Single-point failure

Philip Chien (
Tue, 27 Feb 1996 11:35:23 -0400

The following are some of the conditions under which the tether would be
cut and the orbiter would perform an evasive maneuver:

The tether breaks

the tether departure angle is greater than 60 degrees

the orbiter's attitude rates exceed the nominal autopilot values and cannot
be corrected before the tether departure angle limit is reached

etc. etc. etc.

As far as events which could cause the tether to break I haven't seen a
list, but here's the obvious causes off the top of my head:

1)  accidental firing of the tether cutter mechanism (one of the first
things they checked)

2)  micrometeroid or debris damage (possible, but not statistically likely)

3)  abrasion against something in the deployment mechanism

4)  overstressing the tether beyond its design limits (not bloodly likely)

5)  fault in the tether's manufacture

6)  damage some time since the manufacture

7)  some incredibly high current in the tether which somehow got localized
in one location to burn it (boy am I reaching here)

8)  the piece I removed before flight  (well maybe that's it ....)

9)  space aliens cutting the tether

10) LASER from some adversary country destroying the tether

well, it gets sillier beyond that point, but no sillier than the face on
Mars ...

>2.  Whose responsibility is it if it DOES break?

NASA (via the US government), per U.N. treaties and other international
agreements.  As the launching country (both for the shuttle and the
deployer 'upper stage') the U.S. is responsible for any liabilities or
damages, either to other countries space assets or to any damage on the
ground (a la the Soviet ROSAT nuclear reactor which spilled in Canada)

>3.  What will it take to retrieve the satellite in this scenario? Clearly you
>would need sufficient fuel, and a means to jettison the tether.  Also, someone
>aboard would have to have experience with the robot arm, and a space walk
>would be required.

I don't think the RMS would be an absolute necessity (it isn't installed
for this mission).  The crews have shown great abilities for rendezvousing
and stationkeeping close to another spacecraft without hitting it, and
before the mission the crew indicated that they had developed procedures
for manually grabbing the spacecraft and putting it in to the docking ring
if necessary.

>Perhaps these issues WERE discussed by payload engineers, flight dynamics
>personnel and safety people, and retrieval was either deemed too risky, too
>expensive, or just plain impossible.

I'm certain it was looked at, and evaluated as a non-concern.  yeah there's
a one in kazillion chance that the satellite and tether would run in to
another spacecraft, or even tangle up into Mir's structure, but not enough
to be a concern.  There's been plenty of other debris which has been put up
their accidentally and hasn't been a concern (e.g. HST solar array, SFU
solar arrays, etc.)

>But $400+ million sounds like a lot to
>risk on a 1/10th inch tether without having a retrieval option.  I'm waiting
>to hear a reasonable explanation for this screw-up...

See previous message - the $400 million was spent in Italy, and well used.
Retriving the satellite would not put $400M back in to anybody's pocket, so
it really doesn't mean that much - especially in the long run.

and this thread is getting a tad beyond the charter of the seesat group.
Let's go look for that thing folks!

Philip Chien, Earth News - space writer and consultant  PCHIEN@IDS.NET
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