Lunar Lander: How One Engineer's Persistence Led to Apollo Success

Time and distance have a way of blurring the hard edges, sharp points and intense disputes that accompany innovation.

Time and distance have a way of blurring the hard edges, sharp points and intense disputes that accompany innovation. Years later, what once seemed a ridiculous or foolish idea is thought of as an obvious, "of course, we knew it all along" approach.

One example is the "disposable" approach to the NASA Apollo mission's lunar module lander, which was largely conceived and pushed single-handedly by John Houbolt, who passed away in April 2014. The original moon-landing plan was for the manned third-stage of the launch rocket to "back down" to the moon's surface, then take off and return to Earth, just as in the classic movie Destination Moon dating from 1950. Houbolt's innovative idea called for a mission with a lunar-orbit rendezvous and a leave-behind lunar module descent stage. The lunar lander would include a separable ascent-liftoff stage, a concept that faced intense opposition and even some derision.

However, Houbolt persisted and even circumvented channels by writing a sharp, potentially career-endangering letter directly to the Apollo project head Robert Seamens, who was six org-chart layers above him. Houbolt was certain that the three-stage approach was an unworkable dead end and that his alternative was viable.

Houbolt did his own assessment of both the direct-to-Moon-and-back and alternative Earth-orbit-only rendezvous profiles and said they just wouldn’t work when all of the weight, fuel and risk issues were considered. He made a case for his alternative, but the NASA establishment made an equally good case, at least initially. Many engineers up and down the organizational chain were legitimately skeptical as they criticized his ideas and figures.

Challenging Conventional Wisdom

While we now consider an in-lunar-orbit spacecraft rendezvous as routine, the viability and risk of the more basic Earth-orbit linkup was actually a major concern at the time (rendezvous was so new that Buzz Aldrin's doctoral thesis at MIT was on the subject).

bolt's proposal for a lunar-orbit rendezvous was almost inconceivable and therefore even more challenging, and he proposed it before any Earth-orbit linkup had even been tried. No one knew if such a complex set of maneuvers was possible in Earth orbit; to do it around the Moon seemed nearly impossible. The reality is that orbital "mechanics" and navigation are difficult and unforgiving, especially with a scenario that is so drastically fuel constrained.

To be fair to those who were not persuaded at first by Houbolt, nearly every aspect of the fundamental design and specifics of the Apollo lunar mission lay somewhere between a rough estimate and a wild guess. There were thousands of technical unknowns from basic rocket and capsule structure to propulsion, weight and guidance issues. Engineers stumbled around in the dark, so to speak, and there were legitimate and convincing counterarguments to his approach.

However, as the Apollo mission's proposed structure and design proceeded and became more tangible—and thus the equations of fuel, mass and operational sequences grew more robust—the analysis and risk for a third-stage lunar landing and liftoff showed the overall plan could not succeed. As a result, Houbolt's alternative approach became the one to look at more closely and try to implement – if its many issues could be addressed.

Rethinking Fundamentals

From an engineering perspective, what's most noteworthy about NASA's decision to pursue the independent, disposable lunar module (the ascent stage was discarded after its rendezvous with the orbiting command module) is how it tossed out most of the original fundamental design "givens." The landing vehicle would only have to work in the low-gravity, no-atmosphere moon environment, instead of being the launch rocket's returning third stage. This change had a ripple effect through the entire Apollo project approach, all the way back to its basics. The design effort was restarted with a nearly blank page, as everything about the mission from the Saturn V first stage rocket to the mission's sequence of events, could be re-examined in the context of two fundamentals points.

First, the goal is to land and return at least one astronaut from the moon's surface by 1970 (less than a decade from the date in September 1962 when President John F. Kennedy set the goal).

Second, the escape velocity from Earth's gravitational pull is about 40,000 km/hr (25,000 mph). This number is independent of the weight being launched, but obviously a heavier weight will need more thrust to reach this critical speed.

The consequences to the mission architecture were dramatic. For one thing, the lunar lander itself no longer had to survive a return to Earth and so could be an ultralight, relatively flimsy, one-time use vehicle. For another, the technique of in-orbit rendezvous had to be mastered and extended for lunar-orbit rendezvous where there would be no second chance, and no backup escape plan if something went wrong.

Furthermore, the lift-off engine of the lander's ascent module that would blast the astronauts back from the surface up to rendezvous with the orbiting command module would have no redundancy; it would have to be extremely simple and reliable, and had to work perfectly the first and only time it would be fired. (Watch a video of the Apollo 17 lunar ascent stage liftoff from the Moon's surface.)

Ripple Effect

To understand the impact of payload weight, note that the overall Saturn launch vehicle weighed 6.2 million pounds (2.8 million kilograms) and generated 7.6 million pounds (34.5 million newtons) of thrust via its five F-1 engines. All this was for a lunar module and propellant which weighed a combined 36,100 pounds (16,400 kg). In contrast, the most-optimistic weight estimate of a third stage designed to land directly on the moon's surface and also make the return trip was at least three times those numbers.

A rough guide suggests that every additional pound of payload costs about 100 pounds of booster weight (more structure, more fuel) to get into orbit, and a corresponding increase in thrust. If the entire third stage was to be used as a landing and return vehicle, the overall project would have required two or three synchronized Saturn V launches. Each launch would carry parts for the mission, then the spacecraft would have to be linked up and assembled while in Earth orbit before proceeding to the moon.

As further evidence of the ripple-effect caused by the successful design, less than one-third of the actual lander's weight was its ascent stage, which carried the astronauts away from the lunar surface and back to the orbiting command module. Many subsystems and functions that otherwise would have been required could be taken off the "must have" list, while others could be scaled back and simplified. All that the ascent stage in Houboult's design had to provide was a basic place for the astronauts to stand; short-time life support; minimal navigation, guidance and electronics systems; small braking rockets; and a much-smaller lift-off engine.

The passing of John Houbolt illustrates a case of a lone wolf whose radical ideas were contrary to conventional wisdom, Fortunately, he received vindication fairly early in his career rather than at the end of after his life (or after his passing). In the end, his idea won out: the analysis and physics eventually confirmed that his "crazy" idea was the only one that had a chance of succeeding. And it did.