APOLLO LUNAR MODULE LANDING STRATEGYDonald C. CheathamAssistant Chief forEngineering and DevelopmentGuidance and Control DivisionFloyd V. BennettAssistant ChiefTheoretical Mechanics BranchGuidance and Control Division175APOLLO LM LANDING STRATEGYINTRODUCTIONSTRATEGY CONSIDERATIONSSpacecraft SystemsGuidance and Control SystemLanding Radar SystemWindow SystemDescent Propulsion SystemMission Landing Position RequirementPOWERED DESCENT IESIGNBraking PhaseObjectives and ConstraintsIgnition LogicGuidance with Limited _irottleLanding Radar UpdatingDelta V BudgetDescent Guidance MonitoringSummary of Braking PhaseFinal Approach PhaseObjectives and ConstraintsDetermination of Hi-gateParameter TradeoffsRedesignation FootprintLanding Point Designator Utilization176 ADelta V BudgetStunmaryof Final Approach PhaseLanding PhaseObjectives and ConstraintsNominal TrajectoryDelta V BudgetLUNARLANDINGTOUCHDOWNCONTROLObjective and Constraints - Modesof controlSequenceof eventsDescent engine shut-offManual control of landing velocitiesAutomatic control of landing velocitiesABORTAFaR TOUCHDOWNSUMMARY176 BAPOLLOLUNARMODULELANDINGSTRATEGY1.0 INTRODUCTIONThe landing of the Lunar Module (LM) upon the surface ofthe moon will be the climax of the Apollo mission, althoughthe importance of the return phases'is not to be de-emphasized.The LM landing approach will be the first time that the com-plete LM system will have been operated in the lunar environ-ment. This also will be man's initial face-to-face encounterwith the exact nature of the terrain in the landing area andof the problems of visibility as they may affect the abilityto land the LM; although, these aspect_ of the landing willbe simulated many times in fixed-based simulators and partialpreflight simulators. These simulations are extremelyimportant in the preparations for the mission; but onlyafter the mission is completed will it be known how adequatethe simulations have been.Considering the entire LM descent after separation from theCommand Module in lunar orbit, a theoretical landing maneuvercould consist of a Hohmann transfer impulse on the back sideof the moon with a delta V, or change in velocity_ of 109ft/sec, followed 180 ° later by an impulsive velocity changeof about 5622 ft/sec as the LM approaches the lunar surface_as illustrated in figure i. The flight path angle in thefinal portion of the approach would be zero degrees. Sucha theoretical approach would require infinite thrust-to-weight ratio by the descent engine. This_ of course, isan impossible and impractical approach. A finite thrust-to-weight ratio of the descent engine must be used and theapproach path must account for lunar terrain variations anduncertainties in the guidance system. Since lunar terrainvariations of as much as + 20,000 ft. could be expected, and,also_ uncertainties in the value of the lunar referenceradius_ coupled with guidance dispersions, could add another15,000 ft. to the uncertainty, a conservative safe value of50,000 ft. was chosen as a pericynthion altitude. From aperformance standpoint_ the choice of 50,000 ft. as opposedto either 40,000 or 60,000 ft. was quite arbitrary becausethe difference from the standpoint of fuel requirements wasvery slight, as indicated in figure 2. The initial thrust-to-weight of the LM descent engine will be about three-tenths.Combining this thrust-to-weight with a perigee altitude of50,000 ft. leads to the descent profile, as shown in figure 3.The separation and Hohmann transfer maneuver requires slightly177less delta V due to the pericynthion altitude increase.The powered descent portion approaching the landing area,however, requires a delta V of 5925 ft/sec, which is a con-siderable increase over the infinite thrust requirement. Ascaled trajectory profile of this theoretical LM powereddescent is shown in figure 4, indicating that the entiredescent takes approximately 220 n. m. The LM velocity andattitude is shown periodically along the flight profile.This trajectory has the predominant characteristics of alow flat profile terminating with a flight path angle ofabout 9 degrees. An obvious feature is that the crew, con-sidering the location of the LM window, never have theopportunity to see where they are going. They can lookeither directly up, or, if the LM is rotated about itsthrust axis, can look down at the surface, but they arenever able to see in the direction they are going. Ifthe crew is to perform any assessment of the landing areaor out-the-window safety of flight during the approach, itis obvious that the latter portion of the trajectory mustbe shaped so that a different attitude of the LM can beused during the approach. Shaping the trajectory awayfrom the fuel optimum approach will result in a penaltyin fuel requirements. Both the amount of time the crewwill require to assess the landing area, and the rangefrom which the landing area can be adequately assessedmust be traded off against the amount of fuel involvedin the penalty of the shaping. It soon becomes obviousthat a strategy is needed that will trade off the systemcapabilities of the spacecraft and the crew capabilitiesagainst the unknowns of the lunar environment encounteredduring the descent from the orbit, in order to insure thatproper utilization of the onboard systems can be made togreatest advantage. The development of this strategy,then, is the subject of thispaper.2.0 STRATEGY CONSIDERATIONSThe LM landing strategy can be defined as the science andand art of spacecraft mission planning exercised to meetthe lunar environmental problems under advantageous condi-tions. In order to plan strategy, the objectives, theproblems to be faced, and the characteristic performanceof available systems need to be well known. As indicatedin figure 5, the objectives of the LM landing planningstrategy are to anticipate the lunar environmental pro-blems and to plan the landing approach so that the com-bined spacecraft systems, including the crew, will mosteffectively improve the probability of attaining a safelanding. The major factors that must be considered inthis strategy are the problems brought about by the178orbits/ mechanics of the landing maneuver, the limitationsof the spacecraft systems (including limitations in fuelcapacity and payload capability), and the constraints ofthe lunar environment (including terrain uncertainties,visibility, and determination of suitable landing positions).The orbital mechanics aspects have been discussed in thepreceeding section.