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Comment by mreithub | Hacker News Reader

One thing I don't quite get is their claim that the rocket would need "just some solar panels" instead of a fuel tank when in the paragraph before that they talk about exhausting argon-based plasma.

If there is a propellant, you'd need to store that somewhere first, right? Does each particle of the propellant exit the rocket with much higher energy? Or can Argon be stored in a much denser form than other fuels?

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The difference is 'momentum gained per gram of propellant'.

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.

Thanks to you and the others for clarifying. For some reason it didn't occur to me they were talking about an ion engine. I've heard of them and their slow-but-steady acceleration characteristics before but never actually knew how they work (and for some reason didn't quite make the connection).

Yes, each particle of the propellant exits the rocket with much higher energy. Since you can only get so much energy from chemical reactions, to get to these higher exit velocities the energy from the solar panels is used. You use a higher ratio of energy to propellant mass than chemical fuels use.

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.

Side note: hats off to anyone working with high energy plasma.

If I make a mistake in my code, it generally doesn't escape magnetic confinement and instantly vaporize other portions of my system with the fury of the sun.

Specific impulse or Isp is what you want. And yes, ion thrust takes a lot less propellant. https://en.wikipedia.org/wiki/Ion_thruster

However, this is not really 'new' tech. https://en.wikipedia.org/wiki/Deep_Space_1 used an early version in 1998.

(IAN a physics major) But to elaborate per my understanding, there are 2 important variables you're playing with in rocket engines:

- Specific impulse / Isp (aka how much thrust you get for a given amount of propellant)

- Maximum Thrust

Due to the rocket equation [1], adding more propellant increases the weight of your vehicle, making it harder to move, requiring more propellant, etc etc. So being efficient with your propellant is very good.

That said, given a requirement against a gravity well (e.g. a planet), there's usually a minimum total thrust required for a given maneuver to be successful (low total thrust = maneuver takes longer = more time for gravity to pull you = more propellant required).

Thus far, we generally have two types of engines. (1) High maximum thrust, lower Isp (chemical rockets) & (2) high Isp, low maximum thrust (ion/electric engines). The two are currently very far apart [2, sort by Specific Impulse decreasing, then look at the Thrust column].

As examples (Isp Vacuum / Thrust Vacuum): NEXT ion thruster 4,100s/0.236N @ 6.9 kW, VASIMR 5,000s/5.7N @ 200 kW, Space Shuttle SRBs 268s/14MN.

The hope with VASIMR is that it provides a middle ground where high Isp is available with enough total thrust to actually be useful for something other than slow orbital adjustments. An example of "something useful" would generally be anything beyond Earth orbit that covers large distances, e.g. flying to Mars.

[1] https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation

[2] https://en.wikipedia.org/wiki/Comparison_of_orbital_rocket_e...

This is correct but incomplete: Vasimr may have a higher power density (be it power per thruster mass or per thruster volume) than ion thrusters but I am pretty sure it has lower power density than a Hall thruster, while being also much more complex.

The main (and not yet verified) claim of Vasimr compared to existing and proven tech like Hall thrusters is the ability to greatly vary the specific impulse.

That's what the article claims. That you can run in a less efficient mode that produces more total thrust when you're climbing out of a gravity well, then switch to the lower thrust higher efficiency mode for the long haul.

I thought one of the issues with Hall thrusters was that, in engineering-practical terms, getting enough total thrust out of them was impossible. Or would they scale effectively?

> However, this is not really 'new' tech. https://en.wikipedia.org/wiki/Deep_Space_1 used an early version in 1998.

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.)

They all need a propellant and power feed. But, you can use those same feeds for an array of multiple engines. Thus, 'scaling up' has a wide range of options and trade-offs. Vasimr let's you scale power output a lot from the same engine, but so would an array of smaller engines.

With that in mind the specific techniques are important, but the gaps are often overstated.

Yes, the claim is entirely wrong. The rocket still needs propellant. It just uses electricity to eject the propellant at much higher speeds than usual chemical reactions.

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