[AR] Re: Testing Scaling Law
- From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
- To: Arocket List <arocket@xxxxxxxxxxxxx>
- Date: Fri, 11 Nov 2016 12:16:13 -0500 (EST)
On Fri, 11 Nov 2016, Ed LeBouthillier wrote:
I was thinking about the effects on cost of scaling up a rocket design
and I came to a rough conclusion: The invested cost in the testing of
rockets goes up by the cube of the scale.
What makes you think this? Is this just an off-the-cuff guess, or do you
actually have some reasoning behind it?
The only things that really have to scale somehow with gross mass are pad
structure (which has to hold the thing up before ignition), propellant
infrastructure (which has to hold and move liquid mass nearly equal to the
vehicle gross mass), and structural parts of test stands (which have to
take thrust loads a little higher than gross mass). Plus the safety
distance between the pad and anything valuable. But it doesn't actually
cost very much to make the girders thicker or the pipes fatter or the
wires longer, except where you cross thresholds in things like the size of
the construction equipment needed.
I would expect somewhat the same situation as for rocket hardware itself:
strong scaling with complexity, thinness of margins, and closeness to
leading edge of technology; much weaker (and stairstep rather than linear)
scaling with some mild function of size.
So, by the time you get to developing something like the Apollo Saturn,
it takes 10% of the GDP for many years to get all of the testing
equipment and the testing of the important propulsion components.
Let's have some respect for the numbers here -- it was nowhere near that.
NASA's entire budget for the two or three peak years of Apollo (and it was
quite a sharp peak) was about 5% of the Federal budget, and a much smaller
percentage of GDP.
Henry
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