[AR] Re: Faster Space Transport? (was Re: Zubrin,

  • From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
  • To: Arocket List <arocket@xxxxxxxxxxxxx>
  • Date: Fri, 23 Aug 2019 16:56:23 -0400 (EDT)

On Fri, 23 Aug 2019, William Claybaugh wrote:

Propellant at a propellant depot costs the price of that propellant on the
ground plus the cost of launching it to LEO plus the pro-rata amortization
of the cost of the depot plus the pro-rata depreciation of the depot plus
the cost of losses.

Propellant in an upper stage costs the price of propellant on the ground
plus the cost of launching it to LEO.

True if:

1. The cost of launching an extra kilogram to LEO is the same either way.
2. There is extra room in the tanks for it.
3. The lower stage has the lift capability to carry its extra mass.
4. Reduced upper-stage launch mass doesn't confer some special advantage,
   like seriously reduced structural mass, that's very important later.

Which is to say, if it's just a matter of whether to fill the tank a bit more, that does indeed usually win. Depots et al show to advantage when it's not that simple, when one (or more) of those assumptions is not true.

It's appealing to think (or at least claim) that things *are* that simple. Trouble is, often they aren't.

For example, an important advantage of depots is that they decouple LEO departure mass from launcher payload mass, so violations of assumptions 2 and 3 don't threaten to sink your project. Should we worry about such violations? Yes! They nearly sank Apollo -- which survived only because Wernher von Braun had quietly built a rather bigger rocket than Houston's spacecraft mass estimates would have required -- and Constellation's troubled history conspicuously included repeated discoveries that the rockets were undersized. Let's see, that's 2 out of NASA's 2 previous attempts to reach the Moon that had big trouble with those assumptions -- should we really believe that the current attempt is immune to this?

Depots are not getting any traction because the key players--who are at OMB and the Space Council--know these facts.

Or because they are getting their briefings from folks who quietly aren't discussing the dubious assumptions underlying these "facts".

Henry

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