An Alfvenic reconnecting plasmoid thruster | Hacker News Reader

An Alfvenic reconnecting plasmoid thruster

cambridge.org

35 pointsChuckMcM18 comments

The money quote:

«Because the system-size plasmoid is an Alfvenic outflow from the reconnection site, its thrust is proportional to the square of the magnetic field strength and does not ideally depend on the mass of the ion species of the plasma.»

[Edit: a bit more detail] The exhaust velocities are 20 to 500 km/s, the plasma is relatively low-temperature and density, the exhaust speed depends on the magnetic field (B^2). A device a couple meters long is predicted to produce thrust up to 100N with 10MW of electric power. All this with lightweight ions to conserve both energy and mass. (To compare, current best ion drives produce exhaust at 20-50 km/s only with heavy ions, with thrust well below 1N.)

If this works, this is a big deal. Even if it does not completely remove the tyranny of the rocket equation, it may greatly alleviate its influence. It might allow to quickly move around the Solar system using moderate amounts of reaction mass, provided that a powerful enough nuclear electric plant can fit on board.

This of course isn't going to work for liftoff from Earth or Mars, but alternative mass orbital transit solutions exist at least in theory (space elevator), while there were no alternatives for proper space travel.

The power source has been the big limiting factor in electric propulsion. If your exhaust velocity is several times the mission delta vee, your propellant amount becomes really small. But your power source is probably going to be big. If you use a lower exhaust velocity, you need more reaction mass, yes, but you need less power also. This might in aggregate be better!

Ie, higher exhaust velocity can result in worse performance.

It all depends on your power source, mission delta vee and required acceleration.

Keep in mind that most of that comes from MHD simulation and MHD simulation are well known in the community to get details of reconnection quite wrong [1]. So until somebody has confirmed that with particle-in-cell simulations, hybrid simulations with a good Ohms law or an experimental device, take all performance numbers with a big grain of salt.

Oh an another thing: Very high velocity exhaust gives you large I_sp, so good efficiency in the use of the working gas that you are exhausting to get delta-v. But high exhaust velocity also implies that you need A LOT of (electrical) energy to do that. (Since power requirements scale as P = 0.5 * m/dt * v_ex^2, but thrust only goes up linearly, i.e. F = m/dt * v_ex.)

[1] See figure 1 in https://agupubs.onlinelibrary.wiley.com/doi/10.1029/1999JA90...

Thanks, I was a bit surprised to see reconnection just being assumed to be a reliable design component and wondered what bit of news I'd missed.

I never knew that about exhaust velocity. So as propellant mass efficiency goes up linearly, power use goes up exponentially? does that make fast interstellar travel impossible?

Not exponentially. That would be exp(v). But quadratically, which is faster than linear. I would really hope that at least here people would know that exponentially names a particular function and is not a synonym for "fast".

This is an interesting paper, the article is here: https://interestingengineering.com/physicist-designed-a-plas...

The short version is that using a tokamak like structure you can build a device to accelerate plasma, and with that, when you throw it out the back of your rocket, you get thrust. Really high velocity thrust, like 20kM/sec. Depending on the gas used, you could set up some pretty high thrust there.

And as a bonus you can throttle it up and down!

> specific impulse from 2000 to 50,000 s, power from 0.1 to 10 MW and thrust from 1 to 100 N.

Does anyone here know how to translate that to Mars trip times, assuming reasonable vehicle parameters?

It's complex. 100N of thrust is really nothing. The Falcon 9's Merlin engine can produce 900,000N of thrust, for comparison.

But that specific impulse (Isp) is what's interesting. Isp is fuel efficiency, and for most trips in space fuel makes up most of your mass. A big improvement in Isp means a lot less mass to get where you're going.

Merlin has an Isp of 311s. These folks are talking about an Isp of 50,000s.

Assuming a magic massless 10MW electrical source to get that 100N of thrust in the SpaceX BFR (aka Starship) with a mass of 5,000,000 kg, you'd get 1/50,000 meter/sec^2 of acceleration, it would take 14 hours to get 1 additional meter per second of velocity.

It won't matter a bit, if my math is right.

Your math seems right at first glance. But keep in mind that it will only expel 25kg of working gas in those 14 hours. For a short trip to mars with a heavy rocket this kind of drive might not matter, but imagine a probe that can run this engine for a decade.

Since the thruster described is less than 1m in diameter, you can attach a serious number of them to a ship the size of a Starship.

Also, with such a high specific impulse and the ability to use light ions like helium or maybe even lithium, you may not need to use a 5-kiloton craft to deliver a few tons of payload to Martian orbit. Nuclear fuel has absurdly high power density, even with all the shielding, reactor, turbines, and generators factored in.

The largest electrical power delivered in space from nuclear sources was 3000 watts, which is far less than the 10,000,000 watts they call for just to generate 100 newtons of force.

Generating power in space isn't easy.

Source: https://en.wikipedia.org/wiki/Nuclear_power_in_space

This is the question of launching a conventional fission reactor into space. Nobody just needed 10MW of electric power at space yet.

Of course, a nuclear reactor produces heat, not electricity directly, so we're limited by the Carnot cycle, and will need large radiators, defeating some of the mass savings. OTOH space is very cold, and a suitable gas can be made to work with a seriously higher temperature difference than practical on Earth, giving better utilization of the heat. OTOH reaching the asteroid belt and Jupiter in reasonable time is only possible with non-chemical rockets, and nuclear fission power is the only realistic option now.

> OTOH reaching the asteroid belt and Jupiter in reasonable time is only possible with non-chemical rockets, and nuclear fission power is the only realistic option now.

Nuclear fusion is another feasible option, if you can get around the Partial Test Ban Treaty because an asteroid is going to exterminate humanity or something.

It's not very viable yet because the only form of it we reliably achieve is a bomb. This makes the only useful spacecraft to use it something like Project Orion.

Such a device, while possible, is very fuel-inefficient: you need several kg of plutonium just to ignite the tritium. Peak mechanical and radiation loads are very high.

A thruster which could produce much lower thrust for a much longer time seems much safer and cheaper to implement, even if detonating nuclear bombs in space were not an issue. And a issue it is: you need to do it rather far away from all other spacecraft, which means some weird orbital plane.

I remember this for use in the atmosphere but no follow on news and all went quiet very quick.. hmmm

https://www.newscientist.com/article/dn13840-invention-plasm...

The "plasma-powered flying saucer" in your article doesn't appear to have any relationship to the thruster in the original post other than the word "plasma". The former appears to be an air ionizer [0]; the latter is a repurposed fusion research tokamak.

This is analogous to referring to both bumblebees and the Saturn V rocket as "liquid-powered flying vehicles".

[0] https://en.wikipedia.org/wiki/Air_ioniser

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