[AR] Re: SpaceX Single Stage to Orbit -wings
- From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
- To: Arocket List <arocket@xxxxxxxxxxxxx>
- Date: Mon, 3 Jun 2019 19:42:59 -0400 (EDT)
On Mon, 3 Jun 2019, Peter Fairbrother wrote:
[re F9 first stage:] Vertical landing under rocket power was retrofitted
into the system after early attempts made it clear that the splashdown
concept simply wouldn't work -- the stages broke up during reentry.)
Do you mean the first stages broke up on splashdown? Not meaning to be
pernickety, but if it broke up on reentry, wouldn't the vertical landing
stages break up too?
My understanding (with the caveat that SpaceX is not exactly talkative
about things that go wrong!) is that the breakups were on reentry, or at
least mostly so. The vertical-landing stages do a braking burn just
before reentry, to reduce aerothermal and aerodynamic loads. They also
probably benefit from considerable accumulated experience guiding tweaks
to the design.
Didn't they expect that?
Nope. As of early 2011, they'd taken the parachutes off the first stages
in favor of more telemetry hardware, to get a clearer picture of just what
was going on during reentry.
Though vertical landing under rocket power does seem to work for them, I
wonder about human landings - I know Soyuz uses last-minute braking
rockets, but perhaps that is a bit different somehow?
It's different in one respect: as I understand it, a Soyuz landing with a
complete braking-rocket failure is considered survivable for the crew.
Not pleasant, and possibly injurious, but not fatal.
As any Harrier pilot will tell you :-), a propulsive landing is actually
better than a runway landing in some ways: "it's easier to stop and then
land, than to land and then try to stop". You do need a fallback of some
kind in case of a major propulsion failure (which might just be crushable
structure etc. to absorb impact -- helicopters do a lot of that).
Or did the Russians get it right all that long time ago - on reflection
it doesn't seem that difficult, get radar height, differentiate to get
radar vertical velocity, consult precomputed table of height/velocity,
fire rockets according to table.
If memory serves, they actually use a gamma-ray (!) ground-proximity
detector -- immune to interference, immune to bad weather, insensitive to
the exact nature of the surface.
Henry
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