Fusion Powered Aircraft?

[firefossil]

I was thinking about Project Pluto, the legendarily wacky proposal to build a fission powered supersonic ramjet that after dropping its nuclear payload, can fly for weeks without landing while spewing radiation and smushing people with pressure waves for shits and giggles. It saw a ground test of a prototype engine before being dropped. It was crazy and kind of douchey for a doomsday weapon, and due to the whole 'nuclear reactor farting radiation on everyone as a design feature' isn't practical for much else.

But like let's say we develop a microfusion reactor of comparable scale and output, just fusion rather than fission based. Its unclear to me, would its output still necessarily be irradiating to surroundings, or is that a feature specific to fission's use of dense radioactive elements? The helium would be harmless, but it'd still be spraying energetically. If tolerable, would it be possible in turn to build conventional aircraft that can fly for absurd amounts of time, or are much larger (ie helicarrier sized flying fortresses) than would otherwise be possible with normal fuel?

 

[Cloak&Dagger]

Project Pluto was specifically an open-core design, you can do it with a closed-core design that isn't spewing out radioactive materials, but that's a massive waste of weight for an amount of radiation that isn't relevant.

For a fusion reaction I don't even think you could have an open-core design since you wouldn't be able to get fusion in such a chaotic environment, fusion is much harder than fission.

 

[Avernus]

It wouldn't necessarily irradiate everything; for that matter neither would a fission drive if it was closed cycle.

Ninjaed, I see.

For a fusion reaction I don't even think you could have an open-core design since you wouldn't be able to get fusion in such a chaotic environment, fusion is much harder than fission.

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Worse, if you could pull it off you'd be flying overland with a fusion temperature drive plume. The radiation danger would be somewhat overshadowed by the "instantly vaporized" issue...

 

[Baughn]

If we're talking the likes of ITER, it still produces a massive neutron flux. The containment walls are expected to need replacement every couple of years, and having them spread out across a city as shrapnel wouldn't be a good thing.

 

[Cloak&Dagger]

If we're talking the likes of ITER, it still produces a massive neutron flux. The containment walls are expected to need replacement every couple of years, and having them spread out across a city as shrapnel wouldn't be a good thing.

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It wouldn't really matter. Radiological weapons are a joke, and just operating a reactor for a few hours isn't going to do much, even less a fusion reactor which has far less dangerous waste.

 

[RRoan]

But like let's say we develop a microfusion reactor of comparable scale and output, just fusion rather than fission based. Its unclear to me, would its output still necessarily be irradiating to surroundings, or is that a feature specific to fission's use of dense radioactive elements? The helium would be harmless, but it'd still be spraying energetically. If tolerable, would it be possible in turn to build conventional aircraft that can fly for absurd amounts of time, or are much larger (ie helicarrier sized flying fortresses) than would otherwise be possible with normal fuel?

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The main radiation danger with fusion is from neutron activation, wherein energetic neutrons released by the reaction are captured by atoms in the surrounding environment. This results in said atoms becoming excited (because they just absorbed a bunch of energy) and usually unstable to some degree (because adding an extra neutron to an atom usually creates an unstable isotope). This means that the things around your reactor will, over time, become low level radioactive waste.

Buuuuuuut not all fusion reactions release neutrons. That's why the deuterium/helium-3 and proton/boron-11 reactions are desirable for future power plants. Deuterium/Helium-3 would be more desirable for vehicles, since it has one one of the highest energy densities and it's a relatively easy reaction to sustain. Proton/Boron-11, on the other hand, is (relatively) terrible in those categories but has the advantage of not requiring the strip-mining of celestial bodies.

It may come as a surprise to some people, but most fusion reactions don't actually release electromagnet radiation directly. Aside from some exotic reactions found in large stars (and not relevant to power generation), all the excess energy comes in the form of kinetic energy possessed by the fusion products (which is to say that the fusion products are going really fucking fast). With aneutronic fusion reactions all of the products will be charged, which means that you can control where they go using electromagnetic forces. If the fusion reaction does produce neutrons then some of that energy will be indiscriminately dumped into the surrounding area.

If you can make a fusion reactor light enough and powerful enough to get off the ground, then I don't think radiation would be a showstopper as long as you were using an aneutronic reaction. Neutron-producing reactions would probably be fine for a use case like project pluto, but I'd stay away from them for things that are supposed to last.

 

[Graviator]

Worse, if you could pull it off you'd be flying overland with a fusion temperature drive plume. The radiation danger would be somewhat overshadowed by the "instantly vaporized" issue...

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I do not think there would be enough power in the exhaust to cause problems at cruising altitude. That plume would be a very small amount of material compared to the amount of air it's mixed with, unless the thrust is orders of magnitude stronger than any conventional aircraft.

During takeoff, however, we might need to worry about scorching the runway.

 

[13th Bee]

It would also depend on how quickly it goes through its fuel.

 

[Ralathon]

A fusion powered plane wouldn't be scorching the runway, or flash vaporizing you as it flew overhead. Sure, the fusion plasma itself will be blisteringly hot, but that plasma ends up mixing with the outside air in the engine. By the time it leaves the exhaust, it won't be much hotter than the exhaust of a conventional plane.

After all, the goal is to make the plane fly. And that's going to take the same amount of power whether you use fusion or kerosene as your energy source. So the net heat output of the system will be roughly equivalent.

You'll only start to vaporize things if you try to pull maneuvers that require much more power than conventional flight. Like trying to fly mach 30 at sea level.

 

[Ciber]

Maybe lets figure out stationary building sized fusion before we start talking about this?

 

[firefossil]

You'll only start to vaporize things if you try to pull maneuvers that require much more power than conventional flight. Like trying to fly mach 30 at sea level.

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There is also the "lift larger loads" side of the equation. According to my calculations, it'd take about 9.8 billion gigawatts to keep a Nimitz Class Aircraft Carrier airborne. The nuclear reactors aboard said craft have a maximum output of around 0.2 gigawatts, and there are efficiency issues too. Of course an actual sky carrier or battleship would presumably be made of more lightweight materials, and doesn't have to rely on smaller less efficient reactors like a fusion powered fighter jet would.

In any case, my point is that if you can have an open-core fusion drive that doesn't make everyone murder you on general principle, you could probably field aircraft that are much larger, can fly for much longer periods of time, or both. I mean as this xkcd graph shows:

Fission and fusion have absolutely absurd advantages when it comes to energy density, and ultimately energy density is the biggest limit on how much and long you can keep an aircraft aloft before you start ending up having to carry your weight in fuel.

Huh come to think of it, an open-core fusion drive could also be pretty useful for getting to space, though as a ramjet it'd be useless in space proper.

 

[Ralathon]

Huh come to think of it, an open-core fusion drive could also be pretty useful for getting to space, though as a ramjet it'd be useless in space proper.

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You can use a fusion scramjet to get up to near orbital speeds in the upper atmosphere, lift your nose to put yourself upwards to get into a suborbital trajectory and inject a bit of hydrogen from an onboard tank into your engine for the final boost into orbit.

Once you have fusion power plants that produce enough power to lift themselves, the sky's the limit really. It enables some real scifi shit, like making brachistochrone trajectories inside the solar system viable. Which cuts down travel times from years to mere days. Or enabling interstellar vessels that actually stand a chance of arriving at another star intact.

 

[firefossil]

You can use a fusion scramjet to get up to near orbital speeds in the upper atmosphere, lift your nose to put yourself upwards to get into a suborbital trajectory and inject a bit of hydrogen from an onboard tank into your engine for the final boost into orbit.

Once you have fusion power plants that produce enough power to lift themselves, the sky's the limit really. It enables some real scifi shit, like making brachistochrone trajectories inside the solar system viable. Which cuts down travel times from years to mere days. Or enabling interstellar vessels that actually stand a chance of arriving at another star intact.

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Relativistic travel is kinda stuck with the hard limit that reaching near-lightspeed is around the point you start requiring a ship's weight in mass energy to accelerate or deaccelerate it to that, and that's assuming 100% efficient antimatter-matter conversion based engine rather than fusion which is nearly an order of magnitude less efficient. With a fusion based engine, I think the upper limit for acceleration and deacceleration is going to about around 10% lightspeed, which is still going to be a generation ship, though if you are heading to Alpha Centauri it'll be only two generations.

Returning to the earlier point, I kinda of wonder if fusion engines would lead to bigger aircraft. Economy of scale would mean that bigger aircraft would be able to field bigger more efficient fusion reactors, or depending on the technical specifics, they might be the only aircraft able to field it. Meaning that if the military wanted to move material by air, instead of sending lots of passenger liner sized cargo planes, they'd send a smaller number of ocean liner sized planes.

 

[Ralathon]

I think you'd still want to go as small as possible in terms of aircraft. Even if their fusion drives were more efficient at larger scales. The cube square law is just too strong. As you increase the size of your aircraft by a factor of 2, you increase the surface area and thus lift by a factor 4. But weight goes up by a factor of 8.

So if you try to make really big aircraft you'll be forced to make the plane less dense to keep it airborne. This means less structural reinforcements and a much weaker aircraft. Make it too big and even standard maneuvers will rip it to shreds.

 

[Graviator]

You'll only start to vaporize things if you try to pull maneuvers that require much more power than conventional flight. Like trying to fly mach 30 at sea level.

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Even then, the faster you go, the larger a volume your exhaust gets spread over.

 

[Cadis]

If we're talking the likes of ITER, it still produces a massive neutron flux. The containment walls are expected to need replacement every couple of years, and having them spread out across a city as shrapnel wouldn't be a good thing.

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This is why you make the walls a fluid instead of a solid (this is one avenue they are perusing to deal with the issue of neutron bombardment. It is hard for your walls to erode or become brittle due to neutron flux if they aren't a solid).

 

[Fouredged Sword]

Also in atmo any thrust capable of doing significant damage to the surroundings at cruising altitude would ether push your engine through the front of your plane or push your plane through the air so hard that it explodes due to air friction. It's not like we can't make hypersonic missiles with modern tech. We just can't keep them from exploding on their way to the target.

I am reminded of the problem we had with the first aircraft carrier to be powered by a nuclear reactor rather than diesel. They made the switch halfway through the construction and didn't realize how much power they added to the drive system. They had to throttle the main screws because if someone ever did send all power to the screws it would send the screws out the front of the ship.

 

[Triggerhappy]

Also in atmo any thrust capable of doing significant damage to the surroundings at cruising altitude would ether push your engine through the front of your plane or push your plane through the air so hard that it explodes due to air friction. It's not like we can't make hypersonic missiles with modern tech. We just can't keep them from exploding on their way to the target.

I am reminded of the problem we had with the first aircraft carrier to be powered by a nuclear reactor rather than diesel. They made the switch halfway through the construction and didn't realize how much power they added to the drive system. They had to throttle the main screws because if someone ever did send all power to the screws it would send the screws out the front of the ship.

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Or, y'know, just catastrophically tear apart the turbines powering the drive shafts.

Not my area of expertise, but I expect something in the power transmission system would fail first.

 

[Pineapple]

They had to throttle the main screws because if someone ever did send all power to the screws it would send the screws out the front of the ship.

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That's... rather hyperbolic.

Or, y'know, just catastrophically tear apart the turbines powering the drive shafts.

Not my area of expertise, but I expect something in the power transmission system would fail first.

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All you need to make a jet engine work is heat. You don't need anything exotic.

You can jam heat exchangers into the mix instead of burner cans and use your reactor heat instead of fire, it'll run the jet engine more or less identically to one that burns fuel, and you'll get clean thrust out the back. No need to vent anything.

 

[Triggerhappy]

All you need to make a jet engine work is heat. You don't need anything exotic.

You can jam heat exchangers into the mix instead of burner cans and use your reactor heat instead of fire, it'll run the jet engine more or less identically to one that burns fuel, and you'll get clean thrust out the back. No need to vent anything.

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Uhm? Are you answering the correct person?

Because I'm more or less agreeing with :

That's... rather hyperbolic.

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I don't know a ton about the propulsion system on the Enterprise but I presume that they were steam turbines driven by a secondary steam loop from the reactors.

If there was a concern with the reactors providing too much power it probably would have been felt here at the conversion point from steam to motive power with the reactors being capable of providing far more power than the turbines could safely convert to torque on the propeller shafts.

 

[Pineapple]

Uhm? Are you answering the correct person?

Because I'm more or less agreeing with :



I don't know a ton about the propulsion system on the Enterprise but I presume that they were steam turbines driven by a secondary steam loop from the reactors.

If there was a concern with the reactors providing too much power it probably would have been felt here at the conversion point from steam to motive power with the reactors being capable of providing far more power than the turbines could safely convert to torque on the propeller shafts.

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Oh right i thought we were talking about planes.

As for the carrier; even the kittyhawk and her diesel engines could turn the screws hard enough to break shit, so it's not like that wasn't a known issue.

The problem with the enterprise was that it had such a surplus of power it would rapidly reach a point where it would snap the prop shafts because the hull hit its hydrodynamic speed limit but the engines just kept a pushin. There's something impressive about having so much shaft horsepower that you can hold your own in a fight against the ocean.

I am reminded of a story from a friend of mine who was crew on the kitty hawk. Back when america was in iraq for the first set, the Kitty Hawk was running balls to the wall for the persian gulf and she was running so hard and the ship was shaking so hard that nobody could keep anything on a shelf or a table because it would just rattle out onto the floor.

 

[Gravitas Hunt]

You can jam heat exchangers into the mix instead of burner cans and use your reactor heat instead of fire, it'll run the jet engine more or less identically to one that burns fuel, and you'll get clean thrust out the back. No need to vent anything.

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Thing is, no heat exchanger is going to be as effective as mixing gases together. That's not to say it's impossible to make one that'll work, just that I don't know if we quite have the engineering capability yet.

 

[Fouredged Sword]

Oh right i thought we were talking about planes.

As for the carrier; even the kittyhawk and her diesel engines could turn the screws hard enough to break shit, so it's not like that wasn't a known issue.

The problem with the enterprise was that it had such a surplus of power it would rapidly reach a point where it would snap the prop shafts because the hull hit its hydrodynamic speed limit but the engines just kept a pushin. There's something impressive about having so much shaft horsepower that you can hold your own in a fight against the ocean.

I am reminded of a story from a friend of mine who was crew on the kitty hawk. Back when america was in iraq for the first set, the Kitty Hawk was running balls to the wall for the persian gulf and she was running so hard and the ship was shaking so hard that nobody could keep anything on a shelf or a table because it would just rattle out onto the floor.

Click to shrink...

And the funny part is because a quirk of hydrodynamics the carrier has a higher top speed than any other ship in the fleet. Once you have sufficent power the main impediment to going faster is cresting your own bow wave. A big ship trying will break. The longer your ship, the faster you can go before you start pushing up your own bow wave.

 

[Pineapple]

Thing is, no heat exchanger is going to be as effective as mixing gases together. That's not to say it's impossible to make one that'll work, just that I don't know if we quite have the engineering capability yet.

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We had the engineering capability to make a nuclear jet in the 1960s, it's workable technology.

 

[ZeroXSEED]

I do not think there would be enough power in the exhaust to cause problems at cruising altitude. That plume would be a very small amount of material compared to the amount of air it's mixed with, unless the thrust is orders of magnitude stronger than any conventional aircraft.

During takeoff, however, we might need to worry about scorching the runway.

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Bro, WWII Bomber formation can cause WEATHER ANOMALY, what do you think fusion plume would do to background temperature?