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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. (

by 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?
- todd8 7y ago
  Sounds like the same person, it came up in a class taught by Boyer and Moore.
Rooster61 7y ago
What you are referring to is an ullage burn. Apollo definitely had ullage burns that mitigated this problem before lighting up the rockets in earnest. https://en.wikipedia.org/wiki/Ullage_motor https://en.wikipedia.org/wiki/Ullage_motor Another note is that it wasn't just for the calculation of how the main burn would go. If the propellant was sufficiently distributed in the tank, not enough propellant would actually get to the valves at the back of the tank, and the engine could potentially not even fire.
VLM 7y ago
The word to google for is "ullage" There's been a lot of creative solutions. If you have non-cryogenic fluids you can use pressurized bladders. Lots of strange looking (to me) surface tension designs exist to manage the bubble, as long as the bubble is far from the intake who cares. Supposedly the cryogenic Saturn V third stage simply vented its boiloff to the rear such that the stage was essentially under continuous tiny acceleration, which must have made the navigation calculations interesting and set a time limit on how long you could screw around in low orbit before going to the moon.
- Rooster61 7y ago
  > Supposedly the cryogenic Saturn V third stage simply vented its boiloff to the rear This occured after the ullage burn. The vents were not enough to do it alone, but they did keep the propellant settled between the cut off of the ullage burn and the real burn. https://space.stackexchange.com/questions/31618/why-did-the-ullage-motors-for-the-s-ivb-tli-burn-shut-down-prior-to-s-ivb-igniti https://space.stackexchange.com/questions/31618/why-did-the-... Also, I believe the Saturn V also had baffled propellant tanks to help keep the propellants from getting too unsettled even in zero-g.
tzm 7y ago
Interesting. Here's a short 3D visualization of the Apollo mission that briefly mentions the floating fuel issue / resolution: https://youtu.be/r0ubiU3PRHw?t=63 https://youtu.be/r0ubiU3PRHw?t=63

by todd8 7y ago

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?
todd8 7y ago
Sounds like the same person, it came up in a class taught by Boyer and Moore.

Rooster61 7y ago

What you are referring to is an ullage burn. Apollo definitely had ullage burns that mitigated this problem before lighting up the rockets in earnest. https://en.wikipedia.org/wiki/Ullage_motor https://en.wikipedia.org/wiki/Ullage_motor Another note is that it wasn't just for the calculation of how the main burn would go. If the propellant was sufficiently distributed in the tank, not enough propellant would actually get to the valves at the back of the tank, and the engine could potentially not even fire.

VLM 7y ago

The word to google for is "ullage" There's been a lot of creative solutions. If you have non-cryogenic fluids you can use pressurized bladders. Lots of strange looking (to me) surface tension designs exist to manage the bubble, as long as the bubble is far from the intake who cares. Supposedly the cryogenic Saturn V third stage simply vented its boiloff to the rear such that the stage was essentially under continuous tiny acceleration, which must have made the navigation calculations interesting and set a time limit on how long you could screw around in low orbit before going to the moon.

Rooster61 7y ago
> Supposedly the cryogenic Saturn V third stage simply vented its boiloff to the rear This occured after the ullage burn. The vents were not enough to do it alone, but they did keep the propellant settled between the cut off of the ullage burn and the real burn. https://space.stackexchange.com/questions/31618/why-did-the-ullage-motors-for-the-s-ivb-tli-burn-shut-down-prior-to-s-ivb-igniti https://space.stackexchange.com/questions/31618/why-did-the-... Also, I believe the Saturn V also had baffled propellant tanks to help keep the propellants from getting too unsettled even in zero-g.

tzm 7y ago

Interesting. Here's a short 3D visualization of the Apollo mission that briefly mentions the floating fuel issue / resolution: https://youtu.be/r0ubiU3PRHw?t=63 https://youtu.be/r0ubiU3PRHw?t=63