Why Apollo 11’s Ascent Engine Had to Work the First Time

[00:00:00]On July 21st, 1969, 21 hours and 36 minutes after landing, Neil Armstrong and Buzz Aldrin prepared to leave the moon. Beneath their cabin was a single ascent engine developing approximately 3,500 lb of thrust. There was no second main engine and no spacecraft capable of descending from lunar orbit to rescue them. The engine had been thoroughly tested before the mission, and its basic design had already been operated in space, but Eagle's particular ascent system had remained dormant throughout the journey from Earth, the descent to the moon, and the crews stay on the surface. There could be no short test firing before departure. The next start command had to produce a complete 7-minute ascent burn. Its designers therefore removed almost everything the engine did not require. It had no turbo pump, no throttle mechanism, and no gimbal system. What remained was a pressure-fed rocket engine built around simplicity, redundancy, and propellants that ignited on contact.

[00:01:13]The lunar module's descent stage had completed its purpose. Its engine, landing gear, empty propellant tanks, and surface equipment would remain in the sea of tranquility. Armstrong and Uldren would return in the smaller ascent stage containing the crew cabin, guidance equipment, reaction control thrusters, propellant tanks, and ascent engine. The engine was mounted within the lower portion of the ascent stage.

[00:01:40]Its thrust chamber extended down into the space between the two stages, but the engine could not steer by moving its nozzle. It was rigidly installed with its thrust line passing close to the ascent stages center of gravity. It also operated at only one thrust setting, approximately 3,500 lb. Unlike the descent engine, it could not be throttled. Throttling was unnecessary because the ascent stage did not have to hover or select a landing point. Its objective was to reach a specified altitude, velocity, and orbital plane.

[00:02:17]The guidance system steered by rotating the complete spacecraft with the lunar module's reaction control thrusters.

[00:02:24]This produced a far simpler main engine with fewer mechanical systems that could prevent it from operating.

[00:02:33]The F1 engine required a turbo pump producing tens of thousands of horsepower to force propellant into its combustion chamber. The lunar module ascent engine used no pump at all. Two spherical tanks stored helium at a nominal pressure of approximately 3,150 lb per square in. When the ascent propulsion system was prepared for operation, explosive isolation valves opened the helium paths. The gas passed through filters and redundant regulators before entering a common manifold. From there, it applied pressure to the ascent stage fuel and oxidizer tanks. That pressure forced the propellants through their feed lines, trim orififices, and engine valves. The regulated pressure at the engine interface was approximately 170 lb per square in. The fuel was aerosine 50, an equal mixture of hydroine and unsymmetrical dimethyl hydroine. The oxidizer was nitrogen troxide.

[00:03:35]These were hypergolic propellants. When they met inside the combustion chamber, they ignited chemically. No spark plugs, pyrochnic igniter, or separate ignition fluid was required.

[00:03:47]Opening the engine valves admitted the two liquids to the injector and combustion began. The propellants could be stored for the entire mission without refrigeration. They were also used by the descent engine and the lunar module's reaction control system, simplifying the spacecraft's propellant requirements. Their disadvantages included toxicity, corrosiveness, and the danger of accidental contact. The feed system therefore incorporated check valves, filters, isolation valves, and seals to prevent propellant or vapor from flowing into the helium manifold.

[00:04:23]The absence of a backup ascent engine did not mean that the system had only one path through every component. Helium was stored in two tanks and routed through redundant flow paths. The engine valve package contained dual passages for the fuel and oxidizer with series connected ball valves controlling each flow path. Electrical controls also provided more than one method of commanding the engine. If the normal automatic start logic failed, the crew had procedures for initiating the ascent engine manually.

[00:04:59]Guidance had similar protection. The primary guidance, navigation, and control system normally calculated and controlled the ascent. The separate abort guidance system could provide guidance, navigation, and control if the primary system failed.

[00:05:16]The ascent stage also carried 16 reaction control thrusters arranged in four clusters. These controlled pitch, roll, and yaw while the fixed main engine supplied the forward acceleration.

[00:05:30]These backups could overcome a failed computer, electrical path, or control function. They could not replace a complete loss of main engine thrust. The reaction control system was not designed to lift the ascent stage from the lunar surface and place it into orbit.

[00:05:47]Reliability, therefore, came from simplifying the main engine and duplicating the components most likely to interrupt its operation.

[00:05:56]Eagle's ascent engine was not an entirely untested machine. Development engines underwent extensive firings, including simulated altitude testing.

[00:06:07]Production hardware was inspected, calibrated, and acceptance tested.

[00:06:12]Complete propulsion systems were exercised in ground installations representing the lunar modules flight configuration. Apollo 5 had tested lunar module propulsion during an uncrrewed Earth orbit mission. Apollo 9 then flew the first crude lunar module and operated its ascent propulsion system in Earth orbit. Apollo 10 carried another lunar module to the moon and provided further experience with the complete spacecraft. Eagle's flight engine had therefore been supported by years of component engine and integrated system testing. However, once the flight hardware was installed, serviced, and carried to the lunar surface, the crew could not fire it briefly and then inspect the result. A firing consumed propellant, heated and eroded the ablative thrust chamber, and would immediately begin lifting the ascent stage. The first operational firing on the moon was the departure.

[00:07:12]Apollo 11's planned liftoff time was selected to place Eagle in the correct relationship with Colombia, which continued orbiting the moon with Michael Collins aboard. Before departure, Armstrong and Aluldren transferred the lunar samples, exposed film, and equipment into the ascent stage. Items no longer required were left on the surface to reduce liftoff weight. The guidance computer received the ascent targeting information. Both the primary and abort guidance systems were checked, the reaction control system was configured, and the ascent propulsion system was armed. The connections between the two lunar module stages also had to be severed. Explosive devices released the structural attachments while cutters divided the electrical wiring and other connections crossing the interface. Those events occurred as part of a closely controlled staging and ignition sequence. The ascent engine fired downward into the descent stages open central structure while Eagles separated from its landing platform.

[00:08:17]This arrangement was known as a fire in the hole start. The engine plume, pressure pulse, and debris environment had been reproduced during development testing because the ascent engine could not begin its operation in unobstructed space.

[00:08:33]Eagle lifted off at 124 hours 22 minutes mission elapsed time, approximately 12:54 p.m. Central Daylight Time on July 21st.

[00:08:45]For the first two seconds, the guidance system held the initial attitude while the ascent stage cleared the descent stage. The vertical rise phase continued for approximately 10 seconds to provide terrain clearance. The spacecraft then began pitching toward its required flight path. Since the main engine was fixed, the reaction control thrusters rotated Eagle, redirecting the engine's thrust. The primary guidance system commanded attitude rather than engine power. The ascent engine remained at full thrust while the guidance computer continuously calculated the velocity still required. Once the vertical rise conditions had been satisfied, the computer entered the orbit insertion phase. The trajectory became progressively more horizontal as Eagle accelerated along the lunar surface and gained orbital velocity.

[00:09:37]The crew monitored the primary guidance solution, the independent abort guidance data, and the engine indications.

[00:09:45]If the two guidance systems began disagreeing beyond established limits, Armstrong and Aluldren would have needed to determine which system remained reliable. No such intervention was necessary.

[00:09:59]Pre-m mission planning called for an ascent burn of approximately 7 minutes and 18 seconds. Eagle achieved insertion after about 7 minutes and 15 seconds.

[00:10:10]Post-flight analysis found no ascent propulsion anomaly. The slightly shorter burn was consistent with a thrusttoe ratio marginally higher than predicted.

[00:10:21]At engine cutoff, Eagle was approximately 60,300 ft above the moon. Its downrange velocity was about 5,537 ft pers.

[00:10:33]After the residual velocity was trimmed with the reaction control system, the ascent stage occupied an orbit approximately 9 12x 47 nautical miles.

[00:10:44]The ascent engine had used most of its available propellant, but investigators estimated that approximately 250 lb of usable propellant remained at cutoff.

[00:10:55]Its main task was complete. The subsequent rendevous corrections were performed with the reaction control thrusters.

[00:11:04]At insertion, Colombia was already far ahead of Eagle in lunar orbit. The ascent trajectory had been timed so that the two spacecraft would arrive at the correct relative positions for rendevu.

[00:11:17]Armstrong and Uldren used the lunar module's guidance system and rendevous radar to measure Colombia's direction, distance, and closing rate. Michael Collins independently tracked Eagle from the command module. A sequence of reaction control maneuvers gradually raised and adjusted Eagle's orbit. The two spacecraft approached one another over approximately 3 and 1/2 hours.

[00:11:42]Collins completed the docking, reconnecting the same two spacecraft that had separated before the lunar landing. Armstrong and Uldren transferred themselves, the lunar samples, and the exposed film into Colombia. Eagle had completed the only task for which its ascent stage existed, carrying two astronauts from the lunar surface back to the command module.

[00:12:09]Landing was only half of Apollo 11's lunar objective. The return depended on one compact, pressure-fed engine operating after days of inactivity and completing a continuous burn of more than 7 minutes.

[00:12:25]NASA reduced the risk through ground testing, flight experience, hypergolic propellants, redundant feed and control paths, backup guidance, and a fixed non-throttable engine. At lunar liftoff, those preparations were compressed into a single command sequence. The valves opened, the propellants met, and the ascent stage left the sea of tranquility.

[00:12:50]3 and 1/2 hours later, Armstrong and Uldren were back with Michael Collins.

[00:12:55]Only then had Apollo 11 finished the lunar portion of its mission.

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