Downthread: Tweak burns (Jul 10, 1969)
See list attachedMay 1, 196969-PA-T-67APA/Chief, Apollo Data Priority CoordinationDescent Aborts and subsequent Rendezvous Mission Techniques
On April 28, 1969 we reviewed the Descent Aborts and subsequent Rendezvous Mission Techniques with the crew and the rest of the world. I think most of this is quite complete and agreeable to everyone concerned, with one major exception. I was shocked and ashamed to find that I had badly misunderstood the situation regard- ing the CSM rescue techniques and, although there were plenty of ideas, the detailed techniques were not at all firm at that time. Subsequently, (April 30) a much smaller group of us beat that into the ground too. Therefore, this memo is to document my understanding of the agreements we reached at both of these sessions. I'm sorry it's so long – just a big subject, I guess.
1. Abort after separation if there is to be no DOI
During our meeting we inadvertently got into a lengthy dis- cussion on conditions governing whether or not DOI should be attempted on the first or second opportunity. That, of course, is important but was not our real purpose at this meeting. We did finally con- clude that in the event no attempt is made at DOI, the LM should use the brute force, immediate return technique for getting back to the CSM. The point is the separation velocity setting up the equal period mini-ball orbit is so small that automatic closure is by no means certain. Accordingly, when it is decided to abort, the crew should take positive action to establish a fairly substantial closing rate. The present recommendation is that they should set up a closing rate which in feet per second is equal to eight times the current range expressed in nautical miles. This is the same procedure that should be used for fouled up DOI maneuvers. It is useable until about ten minutes after DOI.
Some of the crew present expressed a concern that the factor “eight” seems excessive under certain circumstances and requested that somebody make sure it is really the best value. I guess this is your job,Mr. Lineberry, if you can find time between now and July to handle it. I think we should all realize, however, that simplicity in procedures may prohibit using the value that is optimum under all circumstances.
2. Abort if no attempt is to be made to initiate powered descent
At one time it was considered impractical to go an extra rev and attempt PDI two hours late, primarily due to fear of an unacceptable rendezvous/abort situation. This has proven to be unfounded. The same rendezvous abort procedures work after an extra rev, although there is an extra cost of about 70 fps for insertion from descent aborts. The extra insertion velocity does make APS propellant depletion more likely for late aborts, but the RCS can be used to make up the difference. Time required to complete a CSM rescue can be increased up to 12 hours and at a cost of 800 fps. This is used to put the CSM in a dwell orbit. But, this is only necessary if the LM experiences many failures and does not seem sufficient justification to scrub the landing attempt. Eight LGC descent abort coefficients for P70/P71 and one for the AGS must be updated in real time. (Incidentally, the current plan is to update these in real time on the nominal mission to account for dis- persions in the CSM orbit.) A platform alignment should be performed by the LM prior to the second PDI attempt. The major open item is for the Flight Dynamics people to establish what Pad and command messages must be sent to the spacecraft and when. (There is also some question of accuracy of the revised descent targeting.) The primary concern deals with time available to do this. Incidentally, these same techniques may also be useable for a DOI maneuver delayed one rev.
3. PDI Abort
A PDI abort is only used if it is known that PDI will not be attempted or possibly, if the DPS engine does not ignite. Considerable thought was given to using an onboard capability for targeting this maneuver. Specifically, the technique was for the crew to initiate the powered descent programs following the nominal timeline through engine ignition and then hitting either the Abort or Abort Stage button to utilize the DPS or APS Descent Abort programs which auto- matically target the abort maneuver. It was finally concluded, however, that this technique by itself was not really adequate because spacecraft systems problems could occur at PDI time which would make it highly desirable not to have to commit instantaneously either to aborting, nor to going around another rev. That is, it seemed almost mandatory to provide an abort opportunity a short time after PDI to provide a little time to think over the situation and decide what to do – go around and try PDI again, or to abort now. Since the delayed abort opportunity was considered a requirement for this purpose, the question boiled down to whether the crew and everyone else should learn and be prepared to use the instantaneous PDI abort technique as well. Since there are some problems not yet worked out with it and special procedures are required, we concluded that it was best to drop use of the onboard technique and to provide a ground targeted abort opportunity at PDI plus 10 minutes. This abort would utilize the standard pre-thrust and thrust programs (that is, P30 and P40 or P42) and PDI Abort Pad message voiced to the crew before DOI. Since this maneuver assumes nominal conditions coming into PDI, the targeting for this burn is essentially known today. Accordingly, Ed Lineberry is to supply the ΔVg values to FCSD to be included in the crew's checklist. Simulations and experience may eventu- ally prove that the Pad message need not be sent.
Incidentally, if DPS ignition does not occur at PDI there is no need for the crew to remove ullage since it is so small.
4. Aborts from Powered Descent
It has been established that a trim maneuver (we've been calling it the “tweak”) is necessary after LM insertion into orbit in order to compensate for known errors in the LGC abort target coefficients and measured dispersions in the insertion conditions. Tweak targeting will be carried out by the MCC (not onboard) based on the best available data source for cutoff state vector – ordinarily the LM PGNCS – and will be relayed to the crew within 1½ minutes after main engine cutoff. The tweak burn is nominally horizontal but spacecraft attitude can be sub- stantially in error with negligible results.
I think everyone agreed to the necessity of the tweak burn but there was considerable discussion on how the post insertion situation should be handled. We finally recognized that the thing that most con- fused the issue was the DPS. For example, plume impingement precludes making large burns while docked, making jettison procedures necessary under certain ΔV circumstances. Systems problems might make it manda- tory that the DPS not be jettisoned, meaning that procedures were needed for both cases – staged and unstaged and so forth. There appeared to be minimal problems associated with the situation if the LM had to stage the DPS in order to achieve orbit. This led us to the final resolution, namely:
a. If the LM achieves orbit using the DPS and the Vgo is less than 30 fps, the CSM will make the tweak maneuver at DPS cutoff plus 12 minutes. This maneuver will be under GNCS control using the SPS or RCS, whichever is called for. In this case, the LM can carry the DPS as far as docking with the CSM if that is considered desirable or it may be jettisoned at any convenient time, provided the act of jettison is carried out without any perturbation to the trajectory. If the DPS is carried along, it may be used for some of the rendezvous maneuvers.
b. If the LM insertion into orbit is on the APS, the LM makes the tweak burn as soon as possible, probably within two minutes after engine cutoff using the RCS and the “average G” program (P47).
c. The significance of “Vgo less than 30 fps” mentioned above is that if the DPS cutoff occurs with more than 30 fps left to be gained, the crew is supposed to Abort Stage and finish the maneuver on the APS. This is a rule we have agreed to for a long time.
d. The LM does not trim any ΔV residuals after main engine cutoff for any descent abort unless the MCC fails to advise the crew within 30 seconds after cutoff that the MCC targeting will be available. The point here is that if the MCC has lost communication, which includes even the high-bit rate telemetry needed for targeting, the course of action is for the crew to trim the residuals as soon as possible. On the other hand, it is advantageous to wait if they are going to make the MCC targeted tweak burn. They should know within 30 seconds after cutoff which of these situations exist.
e. The voice message from MCC consists of only two parameters – TIG and ΔVx.
f. Just as in a nominal mission, the MCC will always update the LM state vector in the CMC based on LM telemetry data regardless of which vehicle makes the tweak burn. However, if the CSM is the active vehicle, the LM crew must update the CSM state vector in the LGC using the target ΔV program, P76.
5. Late Aborts from Powered Descent
Aborts during the first 10 minutes of powered descent utilize variable insertion velocity targeting in the LM guidance computers – both PGNCS and AGS. The subsequent rendezvous sequence is essentially the same as a nominal rendezvous. As a result, standard CSM mirror image targeting can be used to backup the LM and no special procedures are required aside from the tweak burn noted above. However, after approximately ten minutes into powered descent the variable insertion targeting would result in an apogee less than 30 n.mi., which we consider too low. Therefore, aborts after that time are targeted for a standard low orbit – 9 by 30 n.mi. and the rendezvous situation begins to degrade. That is either the terminal phase lighting conditions or the coelliptic differential altitude becomes undesirable. It is recognized that for aborts occurring during an additional 40 seconds into descent the standard rendezvous sequence can be continued since we consider the resultant increase in differential altitude up to 20 n.mi. acceptable. After that point, something else must be done. The something else is as follows – in order to maintain nominal lighting and ΔH, an extra rev is required. Two extra maneuvers are required in the subsequent rendezvous sequence costing a total extra ΔV of as much as 80 fps. (This extra ΔV cost diminishes to zero as the abort is delayed.) The first extra maneuver, called “Phasing,” occurs about 50 minutes* after insertion and is tar- geted by MCC to establish the nominal ΔH and TPI time. The Phasing maneuver is horizontal; its ΔV is a function of abort time. It will be transmitted by voice using the standard External ΔV Pad format. CSI₁ is the other extra maneuver and occurs 180° after Phasing. It is tar- geted onboard using an MCC supplied TIG. Following these two extra maneuvers, the spacecraft goes through the standard CSI/CDH/TPI sequence. All of these maneuvers are, of course, computed onboard.
The CSM performs standard mirror image targeting as usual with one exception. Since the Phasing burn could be excessively retrograde, the CSM backup of Phasing must be limited to about 50 fps. If this occurs and the CSM must execute it, the crew must use some special P32 pro- cedures for CSI₁ to compensate for the inadequate Phasing adjustments. (The complete procedures are being documented thoroughly by MPAD and FCSD.
That's long and maybe confusing. In summary, let me point out the key things. Our problem – the one that took a day to resolve – was to figure out some way to work with both spacecraft so that:
a. The rendezvous situation would be completely acceptable – particularly the lighting and adequate tracking time and
b. That at any point, either spacecraft could take over the active role as the situation dictates and
c. That the technique be relatively simple – especially not loaded with special procedures that differ from nominal.
The solution satisfies these things very well much to the credit of Jerry Bell, Ed Lineberry, H. David Reed, Milt Contella, and probably some others.
The tasks to clean this up are:
a. OMAB – Pin down the precise timeline, ΔV's and TIGsg lighting, ranges, rates and angles – that is the reference trajectory for a few key descent aborts.
b. MPB – Establish the rendezvous navigation tracking schedule and all that goes with it.
* Phasing shall actually occur at a fixed GET corresponding to the CSI time for an abort occurring at 10 minutes into powered descent. This GET time will be on a pre-DOI Pad.
c. FCSD – Prepare the detailed crew procedures – particularly CSM – and identify which specific parts should be given highest simula- tion priority if the crew can give any attention to them preflight.
d. OMAB – Compute the rendezvous maneuver biases which must be applied to one spacecraft solution for use by the other for the various abort modes.
6. Aborts After Touchdown
Current planning includes two “preferred” times for aborts after touchdown. “Preferred” is misleading in that for the first stay/no-stay period, it is preferable to Abort Stage as soon as its need is recognized and then to carry out the rendezvous sequence precisely as described above in Section 5.
Since it is considered undesirable to remain in the insertion orbit through perigee, it was decided to establish a minimum Phasing burn of 10 fps which will always be executed by the LM to raise perigee. This, of course, changes the stay/no-stay decision time about 30 seconds earlier and the second preferred abort time one minute earlier since it reduces the catch-up rate in the parking orbit.
7. Here are some odds and ends of interest to me:
a. All rendezvous navigation, both nominal and following aborts in both spacecraft, will be operated to update the LM state vector regardless of which vehicle is active. This is done because the CSM state vector is known better inertially than the LM.
b. It is important to recognize that after a descent abort there is a very good chance the LM will have a substantial DPS and/or APS capability remaining – particularly the latter. Some of these rendezvous maneuvers can be very large – up to 120 fps. The MCC must be prepared to assess and assist the crew in choosing which engine should be used to avoid all the many constraints the LM has regarding plume impingement and APS restarts. Also, regarding PGNCS minimum burn accuracy and how to use the interconnect, etc.
8. That's it!