|MadSci Network: Physics|
Hi, David. Good question. Seems odd, doesn’t it, that you could actually move things with light? Light is a form of energy, however, so if you can convert the light energy into kinetic energy (energy due to motion) then it makes sense. I’ll mention how it can be done, but I’ll also point out problems people have encountered – it’s a good way to extend an idea into how one actually applies it. Looking through the web for information, I’ve come across two different ways that people use light to generate motion of a vehicle. They are each based on using the light energy to ignite a propellant, which provides thrust to the vehicle. In other words, the ignition of the propellant generates a bunch of fast-moving molecules. By setting up the explosion to occur right next to a shield on the vehicle (capable of withstanding the nearby explosion), all the molecules which “bump into” the shield push the vehicle away from the blast. The two different methods refer to two distinctly different propellants – in one method the propellant is actually the air around the vehicle, while in the other the propellant is a layer of combustible material coating the shield itself. I find the first one interesting, if only from the standpoint that it’s neat to realize people can actually blow up enough air with light to move a real object. It’s done by making the shield into a highly-reflective mirror – actually a bunch of small spherical mirrors. Light energy can propagate freely through the atmosphere if it’s in a big enough beam. For example, think of all the solar energy passing through the air on a sunny day. If, however, you take a big magnifying glass outside, it’s easy to see the effect of the concentrated solar light – it’ll burn most anything. That’s the effect of the spherical mirrors on the launch vehicle. When a large laser beam from the ground hits the mirrors, the energy is focused down into a bunch of very tiny spots near the mirrors. The light is so intense in these spots that it actually ignites the air, creating the thrust needed to propel the vehicle. Using air as a propellant also simplifies the launch process: you get to use free fuel anywhere you go … almost. I say ‘almost’ because it’s pretty clear you can’t use this mechanism if you’re interested in putting something up into the vacuum of space. Without any air, well, you’re not going to get very far. The second method involving a propellant coated onto the shield is, in my admittedly non-expert opinion, probably more practical. I say this because 1) with tailored propellants you can certainly find a material that will combust at lower light energies (reducing the size of the laser you need to drive the propulsion), and 2) every molecule which blows off the shield will contribute to thrust, while only half the molecules from an air explosion will start off towards the shield to contribute to the vehicle’s motion. Although at this level the idea is a pretty simple one to communicate, the practical engineering involved with deploying a real vehicle is very complex. That’s because of several factors including (but not limited to): a) the laser power needed per pound of vehicle, b) keeping the laser beam focused and tracked onto the vehicle, and c) steering the vehicle. (a) Laser energy. As you might expect, the transfer of optical to kinetic energy isn’t all that efficient. In other words, it takes a lot of laser energy to impart even a little kinetic energy. For example, in upcoming tests announced at http://www.space.com/businesstechnology/technology/laser_propulsion_000705.html the program participants mention that they hope to send 50 gram objects 300 meters into the air with a 10 kilowatt laser. NASA and the Air Force speculate that with a megawatt-class laser (1 million watts) they could send about a kilogram weight into earth orbit. Now, peak powers of a megawatt are no problem for lasers on very short time scales, but a sustained megawatt averaged over minutes is. This restriction is serious enough that it’s unlikely anyone will field such a laser any time soon. Conclusion: a kilogram won’t be laser-sent into orbit anytime in the next decade or two unless they find a way to do it with less-powerful lasers. [Tech. note: the previous statement applies if (as I suspect) the megawatt will need to be contained in a high-quality spatial mode. Megawatt “flashlights” could probably be fielded in the near future, but their beam focusing performance would be so poor that I’m guessing they’d be inapplicable to this problem.] (b) Laser focusing & tracking. Not only will a high-power laser system need to keep pointing towards the vehicle as it gets accelerated through its path, it will need to continually adjust its focal range to keep as small a spot as possible at the target. At kilometer ranges, atmospheric turbulence and available beam-steering optics limit the minimum size of a laser spot to about a meter across or so. At tens or hundreds of kilometers out, the laser spot gets even bigger. These spot sizes are only achieved, however, if the transmission optics are continually adjusted throughout the vehicle’s flight. This adjustment must also account for wind and other atmospheric conditions in real time. As difficult as all this sounds, however, the importance of these problems in other areas of science has spurred solutions. It’s merely a matter of incorporating these tremendously complex (and expensive!) elements into the launch process. (c) Vehicle steering & stability. Here’s yet another problem – with the laser giving you a big dumb push from a single spot, how do you manage to steer the craft to where you want it to go? As an example, the earliest flight demos I saw (on a NOVA program or some such years ago) showed great results when the vehicle was guided by a wire. As soon as the vehicle cleared the wire, however, it tipped to one side or another off the laser beam and promptly fell to the ground. Do not judge the viability of the idea by this anecdote: the early tests were to measure conversion efficiency not steering. I bring it up only to spur the thought of, “Just how would you steer one of those things??” Other references: http://sfwrg.org/l002.html http://library.thinkquest.org/03oct/02144/text/propulsion/laser.htm? http://www.npl.washington.edu/AV/altvw21.html For a somewhat different take on the laser propulsion schemes: http://www.islandone.org/APC/Beamed/04.html
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