3 ms·

Reboosting is a capability that ISS supports. If it can handle the G force of acceleration through a reboost, it could presumably handle the same acceleration a

Reboosting is a capability that ISS supports. If it can handle the G force of acceleration through a reboost, it could presumably handle the same acceleration and deceleration for the longer burn time it takes to get to Mars. But in reality it's probably just more economical to send a new ship to Mars. https://en.wikipedia.org/wiki/Reboost https://en.wikipedia.org/wiki/Reboost

dalke 10y ago
ISS reboost is about 0.0185 m/s/s, says https://youtu.be/ZkdfkRC4DbA?t=121 https://youtu.be/ZkdfkRC4DbA?t=121 . Delta-V Earth/Mars is 5,748 m/s, says http://www.projectrho.com/public_html/rocket/appmissiontable.php http://www.projectrho.com/public_html/rocket/appmissiontable... . That's 3.5 days of boost. The ISS mass is 419,455 kg. The minimum energy needed is 1/2 m v^2 or 7E12 J. A Saturn V is about 1E11J, says http://www.ocean.washington.edu/courses/envir215/energynumbers.pdf http://www.ocean.washington.edu/courses/envir215/energynumbe... . So if you attach a few score Saturn Vs to ISS, you might be able to pull it off.
- krallja 10y ago
  The ISS engines might be more efficient than the F-1 was, so the mass could be less than 70 Saturn Vs. Still a ridiculously enormous amount of energy. Makes you wonder how we're ever supposed to build anything significant on or around Mars.
  - Dylan16807 10y ago
    It seems to use UDMH and N2O4 or similar, a hypergolic mix that's convenient for maneuvering thrusters but is not particularly efficient.
    - krallja 10y ago
      That is the only option which makes sense. LOX isn't good for things which wait. Solids have low efficiency. Hypergolics are a good choice for something that has to sit idle for a very long time, and the chemistry has been understood for decades. I guess I was hoping the Russians had figured out ion thrusters in the 70s or something.
- skorgu 10y ago
  What about ion engines? I'm extremely not sure I have this math right but NEXT claims 4190 second ISP [0]. Plugging that into the rocket equation I get: iss_mass = 419600 sep_isp = 4190 delta_v = 5748 htv_payload = 3310 falcon_launch_cost = 62000000 total_mass = iss_mass * math.exp(delta_v / (9.8 * sep_isp)) fuel_load = total_mass - iss_mass dragon_launches = math.floor(fuel_load / htv_payload) total_cost = dragon_launches * falcon_launch_cost I get: Total vehicle mass: 482,646.89 kg Fuel to add: 63,046.89 kg Dragon 2 launches: 19 Total cost: $1,178,000,000.00 Not what you'd call cheap but at least in the realm of the possible. I'm assuming the ISS' solar panels (~120 kW [1]) is sufficient to power the thruster, that a dragon launch costs no more than a falcon launch (certainly false), and that the ion engine is already on board the station. [0] https://en.wikipedia.org/wiki/NEXT_(ion_thruster) [1] https://en.wikipedia.org/wiki/Electrical_system_of_the_International_Space_Station
  - dalke 10y ago
    If there's only one engine then that's 236 mN thrust according to your [1]. Lets call it 4.12 N - put a few thrusters on or improve the technology. The station mass is 419,455 kg. F = ma so the acceleration is 1 µ G. It will take a very long time to get to Mars that way.
    - skorgu 10y ago
      About 18 years assuming I'm using the right formula: http://www.wolframalpha.com/input/?i=acceleration+formula&rawformassumption=%7B%22FS%22%7D+-%3E+%7B%7B%22SpeedAccelerationTime%22,+%22t%22%7D%7D&rawformassumption=%7B%22F%22,+%22SpeedAccelerationTime%22,+%22a%22%7D+-%3E%229.822E-6+m%2Fs%5E2%22&rawformassumption=%7B%22F%22,+%22SpeedAccelerationTime%22,+%22v%22%7D+-%3E%225748+m%2Fs%22 http://www.wolframalpha.com/input/?i=acceleration+formula&ra...

dalke 10y ago

ISS reboost is about 0.0185 m/s/s, says https://youtu.be/ZkdfkRC4DbA?t=121 https://youtu.be/ZkdfkRC4DbA?t=121 . Delta-V Earth/Mars is 5,748 m/s, says http://www.projectrho.com/public_html/rocket/appmissiontable.php http://www.projectrho.com/public_html/rocket/appmissiontable... . That's 3.5 days of boost. The ISS mass is 419,455 kg. The minimum energy needed is 1/2 m v^2 or 7E12 J. A Saturn V is about 1E11J, says http://www.ocean.washington.edu/courses/envir215/energynumbers.pdf http://www.ocean.washington.edu/courses/envir215/energynumbe... . So if you attach a few score Saturn Vs to ISS, you might be able to pull it off.

krallja 10y ago
The ISS engines might be more efficient than the F-1 was, so the mass could be less than 70 Saturn Vs. Still a ridiculously enormous amount of energy. Makes you wonder how we're ever supposed to build anything significant on or around Mars.
- Dylan16807 10y ago
  It seems to use UDMH and N2O4 or similar, a hypergolic mix that's convenient for maneuvering thrusters but is not particularly efficient.
  - krallja 10y ago
    That is the only option which makes sense. LOX isn't good for things which wait. Solids have low efficiency. Hypergolics are a good choice for something that has to sit idle for a very long time, and the chemistry has been understood for decades. I guess I was hoping the Russians had figured out ion thrusters in the 70s or something.
skorgu 10y ago
What about ion engines? I'm extremely not sure I have this math right but NEXT claims 4190 second ISP [0]. Plugging that into the rocket equation I get: iss_mass = 419600 sep_isp = 4190 delta_v = 5748 htv_payload = 3310 falcon_launch_cost = 62000000 total_mass = iss_mass * math.exp(delta_v / (9.8 * sep_isp)) fuel_load = total_mass - iss_mass dragon_launches = math.floor(fuel_load / htv_payload) total_cost = dragon_launches * falcon_launch_cost I get: Total vehicle mass: 482,646.89 kg Fuel to add: 63,046.89 kg Dragon 2 launches: 19 Total cost: $1,178,000,000.00 Not what you'd call cheap but at least in the realm of the possible. I'm assuming the ISS' solar panels (~120 kW [1]) is sufficient to power the thruster, that a dragon launch costs no more than a falcon launch (certainly false), and that the ion engine is already on board the station. [0] https://en.wikipedia.org/wiki/NEXT_(ion_thruster) [1] https://en.wikipedia.org/wiki/Electrical_system_of_the_International_Space_Station
- dalke 10y ago
  If there's only one engine then that's 236 mN thrust according to your [1]. Lets call it 4.12 N - put a few thrusters on or improve the technology. The station mass is 419,455 kg. F = ma so the acceleration is 1 µ G. It will take a very long time to get to Mars that way.
  - skorgu 10y ago
    About 18 years assuming I'm using the right formula: http://www.wolframalpha.com/input/?i=acceleration+formula&rawformassumption=%7B%22FS%22%7D+-%3E+%7B%7B%22SpeedAccelerationTime%22,+%22t%22%7D%7D&rawformassumption=%7B%22F%22,+%22SpeedAccelerationTime%22,+%22a%22%7D+-%3E%229.822E-6+m%2Fs%5E2%22&rawformassumption=%7B%22F%22,+%22SpeedAccelerationTime%22,+%22v%22%7D+-%3E%225748+m%2Fs%22 http://www.wolframalpha.com/input/?i=acceleration+formula&ra...