[AR] Re: Parabolic Nozzle Approximation Function
- From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
- To: arocket@xxxxxxxxxxxxx
- Date: Tue, 6 Dec 2016 15:30:27 -0500 (EST)
On Mon, 5 Dec 2016, Wedge Oldham wrote:
This is how we got to the moon using "slide rules". Then Engineer had
to already know what the answer was. At a bare minimum the engineer
must know the answer to within one order of magnitude before he picks up
the slide rule...
There are, alas, problems which just cannot be addressed that way, where
computational number-crunching is unavoidable. Turns out, in fact, that
going to the Moon is one of them! It's difficult to get good results for
Earth-to-Moon trajectories using patched-conic methods -- the Moon is too
massive and moves too fast, so it perturbs the transit trajectory quite
significantly before any reasonable patch point.
Mind you, the basic point remains valid. Surrendering control of the
details to the software sacrifices insight into the process, which makes
it harder to detect not-too-blatant wrong answers, or places where useful
tradeoffs could be made. Sometimes the payoff is so large that this can't
be avoided, but all too often, the payoff is relatively small -- in which
case, it would be better done as a final refinement step, or not at all,
rather than surrendering right at the start.
Henry
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