XPost: rec.arts.sf.written, sci.space.policy, sci.physics
From: jfindley@cinci.nospam.rr.com
In article ,
rgregoryclark@gmSPAMBLOCKail.com says...
> =====================================================================
> "Jeff Findley" wrote in message
> news:MPG.359094b723ade8e0989ad9@news.eternal-september.org...
>
> In article ,
> rgregoryclark@gmSPAMBLOCKail.com says...
> > The reason why it?s so high for the TSTO is that the first stage has to do
> > a
> > boost back maneuver to return to the launch site. But for the SSTO you
> > just
> > let the Earth rotate beneath you until you are over the landing site then
> > land vertically.
> >
>
> Don't forget Falcon 9 first stages also do a reentry burn to reduce the
> velocity before hitting the atmosphere. On a reusable SSTO version of
> the same, you'll either need to reserve a *hell of a lot* of propellant
> for that burn or you'll really need to beef up the TPS.
>
> Jeff
> --
> =====================================================================
>
> As I recall it doesn't take much of a delta-v for a capsule or rocket stage
> in orbit for a deorbit burn, in the range of 100 m/s. The reason is that
> once it is lowered just a little into the atmosphere, air drag does the rest
> to cause it to fall from orbit.
>
>
You're still not understanding and I don't understand why. The problem
with doing a minimum reentry burn is that when a Falcon 9 first stage
hits the atmosphere at that velocity, it burns up! It doesn't have a
massive amount of TPS like the space shuttle orbiter had. It has the
bare minimum TPS to protect sensitive areas like around the engines.
Look, Falcon first stages are on a suborbital trajectory so they didn't
think they needed a reentry burn at all. Early Falcon first stage
recovery attempts used parachutes with no reentry burn. It didn't work!
The stages burned up on reentry because it was coming in too fast and
had too much aerodynamic heating causing the stage to disintegrate upon
reentry. SpaceX found that they *needed* a reentry burn to get the
velocity down low enough that the stage does not perform a destructive
reentry.
Once you need a reentry burn (for your suborbital stage), that means
your engines need to be restartable. Once you make them restartable,
you might as well do a landing burn instead of trying to use parachutes.
That's the short reasoning behind how Falcon first stages went from a
"simple" landing technique that didn't work to a more complicated
technique which *always* involves a reentry burn to reduce the velocity
of the stage so it doesn't burn up.
You're asserting you can do a minimum reentry burn and somehow the stage
won't burn up on reentry. This is, quite frankly, b.s. You need to:
1. Perform a reentry burn to get the velocity down to the reentry
velocity used by current, suborbital, Falcon 9 first stages. This will
need *a lot* of propellant since you're starting at orbital velocity.
2. Beef up the TPS on the stage so it can handle a higher reentry
velocity without burning up. This will require *mass* to be thrown at
the problem in the form of additional TPS.
3. A combination of the above two solutions would work as well.
There is no free lunch here! You can't take a stage designed to reenter
at a low velocity and expect it to survive a reentry from nearly orbital
velocity. The math doesn't work!
Jeff
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