Nuclear Propulsion - The best way for space travel

Josh Cryer · 2003-03-01 21:55:10

Yeah, and where are you going to build and deploy sails as large as countries?

You clearly have no idea how plasma sails work, which is quite funny, actually, because you're sitting here pretending like you're knowing what you're talking about.

Plasma sails are merely slightly reflective bubbles of gas! All you do is turn on a dang magnet, squirt some helium in the surrouding area, and the magnetic field will inflate, once you've reached your desirable size (30km is plausible by one of the scientists plasma computer models, but there's nothing saying it can't be scaled to thousands), you squirt in some particles that tend to either reflect or absorbe light.

This isn't a conventional solar sail. It doesn't have a physical structure. The whole thing is held together via the magnetic field.

Actually, you'd be right in the middle of the Van Allen belt, if a flux occurred.

Right, and when a flux occurs on earth everyone dies. The only way the belts would be an issue is if the magnetic field was small. It would be quite large for larger application. Even for small probes, the bubble would be so big, I would find it hard to imagine a flux being so large as to put you in the middle of it.

It's not quite safe, Josh.

I'd be willing to wager that it'd be safer than sitting upon the ?pinky finger of god.? We're talking pixy dust here. Not fusion reactions wherein matter itself is destroyed.

The sturdiest titanium is subject to collisions, Josh. It's simple engineering, a collision with a hot, fast object will tear apart your sail.

Oh, soph, but wait, you have no idea of what you're talking about! Surprise surprise! The sail isn't ?built? out of anything. It's just, like I said before, a magnetic bubble. If you're saying that the sail generator is suceptable to collisions, so too would any engine be. But the plasma sail itself is fairly invincible, unlike physical sails made of foil. I feel like a broken record here.

As the sail is pounded with more rays, the material may lose its superconductivity. This is what I meant by degrading materials.

Yes, I know exactly what you're talking about. You're still stuck on Zubrin's magsail concept, because you didn't feel like taking the time to read the sources I gave. We're talking a whole new concept, soph. As the sail is pounded with more rays, the sail will lose some small minute bit of mass (this is why it doesn't have an infinite ISP), as helium and other particles escape the magnetic bubble, but this ishow the thing operates. Nothingw ill degrade, though, because the engine andcargo would be in the middle of the bubble, completely safe from harmful cosmic radiation.

We are not talking about magsails.

We've had fission reactors last for 50 years. With new advances in materials and the fusion process, fusion reactors can probably last much longer.

We don't even know if fusion is possible yet. But let's say that it is, and that we'd be able to contain it and everything. I would find it hard to imagine that fusion reactors wouldn't require constant maintenance.

A question: if you have these immense plasma sails, how do you slow down and stop at a planet? Land? Manuever?

With reflective plasma sails, tacking may be possible, so you could do any kind of manuevering you wanted.

Even if that's not possible, gravity assists and teathers could work. The point isn't that they're extremely versatile like nuclear would be. The point is that they'd be safe, simple and cheap. Thus ideal. Pioneers would be using them.

soph · 2003-03-01 22:06:47

And have any tests been done to show that this works?

Fusion we know can work, look at the Sun. And we're just below breakeven. Wishful thinking.

If you want your own plasma sail thread, have fun, but you are still hijacking a thread.

I'd be willing to wager that it'd be safer than sitting upon the ?pinky finger of god.? We're talking pixy dust here. Not fusion reactions wherein matter itself is destroyed.

Do you know what fusion is? Nothing is destroyed, it is merely fused. Matter can never be destroyed, anyway, it's a law of physics.

We don't even know if fusion is possible yet. But let's say that it is, and that we'd be able to contain it and everything. I would find it hard to imagine that fusion reactors wouldn't require constant maintenance.

We know fusion is possible, because it has been accomplished. We have multiplied our energy ratio many many times over the past decade, as preston said.

And a functional fusion reactor wouldn't need much maintenance, in fact, the only maintenance it would need would be a new wall every few decades, if at all. To stop and start the reaction, you turn on/off microwaves and supply/cut off fuel. It's much safer than fission-no moderation needed. You control the reaction by insertion of fuel (easily controlled by computers weighing in many different performance variables.)

Josh Cryer · 2003-03-01 22:28:42

And have any tests been done to show that this works?

Not yet. At least not for the full concept of a plasma sail. In the next few months I'm going to invest my money to test the idea, though.

Mini Magnetospheric Plasma Propulsion, however, works in the computer models, just like any other successful propulsion concept. So we're certainly halfway there.

Fusion we know can work, look at the Sun. And we're just below breakeven. Wishful thinking.

Mini Magnetospheres we know can work. Look at Jupiter and Saturn. The difference is, of course, we've already mastered plasma inflation, and particle suspension concepts. All that we now need is a particle that works! I would say finding such a particle isn't as difficult as controling the fusion process.

If you want your own plasma sail thread, have fun, but you are still hijacking a thread.

Well, I do have my own thread, but this thread is inviting other concepts. It makes a claim, and I didn't think that this forum was a place where claims could go unopposed on the basis that opposing them would be ?hijacking.? I'm not really hijacking I'm just clearing up unfortunate misconceptions about plasma sails.

But sure, I'll tone it down if you want.

Do you know what fusion is? Nothing is destroyed, it is merely fused. Matter can never be destroyed, anyway, it's a law of physics.

No, energy can never be created or destroyed. Matter can be destroyed by converting it into enthropic energy (which isn't reversable). When you take two hydrogen atoms and push them together to make a helium atom, you lose a proton (or is it a neutron? I can't remember). The point is, yes, matter is destroyed in the sun.

soph · 2003-03-01 22:41:37

http://www.101science.com/lawofcon.htm

"Matter cannot be created nor destroyed"

Proper measurement is significant.

All components of a reaction must be accounted for.

Matter is not destroyed on the Sun, it changes form.

And as I've said, fusion can get more payload around, faster, and with more flexibility than a plasma sail.

Fusion has actually been performed.

Josh Cryer · 2003-03-01 22:50:29

That's just a common semantic argument. That matter and energy are synonymous. If we're going to approach it that way, sure. Matter can't be destroyed. But if we seperate the two, it's obvious that once mass is turned into energy it can't be turned back into mass! It is destroyed, even though the energy lives on.

But anyway, we've created a stable fusion reaction where we got more out than we put in?

Euler · 2003-03-02 03:55:17

When two hydrogen atoms fuse to create a helium atom, none of the protons or neutrons are destroyed. However, the helium atom does have less mass than the combined masses of the hydrogen atoms. This decrease in mass is actually what created the energy due to E=mc^2. Mass and energy are actually the same thing, and it is possible to convert mass to energy and vice-versa. The process is reversible, and scientists have successfully created particles (with the associated antiparticles) by concentrating energy until it creates matter. However, there is greater entropy when there is more energy and less mass, so natural reactions tend to decrease mass and increase energy.

With regards to a fusion reactor, I am sure that it is possible, but the engineering difficulties are greater than soph thinks. With fission, all you have to do is get a bunch of rod of enriched Uranium, and they will naturally heat up when you put them close together. The amount that they heat up can be easily controlled by simply putting dividers between the uranium rods to separate them when you want to slow down the reaction. The heat from the Uranium is then used to boil water, which turns turbines in the same fashion as generating power from coal. It is really quite simple, and anyone who can get enough uranium can quite easily make a working fission reactor that generates power.

With a fusion reactor, however, the reaction happens at an extremely high temperature that would instantly vaporize any material that it contacts. Therefore, the reaction must occur in a magnetic field, and very powerful lasers are required to sufficiently heat up the hydrogen. With fusion, it is necessary to carefully control the output of the lasers and the nature of the magnetic field in order for the reaction to occur. Then you have to also worry about carefully adding more hydrogen to the reaction to keep it going. Finally, you have to actually convert the energy from the fusion into electrical/mechanical energy, which may not be as easy to do as it is in fission. Presumably the engineering difficulties will eventually be solved, but I am not sure if it can ever be made as simple and reliable as fission.

Regarding plasma sails, it is an interesting concept and it would be nice to prove it works. I think that it is a method of propulsion that could become reliable and economical. However, if it relies on gravitational "tacking", then there are limits to the accelerations and velocities that are possible, which could become very inconvenient in the outer solar system. Therefore there will always be a niche for other methods of propulsion that allow more flexibility and maneuverability.

Euler · 2003-03-02 05:51:12

I?ve been doing some more reading on the plasma sails, and it has actually made me more suspicious of the viability of this method of propulsion. There are a number of questions that were not really resolved by anything that I read.

1. What is the shape of the sails? I was under the impression that they would be discs, but I keep seeing pictures of roughly spherical magnetic fields. A disk can be used to steer a spacecraft, while a more spherical magnetic field could only accelerate directly away from the sun.

2. Does the magnetic field require power to generate? If a permanent magnet or something permanently charged could generate the magnetic field, it would be much better than using something with a huge power requirement.

3. What are the reflectivity and absorption of different kinds of dust (in terms of %reflection*area/mass and %absorption*area/mass)? Presumably, if there is a low dust density, then doubling the dust would double the absorption and reflectivity. The same logic would also indicate that (contrary to some opinions) the increased area of the magnetic field in a dusty sail as it got further from the sun would not increase propulsion because the dust is less dense. It would be nice to have at least some values in order to compare thrust/weight ratios with other sails.

4. How much light is reflected vs. how much is absorbed? Reflected light not only is capable of giving twice the impulse of absorbed light, it also has the much more important trait that it could be directed, steering the ship. Absorbed light just pushes directly away from the sun.

5. Can you take down a dusty sail and recover the dust? Or do you have to bring extra dust so that every time you take it down you can set it up again? I suppose that if the sail were a perfect disc, it could be turned edge-on to the sun rather than taking it down, but doing so may be inconvenient.

It seems to me that this technology would only be useful if it creates a high-reflectivity semi-flat disc that does not require much power to operate. If you can't direct the thrust much, the whole idea is pretty useless. If the power requirements are steep, then you might as well use ion propulsion. In addition, the sails would have to have a thrust/weight ratio, durability, and price that are at least competitive with traditional sails. In short, this could be useful, but only there are favorable answers to several questions.

Josh Cryer · 2003-03-02 05:55:00

Heheh, I don't understand most chemical or nuclear reactions, I just know that you can't reverse entropy, and any process which is entropic (ie, all energy processes, like turning mass into energy) cannot be reversed.

I read some stuff that we'll possibly have workable fusion by 2010. Your post about fusion kind of hints at what I was getting at (even though I don't know much about fusion beyond what I read on the front page of fusion sites). Fusion is pretty complex. If it was easy we'd have done it by now.

Josh Cryer · 2003-03-02 06:08:45

What is the shape of the sails? I was under the impression that they would be discs, but I keep seeing pictures of roughly spherical magnetic fields

Both, the sail is a bubble with a disc suspended in the middle. Thinik Saturn, but with the rings within Saturn's clouds, not around Saturn itself. Does that make sense?

Does the magnetic field require power to generate?

From what I read, it only takes a little bit of electricity to keep the magnet magnetized after you power it up. (And also, I think I recall that the best permanent magnets aren't that strong anyway. Maybe superconducters are the way.)

What are the reflectivity and absorption of different kinds of dust (in terms of %reflection*area/mass and %absorption*area/mass)?

We have no idea, no one has really looked in to it! Robert Sheldon attempted to get a grant to look in to this further, but I don't think his grant went through.

The same logic would also indicate that (contrary to some opinions) the increased area of the magnetic field in a dusty sail as it got further from the sun would not increase propulsion because the dust is less dense.

The constant pressure is referring to the solar wind, not photons. The original concept of plasmal propulsion was to exploit the solar wind, this dusty concept is just taking it one step further, and exploiting sunlight.

How much light is reflected vs. how much is absorbed?

See answer above.

Can you take down a dusty sail and recover the dust?

I don't think so, I mean, one might be able to figure out how. Since the dust itself would be on a disc (see my icon!), you might be able to devise a cup of sorts that rotates on a line around the craft. I doubt you could get all of the dust back, but a good deal of it could be salavagable under such a processs. This would be especially desirable if you used manufactured particles like buckyballs.

But sure, your criticisms are highly welcomed. It seems that until further particle studies are done, a full picture can't be made. If we could find highly reflective particle, we would have all ofthe advantages of a solar sail (tacking, etc), if not, obviously it does create some limitations. And interesting concept, is that one-way ships wouldn't need to have a large fuel tank or anything. You'd just ?charge them up? and send them off.

soph · 2003-03-02 08:53:30

Heheh, I don't understand most chemical or nuclear reactions, I just know that you can't reverse entropy, and any process which is entropic (ie, all energy processes, like turning mass into energy) cannot be reversed.

What is heat? Think about it, Josh. You still have matter, the form is the difference. You may not be able to take x and bring it back to y, but you still have the same amount of matter. Otherwise, our universe would be dead by now.

RobertDyck · 2003-03-02 10:40:19

Actually, you don't have the same amount of matter. Fusion, and even fission, convert a small fraction of the mass of atomic nuclei into energy. This can be reversed. Fermilab has a giant cyclotron call the Tevatron (Terra-Electron-Volt cycloTRON). They accelerate protons to a fraction of the speed of light so there is more energy in their motion than their mass, and then they slam the protons into a tungsten foil. The protons punch through, although their path is deflected a bit; the straight beam becomes a cone. However, they are slowed by the impact and the energy of their motion is instantaneously converted into a new subatomic particle. There is no control over which particle is created; any particle that requires that much energy may form. They use magnetic fields to sort the resulting particles and keep the antiprotons.

There is a second means of converting energy into mass. Focus a very high energy laser on an electron stream as it impacts a tungsten foil. It will create an electron-positron pair. The energy from a single photon from the laser plus the energy of the electron's motion must be enough for the mass of both the electron and positron created. Focusing the laser on an electron is not enough; the energy to mass conversion can only occur in the nucleus of an atom. Focusing a laser that intense on a nucleus without the electron tends to cause fission. A proton could emit a positron becoming a neutron, or a neutron could emit an electron becoming a proton. Either way, that changes the atom to a different element. Would the new element be stable, or undergo radioactive decay?

You will notice with each of these matter- to-energy conversions, charge is maintained. You either move a static charge or create both positive and negative at the same time. The total mass-energy of the system is also maintained, but energy is converted to mass using E=Mc^2. The input energy required is slightly larger than the energy to create mass; for example if you use the laser and electron method to create an electron-positron pair then the new particles must have enough energy to get out of the nucleus or they will just fall back in and be destroyed. That means the new electron and positron will have high speed. This means the energy-to-mass conversion is less than 100% efficient, and tends to increase entropy.

dicktice · 2003-03-02 11:59:33

Hello, Euler. Nice discussion regarding plasma sailing. I'd like to comment, referring to your points, according to numbers--
1. Spherical (ion cloud) shape works for me, since the main objective of pioneering space probes is climb out of the Sun's gravity well. The constant thrust achieved eliminates the need to orbit, since it enables straight-line acceleration radially from the Sun.
2. Superconductor magnets are "a natural" for use in space.
3. I imagine interplanetary space to be just about the least dusty environment you could find.
4. Plasma sailing relies on Solar wind particles, not Solar radian photons as does light sailing.
5. "Taking down" or "reefing" the plasma sail would simply require switching-off or reducing the magnetizing current.
6. Navigating a plasma sailing spacecraft may be rather analogous to a square-sailed galleon. You sail down wind until you reach the orbit of your destination, having aimed your radial to reach it slightly ahead and cut your current to arrive at zero radial velocity. Then, using unboard fuel obtained from ice your spacecraft is mostly composed of, accelerate in a vector to match velocity with the destination. To return, decelerate to zero Solar orbital velocity and simply fall radially towards the Sun, using magnet current to back-sail until you arrive at that orbit, etc. By only partially decelerating or accelerating in orbit, plasma sailing can be made to drift either way from directly away from the Sun, like sailing in an ocean current...it's simple, and economically viable. All we have to do is achieve low Earth orbit economically, tether-escape out to the inner asteroids for water, and we're away in my lifetime!

soph · 2003-03-02 12:04:50

dicktice, and why should we expect this to actually work, when no spacecraft has ever been built on this concept?

Fusion has actually been accomplished, not sustained or controlled, but accomplished. We also know more about nuclear physics from experience.

What more do you need for a fusion drive that you don't need for a plasma drive (besides the reactor and D-T/H3)? Water?

And where do you get your power from?

dicktice · 2003-03-02 13:25:34

But, if it did turn out to be viable Soph, wouldn't that be great? "All we are saying is...give plasma-sail a chance!"
G'bye for now.

soph · 2003-03-02 13:46:39

Sure, what I'm saying is, every type of propulsion/power/design, etc. will have its place, espeically once private industry finds its way into space.

Fusion will have its uses, plasma sails will have their uses, too.

Like I was saying, in the carrier thread, fusion can be used to power a permanent outpost in, say, the asteroid belt, that has the option of remaining parked, or mobile. And it could generate a lot of power. Fusion drives would probably be faster, too.

So, this is just an example of why each design would have its benefits.

Josh Cryer · 2003-03-02 16:26:35

Robert, I think you know what I'm saying. I'm not saying tyhat we can't create matter, I'm saying that since all energy exchanges undergo entropy, we cann't create matter with 100% efficiency. Thus, matter is irrevocably changed into energy. ?Destroyed.?

soph, no one said fusion wasn't beneficial. We're just saying, that, hey, there really is a comparable alternative out there. Fusion isn't end all to everything.

soph · 2003-03-02 17:47:57

Isn't that kind of what I said, not only about fusion, but about pretty much everything else?

Euler · 2003-03-02 18:08:40

To return, decelerate to zero Solar orbital velocity and simply fall radially towards the Sun, using magnet current to back-sail until you arrive at that orbit, etc. By only partially decelerating or accelerating in orbit, plasma sailing can be made to drift either way from directly away from the Sun, like sailing in an ocean current...it's simple, and economically viable.

This is easily done by a disc that reflects light, but not by a bubble pushed by the solar wind. With the (non-dusty) plasma sail, it is only possible to create a force directly away from the sun. This means that if you want to decrease your orbital velocity, you need a different way of doing it.

In addition, generating force directly away from the sun is not always the easiest way of getting farther away from the sun. If the force created is much less than the force of the sun's gravity, the vehicle will not be able to get to the outer planets at all. Contrast this with a light-reflecting solar sail, which can use light to gain rotational energy, and eventually reach any orbit.

Since the plasma sail creates a force that directly counteracts the sun's gravity, it can be thought of as a method of artificially reducing the suns gravity. However, this is all that it does. According to some calculations I did, the force of gravity on 1 kg at 1 au from the sun is about the same as the pressure of the solar wind on 6 km^2 of space. A plasma sail would not be 100% efficient, so it would probably need at least 10 km^2 of sail area/kg in order to completely counteract the sun's gravity. If this is achievable, then the technology could be very useful for things like deep space probes. Otherwise, I think its capabilities would be too limited to be very useful as a stand-alone technology. It is possible that it might be used in a hybrid with another method of propulsion that would give it the necessary maneuverability.

soph · 2003-03-03 15:18:57

Finally, you have to actually convert the energy from the fusion into electrical/mechanical energy, which may not be as easy to do as it is in fission.

Actually, you don't. You use the same concept as in a NTR. Let the hot plasma and hydrogen leak out of one end. No need for electrical energy.

You can get high thrust out of a magnetic fusion drive by using hydrogen propellant (in addition to the plasma), and high isp by using pure fusion plasma without hydrogen. With hydrogen, your isp is 10,000 seconds, without, its up to 2 million seconds.

I wonder if you could use 50/50, and get the best of both.

Josh Cryer · 2003-03-03 17:42:08

Actually, you don't. You use the same concept as in a NTR. Let the hot plasma and hydrogen leak out of one end. No need for electrical energy.

How do you power the lasers and or start the fusion reaction?

soph · 2003-03-03 17:49:12

Fuel cells. Once the lasers/microwaves heat the reaction, they are hardly needed anymore-the reaction is a chain exothermic reaction.

Josh Cryer · 2003-03-03 18:26:54

You still need power to contain the plasma so that it doesn't melt the ship to pieces.

Preston · 2003-03-03 19:14:39

A mirror-type fusion rocket might need 10-100MW of power*. This would come from a smallish nuclear reactor, and waste heat from the fission reactor and engine would be radiated by large panels or a liquid droplet radiator. In the steady state, the engine could replace the fission reactor as the power supply. If we are burning D-3He, we could convert some percent of the fusion power at >90% efficiency (easily) to electrical power which would run the ship and engine.

A pulse fusion rocket might use small quantities of antimatter, or if driven by lasers, a small fission reactor would give electricity and power capacitor banks.

Soph, you are misunderstanding the hydrogen propellant concept for fusion: You can add hydrogen flow rate, which means you can simultaneous lower the Isp and increase the thrust, in a continuum. There may be some engineering limits on how exactly this is done.

This plasma sail stuff is much simpler. When I have the time I'll read more about it, though, since I wonder about the performance limits. There is a proposed M2P2 interstellar precusor mission, which has a delta v of 50-80 km/s, but that's with a very small spacecraft. Before, I didn't know that it used propellant, but it does (the magnetized particles that 'inflate' the magnetic field).

The reflectivity of the particles doesn't matter (it is an extremely tenous plasma with small opaque surface area). All they are for is making a magnetic field.

* Santarius, John F., and B. Grant Logan. ?Generic Magnetic Fusion Rocket Model.? Journal of Propulsion and Power 14.4 (1998):519-523

soph · 2003-03-04 10:18:32

You still need power to contain the plasma so that it doesn't melt the ship to pieces.

That's what your magnets do.

Preston · 2003-03-04 13:02:44

A magnetic confinement reactor consists of a few things:
1) High temperature superconducting magnets which require active cooling, since it is 2 hundred million degrees kelvin just a meter or two away
2) Power to run neutral beam injection
3) Microwave heating
4) Joule heating
5) Fusion product heating

Joule heating (from a current induced in the plasma) applies to toroids but I don't think it applies to mirrors.

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