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Where we're going, we don't need rockets.

Wednesday, 23 November 2016 12:59PM RantsScience

Let's be honest. The EM Drive is probably still wishful thinking.

an army of cubesats!

So far in human history, every method of propulsion ever found has involved some application of Newton's 3rd law. That's the famous one - "For every action, there is an equal and opposite reaction." Simply put, what this means that if you want to go somewhere, you have to push off something. If you want to take a step forward, you have to push back on the floor. If you want to drive forward, your tires have to push off the road. If you're flying, your engines - and wings - are pushing off the air. And in the vacuum of space, lacking anything else to push off, you push back on your own stuff which you brought up there with you.

The big problem with the EM drive is that it's not pushing off anything. What it's apparently doing is generating some microwaves, bouncing them around inside an appropriately shaped chamber, and hoping. At no point, as far as I can tell from skimming the paper, does anything actually leave the chamber. It seems to be a closed system.

The problem with that is that while yes, these microwaves can impart a force, in this kind of setup it doesn't seem like they can impart a net force. If the microwaves don't actually leave the chamber, they're going to keep bouncing off, and imparting a force to, both ends of that chamber. In Newtonian terms, it's like pushing off something while at the same time trying to hold on to that thing so you don't lose it. You aren't going to go anywhere.

And while there is a lot of "relativistic" and "quantum" being thrown around, this is the big issue with the EM drive. It's trying to get thrust, without throwing propellant. It's trying to get movement, without any reaction. It's breaking not just one, but several, fundamentally important laws of physics.

Which is not to say that such a thing can't happen - but we really need way more than one pre-publication paper to say that it's "confirmed". There are much better breakdowns of what, exactly, the EM drive is supposed to do, and why it probably isn't doing it, at Ars Technica and on Scott Manley's video on the topic.

The fact is that there are a bunch of pretty kickass new propulsion methods floating around out there right now which do obey the laws of physics, and which actually deserve the media attention the EM drive is getting.

The Electrospray Thruster

ion electrospray thruster

We've had ion thrusters before. They err on the velocity, rather than the mass, side of conservation of momentum, using electricity to accelerate a little bit of mass incredibly fast rather than a lot of mass merely very fast. This one is cheap, and more importantly doesn't use superheated plasma, which makes it actually accessible to people outside of major space programs. It uses little blocks of liquid salt instead, spitting out a stream of ions. It's a prime candidate for something like a Cubesat.

(Cubesats are tiny, rad little satellites, which piggyback into orbit on other satellites' launch vehicles. It's a good way to do cheap space science without investing in a launch of your own.)

The reason electrospray thrusters are so cool is because - the creators estimate - some variant of electrospray thruster could have the capability to take a cubesat out of Earth's orbit and into the rest of the solar system. This would be huge. Right now, we see universities and high schools working on cubesats, but they're limited to Low Earth Orbit, at whatever altitude their rocket ends up. With this? They could end up anywhere in the solar system.

Magnetorquers

magnetorquer

As cool as ion engines are, they're still essentially rockets. They throw stuff out the back to go forwards. But the microgravity and vacuum conditions of space open up a whole bunch of unconventional methods of moving your spacecraft around.

One of the weirdest is a device called a magnetorquer. This isn't propulsion so much as attitude control, but how it works is pretty remarkable. By extending a rod or wire some distance from the spacecraft and running a current through it, you can create an electromagnet strong enough to interact with the magnetic field of the earth itself. With a couple of these running through different axes of your satellite and modifying their field strengths, you can turn in any direction you like without expending any fuel.

Which is not to say that you're getting something for nothing. There still needs to be pretty significant energy dumped into those magnets. And Newton's third law still applies - except that your reaction force is being applied to the planet, imperceptibly changing its own motion, through its magnetic field.

Solar Sailing

solar sail

But the coolest not-rocket of all is The Planetary Society's Light Sail project.

The Light Sail is exactly what it sounds like.

It's a giant sail, propelled entirely by the power of light. No, it's not a solar panel generating electricity. It's actually pushed along by sunlight, exploiting a phenomenon called radiation pressure.

Basically, whenever light bounces off something, it loses a little bit of its momentum. The light doesn't slow down, since it's always travelling at the speed of light. But unlike the EM drive, this doesn't violate any of the laws of physics. Rather than coming from the light's velocity, the energy comes from its frequency. It reflects off the sail very slightly redder than when it arrived. The result is a tiny bit of energy imparted to the side of the sail facing the sun which, in the vacuum of space, creates a tiny but measurable amount of acceleration.

Here's the best thing about solar sails though. As far as your payload is concerned, that thrust is totally free. You don't need any propellant, just a big enough sheet of shiny mylar. The applications of this are pretty astounding. For a small enough payload, we could eventually sail anywhere in the solar system. For a larger one, you're looking at massively extending the life of satellites.

Plus, it looks really cool.

What these have in common is that they're everything the EM drive claims to be. They are cheap, easily deployable, and require minimal reaction mass - always a plus, since carting stuff to space is expensive. The difference is that they work. Provably. Experimentally. Operationally. They're out there letting us do cool science right now - and that's worth a thousand curiosities on a thousand workbenches.


More reading:

The latest EM drive paper.
http://dx.doi.org/10.2514/1.B36120

The Planetary Society's Lightsail Project
http://sail.planetary.org/

The Pioneer Anomaly - an interesting case of radiation pressure.
http://en.wikipedia.org/wiki/Pioneer_anomaly

NASA feasibility study on interplanetary cubesats
https://www.nasa.gov/directorates/spacetech/niac/staehle_interplanetary_cubesats.html

The MIT paper on electrospray thrusters.
http://spectrum.ieee.org/tech-talk/aerospace/satellites/ion-electrospray-engines-give-a-boost-to-tiny-satellites

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