PA/Manager, Apollo Spacecraft ProgramApril 23, 196969-PA-T-62APA/Chief, Apollo Data Priority CoordinationF mission rule regarding DPS gimbal drive failure indication
This memo is to report how it is currently planned to handle an indicated failure of the DPS gimbal drive actuator (GDA) on the F mission.
On the F mission there are two DPS maneuvers – DOI and Phasing. The DOI burn is about 71 fps achieved by 15 seconds duration at 10 percent thrust and about 15 seconds at 40 percent. The Phasing burn is about 195 fps achieved by 26 seconds at 10 percent and about 19 seconds at full thrust. The question to be answered was what should the mission rule be covering a GDA fail light occurring on either of these burns?
From the offset it should be made clear that advice from the MCC during the maneuvers is out of the question due to the communication delay when the spacecraft is operating at the moon. And of course, the DOI burn is performed in back of the moon.
The fail light coming on can mean any one of three things – the gimbal is moving when it is not supposed to be, the gimbal is not moving when it is supposed to be, or the indication itself is at fault. Apparently by far the greatest probability is that the failure indica- tion itself is in error. As you know, there is no direct cockpit readout of DPS gimbal angles. Accordingly, the only way the crew has of determining that the light is in error is by waiting for some other cue such as excessive attitude error on the FDAI and hearing or seeing the RCS jets firing to maintain attitude, as they will when the LM attitude error as controlled by the DPS gimbal positioning exceeds 1°.
If the light comes on during the G mission, the mission rule will almost certainly be to await the second cue before taking any action because even a runaway gimbal cannot create a problem and you unnec- essarily have blown the mission by turning off the GDA if the light is wrong. It is currently intended to use this same rule on F, although it is not so clearly proper for F as G. Specifically, in the event we really do have a runaway gimbal, it is almost certainly possible to continue on with the nominal mission provided the crew deactivates the GDA immediately in all cases. That is, by freezing the DPS gimbal position, it is possible to complete not only the DOI burn, but also the entire Phasing burn using RCS for attitude control. Analysis has shown the RCS propellant required is not excessive and the plume impinge- ment constraints are not exceeded. For example, if the GDA misalignment were l½° throughout the entire Phasing burn, only 15 seconds of RCS would be required of the worst jet. This gives the crew more than 5 or 6 seconds to deactivate the GDA in the worst situation – namely a runaway gimbal moving at 0.2° sec. If the crew does not deactivate the GDA as soon as they get the light, but rather awaits the second cue, mistrim may be too great to permit use of the DPS for the Phasing burn. This would force us either to use the APS for Phasing or to perform a PDI abort, which essentially eliminates the long range rendezvous navigation exercise and results in a non-nominal rendezvous sequence. We don't think this is the case and are getting some computer simulations run to prove it. That is, we expect that even by awaiting the second cue, the resultant misalignment will be within RCS control capability.
In the event of a real GDA failure during the DOI there are some things the MCC can do once the LM appears from behind the moon. Care must be taken, however, to make sure that these tests do not result in further misalignments of the DPS gimbal during the Phasing burn. Certainly the MCC can make an estimate of which direction the mistrim appears to be the largest prior to the maneuver and could recommend that the opposite RCS jets be used for ullage in order to reduce the probability of reaching the plume impingement constraint during the Phasing burn.
The mission rule is currently written this way, with the approval of everyone I know who is interested. The only perturbation I can foresee would result from the analysis noted above showing we might lose the DPS for Phasing if the crew awaits the second cue. In that case, a review might be worthwhile.