After reading the book Apollo, I took a closer look at the lunar orbital chart of LM pilot Fred Haise (a really cool birthday gift from my rocket buddy Erik), with the forlorn inscription near the landing site that they bypassed to return to Earth:
“No LM touchdown, but no LM impact either! Freddo”
On approach to the moon, debate ensued on what type of burn to use to get the Apollo 13 lifeboat to loop around the moon and back to Earth. To preserve options and later course corrections, they decided to start with a partial burn to orient the craft in the general direction of an Earth return path.
Since an earlier explosion took out the main oxygen tanks, they improvised and used the Lunar Module Descent Engine (LMDE) DPS engine — the engine from the lunar lander, designed to slow the LM’s decent to the moon — to instead push the crippled Command Module and reentry capsule in an untested manner:
“Finally, at 2:43 in the morning, Lovell pushed the ignition button and the DPS engine ran at low throttle for thirty seconds, putting the spacecraft into a trajectory that, even without a second burn, would bring it down in the Indian Ocean not quite four days later. Lovell was relieved. He wasn’t completely confident that the burn provided them with a survivable entry [angle], but at least the spacecraft would intercept the Earth’s atmosphere. In his mind, this was much better than the alternative that had just been avoided — orbiting the Earth indefinitely, in a lonely revolution with an apogee of 240,000 miles and a perigee of 3,000 miles, a ‘perpetual monument to the space program.’” (Apollo, p.410)
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