Is Fusion Practical for Spacecraft?

[cjackson]

Whenever controlled fusion is available for space applications, would it be practical for propulsion?

Would it enable man to reach the outer solar system?

[Hornblower]

Whenever controlled fusion is available for space applications, would it be practical for propulsion?

Would it enable man to reach the outer solar system?

As I see it, the challenge is to develop a fusion-powered system in a size suitable for a spacecraft. It could be used to generate electricity for an ion thruster. The primary challenge for starters is to create a controllable reactor of any size.

[WayneFrancis]

Still very early days ... there is the old saying that it is only about 10-15 years away but we are getting pretty close to surplus energy. I guess a bigger issue in space might obtaining enough deuterium. IE you really don't want to scoop up a half a million time more material then you'd actually get fuel from. Here on Earth it isn't that big of a deal to process a half a million litres of sea water to get 1 litre of deuterium...in space while travelling to your destination it might be a bigger issue but maybe someone else knows if that is actually the case or not.

[antoniseb]

Fusion is a practical option... for massive spaceships in the centuries ahead. I'm presuming that by that time we will have fusion technology to be able to use protons, and not need Deuterium. There's no point trying to design such a ship today because we don't know how big such a reactor would have to be. Would it be a tokomak with a kilometer radius? Some other design? Will nanotechnology and carbon nanotubes enable us to build them much smaller and lighter? Who knows now? The time will come.

[jfribrg]

There has been talk of mining He3 from the lunar regolith. Of course this He3 comes from the solar wind so I suppose in theory that it might be possible to collect this material in flight at least until you get close to the heliopause. I suspect that the engineering necessary to collect and extract this material would turn out to be far more difficult than bringing enough deuterium (or tritium) from Earth.

[cjameshuff]

Fusion propulsion has a completely different set of requirements from fusion power...it's entirely reasonable to have a fusion thruster that requires a fission reactor as a power supply. Controlled fusion in that sense is already available.

[swampyankee]

Well, it depends on how one defines "controlled." Properly used explosions -- such as Project Orion -- are controlled, but probably don't meet the intent of the original poster.

Within what I perceive as the original poster's intent, I'd estimate about ten years after successful controlled and sustained fusion is demonstrated in a lab. Since sustained, controlled fusion has been ten years away for about forty years, I'd not advise holding ones breath.

[cjameshuff]

Within what I perceive as the original poster's intent, I'd estimate about ten years after successful controlled and sustained fusion is demonstrated in a lab. Since sustained, controlled fusion has been ten years away for about forty years, I'd not advise holding ones breath.

People have been building devices capable of sustained, controlled fusion for decades. The hurdle for power generation has been producing more power than the fusion reactor consumes. This is not an obstacle for a propulsion system...the competing systems don't produce any power, and can be effective propulsion systems even if they use a fission reactor for power. If the efficiency gain of a fusion rocket over a plain plasma rocket is high enough to compensate for whatever extra equipment might be needed, it doesn't matter if it needs external power.

[jfribrg]

Can someone explain how you can use fission to power a fusion reaction (besides the obvious hydrogen bomb mechanism).

[cjameshuff]

Can someone explain how you can use fission to power a fusion reaction (besides the obvious hydrogen bomb mechanism).

Are you asking how fusion reactors work? How fission power plants produce electricity? Something else?

[jfribrg]

Are you asking how fusion reactors work? How fission power plants produce electricity? Something else?
As I understand it, in order for fusion to work, you need tremendous pressures. In a hydrogen bomb, a fission bomb is used to create these pressures, but this is not "controlled fusion". Other methods involving lasers or electromagnets (and probably other methods that I am not aware of) require enormous amounts of energy and a very heavy containment structure. I don't see how you can use fission as an energy soure in a spacecraft. The weight of a reactor big enough to generate the multi-megawatts necessary to sustain fusion seems vastly prohibitive, and that doesn't include the infrastructure to replace spent fuel rods nor the shielding to protect the rest of the spacecraft from the radiation ( even if the craft is unmanned, we still need to protect the electronics from the radiation). If you are going to be lifting that much weight into space, why not just build a gigantic ion drive with an enormous supply of propellent and skip the fusion.

[cjameshuff]

As I understand it, in order for fusion to work, you need tremendous pressures. In a hydrogen bomb, a fission bomb is used to create these pressures, but this is not "controlled fusion". Other methods involving lasers or electromagnets (and probably other methods that I am not aware of) require enormous amounts of energy and a very heavy containment structure.

You need temperature more than pressure, and it is easier to increase temperature while keeping the pressure very low. The purpose of the heavy pressure vessel is to exclude the atmosphere outside, not contain anything, the interior of an operating fusion reactor is a near vacuum. For a spacecraft, it is the mass of the magnets, cooling and power systems, etc that is a concern. High power plasma drives and even relatively low power ion drives already have similar equipment.



I don't see how you can use fission as an energy soure in a spacecraft. The weight of a reactor big enough to generate the multi-megawatts necessary to sustain fusion seems vastly prohibitive, and that doesn't include the infrastructure to replace spent fuel rods nor the shielding to protect the rest of the spacecraft from the radiation ( even if the craft is unmanned, we still need to protect the electronics from the radiation).

Erm,...there's several dozen nuclear reactors in orbit right now. They are heavy, yes, but they are also capable of high power output for extended periods of time. I'm not sure why you think replacing fuel rods is an issue or a requirement, and shielding requirements can be greatly reduced just by careful positioning, or even by mounting the reactor on a boom (there's no sense in shielding against radiation that's going into open space). Nuclear-electric propulsion is not a wildly speculative sci-fi idea, it's essentially what VASIMR, HiPEP, PITs, etc were designed for.



If you are going to be lifting that much weight into space, why not just build a gigantic ion drive with an enormous supply of propellent and skip the fusion.

Because such a drive puts out no more power than you put in, and total system mass may very well be nearly as much as a system that achieves some amount of gain using fusion.

[IsaacKuo]

It's really not clear if you gain anything using fusion, though. While fusion could boost the overall energy of the exhaust products, the easier fusion reactions put most of that energy into neutrons--which can't be deflected for thrust. The result is that you actually lose energy and thrust compared to not using fusion at all. Worse still, these are high energy neutrons which demand more shielding than the low energy neutrons generated by a fission reactor.

That said, there are a whole bunch of different potential fusion concepts, fission concepts, and fusion-fission hybrids. It's not a simple yes/no question.

There are also non-nuclear propulsion possibilities for manned missions to the outer solar system and even interstellar missions.

[eburacum45]

For some interesting data on real, speculative and fictional propulsion systems you might like to read this Project Rho page
Atomic Rockets Engine List (http://www.projectrho.com/rocket/enginelist.php)

[cjameshuff]

It's really not clear if you gain anything using fusion, though. While fusion could boost the overall energy of the exhaust products, the easier fusion reactions put most of that energy into neutrons--which can't be deflected for thrust. The result is that you actually lose energy and thrust compared to not using fusion at all. Worse still, these are high energy neutrons which demand more shielding than the low energy neutrons generated by a fission reactor.

Those are real issues with fusion propulsion, yeah. But again, my point was simply that fusion power here on Earth is not a prerequisite for fusion propulsion in space...a fusion rocket that requires external power is not an automatic failure, considering that the closest alternative is plasma rockets that also require external power.

As for the neutrons, that actually reminds me of another idea I've had...use fusion as a neutron source for a subcritical nuclear saltwater rocket. This might let you get away with less enriched fission fuel that would be much less of a storage and transportation nightmare. I'm not even going to try to argue that this is practical, but it might well be closer than a straight NSWR.

[PraedSt]

In order to achieve thrust, is it necessary to deflect neutrons? Can't you asymmetrically absorb them instead? We know how to absorb neutrons I think.

[Noclevername]

You could use an aneutronic form of fusion, but that's a much harder feat to accomplish than D/T fusion.

[Van Rijn]

In order to achieve thrust, is it necessary to deflect neutrons? Can't you asymmetrically absorb them instead? We know how to absorb neutrons I think.

Or symmetrically absorb them. It's expected that D-T fusion reactors would have lithium blankets. The blanket would absorb energy while stopping the neutrons. Heat engines could then be used for energy generation. Also, some of the lithium would be transmuted into tritium, which would be used as fuel for the D-T reactor. That's very important because, due to its short halflife, only traces of tritium can be found in nature.

[cjackson]

What if we stick with launching payloads to orbit with chemical rockets, or we build the spacecraft in orbit? Then would fusion reactors or lasers be able to cut flight time to the planets down from years or months to weeks or days?

[Hornblower]

What if we stick with launching payloads to orbit with chemical rockets, or we build the spacecraft in orbit? Then would fusion reactors or lasers be able to cut flight time to the planets down from years or months to weeks or days?

That would depend on what sort of thruster and propellant combination is powered by the reactor. For a trip to the outer planets or beyond, a nuclear powered ion thruster could deliver a lot more final delta v than a chemical rocket with the same mass of propellant, but it would do so by delivering a tiny amount of thrust for a very long time. It is difficult to get high short term thrust and high specific impulse in the same thruster. It would not necessarily reduce the transit time for the inner planets. I don't know whether or not it would speed up a trip to Neptune or Pluto.

When I was in high school in the 1960s, my father showed me an illustration of a proposed nuclear powered ion thruster spacecraft going to Mars. Its acceleration at full power was about 1/10,000 g, and it gradually spiraled out from the Earth and then from the Sun, rather than having an all-at-once insertion into a Hohmann orbit as would be done with a chemical rocket. The trip was slower than with a chemical rocket. The theoretical advantage was a large reduction in the total mass, mostly propellant, which had to be lifted from the ground and into a low Earth orbit to assemble the spacecraft.

[hypergreatthing]

just to add some info:
Fusion has been done since the Farnsworth fusors in the early 60s (http://en.wikipedia.org/wiki/Farnsworth_fusor), which as far as i understand are basically cathode,anode reactions at really high voltages which produce a net loss of energy, but frees neutrons. People make these as a hobby now a days.
The idea of a controlled fusion reactor that generates energy has been a scientific dream for a while. There's laser, magnetic confinement (such as Tokamak, ITER, polywell etc) and a few other methods.
For space travel there's two different usages. For direct engine output (think orion, replace atom bombs with hydrogen bombs and get more thrust) or for generating electricity/energy and using it via ion engines or other.
The real advantage over conventional vs fission or fusion is one: weight of fuel.
If your spaceship has less mass because the fuel has a huge energy density compared to LOX (or whatever rocket fuel you're using), then you need less thrust to accelerate. You'll also be able to travel longer distances because you can carry more fuel, etc.

[cjackson]

Oh and I'm not asking whether it would be practical with today's technology, but rather 100 years or more into the future.

[swampyankee]

Oh and I'm not asking whether it would be practical with today's technology, but rather 100 years or more into the future.

I don't think we can reliably predict what will be practical in ten years, let alone a hundred.