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Date: 18 May 88 01:04:46 GMT
From: thumper!  (Phil R. Karn)
Subject: Re: cooling by radiation

> As I recall, the Space Shuttle is supposed to cool itself while in
> orbit by opening the cargo bay doors, and pointing the cooling array
> at the earth.  Since this is much less efficient than pointing at open
> space, I presume there must be some good reason for this choice. Is it
> because faster cooling might damage something, because this position
> keeps the cooling surfaces out of the sun most of the time, or for
> some other reason?

I believe the reason has to do with thermal control in the payload bay.
Pointing the bay at the earth is much more benign than pointing it
either at the sun or at deep space.

Back when we thought we could launch an amateur PACSAT (packet radio
satellite) from a GAS canister on the shuttle I took a look at the
thermal environment. It can be summed up in one word: horrendous!  If
you design your payload to survive direct sunlight, it will freeze if
the bay is oriented to deep space for any length of time. If you design
it to work in shadow, it will fry in the sun.  And, of course, GAS
customers are peons -- you get no say over the orbiter's attitude, and
they usually can't even tell you what they expect it to be at any given
time during the mission.

You don't get a single microwatt of the kilowatts being generated by the
orbiter's fuel cells. You have to waste half the canister just carrying
batteries -- no lithium batteries are allowed, this is a man-rated
vehicle. You may well end up using most of your battery power just
keeping warm. The best you can do is compromise on the thermal design
and hope that they'll keep it pointed at earth most of the time.

I'll take a free-flying payload (with an unmanned launcher to fly it on)
any day. At least you know what the thermal environment will be so you
can plan for it.  The selling of the shuttle as a platform for easy,
inexpensive, small scale space applications is one of the biggest con
jobs in history.


Date: 20 May 88 23:27:07 GMT
From: thumper!  (Phil R. Karn)
Subject: Re: cooling by radiation

> And of course it would all be so much better for a small experimenter
> on a big shared unmanned platform, right?

Partially yes. Have you ever seen the GAS Safety Manual? I have. Just
read it sometime if you want to get depressed. And my copy was published
BEFORE the Challenger disaster.

I really want my platform to be FREE-FLYING, not just unmanned.  But the
GAS program was not originally set up to actually launch payloads. A
couple of very persistent people finally persuaded NASA to allow it;
remember GLOMR and NUSAT? They could launch their payloads from a GAS
can, but only through the use of a NASA-developed deployment mechanism
that took fully HALF the can's already-cramped interior. (I know -- I
visited Goddard during the NUSAT preparation and and inspected the
actual flight mechanism with spacecraft attached).

Even worse, the NASA safety people insisted that the deployments occur
on the LAST day of a week-long mission. Why? Because they were worried
about the possibility of a collision between the spacecraft and the
shuttle orbiter, and doing it on the last day would minimize this. (As
it turned out, they were able to move up the NUSAT/GLOMR deployment
because the dry cells in the can (remember -- no orbiter power) weren't
expected to last the entire mission. And GLOMR didn't make it out at
all, and had to be brought back and relaunched).

AMSAT has had considerable experience flying small secondary payloads on
unmanned launchers, primarily Delta and Ariane. Yes, there certainly are
interface and safety requirements. We fly solid and hypergolic kick
motors on Ariane, and share space with $100M Landsats on Delta. It's
perfectly reasonable for them to require assurance that our payloads
won't kill somebody or ruin a mission. And with 13 satellites launched
we've built an absolutely perfect track record in this regard.  (The
Ariane launch failure that occurred in 1980 was caused by a first stage
engine defect, and had nothing to do with our payload).

Even if you're allowed to deploy something from a GAS can, however,
you're in an entirely different league. The bureaucracy levels and
safety requirements are orders of magnitude higher, and the service
provided by the vehicle itself is much worse. Standard facilities, like
battery charging and telemetry while on the pad, are not provided.  Your
payload may have to survive for months on the ground before launch
without your being able to touch it. Instead of being deployed in a
known, preselected attitude within minutes of reaching orbit, you get
dumped out at an unpredictable time and attitude chosen by NASA, not
you, and they couldn't care less about the thermal beating your payload
might have to take in the meantime.  The orbit is much lower than that
typical of unmanned launchers, but conventional kick motors are
completely out of the question because of the safety rules.  So either
you resign yourself to a <1 year lifetime, or you take a big detour and
go off to build solar-powered thermal thrusters.  And perhaps you'll
even have time left to work on whatever payload you wanted to fly in the
first place.

These drawbacks are inherent from BOTH the man-rated and the shared-bus
nature of the Shuttle, and would work against anyone trying to use it
for low cost, small-scale space research or applications.


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