Gemini VIII: Control System Problem

From: rk (stellare_at_nospamplease.comcast.net)
Date: 12/24/04 

Date: Thu, 23 Dec 2004 22:55:44 -0600

7.1.2.9 Control system problem. - At approximately 7 hours g.e.t., the two
spacecraft were configured for the platform-parallelism test, which was to
have provided a comparison of the spacecraft and GATV attitude reference
systems. The GATV Attitude Control System (ACS) was active, and the TDA L-band
transponder was off. The spacecraft attitude-control power-switch and
maneuver-control switches were off. The radar was off, and the control mode
switch was in PULSE.

Shortly after sending encoder command 041 (recorder ON), roll and yaw rates
were observed to be developing. No visual or audible evidence of spacecraft
thruster firing was noted, and the divergence was attributed to the GATV.

Commands were sent to de-energize the GATV ACS, geocentric rate, and horizon
sensors, and the spacecraft Orbital Attitude and Maneuver System (OAMS) was
activated.

The rates were reduced to near zero, but began to increase upon release of the
hand controller. The ACS was commanded on to determine if GATV thruster action
would help reduce the angular rates. No improvement was noted and the ACS was
again commanded off. Plumes from a GATV pitch thruster were visually observed,
however, during a period when the ACS was thought to be inactivated.

After a period of relatively stable operation, the rates once again began to
increase. The spacecraft was switched to secondary bias power, secondary
logics, and secondary drivers in an attempt to eliminate possible spacecraft
control-system discrepancies. No improvement being observed, a conventional
troubleshooting approach with the OAMS completely de-energized was attempted,
but subsequently abandoned because of the existing rates.

An undocking was performed when the rates were determined to be low enough to
preclude any recontact problems. Approximately a 3 ft/sec velocity change was
used to effect separation of the two vehicles.

Angular rates continued to rise, verifying a spacecraft control-system
problem. The hand controller appeared to be inactive. The Reentry Control
System (RCS) was armed and, after trying ACME-DIRECT and then turning off all
OAMS control switches and circuit breakers, was found to be operative in
DIRECT-DIRECT. Angular rates were reduced to small values with the RCS B-ring.
Inspection of the OAMS revealed that the no. 8 thruster had failed open. Some
open Attitude Control and Maneuver Electronics (ACME) circuit breakers
probably accounted for the inoperative hand controller noted earlier. All yaw
thrusters other than number 8 were inoperative. Pitch and roll control were
maintained by using the pitch thrusters. 

-- 
rk, Just an OldEngineer
"Engineers abhor extrapolation"
-- Ken Iliff, from _Runway to Orbit_, 2004

Relevant Pages