NASA’s longshot bet on a revolutionary rocket may be about to pay off

I was at a demo for a plasma engine similar to VASIMR back in the 90's at a national lab. It was a long machine running the length of a large room. There was a window in the side where you could see the plasma. When we showed up there was a faint pinkish plasma barely visible in the window. The guy who was showing us around told us they were just getting ready for the demo and we should wait a few minutes. After a few minutes of the plasma being off, one of the engineers gave a thumbs up. Our guide pulled us close to the window and said "Watch this!". Nothing happened. "One more second!", he said. Then we smelled burning electronics. "Shit!", he said and ran towards the power supplies.

My co-founder and I looked at each other and smiled, because it was so nice to be on the other side of that situation. I knew exactly how the guys felt. Usually we were the ones trying to demo an experiment that sort-of, kind-of worked.

You can approximately minimize total mass with this info. Turns out, your spacecraft will in total be heavier, if you go to higher exhaust velocities. You save on propellant but you need bigger solar arrays. Not worth it.

All this talk about "Mars in x days" assumes a fantastic power plant. It's a little bit like you're starting a car company and showing wheels and saying "my car company enables range and acceleration three times that of Tesla" while you don't have any kind of battery or motor yet.

Traditional engines burn fuel, and the expanding, hot fuel pushes itself out the back of the engine at high speed, pushing the rest of the spacecraft forward. The fuel containing the energy is also the propellant.

Ion engines are the opposite in almost every way. You spend a lot of electrical energy (solar, nuclear, whatever) speeding up a tiny amount of propellant (which is usually something non-reactive like argon). You shoot a little bit of fuel at insanely high speeds out of the engine, and it pushes the spacecraft forward a little bit- so you do it for a very long time.

Yes, ion engines still need propellant, but they need a whole lot less of it.

As discussed in the article, this is a very different kind of ion thruster. That category is quite large. In particular, I believe essentially all ion thrusters flown have been electrostatic ion thrusters, including both Deep Space 1 and the Hall thrusters discussed in the article.

https://en.wikipedia.org/wiki/Ion_thruster#Electrostatic_ion...

In contrast, the VASIMR engine discussed in the article is an electromagnetic ion thruster

https://en.wikipedia.org/wiki/Ion_thruster#Electromagnetic_t...

(Apologies if you didn't intend to suggest the tech isn't that new, and just meant that the low-propellant-usage part isn't that new.)

The thing is, pretty much every plasma propulsion device has the theoretical ability to throttle specific impulse. Gridded ion engines and Hall thrusters can for instance directly throttle specific impulse (Isp) by varying the discharge voltage. Other plasma thrusters can do this by other means like increasing RF heating, varying the magnetic field, etc.

Unfortunately, in real life the actual usable range of Isp always ends up much narrower that initially predicted because of a variety of factors that are difficult to predict theoretically (erosion and ionization efficiency in particular).

AFAIK, due to the shear size and complexity of Vasimr, its thrust has never been directly measured and we know very little about its actual performance. I would therefore wait a little and discard sensationalist articles until real, extensive testing has taken place.