[FPSPACE] don a. nelson nasa retired engineer

[gorski]

As memory serves, the point of the thermal protection system on the space
shuttle was so they could use a regular, inexpensive aluminum frame.  The
SR-71 was built to much different specifications--it was slightly shorter,
somewhat narrower, much lighter, and enclosed a heck of a lot smaller
volume.  I don't know what the SR-71's maximum skin temperature was, but I
suspect it wasn't near what the RCC on the leading edges of the shuttle
see, either.

The shuttle was designed to enclose a huge space (not just the crew cabin)
and keep it at a reasonable temperature--after all, sometimes they carry
spacecraft back from space (remember LDEF?).

So I don't think there was a good way to try and imitate the SR-71 design
in the shuttle--even if trying to build the much larger airframe out of
titanium wasn't prohibitively expensive, even for a massive government
program like the space shuttle.  The idea was pretty much that if the
thermal protection system was significantly compromised, even a
high-temperature airframe probably wouldn't hold the spacecraft together
for long.

And the whole point of the thermal protection system being designed the
way that it was is that the tiles do not transmit heat from one side to
the other--you can heat them up to glowing on the one side, and they're
still cool enough to touch on the other.  (I saw this demonstration back
in the late 1980s with a blowtorch and a piece of HRSI--the black tiles
from the shuttle underbelly.)  So it's not like you've put aluminum
directly under a titanium skin, which would transmit a lot of heat to the
airframe--if it's not compromised, the TPS is sufficient to keep the heat
out.


As for safety, the first couple of shuttle missions had ejection seats
(I've heard they were modified SR-71 ejection seats, but I don't know
that).  But they realized that there was a very small part of the ascent
during which they would be usable (and none of reentry--high mach speeds
plus hot plasma means a very quick death).  And, ejection systems weigh a
lot so there's a substantial payload penalty for something which brings
almost no added safety... and, there's not a good way to eject astronauts
flying on the mid deck.  So they pulled the ejection seats out after the
"shakedown" flights of Columbia (first few; I forget how many) and never
researched them further for operational flights.

I've heard some facilities for "bailout modes" exist.  The crew wear
pressure suits ever since Challenger, and while I don't know the details
of the modes in which they're used, I'm led to understand that if the
shuttle is in a somewhat controlled flight at relatively low altitude the
crew can bail out (...my guess is this would have to do with engine
failure modes--you can still control your flight with one engine running
even if you can't develop the kinetic energy to make it all the way across
the Atlantic, or something).



The shuttle is not the "magic elevator" we were supposed to believe in
1985.  It's *safe*, but not in the sense that driving a Volvo across a
parking lot is safe... astronauts do have a high risk job, and they know
it.  It's probably safer than the job of "test pilot" which is on so many
pilot astronaut resumes.


Good luck with your further research!

--Christopher Gorski



On Wed, 23 Jul 2008, howard  hoffman wrote:

}greetings:  is it possible that anyone would know how to contact don
}nelson. i am a retired aircraft mechanic , and i have questions. i was
}shocked that the shuttle airframe was manufactured out of 2024 and 7075
}aluminum alloy , same as most of the aircraft i worked on , such as
}boeing 737 , 727 , dc9 , and md80 aircraft. when i say i was shocked , it
}is not a problem using these materials , but to use them directly
}underneath the thermal protection tiles and carbon fiber is a very big
}""""  no - no !!!!  """"  i say this as we all know that aluminum
}structural strength is limited to around 300f to 500f degrees.  but if
}the shuttle had been manufactured using combinations of stainless steel ,
}titanium , and most important , tantalum , do not misunderstand!!! all i
}am saying is this. the shuttle design of using 2024 and 7075 aluminum for
}weight saving is safe as long as these high strength , high temperature
}metals are used as an additional insulator against those 3.000 degree
}temperatures. especially the leading edge of the wings. here i would use
}stainless steel rails and webs. cover this with titanium sheet. finally a
}tantalum sheet over top of this. not one tantalum sheet , but three ,
}four , or even five sheets of tantalum. probably three sheets of tantalum
}would be allowed , but the end result of all of this is that if the
}thermal tiles on the leading edge were to fail , i am cetain that these
}tantalum sheets would survive. i am also certain that the titanium sheets
}under the tantalum would survive. finally the stainless steel structure
}would maintain strength.  i am not a engineer , but i believe that the
}term ,,,,, """"" fail-safe""""""  is what i am talking about. when the
}sr-71 blackbird program was designed , they went the extra mile to ensure
}the safety of those pilots , even to the extreme measure that if they had
}to bail out at 2,000 mph , that they would not burn up , and at 15,000
}feet they could open their parachutes , and i am not seeing the same
}concern for these astranauts. i think that on the next flight , george
}bush should go up , and if he feels it is not safe , then i suggest that
}the shuttle be grouded until modifications are done. so if you read my
}letter , mr. don nelson , my name is howard , and my phone number is
}412-264-2313.  sincerely ,,,,,,,,, howard from pittsburgh,pa