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Accumulate Fuel at Space Station?



 
 
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  #21  
Old March 14th 04, 10:10 AM
Phil Karn
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Default Accumulate Fuel at Space Station?

wrote:
I would like to know why we don't travel to Mars by first lifting many
loads of fuel to the space station. The actual spaceship would be
assembled in space, at the space station, from parts that are lifted
there the same as the fuel is lifted, by conventional rockets.


There are several reasons. Despite all the pro-station propaganda of the
past few decades that prominently sold the space station as a way point
to the planets, the stark physical reality is that it just isn't useful
for that purpose.

Although many planetary missions begin with an earth parking orbit, its
orbital plane is carefully chosen to coincide with that of the
interplanetary trajectory. To do otherwise would waste a *lot* of fuel
for no good reason. But the space station is in a high inclination orbit
to improve coverage of the earth and to make it easier for the Russians
to reach it from their high latitude launch sites. Since all of the
planets are in orbital planes not far from our own, they're all very
hard to reach from a high inclination earth orbit.

Even if the station were in a more equatorial orbit, the chance of its
orbital plane magically coinciding with that of any desired
interplanetary trajectory anytime during the interplanetary launch
window is extremely small.

Basically, the space station -- as conceived and especially as built --
is pretty much useless for all of the magical things it was once claimed
would do. Acting as an interplanetary way station is just one of them,
and the people who said it would should have known better. It's an
incredible scandal.

Phil
  #22  
Old March 15th 04, 09:56 PM
Paul F. Dietz
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Default Accumulate Fuel at Space Station?

Phil Karn wrote:

Basically, the space station -- as conceived and especially as built --
is pretty much useless for all of the magical things it was once claimed
would do. Acting as an interplanetary way station is just one of them,
and the people who said it would should have known better. It's an
incredible scandal.


Well, one might be able to get around this by launching into a highly
elliptical orbit, changing plane at apogee, then leaving orbit with
another burn at perigee. Seems awfully complicated though, and it
adds at least two more passes through the radiation belts.

Paul
  #23  
Old March 16th 04, 11:36 PM
Dr John Stockton
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Default Accumulate Fuel at Space Station?

JRS: In article , seen in
news:sci.space.science, Phil Karn posted at Sun, 14 Mar
2004 09:10:23 :-

Although many planetary missions begin with an earth parking orbit, its
orbital plane is carefully chosen to coincide with that of the
interplanetary trajectory. To do otherwise would waste a *lot* of fuel
for no good reason. But the space station is in a high inclination orbit
to improve coverage of the earth and to make it easier for the Russians
to reach it from their high latitude launch sites. Since all of the
planets are in orbital planes not far from our own, they're all very
hard to reach from a high inclination earth orbit.



If the initial ISS orbit departure is with a correctly-timed burn taking
it to the Moon's distance (for example), where its speed will be
significantly less than the Moon's speed, which is about 1 km/s, then a
small sideways burn (guesstimate : under 500 m/s) will change the plane
of the orbit to the desired one. Then, after another half-orbit back to
ISS height (or tweaked to lower), another major burn is used for final
departure.

It is well-known that propulsive burns should be done at low altitude;
this way, there is more low time available for a burn, allowing less
thrust and a smaller engine.

The extra week or two for departure does cost consumables; OTOH there is
also the advantage that re-lighting of the main engine is tested near
Earth, and the manoeuvring engine can be tested at the plane change
burn, with auto-return whether it works or not.

I've not worked out the trade-offs. Conceivably, the plane change could
be done by lunar gravity-assist.

***

I believe that I used your software for a while; and that a sort-of-
colleague still does.

--
© John Stockton, Surrey, UK. Turnpike v4.00 MIME. ©
Web URL:http://www.merlyn.demon.co.uk/ - FAQqish topics, acronyms & links;
some Astro stuff via astro.htm, gravity0.htm; quotes.htm; pascal.htm; &c, &c.
No Encoding. Quotes before replies. Snip well. Write clearly. Don't Mail News.
 




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