Is a Fast Rocket in Our Near Future?

If we ever get serious about human space travel outside of Earth's orbit then velocities achieved must be faster. I'm just not sure we need to put people that far out in space, yet. Probes and robots (Curiosity is a robot) seem ideal for answering the questions we have about other planets, asteroids and comets.

Vasimr is a very "cool" engine. Its main benefit is the ability to use only hydrogen and electricity, two of the most abundant things in the universe...

-J

Roll on Nuclear Fusion to generate electricity! Michio Kaku* seems convinced that tamed fusion really is 20 years away - this time. Without it, I cannot see VASIMR type propulsion systems being suitable for interplanetary travel. Solar panels might be useful for Earth-Mars trips, but the further out you go, the less effective they become. No-one tells us how much hydrogen would be needed.

* http://www.youtube.com/watch?v=4gRnezJNFro

Questions: (1) does the fuel have to be stored as hydrogen, or will water itself suffice? The reason I ask, is that Aluminum or Magnesium can be utilized to readily produce a great deal of relatively pure hydrogen (than can easily have the water vapor removed therefrom). Granted that a tank of liquid hydrogen would provide more hydrogen per unit volume and weight than water....

(2) Has anyone considered lithium fission reactor as a power source AND as a source of additional plasma fuel in the form of helium and beryllium? If the reactor were planned correctly, it should not be too difficult to draw off the fission products (maybe even unreactive lithium isotope) as vapors and use them in the plasma jet.

It is not dilithium, but at least it has lithium being used in a sentence with space propulsion.

The thrust of a rocket is dependent on the exhaust velocity of the exhaust gases and this depends partly on the average molecular weight of the exhaust gases. Thus hydrogen is the gas (lowest molecular weight) of choice for this type of "monopropellant" rocket.

The lightest way (but bulkiest) way to store hydrogen is as liquid H2. Not a lot of point of making hydrogen on board from other chemicals, although the ammonia-borane adduct (NH3:BH3), which is a solid, has been looked at as a method of storing H2 for fuel cell powered vehicles. Upon heating it releases H2 in a series of stages.

OK. Suppose they use the ammonia-borane adduct. Use the lithium reactor as a heat source, produce the hydrogen, then use the hydrogen in a turbine to generate the electricity (or at least part of it) supplying the plasma jet. Maybe even the waste heat off the reactor-generator cycle would drive the hydrogen production? I forget the temperature needed, but it might be a bit high for that.