Upthread: It is proposed that we plan a two step LOI maneuver (Dec 15, 1967)
Downthread: Two-stage LOI looks good after C’ (Feb 05, 1969)
PA/Manager, Apollo Spacecraft ProgramAUG 5 196868-PA-T-186APA/Chief, Apollo Data Priority CoordinationRecommendation to retain the Two-Stage Lunar Orbit Insertion (LOI) Maneuver
This memorandum documents the results of our review of the two-stage LOI maneuver which you requested as a result of a recent OMSF sugges- tion that it might be preferable to return to a single burn plan. Participation in this review involved all operational elements of MSC concerned in this matter. The conclusions are unanimous.
Summary – It is recommended that the two-stage LOI be retained in the mission plan for the first flight to the moon. The justification can- not be based on numerical results of analyses. In fact, if you believe in “Three Sigma” system performance and/or that nothing unexpected will happen, it can be shown that a single LOI burn is safe and that a subsequent trim maneuver will not be needed. Except for the 30 pound RCS cost, no advantage or disadvantage can be assigned a specific value. That is not to imply that they are valueless, however. There are very significant considerations and it is our belief the advantages of a two-burn LOI substantially outweigh the disadvantages – including the 30 pounds of RCS.
Background – To insert into a 60 n.m. circular orbit it is necessary to make a LOI maneuver of about 3200 fps which requires an SPS burn of about 380 seconds duration. The acceleration at the end of this maneuver is approximately 10 fps/sec., which reduces the orbital altitude about 7 n.m/ sec. of burn time. This maneuver can be accomplished in one continuous maneuver or can be discontinuous – that is, performed in two steps. The first stage of the two-stage LOI in the present lunar mission plan accom- plishs all but about 150 fps (15 second burn duration) of the LOI resulting in orbital altitudes of 60 by 170 n.m.; the second stage completes the process of achieving a 60 n.m. circular orbit. The second stage is performed entirely in-plane and, with targeting based on lunar orbit MSFN navigation, will reduce the dispersion of the in-plane orbital elements significantly.
Monitoring procedures can be established for a single-burn LOI, which vote two systems out of three to assure a safe pericynthion. Therefore, if at least two systems out of three are working, we are assured a safe maneuver. Based on current estimates of systems performance (G&N, ΔV counter, and SPS engine), the monitoring procedure would cause the crew to shut off the SPS manually, unnecessarily about 20% of the time – that is, before a satisfactorily operating G&N sends “Engine Off.” However, assuming that we can tolerate a dispersion of about ± 10 n.m. around the nominal 60 n.m. circular orbit, in no case (out of 100 runs) was an in-plane trim maneuver required. In fact, the manual intervention often improved the situation, providing a more nominal orbit than the G&N would have achieved. Further- more, current estimates of systems performance (pre-LOI MSFN, G&N during LOI, and DPS during powered descent) assure us that no out-of-plane trim maneuver will ever be needed in lunar orbit prior to descent. (See comment no. 1)
The following is a list of advantages of a one-stage LOI:
1. It nominally saves about 30 pounds of RCS propellant which would be used during LOI₂ for ullage, alignment, and attitude hold.
2. Approximately half the time it reduces the number of SPS burns in a lunar mission by one. (See comment 3a)
3. It reduces the nominal lunar timeline approximately one revolution (two hours).
4. It reduces terminal supercritical helium pressure build up by about 20 psi. (See comment 3b)
5. It reduces cryogenic hydrogen and oxygen consumption about 0.5 pounds and 5 pounds respectively.
The following are advantages of a two-stage LOI:
1. Crew Safety
a. Protects against double failure including undetected systems degradation beyond three sigma.
b. Protects against the unexpected. This is the first attempt to insert into lunar orbit and experience has shown that the unexpected is likely to occur, particularly on first attempts.
c. Provides a more nearly nominal lunar operating orbit, thereby decreasing the ranges of conditions in Descent, Rendezvous, and Abort for which the crew must train.
2. Other considerations
a. Makes lunar operations more nearly nominal by assuring achievement of the pre-planned orbit. (See comment no. 2)
(1) Simplifies procedures – for example, permits use of pre- mission “canned” CSM LM rescue maneuvers and descent abort switchover points.
(2) Reduces dispersions on such things as DOI and CDH burn attitude.
(3) Helps in development and simplifies crew charts and similar operational aids.
(4) Reduces DPS budget a little bit.
b. Keeps timeline constant in the event a trim burn becomes necessary.
c. Makes the mission plan and crew procedures less sensitive to changes in systems performance estimates, development flight experience, etc.
d. Assures two good (i.e., complete) G&N tests as opposed to about an 80% probability of getting one.
e. Avoids a change in the mission plan and all it affects. That is, everyone has been going and thinking two-stage LOI for eight months.
The basic problem, of course, is in weighing these lists of advantages because none of the items on either list is overwhelming. Certainly, the extra consumable costs and the lower supercritical helium pressure margin of a two-stage LOT are affordable; on the other hand, it certainly cannot be said the added risk and greater in-orbit dispersions of a one-burn LOI are unacceptable. These things do not dictate the decision of which way to go. Nor does the nebulous added risk of an extra SPS restart. This system's reliability will have been proven more than adequately in-flight prior to this mission – certainly to the extent that a five-burn mission is not significantly more dangerous than a four-burn. This must be true since a relatively high probability, contingency situation requires as many as four or five extra SPS burns for CSM rendezvous maneuvers to rescue the LM.
It seemed to the review team that the one vs two-burn LOI decision must be based primarily on operational considerations – primarily the signi- ficant advantage of close-to planned conditions as they influence both a nominal flight and ones conducted under degraded conditions. For example, assurance of a nominal CSM parking orbit will help immeasurably in effecting the rendezvous and will substantially reduce the ΔV costs. (Add in a system failure – the rendezvous radar or (beacon) for example – and the advantages multiply.) AND, it reduces communication requirements on a nominal mission. AND, it simplifies everyone's job pre-flight and in-flight at least a little – sometimes a lot.
But most important of all is our concern for the consequences of the many things we will not have thought about but will encounter on the first lunar flight. Anything that can be done to keep the dispersions small and the procedures simple provides that much more tolerance for the unexpected and that much more time and attention that can be devoted to handling them. It seemed to us, the cost of the two-stage LOI is a small price to pay for these intangible but important benefits.
This appendix consists of a number of comments which amplify or explain statements made in the main body of this memorandum. They are separated into three categories: 1) How the out-of-plane is handled, 2) The in- plane situation, and 3) Consumables and other things.
1. How the out-of-plane is handled
We plan to make no plane change in lunar orbit prior to descent. It is expected the pre-LOI MSFN navigation and targeting plus G&N per- formance during LOI₁ will provide a more desirable orbital plane than could be obtained by a trim maneuver based on lunar orbit MSFN navigation and targeting. That is, after insertion into orbit no new information, upon which we would be willing to act, will be available until the sextant observations of the landing site are obtained on DOI day. But more important than that, there is no need to make a plane change prior to powered descent.
a. The three sigma out-of-plane dispersion of the LOI maneuver is currently estimated to be about 0.3°. Major contributors are pre-LOI MSFN navigation and targeting and G&N control through LOI which have been RSS'ed to obtain that value.
b. DPS ΔV required to perform an out-of-plane maneuver is almost exactly proportional to the square of the landing site displacement from the orbital plane. One half degree costs 10 fps by itself; however, when RSS'ed with other descent dispersions, it contributes only about 2 fps to the DPS budget.
Therefore, it is clear the LM has considerably more capability than is needed to handle the out-of-plane situation which will actually become known only when the sextant observations are made – even if MSFN is two or three times worse than we expect today. And, of course, we are extremely anxious to avoid this extra SPS burn in order to avoid impacting the DOI day timeline which is almost unacceptably crowded without it.
2. The in-plane situation
The problems in-plane are quite different. Although the mission techniques are nominally tolerant of fairly large (± 10 n.m.) dispersions in orbital altitude, these dispersions have a highly undesirable affect on the flight from an operational standpoint.
a. Current estimate of the in-orbit dispersion due to pre-LOI MSFN navigation and targeting and G&N performance is in excess of 7 n.m. (three sigma). This could be reduced to about 2 n.m. (three sigma) based on post- LOI MSFN navigation and targeting and G&N performance through the short LOI₂ burn.
b. It may be argued that without actual experience we have no assurance the in-lunar orbit MSFN navigation and targeting will be of a quality to reduce the altitude dispersions as noted above. It is true the analysis that yielded those results is solely based on Langley Lunar Orbitor data whose orbit was not the same as that planned for Apollo. However, if it is not of that quality, the lunar landing is in jeopardy anyway. It is important to realize that MSFN is a vital in-line part of the G&N system – not a backup – and its failure would force switching to an alternate non-landing mission. The point is it either works that well or we don't land.
c. Considering the problems associated with determining the lunar potential, no plans are being made to make improvements in it during the operation. The point is MSFN performance in lunar orbit should remain essentially constant – not significantly better after a day than during the first revolution, although by then we should at least know how well it is working.
3. Consumables and other things
a. It can be shown that by taking advantage of the two-stage LOI targeting flexibility it is possible to eliminate one of the SPS translunar midcourse correction (MCC) maneuvers approximately half the time. Of course, avoiding this maneuver does not save ullage RCS since the MCC does not require ullage.
b. DPS supercritical helium pressure builds up continually from time of loading before launch until the DPS is used for Powered Descent. The loading technique is fixed and the pressure rises there- after with no external control toward its red line value as a function of time at a rate of approximately 8 to 10 psi/hour. Assuming the higher rate and one complete extra orbit, the pressure increase is 20 psi.
c. Assuming a 90 amp load, the extra hydrogen consumed will be less than 0.5 pounds during the extra revolution. The oxygen used for power will be less than 4 pounds; adding metabolic and cabin leak oxygen of less than 0.5 pounds each, the total extra oxygen consumed in this revolution is less than 5 pounds.