What kind of propulsion could help future space missions?

Gigantic hydrogen scooping ramjet.

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The basic problem with space travel other than it being a matter of extreme duration is the weigth of the fuel necessary to propel a sustained flight under power. In most cases thus far there has been a launch, then a boost, then a small rocket extended burn, then a long drift in space. Most destinations that seem to be of interest are vast distances beyond the 250,000 mile distance to the Moon.

Anyone who has ever watched a lift off from Cape Canaveral has seen those two huge cannisters of fuel and rocket engines fall off of the rocket ship about one minute into launch. There is simply no way to carry ten or fifty times that much fuel up into space. Given the sparcity of hydrogen in the weak, weak atmosphere of outer space, I doubt that sufficient hyudrogen could be collected to do much in the way of generating power for extended runs of any rocket motor. Also it would appear that given the teeny amounts of hydrogen that it is possible to collect in outer space, the collection system energy consumption mechanism would use up more energy than it collected, leaving you with a loose/loose situation (burned up a lot of energy collecting fuel that isn't going to do much for you).

The Sun's energy is constant in outer space.

The sail idea is a possibility that has great merit except for possible damage from debris in space which seems quite likely. Debris travels at fantastic speeds and would penetrate the sails like armor piercing rounds in a heart beat, and cannot be planned for or anticipated.

While I do not have any designs for such, some kind of X Ray or Gamma Ray collector and propulsion syustem seems much more plausible to me, though I have no idea how such a system might work..

The technology for another source of propulsion like what you're talking about is maybe a handful of decades away away.

There's actually a number of viable options,

scientists are working on building anti-matter gravity traps, so that they can store relatively large amounts of antihydrogen. Combining regular hydrogen with antihydrogen converts both atoms into energy, a 100% efficient exchange.

the potential of an antimatter engine is almost unimaginable

Then theres the fabled fusion engine. As you know whenever we harness nuclear energy first we make it into a power reactor then a bomb then an engine... Well as far as fusion is concerned we're kinda stuck on the powerplant part... The incredibly powerful magnetic fields necessary to keep the reaction stable and burning takes about 100 times more energy than can be derived from the reaction itself...

While temperatures on a nuclear level whithin the reaction are as hot as the core of the sun, the amount of actual heat radiated from the reaction wouldn't even boil a cup of water

I think it's safe to say that one of these two energy sources will be tapped within fifty or sixty years if we continue at our current rate of progress.

and within 80 one of them will be developed into some sort of space propulsion system

that is if we make it past the bomb part....

(by the way don't be discouraged about the hefty time frame, recent discoveries in genetics have allowed scientists to literally double the lifespan of every organism from yeast cells to monkeys, there are drawbacks, but in the coming decades it may be possible to extend the human lifespan)

I liike the solar sail myself. It is inexpensive, but takes a lot of time to get to speed. There are ways to increase the acceleration. Magnetic hydrogen ramscoops is another good idea. There are alot of problems inherent in the ion drive. Like weight to thrust ratios. And also there is a propellant issue. With solar sails and ramscoops that is not so much the issue.

But wouldn't it be cool if they can come up with a propulsion system based on gravitons?

i dont know much about this, but one night i was on the phone and my tv was on mute (on the science channel)--a depiction of a huge sail was attached to a relatively small spacecraft. a nuclear warhead was sent from the spacecraft towards the attached sail, and then KABOOM! the energy from the explosion is caught in the sail, thrusting the unit forward at a very high speed.

V.A.S.I.M.R. would be the kind of propulsion N.A.S.A. would find useful but they couldn't use the technology because of budget reasons.

The Bussard collector technology is decades from actual uses and 50 years from now they might be feasible but V.A.S.M.R. is ready to go with very little modifications.

Remember the MIR space station? Not Really? Well that's how people are going to remember the ISS.