I've come up with an idea even better than Particle Puff Propulsion. Now that I've thought of it, it seems so obvious.

The basic idea behind runway pulse propulsion is to line up a bunch of nuclear bombs in a long line. The starship travels along this "runway", and bombs detonate one-by-one as the starship passes them. Each detonation provides a forward push to the starship, accelerating it to high speed. The advantage of runway propulsion over a rocket is that the starship doesn't need massive fuel tanks so you don't waste effort accelerating the partially full fuel tanks. However, it seems like you're limited in top speed to the speed of the bomb explosion products--unless you use some sort of "clever sail" which can provide a forward push even when the starship is moving faster than the explosion expansion rate.

Well, once I thought of how to design the "clever sail", it turned out to be pretty easy to do. One of the simplest magnetic geometries does it--a simple torus. Remarkably, a torus magnetic field deflects bomb products in a way that efficiently acheives any velocity, limited only by the speed of light.

http://www.geocities.com/mechdan/torusail/index.html

The theoretical efficiencies are close to the absolute ideal. The costs scale quadraticly with cruise velocity--proportional to the kinetic energy of the starship. In contrast, a rocket's costs scale exponentially with cruise velocity. If you want a rocket to go much faster than the exhaust velocity, you pay dearly for it. With torusail propulsion, you can go as fast as you want and you only pay the going conservation-of-energy rate for it.

What I love about torusail propulsion more than anything else is that the physics and technology involved are very simple so even I can understand and analyze it. The magnetic fields involved are relatively low strength and used in simple ways well within what's already been demonstrated in the lab. There aren't any supercompressed or superheated plasmas involved; no plasma instabilities to worry about. There aren't any novel nuclear bomb design issues to worry about--the fission-fusion-fission device is essentially off-the-shelf.

Torusail propulsion is doable with current technology, and at a reasonable cost. The R&D challenges and expense would be less than that of particle puff propulsion, and yet the potential performance is significantly greater.

Essentially, the bomb goes off when the vessel is directly by its side, sideways on, and the mag field deflects the sideways thrust backwards? is that right?

I've been thinking about bomb-track propulsion since you introduced it before. One thing struck me a few days ago: a nuclear weapon produces a powerful electromagnetic pulse, that is said to burn out electrical circuits nearby, so much so that chips for military use have to be hardened against it. I don't know how much of the bomb's energy goes into the EM pulse, although I suspect its a small proportion.

However presumably the EM pulse travels at c. Is there (a) any way to use this as propulsion in itself, and (b) would the EM pulse have a bad effect on the torus mag field you propose above?

Conceptually, there are ways to design a "nuclear flashbulb" which outputs a large burst of photon energy. However, even if the "flashbulb" itself is efficient, photon thrust is not. For example, using photon thrust to push a vehicle already moving at 10%c only adds kinetic energy to the vehicle at 10% efficiency. (Assuming perfect reflection of all of the photons.) When you get to near-c velocities, then photon thrust can be very efficient. However, the energy costs for acheiving near-c velocities are inherently incredible. For a more reasonable .25c mission, photon thrust is much less efficient than thrust from baryonic matter.

I dunno, nuclear bombs seem awfully inefficient. Especially in space...

Where do you get the idea that nuclear bombs are inefficient? The U238 fission stage of a fission-fusion-fission bomb is by any reasonable measure very efficient--yields around 80% are acheived. It's also cost efficient. There isn't any other energy source which gives you anywhere near the joules per dollar. It's also mass efficient. There's no other energy source with current technology which gives you anywhere near the joules per kg.

How many of those efficient joules actually wind up propelling the spacecraft? If a system is only 20% efficient but puts that 20% into propulsion, isn't that better than a system that is 100% efficient but only puts 2% into propulsion?

A surprisingly large amount. Torusail propulsion can in principle dump well over 90% of the explosion's energy into the kinetic energy of the spacecraft. However, in a plausible system maybe only 50% of the bomb's explosion products are deflected by the torusail.

The overall efficiency could be around 35%, which is actually a VERY good number compared to most propulsion systems in general.

But in any case, energy efficiency isn't as important as overall cost effectiveness. Being 100% efficient with fuel that costs 10c per joule isn't as cost effective as being 30% efficient with fuel that costs 1c per joule.

I'm not sure how critical the position of the vessel relative to the explosion is?

At 20%c, 60,000,000 metres per second, the vessel will traverse 60 metres in one microsecond. (The fastest human reaction is the reaction of your eye blink when something touches your eyelashes, that's 12 milliseconds or 12,000 microseconds. In that time the vessel travels about 450 miles at 20%c.)

I'd guess the bomb timing will be difficult, but perhaps not insuperable. Can nuclear detonations be scheduled with microsecond precision?

On the EM pulse question, I was not refering to the photon output per se. When a nuclear weapon goes off, there is an electromagnetic pulse that induces electric currents in conductors. Apparently, electricity supplies over a wide area can be disrupted. My question was, is there a way of utilising this for propulsion using a magnetic field on the vessel in some way?

To some other posters, please note there is currently a problem with what to do with all the weapons-grade plutonium, highly enriched uranium etc from nuclear weapons decommissioning. The US military has just 'released' about 200 tonnes of HEU, that will have to be down-blended with natural U for release into the civil nuclear market. The costs of security and supervision of this will be stupendous.

The explosion timing is not quite that critical. The explosion products expand outward at around .04c, and the timing window you have is roughly equal to the torus thickness divided by .04c. For example, if the torus thickness is 120m, the time window is 1/100,000 of a second.

Think of it as trying to arrange the slowly outward moving explosion products to happen to be in the path of a fast moving sail when it passes by. The actual speed of the sail doesn't matter.

EM pulse is a pulse of electromagnetic radiation. It's photon energy, just photon energy in the microwave frequency region.

I just can't see the benefit of this type of propulsion system.

First, how are you going to get all these bombs out there? It would cost a fortune to launch all these propulsion bombs and many, many years to get them into position. Not to mention that you would have to make a whole bunch of specialized bombs to get any real efficiency out of them.

I would think it would matter alot. Even if you could design an explosive charge to direct most of it's energy in a focused area, you're going to have to detonate the charges closer to the spacecraft to receive the same impulse. And the faster the craft goes, the closer the charges. Eventually, you're going to be detonating bombs very close to the craft. I would think that radiation and EMP's would be a major concern at this point.

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One thing I love about these bomb drives is that they fail to address the issue of how the crew will cope with these rather impressive bursts of acceleration, to say nothing of how the intend to address the physical stress of explosive acceleration on the ship itself.

A sail designed to take in 90% of a nuclear explosion's energy and convert it into velocity...interesting, and what material do you think you're going to make the connection to the main ship from and not have it sheer off after the first bomb goes off? I'm not a structural engineer, but the dynamic stresses on the contact points would be enough to invoke a coronary, I'd think.

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I still like a big rigged NSWR. Cooling is the main concern, but with a massive Sea dragon hull and a stout enough nozzle with a lot of water cooling it--it should work. Something needs to launch the torussail anyway.

At some point in time--one has to use the brute force approach. We've tried the finesse thing with X-33.

Time for something else.

Actually this has all been considered, and there are solutions.

Not sure the "nuclear runway" would work, but other similar nuclear pulse drives probably would.

One of the most studied was the Orion project, which in theory could reach 10% of light velocity. It's buildable with our current physics and essentially our current technology.

Acceleration could be controlled and buffered for a constant 1G boost with humans on board. Unmanned versions could boost at much higher Gs.

It would be a huge development effort on the scale of Apollo or larger. However there's no reason why it wouldn't work. It's one of the most achievable, highest performance drive systems based on our current technology and physics:

http://en.wikipedia.org/wiki/Project_Orion

There are other concepts such as VASIMR roughly attainable with current technology, but it would require a powerful fission reactor. Not sure how it compares with Orion in terms of attainability and ultimate velocity, but in very gross terms they might be comparable:

http://en.wikipedia.org/wiki/VASIMR

This proposal is for an unmanned space probe, but it could be scaled up to a manned mission. Transfering 90% of the bomb's blast into kinetic energy sounds like a lot of impulse but it actually isn't, once you get up to speed (which is where the bulk of the acceleration time and expense occurs). The amount of impulse for a given energy input is inversely proportional to your current velocity.

BTW, the bomb detonations occur further and further away from the ship as you go along--they detonate AHEAD of the starship, so that the slow moving sideways annular explosion products have time to reach the torus magsail's diameter.

The advantage of torusail propulsion over nuclear pulse rockets and NWSR is two-fold:

1. Torusail propulsion is capable of MUCH faster velocities. NSWR or Orion style propulsion would take a century or more to reach Alpha Centauri. That's too slow for a realistic interstellar mission, IMHO.

2. Torusail propulsion is cheap and efficient compared to rocket designs. The fuel is cheap and costs are linear with overhead. For a rocket, the costs are exponential with overhead.

Issue 1 is a deal-killer as far as I'm concerned. I feel .25c is about the "sweet spot" compromise between practical mission times and reasonable cost. But NSWR, Orion, and other nuclear pulse rockets just can't get anywhere near .25c.

I guess what I don't follow then is how these high efficiencies result when a bomb is far away with only a very small angular portion of the blast actually going in the direction of the sail? How does one direct a large percentage of a nuclear blast in one direction in space?

From the perspective of the starship, the bomb is exploded far ahead, but the bomb sprays a conical blast which conveniently mostly hits the sail.

From the perspective of the runway, the bomb explodes with a roughly annular blast. The outward velocity of the blast is relatively slow. Then the fast moving starship comes and sweeps across a cylindrical swath of space. The sail actually sweeps a slightly greater fraction of the bomb explosion the faster it goes.

Likewise, how do you know what fraction of the bomb products could be magnetically deflected, and what the mass and speed of those products is? Without knowing that I don't see how any performance, efficiency or feasibility could be calculated.

The bulk of a bomb's energy is released as thermal radiation and blast, non of which can be magnetically deflected.

To my knowledge only a fraction of the bomb's energy is released as ionizing radation (neutrons, gamma rays, alpha particles, electrons) and only a fraction of that is charged alpha particles. Anything that's not charged can't be deflected. Electrons are charged but have almost no mass so wouldn't be useful:

The mass of alpha particles is known, but I have no idea what the speed is. I also have no idea how you'd calculate the shape and strength of magnetic field required to deflect them a given amount.

http://en.wikipedia.org/wiki/Nuclear_explosion

The tertiary fission stage is the main one we're mostly concerned with. The U238 casing to a rough approximation absorbs most of the energy of high energy particles and radiation. In addition, it undergoes high yield fission--yields of around 80%. Of that energy, about 80% goes directly into the fission fragments. The remaining 20% is in slow neutrons and other radiation of which a large fraction will be absorbed by the fission fragments and unfissioned U238. To a rough approximation, more than half of the bomb's energy is dumped into the kinetic energy of the outer case's fission fragments and unfissioned U238. Of that, maybe half or more of the fragments are deflected by the toroidal magnetic field.

The electrons actually get deflected in the opposite direction as the positively charged nuclei by the magnetic field. Fortunately, they have three orders of magnitude less mass than the nuclei involved, so they don't cause too much drag.

Calculating the deflection caused by the magnetic field is a simple matter of calculating the Lorentz force, considering the mass ratio of protons/(protons+neutrons) in each nuclei, and estimating the length of time spend inside the magnetic field. Conveniently, this length of time is inversely proportional to the ship's velocity, and the desired deflection is also inversely proportional to the ship's velocity.

Oh--the wiki reference you give has to do with nuclear detonations within Earth's atmosphere. The atmospheric "blast" doesn't exist in outer space. Instead, the plasma goes outward unobstructed by the vacuum of space.

Also, the reason an atmospheric detonation emits so much thermal radiation is because the overwhelming majority of the bomb's energy is immediately absorbed by the atmosphere near the bomb. This causes a powerful fireball, which releases energy in the form of the shockwave and radiating thermal radiation. Neither effect is relevant in outer space.

Got to thinking about runway length, and don't see how it could work from that standpoint alone.

If you boost at 1G, in 90 days you'll be at about 0.25 c (v = a * t). However in that time you traversed 2.96E14 meters (s=1/2 * a * t^2), or 62,000 times the distance to Pluto.

You'd first need to build an Orion ship to take the bombs out that far.