5 ms·

No. Electric propulsion works by bringing ions to very high energies/speeds. This isn't practical in a dense atmosphere, because ions make too many collisions w

by uvdiv 14y ago

No. Electric propulsion works by bringing ions to very high energies/speeds. This isn't practical in a dense atmosphere, because ions make too many collisions with air molecules. Their energy is dissipated almost immediately, as heat. Moreover, it's not useful for escaping a planet's gravity well, atmosphere or not. Ion thrusters are extremely weak. E.g. the Dawn probe has a thrust/weight ratio of 0.000007. At the earth's surface, the gravity pulling it down would be 135,000 times stronger than the thrusters pushing it up. It can't fly. https://en.wikipedia.org/wiki/Dawn_Mission https://en.wikipedia.org/wiki/Dawn_Mission

tocomment 14y ago
Hmm, maybe you can help with my math? (and keep an open mind) Here's what I was thinking. Float an ion thruster powered ship tethered to a weather balloon with solar panels up to 53km altitude (world record) and fire up the thrusters gradually reaching orbital velocity. Here are my calculations: altitude = 53 km Air density = 5.3 kg/m^3 max speed =7 km/s Coefficient of drag = .02 Cross section area = 130 meters I used this equation to calculate the drag: (aird/2) * (speed2) * coeffdrag * area And I got around 340 newtons of drag. If an Ion thruster could produce something greater than that, we might be in business.
- uvdiv 14y ago
  Your calculation is off by 10^6 -- I think you mixed up m/s with km/s? Also the air density should be far lower -- at your altitude (53 km) it's 7.2e-4 kg/m^3 or so. I don't think streamlining makes much difference at hypersonic speeds -- air molecules will hit your craft regardless of how it's shaped, there's no time push them out of the way. So a drag coefficient of 0.02 isn't likely; it's probably at least 1. You could look up stuff on spacecraft reentry to find more accurate figures. https://www.wolframalpha.com/input/?i=density+air+53+km https://www.wolframalpha.com/input/?i=density+air+53+km Put together, I get something like 2 million Newtons or 200 tonnes force. This is about half the thrust of SpaceX's Falcon 9, so clearly this won't work. https://www.google.com/search?q=(7.2e-4+kg%2Fm%5E3+%2F+2)+*+(7+km%2Fsecond)%5E2+*+130+meters%5E2 https://www.google.com/search?q=(7.2e-4+kg%2Fm%5E3+%2F+2)+*+... https://en.wikipedia.org/wiki/Falcon_9 https://en.wikipedia.org/wiki/Falcon_9 The drag power at this speed is 16 gigawatts, which needs about 100 square kilometers of solar panels. https://en.wikipedia.org/wiki/Drag_(physics)#Power https://en.wikipedia.org/wiki/Drag_(physics)#Power https://www.google.com/search?q=(7.2e-4+kg%2Fm%5E3+%2F+2)+*+(7+km%2Fsecond)%5E3+*+130+meters%5E2 https://www.google.com/search?q=(7.2e-4+kg%2Fm%5E3+%2F+2)+*+... Another problem: at this altitude, a sphere with a 130 m^2 cross section has a lifting power at most 0.8 kg (the mass of the displaced air). https://www.google.com/search?q=4%2F3+*+pi+*+(130+m%5E2+%2F+pi)%5E(3%2F2)+*+7.2e-4+kg%2Fm%5E3 https://www.google.com/search?q=4%2F3+*+pi+*+(130+m%5E2+%2F+... This really won't work.
  - tocomment 14y ago
    Wow. Thanks for fixing the math. Gosh I was really sure the atmosphere would be so thin that it just might work. Oh well back to the drawing board. Btw what did you mean about the lift? I was proposing a helium balloon/blimp to supply the lift.
    - stouset 14y ago
      Right, but helium doesn't just magically "go up". It provides lift in proportion to the difference in density between the gas in the balloon and the surrounding atmosphere. As the atmosphere thins, the amount of lift capable of generated by a helium balloon decreases to the point where, at the altitude you've mentioned, it could lift a total of about 0.8kg (according to GP's calculations). The point being that you wouldn't be able to have much of a payload lifted to those heights.
      - tocomment 14y ago
        Oh that's a very good point. So I'd also need a whole lot more helium than I realized. Which would increase the drag even more. Oh well on to the next idea.

by uvdiv 14y ago

tocomment 14y ago

Hmm, maybe you can help with my math? (and keep an open mind) Here's what I was thinking. Float an ion thruster powered ship tethered to a weather balloon with solar panels up to 53km altitude (world record) and fire up the thrusters gradually reaching orbital velocity. Here are my calculations: altitude = 53 km Air density = 5.3 kg/m^3 max speed =7 km/s Coefficient of drag = .02 Cross section area = 130 meters I used this equation to calculate the drag: (aird/2) * (speed2) * coeffdrag * area And I got around 340 newtons of drag. If an Ion thruster could produce something greater than that, we might be in business.

uvdiv 14y ago
Your calculation is off by 10^6 -- I think you mixed up m/s with km/s? Also the air density should be far lower -- at your altitude (53 km) it's 7.2e-4 kg/m^3 or so. I don't think streamlining makes much difference at hypersonic speeds -- air molecules will hit your craft regardless of how it's shaped, there's no time push them out of the way. So a drag coefficient of 0.02 isn't likely; it's probably at least 1. You could look up stuff on spacecraft reentry to find more accurate figures. https://www.wolframalpha.com/input/?i=density+air+53+km https://www.wolframalpha.com/input/?i=density+air+53+km Put together, I get something like 2 million Newtons or 200 tonnes force. This is about half the thrust of SpaceX's Falcon 9, so clearly this won't work. https://www.google.com/search?q=(7.2e-4+kg%2Fm%5E3+%2F+2)+*+(7+km%2Fsecond)%5E2+*+130+meters%5E2 https://www.google.com/search?q=(7.2e-4+kg%2Fm%5E3+%2F+2)+*+... https://en.wikipedia.org/wiki/Falcon_9 https://en.wikipedia.org/wiki/Falcon_9 The drag power at this speed is 16 gigawatts, which needs about 100 square kilometers of solar panels. https://en.wikipedia.org/wiki/Drag_(physics)#Power https://en.wikipedia.org/wiki/Drag_(physics)#Power https://www.google.com/search?q=(7.2e-4+kg%2Fm%5E3+%2F+2)+*+(7+km%2Fsecond)%5E3+*+130+meters%5E2 https://www.google.com/search?q=(7.2e-4+kg%2Fm%5E3+%2F+2)+*+... Another problem: at this altitude, a sphere with a 130 m^2 cross section has a lifting power at most 0.8 kg (the mass of the displaced air). https://www.google.com/search?q=4%2F3+*+pi+*+(130+m%5E2+%2F+pi)%5E(3%2F2)+*+7.2e-4+kg%2Fm%5E3 https://www.google.com/search?q=4%2F3+*+pi+*+(130+m%5E2+%2F+... This really won't work.
- tocomment 14y ago
  Wow. Thanks for fixing the math. Gosh I was really sure the atmosphere would be so thin that it just might work. Oh well back to the drawing board. Btw what did you mean about the lift? I was proposing a helium balloon/blimp to supply the lift.
  - stouset 14y ago
    Right, but helium doesn't just magically "go up". It provides lift in proportion to the difference in density between the gas in the balloon and the surrounding atmosphere. As the atmosphere thins, the amount of lift capable of generated by a helium balloon decreases to the point where, at the altitude you've mentioned, it could lift a total of about 0.8kg (according to GP's calculations). The point being that you wouldn't be able to have much of a payload lifted to those heights.
    - tocomment 14y ago
      Oh that's a very good point. So I'd also need a whole lot more helium than I realized. Which would increase the drag even more. Oh well on to the next idea.