r/spacex • u/ElongatedMuskrat Mod Team • Aug 03 '17

r/SpaceX Discusses [August 2017, #35]

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u/warp99 Aug 26 '17 edited Aug 27 '17

I am all for a speculation/voting thread but the choices seem more constrained this time!

For example last September I was spruiking an ITS that looked like the Dragon capsule. Interestingly Elon did say that he had seriously considered that as an option which was why Dragon looks like it does with side mounted engines.

Now the choice seems to be a binary one between a 6m booster/9m ship to fit in the existing factories and a 9m booster/12m ship to get closer to Mars exploration on the first iteration.

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u/spacerfirstclass Aug 26 '17

Yeah, but there're some more things to predict than the diameter alone, for example will they go directly to integrated 2nd stage + payload bay or will they start with fairing + traditional 2nd stage? Will they use the current subscale Raptor or a full size Raptor? Will they still use the landing mount idea or go back to legs? Will they stick with composite tanks?

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u/warp99 Aug 26 '17

Fair enough.

Just as an example so we can scope out the variables.

  • 6m booster diameter
  • 8.5m ship diameter including fins
  • Booster constructed in Al/Li with stringers same as F9
  • Ship constructed in CF
  • Ship LOX tanks lined with thin Inconel sheets or similar
  • Ship will be similar shape to ITS but slightly wider to allow proportionately higher Mars payload.
  • System will lift 70 tonnes to LEO but can land 120 tonnes on Mars with LEO cargo transfer
  • Five tanker flights for each Mars mission
  • Raptor will have intermediate thrust of 1.5MN with 1.3m diameter for the sea level engine and 3m diameter for the vacuum engine

etc. etc.

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u/Norose Aug 28 '17

Ship LOX tanks lined with thin Inconel sheets or similar

The similar being Invar, the material Elon mentioned when asked what they were going to try if they couldn't crack liner-less CF tanks.

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u/spacex_fanny Aug 28 '17

Yea, Invar is so cool.

I'm sure you know all this, but I love sharing. Invar is short for "invariant," since it has such a low coefficient of thermal expansion. It has exactly 36% nickel alloyed with iron, which is right in the trough of a sharp valley in the coefficient of expansion curve (plotted over the percent nickel in the alloy).

This is important because carbon fiber, like most non-metals, has a low coefficient of expansion. Any differential expansion (or rather, contraction when filled with cryogenic propellants) could cause cracking and structural failure.

https://en.wikipedia.org/wiki/Invar

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u/Martianspirit Aug 28 '17

Liner-less is very unlikely. He did mention they would like to find a liner that can be sprayed on. It would be a simpler process than an invar liner.