Air launched fusion craft?

[Newgame space program]

Is this feasible in any way? Because I'm hooked and want to do it.

[Ember12]

It's been released that starships will be bigger than the VAB, and so will have to be assembled in space.  So getting one under an airplane might be tough.

[Newgame space program]

Who said that all fusion crafts are starships?

 

[MechBFP]

1 hour ago, Newgame space program said:

Who said that all fusion crafts are starships?

 

Fusion engines are going to be MUCH too large for anything smaller.

[Pthigrivi]

39 minutes ago, MechBFP said:

Fusion engines are going to be MUCH too large for anything smaller.

Have you messed around at all with Nertea’s Far Future Technologies? I just used his Tokamak fusion aerospike to build a tailsitting workhorse lifter that SSTO’s 120t payloads. It's real fun. 

Edited February 14, 2022 by Pthigrivi

[K^2]

3 hours ago, Ember12 said:

It's been released that starships will be bigger than the VAB, and so will have to be assembled in space.

Setting aside the question of whether that will be the only way to build fusion engines, I don't think even that's going to stop some players. I'm sure somebody's going to end up building these giants in orbit, then entering atmosphere to land at some suitable clearing to pick up cargo.

Getting TWR for takeoff will be tough, but again, I'm sure somebody will figure it out. We've seen people do space planes powered by ions, after all.

[KerikBalm]

Technically, thermonuclear Orion drives are mostly fusion drives, they can certainly lift you off the surface

[Newgame space program]

On 2/13/2022 at 9:37 PM, MechBFP said:

Fusion engines are going to be MUCH too large for anything smaller.

Have you seen the metallic hydrogen crafts in the ksp 2 trailers?

[MechBFP]

16 hours ago, Newgame space program said:

Have you seen the metallic hydrogen crafts in the ksp 2 trailers?

Yes. Those aren't fusion.

[Davi SDF]

They gave KSP players procedural wings and better performance, no matter the engine and it's size, I will find a way to build a plane large enough to lif it.

Hopefully there are some engines capable for that too though! Like the Fusion Aerospike that was mentioned above, or some kind of Project Pluto engine too.

[Newgame space program]

10 hours ago, MechBFP said:

Yes. Those aren't fusion.

Dude, metallic hydrogen is literally a type of plasma, its fusion.

[K^2]

11 minutes ago, Newgame space program said:

Dude, metallic hydrogen is literally a type of plasma, its fusion.

No. Not at all. Not even close. There is zero connection between the former-speculative-now-confirmed-fictional metallic hydrogen drive, which is still a form of chemical propulsion, and the fusion drive, which is a type of nuclear propulsion. These aren't in the same ballpark. Metallic hydrogen drive is a few times more efficient than a liquid hydrogen rocket. Any kind of nuclear drive is at least thousands of times more efficient.

[Newgame space program]

Oh, sorry I must have been not  interpreting what I read correctly.

[MechBFP]

1 hour ago, Newgame space program said:

Oh, sorry I must have been not  interpreting what I read correctly.

The main difference is that fusion requires the process of fusing atoms together to release energy. Metallic hydrogen is simply an extremely dense version of fuel that creates energy by being burned like a normal fuel.

[BigStar Aerospace]

Okay, so fusion is more than likely out of the question, but we do have fission. So why not fly high into the atmosphere with a very powerful plane then drop a very heavy rocket with an even heavier engine that shoots nukes out the back? Or we can use nuclear thermal reaction engines such as the LANTRN engine that has the extending nozzle on a craft, that would supply a very efficient SSTO quite possibly.

[K^2]

57 minutes ago, MechBFP said:

Metallic hydrogen is simply an extremely dense version of fuel that creates energy by being burned like a normal fuel.

It's slightly more interesting than that. The conversion of metallic hydrogen back to molecular hydrogen itself is extremely exothermic, which is why metallic hydrogen can burn so much hotter, and can even be used without oxidizer at all at a significant fraction of efficiency. But ultimately, that's still a chemical process, not a nuclear one.

The trouble is that it turned out not to be metastable^* after all, so storing it would require the sort of pressures at which even a bottle rocket filled with water would have decent I_SP. Provided you had materials to build the tank out of to withstand that pressure, which don't exist, so that's a problem.

In Kerbal universe, however, metallic hydrogen is, apparently, metastable, and so the kerbals get to rip the benefits humanity has been denied. But honestly, they're making better use of it. We'd probably end up making bombs, or something. So good for kerbal-kind.

 

^*Good example of something that is metastable like that is diamond. Graphite turns into diamond under extreme pressure, but then doesn't turn back into graphite when you release the pressure unless you heat it up. If you do heat up diamonds, they will burn quite nicely in pure oxygen and can, theoretically, make for better fuel than pure graphite. The difference is nowhere near as pronounced as it would have been if metallic hydrogen was metastable, but diamond is still a very good illustration of the sort of thing that people were hoping for with metallic hydrogen.

Since it's not metastable, however, the only place where we think it exists in any substantial quantity is cores of gas giants. It might even be responsible for their extreme magnetic fields. Not something we can make good use out of any time soon, at any rate.

[Pthigrivi]

Im not a nuclear engineer but couldn’t a fusion reactor be used to heat liquid hydrogen to extreme temperatures similar to an NTR?

[Laxez]

19 minutes ago, Pthigrivi said:

Im not a nuclear engineer but couldn’t a fusion reactor be used to heat liquid hydrogen to extreme temperatures similar to an NTR?

I'm not entirely sure, because in a rocket engine two elements burn to create thrust, metallic hydrogen is certainly an element that in its state has a high combustible (or explosive) potential, however in nuclear engines the same reaction does not occur In fact, since there is no combustive element (oxygen) but only the fuel element (hydrogen), in the fusion engine I believe it matters more how much an element expands when it is hit by nuclear particles. having said that I don't know id metallic hydrogen is better or worse than normal hydrogen in nuclear engines, unfortunately I think it's not something we will discover in the near future haha

[SciMan]

It is entirely possible to use a fusion reactor to heat hydrogen and then expand that hydrogen out a nozzle to create thrust.

In fact, doing so grants you higher specific impulse than a Fission thermal rocket (what most people call an NTR, but I'm using my terminology because technically a "Nuclear" thermal rocket could use either Fission or Fusion reactions to heat reaction mass and then expand it out a nozzle to create thrust).

Even a gas-core Fission thermal rocket can't compete with a Fusion thermal rocket.

And in the vacuum of space, you get even greater specific impulse if you entirely skip adding any hydrogen to the exhaust, and just use solely the reaction products of the fusion reaction (which are traveling at a few percent of the speed of light). The trade-off, naturally, is that doing such a thing drastically reduces thrust of the engine. Specific impulse and thrust are inversely related, but both are also directly proportional to the amount of power input.

For something like the Space Shuttle, chemical fuels are what you need to provide the high thrust needed for liftoff, because you can make (relatively) small engines produce a huge output power (in the gigawatts range for each SRB, and several hundred megawatts for each SSME).

For something like a fusion rocket to be feasible to lift off from Earth under its own power, it would need similarly high Thrust, which surprisingly means that Hydrogen might not be the best propellant (and you can just forget about using only reaction products).
Something like Methane would be a much better choice, because thrust depends on mass flow rate much more strongly than it depends on exhaust velocity, and you are much more able to increase the mass flow rate thru the engine with Methane than you are with low-density Hydrogen.

Actually, Water (plain, distilled water) would be a pretty good reaction mass for a nuclear rocket as well, and even with such high-density fuel a fusion rocket could likely get 3000s ISP out of such propellants simply by nature of the high temperatures the fusion reaction is capable of heating that water to.

 

Of course, the big challenge for any of these "surface to orbit" type Fusion-powered rockets is designing a compact fusion reactor and heat exchanger that despite its small size has a power output measured in likely tens of gigawatts (getting that high ISP means you have to dump a lot of power into that reaction mass).

I'm unsure if we'll see anything like this in KSP 2 however, as it seems to me that the Metallic Hydrogen engines are intended to take that niche in the "thrust versus ISP" curve.

Edited February 17, 2022 by SciMan

[KerikBalm]

On 2/17/2022 at 9:24 PM, SciMan said:

It is entirely possible to use a fusion reactor to heat hydrogen and then expand that hydrogen out a nozzle to create thrust.

In fact, doing so grants you higher specific impulse than a Fission thermal rocket (what most people call an NTR, but I'm using my terminology because technically a "Nuclear" thermal rocket could use either Fission or Fusion reactions to heat reaction mass and then expand it out a nozzle to create thrust).

No, it doesn't get you any better, not really. Both fission and fusion thermal drives would be limited by how hot you can make that hydrogen passing by the reactor and out of a nozzle, without melting the nozzle/engine. Solid core NTR is way outperformed by open cycle gas core, not because one fission reaction is more energetic than another, but because one way can reach higher propellant temperature without melting.

For a thermal rocket, Fusion or fission can provide plenty of energy, the mass of the nuclear fuel won't be relevant, just the method of heating the propellant. In that case, fission actually had many benefits

On 2/17/2022 at 9:24 PM, SciMan said:

Even a gas-core Fission thermal rocket can't compete with a Fusion thermal rocket.

How so?

On 2/17/2022 at 9:24 PM, SciMan said:

And in the vacuum of space, you get even greater specific impulse if you entirely skip adding any hydrogen to the exhaust, and just use solely the reaction products of the fusion reaction (which are traveling at a few percent of the speed of light).

Then it's not really what people would call a thermal rocket.

It's not heating a propellant with energy from a separate fuel

On 2/17/2022 at 9:24 PM, SciMan said:

 

 

[Bej Kerman]

On 2/17/2022 at 1:58 AM, K^2 said:

now-confirmed-fictional

Are you sure?

[K^2]

1 hour ago, Bej Kerman said:

Are you sure?

Reasonably. The papers which predicted metastability also predicted a much lower metallization pressure. Scientists have already put these results into question with revised models, but the fact that it doesn't match experimental predictions is fairly conclusive that these predictions were not valid. Moreover, while we still don't have definitive proof that metallic hydrogen has been achieved in laboratory conditions, there were signs of a phase transitions that make it likely that they either hit a metallic phase, some novel phase, or a mixture. Any of these outcomes make metastability rather unlikely based on the results that were obtained.

There could be something interesting going on at extremely low temperatures in conjunction with extremely high pressures. Like, if it turns into a supersolid, which some research suggests it could, there can be a sufficient energy gap to provide metastability at pressures that aren't quite that high so long as extremely low temperatures are maintained, likely going into mK if you want a substantial pressure decrease. But a) this is pure speculation at this point. Other than such states of matter are mathematically conceivable, we have zero reason to think this really takes place, and b) even then, there's only so much "impossible" stabilization you can get out of a quantum effect. You wouldn't be able to go from 500GPa to nearly nothing. You might still be able to make some small beads or wire in ceramic sheathing for its electrical and magnetic properties, but the idea of metallic hydrogen as rocket fuel is pretty much dead at this point.

[Bej Kerman]

14 minutes ago, K^2 said:

Reasonably. The papers which predicted metastability also predicted a much lower metallization pressure. Scientists have already put these results into question with revised models, but the fact that it doesn't match experimental predictions is fairly conclusive that these predictions were not valid. Moreover, while we still don't have definitive proof that metallic hydrogen has been achieved in laboratory conditions, there were signs of a phase transitions that make it likely that they either hit a metallic phase, some novel phase, or a mixture. Any of these outcomes make metastability rather unlikely based on the results that were obtained.

There could be something interesting going on at extremely low temperatures in conjunction with extremely high pressures. Like, if it turns into a supersolid, which some research suggests it could, there can be a sufficient energy gap to provide metastability at pressures that aren't quite that high so long as extremely low temperatures are maintained, likely going into mK if you want a substantial pressure decrease. But a) this is pure speculation at this point. Other than such states of matter are mathematically conceivable, we have zero reason to think this really takes place, and b) even then, there's only so much "impossible" stabilization you can get out of a quantum effect. You wouldn't be able to go from 500GPa to nearly nothing. You might still be able to make some small beads or wire in ceramic sheathing for its electrical and magnetic properties, but the idea of metallic hydrogen as rocket fuel is pretty much dead at this point.

Better give it another 1 or 2 decades

[K^2]

Might be a lot sooner. If they manage to perfect technique with the diamond vice, we'll know for sure in a few years.

[SciMan]

On 2/19/2022 at 3:33 PM, KerikBalm said:

Then it's not really what people would call a thermal rocket.

On the contrary, it is still a thermal rocket, the propellant that you are heating is also the fusion fuel. With a fusion plasma rocket (like one that uses a tokamak or stellarator to contain the fusion plasma), the fusion plasma exhausted from the rocket's magnetic nozzle will not entirely consist of fusion fuel that has undergone a fusion reaction. Instead, it will be mostly the fusion fuels that have NOT been reacted, but have been heated to plasma temperatures by the fusion reaction of the fuel that DOES react.

You see, A thermal fusion reactor works by simply heating the fusion fuels to the point that their THERMAL velocity is high enough to overcome the repulsion forces that exist between atomic nuclei, and simultaneously uses magnetic fields to compress the resulting hot plasma to the point that the rate of collisions between fusion fuel atomic nuclei is high enough to provide the specified amount of (thermal) power output.

I can understand your confusion, for there ARE fusion rockets that are NOT thermal rockets. However, this requires an entirely different design paradigm.
This different paradigm is known as a Beam Core design, which can also be known by the name "Athermal fusion reactor". This is because the reaction happens between colliding particle beams of fusion fuels, with the velocity of the particles being caused by a linear or cyclotron particle accelerator, rather than by thermal heating. Unlike thermal velocity, this velocity is highly directional, which is why you're able to form particle beams in the first place.

You do need to heat the fuel atoms a small amount, but only enough to slightly ionize them so that the particle accelerator can manipulate the atoms by use of electromagnetic fields. The majority of the input energy of the fusion reaction is supplied by the particle accelerators.
The advantage is that you can use fusion reactions that have much lower radiation shielding requirements (D-He3, P-11B, etc), despite those reactions requiring much higher particle energies to cause fusion, because you can ensure that a large portion of the input energy goes directly into making the fusion reaction happen, instead of just being mostly wasted on heating plasma that won't even be able to react before it is expelled out the magnetic nozzle.

 

Oh yeah, and as for how a fusion rocket is able to beat a fission rocket, that's easy. Each instance of the fusion reaction happening outputs more energy than each instance of the Fission reaction happening.

And as for how that translates into higher specific impulse without melting the engine, that's also easy. Since we're ideally dealing with plasmas and not hot neutral gas, instead of using a physical nozzle you can use a "nozzle" constructed of magnetic fields, ideally created by superconducting magnetic coils.

If you've ever seen a rocket engine nozzle that looks like it couldn't possibly work because there's only a skeleton in the right shape, and nothing physical to contain the gas, that's almost certainly a magnetic nozzle of some form.

If you haven't seen one, here's an image:

Magnetic Nozzle

Edited February 28, 2022 by SciMan