6 ms·

A once-classified anomaly nearly killed NASA's first moon astronauts

85 points by hsnewman 7y ago

nmc 7y ago
TL;DR After being jettisoned, the Service Module remained dangerously close to the Command Module during re-entry, so there was a risk that the two could have collided
- netsharc 7y ago
  That was a stupidly long article, repeating things a lot of people already knew, for that little bit of information...
  - throw20102010 7y ago
    It’s written in the modern clickbait style- they tell you that you’ll never believe something, then they string you along as long as they can. It’s frustrating to read and usually ends up being mundane info. Usually by the second paragraph you can spot this type of writing and need to be ready for disappointment. I’m not sure that the whole command/service module separation issue was actually classified, or if it is just a less commonly known part of the story.
  - zizee 7y ago
    I don't know who said it first, but we see a lot of blog posts that could have been condensed into tweets, and a lot of books could be condensed into a blog posts.
  - mannykannot 7y ago
    Its saving grace, IMHO, is that it has a link to the NASA report on the issue: https://archive.org/details/nasa_techdoc_19710017109/page/n3 https://archive.org/details/nasa_techdoc_19710017109/page/n3
    - pbhjpbhj 7y ago
      Apollo 11 Mission Anomaly Report 3 - Service Module Entry Conclusion Tip-off moments applied to the service module at jettison cause the spin vector to be misaligned with the service module X-axis. The rigid body spin motion of the service module excites longitudinal slosh of the propellants in tanks. The sloshing then becomes the dominant force and causes the spin vector to approach a position normal to the service module X-axis. The sloshing can orient the service module spin axis such that the net -X thrusting over a period of 300 seconds can not only reduce the separation velocity of the service module but also reverse its direction. This condition introduces a remote possibility of recontact between the service modules and command module. An optimum separation velocity can be obtained for a range of propellant loads by restricting the spin thrusting to 2 seconds and the X-axis thrusting to 25 seconds of firing time.
      - avian 7y ago
        I love how the solution was to add one time-delay relay into the circuit. It's a 70s version of a one-line patch.
- ncmncm 7y ago
  What makes it the more significant is that they launched 12 with no fix in place. It doesn't reveal how they kept the astronauts' comments looking out the window secret. Weren't they broadcast live? It seems like the fact of the SM and its fragments not falling behind must mean something important about hypersonic flight.
  - perlgeek 7y ago
    > It doesn't reveal how they kept the astronauts' comments looking out the window secret. Weren't they broadcast live? The plasma buildup around the vehicle interferes with radio communication -- though I'm not sure in exactly which phases of the reentry. Or maybe the window orientation made that hard to see?
  - Rooster61 7y ago
    > Weren't they broadcast live? Nope. Communications were split into multiple channels, all of which were monitored at the PAO, or Public Affairs Office console in mission control by a NASA official. It was deliberately designed so that the public only heard what NASA wanted them to hear.
TeMPOraL 7y ago
Huh. When playing Kerbal Space Program, I usually design the spacecraft with a final-stage engine + fuel tank below the heat shield, separated by a decoupler. I deorbit the craft and then detach the propulsive part. As my capsule screams through the air in a ball of plasma, I often see the propulsive part following it through the air at a distance of couple hundred meters, almost to the very end. I always thought this was some KSP aerodynamics bug - intuition told me that the detached part, not being particularly well-shaped, should burn up much quicker and decelerate much faster. Turns out I was wrong - according to the article, this happened multiple times during the Apollo program. I guess this confirms that KSP is not that far from how the Space Race looked like...
- pavel_lishin 7y ago
  I wish the article had gone into why this happened. I also expect the jettisoned engine stages to be much more susceptible to atmospheric drag. Maybe it's because the atmosphere is so thin up there that despite the plasma buildup, it's still basically a vacuum?
  - mannykannot 7y ago
    The NASA report [1] goes into the details. The procedure was to fire the service module reaction control thrusters to separate it from the command module, spin it to give it stability, and then exhaust their fuel in slowing it down. As the service module was not symmetric around its longitudinal axis, however, the initial spin was not exactly aligned along that axis, and would precess with a period of about 10 seconds. This would lead to sloshing of the residual service module propellants, exacerbating the misalignment markedly -- sometimes wobbling beyond 90 degrees. This meant that the thrusters were not achieving the desired change in velocity, and might sometimes even accelerate towards the command module. After modeling the problem for various residual fuel masses, a solution was found in reducing the time spent on spinning the service module to two seconds, and cutting off all thrusters after 25 seconds. [1] https://archive.org/details/nasa_techdoc_19710017109/page/n3 https://archive.org/details/nasa_techdoc_19710017109/page/n3
  - VLM 7y ago
todd8 7y ago
It’s amazing how many engineering problems had to be solved in the space program. I heard of one that involves software from programmers that worked for NASA. (I don’t remember the space craft but I think it was Apollo). After separation from the boosters that got the space craft into orbit, the main engine of the craft had to burn to head off to the moon. This engine could be maneuvered to “steer” the craft, but to propel the craft accurately (without yawing or pitching) the engine had to line up with the craft’s center of mass and the desired velocity vector. The craft actually calculates the center of mass before the main burn by starting the engine and wiggling the engine a bit then checking the resulting change in the crafts attitude (using gyroscopes, etc.) The software was designed to do all of this automatically and testing showed that it worked. However there was a problem. After separation from the booster stages the craft would be in zero gravity so the main engine’s fuel was a big sphere of liquid floating in the center of the fuel tank. The initial burn for the calculation of center of mass could falter if the fuel wasn’t at the base of the tank where the tank connected to the engine. Before the main engine could be fired the maneuvering jets had to be fired just long enough to press the fuel to the aft side of the tank. The software did this too, but they discovered that in some cases the software didn’t do it long enough. Fixing the bug was too risky so close to launch, but the software could wildly miscalculate the center of mass and the subsequent burn of the main engine could send the craft into a fatal rapid tumble. The solution they came up with was to tape a note on the control panel next to the main engine switch telling the astronauts to manually switch on the maneuvering jets to accelerate the craft before ignition of the main engine.
- patchorang 7y ago
  I had a professor that told us about when he found a bug in some of the flight code while he was interning at NASA. They decided it was too late to fix the bug, so they also put a sticky note on the control panel with instructions. Now I'm wonder how many sticky notes were on that control panel.
  - garmaine 7y ago
    Sounds like the same bug?
dsfyu404ed 7y ago
I think this article is over-hyping the danger. Sure, the risk of the command module and the service module crashing is nonzero but it's so tiny. They start off being pushed away from each other by the separation and they have different aerodynamic properties. It's not surprising that they continued on similar paths initially since the atmosphere is very thin minimizing the aerodynamic differences. To make an analogy, reentry is like tossing two empty beer cans onto a windswept lake. Sure they might have a chance to hit each other early on but the chances of that are small and if their aerodynamic properties are different (i.e one isn't totally empty) they will get further apart as time goes on. The Apollo program had a list a mile long of bigger risks to spend their time mitigating. This article is just hand wringing.

85 points by hsnewman 7y ago

nmc 7y ago

TL;DR After being jettisoned, the Service Module remained dangerously close to the Command Module during re-entry, so there was a risk that the two could have collided

netsharc 7y ago
That was a stupidly long article, repeating things a lot of people already knew, for that little bit of information...
- throw20102010 7y ago
  It’s written in the modern clickbait style- they tell you that you’ll never believe something, then they string you along as long as they can. It’s frustrating to read and usually ends up being mundane info. Usually by the second paragraph you can spot this type of writing and need to be ready for disappointment. I’m not sure that the whole command/service module separation issue was actually classified, or if it is just a less commonly known part of the story.
- zizee 7y ago
  I don't know who said it first, but we see a lot of blog posts that could have been condensed into tweets, and a lot of books could be condensed into a blog posts.
- mannykannot 7y ago
  Its saving grace, IMHO, is that it has a link to the NASA report on the issue: https://archive.org/details/nasa_techdoc_19710017109/page/n3 https://archive.org/details/nasa_techdoc_19710017109/page/n3
  - pbhjpbhj 7y ago
    Apollo 11 Mission Anomaly Report 3 - Service Module Entry Conclusion Tip-off moments applied to the service module at jettison cause the spin vector to be misaligned with the service module X-axis. The rigid body spin motion of the service module excites longitudinal slosh of the propellants in tanks. The sloshing then becomes the dominant force and causes the spin vector to approach a position normal to the service module X-axis. The sloshing can orient the service module spin axis such that the net -X thrusting over a period of 300 seconds can not only reduce the separation velocity of the service module but also reverse its direction. This condition introduces a remote possibility of recontact between the service modules and command module. An optimum separation velocity can be obtained for a range of propellant loads by restricting the spin thrusting to 2 seconds and the X-axis thrusting to 25 seconds of firing time.
    - avian 7y ago
      I love how the solution was to add one time-delay relay into the circuit. It's a 70s version of a one-line patch.
ncmncm 7y ago
What makes it the more significant is that they launched 12 with no fix in place. It doesn't reveal how they kept the astronauts' comments looking out the window secret. Weren't they broadcast live? It seems like the fact of the SM and its fragments not falling behind must mean something important about hypersonic flight.
- perlgeek 7y ago
  > It doesn't reveal how they kept the astronauts' comments looking out the window secret. Weren't they broadcast live? The plasma buildup around the vehicle interferes with radio communication -- though I'm not sure in exactly which phases of the reentry. Or maybe the window orientation made that hard to see?
- Rooster61 7y ago
  > Weren't they broadcast live? Nope. Communications were split into multiple channels, all of which were monitored at the PAO, or Public Affairs Office console in mission control by a NASA official. It was deliberately designed so that the public only heard what NASA wanted them to hear.

TeMPOraL 7y ago

Huh. When playing Kerbal Space Program, I usually design the spacecraft with a final-stage engine + fuel tank below the heat shield, separated by a decoupler. I deorbit the craft and then detach the propulsive part. As my capsule screams through the air in a ball of plasma, I often see the propulsive part following it through the air at a distance of couple hundred meters, almost to the very end. I always thought this was some KSP aerodynamics bug - intuition told me that the detached part, not being particularly well-shaped, should burn up much quicker and decelerate much faster. Turns out I was wrong - according to the article, this happened multiple times during the Apollo program. I guess this confirms that KSP is not that far from how the Space Race looked like...

pavel_lishin 7y ago
I wish the article had gone into why this happened. I also expect the jettisoned engine stages to be much more susceptible to atmospheric drag. Maybe it's because the atmosphere is so thin up there that despite the plasma buildup, it's still basically a vacuum?
- mannykannot 7y ago
  The NASA report [1] goes into the details. The procedure was to fire the service module reaction control thrusters to separate it from the command module, spin it to give it stability, and then exhaust their fuel in slowing it down. As the service module was not symmetric around its longitudinal axis, however, the initial spin was not exactly aligned along that axis, and would precess with a period of about 10 seconds. This would lead to sloshing of the residual service module propellants, exacerbating the misalignment markedly -- sometimes wobbling beyond 90 degrees. This meant that the thrusters were not achieving the desired change in velocity, and might sometimes even accelerate towards the command module. After modeling the problem for various residual fuel masses, a solution was found in reducing the time spent on spinning the service module to two seconds, and cutting off all thrusters after 25 seconds. [1] https://archive.org/details/nasa_techdoc_19710017109/page/n3 https://archive.org/details/nasa_techdoc_19710017109/page/n3
- VLM 7y ago

todd8 7y ago

It’s amazing how many engineering problems had to be solved in the space program. I heard of one that involves software from programmers that worked for NASA. (I don’t remember the space craft but I think it was Apollo). After separation from the boosters that got the space craft into orbit, the main engine of the craft had to burn to head off to the moon. This engine could be maneuvered to “steer” the craft, but to propel the craft accurately (without yawing or pitching) the engine had to line up with the craft’s center of mass and the desired velocity vector. The craft actually calculates the center of mass before the main burn by starting the engine and wiggling the engine a bit then checking the resulting change in the crafts attitude (using gyroscopes, etc.) The software was designed to do all of this automatically and testing showed that it worked. However there was a problem. After separation from the booster stages the craft would be in zero gravity so the main engine’s fuel was a big sphere of liquid floating in the center of the fuel tank. The initial burn for the calculation of center of mass could falter if the fuel wasn’t at the base of the tank where the tank connected to the engine. Before the main engine could be fired the maneuvering jets had to be fired just long enough to press the fuel to the aft side of the tank. The software did this too, but they discovered that in some cases the software didn’t do it long enough. Fixing the bug was too risky so close to launch, but the software could wildly miscalculate the center of mass and the subsequent burn of the main engine could send the craft into a fatal rapid tumble. The solution they came up with was to tape a note on the control panel next to the main engine switch telling the astronauts to manually switch on the maneuvering jets to accelerate the craft before ignition of the main engine.

patchorang 7y ago
I had a professor that told us about when he found a bug in some of the flight code while he was interning at NASA. They decided it was too late to fix the bug, so they also put a sticky note on the control panel with instructions. Now I'm wonder how many sticky notes were on that control panel.
- garmaine 7y ago
  Sounds like the same bug?

dsfyu404ed 7y ago

I think this article is over-hyping the danger. Sure, the risk of the command module and the service module crashing is nonzero but it's so tiny. They start off being pushed away from each other by the separation and they have different aerodynamic properties. It's not surprising that they continued on similar paths initially since the atmosphere is very thin minimizing the aerodynamic differences. To make an analogy, reentry is like tossing two empty beer cans onto a windswept lake. Sure they might have a chance to hit each other early on but the chances of that are small and if their aerodynamic properties are different (i.e one isn't totally empty) they will get further apart as time goes on. The Apollo program had a list a mile long of bigger risks to spend their time mitigating. This article is just hand wringing.