In his March 6 column, “Why are we in space?”, Cecil asserts, “you can’t reach the speed of light (or even get remotely close to it) with any technology now on the horizon due to the vast amount of energy required.” That must be true since Cecil said it, but it got me to thinking, along slightly different lines, which is why I posted this under “General Questions” instead of “Comment on Cecil’s Column.”
The energy problem is, in concept, soluble – someday we might have controlled nuclear fusion or even matter-antimatter annihilation. (See Robert W. Forward’s book, “Indistinguishable from Magic.”) The real challenge is more mundane and mechanical. What always amazed me about the spaceships in “Star Trek,” etc., is not that they can go faster than light, but that they can move at all without expelling any reaction mass.
At the present stage of our technology, space travel consists of giving the spaceship one huge PUSH and then simply letting it drift until it arrives at the desired destination; and that PUSH can only be produced by expelling exhaust gases, which have to come from fuel and oxygen the spaceship carries with it, which is why the space shuttle’s fuel tanks are larger than the craft itself. It’s not a true that a rocket “pushes against” nothing; it pushes against its own exhaust gases, and could not move if it didn’t. That’s a major limiting factor, because the initial push has to move the (remaining) fuel as well as the ship.
This also means a CONSTANT-BOOST drive is just about impossible – once you’ve got the ship moving with that one big initial impulse, you’ve used up most of your fuel and all you can do is drift. That’s how they got to the Moon. Every scenario I’ve read for a Mars expedition requires the astronauts to drift through space in free-fall until they reach Mars orbit – which is why they would have to carry at least a two-year supply of food – which, again, limits what can be done. But if they could accelerate towards Mars at a constant one-g, and “skew-flip” and reverse thrust direction at midpoint so they spend the rest of the trip decelerating – then they could reach Mars in weeks (I think – some other doper has to do the math); and they could make the whole trip at one-g and not lose any muscle tone, etc.
So far as I know, every theoretical space drive that has been seriously proposed – nuclear, etc. – depends on expelling reaction mass. I suppose you could make an exception for the Bussard ramjet – which still moves by expelling reaction mass, but the mass is interstellar hydrogen the ship picks up as it goes along. Solves the problem, but only half way – what if you want to travel through an especially “dry” region of space (assuming there is such a thing)? There’s also the “solar sail” but that obviously has limits too – the sun provides a weak vector force in one direction only, away from the sun. What if you want to go towards the sun? A sailboat can use the water’s resistance to “tack” back and forth and, effectively, sail into the wind, but the water has no functional equivalent in outer space.
But I’m sure a lot of you dopers read sf and cutting-edge speculative science stuff. Do any of you know of any proposed space drive, theoretically possible in terms of our current scientific knowledge, which could move a spaceship without expelling reaction mass? Which could move a spaceship at constant boost for a long journey? Or which could do both?