A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2013 Nov 4, 08:40 -0800
Don Seltzer had previously described an event during the Apollo 13 mission when the Sun was used to check the alignment of the IMU because the stars were nearly impossible to see because of the cloud of fine debris and ice particles which was travelling with their spacecraft after the explosion. This was used to ensure that the spacecraft would be pointing in the correct orientation for the "PC+2" burn that would speed their return to Earth. Although it was nowhere near as exact as a star check, which could align the platform to +/- 1 minute of arc, an error of +/- 1 degree was acceptable for this burn. This engine firing took place two hours after closest approach to the Moon or "pericynthion", hence PC+2. At this point the main computer and the IMU were still powered up so this was really not much different from normal operating procedure --except that the Sun was used for orientation instead of the stars. This verified that the platform was properly aligned within those rather large margins of error. In fact, the stars later became visible and identifiable before the burn while the spacecraft was in the Moon's shadow (the ice particles and debris were no longer illuminated), but the Sun check was sufficient and they did not bother with another more accurate orientation check. After that burn, the main computer was shut down because it consumed too much electricity and water (for cooling).
Yesterday Paul Hirose described a different navigation event, which is more famous since it was dramatized in the movie "Apollo 13". Paul, you wrote:
"The emergency technique that used the Earth and Sun to establish a known
attitude is described on this page:
Thanks for the link. That's a great article. But note that this is not the same burn (not the PC+2 burn that Don had described earlier). The visual alignment using the reticle affixed to the LM window and the terminator of the Earth were required for later trajectory corrections because at that point the computer and main inertial unit were powered down. Those burns were performed by manual timing and manual orientation. Only the backup AGC computer with its relatively primitive strapdown IMU was active. It was not considered accurate enough to maintain proper orientation for the whole burn, but manual orientation proved to be easy enough to maintain. The gyrations shown in the movie version were exaggerated to create drama. In fact, the procedure, while seemingly desperate, was very effective and required only ordinary attention from the crew during the burn.
Regarding the three gimbals versus four gimbals debate in the early design phase of the Apollo guidance system, you wrote:
"Sometimes winning an argument merely proves you're the more skillful arguer."
I think there's another lesson here. Hoag's victory, according to Mindell's account, was not merely "skillful arguing". Grumman's case wasn't intrinsically superior but poorly argued. Both Grumman and MIT's Instrument Laboratory (the designer's of the navigation system) were presenting their cases to NASA in terms of "simplicity, low-weight, reliability". Three gimbals was simpler than four and obviously saved weight. The cost was a slight reduction in reliability/redundancy and also some unknown increase in the workload on the astronauts, and after all, that's what they're there for. But this focus missed out on the main issue that Mindell addresses in his book: the man-machine interface. That's where the focus of the argument should have been. Grumman contended that the decrease in redundancy was substantial and dangerous, and on this point they were wrong. If anyone (MIT, Grumman, NASA) had focused instead on simulating how easy it was to fly the spacecraft into gimbal lock unintentionally, then the decision for four gimbals would have been clear. Instead everyone focused on the big issue of reliability. The Grumman engineers supposedly made this their principal issue. They were wrong, and according to Mindell, David Hoag of the IL was able to demonstrate conclusively that their argument on this issue was weak. His argument won the day, but it was the wrong argument to fight. The decision made the spacecraft more difficult to fly, and Hoag and the IL realized this as soon as they observed the vehicle in flight with astronauts always worried about gimbal lock. And indeed, Neil Armstrong himself flew the lunar module "straight into gimbal lock" while ascending to rendezvous in lunar orbit after the first Apollo moon landing.
This great gimbal debate took place nearly fifty years ago in 1964. Many mistakes were being made in the Apollo program at this time, and some of those (not this one) would lead directly to the Apollo 1 tragedy. The Apollo program was insanely rushed. That "before this decade is out" goal was unreasonable, and it meant that the lessons of the Gemini program were almost completely wasted. Most of us recall the crazy speed of the Apollo program fondly and think of it as the "good old days" when NASA could do anything, but the program was wasteful in almost every sense and very risky.
NavList message boards and member settings: www.fer3.com/NavList
Members may optionally receive posts by email.
To cancel email delivery, send a message to NoMail[at]fer3.com