Space Forum / Space History / May 2004

Those died VERY early on, predominantly due to power usage.

One of the things people tend to overlook in their "Usenet-engineering"
of large, complicated space vehicles or structures which are to be built
or assembled over time is the complication brought about by that
stretched-out assembly time-frame.  

In the case of a modular space station, since it is built over a period
of months and years rather than days and weeks, power for operation of
GNC systems and propulsion (and probably ECLSS as well, for segments
with habitable volumes) has to be provided immediately and maintained
continuously(*).  There is a definite balancing act going on, especially
early and mid-way through the sequence when the entire power-production
infrastructure is not present.  In the case of SSF/ISS, full power is
four fully-functioning solar PV arrays with completely operational alpha
and beta gimbals to track the sun fully and completely.  Of course, that
much surface area contributes tremendously to orbital drag; so in
exchange for the electrical power capacity you have to provide
substantial reboost capability.  Early in the assembly sequence, power
is severely constrained.  You just don't have enough juice for stuff
like resistojets so you provide an alternate reboost capability.  By the
time you work out all the operational aspects of that alternate reboost
approach, it becomes obvious that resistojets are a "neat" idea that
appeals to design engineers but not to operations types - you get to use
all that waste water which is otherwise vented in an open-loop ECLSS and
you put it to good use instead.  However, a large part of the reason the
ECLSS is open-loop is due to power consumption to close the loop with a
full-up O2 generator and a Sabatier or Bosch reactor to reduce carbon
dioxide.  Well, if you have the power surplus to run resistojets, you'll
probably have enough to close the ECLSS loop in the first place,
severely reducing your supply of reaction mass for the resistojets.  
Meaning you need that alternate reboost capability anyway.

There are also the ancillary (and not inconsiderable) problems of mass
for the runs of exterior, insulated piping needed to duct liquid water
to the resistojets far out on the truss; power to run heaters in the
lines; assembly sequence complications (another series of lines to
attach to modules via EVA); risks of component corrosion from the water
or urine used as reaction mass; and the risks of external contamination
of PV arrays, TCS radiators and external experiment pallets from the
exhaust.

*NOTE:  A lot of people wonder why ECLSS has to be provided to modules
immediately, even if they won't be opened and occupied for some time.  
There are several reasons: some of the electronic equipment is partially
air-cooled and although it MAY be mounted on cold-plates for liquid
cooling, operating without additional air cooling may result in reduced
functionality or result in operational overload on the liquid cooling
systems (especially prior to connection and operation of the full
external TCS); also, if the module is allowed to heat up, certain
components and materials will off-gas and/or out-gas impurities into the
air, increasing the load on the ECLSS TCCS, possibly beyond its capacity
to purify the air.

And in Mary's case it could be a bag of dog sh.t even...

Pat

So one day we will see the Loonies asking for a C-14 dating and a DNA
analysis of the shitbags on the Moon... But we already know: This will
proof nuthin'. It could be faked.

Harald