Exploring the Unknown: Selected Documents in ... - The Black Vault

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First Steps into Space: Projects Mercury and Gemini
from a posi tion that would place spacecraft 6 forward and below spacecraft 7 at
final rendezvous, and that braking would occur slightly later than nom inal rather
than earlier. This was the crew’s approach to being con servative with respect to
the lighting conditions during the braking maneuver in that, being slightly later,
it would insure that the target would be in daylight during the final approach. A
pitch angle to space craft 7 of 20.8 deg was selected for terminal phase initiate at a
range of 41.06 nautical miles. At this time the START COMP button was pressed,
[7-21] and the initial computer solution produced a value of 31 ft/sec forward, 7
ft/sec up (this value later decreased to 4 ft/sec up at the time of thrust), and 1 ft/
sec right. The backup solution was computed to be 23 ft/sec forward and 2 ft/sec
up, and a notation was made of this anomaly. The crew discussed the problem and
decided that if a backup maneuver had been necessary they would have applied
the nominal thrust of 32 ft/sec. This decision was reached because of the nominal
tra jectories that were indicated, up to that point, on the onboard target centered
coordinate plot. In case the radar or computer had failed, the thrusts that would
have been applied were those necessary to achieve changes in velocity of 2 ft/sec
up and 32 ft/sec forward.
After completion of the transfer thrust, the fuel remaining was 62
percent. At this point, the time system was reset to zero based on the beginning
of the first computer time cycle that occurred 270 sec onds after depressing the
START COMP button (nominally, this time co incides with the end of the transfer
maneuver). The crew used this phase elapsed time (p.e.t.) as a time reference
through final rendezvous. The target was not in sight during the tight-tracking
period from 3 to 5 minutes after the transfer maneuver. During the 3-to-5 minute
tight -tracking period, the analog range rate was 160 ft/sec at 3 minutes 30 seconds
p.e.t. Computations from the onboard computer showed
156 ft/sec. At 4 minutes 30 seconds p.e.t., range rate from the analog meter
was 155 ft/sec, and the computer value was 152 ft/sec. These com parisons show the
close agreement between the analog meter readout and the computer solution and
provided the crew with high confidence in the radar-computer interface.
At 5 hours 23 minutes g.e.t., during the 3-to-5 minute tight -tracking
period, spacecraft 7 lights were barely visible and not suf ficient for tracking. This
time corresponds to a range of approximately 24 miles.
Subsequent to 5 minutes p.e.t., the spacecraft was pitched down to
horizontal, using the direct attitude-control mode, to align the platform. It
was decided that alignment would be conducted during the planned optional
alignment period, from 5 minutes to 10 minutes p.e.t. This decision was based on
the fact that 1.5 hours had elapsed since the last alignment. During this alignment
period (with the platform in SEF, the control mode in pulse, and the flight director
indicator dis playing platform and attitude), very little motion was detected in the
pointers, indicating that the platform had been in good alignment. In addition,
the optical sight and the visible horizon also indicated good alignment before
starting the align period. This excellent performance of the platform provided
the crew with further confidence in the space craft 1GS system. At 10 minutes 20
seconds p.e.t., direct control was selected and the spacecraft was pitched back
up in order to track space craft 7. The radar lock-on light had not extinguished;