Futuristic propulsion of spacecraft

Ryan_m_b

There is no good reason to think this is possible.

qraal

The minimal power (dE/dT) for a voyage is when acceleration - forward or reverse - occupies 2/3 of the total trip time. To the Moon and back, 768,000 km, in six hours ('a few') requires 14.8 m/s² acceleration, which is perhaps a touch uncomfortable for the four hours under thrust.

Total delta-vee - the velocity change - is 213.3 km/s, which is doable by advanced ion drives, but impossible to achieve in just four hours under thrust. Ion drives have strict thrust limits due to arcing from excessive voltage. Plasma drives, particularly VASIMR, have no such voltage limits, but they do have heat-loading issues. Current designs take weeks to reach the Moon, albeit using much less propellant than chemical rockets.

mfb

Why would you need a higher (peak?) power when you accelerate 1/2 of the trip and decelerate the other 1/2? In addition, you can reach the moon with 100W and an ion drive. It just needs ages.
I think the 2/3 are the result of some other optimization process.

qraal

For a given distance, over a given travel-time, there's a minimum power. Kinetic energy depends on the square of the velocity, but the required peak velocity increases for the greater time spent accelerating. Ignore the complications of mass-ratios, and it can be proven that the minimum power to make the trip is when 1/3 the total time is spent accelerating, 1/3 coasting and 1/3 braking. Power being the first derivative w.r.t time of the energy. Do you want a reference?

mfb

A reference would be interesting, indeed.

With 1/3, 1/3, 1/3 for the distance d in time T and uniform acceleration, the maximal velocity v can be determined via 2/3T*v=d <=> v=3d/(2T). The required acceleration is then given by a=9d/(2T).
With 1/2, 1/2, the maximal velocity is given by 1/2T*v=d <=> v=2d/T and the required acceleration is a=4d/T, which is smaller.

As long as the mass of the rocket does not change significantly during the trip, power is proportional to (or at least monotonic in) acceleration. If the mass changes, we need additional data about that. In this case, I would expect asymmetric solutions.

qraal

My reference for the minimum power is Moeckel, W.E. from here...

Comparison of advanced propulsion concepts for deep space exploration

...though you have to extract it from his discussion as he doesn't lay out the algebra involved. In other words, an exercise for the reader.

mfb

In this case, I'll just assume that his calculation involves some other constraints (like max delta v), otherwise see the calculation in my post for the algebra.

GTOM

Well this one is optimistic about antimatter.
http://worldofweirdthings.com/2012/0...re-antimatter/

(Well i know it investigating fringe things, but from what i read i dont consider it a pseudo scientific site, although im not PF mentor, so sorry if i am wrong.)

Is there any theoretical chance, that antimatter can be wrapped in neutral particles somehow, like a proton wraps in the positron?

Also someone had the idea to build giant magnetic sails, and propel them with neutralized particle beams. He thoughts it is better than laser. Can particle beams act like ultra-short wavelength lasers?

Ryan_m_b

I took a brief look and couldn't see anything crackpot about it (though I could have easily missed it) but regardless this is some random person's blog. All they are doing is talking about a few articles they have read, how does that add anything to the discussion? Aside from this nothing they say changes the objections I raised above.
I don't think this is true, positrons are not just sitting in the proton. Regardless all you are proposing is a system whereby the ship would then have to carry a particle accelerator to make the antimatter it is about to use (which would consume hideous amounts of energy).
See here http://en.wikipedia.org/wiki/Magnetic_sail