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KSP taught me that orbital intercept to rescue Watney was nosense. (The book was slightly more believable here). also the whole swing by Earth and head back at

KSP taught me that orbital intercept to rescue Watney was nosense. (The book was slightly more believable here). also the whole swing by Earth and head back at the last minute was wrong. Orbital intercepts between planets requires very precise timing and alignment. A transfer from Mars to Earth is never going to line up with an instant return transfer window back to Mars. On KSP itself, huge weakness for me in the game is that it is virtually unplayable stock. Absolute minimum you need to install a data providing mod like Kerbal Engineer Redux for both building rockets to quickly assess dVs and TWRs etc and in flight to help with figuring out burns etc. In stock you are reduced to having to use spreadsheets to build rockets and trying to land from visual clues only. The stock game badly needs something like KER. KAC is also helpful to quickly find transfer windows etc.

CydeWeys 9y ago
> A transfer from Mars to Earth is never going to line up with an instant return transfer window back to Mars. The whole point of the Hermes spacecraft as used in The Martian, though, is that it has a high specific impulse, low thrust ion drive, that runs continually during the entire orbit (using power from a reactor to do so). Andy Weir, the author, is a programmer, and he actually wrote a physics simulator to calculate the required orbit; see here: http://www.galactanet.com/martian/hermes.mp4 http://www.galactanet.com/martian/hermes.mp4 So it's definitely not a Hohmann transfer orbit since there is thrust applied throughout, and you are consequently reaching your destination faster than such a transfer would arrive. It ends up being a low thrust brachistochrone orbit. So anyway, the point is that, since you're continuously applying thrust anyway, you don't need perfect alignment, it just takes you longer if the alignment isn't correct. Andy Weir is a serious hard science fiction nerd, to the point that you can download the data that he simulated for these orbits and verify for yourself that they are in fact correct for the stated specifications of Hermes' ion drive. Totally agree with you that the Engineer plugin is absolutely essential for playing KSP, though. The game is so frustrating without that because there's so much trial and error with randomly flipping craft thanks to the new aero model if the TWR is off for any of your stages (and it will be until you've played awhile and developed an intuitive sense of what kind of rocket is reasonable -- a proficiency you won't likely reach if you didn't have Engineer from the beginning to help get you there).
- geekpowa 9y ago
  Thanks for the link. That was well awesome. I withdraw my criticism.
  - CydeWeys 9y ago
    I had a similar issue with Seveneves recently. I had a strong science-based disagreement with the idea of Izzy always keeping its anchored asteroid facing into the oncoming atmosphere of LEO, which I was sure was impossible because it would be facing the rear as often as it would the front, because objects in space/thin atmospheres don't fly and don't maintain a constant relative orientation just because they're orbiting something. To me it seemed as dumb as a spacecraft that had to constantly have its engines firing behind it just to stay in orbit. It wasn't until I shared this frustration with a smart friend, and he pointed out that it actually is possible if the space station has a rotational period equal to its orbital period, that I realized I was wrong. The author was indeed being smarter about the science of it than I was, and had done the research. And, forget just the future part of it; the ISS actually currently flies with a specific constant relative orientation through the atmosphere so as to minimize air resistance, using, you guessed it, a rotational period equal to its orbital period, plus occasional corrective actions from gyros and thrusters.
    - TeMPOraL 9y ago
      > the ISS actually currently flies with a specific constant relative orientation through the atmosphere so as to minimize air resistance, using, you guessed it, a rotational period equal to its orbital period, plus occasional corrective actions from gyros and thrusters. And plus solar panels, which get realigned to minimize atmospheric drag when the Sun is behind the Earth. https://en.wikipedia.org/wiki/Night_Glider_mode https://en.wikipedia.org/wiki/Night_Glider_mode
      - CydeWeys 9y ago
        Important to note that night glider mode is dependent on the rotation of the overall space station, so that you can set the orientation of the panels to be edge-on into the wind once and have the overall rotation keep that orientation indefinitely. Without the rotation, you'd have to continuously use the motors to adjust the panels, which would burn them out more quickly, use more power, and since they likely aren't freely rotating, would cause large spikes in air resistance once per orbit when you hit the limit of travel and have to do a large opposite rotation to maintain edge-on orientation.
Hop_David 9y ago
As others have mentioned, The Hermes didn't follow a Hohmann trajectory. It is an ion propelled craft capable of 2 mm/s^2 acceleration. A very weak thrust but powerful in that it can sustain this acceleration for looooong burns. If just the sun is accounted for, Andy Weir's trajectories are okay (I believe). However putting planets in complicates things. Ion engines really suck at climbing in and out of deep planetary gravity wells. Neil deGrasse Tyson's trailer for The Martian had Hermes leaving low earth orbit and arriving in Mars orbit 124 days later. Which is nonsense. With 2 mm/s^2 acceleration it would take 40 days to spiral out of earth's gravity well. Not only does this wreck the 124 day trajectory Weir so painstakingly calculated, but most the slow 40 day spiral would be in the Van Allen belts. This astronauts would be killed by radiation.

CydeWeys 9y ago

> A transfer from Mars to Earth is never going to line up with an instant return transfer window back to Mars. The whole point of the Hermes spacecraft as used in The Martian, though, is that it has a high specific impulse, low thrust ion drive, that runs continually during the entire orbit (using power from a reactor to do so). Andy Weir, the author, is a programmer, and he actually wrote a physics simulator to calculate the required orbit; see here: http://www.galactanet.com/martian/hermes.mp4 http://www.galactanet.com/martian/hermes.mp4 So it's definitely not a Hohmann transfer orbit since there is thrust applied throughout, and you are consequently reaching your destination faster than such a transfer would arrive. It ends up being a low thrust brachistochrone orbit. So anyway, the point is that, since you're continuously applying thrust anyway, you don't need perfect alignment, it just takes you longer if the alignment isn't correct. Andy Weir is a serious hard science fiction nerd, to the point that you can download the data that he simulated for these orbits and verify for yourself that they are in fact correct for the stated specifications of Hermes' ion drive. Totally agree with you that the Engineer plugin is absolutely essential for playing KSP, though. The game is so frustrating without that because there's so much trial and error with randomly flipping craft thanks to the new aero model if the TWR is off for any of your stages (and it will be until you've played awhile and developed an intuitive sense of what kind of rocket is reasonable -- a proficiency you won't likely reach if you didn't have Engineer from the beginning to help get you there).

geekpowa 9y ago
Thanks for the link. That was well awesome. I withdraw my criticism.
- CydeWeys 9y ago
  I had a similar issue with Seveneves recently. I had a strong science-based disagreement with the idea of Izzy always keeping its anchored asteroid facing into the oncoming atmosphere of LEO, which I was sure was impossible because it would be facing the rear as often as it would the front, because objects in space/thin atmospheres don't fly and don't maintain a constant relative orientation just because they're orbiting something. To me it seemed as dumb as a spacecraft that had to constantly have its engines firing behind it just to stay in orbit. It wasn't until I shared this frustration with a smart friend, and he pointed out that it actually is possible if the space station has a rotational period equal to its orbital period, that I realized I was wrong. The author was indeed being smarter about the science of it than I was, and had done the research. And, forget just the future part of it; the ISS actually currently flies with a specific constant relative orientation through the atmosphere so as to minimize air resistance, using, you guessed it, a rotational period equal to its orbital period, plus occasional corrective actions from gyros and thrusters.
  - TeMPOraL 9y ago
    > the ISS actually currently flies with a specific constant relative orientation through the atmosphere so as to minimize air resistance, using, you guessed it, a rotational period equal to its orbital period, plus occasional corrective actions from gyros and thrusters. And plus solar panels, which get realigned to minimize atmospheric drag when the Sun is behind the Earth. https://en.wikipedia.org/wiki/Night_Glider_mode https://en.wikipedia.org/wiki/Night_Glider_mode
    - CydeWeys 9y ago
      Important to note that night glider mode is dependent on the rotation of the overall space station, so that you can set the orientation of the panels to be edge-on into the wind once and have the overall rotation keep that orientation indefinitely. Without the rotation, you'd have to continuously use the motors to adjust the panels, which would burn them out more quickly, use more power, and since they likely aren't freely rotating, would cause large spikes in air resistance once per orbit when you hit the limit of travel and have to do a large opposite rotation to maintain edge-on orientation.

Hop_David 9y ago

As others have mentioned, The Hermes didn't follow a Hohmann trajectory. It is an ion propelled craft capable of 2 mm/s^2 acceleration. A very weak thrust but powerful in that it can sustain this acceleration for looooong burns. If just the sun is accounted for, Andy Weir's trajectories are okay (I believe). However putting planets in complicates things. Ion engines really suck at climbing in and out of deep planetary gravity wells. Neil deGrasse Tyson's trailer for The Martian had Hermes leaving low earth orbit and arriving in Mars orbit 124 days later. Which is nonsense. With 2 mm/s^2 acceleration it would take 40 days to spiral out of earth's gravity well. Not only does this wreck the 124 day trajectory Weir so painstakingly calculated, but most the slow 40 day spiral would be in the Van Allen belts. This astronauts would be killed by radiation.