An engine that can conjure thrust from thin air? We speak to the designer

Chatting to Anmol Taploo about the race to develop tech for satellites

Interview Will satellites be capable of generating their own thrust with propellant created out of thin air one day? Scientists at the George Washington University (GWU) and Princeton Plasma Physics Laboratory reckon so.

They have been awarded more than $1 million from the US government boffinry nerve center DARPA to build prototype air-breathing electric propulsion (ABEP) engines demonstrating the technology is possible to drive satellites with electric power in very low Earth orbit. That electric power could come from solar panels, nuclear sources, or batteries, for example.

Here, The Register speaks to Anmol Taploo, a PhD student at GWU, who is working on a novel way of generating plasma for these futuristic engines [PDF].

The Reg: The idea sounds like something out of science fiction. Where did it come from?

A.T.: The concept of ion engines is pretty old. I think during the '60s there was some talk about not having to carry propellant or fuel with you to space and just utilize [local] resources. For example, let's say, we can utilize water as a fuel on the Moon. What if we could utilize metals as propellants or air for fuel? There were all sorts of theories, and the recent work within the last two decades is when the interest started to grow more.

The Reg: How did you end up working on this?

A.T.: When I started my PhD, I didn't know about this field. My background is in aerospace engineering, and more toward working on gas turbine engines and a little bit about spacecraft. I had some background with aerodynamics. I heard about air-breathing plasma engines, it sounds really cool, interesting, and something that could be potentially novel. So [my PhD advisor] said: Why don't you do this as a project? So for the last five to six years, this has been my PhD topic, which eventually led to the DARPA award, which is known as the Charge Harmony project. We applied for that solicitation and we won the $1 million award.

The Reg: Can you explain how ABPEs work?

​​A.T.: Air goes inside an air-breathing plasma engine and there's an electron source that emits high-energy electrons. Those high-energy electrons interact with the incoming air and ionize the particles; it's a chain reaction called ionization. Then they will be excited and emit light that glows.

Plasma is created from the charges, the positive ions, and the negative electrons. Now you have an axial electric field, and you have a perpendicular magnetic field. The charged particles (ions) get accelerated by the Lorentz force to generate thrust.

The Reg: What are some of the benefits of using an abundant and sustainable energy source to power spacecraft?

A.T.: You already have air in the atmosphere, so if you could use that as a fuel that means that you don't need to carry propellant tanks. You can have a simple [spacecraft] design and your launch cost reduces. The lowest launch cost is SpaceX's Falcon Heavy, it costs about $1,500 per kilogram. Say you're using what, like, three or four kilograms of fuel and have some additional weight so just imagine how much money you're saving per launch per satellite.

The Reg: How low do you need to fly satellites with ABEPs to get enough air?

A.T.: I would say we are the only ones who are aiming for very low orbits, at about 70 to 90 kilometers. These kinds of orbits are known as self-cleaning orbits, or self-deorbiting orbits.

The Reg: You mentioned that you will eventually start a company. What commercial advantages are there to having satellites orbiting closer to Earth?

A.T.: There are three strong business cases with this technology. One is extremely high quality imaging, because you are tens times closer to the earth compared to geostationary satellites in low Earth orbit. You have very high resolution imaging and that's really important. Disaster management, military defense – that's a huge market for sure. Second is communications.

The third and the biggest problem is space debris. Because you're orbiting at low altitudes you have drag. So let's say your satellite goes out of control – I'm not talking about fuel anymore, because fuel is infinite –  it will automatically deorbit because of drag in the altitude and burn upon reentry, so you're preventing formation of space debris.

The Reg: What is your goal in the DARPA Change Harmony project?

A.T.: The next milestone for the DARPA contract is to achieve a thrust-to-drag ratio greater than one.

The Reg: Is this technology only useful for satellites orbiting Earth?

A.T.: No, we have shown that it operates with multiple gasses, so it could potentially also be used for different planets. It's not just for Earth, it has potential to be used for spacecraft orbiting Mars, Venus or maybe Titan-like planets at very low altitudes.

The Reg: When do you think we might be able to deploy this on a real satellite in space?

A.T.: My goal is the next five years. I might be too optimistic. ®

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