Need help figuring out a possible fusion engine

[DHF]

Hello,

I am in the process of writing space based sci fi novella, I made a few posts last year and earlier this year regarding different aspects of the ship and crew. If anyone is interested, I wanted to concentrate on the fusion engine of the ship itself and hopefully come up with a feasible model.

The novella takes place several hundred years in the future and details Earth's first trip to Alpha Centauri. The ship will take 40 years to get there and will be traveling at a peek velocity of 0.12c . The ship will be coasting for most of the journy because there was no way I could work out a constant acceleration trip without making the ship the size of texas just to hold the fuel :)

The question is regarding how the engine might work, Its a Fusion engine but the problem I have is that most types of fusion reaction require hydrogen isotopes that are only stable for a decade or so. This isn't a problem for shorter journeys but for a trip lasting 40 years I couldn't come up with an engine that would still have viable fuel to decelerate with.

That had me thinking about a proton to proton fusion reaction. I know we are no where near that level of design today so this will all be pure speculation. Assuming the civilization in question could develope a proton to proton engine, what might it look like? I know stars get away with it but is this something that is even feasible for us to reproduce and what are some of the ways we might go about it?

If the idea of a proton to proton reaction is completely off the table then what are some of the other methods my ship might utilize to facilitate a 40 year journey?

Thanks for all your help.

 

[mfb]

Proton-proton fusion is extremely challenging and pointless. You can use deuterium-tritium fusion to accelerate and deuterium-deuterium fusion to decelerate. Deuterium is stable, only tritium is radioactive (but you use it within the first years, that is fine).

Deuterium-helium3, Boron11-proton and other fuels are possible, but DT and DD are more convenient. Avoiding neutrons in the reaction could become interesting in the future.

 

[DHF]

ok that is good to know. Is there a reason for using the two different types of fuel? why not just rely solely on Deuterium all the way?

 

[mfb]

DT fusion releases nearly twice the amount of energy of DD fusion (per mass). It also gives a higher fusion power for the same conditions (a factor of 50 more!), which helps to accelerate the ship while it is still heavy due to all the fuel.

You could also load your ship with deuterium and lithium and breed the tritium on the way. That is the same concept fusion power plants on Earth would use. That is probably a better solution for both acceleration and deceleration.

 

[DHF]

that makes sense. using the more volatile fuel first allows them to pack much less fuel mass.

Thank you very much.

 

[DHF]

I like the idea of breeding the Triterium. Triterium has a lifespan of about 12 years yes? so the ship would keep the lithium in storage then a few years before they reach the star system it would begin the breeding reaction.

 

[mfb]

Breeding and fusion would be directly connected and happen at the same time - you need neutrons for breeding, and the DT fusion reaction is the most reasonable source for neutrons.

The breeding reaction is not as efficient as fusion, so starting with some supply of tritium for the acceleration phase could still be interesting, but for deceleration a closed tritium cycle (breed -> fuse -> use released neutron to breed new tritium) is important.

 

[DHF]

In that case a breeding system might not work. The ship will be accelerating for about a year then it will coast for almost 40 years before it decelerates. If the triterium is bred during the acceleration, won't it have decayed by the time the ship arrives at its deceleration point?

 

[newjerseyrunner]

Could you use a proton / antiproton? You'd require a lot less fuel and get much more power. If you calculated the density of interstellar hydrogen compared to how much energy you need, you might be able to not require matter at all. If you brought antimatter with you and sucked up hydrogen from the heliosphere / interstellar space.

 

[DHF]

I had considered an antimatter drive but ultimately discarded it because the trip will take several decades and I feel that it would be too difficult for the ship to safely contain antimatter for that long. Even with your design it would not be safe because the stored antimatter would react with anything it touches so if the containment field were ever to fluctuate at any point during the decades long cost, it would come in contact with and react with the liner of the fuel storage container.

There is also a matter of cost. Economics always have to be considered and I just do not feel that any organization would have the resources to fabricate several thousand tons worth of antimatter. Fusion ultimately won out because we are activly striving to master it today and I feel in a few hundred years it will be a viable fuel. It would also be safe to store and relatively cheap compared to antimatter.

 

[mfb]

In that case a breeding system might not work. The ship will be accelerating for about a year then it will coast for almost 40 years before it decelerates. If the triterium is bred during the acceleration, won't it have decayed by the time the ship arrives at its deceleration point?

You don't need to store tritium for longer than a few days (in reprocessing materials).

You start with deuterium, a lot of lithium and a small amount of tritium for a few days of initial fusion reactions. You fuse this tritium with deuterium, creating new tritium in the process. You can use the produced helium for ion drives, that is probably the most efficient way to use the fuel.
Once you reached cruise speed, you reduce the fusion power to levels necessary to support your ship. That will always keep some amount of tritium on the ship.
To decelerate, you probably want much more power than during cruise time. This can be achieved with a different lithium composition - lower power output for a while, but more tritium so you increase the possible power output quickly. Alternatively, have some small neutron source around to activate as necessary. Either way, it is sufficient to breed enough tritium for a few days of operation, because using this tritium allows to breed more again.

 

[DHF]

Ok gotcha. So keep the reactor on at a trickle of power, just enough to keep producing more triterium.

You also mentioned D-h3 reaction. what would be the downsides of that?

 

[mfb]

It needs much higher temperatures and the reaction rate is much lower. The second issue could be a very fundamental problem, as you get more energy losses from Bremsstrahlung compared to heating from fusion, so the plasma could cool down too fast for stable fusion.
Also, He-3 is quite rare.

 

[DHF]

fair enough. D-T with Tritium breeding it is. Thanks for the help.

 

[DHF]

MFB, I would like to follow up on your previous post regarding the engines. Going with a D-T reaction you mentioned using the H4 for ion engines. So in my design of the ship. the main power would come from a D-T fusion reactor then the H4 produced from that reaction would be channeled to a bank of Ion engines to provide thrust to move the ship.

Do I have that right?

 

[mfb]

Something like that. There are certainly more futuristic options, maybe some of them give a bit more thrust, but that is certainly something that should work centuries into the future.

 

[DHF]

Ok good to know. I doubt I will go into detail about the mechanics of the engine on page because the story is about the crew, however I like to have things sorted out ahead of time. This way if the crew make a passing remark about the engines, I sound like I half know what I about :)

Thanks for the help.

 

[DHF]

Quick question on shielding: to save mass, can I position the deuterium tanks between the engine and the sensitive parts of the ship to act as shielding?

 

[mfb]

The shielding is needed between plasma and (cold) superconducting coils anyway, you also need it to extract heat and to breed tritium. Shielding is not an issue.

 

[DHF]

excellent. thanks.

 

[Khashishi]

I don't think it makes sense to use the fusion reactor to generate electricity to power ion thrusters. It's more efficient to use the reactor to produce thrust directly. The energy requirements of the crew will be small in comparison to the propulsion energy.

The considerations for fusion as a thruster are somewhat different than fusion as an energy source. Exploding hydrogen bombs behind you (similar to Project Orion) might work (but we can probably come up with something better in 200 years). Look at http://galileo.phys.virginia.edu/classes/109.jvn.spring00/nuc_rocket/Dyson.pdf

DT fusion is easier, but neutrons are dangerous for the crew, and neutrons are harder to reflect backwards. If you want to breed tritium, that means you have to capture the neutrons, which means you are losing some thrust relative to just reflecting them. And you also have to carry lithium and some neutron multipliers, which is extra mass. So DD fusion might be more practical.

 

[DHF]

Well using the fusion engines to directly produce thrust was my original idea. MFB suggested using the Helium bi-product in ion thrusters would be more efficient. I would be curious to know the pros and cons of both.

using an orion type idea was the model I started with when I began putting this story together. I discounted it because I felt that 200-500 years later that design was far too crude not to mention this voyage is being funded by a private party and I couldn't rationalise how they would get permission to stockpile several thousand warheads.

I considered several reactions and with MFB's help I settled on D-T because it does seem to be the most efficient. I am less concerned with radiation because the crew will be androids. I am aware that there are issues of neutron activation and embrittlement to be dealt with so shielding is needed but not as much as would be to protect an organic crew.

As for using a D-D reaction, The level of power produced is vastly inferior to D-T, I want the ship to make the journey in under 40 years so speed is a factor. Using D-D I would need to pack extra engines to match the same level of thrust and I would need to pack a lot more fuel. In the end I figured the extra shielding and breeding components of a Tritirum based Engine were worth it.

 

[mfb]

Not more efficient (the conversion of thermal energy to electric energy will be inefficient), but easier. Direct exhaust of fusion products might be possible. DD fusion still produces neutrons, 3He 3He or p 11Be fusion would be better. It is unclear if thermal fusion with those fuels is possible at all, given the large Bremsstrahlung losses.

 

[DHF]

So direct exhaust would be the most efficient use but channeling the H4 to Ion thrusters would be mechanically easier?

 

[mfb]

Well, I guess you would not "channel" them. The helium ions hit divertors, capture electrons, get pumped out, the helium can get stored somewhere. The ion drives then ionize it again and accelerate it using the electric energy from the fusion reactors. As last step, electrons are added to the fast ions to make the total exhaust neutral.

 

[Khashishi]

I don't see why you use a diverter plate rather than diverting the ions directly into space. Today, we have big problems creating materials that can withstand the diverter heat flux, and in space, there isn't any good way to cool the diverter. If you used some magnets to divert the plasma to the rear, then that will be an ion thruster itself. But I think difficulty cooling the engine will make a tokamak-like reactor very difficult to put in space. Even if you can block the neutrons with a blanket, won't the blanket overheat? I mean, some heat is good for creating electricity, but too much and you cook everything on board. An aneutronic reaction might be necessary so most the heat can be diverted into space.

 

[DHF]

As far as heat management goes, I figured it could be dealt with on account of the engines only being on full burn for a limited period. for most of the trip the engines will be on a trickle of power, just enough to maintain the systems and keep breeding new fuel. Initially I had calculated the trip based on a one year 1/4 G Acceleration/Deceleration. I suppose I could shorten the time the engines are on by bumping up the acceleration, If the engines push the ship at a full G then they only need to be on for a little over 3 months at each point. If the components and shielding can handle the heat for a few months, then they have almost 30 years to cool off ( originally I based the trip taking 40 years but I bumped up their cruising speed to .15c).

 

[Khashishi]

I'm not sure what the solution to bremsstrahlung losses is, but maybe in 300 years they'll come up with something. Frankly, I don't think current technology will do. If it is possible to reflect x-rays with high enough efficiency, then maybe it's possible to reflect the bremsstrahlung emission back into the plasma. Or maybe the plasma is super-dense optically thick to bremsstrahlung. This requires some kind of inertial confinement fusion, which is pretty much catalyzing bomb explosions. A plasma without electrons would eliminate most of the bremsstrahlung, but how to store a large charge?

Maybe using some kind of non-thermal fusion is the way. Muon catalyzed fusion is real, but you need an efficient source of muons. The problem with accelerator based fusion is that the scattering cross section of the fuel is much higher than the fusion cross section, but maybe there is some way to prevent the fuel from becoming thermalized. Getting speculative here, but maybe if the fuel is in a Bose Einstein condensate, the scattering can be controlled or there may be some way to react by resonating with a carefully tuned input wave.

 

[DHF]

Well the mechanical issues I am willing to take liberties on. I just wanted to make sure that I did not violate any of the laws of physics while designing the ship. If it is something that is possible but we just haven't built it yet, I can live with that. I already had to make that connection with the deflector in front of the ship. Traveling at .15c, every grain of sand striking the shield is going to be like a very large nuclear explosion. The shield will have to endure this abuse for decades so whatever it is made of, doesn't exist yet. If however the engineers of the time have developed alloys that can absorb or deflect that much energy then those same materials or ones like it could be employed in the construction of the engine to handle the extreme temperatures and pressures.

 

[mfb]

The acceleration phase is orders of magnitude too long to store the excess heat in the ship, you'll need radiators independent of the details. Their size will depend on the details.

 

[DHF]

Yes that isn't a problem, I already included Radiators for the heat :)

 

[GTOM]

I would like to have other kind of questions. Could a D-T engine go off like a bomb, or simply melt down? Would such meltdown make the environment heavily radioactive for a long time? I guess it would be still a low thrust high specific impulse drive, whether it produces thrust directly or powers ion thrusters.

 

[mfb]

You can design an engine based on bombs. Project Orion uses that idea.
A reactor cannot explode like a bomb, there is simply not enough fuel and not enough time - as soon as something goes wrong, the confinement is lost, the plasma expands and the reaction stops immediately. The reactor can handle the power output of several seconds before melting down, so in the worst case the plasma melts the innermost surface of the reactor chamber. Replace it (and let its radioactivity decay over time - that is necessary anyway), done.

 

[Drakkith]

I would like to have other kind of questions. Could a D-T engine go off like a bomb, or simply melt down? Would such meltdown make the environment heavily radioactive for a long time? I guess it would be still a low thrust high specific impulse drive, whether it produces thrust directly or powers ion thrusters.

This was a long-time belief in the Battletech universe, but the developers actually deconstructed this idea in one of their technical source manuals several years ago.

 

[DHF]

Aside from the sheer improbability of cost and availability, this is one of the reasons I strayed away from an Antimatter or Orion type drive. Early on I was in favor of The Orion style design but ultimately I didn't like the idea of my ship sitting on thousands of warheads for decades through deep space. one malfunction and there goes the whole thing. Being able to stockpile several thousand warheads for private use was also out of the realms of possibility even for a sci-fi story.