Sunday, November 8, 2015

What I Learned About Beam Riding in My Swimming Pool

The cost of propulsion for star travel largely consists of having to accelerate the unused fuel and propulsion matter. It’s bad enough to have to accelerate all the fuel and propulsion matter you are going to use for deceleration, but you also have to accelerate, at the early stage of acceleration, all the fuel and propulsion matter you are carrying that will be used in the later stages of acceleration. It is a diminishing amount, but the extra propulsion needed really adds up.

One bright idea to reduce this problem is to do beam riding. You send a beam of something and an antenna to reflect it backwards. Your momentum is mailed to you so you don’t have to carry matter along to shoot it out backwards and increase your own momentum. You still have to carry what you need to decelerate, but that can be a small fraction of what you need to accelerate, if you carry everything with you.

They call this beam-riding, which is actually an old combination of words that used to be used for a way of guiding a missile or torpedo to its target. You shine a beam on the target, and the missile sensor sees it and heads toward it. There are variations of course. But there never was a variation that involved powering a missile with a beam. That is something much newer, and obviously more exciting.

You can do it with photons or with protons. With photons, you just need a big mirror. Protons don’t reflect so well, but perhaps there is some electrostatic way to do it. If you capture the first ones you get the electrostatic charge anyway. Let’s just consider photons.

Suppose there is a big mirror on the back end of your spaceship, and your home planet has a giant laser or other photon source that it shoots at you. You reflect the photons somewhere, and lo and behold, you’re moving. Not exactly Star Trek but at least you’re moving.

You better know where you want to go, as changing your mind halfway through the travel isn’t going to happen. You brought deceleration fuel and propulsive matter, and you can’t just use it to change which star you want to head towards. This should be a solvable problem. You make your mind up once and for all before you head out from your home planet. You need to head out in the direction of the destination star, keeping yourself between the destination and the big laser at home.

I thought this was such a splendid idea that I decided to experiment with it, sort of by accident. I had a power failure at my home a little while ago, and it took hours for the power to be restored. All the clocks were off, and had to be reset. I thought I had done all of them, but I found that I had forgotten the pool pump timer. It was set to run in the dark night, so the pool would stay clean, but be available all during the day. I went to swim in the morning, and it was still running. A hah!

There are jets of water that come into the pool around the edge, with the drains elsewhere. I was floating on the top of the water, and aimed at a jet and put my hands in front of the jet. Just like a beam rider, the jet of water pushed me away. It became extremely obvious that the jet was not cooperative. It wanted to get my hands away from it, but it didn’t care about the rest of me. All the mass was on the destination side of the reflector. Mostly it rotated me, when I just stayed stiff and didn’t try to adjust my direction.

Beam riding has not one, but several instabilities that make design a little more complicated than just having a reflector. The first one happens if you are foolish enough to put the center of mass of your starship ahead of the center of thrust, which is what I am calling the average position of where the momentum gets reflected. It is somewhere inside that parabolic mirror you are using to reflect photons. If the center of mass is ahead, you have an unstable situation from the get-go, and the ship will rotate. You are no longer pointing toward your destination star.

This is obviously easy to fix, you just have to put your starship mass behind the reflector. Maybe you build it in a ring around the outside, but that means you have a very, very, very long circular spaceship space. Maybe with a huge ship you could spin the reflector and make artificial gravity. The other choice is to stick it somewhere inside the reflector, actually in the center of the photon beam. You would like to make the photon beam as intense as possible, so you get your acceleration early on, and can keep the reflector as small as possible. Hopefully, it wouldn’t be intense enough at the outset of your voyage to fry the innards of your internal space. Certainly something can be done about starting a bit farther out. There is an obvious tradeoff between momentum collected in close and momentum collected far out, but most likely it is only the very shortest distances where there might be a heating problem, if there is any at all. Suppose you solve this problem.

Now comes two other instabilities. One is reflector angle. If it tips a bit to one side, there will be a gradual effect to increase the tilt. If you have a tilted reflector, the photon beam is going to reflect to one side, and you are going to someplace other than your destination star. Not a good idea to tilt. Some sort of gyroscopic control is needed and some thrustors along the side are also needed to put that reflector back into a perfectly aligned direction. So you need to carry a smidgen of fuel and maybe four thrustors to keep you aligned. If the reflector is not too strong, you might have to spread out the thrust in more places than four, or else you could build some structure to hold them. Or you could put the thrustors on a structure connected to the internal ship space that you are hanging right there in front of the beam. By the way, that thing is going to act to tilt the reflector as well, so you need to make it nice and circular, as well as reflective.

The last instability comes at the last. If you don’t stay along the centerline of the beam, but somehow get a little bit off to one side, the gradient in photon intensity is going to push you further to the side, as well as tilt you. So you need some side thrust to put you back in the beam, or you need to send a message to the ground crew, or space station crew, manning the giant laser to redirect it a bit toward where you are. This is fine, except you were supposed to stay on the line from the giant laser to the destination star. Maybe having your own side-directed thrustors, balanced around the center of mass of the whole ship, is the better idea.

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