Fusion Engines - ultimate limitations on performance if you do it like Tri Alpha does it

[SomeGuy12]

So, apparently, trialpha has a method to confine the plasma where they accelerate particle beams of more plasma that are then bent inwards by massive magnets to surround the hot core where the fusion reaction takes place. So, essentially, the container holding the incredibly hot fusing plasma is more plasma. They claim this is stable.

So what happens if you keep making it larger? If you keep making the reactor larger, the surface area of the plasma container grows slower than the inner core. If the inner core is all fusing fusion fuel, the power output would grow non linearly with the size of the reactor.

An engine version of the reactor, there would be a more complex set of magnets and plasma flows that would form an extremely narrow, long opening to outside the spacecraft. This would be tuned just right so only incredibly hot plasma at the temperature of the reaction itself could escape at the same rate that new superheated plasma is forming from further fusion.

What would ultimately limit performance? My guess is that light from the reaction and neutrons, as you keep making the engine bigger and bigger and more powerful, would grow proportional to engine power output. Since engine power output grows faster than the surface area of the inner part of the engine, eventually there would be too much neutron impingement and light for the materials to handle and the superconducting magnets would probably quench and the whole thing fails.

So you'd want to do aneutronic fusion to minimize the neutrons, since light is much easier to shield against than neutrons.

Looking at the readily available fuels, you'd use either hydrogen/boron or hydrogen/lithium. The lithium one is almost double the power output so it would be preferred if you had mastered the technology to make these things. Variations in the design would run "fuel rich" where unreacted hydrogen gets heated up and is allowed to escape the engine with the hot stuff, this would give you more thrust at the expense of ISP.

Does anyone know a way to rough out what the power/mass limitations would ultimately be? Even to get a guess? Figuring out the ISP is easier, you just take the velocity of the fusion products and divide by 10. It's about 7% C with Hydrogen-Lithium, so about 2 million. So running at maximum ISP, if your spacecraft were 50% fuel, you'd have a dV of 13594 kps. The limiting factor is acceleration - this kind of performance doesn't help if you have to spend years to burn all your fuel.

To get sci-fi levels of thrust, you need megawatts of fusion power per kilogram of engine. "Sci-fi" means around 1 G.

I think it might be possible, actually. Plasma acts to protect the engine material from the core producing all that energy, so you don't actually touch it, and you might be able to tweak your reaction to produce less neutrons. You might also have gaps in the engine bell to reduce surface area exposed to light and neutrons.

Some rough figuring : assuming the engine is 20% of the mass of your spacecraft, you need 500 megawatts/kilogram of engine expressed in the plasma stream escaping your engine. This sounds a bit much, at 0.1 G it would be 50 megawatts, etc. The best engines on earth can do around 10 kilowatts/kilogram, but jet engines have to touch the fuel mixture they are burning and they have to mechanically spin.

The limiting factor isn't magnet mass, because the engine is very large compared to the surface area of the fusion core. It isn't heat radiator mass, because you would use negligibly light droplet radiators. You might even be able to use a series of plasma mirrors to reduce the amount of energy impinging on your solid, physical mirrors that you have to replace if they are damaged.

Let's see. Maybe the way to get a rough estimate is to say : ok, 1/3 of the mass of the engine is the part that deals with the heat. How much heat can you shed and/or resist per kilogram? If 1% of the engine's power output reaches the engine components themselves, then to hit 50 megawatts/kilogram you need to shed 1500 kilowatts/kilogram. That may not be possible. You might be able to do 15.

So the futuristic engine of the future, when it's blasting out a star-bright plasma exhaust, accelerates the spacecraft at 1 centimeter/second. You gain 864 m/s a day and need 43 years to burn all your fuel, assuming your spacecraft is 50% fuel.

Edited October 11, 2015 by SomeGuy12

[PB666]

So, apparently, trialpha has a method to confine the plasma where they accelerate particle beams of more plasma that are then bent inwards by massive magnets to surround the hot core where the fusion reaction takes place. So, essentially, the container holding the incredibly hot fusing plasma is more plasma. They claim this is stable.

So what happens if you keep making it larger? If you keep making the reactor larger, the surface area of the plasma container grows slower than the inner core. If the inner core is all fusing fusion fuel, the power output would grow non linearly with the size of the reactor.

An engine version of the reactor, there would be a more complex set of magnets and plasma flows that would form an extremely narrow, long opening to outside the spacecraft. This would be tuned just right so only incredibly hot plasma at the temperature of the reaction itself could escape at the same rate that new superheated plasma is forming from further fusion.

What would ultimately limit performance? My guess is that light from the reaction and neutrons, as you keep making the engine bigger and bigger and more powerful, would grow proportional to engine power output. Since engine power output grows faster than the surface area of the inner part of the engine, eventually there would be too much neutron impingement and light for the materials to handle and the superconducting magnets would probably quench and the whole thing fails.

So you'd want to do aneutronic fusion to minimize the neutrons, since light is much easier to shield against than neutrons.

Looking at the readily available fuels, you'd use either hydrogen/boron or hydrogen/lithium. The lithium one is almost double the power output so it would be preferred if you had mastered the technology to make these things. Variations in the design would run "fuel rich" where unreacted hydrogen gets heated up and is allowed to escape the engine with the hot stuff, this would give you more thrust at the expense of ISP.

Does anyone know a way to rough out what the power/mass limitations would ultimately be? Even to get a guess? Figuring out the ISP is easier, you just take the velocity of the fusion products and divide by 10. It's about 7% C with Hydrogen-Lithium, so about 2 million. So running at maximum ISP, if your spacecraft were 50% fuel, you'd have a dV of 13594 kps. The limiting factor is acceleration - this kind of performance doesn't help if you have to spend years to burn all your fuel.

Fusion reactors use steam to carry potential energy (liquid/gas phase transition) from the reactor core to the condensor core the drop turns the turbine and waste heat needs to be transferred to heat radiators. These two transfers are the problem. Fir nuclear reactors we use large ponds in which surface evap rates modulate the temperature of the ponds over huge surface areas. In space cooling occurs over the much slower process of radiation.

In a fusion reactor if all plasma is contained in the core then basiccaly a transfers in the core are through infrared radiation to the liquid evaporation surface of the liquid transfer system. So the reactors has to have a cool down pahse to capture heat its just produced.

Fusion is a l-----------o------------n------------g way from being space ready. My thinking is that using anti-matter to use direct fusion energy as a propellant is more intelligent, but we need better ways of making and storing antimatter. It is possible to shield energy much more easily than getting rid of it once you have it. Solar is still the best power source in the inner solar system, and TNGs in the outer solar system.

[SomeGuy12]

Fusion reactors use steam to carry potential energy (liquid/gas phase transition) from the reactor core to the condensor core the drop turns the turbine and waste heat needs to be transferred to heat radiators. These two transfers are the problem. Fir nuclear reactors we use large ponds in which surface evap rates modulate the temperature of the ponds over huge surface areas. In space cooling occurs over the much slower process of radiation.

In a fusion reactor if all plasma is contained in the core then basiccaly a transfers in the core are through infrared radiation to the liquid evaporation surface of the liquid transfer system. So the reactors has to have a cool down pahse to capture heat its just produced.

Fusion is a l-----------o------------n------------g way from being space ready. My thinking is that using anti-matter to use direct fusion energy as a propellant is more intelligent, but we need better ways of making and storing antimatter. It is possible to shield energy much more easily than getting rid of it once you have it. Solar is still the best power source in the inner solar system, and TNGs in the outer solar system.

Please don't post in my thread. Nothing of what you said has any relevance to the discussion. The type of fusion I am describing, you extract electricity directly from the moving charged particles produced by the nuclear reaction. No conversions. Antimatter produces very difficult to deal with products, trashing performance because the products are not electrically charged, so you need a gigantic, massively heavy engine to get any thrust at all. Antimatter may end up being exclusively for interstellar travel, where you need the additional dV and can afford to wait for years.

As for it being a long way off - no ..... I'm talking about what engineering can do, not what will be plausibly done in the foreseeable future. Fusion is basically just as long a way off as nuclear thermal. I mean, we could build and fly nuclear thermal rockets probably 2 years from now if there was a pressing need, but they have never been flown and there is no plans to do so in even 50 years.

Edited October 11, 2015 by SomeGuy12