Purging the Box,
Thoughts on Implementation
RFGoeke				12 May 05
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To keep the bare detectors and their associated electrical connections
happy, we need to keep the interior of the telescope moderately clean
and dry.

*   Choice: make a hermetic telescope or use an N2 purge?

If we purge

*   Choice: purge just the telescope or the whole assembly?

Either way

*   Choice: electrical, latching valves like we used on ACIS or
    manual?

Current ops thinking is that we are plumbed into the universal N2
purge for LRO instruments as soon as that becomes available, for sure
at the launch pad.  That means that our valves are open weeks before
launch and stay that way, so depressurization happens through those
pipes.

Positives for the electrical approach:
   -- Adding two commands (open/close) to the data system is easy, and we
      can drive the valve easily enough.  It isn't even a reliability issue
      since it is only ground use.
   -- You would use the same command to open/close both valves at the
      same time to avoid any overpressure.
   -- You don't have to get near the box to do the purge.
Impact of electrical approach:
   -- Need to verify flow rate (i.e. >zero)
	 ++ Bubble, bubble, toil and trouble; requires separate vent
	    access to our instrument
	 ++ Set inlet pressure to 1psi max., cap off separate vent and
	    measure outlet pressure
	 ++ Put in thermistor flow guage and read in telemetry
   -- Need way to avoid overpressure
         ++ Could do this by limiting inlet pressure
	 ++ Messier way is by including blowout plug at 2psi or so

Positives for the mechanical approach:
   -- You /believe/ it when you open the valve; in the electrical case
      you have to /trust/.
Impact of mechanical approach:
   -- It either adds mass or you have to remove the valve tree late in
      the game.