The Quest for a Fusion Drive
orbitalindex.com
orbitalindex.com
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.
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.
Was that paper a waste of time?
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.
We do though, you can build a fusion reactor in your garage
https://en.wikipedia.org/wiki/Fusor
What’s hard is making a fusion reactor that produces more energy than it consumes, and does so economically.
From their intro [1]
This synergy has been observed in the development of other fission-fusion devices.
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.
We have to get some form of self-sustaining fusion, i.e., producing energy, before we have a shot at making an engine out of it.
https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propuls...
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.
Could you please elaborate on the effects of high nuclear altitude testing on the belts? Preferably, a link to a paper.
It wouldn't expose the Earth to cosmic rays, but it would damage satellites.
> 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.
Hoyt, Robert P., and Bryan M. Minor. "Remediation of radiation belts using electrostatic tether structures." 2005 IEEE Aerospace Conference. IEEE, (2005). http://web.archive.org/web/20110517200032/http://www.tethers...
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."
https://www.npr.org/sections/krulwich/2010/07/01/128170775/a...
> 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.
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)
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.
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.
(b) We can build bombs and pusher plates now ;)
And while no nuclear thermal rocket engine has yet flown, there have been ground tests.
Explosions are cool. Orion is space-orc technology, extremely cool.
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.
Chemical rockets need oxygen, which is heavy and limits Isp. Nuclear rockets can use pure hydrogen and achieve higher Isp.
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.
Anyway. Way more cool.
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).