Liquid and solid fuels have enabled space travel, but would other propulsion systems be better?

While chemical fuels offer relatively high thrust chemical rockets have very poor fuel economy. Notice how most spacecraft (even the shuttle) basically ride to space on top of a giant disintegrating totem pole of fuel tanks.

The best feasible alternative is Project Orion, which involves throwing nuclear bombs out the back of the spacecraft and using them to push it forward (sort of like putting a can over a fire cracker and letting it explode). However, such a propulsion system is banned under a treaty banning nuclear explosive devices from spacecraft. A similar alternative is a nuclear salt water rocket, which uses fissionable material suspended in salt water in tanks of boron carbide, which spontaneously undergoes fission when ejected. It can achieve about the same performance, but the fuel is very volatile and can easily undergo spontaneous fission INSIDE the tank if anything goes wrong, which basically turns the ship into a nuclear bomb.

http://en.wikipedia.org/wiki/Project_Orion_%28nucl...

http://www.npl.washington.edu/AV/altvw56.html

A less "dirty" (radioactive) alternative (but one that cannot be built today) is the inertial confinement fusion scheme used for the hypothetical Daedalus probe to Barnard's Star. It uses pellets of helium 3-deuterium mix ignited by electron guns mounted around a 100 meter hemispherical reaction chamber, with the resulting explosion being channeled by magnetic fields.

http://www.daviddarling.info/encyclopedia/D/Daedal...

These are the best propulsion systems for the kind of large, fast spacecraft that we will need to colonize the solar system.

A more likely near-future alternative is a Nuclear Thermal Rocket, which works by heating the propellant by running it through a nuclear reactor. The performance is dramatically lower than the systems outlined above, because the reactions happens inside the spacecraft instead of outside so waste heat is a much bigger problem. However, it is still significantly better than chemical rockets for interplanetary trips. An NTR system could probably be built today, an air-breathing variant was proposed for Project Pluto during the Cold War and got as far as a functional reactor being built.

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

http://www.merkle.com/pluto/pluto.html

In the immediate future NASA is currently working with ion engines, which are low thrust but have better fuel economy than chemical rockets.

http://nmp.nasa.gov/ds1/tech/ionpropfaq.html

Ion drives and most nuclear thermal rockets could not achieve the acceleration necessary to escape Earth's gravity. They're good for deep space missions, but bad for surface to orbit vehicles.

The ICF engines used in Daedalus would also have a low acceleration (.33 m/s^2, much too little to escape Earth's gravity well), but perhaps a high acceleration version could be built. Orion and the nuclear salt water rocket could both easily be used from Earth's surface. However, since they basically amount to exploding nuclear bombs behind the spacecraft, they would realistically probably be restricted to deep space missions due to environmental concerns. High acceleration Daedalus-style ICF, even if feasible, would also probably not be used for surface launches because it would amount to a giant bomb going off on the landing pad (though it does have the advantage of being a "clean" system with no fallout). For surface-to-orbit missions it looks like we're stuck with chemical rockets, unless we can figure out antigravity or something similarly out there.

One possible short-cut to space though is to use a jet-rocket hybrid. For this a spacecraft would use a high-performance air-breathing engine (like a scramjet) to build up most of the velocity it needed inside the atmosphere and then use a rocket for the final push into orbit. Since the ship would get a lot of "free" oxidizer from the atmosphere during its ascent the fuel savings could be impressive. I did some calculations for a scramjet-HLOX hybrid spaceplane for my own sci fi setting and it came out with a fuel fraction of 1.1 (1.1 kg of fuel per kg of payload), including both the fuel for the final push to orbit and the fuel for the initial push to Mach 5 needed for the scramjet to begin working efficiently. Those are optimistic but realistic figures as far as I know.

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

You're conflating the energy supply and the propulsion system.

In chemical rockets, they are one and the same (the energy supply is also the reaction mass).  But in ion drives, they are separate; the Deep Space One probe used solar energy from PV arrays to drive the xenon-ion thruster.  This achieved a specific impulse (Isp) roughly ten times as good as the Space Shuttle's main engines.

We're having trouble getting fusion to work at any size smaller than a bomb, but fission could easily perform space propulsion in several ways:

1.  Heating a separate fluid (ammonia or hydrogen) for a rocket.

2.  Generating electricity for an ion drive.

3.  Direct conversion of a liquid nuclear fuel into hot gas (saltwater fission rocket, see link).

Solar sails and other means of propulsion is just not going to cut it, if you want an intergalactic space vessel the only way it can be done now is with a multi generational spaceship that would take 10,000 years or more to reach the next solar system, but as for rocket technology N.E.R.V.A. rockets have a little potential. oh and for the people saying that there is nothing to slow a ship in space, then there wrong if a ship was to get near the speed of light there would still be gas and dust particle's in space that would impact the vessel and slow it down, plus if a space ship travelling at the speed of light entered a dense cloud of gas in space the space ship could be torn apart.

We already use fusion to generate electricity.

The idea of using fusion or fission for direct propulsion is outdated (and probably dangerous): e.g., "atomic" bombs exploding just behind the heavily plated "stern" of the spacecraft to make it move forward.

However, the use of nuclear energy to produce energy that will then drive the spacecraft can be used in a different type of motor: ion drive and photon drive (drive out ions or light in one direction so that the spacecraft moves out in the opposite direction). The idea has been around for decades in serious science fiction, with many of the problems identified (and there are big ones).

Speaking of a "photon" drive, if we could find a material to use as an efficient hard-gamma-ray reflector, and means to produce (or store) anti-matter and matter on the same craft, then we could use an anti-matter photonic drive (shooting gamma rays out one way to push the vessel out the other way). Gamma-ray photons carry a lot more energy than "normal" light photons. And the energy obtained from matter would be the most efficient ( E = m c^2 ).

A completely different approach (already being tested) is the solar sail. Use the energy already contained in sunlight and solar wind, to push a spacecraft. Slow acceleration, but lots of sunlight and you don't have to carry fuel...

There have even been (theoretical) proposals to make this work for interstellar travel, but we are still a century away (my estimate). Satellites around the sun to gather solar energy and turn it into gigantic laser beams; a sail designed to maximize the push from the laser beams. At destination, a large portion of the sail detaches and reflects the laser beam onto a smaller portion (still attached to the craft) in order to decelerate it.

I'm afraid warp drives are still only in TV or movie scenarios. Until we understand how the curvature of space-time is caused, it will be difficult for us to produce it artificially...

Antimatter propulsion will even be better, but in earlier time, may be we will use anti-matter catalyzed fusion.

Here is the possible timeline for possible propulsion development :

chemical-fission-fusion-amat catalyzed fusion-amat.

I would have bet on fusion but I agree with person who put together the ppt presentation we need a breakthrough in propulsion technology.

annihilation reactions are energy most efficient but not feasible in a near future

That sounds like your Job.