Re: SpaceShipOne and reentry heat

From: Hephaestus (hephaestus_at_phreaker.net)
Date: 06/26/04 

To: sci-space-tech@moderators.isc.org
Date:  26 Jun 2004 11:57:25 -0700

Daniel Walker <dw229@hermes.cam.ac.uk> wrote in message news:<Pine.LNX.4.60.0406241716070.10924@hermes-1.csi.cam.ac.uk>...
> Reading these postings about SS1's re-entry and from what we've heard
> recently regarding the space shuttle's TPS and its problems, why have
> spacecraft tended not to use engine-based deceleration to reduce the
> thermal effect of re-entry? If the orbital velocity is a major factor in
> re-entry speed, why not take this off and just drop in? Dropping in from
> 400km would give you a nominal speed of ~2.8km/s if you reached the
> Earth's surface (assuming I've done my sums right!).
>
> I imagine the answers will involve the excess fuel needed, manoeuvring,
> restartable engines, etc., so at what point (what altitude) does the
> weight of thermal protection tiles beat the extra fuel needed for an
> orbital craft? I know there's lots of variables involved, but wondered if
> anyone had considered this?

When you see a rocket launch, the vast majority of it (about 95%) is just
there to get the other 5% into orbit. That includes the fuel, engines, and
so on required to get the payload to the required velocity. Conveniently,
zeroing out the velocity like you propose takes exactly the same amount of
work, so if you have a full size rocket in orbit, it can return the payload
to earth via your mechanism.

Unfortunately, since the payload fraction is 1/20th, you've just increased
the amount of weight that must be launched by a factor of twenty! For example,
for a one ton payload, instead of a 20 ton rocket to launch it into orbit, you
now need a 400 ton rocket to launch the payload plus 20 ton return rocket into
orbit. It's easier to build a TPS, no matter how exotic, than to eat a 20-fold
increase in rocket size.

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