When an explosion rocked Apollo 13's service module on April 13, 1970, the vehicle's important role and the accompanying command module's spacecraft suddenly became crystal clear.
The astronauts lost an oxygen tank directly and the other was severely damaged. The vital engine that would accommodate the astronauts at home was knocked out by the Commission. The three crew members did it at home, but hardly – and only by using the attached moon module as a lifeboat.
Certainly, the moon module had oxygen and water and power. But it did not have enough to easily keep three people in the four days needed to come home. Of course, the lunar module has an engine that can get the astronauts to the Earth's orbit from the moon's neighborhood. But this was far from what the lander was meant to do, and it was a tricky business.
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So, when the Apollo program's lunar landings, which began 50 years ago on July 20, will face the coming weeks, the command module deserves its time in the spotlight. It was the spacecraft's astronauts who were in space and, in most cases, on the journey home again. Specifically, only the command module had a heat shield.
Some historians, like Mike Neufeld, a senior curator at the Smithsonian National Air and Space Museum, have argued that the command module cannot be described as its own spacecraft because it was the attached service module that had all the equipment that made the command module work. (Neufeld thus prefers the term command and service module, he told Space.com, a use that NASA has often employed.)
But whether the isolation or work with his partner was one thing for sure: The command module was inspired by every NASA spacecraft that come before it. The big difference? Apollo's command module was larger and could withstand more heat when the astronauts entered the Earth's atmosphere at higher speeds.
Apollo was the last of three spacecraft programs that gradually got NASA into manned moon missions. Mercury was a simple spaceship with a person who mainly drove on autopilot, although an astronaut could take over at important moments, for example at landing.
Gemini who developed after engineers began working on Apollo, one step larger than Mercury, carrying two astronauts. Spaceships in the Gemini range tested vital milestones, which dock and facilitate space walks, while still in the Earth's orbit.
But it would be Apollo's command module that would fly to the moon. It was developed by North American Aviation. (The company was later known as North American Rockwell and is currently part of Boeing.) The command module had a wider, narrower cylindrical nose compared to Mercury or Gemini spacecraft, Neufeld said. The Apollo design was completely covered by a heat shield, although the thickest part was on the back. Apollo's computer but easily equipped with today's mobile phones, was a wonderful day, based on the fast computed integrated chip, rather than the semiconductor transistors used during Gemini.
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Practically, the command module only flew by itself in the few hours before it went off and ran on the batteries at those times. Otherwise, it was on the service module, which used fuel cells for electric power, a Gemini innovation that Apollo wears, Neufeld said. These fuel cells generated water as a waste product which astronauts could drink, in a first for space flight in the United States.
One of the unique features of the command module compared to previous spacecraft was a navigation station equipped with a television and a sextant, Neufeld said. "It was so that the astronauts could, in theory, navigate at home if they lost contact with the ground," he said.
But the arrangement was not perfect. The navigation station had a gyroscope guidance platform which tends to "drift" or lose accuracy over time. So during most missions, the astronauts needed to adjust the guidance platform from time to time.
This became one of the lesser known problems with Apollo 13. After the first explosion, the resulting debris and oxygen clung from the destroyed tank around the spacecraft into a nasty demonstration of the attraction of gravity. The rotation made it difficult for the astronauts to adapt their guidance platform for the trip home. Instead, the crew used in consultation with mission control measures to adapt to the line between day and night on Earth to safely return.
The command and service module underwent three major design changes during its lifetime, Neufeld said. The first came after Apollo 1 when a deadly sponge killed three crew members on January 27, 1967, while driving an exercise on the launch.
Apollo 1 used the earliest "Block 1" version of the command module, which used built-in outer and inner slots for a denser seal. When a fire broke out inside the spacecraft, the crew couldn't get out. Worse, the inside was filled with flammable objects kept in flammable conditions. These were fire hazards that NASA and its manufacturers had not considered.
In the event of the accident, North America redesigned the Aviation spacecraft "to eliminate the dangers of wires," Neufeld said, removing flammable materials from the module. NASA also switched to the spacecraft's Block 2 version, which had a gap that could be opened in seconds.
Apollo 13 led to another change. The explosion itself, which NASA later understood was caused by a series of ground management and handling problems. The problems that were triggered in a fire in the service module, which blew up one of the oxygen tanks and threw the connection to the other, explained Neufeld.
Oxygen was important not only to keep the astronauts breathing, but also for power, as it delivered the fuel cells. So, after Apollo 13, a third oxygen container was added to the service module on the opposite side of the bay from the oxygen tanks of the fuel cells, Neufeld said. "It gave a bit of acidity if it ever was a problem that knocked out the other two oxygen tanks," he said.
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The latest major change in the command and service module came in adding a quadrant to the service module for Apollos 15 16 and 17. These last missions to the moon were strongly focused on science. That priority meant a busy schedule for the astronaut who remained in the command module while the other two crew members examined the moon.
The command module's astronaut would take pictures and perform experiments while in the spacecraft. Then, on the way home, that astronaut would perform a spacewalk to retrieve movies from a camera that photographed the lunar surface outside the spacecraft, as well as anything else that needed to go back to earth, Neufeld said.
These days, the command module's legacy lives in new spacecraft designed to fly within the next few years. These include two commercial crew structures, SpaceX's Crew Dragon and Boeing's CST-100 Starliner intended to bring crews to the International Space Station. NASA builds its own successor to the command module, a moon screen called Orion scheduled to be tested on its first round moon journey earlier than 2020.