OK, I'm in that unpopular camp that believes that the Breakthrough Propulsion Program should never be funded again -- we should not spend any money on Heim Theory, gyroscopic antigravity, space elevators, antimater engines, Bussard ramjets, black hole engines, UFO research, and zero-point energy. I have a radical alternative proposal.

Instead of using Wired Magazine physics to get to the stars, I'd like to use The Feynman Lectures physics to get to the stars. I propose building a craft powered by atomic fission. The engine would be a high-current linear ion accelerator, consisting of a superconducting niobium cavity resonator like this one, to get a nice healthy relativistic exhaust velocity.

Click to view attachment Click to view attachment

Next, we crack open a good book, like Taylor & Wheeler's Spacetime Physics, and figure out how long it would take to get a real spaceship to Alpha Centuri. What is the relativistic form to Tsiolkovsky's Rocket Equation? (OK, T&W does that for you) How much Plutonium would it have to carry? How much ionizable reaction mass?

I guarentee you, this hypothetical ship will get to the nearest star long before anyone invents a warp drive. Maybe I should ask NASA for $1.6 million, to develop this idea?

Interesting, Don. I do not recall similar idea on The Feynman Lectures physics, I would like to know more details/links on this...

....of course you know that linacs and synchrotrons in thier current state are stupendously inefficient.... like that image of cherenkov radiation though. just something about that blue...

I had an impression that superconducting linacs and some new high-efficiency RF amplifiers like the regotron were yielding high current sources with like 50% efficiency. I could be misunderstanding something, I haven't dug deeply into this. Let me as a nuclear physicist I know about this...

In any case, ion engines are efficient, so the question is how far can the exhaust velocity be pushed.




Sorry, I was making a joke with American cultural references. The Feynman Lectures is just a good book on physics by a master. Wired Magazine is a popular magazine that comes out of the MIT Media Lab crowd, and is basically low-quality, melodramatic fluff.

Hemmmm... all these "researches" don't all have the same science status... Heim theories (discussed elsewhere in this forum) could be tested at relatively low cost, but they gather some scepticism. Space elevators could work, or not, but they could also be more expensive than rockets (weeks would be required to haul a charge) UFOs have, with my opinion, little relation with ETs, etc.

But your point is using fission instead of fusion. Let us pass over the obvious dangers and legal issues. It may be true that uranium contains more energy than fusion materials, at equal weight. The problem is that we don't know other means to extract this energy than making electricity with a thermal engine, and an inefficient one (about 30% for large commercial nuclear plants, probably much less for thermoionic reactors used in former USSR satellites). The sames goes for fusion too, anyway.
After, with this electricity, you propose to accelerate particules. Another efficiency concern.

An ancient proposal was to heat hydrogen into an fission reactor, and to use it as a rocket exhaust. But we are limited by the max temp of the reactor (less than 900°C in a comercial reactor, to be compared with the 2000-3000°C of a chemical rocket). So this method could require to use more fuel weight than a chemical rocket! So to accelerate particules is better, as we can "heat" them at the equivalent of hundred thousand degrees. High atomic weight works better, for a matter of creating an impulse. (this is why xenon is used in ion drives. But lead could do as well, just a bit more complicated).


I describe a fission powered interstellar probe in one of my scifi books, but it is not yet published. anyway it don't use a classical fission reactor.

I guess possibly a superconducting linac (any curved track designs are immediately out I think because of the huge synchrotron losses...right?) could be more efficient than I thought. I was thinking of all the energy costs for the liquid helium etc. but I guess if you're in space you may be able to passively radiate to below the Tc of for instance, niobium....

The opposite is true. Fusion of deuterium yields over 80 kilotons of energy per kilogram while fission of each of the 3 most commonly used fissionables yields less than 20 kilotons/kg. Of course, things become much more complicated when you take the whole reactor masses into consideration - fission has the lead then (fusion ones don't even exist yet!), but who knows if advances in fusion research and plasma confinement will turn the tables in the future.
This is the sort of thing that makes hydrogen-oxygen rockets look not that much superior to storable propellant ones (as it would seem from the specific impulse standpoint) when you take everything into account - additional thermal insulation mass, low LH2 density-> large tank volumes and masses, production cost etc...

"UFO research"?

"Its been suggested that we might have something to learn by studying UFO stories. I disagree. First there is this signal to noise ratio problem. Even if the stories are correct, they are only as useful as science fiction. Science fiction can be useful to give you some mental picture to get you started thinking about the real issues, but it is no more useful than that. Even if UFOs were completely real, which is doubtful, and even if I had a film of one in front of me, it wouldn't be of much help."

--http://www.nasa.gov/centers/glenn/research/warp/warpfaq.html
(the FAQ page for the BPP project's homesite)

In other words, the BPP project was not funding "UFO research".

Why then allude to it at all? Presumably because, along with the other items on your list, their dubious or non-existent scientific merit conveyed the insinuation that the BPP program was doing quack science. That in turn allowed you to rely on "the idiomatic pejorative doing some hatchet work..., conjuring up unsavoury subliminal images" (to quote an ex-Australian Governor-General writing about an entirely different field of endeavour--nothing to do with space or space travel--but dealing with a not-dissimilar situation) to assist your contention that the BPP program "should never be funded again".

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Stephen

Yes true, the equilibrium temp in deep space is about 3°K, far enough to allow for supraconduction in most common materials. But is it really useable? First, these temps are available only far beyond Pluto, that means that the probe will need years before switching its engine on. Second, at such low temperatures, it will be enough of something radiating some heat (and it will be) to heat all the thing up, so that an equilibrium temp cannot be expected below 20-30°K or more, not allowing for common materials. That still makes supraconduction much easier to operate than on Earth. But the ideal would be that better semiconductors are found...

What I think is that such a program is basically something which considers "speculative" or "alternative" ideas, in case some may turn true. But of course, many of these ideas turn false, or even krank. They can even be taken over by nutters who afterward claim to have a "scientific caution" of the BPP... For instance I remember well that when the zero-point energy idea came out in France (under the name of "synergétique" in a popular science popularizing review) when I wrote to the guies they send me a sample of their review, speaking of their discovery being dismissed by... the Jew plot. Must this kind of things forbid to study it? It is a somewhat fuzzy domain of physics of which something may come out, so it cannot be dismissed at once. Even if, I think, with the strength of the standard model and of relativity, the odds for a real discovery about things like zero-point energy are low.

About UFOs, like it or not, there are many good scientists who consider seriously that they could be ET spaceships. So they could think legitimate to study them (rationally of course) in order to reverse-engineer them. (Some establishment scientists were even actualy engaged into this study, although this establishment is not proud of this). My personal "heretic" opinion is that UFOs are not ET spaceships (they are another phenomenon, very interesting but not directly related to space propulsion or ET life) so I doubt that any interesting conclusion could be drawn from their study. Or it would be really a BIG breakthrough...

Don:

Never mind the stars - think what a decent propulsion system could do for us locally!

A couple of thousand tonnes of cargo, once a week to any planet in the Solar System...

Bob Shaw

There's an easy and high efficiency option that solves lots of problems, the Orion concept. Payload sits on top of a stack of shock absorbers and a thick plate, and you simply set-off hydrogen (fusion) bombs underneath it. Technology was worked-out many decades ago, and even had (non-nuke) prototypes flying. Hundreds of tons delivered to Saturn orbit would be easy, so less mass sent interstellar shouldn't be a problem.

George Dyson has a good book on the topic:
http://www.amazon.com/gp/product/customer-...3052158-2742327

And the BBC had this neat documentary about it called "To Mars by Bomb" that was pretty good. This was all 1950's technology, so I'm sure we could do much better today

One nice benefit, this is a productive means of depleting the nuclear stockpiles.

A good Sci-fi book that has an Orion vehicle is the Larry Niven's "Footfall” where Earth fights off an interstellar invader using a large nuclear-bomb powered craft. It was even ground launched from Oregon. That would be one hell of a launch to see (but not up close)!

Fusion releases more energy, but fusion reactors do not exist yet. My goal was to describe a ship ot of technology that has been demonstrated. Fission reactors are up to about 50% efficiency now actually. And super conducting linacs are being built that claim 50% efficiency. Beam currents of about one amp are considered possible now. And of course, nuclear energy is only a few percent efficient at mass/energy conversion. Sadly, it is just the best we can do with existing technology.

I hope fusion power works someday, but the history of this subject is not good. Magnetic confinement of plasma has been described as "like trying to confine jello with rubber bands". And you hve to reach enormous temperatures, much much higher than the core of the Sun (inside the Sun, fusion takes place at a very slow rate per unit volume). I don't know enough about this to say if its is just a fundimentally hard probelm, or if it is just something that academic researchers have been fiddling with ineptly. Sometimes when the economics is right, professional engineers step in and make something work in a surprisingly short time (I see this all the time in the computer field).

The problem with NERVA-style rockets (heating hydrogen in a reactor) is that they do not produce relativistic exhaust velocity. It think nuclear-electric drives like ion or VASIMR drives probably have much higher specific impulse. But to reach a star, I believe you have to try to get an exhaust velocity at some healthy fraction of the speed of light. The ORION drive might not be a bad idea, exhaust velocities of 1000 km/sec or so are apparently feasible, but that's still not reletivistic.

Taylor and Wheeler's book does some math on an idealized case:

1. Theoretically, the most powerful propulsion system is one with an exhaust velocity of the speed of light. The ideal spacecraft would convert fuel mass into pure energy, for example by anihilation of matter and antimatter, and emit the resulting gamma rays in a tight beam behind the ship (there is no theoretically known way to do that though).

2. Assume a time dilation factor of 10. This would allow an astronaut to travel 500 light years in 50 years (in his timeframe).

3. If he wants to just get to the star, without stopping, a 100 ton (empty weight) ship will have to carry 2000 tons of fuel. If he wants to decelerate to a stop, he will have to carry 40,000 tons of fuel. if he wants to also come back to Earth and stop, he will have to carry 32 million tons of fuel!

4. At full velocity, his ship will impinge on about 3.0E+15 atoms per second, each of which will strike the ship with an energy of 9 GeV. This is about 300 times the flux of a powerful proton accelerator. So serious radiation shielding will be needed.

But what I'm getting at is, how do you reach a nearby star without resorting to crank science or concepts that are totally beyond modern engineering possibility (antimatter, etc). And maybe the answer is that it can't be done with what we have now.



Pluto is still just on our front door step, for a journey to a star. Niobium is an old-style superconductor, which requires very low temperatures, but it is being used in new accelerators now because the new high-temperatures superconductors cannot yet withstand high intensity electromagnetic fields or very high frequencies. The linacs are essentially superconductiong cavity resonators, where a radio-frequency amplifier maintains a standing-wave. Here's an actual one, people are building these things now:

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Ehm, didn't catch... thanks for the patient explaination, Don!

How about Zubrin's Nuclear Salt Water Rocket?

http://www.npl.washington.edu/AV/altvw56.html

Intersting idea, sort of a more continuous version of the Orion concept. 3.6 % light speed is not bad.

WAAAAAH! Its Zubrin, it's mad, and it works!!!

At least it is the nuclear propulsion project which looks the most feasible of all. But the main problem is not the highly radioactive exhaust, it is raising nuclear fuel to orbit, with ordinary rockets. Should a lauch failure occur with a tank of this fuel, and it will mandatorily spilt into the atmosphere. I don't believe it could be hardened against this like a RTG... unless the fuel boron tubes he describes are just piled into the rocket, not assembled, and packaged so that each small element would withstand reentry and fall. But after the tubes would need to be manipulated and assembled in space.

This issue is also for all fission projects. Think that even the simplest evaluation tests should be made into space. The codes for calculating the nozzle also fall under nuclear secrecy and laws.

You make a fair point. Nevertheless, the problem will doubtless be solved one day. It will have to be. Manned missions into the outer solar system (or beyond) will presumably not be using RTGs any more than they will be using solar arrays.

Of course, those are decades away. On the other hand if the VSE manages to keep going a long-term lunar outpost may well be set up some time during the 2020s or 2030s. It is difficult to envisage how such an outpost can survive the 14-day lunar night without nuclear power. Batteries alone are surely not an option, it will be expensive to bring the outpost crews back to Earth at each nightfall and then send another lot up again at each sunrise, and building such a base at the south pole powered by solar energy gathered from collectors perched atop some peak of eternal sunlight may or may not be viable, much less be the best place to put such a base to satisfy its various purposes.

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Stephen

I thought nuclear reactor were saver to launch than RTGs, since they only become critical in space, when they are leaving earth at a save speed.
Besides, nuclear warheads are much more radioactive and we count on them to reach their target without exploding first..hmmmm

The problem is not what happens in space: once is space on a non-retrurn trajectory, bye bye radioactivity, even the more radical environmentalists will be grateful to see those nuclear fuels leaving our biosphere forever. The problem is during launch: no rocket is 100% safe, and loading them with nuclear fuels is making sure that one day or another this nuclear fuel will be spilt into the atmosphere. In the case of the RTGs (Cassini, Galileo) cautions are taken (the highly toxic plutonium load is packaged into graphite and iridium casings, intended to withstand a re-entry and fall, but environmentalists however disagreed with their use given the uncertainties). And a RTG is small, here the concern is to haul tons of uranium into a liquid form, which could fall in rain and even become critical. With my opinion it is a difficult problem.

With my opinion, fusion at least is better on this point of view: no fusion fuel (except tritium) is toxic or able to explode spontaneously. So they can be launched from the ground without risk.

In more, fusion fuels could be available is space. Fission fuels are not, unless there would be uranium ore on the Monn or Mars. This is unlikely anyway, as uranium ores on Earth formed into the margins of some granitic intrusions. Nothing such on any other planet into our system.

If the space elevator is constructed one day, it would provide a safe way to send nuclear fuels and nuclear wastes into space.

>Fission reactors are up to about 50% efficiency now actually. And super conducting linacs are being built that >claim 50% efficiency. Beam currents of about one amp are considered possible now. And of course, nuclear >energy is only a few percent efficient at mass/energy conversion. Sadly, it is just the best we can do with >existing technology.

The "50% efficiency" = power in beam / wallplug power claimed for superconducting linacs (SL's)
only counts the electrical power going into the RF generators. Whereas in any real superconducting
cavity a large amount of liquid helium must be continuously liquified and circulated to remove the
heat dissipated in the walls of the cavity by the megawatt-scale RF. Given the huge financial burdens
(400 million USD and 20 years of planning) of building and operating even ONE earthbound
MW-scale SL of length <1 km with <10 mW beam intensities, I seriously doubt that such a
technology can be used for interstellar flight.

Those accelerators are intended to accelerate particules at very high energies, in a "hot" Earth environment. Into space, there is less heat, and we don't need to have teraelectronvolts.

But I agree that all this will be expensive and difficult. At first, we are still very far of building a 1km structure in space. It would be probably easier to build electrostatic accelerators, or other emerging technologies to accelerate particules, which are beginning to be tested into accelerators, which can replace kilometres of microwave cavities with some centimetres of laser device.

By the way, it is better to accelerate few particules at higher speed. But if we reason with a given available power, in this case it is better to use it to accelerate as much mass as possible. So we don't need to have super-high energies. In facts things will be a trade-off between available power and maximum mass, leading to a given energy level for particules, probably at middle range energies.

Fusion power is great, I'm all for it. But so far, that has not been very successful, except in the form of a bomb. Fusion is also dirty (neutrons and tritium are some of the byproducts), but so is fission. I don't think a space elevator is physically possible, just a science-fiction idea.

Obviously, a Russian ion engine has sent a probe to the Moon (SMART-1), so we know that is possible, but the exhaust velocity is not enough for intersteller travel. Maybe a GeV class linac is too difficult. But somewhere in between is a maximum practically achievable exhaust velicity.

So I think so far two feasible suggestions are presented:

1) Atomic power + high-velocity ion drive

2) Explosive fission drive

Is that it?

Rather grim news on the space elevator front -- a new paper claims that inevitable defects mean that carbon nanotubes (which have been proposed as the cable components) would at best be less than 1/3 as strong as predicted, and thus much too weak to serve the purpose. (That's not counting their vulnerability to bombardment by atomic oxygen in space.) This means that there may simply not be any substance in the Universe strong enough to serve the purpose:
http://www.nature.com/news/2006/060522/full/060522-1.html

However, strangely, there has been a moderate breakthrough on the fusion-reactor front -- a technique has been developed to prevent sudden, violent leaks of plasma out of magnetic bottles by (ironically) deliberately introducing some chaotic fluctuations into the strength of the magnetic field and thus producing a slow, steady, controllable leak. "The results, which appear online in 'Nature Physics', could help the US$5.5 billion International Thermonuclear Experimental Reactor (ITER) achieve its goal of generating net energy from fusion.":
http://www.nature.com/news/2006/060522/full/060522-2.html

Don:

VASIMR looks kind of sensible.

Bob Shaw

VASIMR is very intersting technology. It looks like (in theory) it can get plasma energy up to about 1000 eV per ion. Kind of at the low end of what you need for intersteller travel I would think, but an exciting possibility for interplanetary travel. Ion acceleration should be able to get you a specific impulse of 1000 times higher or more.

It is a daunting problem to go to the nearest star though. If you look at Taylor and Wheeler's numbers, even for mass conversion into photonic thrust (which is essentially impossible), you need a huge ship.

In case anyone was wondering what this spacecraft looked like:

http://www.up-ship.com/apr/michael.htm

A MUCH better one (predictably) is Poul Anderson's 1983 "Orion Shall Rise", set on an Earth about a millennium from now that's still struggling to recover from the devastating effects of a nuclear world war -- in large measure because our own civilization thoughtfully used up all of Earth's easily extractable raw materials (including fossil fuels), and then staged that war which destroyed all the industrial machinery that could be used to get hold of the Earth's much-harder-to-extract remaining resources. In it, one nation decides to secretly develop an Orion system in order to obtain cheap access to industrial raw materials in space -- and also to establish its own world supremacy. Partway though the book, the plan is found out. Duck, everybody!

Here's another idea:
As someone here previously mentioned, the problem limiting power output from a nuclear fission reactor is operating temperature. Power is proportional to temperature, but beyond a certain temperature, the core melts.
There has been an idea advanced called the gaseous-core fission reactor, where the core is undergoing fission in a plasma state, so operating temperature is no longer a limitation. I remember reading a book by a propulsion engineer named Hunter back in the sixties (which I wish I could find again), and he worked out some of the performance aspects of such a reactor. You'd have the uranium in the form of a plasma that reaches criticalilty inside a chamber with "glass" walls, that it does not come into contact with. Instead the plasma simply reaches criticality and then gets exhausted out the back of the ship before it comes into contact with anything it can melt. But unlike in Orion, the fissioning material does not do the pushing. Instead - and here's the key point: the plasma heats fuel in a surrounding chamber by the radiative flux going through the "glass" walls. The heated fuel then exhausts out the back (along with the uranium plasma) and produces thrust.
He calculated that such a ship would be capable of speeds of 100,000 - 1,000,000 feet/second. It could do 1 AU (Earth-Sun distance) in about 11 days. At such speeds you don't need to travel along a long curving Hohmann trajectory to reach another planet, and you don't have to wait for the planets to come into position. The trajectory of the ship is esssentially a straight line from one planet to another. Such a ship may still be way too slow to travel to Alpha Centauri, but it would open up the solar system to commerce. Pretty cool.

It's interesting that 86 percent of the energy from a fission event is carried away by charged particles, mostly the large fission fragments. Being charged, they can be moved around somewhat by electromagnetic field.

Whatever became of the fission fragment triode? Wasn't that supposed to be an extremely efficiently means of converting nuclear energy directly into electricity?

The problem with fission fragments is their very large mass. Something on the order of 100 a.u. This gives a very poor velocity compared to the immense energy the nucleus has (something like 80 MeV on average). It would be much more feasible to use this to heat smaller molecules (like the H2O proposal before) than to collimate the nuclei themselves as rocket exhaust.

But in the context of intersteller travel, I don't think any kind of thermal expansion of gas can hope to achieve the relativistic exhaust velocities you need. Schemes like Orion also essentially use the fission fragments directly for exhaust. But if the fission-fragment triode can be perfected, to make electricity efficiently, then I think high-speed ion drives might be appealling.

The kinetic energy of fission fragments are around 170 MeV. I bet even for Strontium and Krytpon nuclei, they are booking along pretty fast!

Yes, the total kinetic energy of both fragments. I was implying the average energy of one fragment.