3 ms·

The amount of power is a few orders of magnitude too low to propel these kind of masses electrically. Basically you need at least 10 kW per Newton.

Retric 10y ago
If it only saves 2 weeks that's still useful. Don't forget if your using solar power you have much less power the closer you get to mars. So, you effectively get a free surplus at the start of your journey. The other option, is to have a pure transit vehicle from earth orbit to mars orbit and then a 3rd vehicle to land with. This, would work really well if we wanted to really colonize mars. But, if your only sending say 10 people the R&D is not worth the fuel savings. PS: Let's say it's a 20,000 lbs craft or ~1/10th of the shuttle and your getting ~8.8 N or 1 lbs of thrust for 120 days. That's 9.8 m/s / 20,000 * 60 * 60 * 24 * 120 = 5k m/s. If you already need the panels while in mars orbit, that seems like a no brainier. Finally, If that's 200kw in mars orbit then the numbers seem even more reasonable as that's 460 kw near earth and the extra 260kw ~= 26 newtons which could add ~5km/s to a 60,000 lb spacecraft.
- Gravityloss 10y ago
  The craft's mass seems to be 15x that. http://imgur.com/a/vwG1S http://imgur.com/a/vwG1S Maybe for course corrections where you need to fine tune the orbit a couple of mm/s here and there? For unmanned missions it does make good sense as the mass doesn't grow so strongly if the trip time gets longer.
  - Retric 10y ago
    Thanks, I agree ION drives are not going to work well on a 150+ton craft without a lot more power. Thinking about that I think I figured it out. The earth is going 30 km/sec around the sun, so that's probably where the 30 km/sec comes from. 30 km/sec is well past single stage to orbit and they would not need a booster if they could get into that range. Mars is doing 50km/sec around the sun, but you can cheat in a few places as your earth orbit can add ~8km /sec from orbiting around earth. That's 50 - 30 - 8 ~= 12km/s plus delta V get out of suns and earths gravity well. But you can also play with orbital mechanics to save some energy and you can also subtract the orbital velocity around mars. Anyway, I am not sure how much delta V you need, but it's well under 30km/sec. PS: Video had refueling in what looked like LEO, but if your in a higher orbit the craft needs less fuel to get to Mars from that orbit.
    - Retric 10y ago
      PPS: And not surprisingly I messed up the math but here are some real numbers: https://en.m.wikipedia.org/wiki/Delta-v_budget https://en.m.wikipedia.org/wiki/Delta-v_budget
dogma1138 10y ago
The other problem is that you need to slow down, with ION drives you'll have to start you slow down burn about mid way increasing the travel time considerably. With traditional rockets you can accelerate, get a gravity assist boost and save enough fuel to slow down quickly when you are at the orbital insertion point.
- Gravityloss 10y ago
  If you do a direct re-entry you don't need to slow down with propulsion at either end. :)
  - dogma1138 10y ago
    You aren't going to be doing direct reentry, mars' mean orbital velocity is about 25km/s, your space craft would be traveling at probably 15-20km/s. MOI requires a slow down burn there isn't any way around it, this isn't KSP.
    - Gravityloss 10y ago
      SpaceX is not going to first enter Mars orbit, looks like they're landing the whole craft on the surface. For example, Mars Science Laboratory did the same, and it didn't need to spend fuel trying to enter orbit. http://mars.nasa.gov/msl/mission/timeline/approach/ http://mars.nasa.gov/msl/mission/timeline/approach/ The closing velocity of spacecraft and Mars is nowhere near 25 km/s. As another example, Apollo did a direct entry to Earth as well. Even when entering orbit, you can sometimes use aerobraking or aerocapture, though it's either limited in delta vee or dangerous if you go lower.

Retric 10y ago

If it only saves 2 weeks that's still useful. Don't forget if your using solar power you have much less power the closer you get to mars. So, you effectively get a free surplus at the start of your journey. The other option, is to have a pure transit vehicle from earth orbit to mars orbit and then a 3rd vehicle to land with. This, would work really well if we wanted to really colonize mars. But, if your only sending say 10 people the R&D is not worth the fuel savings. PS: Let's say it's a 20,000 lbs craft or ~1/10th of the shuttle and your getting ~8.8 N or 1 lbs of thrust for 120 days. That's 9.8 m/s / 20,000 * 60 * 60 * 24 * 120 = 5k m/s. If you already need the panels while in mars orbit, that seems like a no brainier. Finally, If that's 200kw in mars orbit then the numbers seem even more reasonable as that's 460 kw near earth and the extra 260kw ~= 26 newtons which could add ~5km/s to a 60,000 lb spacecraft.

Gravityloss 10y ago
The craft's mass seems to be 15x that. http://imgur.com/a/vwG1S http://imgur.com/a/vwG1S Maybe for course corrections where you need to fine tune the orbit a couple of mm/s here and there? For unmanned missions it does make good sense as the mass doesn't grow so strongly if the trip time gets longer.
- Retric 10y ago
  Thanks, I agree ION drives are not going to work well on a 150+ton craft without a lot more power. Thinking about that I think I figured it out. The earth is going 30 km/sec around the sun, so that's probably where the 30 km/sec comes from. 30 km/sec is well past single stage to orbit and they would not need a booster if they could get into that range. Mars is doing 50km/sec around the sun, but you can cheat in a few places as your earth orbit can add ~8km /sec from orbiting around earth. That's 50 - 30 - 8 ~= 12km/s plus delta V get out of suns and earths gravity well. But you can also play with orbital mechanics to save some energy and you can also subtract the orbital velocity around mars. Anyway, I am not sure how much delta V you need, but it's well under 30km/sec. PS: Video had refueling in what looked like LEO, but if your in a higher orbit the craft needs less fuel to get to Mars from that orbit.
  - Retric 10y ago
    PPS: And not surprisingly I messed up the math but here are some real numbers: https://en.m.wikipedia.org/wiki/Delta-v_budget https://en.m.wikipedia.org/wiki/Delta-v_budget

dogma1138 10y ago

The other problem is that you need to slow down, with ION drives you'll have to start you slow down burn about mid way increasing the travel time considerably. With traditional rockets you can accelerate, get a gravity assist boost and save enough fuel to slow down quickly when you are at the orbital insertion point.

Gravityloss 10y ago
If you do a direct re-entry you don't need to slow down with propulsion at either end. :)
- dogma1138 10y ago
  You aren't going to be doing direct reentry, mars' mean orbital velocity is about 25km/s, your space craft would be traveling at probably 15-20km/s. MOI requires a slow down burn there isn't any way around it, this isn't KSP.
  - Gravityloss 10y ago
    SpaceX is not going to first enter Mars orbit, looks like they're landing the whole craft on the surface. For example, Mars Science Laboratory did the same, and it didn't need to spend fuel trying to enter orbit. http://mars.nasa.gov/msl/mission/timeline/approach/ http://mars.nasa.gov/msl/mission/timeline/approach/ The closing velocity of spacecraft and Mars is nowhere near 25 km/s. As another example, Apollo did a direct entry to Earth as well. Even when entering orbit, you can sometimes use aerobraking or aerocapture, though it's either limited in delta vee or dangerous if you go lower.