First-Hand:The Lunar Module (LM) Full Mission Engineering Simulator (FMES)

Submitted by John H (Jack) Sachleben


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The Lunar Module Full Mission Engineering Simulator (LM-FMES) was designed, developed, and operated by Grumman as part of the Apollo Lunar Lander project contract with NASA. The specs were full mission, man-in-the-loop, out of window displays and the ability to substitute flight hardware for computer modules as needed. This document contains my recollections of the design and development of the simulator computer components and interfaces along with the simulator control system.


My 34 year Grumman engineering career started in 1952 with a summer job soldering connectors for F9F-2 fighter, invaluable for learning how to deal with people; old timers, lead men, and inspectors. The next summer was spent with a support group of technicians in a testing facility that also supported the new analog computer lab. For the remainder of my breaks, I worked in the analog computer support group in the Research Department. After finishing my 5 year BEE at Cornell which included building a vacuum tube from scratch and testing it against published 6SN7 specs, I accepted an offer to work in the same group.

My first assignment was to determine the frequency response of the first motion seat installed by Grumman. This device was hydraulically powered with 3 pillars welded to the building frame and wire wound multi rotation potentiometers as the feedback devices. I thought the test was going well, but still hadn’t reached the 3db point when my boss came running in asking what I was doing, since the fight test structural engineers on the floor below were complaining that the lights were swaying and dust was falling from the ceiling. After modifying the feedback circuit, my first experience with D/A, the simulator was used for carrier landing studies, flight control studies and early LM docking and landing studies. The first display was a rear projection screen using a model of a carrier for night time landing simulations. Final tests were “flown” by pilots who finally agreed that simulators were a good idea.

The analog computers were used in support of product design efforts, proposal support, studies of new capabilities, and anomalies with production products. Interesting examples included pilot-in-the-loop evaluation of the first stick feel system to modify response to control stick inputs during transition to supersonic flight and a study to verify that a pilot could shoot himself down. This study was performed after a F9F-9 pilot fired his cannon, dove to a lower altitude, leveled out, and flew into his own bullets. The resulting arguments by design and other engineers over whether it was possible led to the study that showed that the pilot should have flown into even more bullets than were found in the wreckage and resulted in changes in flight procedures to turn before diving after firing guns.

By the early 1960’s man-in-the-loop simulations were common during proposal and early design phases to verify input and output interfaces, pilot workloads, etc. The proposal efforts for the LM, originally known as the LEM, included motion and fixed base studies of both the landing (separation from the Command Module (CM) thru landing) phase and the docking (lift off from the moon thru docking with the CM) phase. Both of these phases lasted about two hours, the maximum duration that analog computers would run before needing readjustment to eliminate drift. Some of the docking studies were conducted using a mockup of the LM located inside a spherical fiberglass radome mounted on the roof, 120 steps above the top of the elevator. A starfield generator projected stars onto the inside of the dome and a model of the CM was included. Two interesting visits during the mockup development included a NY state radiation inspector who insisted that we mount warning labels next to the dashboard displays that utilized radon dials. Since this event happened on a Saturday morning after a celebration, the inspector soon agreed that a warning outside the door was acceptable and preferable to the alternative of being thrown off the roof. A few days later a PHD in Psychology showed up and demanded the color of the phosphorescent displays be changed for human factor reasons. We explained that the color he wanted used ten times more power and therefore more launch weight than the color selected, and after reaching a stalemate kicked him and the first human factors problem in our experience up to management.

FMES Design

On day early in the LM development the Grumman VP of Engineering addressed a group of about 30 of us engineers and stated that he had an impossible job for us with no money limitations. We all laughed out loud and told him under those conditions we could do anything. He then explained we were to design and build a LM full mission simulator including man-in-the-loop with out of the window displays that was also capable of including flight hardware in place of computer models when the hardware was available. At that, we stopped laughing and got to work. The system design was based on the engineering organization, resulting in separate rooms for flight hardware mounting and interfacing, the mock-up and out of window displays, and the analog and digital computer rooms. I was given responsibility for the interfaces between rooms and the system control functions.

Since the analog computers of the time worked in parallel, provided outputs and accepted inputs of +/- 100 volts, interfaced easily with displays and controls, and used time as the only independent variable, they were a natural for “flying” the spacecraft. However, for calculating orbits, desired attitudes, and required main engine thrusts, digital computation was necessary. We used an IBM 7094, the last of the discrete component mainframe computers with a single external interrupt for this type of calculations. Therefore we had to add analog/digital and digital/analog capability to use the 7094 as part of the FMES. We discovered that IBM had just come out with its first capable process control computer, the 1800, that contained and supported ADC and DAC conversion. Once we negotiated an improved delivery position based on National Need, the remaining problem became interfacing the 1800 and 7094. I was familiar with IBM’s procedures for handling special orders since my Dad was an RPQ coordinator in NYC for IBM. However, IBM had switched to the 360 type of microelectronic circuits, known as SLT, for the 1800, and dismantled all 7094 design and modification capabilities. As a result, IBM was unable to design the interface in a timely matter. The solution involved my carrying some IBM 7094 repair manuals to San Jose to help design the interface so that the 1800 could add an id to each interrupt sent to the 7094 and decode the id attached to the outputs from the 7094. The FMES system control system was designed as an extension of the analog control system consisting of three push button lighted switches labeled “Reset”, “Hold”, and “Operate”. The switches were wired so that the system was in “Reset” until all switches were in the “hold” or “operate” position. This allowed any room or device connected to the simulator to force the system to the start or zero time (Reset) or stop the simulation at any time (Hold). When the last switch went to the “operate” position the simulation started.

Simulator System Integration

Interfaces between the computer rooms and the hardware lab, and the mockup and display generation system were shielded cables and an intercom. Each station was responsible for interfacing with the computer signals leading to a problem with the engineer responsible for installing the Nike Radar pedestal that positioned the TV camera over the 3D model of the moonscape below. He was a veteran power engineer who decided to provide a ground plane consisting of 1” copper rods 20 feet long on a grid in the floor of the hanger used for the mockups. The only way to convince him that we had to have a single signal ground for the simulator was to use one of the rods after showing him the noise between the installed rods, mainly ambient noise from the 400 cycle aircraft power available throughout the hanger. As expected the most difficult interface was between the pilot and the projected TV display in the mockup window. The camera, mounted on the end of a boom from the pedestal was free to rotate in three dimensions using spacecraft attitudes from the analog simulation while boom positioning was calculated digitally. Camera shaking from the long moving boom was magnified by the camera, particularly as the capsule neared the surface. Careful filtering and detailed analysis of possible ground loops resulted in realistic views.

The interfacing to hardware was performed by hardware test engineers and designed for prototype modules. Therefore no protection to limit voltage/current excursions was used on the analog signals meaning that +/- 100 volts at significant currents could appear when computer failures, i.e. vacuum tube failures, occurred. We agreed that any such occurrence would be just another test of the prototype hardware. Had we realized that all flight hardware would later be flown on FMES before being installed in the production Landers; different interfaces would likely have been used.

FMES Operation

Since FMES was modular by function and design, the system became operational when the first component came on line. An existing analog computer program was upgraded to represent the latest design vehicle and to provide the required outputs for the1800, the display and mockup, and the hardware lab. When this component was operational, integration of the other components began as they became available. Since analog programming was performed by wiring removable telephone jack boards, rapid changeover to different configurations was practical, allowing for parallel integration of the different modules. In fact, the integration of the various modules was an important objective of the FMES project.

As the different components came on line, LM system testing began to help answer questions concerning pilot workload , visibility during landing, etc. We were reassured as we watched the first televised close by flyover of the moon and compared it to the display in the mockup cockpit and could not tell the difference.


FMES was used throughout the development and building of the LM. In fact, at the direction of NASA, the FMES was used to perform preinstallation tests on all flight equipment before installation in a vehicle requiring round the clock operation for an extended period of time. In addition, FMES was activated during Apollo 13 to help with the planning of the LM rescue activities.


For me, FMES was a dream assignment. What better than a project to do something for the first time to help meet your countries first priority, sending men to the moon and bringing them safely home.

The final satisfaction was listening to the first manned landing on the moon while sunning on our dock in NH provided by all the overtime on the FMES project.