Newsgroups: sci.space.history,sci.space.shuttle
From: Henry Spencer <henry@zoo.toronto.edu>
Subject: Re: CHALLENGER Accident Question
Date: Sat, 25 Apr 1998 17:58:51 GMT

In article <cbingmanErwDon.6yp@netcom.com>,
Craig Bingman <cbingman@netcom.com> wrote:
>>I think it would have had to be considerably higher, not just a little
>>higher.  However, yes, in principle the orbiter could survive such an
>>accident if aerodynamic pressure was low enough.
>
>I'm not sure, but might it be there isn't a point where the solids are
>still attached and burning, and the aerodynamic forces are low enough for
>the stack to survive a burnthrough of the attachment points for the solids.

Quite possibly; I don't have the sort of detailed numbers that would be
needed to say for sure.  The orbiter is definitely pretty well out of the
atmosphere by SRB burnout time, but it's also moving pretty fast, and I
don't know what the dynamic pressure is like, and that's the real bottom
line.

>...As far as I know, there is no reaction control activity
>while the main engines are burning in a normal launch...

Correct.  Not only would it be wasteful, it would be completely useless,
since the RCS thrust is so tiny compared to that of the main engines.

>I sort of have a question at this point.  There was some discussion of
>using solids for launching human payloads at one point.  I believe this
>was to be a variant on the shuttle first stage engines.  Has anyone ever
>"shut down" a segmented solid by pyrotechnically cutting it apart at the
>segments?

You can shut down most solids by blowing open vents in the casing, because
modern solid fuels don't burn well except at high pressure.  In fact, most
solid-fuel ICBMs do shut down their solid stages that way, to minimize
trajectory errors.

The original Titan III SRBs had a shutdown system of that kind (you can
see the outlines of the shutdown ports on the SRB nose cones), because of
the intended use to launch Dyna-Soar.  Dyna-Soar's main abort mode was to
separate from the booster, but it was thought desirable to terminate
booster thrust to make sure that the loose booster didn't catch up with
the escaping Dyna-Soar.  (The shutdown system was deleted when it became
clear that neither Dyna-Soar nor MOL would ever fly.)

>I somehow doubt that this would be useful for the shuttle, but in
>an all solid fuel, or solid first stage situation, it might at least shut
>the damned thing down to the point that you could pull the crew off.

NASA repeatedly looked at SRB thrust termination for the shuttle.  Trouble
is, it's a fairly violent process, and the conclusion consistently was that
the orbiter and ET were not strong enough to survive SRB shutdown.  Note
that this isn't quite the same situation as with Titan, because for the
shuttle you have to shut down the SRBs *before* you can safely do any sort
of separation or ejection.
--
Being the last man on the Moon                  |     Henry Spencer
is a very dubious honor. -- Gene Cernan         | henry@zoo.toronto.edu

Newsgroups: sci.space.shuttle,sci.space.history
From: henry@spsystems.net (Henry Spencer)
Subject: Re: Man rating process (was : Re: Shuttle operational costs question)
Date: Thu, 10 Feb 2000 21:50:11 GMT

In article <87uqk3$85a@teal.sni.net>, Bruce Ediger <bediger@csn.net> wrote:
>There was an urban legend floating around Martin in the late '80s that
>Titan III (B, C, D, E) and T34D were "man rated" because the Air Force
>wanted to use Titans to fly the DynaSoar thing.

No myth about it, as far as the IIIC went anyway.  For example, if you
look at good photos of the early test IIICs, you will see the thrust-
termination ports on the noses of the SRBs, which are there to shut them
down and give an aborting Dyna-Soar a better chance to get clear.  The
thrust-termination system was deleted on the production IIICs, since it
was clear by then that they weren't going to be used for manned flight,
but it wouldn't be at all surprising if less obvious bits of engineering
practice from those days lingered.

(Since somebody will surely ask:  NASA approved SRBs for the shuttle on
the assumption that emergency thrust termination *would* be possible, but
did not revisit the decision when that turned out to be untrue.  The
problem is that thrust termination of large solids is a somewhat violent
process, and the ET and orbiter aren't strong enough to survive it.)
--
The space program reminds me        |  Henry Spencer   henry@spsystems.net
of a government agency.  -Jim Baen  |      (aka henry@zoo.toronto.edu)