What is the reason that Red Dragon is testing supersonic retropropulsion on a Mars mission. Is the Earth's upper atmosphere too different to that of Mars to validate this method as a viable way of landing on Mars?
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1$\begingroup$ My uninformed guess would be that there is pretty high confidence that it will work - based on their computational fluid dynamics modeling - excellent YouTube presentation here - which they're good at because they use Tesla GPUs (partially humor) - and other things. In a complex system with a commercial budget (as opposed to a moon-shot budget) you try to test as many things at the same time as you can with a good chance of success. There may of course be substantial prior research out there as well. $\endgroup$– uhohCommented Jul 28, 2016 at 7:17
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$\begingroup$ Related: How is retropropulsion by the Falcon 9 first stage being used to design retropropulsion entry on Mars? $\endgroup$– Martin SchröderCommented Aug 1, 2016 at 22:58
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$\begingroup$ These pdfs are related; A Survey of Supersonic Retropropulsion Technology for Mars Entry, Descent, and Landing and Maximum Attainable Drag Limits for Atmospheric Entry via Supersonic Retropropulsion. $\endgroup$– uhohCommented Jul 9, 2017 at 6:30
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They have been testing supersonic retropropulsion on Earth: the Falcon 9 landings do their reentry burn in circumstances not too different from Mars' atmosphere.
I've seen no announcement of a Dragon supersonic retropropulsion test on Earth, but that doesn't mean there won't be such a test. SpaceX does a lot without publicly announcing it.
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The Martian atmosphere is very much thinner than Earths. So SRL on Mars needs to happen in the equivalent of the density of Mars's atmosphere.
Thus when the Falcon 9 first stage is coming back for a landing on the ASDS ship (No longer a barge! Don't be a barge-ist on this site!) or back at LZ-1, they are collecting reentry data to share with NASA as it passes through similar densities.
There was a great presentation by Larry Lemke, at SETI talks where he talks about EDL (Entry, Descent, Landing) on Mars, and that there is a 'sporty' way to do it with a lifting body capsule, where you would actually fly down closer to the surface, into the densest part of the atmosphere, and then fly up, to stretch out how long you have to decelerate via aero breaking.
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$\begingroup$ "The Martian atmosphere is very much thinner than Earths" - I realise that, but there will be an altitude on earth where there is an identical density, it will just not be the same altitude on both planets. The rate of change of density will not be the same however. $\endgroup$– Mike HCommented Jul 29, 2016 at 4:26
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$\begingroup$ I'm looking at the video of LL's SETI talk. Slide 5 at about 17:00 - there seems to be a return-to-earth rocket, fully fueled, sitting inside the Dragon capsule which has propulsively landed. Is there some place I can read more about that - especially the math? Oh, the answer to Phil Harwood's question at 14:50 https://www.youtube.com/watch?v=Nz60GcmKOvc - Indeed a ship. $\endgroup$– uhohCommented Jul 29, 2016 at 11:00