To celebrate the 45th anniversary of the Apollo 13 mission, Universe Today is featuring “13 MORE Things That Saved Apollo 13,” discussing different turning points of the mission with NASA engineer Jerry Woodfill.
For our final installment of this series of “13 More Things That Saved Apollo 13,” we’ll look at an event that has not been widely addressed, but it may have been one of the most crucial scenarios which might have ended in disaster and death for the crew in the final minutes of the rescue.
It starts with an atomic electrical power generator called SNAP-27.
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These devises enabled the Apollo Lunar Surface Experiment Package (ALSEP) to operate on the Moon for years after astronauts returned to Earth. They were deployed on Apollo 12, 14, 15, 16 and 17 and included seismometers, and devices to detect lunar dust and charged particles in the lunar environment.
SNAP stands for Systems Nuclear Auxiliary Power and its fuel was plutonium-238 (Pu-238). It was a type of Radioisotope Thermoelectric Generator (RTG) that provides electrical power for spacecraft by converting the heat generated by the decay of plutonium-238 fuel into electricity. Approximately 8 lbs of plutonium was used for each mission and it was it was transported to the Moon in a thermally insulated cask attached to the side of the Lunar Module.
“The cask was so strong and impervious that firing the container with a cannon into a solid brick wall would not break it,” said NASA engineer Jerry Woodfill.
Unfortunately, Woodfill added, as the political climate for anything atomic has grown acrimonious, the application of atomic energy to space exploration has been thwarted.
“Despite a remarkable atomic safety record, a small but powerful political coalition has successfully opposed such harmless devices as NASA’s Apollo SNAP-27 generator,” said Woodfill. “The scare-factor attributed to NASA’s Apollo atomic power generator was based on the threat of a launch pad explosion or exaggerated claims that an accident would contaminate Earth’s atmosphere and ultimately bring death to many. It is amazing that such groups can ignore obvious day-to-day deaths in automobiles yet alarm the public with false atomic threats.”
Woodfill said that the opposition to RTGs has been most unfortunate for the sake of human and robotic exploration of the solar system.
“The limitation of traditional rocket fuels handicaps improvement in propulsion,” he said, “and for the past five decades, little progress has been made in rocket engine specific impulse improvement known as the ISP.”
Additionally, for several years NASA has been facing a shortage of RTGs for powering robotic spacecraft limiting the scope and lifetime of missions going to the far reaches of our Solar System.
For Apollo 13, the SNAP-27 device should have ended up staying on the Moon, but of course, the lander did not land so it, along with the atomic generator, was going to reenter Earth’s atmosphere and end up somewhere on our planet.
It wasn’t long after the accident on Apollo 13 that NASA was contacted by the Atomic Energy Commission (AEC) about where the LM would be reentering and burning up in Earth’s atmosphere.
However, as Apollo 13 approached Earth, their flight path kept deteriorating, despite the crew’s efforts. As we discussed in Part 9 of this series about the potentially fatal gimbal lock, without the Command Modules thrusters and computer navigation system to steer, only the lander’s were available, and manually flying the crippled Apollo 13 spacecraft stack and keeping it on the right trajectory was a huge challenge.
Woodfill said that any ‘tinkering’ with the reentry geometry was altogether ill-advised considering how very ‘iffy” the angle and entry path had become, but AEC officials were pressuring the retro officers about the orientation required for the LM’s reentry to put it into a deep trench in the Pacific Ocean.
Woodfill said that from his perspective of decades of study about the mission, the need to “deep-six” the SNAP-27 generator was almost responsible for having the Apollo 13 rescue end in tragedy. There was confusion among those in Mission Control as well as the crew about the orientation the spacecraft at reentry. However, Woodfill said, an inadvertent ‘mistake’ by Lovell may have actually saved the crew.
“There was a significant debate between the two most knowledgeable retro officers about jettisoning of the lunar lander,” he said. “So uncertain was the scenario of positioning the command ship for LM jettison that the men held exactly opposite views of the result of selecting the position wanted by the AEC scientists. Added to the peril was Lovell’s brush with ‘running the ship aground’, i.e., into gimbal-lock trying to please the AEC.”
A 2009 research paper for AIAA adds insight into the danger of these moments prior to LM jettison and Lovell’s error. “Attempts to perform rapid analysis in a high pressure, time critical spacecraft emergency can lead to errors in analysis and faulty conclusions,” the paper reads. “For example, the spacecraft was maneuvered to the wrong LM/CM separation attitude, ~45 degrees on the north side of the CM ground track rather than the desired 45 degrees on the south side of the CM ground track. This attitude was close to CM IMU gimbal lock and complicated manual piloting.”
Mission transcripts reveal the confusion and the difficulty the crew faced. As Lovell was trying to maneuver the stack into the correct orientation for LM jettison he radioed:
Lovell: We’re having trouble maneuvering, Joe, without getting it in gimbal lock… You picked a lousy attitude, though, to separate.
Capcom: Well, we apologize. Just take your time. Jim, we’ve got time now.
Lovell continued to struggle as the ship continually approached gimbal lock and he questioned the procedure:
Lovell: Houston, why can’t I stay in PGNS ATT HOLD for the LM attitude hold?
Capcom: Stand by on that, Jim.
Lovell: I want to get way over here, Joe, to prevent going into gimbal lock. I have the yaw at about – I’d say about almost 50 degrees.
Capcom: Roger that. Just stay out of gimbal lock and that 45-degree isn’t critical – the out of plane, that is.
Nonetheless, an Apollo 13 post-mission report reveals that shortly before LM jettison the Retro Officer Chuck Deiterich advised the Flight Director that the LM was not in the correct orientation for separation. “The telemetry indicated that we were yawed 45 degrees North instead of 45 degree South,” the report says, so the ship was 90 degrees out of yaw attitude prior to LM jettison.
However, the LM closeout was underway, and there was no chance to use the thrusters to change attitude. The report continues, “No correction action was taken, because the separation was a minimum of 4,000 feet at entry interface, and more likely was going to be 8000 feet or greater. Therefore, no attempt was made to change the attitude.”
“Because the LM’s guidance computer was maintaining the jettison attitude, the crew could no longer steer the assemblage until the LM release,” explained Woodfill. “And then a terribly threatening event arose. In order to preserve the desired attitude to assure that the SNAP-27 plutonium landed in the ocean, the LM’s computer was moving the command ship’s platform into gimbal lock. It was too late to re-enter the LM. The time to unlatch the hatches would be too great.”
But despite the likely loss of control, somehow the LM was jettisoned just prior to the Command Module reaching gimbal lock.
“Had not, it was later discovered, Jim Lovell actually have mistakenly placed the attitude 90 degrees from the desired jettison position, a potentially fatal gimbal lock would have happened,” Woodfill said. “It was as though despite the disagreement between the retro experts and the resulting confusion between Mission Control and the crew, and then Lovell’s error, neither of the miscues of the entire scenario resulted in the dreaded gimbal-lock. Plus, the SNAP-27 ended up in an optimum location in the Pacific Ocean. Indeed, two mistakes made a right. The entry capsule’s guidance platform became stable and ready for reentry.”
However, Deiterich told Universe Today that with respect to the LM jett attitude, the landing point was not greatly affected by north or south. But to assure maximum separation during entry, the southerly direction was actually opposite the northerly direction the crew would fly.
“When I realized they were closing out, I told Kranz we would buy the current attitude,” Deiterich said via email. “The inplane separation velocity was enough to assure reasonable down range separation. We were just being thorough. Knowing is why we accepted the jettison attitude. I remembered the A10 Ascent stage jett and how the pressure between the CM and ASC pushed the ASC away so I picked this as a way to jett the LM on A13.”
Both during the mission and the crew debriefing the puzzling topic of that SNAP-27 disposal caused confusion. Days later during the debriefing, the crew seemed at a loss to understand what was going on with regard to ground control’s insistence on assuming such a particular jettison orientation for the lunar module. Somehow, they didn’t seem aware of the issues with the SNAP-27 atomic generator, an issue that likely would not threaten Earth but in every way threaten the lives of Lovell, Swigert and Haise.
“We were very close to gimbal lock,” Lovell said in the mission debrief. “I questioned whether the LM SEP attitude was that critical. Was it so critical to be at that attitude, or would it have been better to stay away from gimbal lock in the CM?”
Lovell was worried that they didn’t have any backup help of navigating — the Body Mounted Attitude Gyros, or BMAGs. “We didn’t have the BMAGs powered up,” Lovell said in the debrief. “If we had gone into gimbal lock, we would have had to start from scratch again.”
Deiterich agreed, especially since the crew was pressed for time as time for reentry was rapidly approaching. “Maneuvering the LM with the CSM attached was not easy,” Deitrich said via email, “thus Jim tried to keep any maneuvering out of plane to a minimum, once there he was reluctant to move away and also the whole process was brand new and time could then become a factor.”
Woodfill said the entire team in Mission Control helped save the crew – the EECOM (Emergency, Environmental, and Consumables Management) and the lander’s TELMU (Telemetry, Electrical, EVA Mobility Unit Officer) dealing with the spacecraft environmental and power systems, and the ‘Trench’ team of the FIDO flight dynamics officer who was responsible for the trajectory, the GUIDO guidance and navigation officer who was charged with assessing the crafts’ ability to steer itself under astronaut control, and finally, the RETRO whose responsibility was entering Earth’s atmosphere via retro-rocket firing.
“Considering Apollo 13’s myriad of challenges, it would be a toss-up between the groups if a vote were taken akin to voting for the outstanding “player” in a Monday Night Football game,” he said. “But there is no doubt with regard to the final minutes of the contest who would win the vote. It would be the latter group dealing with guidance and reentry. This is especially so considering the number of times the group thwarted loss of guidance. Without them, Apollo 13 would have lost the game to the formidable adversary gimbal-lock.”
And what happened to Apollo 13’s SNAP-27? In the book “Thirteen: The Flight that Failed”, Henry S.F. Cooper said that the plutonium apparently survived reentry and landed in the Tonga Trench south of Fiji in the Pacific Ocean, approximately 6-9 kilometers underwater. It exact location is unknown but monitoring of the areas has shown that no radiation escaped.
Previous articles in this series: