On Sun, 16 Feb 2003, John Carmack wrote:
> I don't see why that is a problem for getting the vehicle back in the air
> with four man-hours of effort -- you would just need more heat shields than
> vehicles, and a pipelined refurb process...
I'm not sure it is fair to sweep the man-hours under the carpet :-) like
that -- you still have to pay for them. (Suppose you could somehow do the
refurb in 15min while still on the vehicle, but it required 100 guys; is
that really any different from needing 5 hours work by 5 guys in another
building?) Not counting such off-line activities as part of routine
turnaround strikes me as reasonable only if they are relatively rare,
not something that's needed on every flight.
In any case, the point may be moot. Bear in mind that if your heatshield
swap operation takes four guys for fifteen minutes -- which seems
plausible as a first guess, given a vehicle big enough to need power
handling of the heatshield plate -- that's one of your four allowable
man-hours gone already, which is a lot to spend on this.
Operating with a four-man-hour turnaround is a whole different world, one
where a design change that saves a whole man-hour is a Big Deal and worth
a lot of development work. The little bits of time overhead involved in
setting up for, and tearing down after, even relatively minor procedures
start to loom large; it starts to look better to eliminate them entirely.
If four guys have to climb down/out/up, put away tools, pick up new tools,
check the paperwork, and move their gear and themselves into position for
the next operation, and it takes them only three minutes to do all this,
that's 5% of your total man-hour budget gone right there!
Things like swappable ablative heatshields, which may be perfectly
reasonable at forty man-hours, become a lot less appealing at four. When
the man-hour budget is that small, the mere fact that a design approach
requires an operation to be done on the vehicle as part of turnaround --
no matter how simple and straightforward that operation is -- becomes a
big black mark against that approach.
Henry Spencer
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