Can any of you Rocket heads explain this?

Discuss life, the universe, and everything with other members of this site. Get to know your fellow polywell enthusiasts.

Moderators: tonybarry, MSimon

Skipjack
Posts: 6102
Joined: Sun Sep 28, 2008 2:29 pm

Re: Can any of you Rocket heads explain this?

Post by Skipjack »

paperburn1 wrote:so it looks like Musk is going after that low hanging fruit, Elon Musk stated that SpaceX were instead working towards a potential staged cycle engine. The forthcoming engine currently under development by SpaceX has been named "Raptor". Raptor will use liquid methane as a fuel, and was stated as having a sea-level thrust of 6,700 kilonewtons (1,500,000 lbf). Since the initial announcement of Raptor, Musk has updated the specification to approximately 230 tonnes-force (2,300 kN; 510,000 lbf)—about one-third the original published figure—based on the results of optimizing for thrust-to-weight ratio.
The Raptor is actually a full flow stages combustion engine, the most efficient type we know to date. SpaceX is indeed optimizing for Thrust to Weight as well as efficiency (Isp, how much fuel the rocket needs to provide a certain amount of acceleration). The problem is that rocket engines get dis- proportionally heavier once they surpass a certain thrust. This is why SpaceX is optimizing for thrust to weight as well. If a total of 3 engines that do 230 tonnes of thrust each weights less than 1 that does 600 tonnes, then it is clearly better to have more, smaller engines.
This also gives you some advantages when it comes to engine out capability (losing one of many engines is not as troublesome as losing one of few) and throttling. The latter is particularly important for powered landings. There is also the possibility to build smaller launch vehicles with those lower thrust engines. They might replace the 3 core Falcon Heavy with a single core Raptor based rocket. They recently got a grant for the Airforce to specifically evaluate their raptor methalox engine for use as an upper stage for (presumably) Falcon9 or Falcon Heavy which could significantly increase payload to higher orbits compared to the lower Isp kerolox Merlins.
What does get more complex is the plumbing to get the fuel to that many engines. I am sure that SpaceX knows what they are doing, though.

DeltaV
Posts: 2245
Joined: Mon Oct 12, 2009 5:05 am

Re: Can any of you Rocket heads explain this?

Post by DeltaV »

paperburn1 wrote:The forthcoming engine currently under development by SpaceX has been named "Raptor". Raptor will use liquid methane as a fuel...
Methane?! Mars is a decoy. Musk is working on establishing Skynet Core on Titan, where the Human Resistance can't get to it.

paperburn1
Posts: 2466
Joined: Fri Jun 19, 2009 5:53 am
Location: Third rock from the sun.

Re: Can any of you Rocket heads explain this?

Post by paperburn1 »

YES, I see its all coming together now. (muh hah hah hah ha) Yes I plan on being a boot-spit lackey of the new machine order.
I am not a nuclear physicist, but play one on the internet.

ladajo
Posts: 6204
Joined: Thu Sep 17, 2009 11:18 pm
Location: North East Coast

Re: Can any of you Rocket heads explain this?

Post by ladajo »

The rocket engine argument about size, weight, count is very similar to the how many outboards and how big should they be for my boat questions.

It is a balance of power, weight, and reliability.
The development of atomic power, though it could confer unimaginable blessings on mankind, is something that is dreaded by the owners of coal mines and oil wells. (Hazlitt)
What I want to do is to look up C. . . . I call him the Forgotten Man. (Sumner)

Diogenes
Posts: 6958
Joined: Mon Jun 15, 2009 3:33 pm

Re: Can any of you Rocket heads explain this?

Post by Diogenes »

paperburn1 wrote:point to ponder, when feeding your turbo pump exhaust back into the reaction chamber it has to be at a much higher pressure than what is in the chamber making the pump more acceptable to failure.

....


This is why I can not teach, i though I had hit the high points but after rereading my posts I clearly missed the question.


I'm still not clear on this. The documentary about the Russian engines talk about higher temperatures and pressures. I get how higher temperatures and higher pressures will improve efficiency. What I don't grasp is how feeding the exhaust stream of the turbine's combustion chamber into the rocket's combustion chamber will result in higher pressures.


I would think you could make the pressure whatever you want it to be by simply feeding fuel and oxidizer in at the higher pressure level, i.e. using a turbo pump designed for higher pressure.


How does feeding the exhaust gases into the rocket combustion chamber result in higher temperatures or pressures?
‘What all the wise men promised has not happened, and what all the damned fools said would happen has come to pass.’
— Lord Melbourne —

93143
Posts: 1131
Joined: Fri Oct 19, 2007 7:51 pm

Re: Can any of you Rocket heads explain this?

Post by 93143 »

I don't have a formal education in rocket design, but I should have enough expertise in the general area to figure out something like this. Lemme take a crack at it:
Diogenes wrote:What I don't get is how dumping exhaust gas from the turbine preburner into the combustion chamber boosts pressure.
It doesn't. But increasing pressure only helps performance so much, and it helps less and less the higher the pressure gets. With an open cycle, there's a crossover point where losing more reaction mass to boost the pressure is no longer a net win (and it depends on how far off stoichiometric you're running the gas generator, and thus on how hot you're willing to run the turbine). But with a closed cycle, there's no such point, so you can crank the pressure as high as you dare.

paperburn1
Posts: 2466
Joined: Fri Jun 19, 2009 5:53 am
Location: Third rock from the sun.

Re: Can any of you Rocket heads explain this?

Post by paperburn1 »

Remember the pressure in the combustion chamber is running around 100 to 200 atmospheres depended on fuel type so your pump pressure has to be that much higher for exhaust to flow into the chamber at a speed high enough to power the pumps. The Turbo pump fails leading to a cascade failure of the rocket Motor I think is the answer your looking for.
I am not a nuclear physicist, but play one on the internet.

Diogenes
Posts: 6958
Joined: Mon Jun 15, 2009 3:33 pm

Re: Can any of you Rocket heads explain this?

Post by Diogenes »

93143 wrote:It doesn't. But increasing pressure only helps performance so much, and it helps less and less the higher the pressure gets. With an open cycle, there's a crossover point where losing more reaction mass to boost the pressure is no longer a net win (and it depends on how far off stoichiometric you're running the gas generator, and thus on how hot you're willing to run the turbine). But with a closed cycle, there's no such point, so you can crank the pressure as high as you dare.
paperburn1 wrote:Remember the pressure in the combustion chamber is running around 100 to 200 atmospheres depended on fuel type so your pump pressure has to be that much higher for exhaust to flow into the chamber at a speed high enough to power the pumps. The Turbo pump fails leading to a cascade failure of the rocket Motor I think is the answer your looking for.


Okay, this is getting there. Not only do you get to use ALL of your reaction mass, you get to use your turbo pump in a region of better efficiency, albeit a region where it is more likely to fail.


The benefits from higher rocket combustion chamber pressures falls off, but this same equation doesn't apply to the turbo pump, and so it is therefore still beneficial to make it run harder.


Or in simpler words, very high turbopump temperatures/pressures still produce worthwhile benefits, and it's exhaust stream can still be used as reaction mass.


I think I am starting to get it.
‘What all the wise men promised has not happened, and what all the damned fools said would happen has come to pass.’
— Lord Melbourne —

Post Reply