Date: 25 Jun 93 18:24:34 GMT
From: Dani Eder <eder@hsvaic.boeing.com>
Subject: Alan Shepard-- DCX vs. The Shuttle
Newsgroups: sci.space

In regards to the hydrogen embrittlement issue:  The space Shuttle
fuel cells use liquid hydrogen and liquid oxygen to make water
and electricity.  To my knowledge the tanks are reused, and I don't
remember any problem occurring with them.  

[pause for me to look in my Shuttle reference data]

According to my copy of the Shuttle Operational Data Book, the
hydrogen tank is made of 2219 Aluminum alloy, the same stuff the
External Tank is made of, and there is a constraint of 25 cycles
at 30% loading and 100 cycles at >90% loading.  My guess is that
the pressure cycles are limited because these are supercritical
tanks (the pressure and temperature are above the critical
point for H2), and they run at fairly high pressures (320 psi
max operating pressure, 480 psi burst pressure).  Since the margin
between operating pressure and burst pressure is only 1.5, the
fatigue life from crack growth must be small to make sure you never 
get a failure, so they spec a 100 cycle maximum.  Commercial
aircraft are designed with more like a factor of 2.0 in
strength, so you get service life of tens of thousands of pressure
cycles (in the fuselage).  Fatigue life is highly non-linear
with the factor of safety you use.

I see no problem with H2 embrittlement.

Dani Eder

-- 
Dani Eder/Meridian Investment Company/(205)464-2697(w)/232-7467(h)/
Rt.1, Box 188-2, Athens AL 35611/Location: 34deg 37' N 86deg 43' W +100m alt.