Friday, December 28, 2007

Defending Star Wars

I've been reading Insultingly Stupid Movie Physics by Tom Rogers. Much if it is pretty good but I think that he is pretty hard on Star Wars. When ranking Wars vs Trek, he gives Trek the better grade, saving his worst grades for Star Wars. The thing is that there are major flaws in his figuring.

Rogers devotes a section to why it will not work for a large space ship (such as a star destroyer) to shoot a smaller ship (such as an X-Wing fighter). He points out that in WWII, a battleship could shoot a kamikaze or dive bomber and, if they hit it far enough out,  the plane would fall into the ocean. On the other hand, if you shoot apart a fighter in space then the parts will continue on and hit your spaceship.

I have a lot of problems with this entire section. The first one is that shooting down a plane seldom causes it to break apart. More likely you will damage control surfaces or cause a wing to lose so much lift that the plane goes out of control. Or you might kill the pilot. I mention this because Rogers makes similar complaints about movies.

My second problem is the mechanics of space warfare. You are dealing with three dimensions and vastly different relative speeds. A battleship cannot take evasive action because of a combination of its mass and the physics of moving through water. Ships can really only move in one direction because of water resistance. Spaceships have no such constraints and could conceivably change course much faster. Not that you see this happen in Star Wars. You do see it in Star Trek but they do it wrong, swooping like an airplane.

Anyway, chances are that the ship being attacked is under power and therefore accelerating. That means that the fighter must aim for where the spaceship will be. There are two ways of doing this. The fighter can do one short, strong burn (acceleration) or a longer, slower one. If you shoot a fighter while it is accelerating then it will fall short.

Rogers ignores this and seems to be going from the assumption that the fighter is not under power and will strike the larger craft through inertia. I will confine myself to that assumption. So you shoot a fighter and it bursts apart. Will the pieces continue straight on? This is not likely if it exploded. In space, every action  has an equal reaction. If a ship is hit with a projectile or it blows up for some reason then its trajectory will be altered.  At long distances it doesn't take much deflection to make a projectile miss its target (the book has a section on this).

Let's assume that you didn't use a projectile to shoot the fighter, you used a laser or other energy weapon that carved the fighter up into pieces without any impact. What happens next?

Rogers' assumption is that the pieces of the fighter will do as much damage as the fighter as a whole. This is an unfounded assumption. By slicing up the fighter you have converted it from a slug into a load of shot. While the two have the same mass, they do not have equal penetrating power.

Mythbusters proved this once with a frozen chicken. The myth involved testing the impact resistance of windshields of high-speed trains. The French borrowed an air cannon that can launch a chicken at high speeds but none of their windshields could withstand the impact. They asked what they were doing wrong. The answer was "first thaw the bird". The Mythbusters were testing to see if a frozen bird had more penetrating power than an unfrozen one.

This took several tests. At one point they used a high-speed camera and determined that both birds expended all of their kinetic energy int he same amount of time so they must have the same penetrating power. Fans objected and they ended up gluing multiple panes of glass together and shooting the bird at that. The unfrozen bird penetrated several sheets but the frozen one went through all of the sheets of glass. The conclusion - a frozen bird penetrates better.

The same will be true for a fighter that is structurally intact versus one that is in pieces.

But none of this really has much to do with Star Wars. Rogers singles out Star Wars as a bad example without specifying which scene offends him. I can think of four extended space battles (two in the original Star Wars, one in Return of the Jedi, and one in Revenge of the Sith). In three of them the target is too large to be seriously affected by the impact of a fighter. The object is to keep the fighters from delivering their payload.

The very first space battle is the only one where a collision would be disastrous. This is fought between the escaping Millennium Falcon and some tie fighters. The tie fighters are making strafing runs and the Falcon is dodging which invalidates the entire argument.

So why pick on Star Wars anyway? It has more to do with reputation than what you actually see on the screen. Trek had advisers who were supposed to keep things honest and sometimes wrote entire episodes around some obscure theory of physics. You can do that when you have hundreds of hours of programming. Star Wars is limited to a half dozen movies (I'm not going to count the games, books and other spin-offs for either).

In fact, one of the most accurate accounts of how physics actually work in a space battle is in a Star Wars novel - The New Jedi Order: Destiny's Way by Walter Jon Williams. Williams also wrote the Dread Empire's Fall trilogy which has the physics of large-scale space battles as a main plot point. I recommend this to anyone who wants to know how it will really be done (assuming that space battles ever happen).

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