I saw this headline and was immediately interested: “'Impossible' rocket drive works and could get to Moon in four hours”
“Interplanetary travel could be a step closer after scientists confirmed that an electromagnetic propulsion drive, which is fast enough to get to the Moon in four hours, actually works.”
I have to admit that I had to google what an EM propulsion drive is and how it works. No offense to the author of this article, as her explanation fulfilled the requirement of making the reader think they understand, but I needed a little bit more than the following: “It produces thrust by using solar power to generate multiple microwaves that move back and forth in an enclosed chamber. This means that until something fails or wears down, theoretically the engine could keep running forever without the need for rocket fuel.”
Actual EM Drive Prototype
Upon further inspection, the RF resonant cavity thruster, if it actually works, could deliver on these grand boasts of interstellar travel. It’s a pretty cool concept that I’m pretty sure violates the law of conservation of momentum, but that just makes it more exciting!
Some have even stated the following: “Even the closest star system, Proxima Centauri, can be reached within only 30 years as opposed to the a few centuries with other technologies such as Nuclear Pulse Propulsion.”
The real problem with this system has to do with the creation of microwaves, which is not done (directly) with solar power. Rather, this is done with a magnetron that is powered from good ol’ fashioned electricity, just as your microwave oven is. The logical assumption that Sarah Knapton, and many other people, make is that if something needs electricity and it’s in space, you can just slap some solar panels on that bad boy. I mean, the Sun’s in space so there’s plenty of sunlight up there, so let’s use it! Am I right?!
Well, you’re wrong. In order for solar panels to do their thing, which is convert photons into electrons/holes, it must be provided photons. Only a small percentage of the total power emitted from the sun actually hits an object in space some finite distance away. Here on Earth, we get about 1,400 W/m^2 worth of power in solar irradiance. However, the farther you get from the Sun, the fewer photons hit you. By the time you get to Mars, you’re down to less than 600 W/m^2 of available solar power, less than half here on earth. And that’s just to Mars. At Pluto’s distance, you wouldn’t even get 1 W/m^2. (Check this out if you'd like to have some fun with these numbers.)
But is there enough power out there to get us to the nearest star? Not even close. Nearest star: Proxima Centauri, 4.2 Light Years away. That’s about 4 x 10^13 kilometers. Let’s assume the efficiency of the solar panels we’re going to use on our trip is unaffected by the fluence of photons (even though in real life, it is not). Let’s also assume that we’re going to use multijunction cells of InGaP/GaAs/InGaAs that can achieve efficiencies of nearly 38%. And let’s use panels the size of, oh I don’t know, a football field (~5350 m2). Now, the EM Drive needs 2.5 kW to apply the thrust, but some think it will eventually only need around 700 watts, so let’s use that. And… We’re OFF!
On this trip, we would get about .0292% of the way to our destination before the sun could no longer provide enough power for the microwave generator. So, while the mode of propulsion may somehow get around the law of conservation of momentum, the power source can’t get around the first law of thermodynamics: cannot create energy out of nothing.
So if this propulsion system can produce net thrust, interstellar travel would still require a source of energy for the engine system. As stated in the article, the non-reliance on heavy fuel as a source of energy is really why this mode of propulsion for interstellar space travel is interesting in the first place. Like many other systems, the EM propulsion drive is very interesting scientifically, but will probably not deliver on the headline-grabbing possibilities.