See list attachedAUG 5 196868-PA-T-185APA/Chief, Apollo Data Priority CoordinationPropulsion system to be used on the “D” Mission Rendezvous CSI Maneuver
One of the planned rendezvous maneuvers (CSI₁) on the “D” mission is nominally zero. Since it is intended to make this maneuver based on the real time situation, some logic must be established to govern when and how the maneuver would be made. This memo is to describe the proposed logic.
If the computed value of the CSI₁ maneuver is less than 1* fps, the maneuver will not be executed at all. If the maneuver is greater than 1 fps but less than 6* fps, the spacecraft will be oriented with the minus X-axis in the direction of the velocity vector and the maneuver will be carried out using four jet RCS. The reason for this orienta- tion is to avoid losing rendezvous radar lock. This means, of course, that the maneuver may be executed in either ± X direction with equal probability.
The 6 fps upper limit is necessary in order to conserve RCS propellant as well as to remain within jet impingement constraints. If the CSI₁ is in excess of 6 fps, the DPS will be used at 10% thrust (even though rendezvous radar lock may be lost).
There was concern about using the DPS to carry out small maneuvers from the standpoint of how the PGNCS would work as well as whether a short burn for CSI would preclude use of the DPS for the 60 fps CDH maneuver approximately 30 minutes later. Harry Byington checked into this and has determined that the propulsion people intend to adopt the following DPS constraint for the “D” mission: the DPS may be used provided at least 30 minutes has elapsed since the previous burn, no matter how short it was. In other words, we have no problem there. It has also been deter- mined that the PGNCS does not limit us either. Although the DPS thrust program (P40) does not have short burn logic like the SPS and APS programs have, including start up and tail off characteristics, it is capable of handling the task. Just before igniting the engine, the PGNCS determines the duration of the burn required to achieve the desired ΔV, assuming the engine will operate at a constant 10% level for the entire period. (This, of course, is not an accurate assumption.) If the burn duration is less than six seconds, as it certainly would be for CSI₁, the PGNCS would com- mand a timed burn. It will simply turn on the engine for the duration of time computed and then will turn it off regardless of the ΔV obtained. Ordinarily, this will result in an underburn since the slow thrust build up characteristics would not be compensated completely by the tail off. The difference should be well within trim capability, though.
Of course, if CSI₁ turns out to be as big as 5 fps it's bad news! Some- thing is not working right, the implications of which may make all this unnecessary. However, it is interesting to know that there is a capability to make maneuvers of any size.
* I selected these values to illustrate the point. They're probably not far off. MPAD is in the process of determining the proper values, OMAB – the first based on rendezvous considerations; G&PB – the second based on engine characteristics and consumables. (Task assignments are needed.)