Apollo 13 questions

[photogineer]

Avid space fan here, but not as knowledgeable as I would like, so I'm always grabbing any books or other things I can get my hands on, and asking questions when I can't, so here is one for the crowd, well really a three parter:

1. In the Apollo 13 movie they had a problem with the trajectory returning from the Moon shallowing out, was that a real problem or added for drama? By shallowing I assume they mean hitting the atmosphere at such a high angle they passed through it before fully re-entering and ended up in some weird elliptical orbit, is this correct?

2. Assuming it was a real event, with the spacecraft in an unpowered coast, what would cause the trajectory to deviate like that?

3. In the movie they had the astronauts transfer ballast to the command module once they realized they were shallowing, what effect would this have since the spacecraft total mass wasn't changed? What was the intended purpose of that?

4. (Ok, I lied, it's a four parter, but I just thought of this one!) If they did in fact skim the atmosphere as I described above in #1, wouldn't that put them in an elliptical orbit that would continue to skim the atmosphere on each pass until they slowed enough to deorbit? If not, why?

Thanks everyone,

Matt

[Deathsoul097]

1) Yes, it did happen, and yes they would. (Or they would just burn up and die from prolonged thermal exposure)

2)The crew at NASA miscalculated the burn as they had forgot that the crew didn't land, and accidentally calculated the mass of the moon rocks they would have been carrying for the burn to return home and finalize the trajectory. This put them into the shallower trajectory mentioned above.

3) No clue. I don't remember that happening in the movie...

4) Yes it would, but they didn't have the resources to sustain that kind of trajectory for that amount of time, due to limited supplies of food, water and air, so the crew would either die from starvation, oxygen deprivation, de-hydration or burning up in the atmosphere due to the heat shield not being designed to take that kind of punishment, being ablative.

[Kerbin Dallas Multipass]

1. A too shallow angle can cause the capsule to skip, or bounce off the atmosphere

2. To my knowledge this skipping is mostly an aerodynamic effect.

The capsule is trimmed in such a way that it creates a bit of lift in the early reentry phases. The capsule sort of self-aligns and surfs down slowly, keeping reentry heat relatively constant (i do not know if the apollo capsule had RCS or torque to help out)

3. Center of gravity and total mass is important in this calculation, so it sounds plausible that Houston wanted them to put some ballast where they were supposed to put the moon rocks.

4. Yes, but this could take several orbits. We're talking days or weeks here. Not much help to the astronauts.

[goldenpeach]

3) It did happen in the movie.

I don't know why they did that, maybe to have the mass the reentry program was calculated for(remember that the apollo spacecraft was a lifting body and did, in fact, two reentry).

[FenrirWolf]

Moving mass around was beneficial because only the CSM was going to actually return alive. Before reentry they restarted the command module and jettisoned the LEM which they had been staying in until that point. So from what I understand, taking things out of the LEM and putting them in the CSM made it so the CSM now had the amount of mass (or at least a closer approximation to it) that they had calculated for re-entry.

[rkman]

wouldn't that put them in an elliptical orbit that would continue to skim the atmosphere on each pass until they slowed enough to deorbit? If not, why?

Matt

It would but due to limited battery time they'd run out of life support before reentry.

[photogineer]

Thanks for the answers guys, if you don't mind I have some follow-up questions:

Regarding the shallowing, I understand your point about the mass, I guess sometimes we take KSPs magic ability to know everything about a vessel for granted so does that mean that the shallowing occurred because of the incorrect burn calculation and it just took them a while for the error to grow to a level they could detect? Nothing was actively altering the course during their coast? I only ask because (in the movie, maybe this is just a dramatization) during the part leading to reentry, the ship was shallowing again, and they just had to live with it. At this point they had already discovered the mass error, and moved ballast and done a correction burn, why would it still be occurring?

Regarding the ballast, was the CM heat shield actually used as a lifting body? I had thought that the skip re-entry was only considered and not actually implemented for any Apollo missions?

Thanks,

Matt

[JumpsterG]

The capsule did act as a lifting surface, and ballast would be used to shift mass around it could be rotated around in order to "steer" the capsule when moving through atmosphere. Someone else on these forums posted a link to the following video a while back and I bookmarked it for explaining the process very explicitly.

Around 6:40, it describes the atmospheric approach (elaborating on what can go wrong later in the video) and around 10:10, it describes the use of the capsule as an aerodynamic surface.

[Edit] I should clarify, the video does not explicitly cover Apollo 13, but does describe how most things were done throughout the Apollo program.

[Edit2] Fixed the "ballast used to steer" error. Rewatching the video shows that rotating the vehicle is what is done, not shifting mass around within.

Edited December 5, 2013 by JumpsterG

[Nibb31]

After the free-return burn, they had calculated the re-entry trajectory for the weight of the CM at the end of a nominal Apollo mission. Of course, on a nominal mission, the weight of the CM would have included a certain amount of moon rocks and samples brought back from the surface, while most of the trash and used equipment was jettisoned with the LM.

Luckily, someone at Houston spotted the mistake before they separated from the LM. They decided that it would be preferable to move some ballast from the LM into the CM, in order to approach the weight that was used for the calculations, rather than to rerun the calculations minus the weight of the rocks. Those calculations took several hours in those days, and would have probably required an adjustment burn, which they wanted to avoid.

The shape of the Apollo capsule did have a positive lift/drag ratio when pointed at a certain angle, a bit like a wing:

The following diagram shows (at an exagerated scale) what the re-entry profile looked like:

Because the Apollo CM passively took the aerodynamic angle shown in the first diagram when it was 'heads down', the method for steering the lift up or down was to use thrusters to roll the CM left or right.

- To reduce lift (phase 'I'), they would typically roll 2 minutes left, then 2 minutes right in order to maintain the trajectory

- Then to provide more lift and go up (phase 'II'), they would roll the CM to a straight 'heads down' attitude.

This trajectory allowed the capsule to lose speed in the upper atmosphere while not putting too much stress on the heat shield and not taking too many Gs. A direct dip into the lower atmosphere (without phases 'II" and 'III' in the diagram) would have caused a much more brutal deceleration and generated more heat for a longer period.

Of course, because the Apollo 13 CM was lighter than it should have been, without the extra ballast it might not have dipped low enough into the atmosphere (in phase 'I') to be captured, resulting in an even worse day for the crew.

Fascinating, isn't it?

Edited December 4, 2013 by Nibb31

[photogineer]

The capsule did act as a lifting surface, and ballast would be used to shift mass around in order to "steer" the capsule when moving through atmosphere. Someone else on these forums posted a link to the following video a while back and I bookmarked it for explaining the process very explicitly.

Around 6:40, it describes the atmospheric approach (elaborating on what can go wrong later in the video) and around 10:10, it describes the use of the capsule as an aerodynamic surface.

[Edit] I should clarify, the video does not explicitly cover Apollo 13, but does describe how most things were done throughout the Apollo program.

After the free-return burn, they had calculated the re-entry trajectory for the weight of the CM at the end of a nominal Apollo mission. Of course, on a nominal mission, the weight of the CM would have included a certain amount of moon rocks and samples brought back from the surface, while most of the trash and used equipment was jettisoned with the LM.

Luckily, someone at Houston spotted the mistake before they separated from the LM. They decided that it would be preferable to move some ballast from the LM into the CM, in order to approach the weight that was used for the calculations, rather than to rerun the calculations. Those calculations took several hours in those days, and would have probably required an adjustment burn, which they wanted to avoid.

The shape of the Apollo capsule did have a positive lift/drag ratio when pointed at a certain angle, a bit like a wing:

The following diagram shows (at an exagerated scale) what the re-entry profile looked like:

Because the Apollo CM passively took the aerodynamic angle shown in the first diagram when it was 'heads down', the method for steering the lift up or down was to use thrusters to roll the CM left or right.

- To reduce lift (phase 'I'), they would typically roll 2 minutes left, then 2 minutes right in order to maintain the trajectory

- Then to provide more lift and go up (phase 'II'), they would roll the CM to a straight 'heads down' attitude.

This trajectory allowed the capsule to lose speed in the upper atmosphere while not putting too much stress on the heat shield and not taking too many Gs. A direct dip into the lower atmosphere (without phases 'II" and 'III' in the diagram) would have caused a much more brutal deceleration and generated more heat for a longer period.

Of course, because the Apollo 13 CM was lighter than it should have been, without the extra ballast it might not have dipped low enough into the atmosphere (in phase 'I') to be captured, resulting in an even worse day for the crew.

Fascinating, isn't it?

Wow, thanks so much for the detailed answers guys, I think that just about clears up my questions. Very informative, and that video is excellent. Fascinating indeed. Now I'm off to find another book on the subject.

Thanks again,

Matt

[SFJackBauer]

This information comes from a respected website (IEEE), so:

Although The PC+2 burn had been right on the money, the Trench was increasingly unhappy about the Odyssey and Aquarius's trajectory. Something was pushing the spacecraft off course (afterwards it would be determined that a water vent on the Aquarius had been acting like a little rocket jet, gently sending Apollo 13 in the wrong direction)

Original article here:

http://spectrum.ieee.org/aerospace/space-flight/apollo-13-we-have-a-solution-part-3

Part 1: http://spectrum.ieee.org/aerospace/space-flight/apollo-13-we-have-a-solution

Part 2: http://spectrum.ieee.org/aerospace/space-flight/apollo-13-we-have-a-solution-part-2

Also

http://www.universetoday.com/62339/13-things-that-saved-apollo-13/

[rdfox]

It's both, actually. The water vent was pushing the spacecraft off-course, *and* the lack of ballast in the CM would have resulted in an incorrect trajectory on re-entry. The first was resolved with a small midcourse correction burn using the LM's RCS engines (we're talking on the order of ~5m/s or less here) to push the spacecraft into a trajectory that would have it in the entry corridor whether or not the drift continued, while the second was resolved with loading stuff into the CM.

There are actually a lot of items deliberately glossed over in Apollo 13 to keep the movie's runtime down to a reasonable length!