Fun Science Fact #35: The EM Drive may be real. Take that, Newton!
So apparently, the EM Drive is maybe possibly real. NASA’s Eagleworks Laboratory has gotten a paper through peer review describing a vacuum engine that produces reactionless thrust. This appears to violate Newton’s third law (that whole “equal and opposite reaction” thing,) but there are some interpretations of quantum mechanics that might explain what’s going on. I don’t care so much about how it works, though. I’m an engineer. I want to know what we can do with it.
All known space drives (with the exception of solar sails) operate by throwing stuff out the back at high velocity. Thrust is then imparted to the vehicle in the same way that thrust is imparted to your shoulder when you fire a rifle. There’s a big drawback to this approach: pretty soon, you run out of stuff to throw out the back. As a result, there are very severe limits to how fast we can accelerate a space probe with conventional rockets. We can get out of the solar system (barely) but to reach the next nearest star at the velocities we can reasonably achieve would require a minimum of 50,000 years or so.
In order to do much better that this, we would really like a propulsion method that doesn’t require so much fuel, and which therefore allows us to work with much lighter probes—enter the EM Drive. It operates by bouncing microwaves around in a reaction chamber, which interact with quantum vacuum fluctuations in the blah blah blah blah. Bottom line: it generates 1.2mN of thrust for every kW of power you pump into it. What does that get us?
Well, this is actually a very small amount of thrust. Let’s assume we’re powering this engine with a 1MW fission reactor. These things are about the size of a trash can, and weigh roughly 500kg. Let’s assume also that we’re attempting to move something like the Cassini probe that we sent to Saturn a while back. Without fuel, that weighed about 2,100 kg. So, we have a total vessel weight of roughly 2,600 kg. Our 1MW reactor gets us 1.2N of thrust. How much acceleration does this produce?
Not much, as it turns out—0.00046 m/s2, to be precise. So, after an hour of continuous thrust, our probe would be traveling at 1.7m/s—a comfortable walking speed. This doesn’t seem all that promising, does it? However, because this engine doesn’t require reaction mass, we can leave it running as long as the nuclear reactor holds out. Marine reactors (the kind we put in submarines and such) can run up to 33 years without refueling. Leave our engine running for that entire time, and we’ve got our probe up to roughly 480,000m/s. That gets our travel time to Proxima Centauri down to only 2700 years! Don’t forget to pack a snack.
Okay, so that’s not great, but what if we scale up the power a bit? Thrust for this engine appears to rise linearly with the amount of juice we pump through it. So, replace our dinky 1MW reactor with the kind we use on aircraft carriers. Those are a bit heavier, of course (like 8,000kg instead of 500kg) but they also produce 700MW, which is quite a bit more power. Hook this up to our Cassini probe and we have a total vehicle weight around 10,000 kg, coupled with thrust of 840N (and yes, I know it’s a big assumption that thrust will continue to scale with power at these levels, but go with me on this.) Our acceleration is now 0.084 m/s2, giving us a maximum cruising speed of about 87,000,000m/s. Fun fact: this velocity gets us to Proxima in only 15 years! We can’t actually get to Proxima in that time, of course, because reaching this velocity takes us 33 years of continuous thrust. Also, we’d probably like to slow down at the other end of the trip. Zipping through the Proxima system at a significant fraction of the speed of light would make for a very anticlimactic trip. So, we accelerate for a while, shut down the reactor, then turn it back on and decelerate for arrival. At the end of the day, we’ve got a probe orbiting another star in about 40 years, give or take.
Again, lots of assumptions here. First, we’re assuming that our probe isn’t going to bump into anything during its trip, because a pebble at relativistic speeds has the kinetic energy of a nuclear bomb. Also, we have to make sure our Cassini probe has enough juice at the other end to get a signal back to us over four light-years. These are details, though. Also, on the upside, we could conceivably scale this up with multiple reactors and multiple engines to power something even bigger. Conceivably, something like this could get get humans to another star—I mean, as long as we don’t mind using babies for astronauts, we could possibly even get them there before they die!
Anyway, the bottom line is that the EM Drive, if real, is kind of a big deal. Stay tuned for further developments.
