Whatever happened to transmitted power designs, like using a ground-based laser to lift a payload? If you can leave the powerplant on the ground and only send the power, you no longer have to lift the fuel, just reaction mass.
I don't think that gets you much, as the fuel normally doubles as the reaction mass. So take hydro-lox. The output is water and heat, which equates to steam, which equates to propulsion. Now you could just fill a tank with water and use ground based lasers to heat it into steam, and save the complexity of handling cryogenic materials. But you need a laser powerful enough to convert a rocket full of water to steam over the course of a few minutes. And be able to hold that laser on the target.
Edit: The Saturn V held 3.2 million liters of fuel. And I think it takes about 2600 joules of energy to boil off 1 liter of water. So that is 8.3 billion joules of energy. If the flight time to orbit is 5 minutes, then a 27 megawatt laser should should do the trick.
Second edit -- I didn't see the "k" in front of joules on my random web searches for number of joules to boil off 1 liter of water. So that would be a 27 gigawatt laser (10 times the energy need for time travel). And, according to Retric below, I'm off even further. Point I was originally trying to get at is using a laser is more than a shade past impractical.
Your off by ~61,545 times. As, you don't want the energy to boil water, you want the energy you get from burning hydrogen.
1 liter of water is 11.19% hydrogen or ~0.1119kg. Hydrogen is 143 MJ/kg so 1 liter of water ~= 143,000,000J * 0.1119 = 16,001,700J.
So, you want a 1,661,715 megawatt laser. Good luck with that.
PS: You might be able to do laser assisted rocketry where you hit the combustion chamber with energy to increase exhaust velocity. But even that would take insane accuracy and a 100+ terawatt laser to be useful.
Ah, I didn't do the calculations myself, just did a couple random Google searches for how many joules it takes to boil off one liter of water (not how much it takes to bring a gallon to boiling point, which is of course much much less). Here's one site I used: https://www.physicsforums.com/threads/energy-required-to-boi..., but I didn't see the "k" in front of joules. Thanks.
Your still confusing a state change (boil) with a chemical change (burn). The reason this is important is how fast stuff comes out the bottom of your rocket is really important and pushing it faster takes more energy. To beat chemical rockets you need to use more energy to push harder.
One method of laser propulsion is using the laser to ablate a metal reaction mass. Since metal is much denser than water, and is converted into plasma, a much smaller reaction mass can be used. This method has a specific impulse of about 5000s, an order of magnitude higher than chemical rockets.
Rocket exhaust is normally plasma. Also, density is not really much of an issue unless it's really low. Rockets use liquid hydrogen which is normally ~1/14th the density of water.
That would be useful for station-keeping for the ISS. I don't think NASA would let you point a laser strong enough to ablate metal at their station, though.
The big advantage would be that we're not limited to the exhaust velocity of combustion (which is inherent to using your reaction mass as fuel). If we can lase it hot enough to double the exhaust velocity, the rocket equation says we only need about a square root of the original amount of propellant and power, because almost all that fuel was lifting the rest of the fuel.
To move from the 85% propellant of rockets to 15% (somewhere between a fighter jet and a train, according to the article) we have to increase exhaust velocity of whatever we're pushing down to push us up by 12x.
But how much benefit is that? Remember, rocket fuel doubles as reaction mass. This design STILL requires reaction mass, which will be essentially equal to the weight rocket fuel would have been. So all that's saved is the engines' own weight, which is a tiny fraction of the whole.
It takes a kilowatt laser to lift a gram. A megawatt laser to lift a kilogram. A gigawatt laser to lift a metric ton.
Biggest chemical laser so far is about a megawatt. The laser diode people are making real progress, into the kilowatt range (http://teradiode.com/technology/) but gigawatt laser array are still a ways off.
