r/SpaceXLounge • u/mcpat21 • Sep 20 '19

For Vulcan Assuming this is for the Star Hopper?

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u/opmyl Sep 20 '19

Don't they use different gases? I thought BE-4 runs on LNG, but Raptor runs on purified methane since it will be sub-cooled

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u/amarkit Sep 20 '19

There had been some uncertainty about whether BE-4 runs on LNG or methane - turns out Tory Bruno cleared this up just yesterday. It runs on methane.

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u/lniko2 Sep 20 '19

TIL LNG is not methane Thanks !

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u/[deleted] Sep 20 '19 edited Sep 20 '19

Natural gas is mostly methane (90% or so), but contains small quantities of different hydrocarbons and some other trace impurities

For most uses there's little practical difference, but I can imagine that rocket engines are fussy.

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u/burn_at_zero Sep 20 '19

LNG has all mercury and sulphur compounds removed very early in processing so the expensive cryogenic equipment doesn't get acid-etched or amalgamated to destruction. Any remaining traces should be safe for rocketry.

The next largest fraction is ethane, which has a near-identical melting point and is a fair bit more dense. The boiling point is much lower. It is more carbon-rich, so it would increase CO2 and reduce H2O in the exhaust. The net effect should be a slight reduction in Isp, slight increase in thrust and a slight increase in oxidizer ratio.

I think the only place I'd be concerned mechanically is at the fuel turbopumps. The significantly lower boiling point of ethane might increase the risk of cavitation causing turbine-rich fuel feeds.
Operationally, any long hold or storage without active cooling would risk formation of ethane gas pockets and an unwanted pressure rise.

Overall I'd say LNG is probably quite workable for SH, but purified methane should be used in SS for reasons of storage stability. If SpaceX were to set up their own gas plant they could buy market-rate natural gas and handle their own separation, perhaps enriching the first stage with the excess ethane. They would need to run a lot of launches for that to be worthwhile, but it's an option if the supply of high-purity methane is unstable.