Reactionless Drives

Fiction & Fact. Essay. 1900 Words, 10-minute read.

A NASA concept illustration of a warp drive ship, the IXS Enterprise. Mark Rademaker/NASA image.

By Ray Tabler

A staple of science fiction is the reactionless drive. That is, a means of propulsion which moves a space ship without the use of rockets or jets. As Matthew McConaughey said in the movie Interstellar, “Newton’s third law – the only way humans have ever figured out of getting somewhere is to leave something behind.” Every action has an equal and opposite reaction. This is how rocketry works. The boom goes one way, in the form of rocket exhaust, and the space ship goes the other, if everything is working right.

There’s a cruel bit of math called the rocket equation. I won’t torture you with the details. The executive summary of the rocket equation is that the faster you want your rocket to go, the more rocket fuel you need to achieve that speed. As velocity increases linearly, the fuel requirements increase exponentially. The whole business gets very expensive very quickly. See what I mean about cruel?

We live in the real world, at the nonexistent mercy of the rocket equation. Fortunately, rocket scientists have gotten pretty good at dealing with that constraint. That’s the way we got to the Moon. If we don’t get sidetracked, and we’re lucky, it’ll take us to Mars before long.

Hard sci-fi cleaves to the rocket equation, because that, and the parlor trick of a gravity sling shot, are the only proven ways to move about in space. But many science fiction readers will overlook a bit of cheating to move the story along. Which is where the reactionless drive comes in.

This essay discusses imaginary propulsion methods for writers and readers of science fiction, who for dramatic reasons need to sidestep the rocket equation and its merciless reign. Handwavium is the order of the day. Because I don’t actually know how to build any of these wonderful devices. If I did, I’d be hallway to Alpha Centauri by now.

Gravity Drive – If we could come up with a way to manipulate gravity, we could go almost anywhere. Not only would that allow flight in an atmosphere without wings, but a space ship could push, or pull, against the mass of a planet or star without need for rockets. Or, so the fictional theory goes. In Larry Niven’s Known Space series an aggressive alien species, the Kzinti, utilized a gravity drive to travel between the stars. The Kzinti gravity polarizer would go faster than light (FTL) but it got the job done. That is until a different alien species sold humanity an FTL drive, throwing a money wrench into the Kzinti plans for conquest. Which is not to say that your fictional universe can’t have an FTL gravity drive. Niven just didn’t choose to gift the Kzinti with one until it was too late.

Warp drive – A warp drive distorts the fabric of space time itself to move around. Star Trek Enterprise famously used a warp drive for traveling interstellar distances, with secondary “impulse” propulsion for shorter trips. (Impulse propulsion is some kind of fusion-powered plasma rocket.) The concept of warping space to move about originally came from a 1957 John W. Campbell novel, Islands in Space. Star Trek and other books and stories utilized the idea.

Ironically, Mexican physicist Miguel Alcubierre was inspired by Star Trek to investigate the possibility of such a device. And found a way to make it work in the real world. It involves creating a field which expands space time behind the space ship, and shrinks space time in front. That would propel the craft forward in a warp bubble, potentially faster than the speed of light (effectively) without violating the cosmic speed limit. Now, creating the warp bubble would require a lot of energy, more than we can manage to wield today. But the math and the physics say that it should work. Which is more than most of the other schemes covered herein can say.

The Enterprise relied upon antimatter to power its warp cores, which would provide some high-density energy, to be sure. Star Trek has an uncanny knack of predicting future technology. Desktop computers, cell phones, and computer tablets all arguably appeared on the show before in reality. A working Alcubierre drive, would continue that trend.

Hyperspace – As we presently understand things, an object with mass can get closer and closer to traveling at the speed of light, with exponentially increasing energy expended, but not actually achieve that velocity. A fictional dodge around that constraint is a detour through hyperspace. Hyperspace is some other space time, separate from our normal existence, wherein the distance between origin and destination is considerably shorter. Or, maybe the rules are different enough that effective FTL is allowed. Then the ship drops back down to regular old space, and the story continues.

Any number of science fiction universes have folded hyperspace into their universe-building. The Millenium Falcon, in Star Wars, in afflicted with a sticky hyperdrive, which fails to perform when drama demands. The FTL in Larry Niven’s Known Space eerily pinches viewports closed, as if they don’t exist. The human mind refuses to recognize that there’s anything outside the ship when in hyperspace. It’s dangerous to engage the hyperdrive in Known Space too close to a stellar mass. The shocking reason why is disclosed in one novel. But I won’t spoil the story. In Jack McDevitt’s Alex Benedict series, ships can get stuck in hyper space.

Worm holes – Traveling instantaneously from one point in space to another, distant spot is accomplished through worm holes. It might be argued that wormholes are simply very short trips through hyperspace. And I can’t say that’s not the case, complicated mathematics (which I don’t begin to understand) aside. I’ll let the people who (think) they know what they’re talking about debate the point. As a practical matter, in general, in science fiction stories, hyperspace-capable ships can engage the drive pretty much anywhere they want (beyond some hyper limit around stars or planets). Whereas, worm holes typically stay in one spot (with slow drifting in some universes), and have to be trudged out to in order to go through them.

Wormholes can be artificially formed, as in the Dune series. Genetically-modified, and drug-saturated, Guild Navigators “fold space” to jump from one star system to another in the blink of an eye. Or they can occur naturally, often due to higher dimensional forces between stellar masses. Ships in Jerry Pournelle’s CoDominium series engage an Alderson drive at points where the gravitational potential between stars allow jumps, then have to spend weeks or months crawling across the destination solar system to approach the next jump point.

Worm holes, whether they truly exist, are great for dramatic purposes. A worm hole is a strategic asset, similar to an oceanic pinch point like the straits of Gibraltar between Spain and North Africa, Malacca Strait near Singapore, or the Strait of Hormuz, through which a good chunk of the world’s oil passes. Control of conduits like that comes with real power.

There’s no rule which says that you can either FTL or wormholes. Many science fiction stories have both at once. Star Trek Deep Space Nine had both warp drives and a working worm hole to the other side of the galaxy. The Jolly pirates of Craig Alanson’s Expeditionary Force series use hyperdrive star ships to travel between nodes in an artificial worm hole network, left behind by a mysterious, elder race.

The Bussard Ram Jet – Still a slave to the rocket equation, the Bussard ram jet takes advantage of a loop hole. Instead of carrying fuel along, a Bussard ship uses an enormous electromagnetic field ahead of the craft to scoop interstellar hydrogen up and funnel that material into a fusion reactor, that squirts the exhaust backwards for thrust. The electromagnetic scoop would probably be tens of miles in size. There’s no reason a Bussard wouldn’t work as expected, as long as the hydrogen floating around between stars is dense enough to make it practical. But, we haven’t gone and built one yet. So, we don’t know. Oh, and the human race has to figure out fusion power first too. Piece of cake.

Solar Sails – The wind of particles sleeting off of a star like our sun in, relatively speaking, considerable. Coronal mass ejections periodically play havoc with communications and power grids here on Earth. If you can build a sail big enough, and light enough, that wind will accelerate a space craft to hundreds or thousands of miles per second. Still not anywhere near light speed, let alone FTL, but faster than anything we’ve built to date.

The physics on a solar sail vessel are pretty solid. We could build one today, given enough capital and the will. NASA has successfully tested the concept in orbit. But you’re still looking at a slow trip to even the nearest star, a generation ship or suspended animation (once we figure out how to do that) if people are aboard. Although, the velocity can theoretically be boosted (to some degree) by aiming a big laser at the sail to augment the solar wind.

Ion Drive – What you really get from rocket exhaust is momentum. Which means that even particles small in mass can be useful if they’re flung out the back door fast enough. An ion drive shoots ions out of its nozzle at significant fraction of the speed of light. That compensates for their tiny mass, and you can carry the ion supply in a very compact form, which allows for continuous thrust. Again, it takes a good long while to get up to speed with an ion drive, which limits the practicality for human-crewed ships.

Em Drive – I don’t understand the Em drive. I readily admit that. Supposedly, bouncing microwaves around in a resonant cavity will produce thrust without any input but electricity. If it works, the Em drive would be a game changer. Critics claim that results to date have all been false positives due to measurement errors. I’m not qualified to judge that. But I do recall an earlier controversy which seems to rhyme with the Em drive. The Dean drive was invented in the 1950s. It reportedly transformed rotary motion into linear movement, resulting in a decrease of weight when the whole thing was resting on a scale. Much testing did not produce the promised antigravity device. Physicists who examined the situation claim that what effects which were observed could be explained by friction with the surface the device rested upon. So, no gravity-defying break through. Best check out these reactionless drives carefully. It’s a tricky road to travel.

I will list references below or in the comments, for readers who want to explore reactionless drives in more detail than given here. Be advised, this particular rabbit hole is dark, and deep, and full of quicksand.

The Specter of Reality –Readers and writers of science fiction don’t have to wait for reality to catch up. It’d be pretty boring if we did. Advanced and FTL propulsion might never pan out. How real might any of these reactionless drive be? Depends. Beyond the theoretically plausible Alcubierre warp bubble concept, the physics is shaky, to put it mildly. But we don’t really know what the future might bring. Star Trek predicted cell phones, desktop computers, and the warp drive. So, betting against human ingenuity is a risky business. History is full of things that were impossible, until they weren’t.

END.

Reference Links:

Rocket equation https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation

Reactionless drives Reactionless Drives – Atomic Rockets

Gravity drive https://en.wikipedia.org/wiki/Anti-gravity

Warp drive https://en.wikipedia.org/wiki/Warp_drive

Alcubierre drive https://en.wikipedia.org/wiki/Alcubierre_drive

Hyperspace https://en.wikipedia.org/wiki/Hyperspace

Worm holes https://en.wikipedia.org/wiki/Wormholes_in_fiction

Bussard ram jet https://en.wikipedia.org/wiki/Bussard_ramjet

Solar sails https://en.wikipedia.org/wiki/Solar_sail

Ion drive https://en.wikipedia.org/wiki/Ion_thruster