Downthread: Descent Aborts – Part II (Oct 25, 1968)
See list attachedOctober 21, 196868-PA-T-226APA/Chief, Apollo Data Priority CoordinationDescent Aborts
We have finally started mission techniques meetings on lunar landing descent aborts. At the risk of losing whatever confidence you might have in my judgment, I would like to describe a technique we are probably going to propose for aborts early in the descent phase. That is, within about 25 seconds of commanding the DPS to full thrust. It is a technique that Joe D. Payne and Floyd Bennett have been suggesting for quite a while, but which most of the rest of us had been unwilling to accept.
First of all, I don't think anyone will argue about what should be done between initialization of powered descent and DPS throttle up after the trim gimbal period (currently set for 26 seconds). The ΔV acquired during that period only drops the apogee down to about 40 miles so the best thing to do is probably just shut off the engine and sit tight. That is, no immediate abort maneuvers are required unless it is necessary to get away from a hazardous DPS stage.
After going to full throttle, though, there is a short period (roughly 25 seconds) during which aborts become a little difficult to handle. In this region the trajectory rapidly becomes suborbital, making an immediate abort maneuver necessary to achieve a safe orbit. The problem is that the spacecraft is oriented retrograde to perform the descent maneuver, which is exactly opposite to the direction required to get back into orbit. This causes the problem. Namely, if we want to abort on the DPS, you have a choice of:
a. Either turning off the engine, reorienting the spacecraft about 180°, and reigniting the DPS to make a posigrade burn into orbit – and no one wants to turn off the engine! or
b. Leave the DPS engine on as the spacecraft is being reoriented. Unfortunately, in order to avoid gimbal lock this attitude maneuver must be made in the pitch direction and leaving the engine on causes us to acquire a large radial velocity during the attitude maneuver which must be removed. To do this the spacecraft would go through a pretty wild pitch profile rotating almost a complete revolution from the time of abort to the time of engine shutdown. The reason for this is that attitude change is made at a rate of only 10 degrees a second, which means the engine would thrust with a component in the radial direction for a long time. As you can imagine, there are also considerable problems in the guidance equations, which would cause the engine to be shutdown prematurely under certain circumstances.
Abort Staging with the APS is not much better since it was felt necessary to provide an immediate separation maneuver (currently coded to be three seconds or 30 fps) to get away from the DPS before reorienting to posigrade attitude. And, you can't leave it running for the same reasons as the DPS. So you see, even for an APS abort, we end up turning the engine on, then off, and then back on, which we don't want to do.
Let me point out that after about 25 seconds at full throttle, the hori- zontal velocity required to get back into orbit when combined with the radial velocity picked up during the attitude change results in a guidance and attitude control situation considered acceptable. That is, it is not necessary to turn off the engine during the pitch over to posigrade atti- tude. So our only concern is with aborts during the first 25 seconds after throttle up, when it is neither acceptable to leave the engine on nor to turn it off for fear that it won't start again.
Standby for Payne's solution!
It is proposed that in the event of an abort recognized in that trouble- some period to continue operating the DPS in the retrograde direction until we have reached the time it is possible to make the attitude change to the posigrade direction without turning off the engine! If the DPS is the system that isn't working and it is necessary to “Abort Stage” and use the APS, it is proposed to burn the APS in the retrograde direction as long as necessary to reach the point when we can pitch to the posigrade direction without turning off the APS.
This solution, you see, avoids the need for turning off an operating engine and makes the procedures for both DPS and APS about the same in this time period as they are after this period. The thing that takes awhile to get used to is burning in a retrograde direction lowering the orbit still farther after a need for an abort has been recognized. How do we rationalize doing a thing like that? We currently feel that the advantages of the simplified, standardized procedures and particularly of not shutting down a running engine sufficiently justify thrusting to a situation a little worse than that which existed at the time of abort recognition. And, of course, we do have a tremendous propellant surplus if we abort at this time. Furthermore, aside from some problem associated with throttle up, the probability of an abort being required in this 25 second period seems awfully remote making it very difficult to justify development of a unique set of abort procedures and training to use them. In effect, this proposal creates two rather than three abort zones. No abort maneuvers are required prior to DPS throttle up since the LM is still orbital. Procedures after throttle up are all the same. There is no discrete point in the descent required special techniques.
Formulation of the LUMINARY DPS abort program (P70) is completely compatible with this procedure. That is, for a DPS abort the crew would always delay taking abort action until 25 seconds after throttle up. A program change will be necessary to support this procedure in the APS abort program (P71) so that if the crew hits “Abort Stage”, the APS will light off and separate, maintaining a retrograde attitude until 25 seconds after DPS throttle up time. Then it could go into the abort guidance as currently programmed. Specifically, the change is to have the spacecraft perform a continuous retrograde APS burn as opposed to a three second burn followed by an attitude change and reignition.
Mal Johnston of MIT was at our meeting and will discuss this with our friends in Boston. We'll talk about it some more next time after think- ing it over a couple of weeks. I'd be interested in your comments.