NavList:

Frances-

I went back to my stash of Apollo stuff, and dredged up part of the Apollo spacecraft press kit dealing with the Command Module guidance system. It’s about 18 pages.  I apologize for the quality of the scanned photos.

The main difference between the system in the Command Module and the Lunar Module, is that in the CM they had the luxury of being able to turn the spacecraft to whatever direction they wanted to be able to see a particular star. On the other hand, once the LM landed it was fixed and they would have to point the telescope instead. So they designed a system with a 60^o viewing angle, which they could rotate to one of 6 different directions around the vertical axis.  (Even if you are limited to looking no higher than 60^o above the horizon, there are plenty of really good navigation stars out in space, so it wasn’t necessary to view the entire hemisphere.)

The scene in the movie that you are referring to was when they had to perform a 14 second mid-course correction firing of the LM engine to tweak their speed a bit so they could stay on course.

But as Apollo 13 headed back to Earth, the Reentry (RETRO) and Guidance, Navigation and Control (GNC) officers looking at the trajectory analysis noticed the spacecraft was coming in too “shallow,” that is, Apollo 13 was headed to skip off the atmosphere and out into space forever. Something seemed to be “blowing” the spacecraft off course. Later, it was discovered that cooling vapor from the lander was responsible. Since no lander had been present for previous missions on a return trip from the Moon, such a mysterious “wind” had never been encountered prior to Earth re-entry.

Another burn was needed, but no help from the guidance system would be available, as powering the lander’s guidance system, its gyros, the computer, etc. would use too much electrical power.

Here’s where the backup navigation approach that Lovell experimented with on Apollo 8 came to the rescue.

“If a ‘dead-reckoning’ approach could be used, no electricity would be needed,” said Woodfill. “Simply point the vehicle correctly, start the engine and stop it based on Mission Control’s prescribed time for its operation.” Lovell eyed up the Earth’s terminator line and controlled the “yaw” of the spacecraft, Haise controlled the “pitch” and Swigert timed it with his accurate Omega Speedmaster watch.

Woodfill said he enjoyed Hollywood’s re-enactment of the procedure in the “Apollo 13” movie. Though the spacecraft gyrations about the heavens are wholly exaggerated, the scene where Tom Hanks, Bill Paxton, and Kevin Bacon set-up and execute the terminator burn is generally accurate.

Paul Dolkas

From: NavList@fer3.com [mailto:NavList@fer3.com] On Behalf Of Francis Upchurch
Sent: Friday, June 10, 2016 5:14 AM
To: paul@dolkas.net
Subject: [NavList] Re: Neat video about LEM stellar alignment scope