From: prb@access.digex.com (Pat)
Newsgroups: sci.space
Subject: Re: Terminal Velocity of DCX? (was Re: Shuttle ...)
Date: 15 Dec 1992 20:25:53 GMT
Organization: UDSI
NNTP-Posting-Host: access.digex.com

In article <1992Dec9.133030.6288@ke4zv.uucp> gary@ke4zv.UUCP (Gary Coffman) writes:
>In article <1992Dec7.194132.19219@wuecl.wustl.edu> gene@wucs1.wustl.edu (_Floor_) writes:
>>In article <1992Dec5.165219.18302@ke4zv.uucp> gary@ke4zv.UUCP (Gary Coffman) writes:
>>] always mean higher reliability. A truck engine is usually good for
>>] a million miles while a formula one engine may last 100. They both
>>] put out roughly the same amount of power, but one masses a lot more
>>] than the other.
>>] 
>>
>>What?!? What kind of a comparison is that? I don't really think this is
>>an appropriate example of your point. A formula one engine is tortured
>>by blipping rapidly back and forth between 3,000 and 13,000 rpm continually
>>for two hours. It isn't the size that's the factor, is the use! I think
>>a tree trunk versus a twig is a better example of your scale/reliability
>>dependence point.
>
>Actually, both engines have narrow torque bands. Thanks to multi-speed
>gearboxes, both engines are kept near their torque peak at all times
>in service. The RPM levels are grossly different, 1800 for the truck
>and 12,500 for the F1 engine. That's because the smaller engine has
>to wind tighter to make the same power. IE it's stressed harder because
>it has to be small to maintain an acceptable mass ratio in a race car.
>

Okay, we all agree race car engines are more heavily stressed then
trucks.

>Power to weight ratios are critical to good race car performance as they
>are to good SSTO performance, and for the same reason. Light high output
>engines are less reliable than heavy high output engines. The margins
>are thiner in the racecar than the truck. The truck can afford to throw
>away a couple of thousand pounds of payload for a more reliable engine,
>the race car cannot.
>

The truck also takes a huge mass penalty against speed in order to improve
structural reliabilty.  Racing yachts are designed  for max speed and
equipment reliabilty, of exactly one heat.  a freighter or a destroyer
are designed for much tougher conditions.


>An SSTO has to haul all of it's engine and structure mass to orbit so
>these have to be lighter than a staged rocket that can discard engines
>and structure along the way. This is inherent in SSTO design. So a
>staged rocket can be made to have lower stresses than a SSTO for the
>same payload.
>

Certainly, a staged rocket designed with clean sheet approach can carry
more weight to orbit at a better mass ratio then an SSTO.   That does not
mean that you want to take that approach,  if an SSTO CAN!!!  important point
deliver cargo to LEO  for significantly less cost then any staged rocket
then why go with staged rockets for routine access to space?

Gary,  use your brain.    Just because a staged rocket can be built with
greater margins then a SSTO, does not mean that is the way to go.  

Right now,  all staged rockets are vintage 60's/50's designs.  they have
absolutely no margin for safety.  neither does the shuttle.  All the
models for rocket costing indicate a major cost item is in the building/
stacking operation related to launch.   The russians turn these guys out
like sausages and still expand thousands of hours on stacking ops.  

If a 747 had to change engines every time it flew,  a ticket would
cost 10 times as much.  asirlines are cheap to fly, cuz most items only
need routine maintenance.   literally, you can gas up a 747, clean the
cabin and turn right around.  i've seen it done.  

You seem stuck on stress the same way Bi-Plane builders were in the 20's.
Bi-Planes generate the same lift, for half the stress,  yet all
planes are mono-wing.   Why,  because improved materials and design
methods allowed  adequate safety margins.   That's what we need,  
adequate margins of 140% of expected load.  

Sure,  a NASP, could probably be built with  170% margin of load,  but why?
and if it costs 4 times what DC-1 could deliver, then you are wasting money.

My belief is that the DC-1, will deliver loads to orbit cheaper then anything
else flying.  if it can,  then we are in great shape.


>Shuttle is only high stress because it emphasizes large payload in
>what is essentially a 1.5 stage design. Thus the SSME have to push
>much harder than they otherwise would have to. With a 2 or 2.5 stage
>design, the engines could operate at lower stress levels since they
>would be pushing less dead weight. The wings impose a mass penalty,
>but that's offset by not having to carry landing fuel and it's tankage 
>for VTOL operation.
>

Okay,  so lets say we had a staged shuttle,  kinda like Von Braun's
concept.  Winged vehicle on a Saturn S-1, S-2.   Certainly, it would
have been much more reliable.  Certainly it would have carried more
payload.  Wehrner may have been a nazi (Sorry dennis)  but he was a
damn good rocket designer.  

So we got werners design.  do you know how much it would cost to fly?
probably on the the order or 2-3 billion per mission.  Saturn missions
were costing NASA around  500 million per shot in 1968.  it's why they
started scrubbing them.

Everyone who has looked at McDac's cost figures,  thinks they are reasonable.
the big question is wether or not the engineers can deliver payload.

I am sure i could really make my truck reliable by changing the suspension,
engine and drive train after hauling every load,  but what for?  it starts
when i stick a key in, so why should i complain?

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

pat

Newsgroups: sci.space