3 ms·

That presumes a constant velocity. Any extra-solar ship would certainly need a different acceleration profile. Ion engines come to mind: low acceleration, but c

by mabbo 10y ago

That presumes a constant velocity. Any extra-solar ship would certainly need a different acceleration profile. Ion engines come to mind: low acceleration, but constant for a very long time. If you're going a far distance, they're ideal.

spuz 10y ago
Unfortunately, not ideal enough. But there are other options: https://www.youtube.com/watch?v=EzZGPCyrpSU https://www.youtube.com/watch?v=EzZGPCyrpSU
dalke 10y ago
While you have a valid point, here's what it works out for ion engines. Assume you want 1,000 years to Alpha Centauri and your ship boosts the entire way. (It'll take a few pictures as it flies through the system.) s = 1/2 a t^2 a = 4.22 light years * 2 / ((1000 years)*(1000 years)) = 8 E-5 m/s/s Sounds great, right? But the final speed is: v = a * t = 4.22 light years * 2 / (1000 years) = 2.5E6 m/s = 0.8 c (!) Let's say your engine has a ISP of 8000 s, which is about 10x better than anything we have. Then from the rocket equation you'll need a dry weight (payload, engines, etc.) to propellant ratio of e^(2500000 / (9.8 * 8000) = 70,579,094,313,732 = 71 quadrillion eg, 1 gram of spacecraft requires 70 kilotons of propellant But wait, there's more! It takes power to push 70 kilotons of propellant backwards at 80,000 m/s. E = 1/2 * m * (v^2) = 2E20 Joule Since E = m c^2 => total annihilation of over a ton of antimatter Almost certainly more as that assumes 100% efficiency. And of course that extra ton isn't propellant, so it hits you in the rocket equation. Note: I am not a rocket scientist and likely made mistakes. Even if I'm off by a few zeros, it's enough to show that an ion engine isn't going to get something to Alpha Centari within a few centuries.
- dalke 10y ago
  Oops! There's a couple of missing zeros. The 2.5E6 m/s = 0.8 c should be 0.8% c. That mistake doesn't continue to the rest of the calculation.

by mabbo 10y ago

spuz 10y ago

Unfortunately, not ideal enough. But there are other options: https://www.youtube.com/watch?v=EzZGPCyrpSU https://www.youtube.com/watch?v=EzZGPCyrpSU

dalke 10y ago

While you have a valid point, here's what it works out for ion engines. Assume you want 1,000 years to Alpha Centauri and your ship boosts the entire way. (It'll take a few pictures as it flies through the system.) s = 1/2 a t^2 a = 4.22 light years * 2 / ((1000 years)*(1000 years)) = 8 E-5 m/s/s Sounds great, right? But the final speed is: v = a * t = 4.22 light years * 2 / (1000 years) = 2.5E6 m/s = 0.8 c (!) Let's say your engine has a ISP of 8000 s, which is about 10x better than anything we have. Then from the rocket equation you'll need a dry weight (payload, engines, etc.) to propellant ratio of e^(2500000 / (9.8 * 8000) = 70,579,094,313,732 = 71 quadrillion eg, 1 gram of spacecraft requires 70 kilotons of propellant But wait, there's more! It takes power to push 70 kilotons of propellant backwards at 80,000 m/s. E = 1/2 * m * (v^2) = 2E20 Joule Since E = m c^2 => total annihilation of over a ton of antimatter Almost certainly more as that assumes 100% efficiency. And of course that extra ton isn't propellant, so it hits you in the rocket equation. Note: I am not a rocket scientist and likely made mistakes. Even if I'm off by a few zeros, it's enough to show that an ion engine isn't going to get something to Alpha Centari within a few centuries.

dalke 10y ago
Oops! There's a couple of missing zeros. The 2.5E6 m/s = 0.8 c should be 0.8% c. That mistake doesn't continue to the rest of the calculation.