2013-08-18

08-18-2013: BSinSF - Sublight Propulsion

Quick Peacekeeper update: My wife is about 1/3 of the way through the manuscript making very good change suggestions as she goes.

This week's BSinSF (Bad Science in Science Fiction) post concerns sublight propulsion systems. Faster than light (FTL) propulsion systems don't exist and therefore any novel that uses them is in violation of real science. But, then again, the Star Trek communicator was pure fiction in the 60's—now we have cell phones.

If you think about it, every spacecraft propulsion system in use today is based on Newton's third law which is usually stated as: "For every action, there is an equal and opposite reaction." Basically, we throw mass out one side of the spaceship and the ship moves in the other direction. Simple. But this simple concept creates some major problems for the science-minded SciFi writer.

The first has to do with the fact that every spacecraft ever built must carry a supply of mass (correctly referred to as propellant) that will be expelled to produce thrust. This propellant is part of the mass of the ship and must itself be accelerated. It seems like a colossal waste and it severely limits how fast a ship can move. Look at what it takes to put something into an orbit only 230 miles (370 kilometers) above the Earth. Rockets are ridiculously inefficient and it takes about a kilogram of fuel to put each gram of mass into space.

Another problem that is often forgotten is the fact that once you get moving, you will eventually have to slow down again. This at least doubles the amount of propellant you need to carry. Then there's the question of accelerating all this propellant—this is what the fuel does. For chemical rockets, the fuel is the propellant but if your ship is nuclear powered your fuel and propellant are separate items.

The bad science part of all this is illustrated in the following example. Let's say you are writing a story and you have a fleet of ships moving around a star system. Wanting to sound like you know what you're talking about you state that the ship uses a 'fusion thruster' or a 'plasma engine'. Your warships, of course, are large, impressive vessels armed with death-dealing weapons of incomprehensible power. They accelerate out of Earth orbit to meet the incoming fleet of enemy ships that have been detected crossing the orbit of Mars. A few hours later, the battle begins with ships maneuvering around each other to gain the slightest tactical advantage. Sounds great—right?

Let's run some very simplistic numbers: The distance between Mars and Earth varies between 54.6 and 225 million kilometers. Let's assume they are 100 million kilometers apart at this point in time. To make the numbers even simpler I'm going to ignore the relative motions between Mars and Earth as well as other simplifications. Let's assume your fleet accelerates for the entire trip, spinning around at the half-way point so by the time you meet the enemy fleet your relative velocity is zero. That means you're accelerating for 25 million kilometers. Your ships have some really bad-ass engines so they can accelerate your vessel at 1 gravity. The math says it will take you just under 40 hours to meet the enemy, not just a few. But wait, we should figure out how much propellant this will take.

The retired space shuttle masses about 68,500 Kg. Using the above scenario as well as the theoretically best engine possible and some equations available on the web I've determined that the amount of propellant your ship needs is in excess of 1.2 million kilograms! And that's just to get to the scene of the battle. After the fight is over the fleet will be stuck because they forgot to save enough propellant to get back home. Cutting down the acceleration will save on propellant but will lengthen the travel time. If you stick with pure known physics your novel is going to read like the adventures of Columbus.

So what is an author to do? Cheat. I hate to say that because I'm a big fan of getting the numbers right. But if you want to write a futuristic space opera with star-spanning empires and battle scenes, the only alternative is to cheat. If you do, at least don't blatantly violate the known laws of physics. The best way around this is to not explain the workings of your propulsion system at all. It's just there.

In the Galactic Alliance series, I utilize a reactionless drive. It latches on to the fabric of space and moves the ship with pure energy alone. But there are a vast number of problems with reactionless drives. Luckily, unless you're well-versed in relativity and have a firm understanding of the conservation of momentum, you won't know just how bad this idea violates the laws of nature.

If you want to at least try to keep your numbers right, do the research and run the math. Don't say your megaton battle cruiser fires its fusion thrusters to travel 10 billion kilometers to do battle with the enemy without mentioning the fact that it has to slow down and it has consumed a whopping terra-ton of propellant. Sometimes, you have to cheat science by creating your own—just make sure the rules it uses are consistent and don't blatantly violate known science.

Here are some more quick science notes that seem to be always forgotten:
·         Unless you have super-science your ships will have inertia. Once they get moving, they have to expend energy to stop.
·         Traveling for long times at near-light speeds will affect how time passes inside the moving ship.
·         Spaceships are not airplanes and will never fly like one. Forget all about how Star Wars and Battlestar Galactica ships operate in space. There is no up or down—ships can spin on their axis while continuing to travel in a given direction.
·         Orbital dynamics are tricky and you should at least be familiar with how a ship behaves in orbit.


For further reading (and there's a ton of it here that will make your brain hurt) I must recommend the Project Rho website. There is so much information on this one site that you can spend all week going through it. I strongly suggest you do so. I also have an Excel spreadsheet (soon to be a Java program) that does a number of these calculations for you. You can find it at the bottom of my web page at http://www.dougfarren.com/