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

  • From: Peter Fairbrother <peter@xxxxxxxxxx>
  • To: arocket@xxxxxxxxxxxxx
  • Date: Sat, 24 Aug 2019 22:12:54 +0100

On 24/08/2019 19:02, William Claybaugh wrote:

Peter:

If building a depot requires the development of a lower cost propellant only launcher than that is a further argument against a depot since it further increases the required investment.

Eh? Just send it up in eg a Falcon 9 first and second stage. As there is no payload the unused fuel in the second stage will be plentiful.

No payload shroud, so it will need a new hat - but that's all.

I concur that there is a “depot” that is indistinguishable from refueling from a tanker and that such a depot has little or no amortization, depreciation, overhead, or profit charges.

You’re correct that overhead and profit should be added to the cost of propellant at a depot, but that just seems to be gilding the lily....

To go back to your original post:

"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 [plus profit and overhead]."


Hmmmm, in the second part, are you referring to launching propellant in eg a Falcon upper stage with no payload? I got the impression not, that you were referring to launching propellant as an integral part of a payload - but I may be wrong.



Accepting that cost analysis, still in many cases propellant at a propellant depot can be cheaper than propellant in an upper stage.

Why? Because the cost of launching it to LEO in a Falcon second stage, {which makes up 90% of the cost and is the only real cost of interest - the rest can be ignored as lost in petty cash}, can easily be cheaper than the cost of launching it in an upper stage. Suppose you want a half load?

Another advantage or two of a depot is that you can aggregate propellent loads from several propellent launches before you send up the payload with the rockets to use the propellent, so you can guarantee availability when you launch the main payload without worrying about vagaries like failed propellent launches or launch delays.

And you can have tugs. Yes that's some new hardware, but it means that the propellent launches (and payloads) don't have to have their own manoeuvre and vernier thrusters.


The main disadvantage of a depot is of course its fixed orbital plane. Equatorial seems sensible, but there are other possibilities.


Peter Fairbrother

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