Single-point failure

Rob Matson (
26 Feb 1996 18:37:47 U

                      Subject:                              Time:  18:04
  OFFICE MEMO         Single-point failure                  Date:  96/02/26

On 2/26 Ray Talipsky wrote:

> The momentum transfer gave the satellite an 80 foot per second
> boost and shot it into a 324 x 171 nm orbit.  NASA would not risk
> putting a national asset and the astronaut lives in harms way to
> retrieve an italian satellite!

Assuming the Shuttle had the necessary fuel reserve (and I gotta believe an 80
fps velocity change is well within the fuel margin), there would have been no
more risk to the Shuttle and its astronauts than for any of the prior
satellite retrieval missions.  That is, if it weren't for the tether.  That's
the whole point.  NASA is extremely anal about covering all the bases when it
comes to safety -- as they should be.  I didn't sit on any of the design
reviews for this experiment, but some of the questions I would have asked (if
*I* owned the satellite) would have been:

1.  What are some of the events which could cause the tether to break?  How
likely are they?

2.  Whose responsibility is it if it DOES break?

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.

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.  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...