From: Bruce_Dunn@mindlink.bc.ca (Bruce Dunn)
Subject: SSTO vs. 2 stage
Organization: MIND LINK! - British Columbia, Canada
Date: Wed, 16 Dec 1992 13:43:48 GMT

> Pat writes:
>
> Gary.  demonstrate how staged rockets can deliver lower costs then SSTO,
> and
> you will have a point.


        Although I am strongly in favor of spending money on the DC-X and
successors, I will take a quick crack at demonstrating this.

        In a posting a couple of weeks ago, I presented the results of
calculations which showed the result of using a DC-1 as the upper stage of a
two stage vehicle, rather than as an SSTO.  The first stage I assumed was a
reusable stage approximately the same size as the upper stage, but much
heavier and less stressed.  It used kerosene/LOX in existing engines
(ex-Soviet RD-170), had full engine out capability, and would likely cost
considerably less to develop than the DC-1.  A two stage vehicle, with the
DC-1 as an upper stage, delivered approximately 5 times the payload to LEO as
the DC-1 in SSTO mode.

        If one assumes that development costs are "sunk" and not to be
applied against flights (or alternately assumes that large amounts of
material are being launched and thus development costs can be spread over
hundreds of flights), then the cost of launching revolves around the cost of
refueling and refurbishing the reusable vehicles.  As pointed out by many
posters, this is largely driven by manpower requirements.

        If a DC-1 in SSTO mode can launch 10 tons to LEO and the two stage
vehicle using the DC-1 as an upper stage can launch 50 tons to LEO, then we
have the following costs for launching 50 tons:

DC-1 in SSTO mode:  5 flights needed, incurring 5 DC-1 stage turnaround costs

DC-1 in 2 stage mode:  1 flight needed, incurring 1 DC-1 stage turnaround and
1 lower stage turnaround cost.

       I will assert that turnaround cost of the lower stage will not exceed
the turnaround cost of the upper stage (the lower stage is much less stressed
than the DC-1 upper stage, and uses cheap kerosene and LOX as propellants).
Therefore, as far as turnaround costs go, the two stage vehicle costs 2/5, or
40% as much as the SSTO.  This will be offset somewhat by the fact that
facilities, spares etc for two different stages must be maintained, but I
doubt that this will eat away all of the apparent advantage of the 2 stage
design.

        I will further note that using the DC-1 as an upper stage does not
prevent it being used in SSTO mode for lighter payloads when this would be
desirable.  It also offers the opportunity to boost individual payloads 5
times greater than that achievable in SSTO mode.  A two stage design is thus
more flexible than the SSTO.

        A final advantage to the two stage design comes from the time
necessary to perform turnaround.   Since the lower stage can be inspected and
prepared for another flight while the upper stage is making an orbital
delivery, the frequency of flights will be governed by the turnaround time of
the upper stage after it gets back.  Say that turnaround time is 1 week.  If
you spend your money on two DC-1 vehicles and operate them in the SSTO mode,
then it will take 5 weeks to put 100 tons in orbit (two flights delivering 10
tons each per week).  If you spend your money on one DC-1 and 1 lower stage,
then the same 100 tons can be put into orbit in two weeks (two flights, each
delivering 50 tons).  This ability to put up a lot of payload in a very short
time may be of advantage for situations such as trying to fill the
propellants tanks of a lunar expedition vehicle with cryogenic propellants.



--
Bruce Dunn    Vancouver, Canada   Bruce_Dunn@mindlink.bc.ca
Newsgroups: sci.space