Blocked Out: How NASA Chose Apollo's Command Module

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Between the pictures taken during missions or on the launch pad, we’re all pretty familiar with the Apollo command module. The gumdrop-shaped, blunt bodied capsule was the mothership that stayed in orbit while the lunar module descended to the surface and was also the only part of the Apollo spacecraft able to keep astronauts alive during their fiery reentry into the Earth atmosphere.

What we don’t hear about too often is that there were actually two versions of the command module, one designed for Earth orbital missions and one for flights to the moon.

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In its early life, the Apollo command module’s development was fraught with difficulties stemming from NASA’s inability to choose a mission mode, or more simply the way Apollo would go to the moon. When President Kennedy set the United States on a course for the moon on May 25, 1961, there were two mission modes NASA was seriously considering for Apollo. Direct ascent, the leading method, involved sending a spacecraft directly to the moon. It would land vertically and launch again from the surface for the return to Earth.

The other method, Earth orbit rendezvous, involved launching the pieces of a lunar spacecraft and assembling them in Earth orbit before setting course for the moon.

There was a third option that, towards the end of 1961, wasn’t a favorite among NASA engineers or managers. Called lunar orbit rendezvous, it involved leaving part of the spacecraft in lunar orbit while a smaller dedicated lander descended to the moon’s surface.

With the mode decision pending, NASA accepted bids from 12 aerospace companies hoping to build at least the main Apollo spacecraft if not the portion that would land on the moon as well, be it a landing stage or a separate landing laboratory. But this posed a challenge for the contractors since the mode decision would eventually shape the spacecraft. The spacecraft wouldn’t decide the mode.

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Still without a mission mode selected, NASA awarded the contract for the Apollo command module to North American Aviation on Nov. 28, 1961. NAA began building the vehicle NASA wanted: a blunt-bottomed truncated cone that could support three men for two weeks.

More than six months later, on July 11, 1962, NASA announced its mode decision: Apollo would use lunar orbit rendezvous. The kicker for North American Aviation came another six months later when NASA announced that Grumman Aerospace would be building the lunar module. Not only had NAA suddenly lost the honor of landing on the moon, it would have to go back and alter its spacecraft to make it compatible with a new vehicle built by a different contractor.

Most notably, NAA would have to add a docking tunnel to the Apollo command module so astronauts could transfer between the two spacecraft. This wasn’t something the company had built into its original design.

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Working within the new Apollo mission framework led to two very different command modules. One was further along in its development and one was equipped for missions to the moon. And of course, they weren’t the same spacecraft. What emerged from this near duplication of hardware was a block concept. The Block I would be the original command module, the one unable to dock with a lunar module. Since it would be ready first and have almost all the same systems as the lunar version of the spacecraft, it would be a perfect vehicle for astronauts to train with in Earth orbit. The lunar mission-capable command module with the docking tunnel would be the Block II version. North American Aviation presented the block concept to NASA at the beginning of 1964; the agency signed off on the idea on Jan. 24.

Though they were built for Earth orbital tests, no crew ever flew in a Block I. The first Block I built for a manned mission was spacecraft 012. It was the spacecraft that claimed the lives of the Apollo 1 crew in the fatal prelaunch fire on Jan. 27, 1967.

After the fire, there were so many changes made to the Apollo command module that NASA finally cancelled all manned Block I flights. There were enough differences between this model and lunar version that it hardly seemed worth updating and testing the spacecraft that wouldn’t be going to the moon. Some Block I spacecraft were retrofitted with Block II parts and launched on unmanned missions, but all manned flights launched in updated Block II spacecraft.

The first complete Block II spacecraft made its debut with Apollo 7, but 50 years ago this month the SA-5 mission launched on the first Block II capable Saturn I rocket on January 29, 1964. SA-5 (SA for Saturn-Apollo) was a launch vehicle development test. It was the first orbital flight of a live S-IV upper stage, the first use of stage separation hardware, and the first time launch complex 37 was used. The mission was entirely successful.

The launch of the first Saturn I Block II vehicle with eight aerodynamic fins at the bottom of the S-I stage (first stage) for enhanced stability in flight.
NASA

Sources: Chariots for Apollo; The Apollo Spacecraft Chronology volumes 1 – 4.