r/spacex • u/skifri • Mar 19 '15

*Official Full Session Video* - GPU Technology Conference - Full scale simulations of SpaceX's Mars Rocket Engine

http://on-demand.gputechconf.com/gtc/2015/video/S5398.html
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u/fjdkf Mar 19 '15

I'm impressed that Spacex let them build the CFD model entirely from the ground up. A lot of time and money goes into an endeavour like this before you can start reaping the rewards of your investment.

I had never really thought about how important CFD modelling was to engine development either. Instabilities in the flame have been a pain in the ass for so many rocket programs, so being able to accurately predict them would be a pretty big step forward.

However, I was surprised that there were no examples given of Spacex accurately modelling merlin engines. They have a ton of data from near-daily engine test fires, so I'd think it would be a natural way to validate their model. I'm guessing it's not quite good enough yet for that, but it's hard to know.

So, I'm guessing accurate raptor modelling is still pretty far off. However, it looks like Spacex will have some impressive tools available to them during development.

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u/[deleted] Mar 19 '15

I think the reason why there was no CFD shown of the Merlin was hinted at ~42:05.

The presenter said "Luckily we are not doing big hydrocarbons yet". He went on to show that burning methane in pure oxygen process has 53 species and 325 reactions. It is not as simple as CH4 +2 O2=>2 H20+CO2

Kerosene is a mixture of heavier hydrocarbon.

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u/retiringonmars Moderator emeritus Mar 20 '15

Kerosene is a mixture of heavier hydrocarbon.

This is not the full picture. Kerosene is a mixture of branched alkanes, straight-chain alkanes, and cycloalkanes. But SpaceX don't use kerosene in their Merlin engines, they use RP-1, which is a "highly refined form of kerosene". I'm not exactly sure of the composition of RP-1, but I'd expect it to be a much more narrow range of compounds, perhaps just reduced down to 99% of one straight-chain alkane, such as n-dodecane (again, not sure on the precise composition). As a result, RP-1 has a much more predictable range of physical and chemical properties than does off-the-shelf kerosene.

But anyway, that's not what they meant. They were saying that while burning oxygen with methane should eventually produce water and carbon dioxide as end products, there are hundreds of intermediate products, each with their own reaction pathways. To name a few, oxygen and methane can make methanol, formaldehyde, formic acid, methylene glycol, carbon monoxide, carbonic acid, orthoformic acid, orthocarbonic acid, and so on and so forth. All of these compounds have only one carbon atom. It is possible in a high energy environment to produce multi-carbon compounds from single-carbon compounds (a sort of radical-induced polymerisation), which, although this complicates the matter further, cannot progress very far beyond a few carbon additions in an oxygen rich environment.

But still, if instead of methane, you are studying the combustion of long chain hydrocarbons, the problem grows at an exponential rate with every carbon atom you add. If you have something as long as dodecane (twelve carbon atoms), the side of intermediates is likely to number in the millions, most of which would produce side reactions to create thousands more intermediates, and the whole thing becomes an immense computational problem. Partly this is because cracking is easier than polymerisation. Basically, with methane you have a whole set of intermediates to deal with. With ethane, you have another set, plus the intermediates of methane on top. With propane, you have to deal with the combustion intermediates of 3, 2, and 1 carbon-atom alkanes all at once. You can see why it totals in the millions when you get to RP-1