscross wrote: post #75
burfog wrote: post #71

Looking at the list of cycles on Wikipedia, I'm surprised that nobody seems to have used preburners to pressurize the tanks. Use a fuel-rich preburner to pressurize the fuel tank, and an oxygen-rich preburner to pressurize the oxygen tank. Mixing should be limited even if nothing special is done, due to the temperature and phase of matter and short timeframe. One could of course pay the weight penalty of a piston (ne…

> I'm surprised that nobody seems to have used preburners to pressurize the tanks This combines the worst of both worlds. You get the weight and complexity of turbopumps. And you get the weight, explosion risk and leakiness of pressure vessels.

Skip the turbopump. That is the whole point of pressurizing.

Pressurizing has weight advantages. You can use a balloon tank. The tank no longer has to have the rigidity to support itself.

Rockets with typical cycles have used balloon tanks. Even modern ones like the Falcon 9 are partially that way, with just enough rigidity to be erected empty on the pad. The Falcon 9 uses helium to pressurize; that could be changed to preburner exhaust even if you did keep the moderate pressure and the turbopumps.

Re: The Tech Behind SpaceX’s New Engine

#77

Post by Symmetry »

SECProto wrote: post #70
Symmetry wrote: post #31

Earlier quoted context omitted.

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 thru…

> 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 mix…

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-ra...

Re: The Tech Behind SpaceX’s New Engine

#78

Post by jessriedel »

jbkkd wrote: post #46
prando wrote: post #24

https://www.netflix.com/title/80119093 My rocket-science knowledge is abysmal, but I thoroughly enjoyed this article and the Netflix documentary that I have linked to was incredible. Anyone even remotely interested in rockets should check it out :).

Unavailable for me in the UK, what's the name of the documentary?

"Cosmodrome".

Re: The Tech Behind SpaceX’s New Engine

#80

Post by rbanffy »

burfog wrote: post #76
JumpCrisscross wrote: post #75

Earlier quoted context omitted.

> I'm surprised that nobody seems to have used preburners to pressurize the tanks This combines the worst of both worlds. You get the weight and complexity of turbopumps. And you get the weight, explosion risk and leakiness of pressure vessels.

Skip the turbopump. That is the whole point of pressurizing. Pressurizing has weight advantages. You can use a balloon tank. The tank no longer has to have the rigidity to support itself. Rockets with typical cycles have used balloon tanks. Even modern ones like the Falcon 9 are partially that way, with just enough rigidity to be erected empty on the pad. The Falcon 9 uses helium to pressurize; that could be changed…

This is one of the reasons SpaceX uses supercooled fuel and oxydizer (and one of the reasons for going methalox in the first place) - the tanks self-pressurize, so they can skip the Helium system.

Pumps are still good because you really don't want the tanks to pressurize at the same pressure as the combustion chamber. If you do, you'll see a rapid disassembly, but your claim on it being unscheduled will be questioned.

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