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Comment by rich_sasha | Hacker News Reader

Surely a n00b question, but: why superconducting electromagnets?

I get that means zero electrical resistance and stronger magnetic field at rest, but ultimately the engine is doing work accelerating ions, which will be "felt" as resistance in the electrical system. The total energy must be conserved. I would imagine (?) that work done accelerating the ions is far greater than the losses to heat in the magnetic coils.

Or maybe that's not right?

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Here's a link to the actual paper [1]. According to the paper, the magnetic part of the thruster consumes between 10 and 100 kW if it uses regular magnets, and only 0.2 kW with these superconducting magnets. Most spacecraft use solar panels, and they claim the largest satellites produce 30 kW of power. It follows that 10 kW is an unacceptable level of power consumption, while 0.2 kW is ok. They admit that nuclear powered spacecraft don't necessarily need this new technology, because 10 kW might be negligible for them.

[1] https://www.sciencedirect.com/science/article/pii/S277283072...

Wouldn’t it still be better to use more efficient engines? could they not just have more of them?

You’d get more thrust, but lower delta-v for the same total mass, because you’re replacing propellant mass with fixed mass.

That is an economic question. What does efficiency costs vs what does providing more power cost?

There are several ways a spacecraft engine can be efficient. The fundamental thing is the tyranny of the rocket equation: in order to get faster you need fuel, and then you need fuel to push that fuel, into a compounding way. The best way to address this tyranny of the rocket equation is to have an engine that expels something at a very high speed. The best chemical rockets push hot gas at about 4.5 km/s. The engines discussed here are "magnetoplasmadynamic thrusters" [1]. They push plasma with a velocity in excess of 15 km/s. Some studies suggest even 100 km/s. They are basically the most efficient rocket engines our current technology can build.

The next type of efficiency is how much of the source energy is converted into the energy of the plasma jet. Is it 50%, 90%, 99%. That I don't know, and obviously you want as much as possible.

But this project is addressing a different problem. In order to create this plasma jet, you need a very strong magnetic field to contain the plasma while it is being accelerated. The field itself does not provide the propulsion, it is there only for containment. You don't want to expend a lot of energy maintaining this field. Superconducting magnets are much better than regular electromagnets, so it's no surprise that containing this plasma with superconducting magnets requires a fraction of the power of regular magnets. But someone still needed to do the actual research. What works on paper doesn't translate always in things that work in real life. These guys did just that, and now are ready to send a technology demonstrator in space.

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

Power consumption may not be the only concern but also dissipating the heat. You don't have air or water around you to dump the heat into so you're limited by what you can radiate (is my understanding). Trying to radiate out 200W of heat vs 10-100kW is a dramatically different challenge.

Fill a surrounding container with a volumetric shit ton of nitrogen gas (perhaps with stepped pressure gradients) and you might be able to radiate a significantly larger amount by increasing your surface area without drastically increasing your weight (or use nuclear power to mitigate the weight concerns).

But yes, today’s design space generally prefers as low a power envelope as possible to not have to worry about dissipation.

Wouldn't the limiting factor be the outer layer? I'm not sure what layering would give you, or what adding a container would give you. For short bursts this may make a difference but the steady state means the system as a whole needs to radiate out that energy somewhere. More mass gives some kind of ballast I guess but kilowatts of energy is just a lot to get rid of.

It's not about ballast. Black body radiation, which I'm sure you know is the only way heat is dissipated in a vacuum (unless you're the Sun & even then it's still the primary way), is defined as:

P = ε * σ * A * T^4

That's the constant, the emissivity of the material, the surface area A, and temperature.

We want to increase the temperature differential above what the natural black body radiation can dissipate for a given device (let's take a space probe) and assume we can't change the material. The only thing we can play with is surface area.

The point of the nitrogen gas is to leverage normal convection / conduction to dissipate into a larger gaseous volume which then naturally also has a larger surface area. This would then give you more surface area to dissipate across. The reason I'm thinking the layering with different pressure gradients might be useful is that it reduces the overall weight because you need less overall gas for a given dissipation profile (i.e. you need high density beside your heat source to transfer energy away quickly but less density further away because there's a larger surface area growing with the square of the distance away from the heat source).

[1] https://en.wikipedia.org/wiki/Stefan%E2%80%93Boltzmann_law

Convection depends on buoyancy and hence gravity, and conduction in nitrogen gas is extremely low, so I don't think your idea would work. You need a conventional radiator with forced circulation.

Fractal branching structures (trees), which maximise radiating surface area, don't make good pressurised vessels.

Inflatable envelopes in space are challenging (micrometeoroid damage). The envelope, and thus the gas, need to be hot, lest T^4 swamp R^2. It's feasible to cool a reactor to low-pressure gas (Mars atmosphere [1]) using forced convection, but that's cold gas and IIUC the heat exchanger is comparable to the size and mass budgets of a radiator. So attempting a steelman of a set of forced-convection radiators on poles, hosing hot gas against the envelope... hmm. Big picture, this attempts a low-mass heat transfer construct, using a large-scale low-pressure gas flow, where the cold-side surface is nearly as hot as the hot-side, and is extremely mass constrained (thus flat-ish). Seems hard? Fun thought experiment - thanks!

[1] Colgan Mass Optimization [...] Forced-Convection Heat Exchanger for Mars Surface Waste Heat Rejection 2023 https://asset.library.wisc.edu/1711.dl/FYJCG5YME7O4X82/R/fil...

VASIMR is one of the few test articles with flight hours of this category (or maybe a sibling category), and when you factor into account the thruster and the power supply, its thrust to weight ratio for the VX-200 prototype was unworkably low. Not "too low for launch", I mean "too low for a steady burn of months to attain meaningful progress towards interplanetary or orbital objectives".

You have to simultaneously solve for power to weight ratio issues on solar panels, power supply, and the actual thruster. The best solutions right now for most purposes are relatively low Isp (low propellant efficiency) Hall thrusters.

In space, heat dissipation is remarkably harder than it is on Earth, so getting to, for example, 97% thermal efficiency rather than 94% would be a big deal in terms of the mass of the heat dissipation system, and I think this and reducing the mass of the actual electromagnet is the advance here, if there is any.

One of the biggest things that always bothers me in sci-fi, even harder sci-fi like The Expanse, is the missing heat sinks. All those ships would need massive radiators or they’d melt.

The starship in Avatar got that right. Rest of the film is comic book level and derivative but the ship is good.

It's worse than you think. Here's a really interesting article on what it would take to get Epstein drive performance:

https://toughsf.blogspot.com/2019/10/the-expanses-epstein-dr...

This thing would produce more power than all of civilization does today. No radiator could handle it. They use laser fusion, exploding D-He3 fuel pellets well behind the ship, with a giant magnetic nozzle. A tungsten heat shield protects the ship, and reaches an equilibrium temperature without active cooling.

In the Expanse though The Drive is always on, which means the ship is tossing propellant mass (efficiently) out the back of it - which means it can dissipate most of it's heat that way.

The Drive in the Expanse is _nonsensically_ powerful; Turning it on at mass fractions that seem compatible with their ship designs would be akin to a nuclear exchange for anything within thousands of kilometers of the rear end.

Talking about thermal and propulsive efficiency for a rocket is a little tricky because there are multiple useful metrics, but in this context "waste heat" is anything that doesn't get tossed out the back.

The magnetic field here is acting as a nozzle. The circuit driving the field is not doing work on the ions, and any power loss is just parasitic. It's generally not worth it for small thrusters, because the extra efficiency gained by having a magnetic nozzle is offset by the parasitic mass and power cost (here they eliminate the power cost).

Generating a strong field requires a lot of current. If you have any resistance at all, that's a lot of heat dissipation, which will require cooling.

Less energy use, lower mass? I know superconducting magnets are a goal for both aircraft propulsion systems and use in wind turbine generators, just for the mass reduction.

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