Solar flare-style rocket thruster ‘could send astronauts to outer solar system’ | Hacker News Reader

Solar flare-style rocket thruster ‘could send astronauts to outer solar system’

imeche.org

82 pointsdTal32 comments

We already have a myriad of high specific impulse electric thruster technologies, some of them have operated for decades in space and are standard equipment used on commercial satellites, used since 1997! The exhaust speed ranges from 10 to 40 km/s.

All these technologies are limited by their power source, not by the thruster. That's the actual problem.

If you use higher exhaust velocity, you need less propellant but need more power. After a certain point, power source mass dominates the propellant mass. The total system gets worse with higher exhaust velocity than that.

For clarification, F=MV but kinetic energy = 1/2 MV^2. So to use 1/Nth the mass you need N times as much velocity. Which that takes 1/N * (N^2) = N times as much energy.

Aka 1/100 th the mass = 100 times as much total energy.

And you're probably limited to fission or fusion power sources if you want enough power to achieve high enough thrust for crewed spaceflight with one of these drives. Problem then is getting rid of the waste heat - you'll almost certainly be limited by radiator area.

Solar panels provide a lot of powder for an Earth > Mars trip. You get ~1/2 the sunlight in Mars orbit but it’s still 24/7 365. So your ultra light ~40% efficient panels are averaging something like 350w/m2 * 520 days, which is worth hundreds of pounds of hydrogen + LOX.

Scott Manley has a nice overview video covering "far future rocket technologies"

https://www.youtube.com/watch?v=QEZv_OXA_NI

Link to the original publication: https://www.cambridge.org/core/journals/journal-of-plasma-ph...

Full paper: https://arxiv.org/abs/2011.04192

I don't have the faintest idea how to physically imagine magnetic reconnection. I've only seen it described in terms of "field lines", which are pretty much imaginary. Can anyone explain it in more concrete terms without requiring intimate familiarity with Maxwell's equations?

Edit: To be clear about the unreality of field lines, since that's apparently necessary: magnetic fields are not made of finite numbers of discrete 1D elements. They are continuous. The placement of individual "field lines" in a drawing is purely arbitrary. This part I understand quite well.

Field lines aren’t imaginary. A field line is simply a contour along which the field has equal strength. Reconnection is when unconnected contours of the same potential merge and then diverge in a different topology. For reasons we don’t fully understand, but are learning to engineer, this can result in a very powerful acceleration of parts of the plasma medium.

That's not true.

Magnetic field is a vector field, not scalar field.

Magnetic field line is visualization of the direction of the field, not related to the strength. The strength shows in the density of the field lines.

Even if it wasn't a vector, contours of 3d scalar fields are 2d, not 1d.

You are quite right, my apologies.

You needn't think of them as imaginary, you can make a viewer and see them yourself pretty easily. Here's a randomly-chosen example, one of many similar on YouTube: https://youtu.be/VkSQX5VpYpQ

This is misleading: The reason that iron fillings form lines instead of just spreading evenly is that nearby fillings interact magnetically with each other, rather than anything to do with the field of the large magnet having "lines". Magnetic fields are continuous over space.

Thank you for the correction! Today I Learned.

“Long-distance travel takes months or years because the specific impulse of chemical rocket engines is very low, so the craft takes a while to get up to speed,” she said. “But if we make thrusters based on magnetic reconnection, then we could conceivably complete long-distance missions in a shorter period of time.”

I strongly doubt she said this, as it is factually incorrect. While the specific impulse of chemical engines is low, their thrust is high so they get up to what speed almost instantly.

And there's a catch with increasing specific impulse. Specific impulse is proportional to exhaust velocity. The energy required to expel the exhaust is proportional to the square of exhaust velocity.

Because nothing is 100% efficient, high specific impulse means dissipating a lot of waste heat. Increase the thrust too much and you melt your vehicle.

> they get up to what speed

That looks like a Freudian typo. They get up to their top speed faster, but that top speed is lower. Their high thrust comes from spewing out lots of mass at relatively low exhaust velocity, i.e. low specific impulse. And delta-v is directly proportional to specific impulse:

https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation

The lady's quote may have been a reference to gravitational assist, which must currently be used to get planetary probes to destination with puny chemical rockets:

https://en.wikipedia.org/wiki/Gravity_assist

It gets the job done... eventually, after long detours around another planet or two than the one you actually want to reach.

Wondering if she is talking about focus fusion like these guys do. https://lppfusion.com/technology/fusion-energy-generator/

It's a concept, not even a proof of concept so far.

Also, how is this better than existing ion thrusters? Isn't it just a bigger ion thruster?

> Also, how is this better than existing ion thrusters? Isn't it just a bigger ion thruster?

Besides just operating on a different principle, the theoretical exhaust velocity is about ten times higher than the highest actually flown ion thruster.

So flying the same mission with this thruster would require significantly less propellant. But you don't get to throw something out the back of the spacecraft faster for free. Kinetic energy scales as the square of velocity, so ~10x exhaust velocity is going to mean ~100x power draw.

That still seems like a win, though. Getting 10x further on the available reaction mass in exchange for needing to wait for the batteries to recharge sounds like a great deal for big trips.

It's not. Modern electric propulsion is generally more limited by power than propellant.

I feel like I'm missing some key piece of physics here. This seems like an ion engine but with magnets propelling the exhaust rather than electric fields.

Is that it?

That basic description already fits hall-effect thrusters. I think the idea here is not a new electric thruster but bleeding off plasma from a fusion reactor to power a rocket.

It depends how you look at it. At one level yes it’s using fields to accelerate charged particles, but it’s using a completely different mechanism. It’s a bit like saying a turbofan and a ramjet are the same thing because they’re both jet engines.

First comment on that article is perfect.

>"The new concept would accelerate the particles using

magnetic reconnection

– a process found throughout the universe, including the surface of the Sun – in which

magnetic field lines converge, suddenly separate, and then join together again

generating lots of energy

. Reconnection also occurs inside doughnut-shaped tokamak fusion devices."

(Side note: I think that there's something here with respect to understanding Energy -- itself...

Also, I think the word "Energy" is etymologically interesting... because we see the same root: "En" -- in words like 'Entangle', 'Entropy' and 'Enharmonic'... is there a relationship there? Or is it mere coincidence?)

Future ToDo: Study Magnetic Reconnection in the context of understanding Energy...

I doubt this will have regions of magnetic instability within the corridor and that prolonged exposure of energy to these regions of subspace will caused them to rupture resulting in subspace rifts.

Sounds like a bot to me.

Maybe? Looks like that user has been registered since 2013 and has several thousand karma. I guess they could have assigned a bot to the already active account, but that seems like a long time to run a fairly successful bot on a web forum.

Maybe it's a bit of a Rorschach test, but I thought the comment sounded suspiciously like a reference to a Star Trek: TNG episode with a similar plot about warp drives[1].

[1] https://memory-alpha.fandom.com/wiki/Force_of_Nature_(episod...

Gosh sorry guys. Something about the techno babble in the article made me want to make a Star Trek reference. Bad judgment!

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