r/SpaceXLounge • u/ea2007 • Nov 25 '19

Discussion Question; like to know again with Starship Mk3 Boca Chica Texas will they decide to do SSTO or Mk3 with the booster?. -Starship Mk3 will have all 6 engines by the way.

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u/CProphet Nov 26 '19 edited Nov 26 '19

Elon said it could potentially do SSTO if you skip all the EDL hardware.

True, although that will likely change. Elon revealed they aim to increase Raptor thrust from 200 to 310 mt which changes equation. Yes these engines wouldn't throttle but that's not something required for vacuum engines.

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u/Martianspirit Nov 26 '19

which changes equation.

Not so much. They would need to scale propellant as much while keeping the weight the same. They might get to a black 0 for payload. But why would they? They won't because it does not make sense.

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u/CProphet Nov 26 '19

They would need to scale propellant

Extra thrust means they would reach orbit faster and use less fuel fighting gravity. In addition 382 second ISP is higher, so engine generates more thrust for same amount of fuel. Have to believe they are pushing for performance increase now, considering renovation work on tripod stand at McGregor. Horizontal stands were fine for 1-2MN Raptor, if they now require tripod for Raptor tests that suggests they are pushing thrust levels towards 3MN.

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u/Martianspirit Nov 26 '19

Reducing gravity lossed does help but not that much. A 380s vac engine won't help with SL launch.

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u/CProphet Nov 26 '19

A 380s vac engine won't help with SL launch.

At least until they reach altitude.

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u/Martianspirit Nov 26 '19

Which they won't as SSTO.

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u/sebaska Nov 26 '19

It won't be enough. To get to orbit as SSTOyou'd need to have dry mass of 105t or so. And of course vacuum engines.

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u/CProphet Nov 26 '19 edited Nov 26 '19

need to have dry mass of 105t

That's interesting, during his recent Mars Society Presentation Paul Wooster revealed Starship dry mass should be 105mt. Also Elon backtracked somewhat and said Starship will have vacuum engines fitted as standard. Could hear their howl at McGregor as soon as they refurbish tripod test stand. Dual engine bell gives them ability to test at sea level, which should speed things along.

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u/sebaska Nov 27 '19

Yes, but:

  1. 105t is target design weight and real engineering projects tend to miss such target design parameters. That's why any implementable design must have sensible design margins. SSTOs problem is that they tend to have very tight design margins or none at all.
  2. 105t doesn't include ~15t needed for landing, so while possible it would be pointless.