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Meeting	ACGS Committee Meeting 124 - Williamsburg, VA - October 2019
Agenda Location	7 SUBCOMMITTEE B – MISSILES AND SPACE 7.4 Development of the Apollo Lunar Module Digital Autopilot
Title	Development of the Apollo Lunar Module Digital Autopilot
Presenter	John Tylko
Affiliation	MIT
Available Downloads*	presentation
	Downloads are available to members who are logged in and either Active* or attended this meeting.
Abstract	Midway through the Apollo program, NASA challenged its contractors to evaluate options for improving performance while reducing operational risk. Digital flight control offered significant benefits over the analog systems originally under development, including the ability to control the combined Apollo spacecraft “stack” using the Lunar Module’s Descent Propulsion System which had been designed with limited gimbal control authority. Grumman’s engineers initially challenged MIT’s approach, arguing that a digital autopilot would consume the limited computational capacity of the Apollo Guidance Computer. MIT prevailed when both teams presented their arguments to NASA’s technical leadership. Taking Michael Athans’ first class on optimal control theory, Instrumentation Laboratory engineers George Cherry and Bill Widnall applied Pontryagin’s recently published maximum principal to develop a minimum-time thrust-vector control law which could be successfully implemented in the Apollo Guidance Computer. Making extensive use of both digital and hybrid simulation, including Grumman’s hardware in the loop simulation, they refined and validated this design over the next three years. The Lunar Module Digital Autopilot was first used to control the combined spacecraft during the Apollo 9 mission in 1969. It proved essential in saving the Apollo 13 mission a year later, after the spacecraft’s primary propulsion system was disabled, necessitating two critical burns of the LM Descent Propulsion System to put the spacecraft back on a free return trajectory and to accelerate the return to Earth.