🚀 go-pugleaf

In article <ewright.726733131@convex.convex.com> ewright@convex.com (Edward V. Wright) writes:
>In <1993Jan9.030346.9714@ptdcs2.intel.com> greason@ptdcs2.intel.com (Jeff Greason ~) writes:
>>1) A thermal protection system, capable of surviving multiple reentries
>>   (many) suitable for reuse.  On previous vehicles, this is ablative and
>>   non reusable, or (on Shuttle), heavy and only marginally reusable.
>
>Nonsense.  The first US ICBM warhead (Atlas) used a nonablative copper
>heat shield.  Copper's too heavy for SSTO, but there are many newer
>refractory metals that would work just fine.  The problem is not that
>there's no TPS available, it' choosing from among several alternatives.

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. It tries to *penetrate* the atmosphere as rapidly 
as possible to avoid interception. It's dwell time is very short and it's 
protected mostly by the opaque plasma sheath that forms a boundary between it 
and the atmospheric friction. Copper is a very good conductor of heat,
second only to silver, but the passage through the atmosphere is so brief
for a warhead that the heat doesn't have time to migrate deeply into the
vehicle and prematurely cook off the conventional explosive lenses.

A lander presents a blunt surface to the atmosphere and tries to shed
as much velocity as possible by atmospheric braking. The dwell time is very
much longer, and the heat loading much higher. Shuttle designers originally
considered a titanium skin for the Orbiter, but even a metal as refractory
as titanium wasn't up to the job (besides driving up fabrication costs
dramatically), so they chose to use refractory silicates in the form of
tiles. These are very poor conductors of heat, you can place your bare
hand against one side of the tile while playing an oxy-acetylene torch
on the other and not notice a temperature rise. There has been much progress 
in artificial ceramic refractory materials since Shuttle was designed, and 
better choices are likely available now. Indeed, special refractory blankets 
have replaced tiles in certain non-critical areas on Shuttle. But copper, or 
any other metal won't do for a lander as opposed to a hypersonic plunger like 
a warhead.

Gary
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