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When an explosion rocked Apollo 13‘s service module on April 13, 1970, the vehicle’s vital role, and that of the attached command module spacecraft, suddenly became crystal clear.

The astronauts lost one oxygen tank instantly, and the other was badly damaged. The vital engine that was supposed to bring the astronauts back home was knocked out of commission. The three crewmembers did make it home, but barely — and only by using the attached lunar module as a lifeboat. 

Sure, the lunar module had oxygen and water and power. But it didn’t have enough to easily sustain three people for the four days needed to get home. And sure, the lunar module carried an engine able to get the astronauts to Earth orbit from the moon’s neighborhood. But this was far from what the lander was designed to do, and doing so was a tricky business.

Related: Apollo 11 at 50: A Complete Guide to the Historic Moon Landing

So, while the Apollo program’s lunar landings, which began 50 years ago this July 20, will be front of mind in the coming weeks, the command module deserves its time in the spotlight. It was the spacecraft astronauts sat in while rocketing to space, and in most cases, on the ride home again. After all, 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 allowed the command module to function. (Neufeld thus prefers the term command and service module, he told Space.com, a usage that NASA has also employed frequently.)

But whether in isolation or working with its partner, one thing was for sure: The command module was inspired by every NASA spacecraft that came before it. The big difference? Apollo’s command module was bigger and able to withstand more heat as the astronauts came into the Earth’s atmosphere at higher speeds.

Development 

Apollo was the last of three spacecraft programs that gradually got NASA to crewed lunar missions. Mercury was a simple, one-person spacecraft that mainly ran on autopilot, although an astronaut could take over at crucial moments, such as during landing.

Gemini, which was developed after engineers started working on Apollo, a step larger than Mercury, carrying two astronauts. Spacecraft in the Gemini series tested vital lunar-mission milestones, such as docking and facilitating spacewalks while still in Earth orbit.

But it would be Apollo’s command module that would fly to the moon. It was developed by North American Aviation. (That company was later known as North American Rockwell and is today part of Boeing.)

The command module had a wider, flatter cylindrical nose compared to the Mercury or Gemini spacecraft, Neufeld said. The Apollo design was completely covered in heat shield, although the thickest part was on the back end. Apollo’s computer, though easily outpowered by today’s cellphones, was a marvel of the day, based on the fast-calculating integrated chip, rather than the semiconductor transistors used during Gemini.

Related: How NASA’s Apollo Astronauts Went to the Moon

Practically speaking, the command module flew by itself only during the few hours before reentry, running on batteries at those times. Otherwise, it relied on the service module, which used fuel cells for electrical power, a Gemini innovation that Apollo carried on, Neufeld said. Those fuel cells generated water as a waste product, which astronauts were able to drink, in a first for U.S. spaceflight.

One of the command module’s unique features compared to earlier spacecraft was a navigation station equipped with a television and a sextant, Neufeld said. “This was so that the astronauts, in theory, could navigate their way back home if they lost contact with the ground,” he said.

But the arrangement wasn’t perfect. The navigation station had a guidance platform based on gyroscopes, which tend to “drift” or lose accuracy over time. So, during most missions, astronauts had to realign the guidance platform from time to time.

This became one of the lesser-known problems of Apollo 13. After the initial explosion, the resulting debris and oxygen from the destroyed tank clung around the spacecraft in a nasty demonstration of gravity’s attraction. The clutter made it difficult for the astronauts to align their guidance platform for the journey home. Instead, in consultation with mission control, the crew used measures such as aligning with the line between day and night on Earth to safely return. 

Design changes 

The command and service module underwent three major design changes during its lifetime, Neufeld said. The first came after Apollo 1, when a fatal ground fire killed three crewmembers on Jan. 27, 1967, while they were running a practice liftoff on the launchpad.

Apollo 1 used the earliest “Block 1” version of the command module, which used nested outer and inner hatches for a tighter seal. When a fire broke out inside the spacecraft, the crew was unable to get out. Worse, the inside was filled with flammable items held in flammable conditions. These were fire hazards that NASA and its manufacturer hadn’t considered.

In the wake of the accident, North American Aviation redesigned the spacecraft “to eliminate the dangers of wiring,” Neufeld said, and remove flammable materials from the module. NASA also switched to the Block 2 version of the spacecraft, which had a hatch that could be opened in seconds.

Apollo 13 prompted another change. The explosion itself, NASA later realized, was caused by a series of wiring and handling problems on the ground. Those issues triggered to a fire in the service module, which blew up one of the oxygen tanks and tore away the connection to the other, Neufeld explained. 

Oxygen was vital not only to keep astronauts breathing, but also for power, because it supplied the fuel cells. So, after Apollo 13, a third oxygen tank was added to the service module on the opposite side of the bay from the oxygen tanks by the fuel cells, Neufeld said. “It provided some backup oxygen if there was ever a problem that knocked out the other two oxygen tanks,” he said.

Related: Why the Lunar Module Looked So Much Like a Moon Bug

The last major change to 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 heavily focused on science. That priority meant a busy schedule for the astronaut who remained behind in the command module while the other two crewmembers explored the moon. 

The command-module astronaut would take pictures and perform experiments while still inside the spacecraft. Then, on the way home, that astronaut would perform a spacewalk to retrieve film from a camera photographing the surface of the moon from 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 on in new spacecraft being designed to fly within the next couple of years. Those include two commercial crew vehicles, SpaceX’s Crew Dragon and Boeing’s CST-100 Starliner, each designed to bring crews to the International Space Station. NASA is building its own successor to the command module as well, a lunar spacecraft called Orion, scheduled to be tested on its first round-the-moon journey no earlier than 2020. 

Follow Elizabeth Howell on Twitter @howellspace. Follow us on Twitter @Spacedotcom and on Facebook.  

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