In terms of fusion drivers I've always liked NASA's PuFF[1] or Pulsed Fission Fusion. The idea is to use a normal z-pinch pulsed fusion setting, which won't normally be self sustaining. But if you surround the D-T mix with depleted uranium it'll adsorb a lot of the fast neutrons from the fusion and trigger a round of fission generating more energy. The recoil against the magnetic compressor/nozzle should be enough to generate enough energy for the next pulse and the reaction byproduct are emitted to become your propellant.
Fusion on earth (that people talk about) is about generating electricity. Fusion on spacecraft is about shooting out exhaust gas as quickly as possible and storing energy at high density. The rocket equation relates the exhaust rate to the propelent mass requirements, which determines the feasibility of the spacecraft. It's a completely different optimization problem, and one that has historically favored nuclear energy sources in general.
To clarify further, from TFA: "While early versions of the Helicity Drive wouldn’t exceed the Lawson criterion (more energy out than in) and would require external (presumably solar) power, the energy added to the exhaust from fusion would increase the system’s exhaust velocity and ISP (giving somewhere in the neighborhood of 10,000 s isp, vs a maximum of ~400 for chemical rockets)"
So it is not able to generate more energy than it takes to operate, just like the fusion devices we can already create on earth, but can create really fast exhaust which makes the rocket more efficient.
In 1998, Robert A. Freitas published a detailed design of an artificial red blood cell with several useful properties, (stores 236 times as much oxygen as a natural red blood cell, indefinite shelf life) but with the minor downside that it would require atomically precise diamond fabrication to actually construct. https://www.tandfonline.com/doi/pdf/10.3109/1073119980911768...
I'm similarly always a little surprised at just how inefficient photosynthesis is compared to a solar cell. Once I realized it was more than an order of magnitude I lost a lot of interest in biomass (much less ethanol) as a fuel source for sure...
There is value in creating designs that are feasible if some technological breakthroughs is made. If we manage to make fusion work, then someone can then capitalise on the existing designs. It's also a great motivator for people to work on those hard problems that could enable many fantastic possibilities. It's also just fun to let your imagination not be constrained by what is possible today, but what might be possible in the future.
For a power plant, you need to get more energy out than you put in. The best we've done is output that's about 70% of the input energy.
But a plasma rocket works without any fusion at all. If you get that same 70%, then you're doing 70% better than a plasma rocket without fusion.
On top of that, a power plant needs to run reliably for thirty years or so, withstanding intense neutron radiation, and breeding new tritium fuel from lithium. A fusion rocket might only need to fire for several days for a trip to another planet, and won't breed its own tritium.
The NASA Puff proposal would be more aptly described as a boosted fission engine.
From their intro [1]
This synergy has been observed in the development of other fission-fusion devices.
You may be wondering what "other fission-fusion devices" were ever developed. The answer is of course, nuclear bombs.
The first fission-fusion device was not a thermonuclear bomb, but a boosted fission one [2]. In such a device, there's a little bit of Deuterium and Tritium in the center of the Plutonium pit. When the Plutonium starts undergoing fission, the temperature increases to tens of millions of degrees, and the D-T fusion reaction happens; the energy produced by that reaction is negligible. What the reaction does is release a huge amount of neutrons that go and split nuclei of Plutonium, therefore boosting the yield of the fission in the bomb.
The idea with Nasa's Puff proposal is the fusion is triggered first, using a plasma confinement device, similar to a Tokamak, but linear, not toroidal. The device is well below breakeven. But you don't care about the energy produced by the fusion, only that enough neutrons are released to go on an split a bunch of fissile nuclei. Few people are aware, but not only Uranium and Plutonium (and other heavy nuclei) can undergo fission, but also Lithium. Lithium can't sustain a chain reaction, but if external neutrons (or protons) are provided, it is happy to split and release a respectable amount of energy in the process. Pound per pound about as much energy as Uranium and Plutonium.
The end result is a very hot plasma of millions of degrees that is expelled and provides huge thrust and specific impulse. Of course the ejected plasma will be radioactive, and that's why this design can't be used on Earth to produce energy, or other useful things.
But in space, blowing some radioactive stuff is not a problem: it will simply spread out and go on and on forever and ever. 99.9999999% of it will not encounter anything for billions of years. The few particles that will, by luck of the draw, encounter something will be no different than other cosmic particles coming from supernovas, or other sources.
Ofcourse we can - that's what a hydrogen bomb does!
The Teller–Ulam design and the NIF do the exact same job - they heat up and compress hydrogen to trigger fusion. Its just that NIF and has to fuck around with lazers that are perfectly focused on tiny gold pellets with micrograms of hydrogen. The hydrogen bomb deals with literal tons of material at once.
If you dont care about safety and environment, you concrete over one of the great lakes, heat them up by exploding thermonuclear bombs and use the steam to generate elctricity.
There are a lot of reasons why this is a bad idea, not least of which is the effects of the resulting electromagnetic pulses on satellites, the earth, and the equipment on board the vehicle itself.
There is also the radioactive fallout that would affect human populations all over the world.
Detonating nuclear weapons at high altitudes or in space would also disturb or damage the van allen belts and expose the surface of the earth to high levels of radiation from solar winds and cosmic rays.
And of course there is the fact that it's a violation of international law to bring nuclear weapons to space or to test or detonate them in the atmosphere or in space.
Sure, those are problems if the Orion method is used inside the orbital region.
However, if used after escaping from Earth's gravity well to accelerate towards distant destinations, the only remaining issue is international law. Electromagnetic pulses decay as the inverse square law, radioactive fallout would be outside the Earth's gravity well and disperse harmlessly in space, the Van Allen belts are also not near enough to be affected.
The only real risk would be from accidents while lifting that much fissile material to space, which could be mitigated by proper containment vessels (which do add weight and reduce effective payload, but the total lift may still be worth the cost)
I've always wondered if we could mitigate this issue by burning the spacecraft into a parking orbit around the L2 Lagrange Point on the far side of the moon using conventional propellant, and then beginning detonation there. Irradiating Earth is of course a bad idea, but the Earth Moon L2 point should be plenty far enough away, provides a stable spot from which to begin pulsing in whatever direction they like as the moon orbits, and has the added benefit of the moon itself acting as a shield for the Earth from the radiation.
It doesn't really matter if we irradiate the hell out of the far side of the moon. It's been pelted by solar wind and cosmic radiation since time immemorial. A bit of radiation from a few nukes should be negligible by comparison.
The Moon is very far away. Can we really build strong enough fusion bombs that they would irradiate people down to earth even if they were at half of the distance between earth and moon?
> Detonating nuclear weapons at high altitudes or in space would also disturb or damage the van allen belts and expose the surface of the earth to high levels of radiation from solar winds and cosmic rays.
Could you please elaborate on the effects of high nuclear altitude testing on the belts? Preferably, a link to a paper.
> Code-named "Starfish Prime" by the military, it literally created an artificial extension of the Van Allen belts that could be seen across the Pacific Ocean, from Hawaii to New Zealand.
The belts are filled with captured charged particles which deflect cosmic rays and protect the earth from them. If you blow them away, those rays will get through. One of the Project Orion proposals outlined in the linked wiki is about how to mitigate the impact somewhat by launching from unpopulated areas so that humans are not affected by the resulting radiation that gets through the hole you punch in the earth's protective charged particle shield.
I think you've misunderstood what the Van Allen radiation belts are. Here's a direct quote from the first paragraph on Wikipedia.
> A Van Allen radiation belt is a zone of energetic charged particles, most of which originate from the solar wind, that are captured by and held around a planet by that planet's magnetosphere. Earth has two such belts, and sometimes others may be temporarily created. The belts are named after James Van Allen, who is credited with their discovery.[1] Earth's two main belts extend from an altitude of about 640 to 58,000 km (400 to 36,040 mi)[2] above the surface, in which region radiation levels vary. Most of the particles that form the belts are thought to come from solar wind and other particles by cosmic rays.[3] By trapping the solar wind, the magnetic field deflects those energetic particles and protects the atmosphere from destruction.
To recap:
- The radiation (aka charged particles, aka cosmic rays) is captured by the Earth's magnetosphere.
- The particles are deflected and steered around/away from the planet.
- You can't blow up a magnetic field lines with a bomb since they originate from within the planet's core.
- You can change the velocity of the charged particles themselves by detonating a nuclear device nearby, but...
- Such a device wouldn't affect more than a small number of particles (many would be in Earth's shadow), and would only affect them with pressure from the X-rays and other light released from the explosion because there's no air in space that can carry a pressure wave.
- The Orion proposals are talking about mitigating the damage from detonating nukes in atmosphere, specifically fallout.
I think that there's no chance we'd ever take the risk of launching from Earth into orbit using an Orion drive, the fallout cannot be contained. However, I do think that we should explore using an Orion drive in the area beyond Earth orbit.
There are proposals to hang a tether between the belts and use the charge gradient along the tether to generate electricity -- using the belts as a giant battery.
Fallout, and the particles captured in the belts are essentially the same thing from different sources. Yea it's not "blowing up" the field, it's dispersing the captured particles. The earth's magnetic field is not evenly, statically distributed. The lines of force could be seen as moving, they are not like in a fixed place. Captured particles set up an additional field which deflects more particles from coming through. Dispersing them means new particles can come through. For example, this deflection effect is thinner at the poles which is why more particles come through there, leading the the northern and southern lights. That's why during periods of high solar wind intensity such as solar storms, the lights sometimes occupy a much larger part of the sky and move closer to the equator.
If you read the whole wiki page, there is a more detailed explanation including simulations showing how it works. From the first paragraph, "Most of the particles that form the belts are thought to come from solar wind and other particles by cosmic rays. By trapping the solar wind, the magnetic field deflects those energetic particles and protects the atmosphere from destruction."
The thermonuclear Orion could, on paper, achieve up to 10% the speed of light on an interstellar trajectory at least for a small payload. It's possible that it could send a probe to the Centauri system that would arrive in 40-50 years, short enough to be feasible on human time scales.
I never got the fascination for Orion. Why on earth should a spaceship carry a gigantic pusher plate? Carry hydrogen instead and put through a conventional fission engine. That gives you a much more controllable engine and should save you a lot of dry mass.
Because despite having insane amount of energy in Uranium, you can't tap into it.
Uranium melts at 1000 degrees centigrate, so thats your rough upper limit for how much your reactor can heat hydrogen. If your Uranium melts, it will be leaving the rocket with the hydrogen and spreading radiation everywhere. Basically like Orion but worse.
A normal chemical rocket reaches higher temperatures.
A nuclear bomb will heat anything to millions of degrees. Much more efficient, and any exhaust is moving so fast it will escape the solar system without bothering anyone, provided its pointed the right way.
As far as I know the iSP (specific impulse) of Orion is a lot higher, which may result in better overall performance.
There are some high temperature nuclear rocket engine designs though, like nuclear gas core rockets. They're crazy reactor designs you would never try running down here inside a biosphere but have energy densities far higher than anything else save a bomb.
Yeah the reason is that Richard Nixon cancelled Project Rover, which had long since demonstrated that a safe and reliable nuclear engine was totally feasible
The concept described there sounds like BS. If you're not achieving breakeven, it's probably much easier to just heat plasma with the drive energy and expel that (VASIMIR, if you've heard of that). The plasma stays much cooler than a fusion plasma, so radiation from it should be less -- and this radiation is a big problem, since it heats the vehicle, and that heat has to be radiated, and radiators are big and heavy.
The problem of cooling is the bete noir of all high Isp propulsion systems. This is why I liked laser propelled systems, since it's possible to cool an object with a laser beam (anti-Stokes scattering).
[1]https://www.nasa.gov/puff