Apollo 13: Mission Control, Trajectory Burn
Apollo 13 is one of the most famous NASA missions of all times, perhaps just as famous as the decade defining Apollo 11 — when Buzz Aldrin and Neil Armstrong walked on the moon, as promised by the late president John Kennedy. Maybe that has to do with the 1995 Oscar nominated movie Apollo 13, featuring Tom Hanks and Kevin Bacon. In this text, we will go over some aspects of the mission that made it so notorious, especially the work done by mission control during the free return trajectory maneuver.
There are many technical terms, and if there’s any left without explanation, I suggest checking NASA’s Apollo glossary available here.
Apollo 13
Apollo 13 was scheduled for April 11th, 1970, the first Apollo mission of the new decade. After the notorious success of Apollo 11, the subsequent launches featured increasingly difficult and risky lunar missions.
The landing point for Apollo 13 was in the hummocky Fra Mauro Formation, near the Imbrium Basin — scientists believed it had been formed by a gigantic cosmic collision, and possibly samples of the material that could be found at the site would establish the date of the Imbrium event. Also, the mission included the deploy and activation of an Apollo Lunar Surface Experiments Package (ALSEP).
The commander of the mission was Jim Lovell. One of the most experienced astronauts at the time, he had participated in two Gemini flights and had also been one of the first humans to orbit the Moon on Apollo 8. Fred Haise was the Lunar Module Pilot. He had spent years working on the LM, and he knew it as well as any other engineer. Ken Mattingly was supposed to be the command module pilot, but due to his exposure to measles he was replaced by Jack Swigert, a member of the backup crew. Unlike Mattingly, Swigert hadn’t had a lot of contact with the crew but he was no stranger to the teams on Mission Control.
I don’t mean to spoil this story, but I’m sure the reader is wondering: Yes, this is where “Houston, we’ve had a problem” is from.
Let’s talk about Houston.
Mission Control Center — Houston
Mission Control Center (MCC) was the name given to building 30 of the Johnson Space Center in Houston, Texas. It consisted of two wings, connected by a large lobby. One wing is where most administrative and operations tasks took place. The other wing was a large, windowless concrete block, almost the size of a football stadium. It contained a Recovery Control Room, a Simulation Control Room, many staff support rooms, all sorts of technical equipment and, most important, the Mission Operations Control Room (MOCR). While the astronauts controlled the spacecraft, in the MOCR were the men who commanded the mission, called the controllers. There were actually two MOCRs: one in the second floor and one in the third, and both had a similar setup, but most Apollo missions, including Apollo 13, were run out of the third floor, which is now a National Historic Landmark.
The enormous room was laid out like a small auditorium. Huge displays were on the front wall usually showing a map of the earth and the moon as well as occasional trajectory plots. Facing these displays, were four rows of working surfaces, on top of which where stacked flight plans, books of mission rules and logs, and bulky computer screens. The forth row was occupied mostly by senior management and the Public Affairs Officer (PAO), who occasionally makes appearances on the audios we’re about to explore. In the third row were the INCO (Instrumentation and Communication Officer), the O&P (operations and procedures) and the Flight Controllers. The ‘flights’ are probably the people with the biggest amount of responsibility, as they’re ones in charge of most decision making. In Apollo 13 they were Gerry Griffin, Milt Windler, Glynn Lunney, all led by Gene Kranz, and each had its assigned team according to their shifts.
First in the second row was the physician, or the Surgeon, who monitored the health of the astronauts in flight. Next to him was the spacecraft communicator, known as the CapCom (the name comes from Capsule Communicator). The CapCom was the link between the astronauts and the controllers; he was the closest to a member of the crew on the ground, and for that he had to be an astronaut too. The CapCom during the part here narrated was Jack Lousma. Next to him were the “system guys”: the EECOM (Electrical, Environmental, and Communications Officer), the GNC (Guidance, Navigation and Control Officer), the TELMU (the LEM’s EECOM) and Control (the LEM’s GNC). Those were some of the most difficult positions in the MOCR, since they had to deeply understand the whole spacecraft and how they affected everybody else’s work. In Apollo 13, Sy Liebergot and Clint Burton would share the hot seat as the mission’s EECOM.
The front row was know as the trench, where the engineers were in charge of Flight Dynamics. Retro was responsible for calculating the trajectories and maneuvers on the way back to Earth, including emergency operations. FIDO was the Flight Dynamics Officer, regarded as the coolest job in the house; he monitored the trajectory and planned the maneuvers. Guido, the Ground Navigator, constantly monitored the spacecraft’s position, and suggested stars for the crew to check their location.
What happened so far
The first two days of the mission were more or less uneventful apart from a couple of glitches. The launch went smoothly, except for the center engine of the second stage of the Saturn V that shut down unexpectedly. However scary that may sound, Milt’s team handled it quickly and soon the Apollo 13 was in trans-lunar injection (that means the ship left earth’s orbit in the direction of the moon).
During the first shifts, Jack Swigert worked with the controllers to set up and calibrate the Command Module. Gene Kranz comments that Swiggert seemed like “a very capable stand-in for Ken Mattingly.” Fred Haise presented to the world the inside of the spacecraft in the TV transmission.
The command module had three fuel cells that worked by mixing cryogenic oxygen and hydrogen in order to provide electricity and water — the water was used both for cooling systems and for drinking. The oxygen and hydrogen were stored in a liquid state at temperatures below 300 degrees Fahrenheit and got easily stratified. To stir up the mix, the crew periodically activated fans internal to the tanks. Fifty-five hours after the launch, seconds after Fred Haise turned on the fans, there was a loud bang. The oxygen tank had exploded, causing damage to several systems in the command module. And Jack Swigert famously voiced “Okay, Houston, we’ve had a problem.”
The accident caused two out of the three fuel cells to die, and that alone meant they weren’t going to land on the moon. That also meant that the astronauts were going to be significantly short of power, oxygen and water, and they had to work with Mission Control to figure out ways to save power. As it had been anticipated for situations like this, the crew had to resort to the lunar module as a lifeboat.
The astronauts were 200,000 miles on the way to the moon, and the mission was off. The new plan now was to get the crew back to Earth alive. After heated discussions, the Mission Control decided that their best chance was to get the ship on a free return trajectory. A ship following such a trajectory would naturally go around the moon and get slingshot by its gravity back to Earth, without the crew having to press a single button.
Curiously, up to Apollo 12, the flights had adopted a free return trajectory on the way to the moon, in case emergencies like this would happen. However, that had just been changed to a hybrid trajectory, which would allow a smoother approach to the moon. Thus, the ship had to be maneuvered to the free return trajectory, using only the Lunar Module thrusters. On top of that, oxygen, water and energy were critical, and the teams were working on saving consumables as much as possible.
Trajectory Burn
Okay, this is a good part to start with.
Here CapCom Jack Lousma mentions quite a few things. First of all, he informs the crew that the team down on Mission Control is regrouping and working on a number of fronts to help them. [mention something about the regrouping]. PTC means passive thermal control: to ensure that the parts of the ship are evenly heated by the sun, the crew must occasionally rotate the ship. That’s important to avoid unnecessary tensions in the structure. However, doing so with only the lunar module thrusters, considering the current limitations, can be quite tricky. Also, Lousma says that backup crew for Apollo 14 — consisting on CDR Gene Cernan, CMP Ronald Evans and LMP Joe Engle — are working on the simulators to help test possible maneuvers and figure out alignment procedures for looking at stars out the window.
Knowing the ship’s attitude — a technical term for angle in space — is crucial for the precise execution of the planned maneuvers. When shutting down the command module, Swigert very carefully transferred the attitude data from the CM computer to the Lunar Module, which didn’t have as precise an instrumentation. Nonetheless, by using the location of particular stars on the celestial sphere, in addition to the computer data, it’s possible to compute their attitude and position with high accuracy. However, the ship was involved in a cloud of debris from the explosion that, illuminated by the sunlight, made the stars really hard to find.
The MOCR was in complete Chaos. There was paper, cigarettes and coffee everywhere. Kranz’s subordinates, the white team, was working in another room on pinning down the causes of the explosion. Lunney’s black team was on the Flight Dynamics, working on the return trajectory plan and solving equations for the next maneuver. The systems team was working on power down procedures, to save as much energy as possible.
At time 60 hours and 23 minutes, Lousma instructs the crew on the next move:
The astronauts and the controllers agree on the time of 61:30:00 for the planned maneuver. The tricky thing is that the ‘pings’ — or Primary Guidance and Navigation System (PGNS) — is required for the move, but also it should be powered down as soon as possible to save resources.
Following that, the CapCom talks the crew through an extensive checklist to set the ship ready for the DPS maneuver, and also to power down components in order to save energy and water. Here DPS means Descent Propulsion System, which is the rocket of the lunar module that will be used for the trajectory burn.
At 60:52:16, Jack Lousma arrives with the PAD. The Preliminary Advisory Data, or PAD, was a pre-printed form for the crew to fill in on the computer with the required data — such as time of ignition, attitude, etc — for the next maneuver. Here it goes:
Allow me to translate this for you, esteemed reader. It states its purpose: a short burn to return Apollo 13 to a free-return trajectory; and its scheduled time: 61 hours, 29 minutes, 42.84 seconds. As well, there is the change in velocity across all axis: x: -21.3, y:+4.1, z: -31.2; and its total resultant: 38 feet per second. The spacecraft attitude in relation to the guidance platform is: Roll, 120°; Pitch, 298°. Finally, the burn duration is 31 seconds, the first 5 seconds at 10% thrust and the remainder at 40%.
After a bit more narrated checklists, using the TTCA (Thrust/Translation Controller Assembly), the Apollo crew sets the aircraft to the correct attitude, apprehensively watched by anxious controllers. Jim Lovell gloriously tells them: we’re at the attitude; the CapCom confirms.
At 061:28:53, the CapCom Lousma finally gives the Go for the burn.
Immediately after the move, Lovell is required to enter Verb 16 Noun 85. This displays the difference between the change in velocity and the expected velocity, better known as the residuals. “No trim required” says the CapCom, which means that no correction is needed, the maneuver has been successful. The crew was on its way. If nothing else was done, the spacecraft would splashdown in the Indian Ocean in 90 hours. However, the ship was still 30 hours short of consumables. It was required another maneuver to speedup the return as well as to have a more convenient splashdown target. The work was far from done.
Bibliography
Woods, David. Kemppanen, Johannes. Turhanov, Alexander. Waugh, Lennox J. Apollo Flight Journal. Corrected Transcript and Commentary. NASA History Division.
Kennedy, Garry. Apollo Glossary. Apollo Lunar Surface Journal.
Chaikin, Andrew. A Man on the Moon: The Voyages of the Apollo Astronauts, Penguin Books, 2007. Chapter 7.
Cox, Catherine Bly. Murray, Charles. Apollo. 1989. Chapters 19 and 20.
Ehrenfried, M. V. Apollo Mission Control. The Making of a National Historic Landmark. 2018. Springer.
Kranz, Gene. Failure Is Not An Option. Mission Control from Mercury to Apollo 13 and Beyond. 2001. Simon and Schuster.
Apollo 13 Timeline. NASA History Division
