NetNews Usenet Archive 1992 #31/NN_1992_31.iso/spool/sci/space/17975

Text File | 1992-12-21 | 5.4 KB | 129 lines

Newsgroups: sci.space
Path: sparky!uunet!zaphod.mps.ohio-state.edu!cs.utexas.edu!convex!convex!ewright
From: [e-mail] (Edward V. Wright)
Subject: Re: SSTO vs. 2 Stage
Sender: [e-mail] (news access account)
Message-ID: <[e-mail]>
Date: Mon, 21 Dec 1992 17:34:03 GMT
References: <[e-mail]>
Nntp-Posting-Host: bach.convex.com
Organization: Engineering, CONVEX Computer Corp., Richardson, Tx., USA
X-Disclaimer: This message was written by a user at CONVEX Computer
              Corp. The opinions expressed are those of the user and
              not necessarily those of CONVEX.
Lines: 113

In <[e-mail]> [e-mail] (Bruce Dunn) writes:

>Wright:
>The 2-stager is a *much* larger vehicle, so your vehicle fabrication
>costs will be much larger.

>Larger is a slippery word.  Measured by dry mass, the lower stage is four
>times larger than the DC-1.  Measured by fueled mass, it is two times larger.
>Measured by linear dimensions, it is actually smaller than the DC-1 (it uses
>much denser propellants).

Fabrication costs tend to be proportional to dry mass, not fueled mass
or linear dimensions.  So you can expect them to be four times larger.


>Fabrication costs are not only affected by size, but also strongly affected
>by complexity and by how closely material limits are pressed....  In fact
>therefore, the fact that the lower stage has a higher
>dry mass than the upper stage (even though it has smaller linear dimensions)
>is an indication that it is likely to be less expensive than an upper stage.

The patented Gary Coffman "stress" argument.  Hope you paid your royalties
on this one. :-)  But, in fact, there is nothing to suggest that your
lower stage would be less complex or press material limits less closely.


>I am assuming that the first stage will return to the launch site under its
>own power.

That requires a significant delta-v.  Let's assume your first stage
accelerates its payload to 1/3 orbital velocity.  After separation,
the first-stage must kill its forward velocity.  That's another 1/3
orbital velocity.  Then it must do a ballistic shot back the way it
came.  Another 1/3 orbital velocity.  Since 1/3 + 1/3 + 1/3 =1, your
vehicle needs enough delta-v to put itself into orbit.  If its burnout
velocity is lower, the total delta-v is lower, but the benefit of the
first stage is also less.


>Mating the two stages will take time and manpower, but if the system is
>correctly designed the actual labor need not be too much.  The DC-1 will have
>to be transported in any case from its landing spot to its launch cradle.
>The use of a lower stage would merely mean that the DC-1 would be placed on
>the lower stage, rather than on the launch cradle.

No, not quite.  The upper stage can't simply be placed on top of
the lower one.  It's got to be carefully aligned and attached with
explosive bolts to make sure it stays there during launch.  Very
careful handling is necessary, not only to avoid damaging one of
the vehicles, but also because you're dealing with explosives.

Once the vehicles, you no longer have access to the second-stage
engines.  If a problem is detected before launch, you have to
disassemble the stack again.

Consider commercial airlines.  If they had to repaint an
airliner after each flight, the cost of an airline ticket
would double.

If they had to mate the airliner with a first stage after
every flight, the cost of an airline ticket would probably
go up by a factor of ten.

That's why no one operates two-stage airliners any more.


>Reply:
>OK, in this case two flights of the two stage vehicle can deliver the desired
>total payload in a time of 2 days, as compared to 5 days and 10 flights for
>the DC-1.  The advantage remains - what is your point?

My point is, a single-stage vehicle can be turned around, potentially,
in less than an hour.  Unload the passengers, download the inflight
maintenance log, refuel the vehicle, and load the next batch of
passengers.  Airlines do this all the time.

A two-stage vehicle will add days, if not weeks, to that turn-around
time.  How often a vehicle flies is the most important factor in
determining how much it costs to operate.


>Should the DC-1 have engine ignition trouble on staging, it has both the
>fuel and thrust to land at the launch site.  With only some engines working,
>it can burn off fuel until it has a thrust to weight ratio of greater than
>1, hover to get its weight down further,  and then land.

At what altitude above (or below) water? :-)


>       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).

I can assert that the cost of a luxury car is the same as a compact 1/3
the size.  I don't believe it, but I can assert it.


>Assuming, as a worst case, that the turnaround costs of the lower stage equal
>the turnaround costs of the upper stage,

Your "worst case" would be overly optimistic, even as a best case.


>        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.

No, for the development cost of this vehicle, you could build a "big DC"
five times the size.