Here’s the setup.
Okay, how do they steer the spaceship by only rolling it?
How the hell do they steer; just by rolling?
Great explanation starts at the 10 minute mark.
If you don’t want to watch 10 minutes of vid, the very short version is the capsule was geometrically symmetrical, but not weight distribution symmetrical.
Which had the effect that there was a definite top and bottom and they could “bank” left or right. Which aerodynamically steered the vehicle left & right.
Not much, but enough for their needs.
Fascinating film, thanks for posting that.
Now I have kinda the opposite question. Suppose the capsule is exactly on course and heading right for the landing zone. How does the offset CG not send it off course? Do they just put the capsule into a continuous roll so the offset lift forces cancel out over the course of the re-entry? Seems like that would be an uncomfortable ride for the crew.
I think your course is for the way the capsule will behave with the center of gravity that exists.
Yes it stayed in a continuous roll if no course correction was needed. It was not any more uncomfortable than pulling the usual 6-7 g.
Note that while the previously-listed video is very informative, it is not accurate regarding the final chosen reentry trajectory. No Apollo manned mission used the “skip” reentry described at 14:50 into the video:
This was investigated, the guidance software developed and tested on the unmanned mission Saturn IB mission AS-202: AS-202 - Wikipedia
I don’t think it was tested on the unmanned Saturn V Apollo 4 and 6 missions, but am not sure. Skip reentry was definitely not used on any manned mission. It was workable, and probably was available in a contingency to further extend the downrange track. However due to the limited computing power and software modeling of that era there was the possibility of significant error build-up which could have resulted in an imprecise landing target zone, possibly requiring sitting in a bobbing spacecraft on the ocean for hours until they were located.
The center of gravity would be affected by how the masses in the capsule are distributed, so everything should be in it’s place. How is the off CG determined? Is there a mass in the capsule that determines this? Is there a 500 lb bag of rocks that is tied to a particular strut a particular distance from the nose?
Thanks.
This was my question, too.
I figures it was more than just “Mike and Buzz, raise your arms up and to the left!”
Thanks, joema.
From wikipedia:
Did they consider that some mass would be moved from food storage to waste storage, or is insignificant compared to the center of mass?
The CM weighed 13,000 pounds (5,897 kg), and Apollo 11 brought back 48 lbs (22 kg) of moon rocks. I don’t think it was extremely sensitive to center-of-mass issues from movable items. I recollect only very general care was required.
A much bigger concern in stowing equipment was safely securing it for reentry and splashdown. On Apollo 12 a camera shook loose at splashdown and hit astronaut Alan Bean on the head, knocking him unconscious.
Thanks again.
The weight of the moon rocks became an issue with Apollo 13: as they approached Earth, they were drifting slightly off-course, and couldn’t figure out why.
Then the lightbulb went on: the planned re-entry trajectory assumed that they would be carrying about 50 lbs of moon rocks, which of course they had never collected. Once they adjusted for that difference, they stopped drifting off-course.
In googling for the answer, I find SDMB threads.
Thanks, all. I figured the CM must have had thrusters, because in that great video Bear_Nenno linked to, they showed it rotating into position (shield forward) after jettisoning the service module, but BEFORE it encountered any atmosphere (where it could have made these adjustments in some other way).
Thrusters can change attitude, but not vector. Or at least their impact on vector is all-but negligible.
The aerodynamic forces created by the off-center CG don’t affect attitude but do affect vector.
One of the hard things about space versus not-space is getting used to the idea that where you’re pointed has nothing to do with where you’re going. And therefore that changing where you’re pointed doesn’t change where you’re going. Or vice versa.
That was the movie. In reality it’s impossible for a slight mass difference to cause a trajectory change in the vacuum of space. The trajectory flight controllers knew this and would never have speculated a slight mass difference as the cause of a shallowing entry angle. The actual cause was traced to slow venting of gas from the damaged vehicle: Apollo 13 (1995) corrections - entry 84871 first
However someone at NASA (maybe fatigued or during initial brainstorming of any possible cause) momentarily conjectured the missing moon rocks as causing a trajectory-affecting mass difference. That speculation was recorded somewhere, so the movie script didn’t make it up.
Not only that, but shifting the weight from the LM to the CM couldn’t have made any difference while the two vehicles were still docked, but the movie treated this as a fix to an already existing problem.
I am still curious how the deviation of the CG was made, or if it was an artifact of construction that was exploited.
I’m sure it was intentionally constructed to offset the center of mass about one foot from the center of pressure during reentry. There was no movable apparatus, nor did the astronauts provide that difference by repositioning items in the cabin.
During Apollo development they were counting every ounce of weight, and even had to adopt then-new procedures such as chemical milling to produce thin, lightweight structures on the LM.
Since the CM requirement for offset center of mass was known early in the design, I doubt they would add unnecessary weight by using ballast. More likely it was just a mechanical design offset of the basic structure.
Ballast was used to maintain CG on some Service Modules, since there was an empty equipment bay not utilized until the last three missions. It wasn’t worth it to redesign the entire thing, they had the payload capability.