[amsat-bb:74903] AO-40 Update, 2004-02-03

["Stacey E. Mills" <w4sm@xxxxxxxxx>]

The AO-40 command team has established a routine of trying to cycle the 
main battery off (aux. battery on)  and then the S2 Tx ON every orbit, 
using simple machine codes.  Following this, the sequence to disconnect all 
transmitters is sent, to protect them from low voltage.  If we have 
approximately 10 volts on the main buss, then these commands should be 
making it through, but the S2 transmitter was not designed to run below 20 
volts and is not coming on.  The battery relay has been tested in the amsat 
lab, where a duplicate exists, and it will cycle reliably at 12 volts, but 
not lower.   If we have less than 10 volts, then the commands will not be 
received because the IHU-1 and/or command receivers are insufficiently 
powered.  Either way, the IHU-1 is not currently running IPS.  The machine 
code commands only function in reset mode.   We assume that we currently 
have less than 12 volts and that either the IHU-1 and relay are not 
functional (<10 volts) or the relay isn't functional (<12 volts), because 
cycling the relay should get us out of this situation by disconnecting the 
main battery.

With regard to the stability of the attitude/spin, this will not be a 
concern for a very long time.  We are currently rotating at 3.5 RPM.  The 
spin decay rate is extremely slow.  It will take approximately 4 years to 
drop this to 3.0 RPM.  We can magnetorque at speeds as low as 1.5 RPM.  The 
mystery effect will decrease ALON approximately 11.5 degrees/week.  It does 
not affect ALAT, though ALAT will change slightly as the orbit precesses.

MAIN BATTERY NOTES (and conjecture):
The main batteries consist of three packs housed in sheet aluminum cases 
and bolted to the radial braces between panels 1/6, 2/3 and 4/5.  The cells 
within the packs have threaded metal binding posts and the cells are 
connected by thick metal straps with strain relief "U's" in them.  The pack 
at 2/3 consists of 7 cells and is the negative end of the chain.  The pack 
at 1/6 consists of 6 cells and is in the middle of the chain.  The pack at 
4/5 consists of 7 cells and is at the positive end of the chain.   The main 
battery pack at 1/6 is the closest battery to the "flaky" heat pipe 
thermistor, though it is located "below" this heat pipe near the omni end 
of the spacecraft.  Main battery packs 4/5 and 1/6 lost their thermistors 
during the 400N incident.  Whether this was due to trauma to the battery or 
damage to the cabling is unknown.  If a short to ground occurred in the 1/6 
battery pack it would pull the cells on the negative side of the short in 
this pack to zero, as well as all cells in the 2/3 pack.   Depending on the 
location of the short and the status of the cells in pack 4/5, this could 
pull the main buss voltage to half normal (14 volts) or even 10 volts or 
below. <conjecture> A short at this location might have generated enough 
localized heat (or even some hot metal spatter)  to damage the thermistor 
on the flaky heatpipe or, more likely, its wiring.  This is appealing 
because it would represent a single point failure, rather than a failure 
cascade.  One piece of evidence that doesn't clearly fit with this theory 
is that the cells in pack 2/3, the one main battery pack that still has a 
thermistor temperature sensor, do not appear to get warm following the 
voltage drop.  We do not know how much capacity remained in these 
cells.  It is possible they contained relatively little energy. </conjecture>

As several of you indicated, we are in a waiting game for the main battery 
to develop one "open" cell.

--W4SM for the AO-40 Command Team



-- 
  _______________________________________________________________________
  Stacey E. Mills, W4SM    WWW:    http://www.keplerian.com
   Charlottesville, VA     PGP key: http://www.keplerian.com/key
  _______________________________________________________________________

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