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Microwave Power Beaming May Launch Space Planes into Orbit

A spaceplane that uses ground-based microwave power as an energy source could be a very efficient way of launching satellites

3 min read

Evan Ackerman is IEEE Spectrum’s robotics editor.

Microwave Power Beaming May Launch Space Planes into Orbit
Image: Escape Dynamics, Inc.

A big part of the reason that it’s so expensive to send objects into space is that in order to get them there, we currently use messy, chemically-powered rockets that shed pieces all over the place on their way into orbit, and then smash themselves to bits on their way back down. A much more cost-effective (and elegant) solution would be a reusable single stage vehicle that goes up and comes right back down intact and ready to be refueled and reused.

To accomplish that, a more efficient source of power is needed. Rockets that have to haul their own fuel and oxidizer just aren’t going to cut it. Escape Dynamics thinks it has a solution in the form of a spaceplane that can launch vertically and make it to orbit in one shot, and is powered entirely by microwaves beamed from the ground.

The new spacecraft’s answer to conventional rockets’ fuel tanks is a big tank of hydrogen. And rather than extracting the hydrogen’s energy via a continuous controlled explosion like a conventional rocket does, Escape Dynamics is heating the hydrogen directly using a heat exchanger powered wirelessly by microwaves. Essentially, the spaceplane acts as a big microwave absorber targeted by a bunch of highly focused microwave emitters located on the ground. The microwaves heat the spaceplane, which heats the hydrogen, which gets fired out the back to provide thrust.

The key here is efficiency: Since everything that the spaceplane is carrying is pure propellant mass, and the energy is being transmitted from the ground, you get a lot more oomph. Specifically, Escape Dynamics is talking about specific impulse (for rockets what liters per 100 kilometers or miles per gallon is for cars) somewhere between 600 and 750 seconds (or better) with a propellant mass of around 75 percent, as opposed to about 90 percent for a conventional rocket.

Escape Dynamics’ concept video above is suspiciously snazzy, but they’ve put together some technology demos as well:

What we’re wondering at this point is whether the microwave power transmission is really going to be a viable to way to power a spaceplane. Setting aside the fact that a square-kilometer-size array of microwave emitters seems a bit dangerous (I’d hate to be a frozen burrito anywhere in the vicinity), it’s unclear how efficient this is actually going to be. The only actual number that we’ve been able to find is this one:

“The heat exchanger was designed to capture more than 90% of the incoming microwave energy and heat the propellant flowing through it towards the nozzle.”

That’s reasonably efficient, but it’s not a measurement of how much energy is fed into the system; most notably, we don’t know how much power is lost between the emitter and the receiver, but it’s probably a lot. Back in March, we wrote about some long-range microwave energy beaming experiments by JAXA and Mitsubishi. Having asked the same question of those researchers, we were able to find that as of last year, JAXA expected a 1.6-kilowatt microwave beam to yield a rectenna output of about 350 watts from a 50 meter test. By 2018, they’re hoping to have a small satellite transmit a few kilowatts from orbit to ground, but that’s a much easier direction to go in. 

Whatever the efficiency turns out to be, though, the point is that microwave rockets will use electricity, and that electricity can be harvested from renewables over a period of time and stored for use when it’s needed. So even if it’s very inefficient, it’s not at all inefficient where it matters, which is on the spacecraft itself. : that efficiency remains constant, because it’s just propellant (reaction mass), not fuel (energy storage).

From the sound of things, this launch system isn’t intended for heavy lifting. The maximum payload that it’ll be taking into orbit is just 200 kg. And the development won’t be cheap, either. It’s estimated that it will require about a billion dollars to take the project from its current state to a prototype spaceplane in orbit. The immediate next steps for Escape Dynamics researchers are: seeing if they can use microwaves to power an engine over a few hundred meters outdoors, and then attempting microwave-powered drone flights. As for the spaceplane itself, if everything goes according to plan (which nothing ever does), it could enter orbit as early as 2020.

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