Is Fusion Practical for Spacecraft?

[cjackson]

Would fusion make for practical applications in high speed interplanetary propulsion?

Or would beamed propulsion be better?

 

[QuantumPion]

That really depends on what your definition of practical is. Since neither technology exists today, and regardless of what technology you use interplanetary space travel would still take hundreds of years at the very minimum, you'll have to narrow the parameters of your question.

 

[cjackson]

Travel within the Solar System, let's say.

 

[D H]

Neither technology exists today. Einstein's words are very applicable here: "If we knew what we were doing, it wouldn't be called research, would it?"

 

[Ryan_m_b]

Would fusion make for practical applications in high speed interplanetary propulsion?

Or would beamed propulsion be better?

That really depends on what your definition of practical is. Since neither technology exists today, and regardless of what technology you use interplanetary space travel would still take hundreds of years at the very minimum, you'll have to narrow the parameters of your question.

I bolded the point I think most pertinent. Whilst we can speculate on nuclear fusion propulsion and beamed propulsion without actually having either of them we really can't say because we don't know the ins and outs of how they would work.

 

[turbo]

We can't achieve break-even fusion in controlled situations on Earth in labs. How can we expect to use fusion for space-flight? It's hard to to manufacture an excuse for this...

 

[D H]

We can't achieve break-even fusion in controlled situations on Earth in labs. How can we expect to use fusion for space-flight? It's hard to to manufacture an excuse for this...

In its favor, fusion as a propulsion source is in a sense an easier problem than is fusion as a source for electrical power. The first is creating oomph, the latter, zzzzt. To make electrical power, you have to constrain the natural tendency to create oomph and add a whole lot of extra infrastructure to create zzzzt. That's not to say that there aren't boatloads of unsolved problems with using fusion for spacecraft propulsion.

The same goes for beamed propulsion. Prototypes of beamed propulsion system are about as applicable to solving the problem of space propulsion as is a tokamak.

Beam propulsion is not a solution to space propulsion, period. Beamed propulsion solves the problem of lifting tiny shiny objects a few hundred meters straight up. Orbital altitude is a few hundred kilometers straight up. Radiation pressure is an incredibly weak force. An absolutely humongous laser is needed to something bigger than a tiny, shiny flat piece of metal. Using a laser to send an object straight up is not getting things into space. At least not for long. The energy difference between sitting still on the surface of the Earth versus being in LEO is mostly kinetic.

 

[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?

 

[D H]

We don't know how to build fusion reactors for space propulsion. We don't even know how to build viable fusion reactors on the ground (discounting behemoths like tokamaks that currently consume a lot more power than they create). We don't know if the concept is at all practical. The same goes for beamed propulsion. Worse, in fact. There are many reasons to say that that concept is not practical, period.

These are two of many proposed technologies that are at a perpetually low technology readiness level. Pick your poison, they've all been discussed at this site. There is no way to know ahead of time which, if any will be the "next big thing" in space propulsion. The next big thing in the near future won't be either of these two, and the next big thing in the far future will most likely be something we have not dreamed of yet.

 

[tasp77]

Even if no fusion reaction occurred at all, zapping a fuel pellet with something intended to initiate fusion would drastically heat and vaporize the pellet, and give you an interesting specific impulse. Imagine an RL-10 with a exhaust temperature in the millions of degrees versus 5000K!

Now whether or not that specific impulse figure is worth the bother of carrying around the zap generating equipment, I dunno. You need a power source for that part too, of course. If there was some fusion reaction, even far less than unity in comparison to the power input, you would be further heating the pellet, and increasing the specific impulse.

There would be many tradeoffs to wade through here; how much mass does it take for the zap unit, how big is the pellet, how efficiently can it be heated, does any fusion occur, how fast can the device zap pellets, and how big is the payload mass we are accelerating? There would be some possibilities that would not be useful, like an extremely high specific impulse with a very low firing rate and an enormous power supply. If there would be some more favorable permutations, you might have something . . .

 

[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.

 

[turbo]

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

We don't have such crystal balls. There may be some technology that will leap-frog the dream of fusion propulsion, but we don't know, yet. Bush killed NASA's breakthrough propulsion program. Granted, some of the ideas (extracting and using energy from the quantum vacuum) were pretty "out there", but I don't think W could have wrapped his brain around those, anyway. It was an easy place to cut. Too bad, because if we really want to send humans out of Earth's environs, we need to get away from launching chemicals, combusting them and tossing them out the back of a rocket. It costs 'way too much to get those materials in space to begin with.

 

[D H]

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

Think back to 1911. There was no way back then to predict the world of 2011, and there is no way now to predict the world of 2111.

 

[Ryan_m_b]

It boggles my mind that people use phrases such as "in one hundred years" in a "legitimate" way. No offence intended but as D H points out it's ultimately futile.

 

[tasp77]

Just batting around ideas here, the flash neutron source used as triggers in nukes, how collimated (if not classified info) is their output?

If aimed at a very tiny piece of DU foil, you would get a tiny bit of fission. Enough to vaporize the tiny piece of foil to an interesting degree, I dunno. And it occurs to me those neutron sources probably aren't engineered for more than one burst (wink). This would not be a particularly clean source of energy, but there might be more suitable isotopes for testing. Or maybe plating the foil with something else. Not sure what kind of specific impulse might be theoretically possible, and in actual use it would be much lower depending on engineering.

 

[Janus]

It boggles my mind that people use phrases such as "in one hundred years" in a "legitimate" way. No offence intended but as D H points out it's ultimately futile.

Case in point: In 1985, I bought a "Radio and Electronics" mag which tried to predict what the world of 2000 would look like, a mere 15 years later. They missed way more often than they even got close.

 

[turbo]

Case in point: In 1985, I bought a "Radio and Electronics" mag which tried to predict what the world of 2000 would look like, a mere 15 years later. They missed way more often than they even got close.

Did they mention "flying cars"? Flying cars were a yearly theme in pulps since the early 50's, IIR. At least George Jetson's vehicle (and his maid) were fanciful.

 

[Janus]

Did they mention "flying cars"? Flying cars were a yearly theme in pulps since the early 50's, IIR. At least George Jetson's vehicle (and his maid) were fanciful.

No, but they did predict that at on some major highways your car would "drive itself" under the control of a central traffic control system. The one "hit" That I remember was a system equivalent to GM's On Star.

 

[aquitaine]

No, but they did predict that at on some major highways your car would "drive itself" under the control of a central traffic control system. The one "hit" That I remember was a system equivalent to GM's On Star.

I remember seeing commercials for that type of prediction in a documentary once. I'm pretty sure from the tail fins it was made sometime in the 50's. That being said soon enough we actually will have cars that can drive themselves. Even today the current model of Ford Focus can park itself.

 

[cmb]

Would fusion make for practical applications in high speed interplanetary propulsion?

Or would beamed propulsion be better?

Humans have already demonstrated fusion energy can be generated, in the form of atomic bombs. Though it is not 'controlled' power, it can still be harnessed as a means of pulsed propulsion.

Beamed propulsion is only an idea and would seemingly make little sense past some limiting distance away, whereas pulse propulsion is a 'demonstrated' technology*. Besides, how would a beamed propulsion device turn around and come back?

Project Orion, which aimed to use nuclear pulse propulsion, was canned for political reasons, but reached a demonstrator stage, of sorts, at least using conventional explosive pulses. Some clips from a recent BBC programme on it can be found here:



There seems to be no particular technical reason why this couldn't be built using H-bombs starting tomorrow, but whether anyone would let you build it, let alone launch it, is a whole different matter!

*I'll add that when I say 'demonstrated' I'm meaning no more that essential 'proof-of-principle'. Unless someone builds it, no-one will know if it is viable.

Here's a schematic of what was being aimed for, from the Orion project:

 

[D H]

Project Orion, which aimed to use nuclear pulse propulsion, was canned for political reasons, but reached a demonstrator stage, of sorts, at least using conventional explosive pulses.

The political reasons were several. The 1963 nuclear test ban treaty. A public that did not understand but certainly did fear anything "nucular." A nascent space agency that was very aware of public sentiment and very much wanted to distant itself from its military roots.

The term "political reasons" is a bit belittling. Except for a few privately launched communications satellites, everything done in space is ultimately done for "political reasons." The scientific return on investment (ROI) on those unmanned probes sent to Mars is small compared to the ROI from monies spent on medical research or energy research. The scientific ROI from human spaceflight is arguably even smaller; just ask any astrophysicist.

In any case, Project Orion is considerably different than the fusion-powered spacecraft hinted at by the OP. Orion would have been an incredibly inefficient means of propulsion. An efficient thrust system has a highly focused exhaust stream that is ideally at zero kelvin. The thrust from Orion would have been spread out over two pi steradians and the exhaust temperature would have been very hot temperature-wise. (That it would also have been "hot" in the radioactive sense was a political problem.)

 

[cmb]

The term "political reasons" is a bit belittling.

To be specific, I do not personally know what all the exact reasons were. I wrote 'political reasons' because of what Freeman Dyson wrote in his 1965 Science article "Death of a Project":

"The technical findings of the project have not been seriously challenged by anybody. Its major troubles have been, from the beginning, political."

"The story of Orion is significant, because this is the first time in modern history that a major expansion of human technology has been suppressed for political reasons."

http://www.sciencemag.org/content/149/3680/141.full.pdf

 

[D H]

To be specific, I do not personally know what all the exact reasons were. I wrote 'political reasons' because of what Freeman Dyson wrote in his 1965 Science article "Death of a Project":

"The story of Orion is significant, because this is the first time in modern history that a major expansion of human technology has been suppressed for political reasons."

If you read the article (and you are the one who supplied the link), it describes those political reasons in details.

That quoted phrase from the article is baloney. It might have been the first time that one of Dyson's pet projects was "suppressed for political reasons." There are plenty of predecessors in science that were suppressed for various "political reasons." Just to name a few, chemical warfare, biological warfare, eugenics, human experimentation. Like it or not, some view any use of nuclear technology as worse than any of these. Putting a huge nuclear arsenal in space? Of course it was suppressed for political reasons.

 

[Ryan_m_b]

I feel it is important to note that even disregarding the political factors involved in Project Orion there is no guarantee that the thing could or would work. I often get into discussions with people who pretty much assert that the only thing needed to finish Orion is building it but (correct me if I'm wrong D H) whilst a lot of theoretical work was done and some pictures were drawn there is no firm design.

 

[cmb]

That quoted phrase from the article is baloney. It might have been the first time that one of Dyson's pet projects was "suppressed for political reasons." There are plenty of predecessors ...chemical warfare, biological warfare, eugenics, human experimentation.

It's easy to recognise in Freeman Dyson's writings, etc., an overwhelming desire to apply science for human betterment and growth, both literally and intellectually.

I rather suspect he'd not view the examples you've given as particularly good opportunities for 'major expansion of human technology'!

 

[cmb]

I feel it is important to note ...there is no guarantee that the thing could or would work.

Absolutely.

But the point was that it was closed down after getting over the hurdle that everyone thought a pulsed propulsion system would not work at all.

It's like someone giving you a chance to start a project they don't actually want you to do because they figure it'll fail at the first stage and the egg will be on your face. For Project Orion, it got past the first few hurdles to the astonishment of all the nay-sayers, and suddenly it began looking like a project that might actually work!

 

[D H]

I feel it is important to note that even disregarding the political factors involved in Project Orion there is no guarantee that the thing could or would work. I often get into discussions with people who pretty much assert that the only thing needed to finish Orion is building it but (correct me if I'm wrong D H) whilst a lot of theoretical work was done and some pictures were drawn there is no firm design.

Exactly. The only working prototypes were small dinky things that used convention explosives. Switching to different technologies and scaling up by many orders of magnitude is something that engineers at least have learned doesn't work on the first try. Or the second. Or the third. Or the ... And that's just the explosion and the pusher plate. I can imagine that the physical integrity of the pusher plate would be another problem. There would be one heck of a shock impulse going through the plate. I would think that that plate would have to be pretty much deformity-free.

Then there's the huge logistics problems: How to get all of that stuff to LEO. Ain't no way, no how that no country is going to allow an Orion-style rocket to launch from the ground. (Pardon the bad English. See [post=3650542]this post[/post].) The vehicle would have to be launched piecemeal by conventional rockets.

There would still be huge political problems. People protest launches of satellites that contain radioisotope thermoelectric generators. More recently, look right here at this site at the response to a barely measurable leak of radioactive iodine. One can only imagine the public's response to launching hundreds of full-fledged nukes.