NetNews Usenet Archive 1992 #31/NN_1992_31.iso/spool/sci/space/18020

Text File | 1992-12-22 | 7.4 KB | 167 lines

Newsgroups: sci.space
Path: sparky!uunet!gatech!rpi!clotho.acm.rpi.edu!strider
From: [e-mail] (Greg Moore)
Subject: Re: SSTO vs. 2 Stage
Message-ID: <[e-mail]>
Nntp-Posting-Host: acm.rpi.edu
Organization: The Voice of Fate
References: <[e-mail]> <[e-mail]>
Date: Tue, 22 Dec 1992 07:31:42 GMT
Lines: 155

In article <[e-mail]> [e-mail] (Edward V. Wright) writes:
>In <[e-mail]> [e-mail] (Bruce Dunn) writes:
>
>>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.
>
    What if you assume much lower than 1/3 orbital velocity.
What if you basically use the DC-0 to lifht the DC-1 up to say
100 miles with limited velocity in the horizontal vector.  What
effect would this have?  Is this even practical?  (If someone
who doens't want to do the calculation wants to email the
equations, I'd gladly do them.)

>
>>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.
>
    Are the explosives any more dangerous than dealing
with LOX? LOX spills can be nasty.  Perhaps have a single bolt
in the center with a corresponding spot in the center
of the DC-1 to accept it.  The rest of the DC-1 just sits
on top of the DC-0.  (Or radically, eliminate the bolt
altogether.  REly on gravityto hold things in place. :-)

>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.
>
    This assume how you design the DC-0.  Since we
are looking at a simple to maintain vehicle here, and not
looking at pushing the envelope, leave the DC-1 engines exposed
just like they would be in the launch cradle.  (I.e, if the
launch cradle can leave the engines exposed enough to allow
maintaince, the top of the DC-0 can be built in a similar fashion.)

>Consider commercial airlines.  If they had to repaint an
>airliner after each flight, the cost of an airline ticket
>would double.
>
    Umm, yeah? So who's repainting the DC-1?  In fact,
many airlines like to keep as much of the aircraft unpainted
as possible.  Saves weight and is easier to maintain.

>>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.
>
    So, let's see, your SSTO vehicle takes an hour to turn
around.  NOw a simpler (in theory) DC-0 stage should take less time.
Even if it takes more time it's not a total loss.  Part of that time
your DC-1 will be in orbit.  So, the limiting factor becomes
stacking.  That's the key.  If you have a simple stacking design
(say a minimal of umbilicals and they are auto-connecting), this
could be minimized.  Since the TSTO will take 5 times as much
to orbit, this means that we only have to equal the turn around
time of a DC-1 five times, namely the one hour you mention above.
That means we have to beat a turn-around time of 5 hours.
However, if we include the on-orbit time of the same DC-1, that
number goes way up.  (I.e., just as the turn-around time of a
747 from NY-London-NY may only be 1 hour in London, the total
time to do this takes over 13 with flight 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.
>
    I agree with the second statement.  The first though, even
with my own additions above, I'd claim there are not enough data
points.  We have no real numbers for DC-0.  For DC-1, we have numbers
that McDac has come up with. But these are tentative at best.  (
Though better than the DC-0 numbers I'd claim.)

>
>>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? :-)
>
    Umm, isn't this what the DC-1 is SUPPPOSED to do in any
case, forgetting the DC-0 stage?

>
>>       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.
>
    But, you do assert that DC-1 will be cheaper and easier to
operate than the shuttle.  The only difference in your assertion here
and the above assertion about a DC-0, is taht McDac has provided
some numbers on a DC-1.  If McDac is wrong though... I've got
a compact for sale. :-)
    Remember, we are all arguinh numbers here.  The question is
how reliable are they?
    In reliability order (just to provoke a few flames :-)
I'd say, the reliability of the cost figures for some craft goes
as follows:
    Shuttle > DC-X >> DC-Y > DC-1 >> DC-0.

I base this on the following:
    Shuttle is flying.  Though we often can't agree on a cost,
we have numbers to argue about.
    DC-X is being built. I'm sure Mc-Dac has a good handle on
this cost.
    DC-Y cost figures are much less reliable since we don't know
what if any problems will show up in DC-X.  If few to none, then
the figures are probably pretty good.
    DC-1 figurs should be a simple extrapolation from DC-Y since
it would basically be a production model of the same craft.
    DC-0 (or DC-1 lower stage) numbers are at this point since
no real numbers have been generated.

    Note, this says NOTHING about which would be cheaper.
(In fact that is a pointless argument since DC-X has no
orbital capicity, DC-Y is a prototype.  Only when DC-1 flies
can REAL numbers be compared about cost to orbit.)

>
>>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.
>
    Careful, you're asserting again. :-)