At 3:08 p.m. EDT on July 20, 1969, out of contact with Earth over the far side of the moon, the computer that guided the Apollo 11 Lunar Module (LM) Eagle (image at top of post) opened valves in its descent propulsion system, causing nitrogen tetroxide oxidizer and aerozine 50 fuel to come together in its descent rocket engine. The propellants were hypergolic, meaning that they ignited on contact with each other.
The descent engine fired for a little more than 12 minutes. At the beginning of the burn, Eagle, Apollo 11 Commander Neil Armstrong and Lunar Module Pilot Edwin Aldrin were in a 54-by-66-nautical-mile lunar orbit. At its end, the 16.5-ton, 23-foot-tall lunar lander and its occupants were in an elliptical orbit, the lowest point of which was 50,000 feet above the moon’s Earth-facing nearside hemisphere.
Apollo 11’s target landing site was known officially as Site 2. Selected because it was flat and equatorial, Site 2 was a 10-mile-long east-west-trending ellipse on the moon’s Sea of Tranquility centered at 0° 42′ 50″ north latitude, 23° 42′ 28″ east longitude. Eagle reached 50,000 feet about 260 nautical miles and 12 minutes of flight time east of Site 2, at which point its computer ignited its descent engine again to begin braking and final descent.
As the LM dropped below 7000 feet, its computer fired attitude control thrusters to tip it slowly upright so that it pointed its descent engine and footpads at the moon. This maneuver also aimed Eagle‘s twin triangular windows forward so Armstrong and Aldrin could see Site 2 up close for the first time.
The astronauts immediately realized that they had a problem. They should have been above the eastern edge of the Site 2 ellipse, about five miles from their target landing point. Instead, they had already flown past the center of their target ellipse and were descending toward its southwestern edge.
Apollo 11’s flight plan called for Armstrong to let the computer do the flying until Eagle was about 500 feet above the moon and 2000 feet east of the touchdown point. He would then take manual control and descend almost vertically to the surface. The veteran civilian test-pilot quickly realized, however, that Eagle‘s computer was steering it toward a boulder-strewn crater the size of a football field. This was later identified as West Crater, located near the southwest boundary of the Site 2 ellipse.
His heart rate increasing from 77 to 156 beats per minute, Armstrong assumed manual control early. Gripping his hand controller, he leveled Eagle‘s descent, then scooted the LM almost horizontally across the black lunar sky. While Aldrin read off descent and translation rates, Eagle‘s overworked computer flashed alarms and Capcom in Houston warned that Eagle was running low on propellants. Armstrong flew past West Crater and an adjacent smaller crater, then lowered to a safe touchdown just inside the Site 2 ellipse. At 4:18 p.m. EDT, he radioed his immortal words to hundreds of millions of people: “Houston, Tranquility Base here – the Eagle has landed.”
Flight controllers estimated that Eagle‘s descent stage contained only enough propellants for about 25 seconds of flight when it touched down at Tranquility Base. After the flight, more careful analysis yielded an estimate of 45 seconds, demonstrating that the system for estimating available propellants in real time left much to be desired.
Mission rules called for an abort if fewer than 20 seconds of propellants remained. What if, as Armstrong anxiously sought a safe place to land, flight controllers on Earth had mistakenly estimated an even slimmer propellant margin? They might have done as the rules dictated and called on Armstrong to abort the Apollo 11 lunar landing.
In June 1966, Charles Teixeira, with the Engineering and Development Directorate at the Manned Spacecraft Center in Houston, completed an Apollo Program Working Paper on the hazards of an abort during the 45-second period spanning from 65 to 20 seconds before planned touchdown. He assumed that the LM would be no more than 338 feet above the moon at 65 seconds and about 100 feet high at 20 seconds.
If an abort were initiated, then the LM’s descent stage engine would shut down. Nearly simultaneously, four explosive bolts linking the descent stage with the ascent stage would fire. A fifth pyrotechnic device would drive a guillotine that would cut the wiring umbilical linking the two stages. The ascent stage engine would then ignite to propel the astronauts toward lunar orbit. The abandoned descent stage, meanwhile, would fall to the lunar surface.
From abort initiation to ascent stage ignition, the abort procedure – which, apart from occurring at altitude, paralleled the normal LM ascent stage launch procedure – would last from two to four seconds. During that time, the ascent stage would follow the same path as the descent stage; that is, it would fall toward the lunar surface.
Teixeira assumed that, following an abort during the 45-second period, the four-legged descent stage would strike the moon with enough force to rupture its propellant tanks, while an abort at 20 seconds or after – in other words, at or below 100 feet – would leave its tanks intact. If they ruptured, either of two events might occur. First, the nitrogen tetroxide and aerozine 50 remaining in the tanks might boil and evaporate rapidly in the lunar vacuum. Evaporation would rapidly cool and freeze the propellants, and they would remain safely separated.
Alternately, the propellants might come together. This might occur, Teixeira wrote, if after impact enough of the descent stage structure remained intact around the ruptured tanks to contain the two propellants as they boiled. The result would be an explosion that would drive gases and fragments of the descent stage outward at several thousand feet per second. Teixeira estimated that the blast front would envelope the LM ascent stage within one-tenth of a second.
The extent of the damage this was likely to cause would depend mainly on how long the abort procedure lasted; that is, how quickly the ascent engine could ignite. The faster the ascent engine ignited, the farther from the descent stage the astronauts would be by the time it impacted and exploded.
For a two-second abort procedure, gas pressure from the explosion would damage the ascent stage if the abort began between 32.6 and 20 seconds before planned touchdown. If the two-second abort began between 44 and 20 seconds before planned touchdown, the ascent stage would stand a greater than 20% chance of being hit by a descent stage fragment.
For a four-second abort procedure, gas pressure from the explosion would damage the ascent stage if the abort began between 53.7 and 20 seconds before planned touchdown. The ascent stage would stand a greater than 20% chance of being struck by a descent stage fragment if the four-second abort began between 65 and 20 seconds before planned touchdown; that is, throughout the period Teixeira considered.
Teixeira called the “critical time spans” during which damage was likely to occur “rather short.” He acknowledged that the risk of a descent stage explosion during a near-surface abort might not be great enough to justify “elaborate remedial action,” such as a major redesign of the descent stage.
He recommended, however, that a descent stage propellant dump “at as high a rate as safely possible” become a part of the LM abort procedure. After due consideration, NASA elected not to follow his advice. Had Armstrong been forced to abort the Apollo 11 landing, Teixeira’s recommendation might have come back to haunt the U.S. civilian space agency.
Hazards Associated with a LEM Abort Near the Lunar Surface, NASA Program Apollo Working Paper No. 1203, NASA Manned Spacecraft Center, June 24, 1966.
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