Re: How does the corona ion engine work? What are the involved principles?

Wow! I hadn't heard of either Sky Stations or corona ion engines before I read your question, but it's a neat idea. For anyone else reading this answer, Sky Station International wants to put a solar-powered blimp in the stratosphere above major cities. They'll be able to offer wireless portable Internet and voice communications; something like a cross between cell phone and satellite communications.

The problem is that the winds in that part of the stratosphere can reach 30 meters per second (~60 mph): the blimp has to fly at that speed 24 hours a day, forever, to keep from being blown away from the city. You need an engine to provide thrust, and The Sky Station people want to use a "corona ion engine" for that purpose.

My first reaction to this was "No way!" Ion engines work by using electrostatic charges to accelerate ions (atoms with electrons ripped off) the same way your TV set accelerates electrons. The ion engine then lets the ions fly out the back of the engine, providing a reaction thrust like a rocket. They're usually designed as highly fuel-efficient but very weak thrusters for spacecraft: fuel efficiency is high because the ions leave at very high speed, but they produce only millinewtons of force (hundredths of an ounce). Also, they only work in the vacuum of space and require huge amounts of electrical power.

But apparently larger ones can be made: the Sky Station web site doesn't give many numbers, but the estimates I've made show that it could be done. The drag force on a blimp 25 m in radius flying at 30 meters per second at an altitude of 25 kilometers is roughly 2000 newtons (90 pounds force); I assumed a drag coefficient of 0.1. Since power = force * velocity, you need to supply 58 kilowatts (75 horsepower) of mechanical power to keep the blimp moving. The Sky Station people cover the upper surface of their blimp with solar cells: 150 kilowatts of electricity are available to power the engines. So if the engines have an efficiency of 40%, they could keep the blimp running on solar power alone. How could you turn this electricity into thrust?

So now we come to your question: what exactly is a "corona ion engine"? I spent several hours doing an extensive Web and scientific literature search, and found only one article:

Christenson, E. A.; Moller, P. S. "Ion-neutral propulsion in atmospheric media". AIAA Journal, vol 5, no 10, p. 1768-73.

I haven't had time to read the article, and the abstract is kind of vague. It looks like you use a very strong electric field to rip electrons from air molecules entering the front of the engine, thereby ionizing them. This is the "corona effect", and is what makes the air around high-voltage sources glow pale blue, and causes St. Elmo's Fire and the beginnings of lightning bolts. Now that you've ionized the air, you can use electromagnetic fields to push the ions rapidly out the back of the engine. So the whole thing looks like a jet engine, but with zero moving parts, so it's very reliable. I suppose a clever design would use the same fields to do the corona ionization and accelerate the ions.

The problem with the design is that you need to put lots of energy into the air to ionize it, and that energy is lost as heat. The article above says the maximum efficiency of their corona ion engine is 1%, much less than the 40% Sky Station would need. However, the article is 30 years old, and modern designs are probably much more efficient.

If you have access to a university library, you could read the article above. The Science Citation Index can help you locate more recent articles on the same subject which list this one in their bibliography.

One last thing: some discussion at the Intellectual Snobbery Society mentions an article in the December '96 Wired magazine on the Sky Station. The article claims the corona ion engine "eats ozone". I find this odd, because the corona effect usually creates ozone (this is why high-voltage electricity and thunderstorms smell strange.) However, the electro-chemistry taking place in the stratosphere could be very different.

I'll try to learn more about this if I have time.