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The Tech Behind SpaceX’s New Engine

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Re: The Tech Behind SpaceX’s New Engine

#111

Post by jessriedel »

ballooney wrote: post #86
rbanffy wrote: post #80

Earlier quoted context omitted.

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…

This is completely, completely wrong. Really honestly confused nonesense. Supercooling lowers the ullage pressure, if anything increasing the need to pressurise (self or otherwise), because the self pressurisiation (the vapour pressure) is lower. So you have to do additional work to feed the pumps. The only reason to supercool is to increase the density. Your comment is quite incorrect.

I really appreciate your expertise and that you're taking the time to correct the sometimes overeager amateurs in this thread. But can I suggest you tone down the unnecessary invectives? It's true that there ought to be some social penalty paid when people give confident wrong answers -- it's a pet peeve of mine -- but I think it's sufficient to just say something like "Your comment is incorrect. Here's why, in detail. Please don't project so much false confidence next time." That still stings for the other person to hear, as it should, but it leaves them much more likely to want to learn more. And it's much less likely to escalate into a fight that distracts from the main subject.

Re: The Tech Behind SpaceX’s New Engine

#112

Post by ballooney »

buboard wrote: post #110
ballooney wrote: post #83

Earlier quoted context omitted.

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.

does the mass of the exhaust fuels count as well? whats the percentage of thrust being lost in a gas generator?

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.

Re: The Tech Behind SpaceX’s New Engine

#113

Post by Hypx »

btilly wrote: post #97
Hypx wrote: post #90

Earlier quoted context omitted.

That's not quite correct. SLS is still slated to launch June of next year. It may get delayed to 2021 if problems are found. Also, Orion already flown in 2014, and will undergo an abort test in April.

You are right. After poking around for a bit, I found that SLS+Orion can launch without the upper stage. The upper stage allows more to be carried on the launch. That said, I'm still on the side of betting with history. In most organizations, the deadline is the first date that nobody can (yet) disprove. When a deadline depends on problems not happening on this project that historically have been common, I think it i…

Most of the structural and integration tests are already complete though. We're far into the "beta testing" phase as it were. Also, the BFR isn't anywhere near the state that the SLS is in right now.

Re: The Tech Behind SpaceX’s New Engine

#114

Post by erobbins »

ballooney wrote: post #83
CarVac wrote: post #9

Earlier quoted context omitted.

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…

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.

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?

Re: The Tech Behind SpaceX’s New Engine

#115

Post by ballooney »

jessriedel wrote: post #111
ballooney wrote: post #86

Earlier quoted context omitted.

This is completely, completely wrong. Really honestly confused nonesense. Supercooling lowers the ullage pressure, if anything increasing the need to pressurise (self or otherwise), because the self pressurisiation (the vapour pressure) is lower. So you have to do additional work to feed the pumps. The only reason to supercool is to increase the density. Your comment is quite incorrect.

I really appreciate your expertise and that you're taking the time to correct the sometimes overeager amateurs in this thread. But can I suggest you tone down the unnecessary invectives? It's true that there ought to be some social penalty paid when people give confident wrong answers -- it's a pet peeve of mine -- but I think it's sufficient to just say something like "Your comment is incorrect. Here's why, in detai…

All fair, valid and noted. It’s the eve of Opportunity’s demise, and i worked on mars EDL, and the infinite tide of javascripters making statesmanly-yet-quite-mistaken proclamations on physics can feel quite disrespectful to the actual engineers who work on this stuff now, worked on the stuff on both sides in the cold war, and everyone back to goddard, tchiolkovsky and moore. But you are right. More whisky and no more HN.

Re: The Tech Behind SpaceX’s New Engine

#116

Post by avmich »

newnewpdro wrote: post #106
JumpCrisscross wrote: post #20

“Benefits of the full-flow staged combustion cycle include turbines that run cooler and at lower pressure, due to increased mass flow, leading to a longer engine life and higher reliability.” TL; DR Full flow lowers turbine temperatures at the expense of parts complexity. Given turbopumps are the devil’s ass part of rocketry, this has been a sought-after technology. The pay-off isn’t so much efficiency as much as lon…

I'm confused; wouldn't the oxidizer-rich half run hotter? Isn't that challenge the Russians overcame but the US punted on back in the days of the space shuttle? Your post and the referenced wikipedia article claims the turbines run cooler. Wouldn't that only be true for the fuel-rich side?

> wouldn't the oxidizer-rich half run hotter?

You try running it as hot as possible, but without burning metal. You have about twice as much liquid oxygen by volume than kerosene in a typical engine, but you can heat kerosene to higher temperatures without burning. So... I don't think you'll get higher temperatures with oxygen gas generator?

Re: The Tech Behind SpaceX’s New Engine

#117

Post by avmich »

erobbins wrote: post #114
ballooney wrote: post #83

Earlier quoted context omitted.

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.

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?

> 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?

Re: The Tech Behind SpaceX’s New Engine

#118

Post by paulsutter »

To model Raptor's hypersonic turbulent combustion SpaceX used an internally developed simulator, which uses wavelet compression to vary resolution across many orders of magnitude in both time and physical dimensions:

https://www.nextplatform.com/2015/03/27/rockets-shake-and-ra...

Here is a fantastic talk from the NVIDIA conference:

https://www.youtube.com/watch?v=txk-VO1hzBY

Re: The Tech Behind SpaceX’s New Engine

#119

Post by avmich »

Gibbon1 wrote: post #50
Tuna-Fish wrote: post #41

Earlier quoted context omitted.

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 accele…

> 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.

Isp often defined as "thrust per unit of mass flow". That is, how many Newtons of thrust engine gets from each kilogram per second of propellant spent. Since N = kg * m / s^2 , and mass flow is in kg / s , Isp becomes (kg * m * s) / (s^2 * kg) = m/s , i.e. Isp has units of speed. In vacuum Isp is equal to the speed of gases flying from the engine.

Re: The Tech Behind SpaceX’s New Engine

#120

Post by ridgeguy »

ridgeguy wrote: post #92
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 :).

Excellent, thank you. There's a parallel between Sergei Korolev and Elon Musk, I believe.

To amplify just a bit - I watched the linked video. The parallel that struck me was the description of Korolev as an amazingly multitalented person. I regard Musk as a polymath as well. It interested me that these two chose work that requires understanding of many disciplines (fluid dynamics, combustion chemistry, materials science, control systems, etc.)and were so effective as individual contributors and as leaders.
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