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Using Lasers and Water to Propel Spacecraft?

  • Thread starter WannabeRocketScientist
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I was going through old books I have bought, and came across a book I bought back in elementary school, Future Space. While it is a lower-level text, I flipped to the page of one "future space-station" I had remembered from when I last read it. The station is called the "Powerhouse in Space" and consists of large solar panels to collect large amounts of solar energy to turn into electrical power to run a massive laser. On either side of the station small cone-shaped spacecraft containing electronics and experiments are docked, their only fuel being water. The laser is turned on, flash-boiling the water in the tanks on the cone-craft and the cones shoot off at high velocity due to the pressure and steam created by the flash-boiling of the water, similar to the thrust of a chemical rocket. The laser beam is supposed to be powerful to beam power across millions of miles across space, and the cones themselves supposedly capable of crossing the solar system in a few days.

My question is: Is this feasible now? If so, is the thrust created by superheated steam really enough to propel a small spacecraft across the solar system in a matter of days, rather than years? I am unsure if we have lasers powerful enough to do what is described here, but if we don't, is it possible that we could build one this powerful in the near-future?

It just seems to me that the thrust couldn't be enough to do what is described here, but I have yet to do the calculations myself. If it is true, this could very easily revolutionize our study of our solar system, making it possible to send inexpensive craft into deep space quickly and cheaply. Perhaps, if it did have the velocity to cross the solar system in less than 7 days, it may be feasible to send a larger craft to Proxima Centauri and beyond within "somewhat" reasonable time-frames (certainly not thousands of years).


I believe something like this was proposed by Dr. Aurthur Kantrowitz? of MIT? back in the 1980s and I know it was given some serious study--though probably no serious funding. There is no reason to believe the basic concept would not work, though some of the details may need to be addressed.

Can we shoot something through the solar system in a few days? If a propulssion system is the only consideration then yes, we certainly can. But now should it encounter a grain of sand then it will be blown to shmitherines. And the shrapnel thus spewd out will go darting through the solar system like a cloud of buckshot--many of them on new and unexpected trajectories probably unknown till somebody gets one douzy of a flat tire or cracked windshield.

Would we use water as a propellant? I certainly hope not!!!!! In Earthside thinking water may be just about the most common and cheapest thing we know--but in Space water is one of the rarest and most precious things we know, far, far more expenssive than gold. It rankles me to no end when somebody suggests ice on the Moon could be used as rocket fuel. If you want to get off the Moon use a maglev system but leave the water for drinking. Hydrogen is a lot more common than water, so if we froze a block of hyrogen and zapped that with a laser it would be a much cheaper propellant.

I believe this system is proposed only for non-living cargoes as any living creature will be crushed by the sudden accelleration of gs unless a much lower system is used.

The laser could be used as electric cables are used here on Earth to transport power from a generating station near the sun to a bank of solar cells in the outer solar system. After a few minutes the purchased allotment of power will be recieved and stored and the laser will shift to the next customer. (Of course a solar cell is a solar cell and light is light and some spaceship is probably going to sneak up to the laser beam, stick their bank of solar cells into it, and steal a little free power, leaving the customer further down the line with less than what he paid for.)


The "deal killer" on this idea (and other laser power schemes) is the spreading of the beam (and power required) across long distances. For instance:
a beam generated by a small laboratory laser such as a helium-neon laser spreads to about 1.6 kilometers (1 mile) diameter if shone from the Earth to the Moon.
Even the most powerful beams we have (housed in a ten-story building the size of three football fields https://lasers.llnl.gov/about/nif/) couldn't power such a craft for any distance.


Quite a lot of research on this:


Generally it is over moderately small ranges, for example launching from earth to orbit. Some schemes use microwave energy instead of lasers. I have also heard it discussed for station keeping of arrays of satellites. Another very different and nearer at hand example could be to keep an aircraft permanently in flight without refueling.

Hydrogen is generally chosen as the propellant rather than water. There is some really basic reason for this that any 1st year rocket scientist could tell you, but I forget what it is. It could be the energy you waste converting water from liquid to gas, or maybe it has something to do with the advantage of using lighter molecules. Perhaps a hydrogen molecule moves faster at the same temperature/pressure giving you a higher possible ISP?.. sorry I dunno and am too lazy to look it up :)

I myself was most enthusiastic about this for launching to orbit: The rocket would not need to carry such explosive fuel, could potentially be airbreathing (eg just heat up the air) and could have more energy than any chemical reaction could provide anyway.
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