🚀 go-pugleaf

In article <1993Jan12.171525.7437@ke4zv.uucp> gary@ke4zv.UUCP (Gary Coffman) writes:
...[others]
>>>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

	Actually, tungsten (and most refractory metals) wouldn't work
very well, since they oxidize at catastophic rates when they get too
hot (in air, of course), not to mention the weight.  And, while
titanium is more heat tolerant than aluminum, it still will crap out
at reentry temps.  I have heard of some really exotic plated light
metal alloys for making actively cooled honeycombs for the NASP.  They
looked at using our tunnel for some sample tests in combusting
environments, but we couldn't get the heat transfer rates they wanted
in the required time frame.  Our tunnel typically runs heat transfer
rates of 1.0-1.5 MW/m^2, and they were looking for more like 3-5
MW/m^2.  The upshot is that if you don't have a skin which can run
very hot in an oxidizing atmosphere (lots of atomic oxygen!), backed
up with good insulation, you'll have to go to either active cooling or
ablative cooling, depending on the length of exposure.
	We use water cooled nickel walls for our combustor, which have
proven very durable.  I have also tried mild steel with a nickel
plate, to save money, and it has done pretty well also.  The nickel
superalloys are heavier than titanium, but they will hold strength to
higher temps, and will resist oxidation, unlike tungsten and such.
-- 

Brad Whitehurst    |   Aerospace Research Lab
rbw3q@Virginia.EDU |   We like it hot...and fast.