Futuristic propulsion of spacecraft

[mfb]

Combine a better ion thruster with a nuclear reactor and a lot of Xenon. Or better multiple reactors with multiple engines. Then wait.

At an exhaust velocity of 210km/s, you need e^5 or roughly 150 times the final ship mass as propellant. You can escape the solar system for free with clever gravitational slingshots - this will also give a few km/s final velocity, but that is negligible here.

It is hard to find numbers for power/weight of nuclear reactors. Thermoelectric generators reach 5W/kg. Let's be pessimistic and assume a nuclear reactor just doubles this number to 10W/kg. To power our ion thruster with 250kW design power, we need a reactor with a mass of 25 tons. Let's add 5 tons for radiators, support structure and so on. Therefore, a possible last stage of the rocket will have an initial mass of 90 tons, roughly 60 tons of xenon. With a thrust of 2.5N, it will have an acceleration of 3*10^-5 N, later going up to 10^-4 N. To get its velocity change of 210km/s, we have to wait ~150 years. With this design, the rocket will need 5 stages, for a total acceleration time of ~750 years. I have no idea how to design a rocket that will work for this timescale...

Everything scales with power density. If you believe the various claims for possible small-scale nuclear reactors (at least one of them), you might get something like ~20 MW electric power within 20 tons, a power density of ~1000W/kg. At that level, radiators are certainly important, but this thing can power 80 ion thrusters, and you get the same thing done in just 10 years.

The overall mass of the ship would be ~7000 tons, that is within the reach of current technologies. The ship would be assembled in orbit, the heaviest parts are the nuclear reactors. A Delta IV can launch 22 tons, Falcon Heavy aims for 50 tons, SLS for >100 tons. It would be extremely expensive, but possible to build such a thing within a reasonable timescale.There are certainly more futuristic ideas. Vasimr aims for an even higher exhaust velocity, and all the various fission and fusion concepts might give a much better performance as well.
http://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsion) (10 000km/s with fission bombs)
http://en.wikipedia.org/wiki/Project_Daedalus (35 000km/s with fusion)
http://en.wikipedia.org/wiki/Project_Longshot (10 000km/s, fission to ignite fusion)
http://en.wikipedia.org/wiki/Project_Valkyrie (antimatter to ignite fusion)
...

 

[Chronos]

I would be very reluctant to volunteer for a manned mission.

 

[mfb]

Why? Just 1200 years until your remains will fly past Proxima Centauri! Assuming the ship is flying in that direction.

On a planetary scale, this looks more interesting. With the optimistic fission reactor design, you get ~10km/s per month (with 5% of the ship's mass ejected in that time). This allows missions to Mars within a few months, and to Jupiter within a year or so.

 

[D H]

This thread has little to do with astrophysics. A more appropriate venue is aerospace engineering, which is where I have moved it.

 

[Superposed_Cat]

Was this thread started by an engineer by chance?

 

[Lsos]

The point is you have to keep antimatter caged up and hope that the storage mechanism never fails. The energy we are potentially talking about here is horrendous, if you manage to bring antimatter production down to reasonable prices then you've proposed a system whereby biosphere destroying devices are available for an unreasonable but possible price. Think of it this way: any vehicle fitted with a few grams of antimatter will release a Hiroshima scale explosion when damaged. A few kilograms and you've got the release of >Tsa Bomba scale explosion when damaged.

The potential harm of what you are proposing more than outweighs it's uses IMO.

I don't think antimatter necessarily has to be unsafe. The fact that it's so energy-dense means that it's going to be small and therefore much easier to contain with less things to go wrong. Also, there's no need to contain it inside the spaceship.

I'm visualizing a magnetic "bottle" held a distance away from the spaceship. You can design the bottle so that if the #### hits the fan, the explosion ejects the antimatter away from the rest of the ship.

But this might all be a moot point. If you're out in deep space in a tin-foil spaceship, it might not really matter much if the fuel explodes with a ton or 100 million tons of TNT.

 

[Superposed_Cat]

http://en.wikipedia.org/wiki/Category:Spacecraft_propulsion