On Sun, 22 Dec 2002, Michael Wallis wrote:
> ...Planning to build three is essential to
> ensure you can succeed in a risky venture, IMHO.
I would agree except that I'd add something about "time-critical". #2
makes the program robust against a loss; #3 makes the *schedule* robust
against a loss.
The X-15 program built three because they thought they needed two flyable
for a reasonably paced test program, and they expected that at any given
time, one more would be in the NAA Body And Fender Shop after a major
accident. I think this is still a valid assessment.
The schedule-destroying effect of losing (permanently or not) 1 of 2 is
not just that it cuts your active test fleet in half, but also that with
only one left, people get ultra-cautious and that slows things down more.
Losing 1 of 3 still leaves some room for error, so it makes people much
less nervous.
Henry Spencer
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