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Accumulate Fuel at Space Station?



 
 
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  #12  
Old February 16th 04, 01:41 AM
Gordon D. Pusch
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Default Accumulate Fuel at Space Station?

Markus Redeker writes:

(Gordon D. Pusch) writes:

Rick Jones writes:

[...]
Only because certain electrical components within the tank were overloaded
and caught on fire.


Why were these electrical component _within_ the tank?


The electrical components in question we

1.) The thermostat that sensed the tempertaure inside the supercritical
oxygen tank, which necessarily had to be inside the tank to sense its
internal temperature. It was this component that was overloaded;
it was originally rated to run at 28 VDC, and when the tank heater
design voltage was raised to 65 VDC to allow them to run directly off
the launch-pad DC service, the tank thermostats were accidentally
neglected when all the other components were upgraded.

2.) The circulation pump that circulated the supercritical oxygen through
the tank heater when the thermostat indicated that the tank was too cold.
it is possible that this latter

Note that, while the ultimate cause of the accident was the thermostat
contacts welding themselves closed, it was the _TEFLON ELECTRICAL
INSULATION_ around the wiring to the above devices that initially
caught fire. (In a supercritical oxygen environment, even relatively
"inert" substances such as teflon burn quite cheerily!)


Would they be necessary in a tank of a space station, too?


Unlike the Apollo Service Module, the ISS does not appear to use
"slushed" supercritical oxygen cooled by the excess heat capacity
of liquid hydrogen, so it presumably does not need internal heaters
nor stirring fans...


-- Gordon D. Pusch

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  #13  
Old February 25th 04, 07:14 PM
Henry Spencer
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Default Accumulate Fuel at Space Station?

In article ,
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...


The idea is an old one -- early studies simply *assumed* that even a lunar
expedition, never mind one to Mars, would have to be done that way -- and
things will undoubtedly be done that way (using some space station, not
necessarily the current one) eventually. The question is whether it's
worth doing this for initial, relatively modest expeditions.

The answer depends quite sensitively on the assumptions you make, like
whether the current space station is used (it's in a poor orbit and is not
equipped for the job), just how big your expedition is, and whether your
goal is a one-shot program like what Apollo became, or something with an
orderly growth path.

There are two classical problems with this concept. First, it incurs some
up-front investment which isn't repaid quickly, so it scores poorly by the
standards of people who propose one-shot programs (some of whom don't
realize that that is what they are proposing...). Second, it involves you
immediately with the extremely expensive and inefficient bureaucracy that
runs current US manned spaceflight, and some advocates of NASA Mars
expeditions fantasize that their schemes can somehow avoid such
involvement if only they avoid using the shuttle and the station.

...the thrust required might be only
1/20 of what it would be if launched from earth, so a small propulsive
system will do the job.


Yes and no. There are still efficiency losses if thrust is too low, so
you don't want to make the propulsion system too small.
--
MOST launched 30 June; science observations running | Henry Spencer
since Oct; first surprises seen; papers pending. |
  #14  
Old February 25th 04, 07:16 PM
Henry Spencer
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Default Accumulate Fuel at Space Station?

In article ,
wrote:
OTOH, if the fuel was two components, say oxygen and hydrogen, then
neither oxygen tanks not hydrogen tanks are explosive on their own. To
keep them cold in space mainly requires sheilding from sunlight.


True for oxygen, rather tricky for hydrogen (which has to be kept so much
colder, and which can tolerate so much less heat leakage, that being able
to store it without active refrigeration is questionable).

Kerosene, of course, is no problem.
--
MOST launched 30 June; science observations running | Henry Spencer
since Oct; first surprises seen; papers pending. |
  #15  
Old February 25th 04, 07:24 PM
Henry Spencer
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Default Accumulate Fuel at Space Station?

In article ,
nafod40 wrote:
We could send up water, then just let solar array-powered electrolysis
slowly do its magic to make the fuel. Two years for a bag of fuel? No
problem, no rush.


Such schemes have been looked at repeatedly, but unfortunately they tend
to require inordinately large amounts of power. Water electrolysis needs
roughly 10kW-hr/kg, and that is a *lot* of power by space standards.

Electrolyzing 80t of water -- enough fuel for an Apollo-sized lunar
mission, nowhere near enough for a Mars expedition -- in a year requires
90kW of continuous power. (Nearly double that if your electrolysis plant
runs only when the station is in sunlight.) The full continuous design
power of ISS when completed, assuming no further major Russian
contributions, is under 80kW, and nearly half of that is required for
station systems.

That way no volatile components in the launch. You could freeze the
water, and use it as part of the structure of the launch vehicle to
reduce weight.


Ice is not a good structural material, unfortunately.

Alternate launch techniques such as rail guns? The
payload would certainly tolerate the G's.


So would most any liquid fuel.
--
MOST launched 30 June; science observations running | Henry Spencer
since Oct; first surprises seen; papers pending. |
  #16  
Old February 25th 04, 07:37 PM
Henry Spencer
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Default Accumulate Fuel at Space Station?

In article ,
Gordon D. Pusch wrote:
Note that, while the ultimate cause of the accident was the thermostat
contacts welding themselves closed, it was the _TEFLON ELECTRICAL
INSULATION_ around the wiring to the above devices that initially
caught fire. (In a supercritical oxygen environment, even relatively
"inert" substances such as teflon burn quite cheerily!)


Normal Teflon is not too bad even in such an environment... but the Teflon
in the Apollo 13 oxygen tank had been roasted severely when the tank
heaters were earlier used (improperly) to try to empty the tank on the
pad. The result was a crumbly material that was very flammable in
supercritical oxygen.

Unlike the Apollo Service Module, the ISS does not appear to use
"slushed" supercritical oxygen cooled by the excess heat capacity
of liquid hydrogen, so it presumably does not need internal heaters
nor stirring fans...


The conclusion is generally correct -- bulk storage of LOX for propulsion
use would store it at or around its boiling point, and wouldn't need
anything fancy inside the tank -- but the first part is messed up.
Supercritical oxygen isn't slush, and the Apollo oxygen tank wasn't
cooled by the hydrogen tank.

"Supercritical" just means that storage pressure is high enough that the
distinction between gas and liquid has vanished. This avoids the various
complications of managing a tank which contains both liquid and gas in
free fall, so supercritical storage is generally preferred when quantities
are small and easy trouble-free operation without gravity is wanted. The
price is that the high-pressure tanks are relatively heavy, so it's not
practical for bulk storage.

The S-IVB's LOX tank was cooled by heat leakage from the LH2 tank, but the
Apollo SM oxygen tanks weren't. They just relied on good insulation, plus
the fact that oxygen was constantly in use and hence a modest boiloff rate
was perfectly acceptable.
--
MOST launched 30 June; science observations running | Henry Spencer
since Oct; first surprises seen; papers pending. |
  #17  
Old February 25th 04, 08:10 PM
Henry Spencer
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Default Accumulate Fuel at Space Station?

In article , Markus Redeker wrote:
That said, ISS isn't in a good orbit to use as a stepping stone,


Which orbit would you prefer?


The best orbit for an orbital assembly base is equatorial, although that
requires an equatorial launch site (not just near the equator, mind you,
but very nearly on it -- even Kourou, at 5.2degN, is not really suitable).
The big win is launch windows once per orbit instead of once or twice a
day, and there are some lesser issues like lower radiation dose too.

If it's a US project and you're not prepared to build a new launch base
(or operate from floating platforms like Sea Launch), then a 28.5deg orbit
like that originally planned for the space station is best. It doesn't
have the unique advantages of an equatorial orbit, but it does maximize
payload capacity for launches from the Cape.
--
MOST launched 30 June; science observations running | Henry Spencer
since Oct; first surprises seen; papers pending. |
  #19  
Old February 29th 04, 06:40 AM
Gordon D. Pusch
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Default Accumulate Fuel at Space Station?

Anthony Frost writes:

In message
(Henry Spencer) wrote:

In article , Markus Redeker wrote:
That said, ISS isn't in a good orbit to use as a stepping stone,

Which orbit would you prefer?


The best orbit for an orbital assembly base is equatorial,


The big win is launch windows once per orbit instead of once or twice a
day, and there are some lesser issues like lower radiation dose too.

If it's a US project and you're not prepared to build a new launch base
(or operate from floating platforms like Sea Launch), then a 28.5deg orbit
like that originally planned for the space station is best.


Would there be any advantage in using a 23.5 degree orbit, arranged to
be on the ecliptic, if your assembly is for interplanetary trips?


The problem with any such inclined orbital plane is, it won't stay there.
The perturbation exerted by the Earth's equatorial bulge will cause the
plane of any orbit except 0 degrees and 90 degrees to precess. Hence,
your "23.5 degree orbit" will only occasionally be well aligned with the
ecliptic, and can be as far off alignment with the ecliptic as 47 degrees!


I can't work out if being able to not have your assembled craft do a
plane change is actually an advantage, let alone worth the payload and
timing hit for delivering stuff to the assembly station.


Plane changes are only a big deal if you are going from one orbit deep
in a gravity well to another orbit deep in the _SAME_ gravity well.
For translunar or "escape" transfers, the "plane change" can be handled
during one of the mid-course corrections, after one is no longer deep in
Earth's gravity well.


-- Gordon D. Pusch

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