Advanced Propulsion

Decided to write an overview of what I think the most promising advanced propulsion systems are.

First, some info about how to measure the performance of a propulsion system. Spaceships don't have a "range" like sea-ships or airplanes. Instead, they are limited by their Delta-V, which is the total change in velocity they can achieve. This Delta-V limit puts limits on wether a ship can, for example, escape Earth orbit and move to a higher orbit about the sun to go to Mars. The main way to gauge a performance of advanced propulsion system is Delta-V. Delta-V is generally determined by two factors: mass-ratio and exhaust velocity. Mass ratio is the ratio of the mass of the rocket with propellant to the mass of the rocket when it is done shooting out the propellant.

As for how fast the spaceship can get to a certain spot, it depends on both how much extra Delta-V it has and how much thrust it produces. For ordinary chemical rockets, the thrust is more than adequate, but for many advanced propulsion systems, it takes a long time to actually get to the needed Delta-V.

Now to start with systems that are feasible with current technology, but need some work to become practical. I'll post more advanced propulsion systems and surface-to-orbit launching systems later.

Electric ion: Accelerates plasma ions through an electric potential (similar to a CRT display). Requires large amounts of electricity (either from solar panels or fission reactors) and has somewhat limited thrust, but has massive exhaust velocities.

Magnetic plasma: Like the electric ion thruster, but instead of an electric potential, it uses a magnetic confinement field to force the plasma out the back. A highly promising example, VASIMR, will be tested on the ISS and could revolutionize space transport. VASIMR allows you to dynamically choose the tradeoff between thrust and exhaust velocity. Like the electric ion thruster, it can provide extremely high exhaust velocities.

Solar sail: Absorbs sunlight and emits light back out, making use of the radiation pressure of light. Same force that causes comet tails to point away from the sun. Exhaust velocity is infinite, as it does not use propellant. The sail's thrust to mass ratio is limited by how light the sail can be made. Nano-technology could allow for really light materials to be made. Another method of increasing the thrust, and to provide thrust throughout an interstellar journey, one option would be to shoot a laser from Earth at the sail.

Thermal rocket: Heats a propellant to cause it to expand and exit the rocket. Exhaust velocity is dependent on how hot the propellant can be made, and is significantly higher than conventional rockets. The exhaust velocity is not as high as ion propulsion, but the thrust is much higher. The propellant can be heated either by solar reflectors or by nuclear fission. This type of rocket is clearly well understood scientifically, and is simply a matter of engineering.

I've always imagined that any interstellar craft would be massive things, carrying maybe 100,000 people - essentially an entire city. I think this would be necessary due to the amount of fuel and other supplies that would need to be carried, as well as due to the duration of the journey itself. A long journey (centuries) means that the craft needs to be as self-sufficient as possible, which requires more space.

The Orion (nuclear) rocket had a design capable of carrying 100,000 people - that's similar to what I envision for interstellar travel (maybe powered by antimatter?). What are your thoughts on this? What would be the best propulsion system for an interstellar ark?

http://en.wikipedia.org/wiki/Interstellar_ark

I don't think an interstellar journey will be really long. We could use pretty low tech laser driven solar sails to travel to hundreds of stars in our neighborhood within several years. And that will only get better with time.

Isn't c still a limiting velocity with solar sails? Even if there are hundreds of stars within our neighborhood, the likelihood of finding a habitable planet within 10 light years is probably extremely low. We'd probably have to increase our range to hundreds of light years before finding a suitable planet, which is why I think the journey will take a very long time. Of course, this is all based on speculation on the frequency of earth like planets.

Nope. c is only a limiting velocity from the point of view of Earth. From the point of view of the spaceship occupants, they can go as fast as they like. Now, it will take increasing amounts of laser energy to accelerate them as they approach relativistic speeds because redshift will lower the power of the laser beam, but theoretically, there is no limit on the speed. The only limit is how much power can be provided from Earth.

Oops sorry I've been kinda busy. However, c isn't a limiting velocity from the point of view of people on board the starship. So, they can get there in as little time as they like.