[ExI] SpaceX launch

[spike]

... On Behalf Of David Lubkin...

>...I love that the launch was delayed a day by a crack in a nozzle that
they fixed by trimming the nozzle 4"...

This is a perfectly reasonable way to deal with this kind of problem.
Experiment: take a piece of paper, make a 3 inch tear, take hold either side
of the tear and pull apart.  Note how little force it takes to increase the
length of the tear.  This is kinda analogous to crack propagation by stress
concentration.  Aluminum is especially bad for this.

Now cut 4 inches off to completely take away the tear, making sure to
include the end of the tear where the stress concentration was.  Now take
either side and pull, note how much force is required to start a new tear.

Cutting 4" off the nozzle would reduce its performance some, cost ya a few
percent, since the exhaust would be more underexpanded on exit (which sounds
like a contradiction in terms) but the nozzle would still work.  You just
need to have had enough margin to start with.

>... Imagine the launch delay if this had been NASA...

Ja.  NASA seldom went with the poor man's approach to anything.  In the
nozzle example, they sometimes used nozzles which circulated cryo-fuels thru
the nozzle itself, which heated and turned the fuel to gas before injecting
into the combustion chamber, while cooling the nozzle simultaneously.
Cooool, high performance, but a very expensive approach, and of course if
you get a crack anywhere in that kind of nozzle it is unlikely to be
repairable, so that one goes to the scrap heap, which is already piled high
with expensive hardware.

If you ever get a chance to see 60s era NASA space hardware, you will marvel
at how high techy everything is.  The term gold-plated is an understatement:
they actually used 20 karat gold as solder in a bunch of their circuit
boards.  Makes beautiful hardware, do let me assure you, but that approach
is expensive as all hell.  I am happy to see alternatives coming up. 

Good luck and nobodyspeed SpaceX!  {8-]

spike