From: gary@ke4zv.uucp (Gary Coffman)
Subject: Re: Let's be more specific (was: Stupid Shut Cost arguements)
Reply-To: gary@ke4zv.UUCP (Gary Coffman)
Organization: Destructive Testing Systems
Date: Tue, 12 Jan 1993 17:15:25 GMT

In article <ewright.726776389@convex.convex.com> ewright@convex.com (Edward V. Wright) writes:
>In <1993Jan11.154812.235@ke4zv.uucp> gary@ke4zv.uucp (Gary Coffman) writes:
>
>>A warhead re-entry vehicle is not a good model for a lander. A warhead
>>doesn't attempt to do atmospheric braking. It's shaped like a long narrow
>>cone, or hypersonic bullet.... A lander presents a blunt surface to the 
>>atmosphere and tries to shed as much velocity as possible by atmospheric 
>>braking. 
>
>Ah, which lander are you talking about.  The Delta Clipper *is* shaped
>like a bullet.  It does not present its blunt surface (base) on entry.
>It makes a nose-first, high-angle-of-attack entry modelled after an
>ICBM-warhead trajectory.  This was chosen because of the large amount
>of data available from computer modelling of missile warheads.
>
>It seems rather presumptuous to claim you know more than McDonnell
>Douglas engineers working on the project about what is and isn't
>possible when you are uninformed on such basic facts as this. 

I don't have the direct pipeline into MacDD that Allen seems to have,
but from what little description he's posted, I don't see anything
about Delta Clipper retaining a 30 km/sec velocity to near ground
level. Nor would I consider a 1,279,000 pound (104,100 pound dry), 
40 foot in diameter, 127 foot tall vehicle "a bullet". Certainly not 
when compared to a 8 inch in major diameter, 4 foot long, 80 pound 
nuclear re-entry vehicle.

From the descriptions I've seen posted, DC-Y is supposed to enter
nose first, aerobrake down to under 1 km/sec, then turnover and
descend on it's rocket exhaust. From Alan's latest post, the vehicle
is supposed to be subject to only 95 psf Q at turnover. If that's a 
faulty description, please post the actual flight profile.


>>Shuttle designers originally
>>considered a titanium skin for the Orbiter, but even a metal as refractory
>>as titanium wasn't up to the job 
>
>Yet Another Historical Error.  Refractory metals were up to the
>job until NASA doubled the size of the Shuttle orbiter to meet 
>military payload demands.  (And some engineers at Rockwell still
>felt that refractory metals were viable, given sufficient ingenuity.
>Langley, and possibly other NASA centers have since come up with 
>refractory metals which they believe can do the job.  Some of these
>were considered for use on the fifth orbiter.)

Even the SR71 uses fuel to help cool it's titanium skin, and it travels
more than four times slower than a re-entry vehicle. The only practical
metal more refractory than titanium is tungsten, and no one would seriously
consider a tungsten skinned vehicle. It would weigh too much, not to mention
the fabrication difficulties or the cost. Using titanium instead of aluminum
to *back up* a lighter heat shield is certainly practical, but with the 
extremely good insulating qualities of available ceramic refractories, it 
isn't necessary. An active cooling system could negate the need for a ceramic
shield, but would likely weigh more and be prone to breakdown. A heat shield
has to do more than not melt, it has to protect the internals of the vehicle
from the extremely high heat loads. All metals are good heat conductors, so
they make poor heat *shields*. That's why *insulating* ceramic refractories
are a preferred solution. Honeycomb composite structures are another good
solution, but remain very expensive to fabricate.

Gary
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