Spaceflight Insider

Jack Crenshaw: the space pioneer you never heard of

The crew portrait of Apollo 13. Commander Jim Lovell, Command Module Pilot Jack Swigert, Lunar Module Pilot Fred Haise. Photo Credit: NASA

The crew portrait of Apollo 13. Commander Jim Lovell, Command Module Pilot Jack Swigert, Lunar Module Pilot Fred Haise. Photo Credit: NASA

Despite their scientific justification of gaining samples of the lunar regolith, the Apollo expeditions had two objectives laid down by President John F. Kennedy in 1961: to land a man on the Moon and return him safely to the Earth. A critical part of the second objective, returning safely to the Earth, was a trajectory that would bring the spacecraft back from the Moon and re-enter Earth’s atmosphere.

Dr. Jack Crenshaw was one of the pioneers in designing the figure-8 trajectory which became known as the free-return trajectory. The early Moon missions set off for our closest celestial neighbor in the free-return trajectory which, if uncorrected, would loop them around the Moon and return them straight to Earth.

In order to orbit the Moon and land on it, the Apollo spacecraft’s Service Propulsion System (SPS) engine had to fire in order to slow the vehicle down and settle into lunar orbit. Delicate calculations were needed in order to ensure the orbit would not bring the craft into a collision with the lunar mountains.

Jack Crenshaw. Photo Credit: Jack Crenshaw

Crenshaw. Photo Credit: Jack Crenshaw

Crenshaw graduated from Auburn in 1958 and applied to the National Advisory Committee for Aeronautics (NACA)—the precursor to NASA. He received a reply with NACA letterhead—with the “C” crossed out and replaced by an “S”. That was Crenshaw’s first introduction to the U.S. space agency.

Crenshaw joined NASA Langley in April of 1959, where he was slated to work with one of the wind tunnels. Crenshaw protested that he was a physicist and knew nothing about aerodynamics. He asked where he could use his expertise in advanced dynamics. From there, was placed in the Theoretical Mechanics Division with Clint Brown.

Later that year, Brown created the Lunar Trajectory Group, led by Bill Michael, which was to study lunar trajectories for manned and unmanned missions (and they were all ‘manned’ in that era). The team in which Crenshaw found himself included Bob Tolson, John “Gap” Gapcynski, and Wilbur Mayo. They were tasked with designing a mission based on the old Scout solid-propellant rocket, to send a “Brownie” class camera to photograph the far side of the Moon.

“We wanted a circumlunar trajectory,” Crenshaw said, “because we wanted to take a photograph of the back side of the Moon and (unlike the Russians, who used a TV camera) recover the film. Later, we used the circumlunar trajectory in Apollo because we wanted a fail-safe return to Earth—even in missions that were nominally intended for landings—in case something went wrong (as it did in Apollo 13).”

As Crenshaw says on his website, “When Scout’s projected payload at the Moon went from a few hundred pounds, through zero and negative, those plans were abandoned. However, the effort left us more than ready for Apollo when it came along.”

Bill Michael came across a paper by Hans Lieske and Bob Bucheim of RAND Corp., who had studied the circumlunar trajectory. Michael asked Crenshaw to study it in more detail, using a computer simulation of the Restricted Three-Body Problem . The study was later expanded to include a full 3-D, N-body simulation.

Michael and Crenshaw authored the paper Trajectory Considerations for Circumlunar Missions, which was presented at the Institute of Aerospace Sciences 29th Annual Meeting in New York in Jan. of 1961.

“I can’t say for certain that Bill Michael and I—or even Lieske and Bucheim—were the first to ‘invent’ the circumlunar trajectory,” Crenshaw said. “In recent years, I’ve tried very hard to find a copy of Lieske and Bucheim’s paper, but the best I’ve been able to do is a vague reference: ‘Beginnings of space studies and efforts at RAND during the mid-1950s; work with Hans Lieske, Al Lang, and Bob Buchheim.’”

Sketch of the Free Return Trajectory. Photo Credit: Karen Wehrstein

Sketch of the Free Return Trajectory. Photo Credit: Karen Wehrstein

Unfortunately, in those days, a lot of research went unpublished. Crenshaw explains, “We were all too busy doing stuff to write it down for posterity. This was especially true on the West Coast, where a lot of studies got passed around, scientist to scientist. Many of the ‘think tanks’ were doing studies for classified USAF programs, which—for reasons I can’t begin to fathom—included missions to Mars.”

The difficulty in plotting a circumlunar trajectory is that it is very sensitive to errors in the initial conditions.

“The chances of Apollo astronauts leaving the Earth, swinging past the Moon, and returning to a safe re-entry/splashdown without midcourse guidance are slim to none,” Crenshaw said. “Even so, by using the circumlunar trajectory (the Free Return) as our baseline, we were giving the astronauts at least a fighting chance of getting home alive.”

During the eventful flight of Apollo 13, when an oxygen tank exploded and the command module Odyssey had to be powered down, Commander Jim Lovell was able to save the mission by applying midcourse corrections manually with no help from the Apollo flight computer.

“Ironically enough,” Crenshaw said, “Apollo 13 was the first Apollo mission that did not leave the Earth on a Free Return trajectory. I’m not sure exactly why, but I expect it was because they needed a different trajectory to reach the landing site in the Fra Mauro highlands.”

In fact, none of the Apollo lunar missions after Apollo 12 used the Free Return trajectory, so that they could better target their landing sites.

“So after the accident, the first thing the Apollo 13 astronauts had to do was to get back on a Free Return trajectory.”

After leaving NASA, Crenshaw worked at GE in Daytona, studying abort trajectories. There, he came up with two classes of maneuvers called “Fast Returns”. If a spacecraft did not land on the Moon, it would still have a lot of unspent fuel in both the service module as well as the lunar module. In order to make a faster return trip, that fuel could be used to get home quickly.

Apollo 13 mission patch image credit NASA

The Apollo 13 insignia. Image Credit: NASA

“In the movie Apollo 13, there’s a scene where the Gene Kranz character played by Ed Harris first hears of the accident from his trajectory specialist,” Crenshaw relates. “Kranz says, ‘Can we do a fast return?’ Answer: ‘No, we’re too far out for that. We’ll have to get on a free return.’ That exchange struck me because I had designed both trajectories. I thought, ‘These guys just told my life story.’”

It was fortunate, in fact, that the Apollo 13 crew did not attempt the Fast Return. To do so, they would have had to use the SPS engine, which was likely damaged in the explosion. Lighting the damaged engine may well have destroyed the spacecraft.

After his time at NASA, Crenshaw worked for General Electric on their NASA Apollo Study Contract in a small team that was responsible for trajectory work. Lieske, who had studied lunar trajectory and provided the basis for Crenshaw’s and Bill Michael’s 1961 paper, was the systems engineer for trajectories. His job was to supply Lieske with trajectories.

Crenshaw said, “GE has always tended to encourage competition between groups, and the Apollo effort was no exception. During my stay there, there were no less than eight departments, all vying to be THE GE trajectory analysis group. Hans sought to have one of his own and cooked up a devious scheme to get it. One day he off-handedly asked me to look at the problem of Return from the Moon. As far as I know, no one had yet done that one. I said I would, but there was no sense of urgency about it, and we certainly had more pressing problems to study. Well, it turned out that Hans was complaining to management that my team wasn’t responsive enough to his needs. He had contracted with another company—Space Technology Labs (STL) in Houston—to study the same problem. His hope was that he would get results from STL before he got them from us, so he could say, ‘See, I’m having to go outside the company because I can’t get trajectories from Crenshaw.’ Somehow or other, my manager heard about the plan. So we jumped on the Return from Moon problem and got the results. Hans never got them from STL. Management found him out, and he was fired.”

A few months later, Crenshaw attended a conference at the American Institute of Aeronautics and Astronautics (AIAA) conference, and he saw that there was a paper by Dr. Paul Penzo on trajectories for Return from the Moon.

“After his presentation, I approached Paul and told him I had also studied the problem. Over lunch, I told him the story of Hans Lieske and his scheme to use STL as a weapon against me. Paul said, ‘There’s more to the story than you think.’ I asked, ‘How so?’ He said, ‘I was the guy at STL!’”

Because he had not studied the problem before, Penzo had not been able to produce results on Lieske’s tight schedule, but the problem intrigued him and he continued to study it which brought him to AIAA.

“Paul and I had a good laugh over it, and [we] became fast friends, though we’ve not met since the Apollo days,” Crenshaw said.

According to Crenshaw, Bill Michael was the first to analyze Lunar-Orbit Rendezvous for the Apollo missions.

“He published his work in a NASA Tech Note authored by yours truly. Thanks to a gee-whiz article in Life Magazine, John Houbolt got credit for that idea, and has been trying ever since to get big-bucks NASA payments for ‘his’ idea.”

In 1962, Crenshaw developed a technique known today as Universal Variable. “Instead of separate sets of equations for elliptic, hyperbolic, and parabolic orbits, this formulation unifies the equations in a single set of new functions I called the Unified Functions. Despite the fact that I published them first—and used them in all my studies for GE—Richard Battin got the credit, and they became known as the Lemmon-Battin functions.”

In 1961, Crenshaw had studied the motion in near-circular orbits and developed a theory for the motion of a spacecraft relative to a reference circular orbit. That technique is widely used today for geostationary missions, and it has become known as the Clohessy-Wiltshire equations.


Collin R. Skocik has been captivated by space flight since the maiden flight of space shuttle Columbia in April of 1981. He frequently attends events hosted by the Astronaut Scholarship Foundation, and has met many astronauts in his experiences at Kennedy Space Center. He is a prolific author of science fiction as well as science and space-related articles. In addition to the Voyage Into the Unknown series, he has also written the short story collection The Future Lives!, the science fiction novel Dreams of the Stars, and the disaster novel The Sunburst Fire. His first print sale was Asteroid Eternia in Encounters magazine. When he is not writing, he provides closed-captioning for the hearing impaired. He lives in Atlantic Beach, Florida.

Reader Comments

But who is Richard Crenshaw in the URL to this article?

Hi Eerik,

It looks like the link is related to Richard, I am not aware that in Jack W. Crenshaw, the W stands for Richard.. 🙂

After all of this year, Dr. Chrenshaw is still active working on trajectory calculations. I have the privelage to work with Dr. Crenshaw. He is my mentor in Aersopace and more importantly my friend.. One thing that the author forgot to mention is that Jack has coined a new acronym for the KISS priciple: Keep It Short and Simple…

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