Is it possible to build a space drive that doesn't expel reaction mass?

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?

Not unless we find a way to violate the theory of conservation of momentum. And since that ranks right up there with the conservation of energy and second law of therodynamics, I doubt we would be doing it anytime soon.

Basically, the conseration of momentum means, in order to go somewhere, we can either throw a lot of mass very slowly in the opposite direction or a tiny bit of mass very very quickly.

m1v1 + m2v2 = 0.

The only way to get around it is with wrp drives and hyperspace and the like.

Wouldn’t that be, you know, dangerous?

But if you’re looking for a “clean” propulsion system, solar sail is about it. Sure, it would take forever to accelerate. But if your destination is another star system (I can dream, can’t I?), then you’ve got built-in brakes – the destination star imparts reverse thrust.

I suppose you could use an orbital mirror or laser to focus on your sail, and that way you could drive toward the Sun. But again, why would you want to?

What about somehow creating an artificial gravity field just ahead of the space ship, so that it’s constantly “falling”–like deliberately poking the rubber sheet just ahead of the billiard ball, yaknowwhuttamean?

That’s kind of how Star Trek’s warp drives worked, Doghouse Reilly. The idea was that by warping space artificially (which is what gravity does naturally) you could create your own acceleration. That doesn’t explain how The Enterprise was able to go faster than light, however.

A solar sail can tack toward the sun, sort of. At least it can move you to a lower orbit. A reflective sail always feels the force at right angles to the surface, so if you tilt the sail at 45 degrees you can get rid of some of your orbital speed. You are also pushed outward, but that just makes your orbit a long narrow ellipse instead of the near-circular one you would have by following the earth. Once the inner end of your orbit is as close in as you want, changr the sail position to round up the new orbit.

Of course the thrust would be very low with any sail of reasonable size, so the change would take a long time. I’ll leave the time calculation as an exercise for the reader.

This is not true. You can tilt the sail to change the direction of force. If you tilt it almost edge-on, the force will be almost 90 degrees away from sunlight. Also, to go towards the sun you don’t fire an engine towards the sun. You point your engine “forward” (in terms of orbital motion around the sun) to kill your orbital speed and let the ship drop towards the sun. So solar sails can be used to send a spacecraft from earth to Mercury. In fact it’s probably the best way to do it, because solar sails work better the closer you get to the sun.

As already pointed out, you have to use some kind of reaction mass for propulsion if you don’t want to violate conservation of momentum. We have no reason to suspect it’s possible. So the only way to propel a spacecraft without losing mass is to obtain reaction mass elsewhere. A Bussard ramjet is one possibility, but the problem is that the ram scoop acts as a brake and the captured atoms may not produce enough thrust to compensate. Another possibility is to shoot a laser beam from earth (or any stable platform) and use the beam to push a light sail. That’s about it.

As for artificial gravity, there’s no reason to think it’s possible. Relativity doesn’t allow for it.

The hitch with the whole warp drive thing is that since you’re warping space, you’re not really travelling superluminally, it just looks that way. Which is why the crew of the Enterprise never experienced any relativistic effects while hopping through the galaxy. Don’t ask me to explain it further, I don’t have a copy of the Enterprise Owner’s Guide lying around, but I’ve heard it discussed other places.

Additionally, you might check this out. It looks to have some interesting tidbits on it - and it’s on a NASA page, so I assume it’s fairly reputable.

Well, if mass increases as velocity approaches the speed of light, then shouldn’t you be able to expell an ion at, say, 99.9% of the speed of light and be ‘pushing’ against a much larger mass than just the rest mass of the original ion?

Star Trek ships could be sending out one atom a second with a reletivistic mass of thousand of tons, that would still fall in with the conservation of momentum, right?

A thought: with action-at-a-distance, a ship could conceivably push on a body of mass, eg. a star, with which it wasn’t in contact. This would violate relativity but not Newtonian mechanics, which makes it less unlikely in my book. (Unlikely meaning ‘virtually inconceivable’, but still …)

Star Trek warp engines don’t create gravity in front of themselves to pull them along. The warp field essentially squishes space in front of it. Think of a tablecloth that is getting bunched-up (hence the ‘warp’ in the warp drive). In essence they shorten the distance to their destination. Not all at once but instead of travelling a meter to go a meter they travel a centimeter to go a meter (just an example). That’s why they don’t all get creamed when they drop out of warp drive. Even their inertial dampeners that protect them from acceleration wouldn’t help dropping from superluminal speeds to a leisurely few percent of light speed in a few moments (technically they aren’t going faster than light speed which is impossible even in Star Trek…some science even they wouldn’t break for the sake of a script).

SCSimmons, what exactly do you mean by “action-at-a-distance”? I asked for ideas which are possible in light of our CURRENT scientific knowledge. Is there some kind of theoretical technology I’ve never heard of, which would allow a spaceship to “push against” stellar bodies that are light-years away?

Similarly, how is it possible, in light of current knowledge, to create a “Star Trek” style “space warp” along which the ship can (I suppose) slide like a marble down a chute? And what exactly is that “impulse power” engine the Enterprise used for sublight speeds? Apparently impulse power doesn’t involve a space warp, but what does it involve?

Several contributors to this thread have mentioned the potential of the Bussard ramjet. Does anybody know how much speed or, more importantly, acceleration you could reasonably expect to get out of a Bussard ramjet? This is an important question. In Robert Heinlein’s novel “Have Space Suit, Will Travel,” the hero, Kip, gets kidnapped by malevolent aliens who do not have FTL capability but DO have a constant-boost drive that can go at a steady 2 or 3 g’s. Kip calculates that if they have that, they have a star drive: One could get from Sol to Alpha Centauri in just a couple of years. Relativity would do nothing but help – the time-dilation effect would reduce the perceived duration of travel and reduce the amount of provisions, etc., you would need to bring along. Now, is there any possibility a Bussard ramjet could be used that way? To achieve constant-boost over interstellar distances? And, if so, how many g’s could you expect to get out of the drive?

The problem is the law of conservation of energy (and mass since emergy is mass). You would need to transfer a great deal of energy (hence mass) to that ion moving at 99.999% c somehow and you would lose that reaction mass.

IIRC Startrek impulse engines are fusion engines converting H to He and somehow doing it in a way that produces much more energy then is put in.

Photons have momentum, so a spotlight could be used for thrust without expelling reaction mass.


Energy is mass. It doesn’t matter if you throw out pure energy or mass out the tailpipe, your mass will decrease (incidently by the exact amount of the thrust, even though 100% of your thrust may not be exactly in the direction you want to go).

You use a spotlight powered by a battery, the battery will decrease in mass as light is being put out.

True, but the “push” you get from expelling photons is incredibly tiny. You gotta generat a lot of energy to get a minuscule push – thats why it’s better to use a solar sail and either use the light from the sun to give you the photon momentum, or else to build a big laser on a planet somewhere and use it to push on your sail – that way you dont have to carry a big laser, dump its excess heat, and worry about accelerating all that mass. Incidentally, the ship in Larry Niven’s story “The Warriors” is a photon drive of the sort you suggest.
For my money, give me a drive in which ions are accelerated to appreciable relativitstic velocitities. You don’t need 99.9% c to make this work. But it’s a way to get a lot of oomph without a huge reaction mass. The hitch, again, is hat you need a wonking ig amont of power. But I’ll bet your accelerator will be a lot more fficient than a laser, and not need as much cooling.

IIRC, the warp engine works by dissolving the space-time fabric in front of the ship and recreates it behind. Since the ship isn’t actually moving, it can exceed the speed of light.

There was an experiment I read about years ago involving a pair of gyroscopes on a rotating arm, and a guide that forced the gyros to move downward, and this apparently reduced the rest mass of the assemble compared to when the gyros were stopped. It was theorized that something like this could be used to produce a reactionless thrust. I can’t find it on the web, though. Anyone else recall this?

IIRC, the warp engine works by dissolving the space-time fabric in front of the ship and recreates it behind. Since the ship isn’t actually moving, it can exceed the speed of light.

Wow. Is there a record for the longest time between double-posts? If not, I nominate Urban Ranger!