Nuclear Pulse Propulsion – Orion and Beyond (2000) [pdf] | Hacker News Reader

Nuclear Pulse Propulsion – Orion and Beyond (2000) [pdf]

ntrs.nasa.gov

I think I like the recent work on pulsed fission-fusion better. Basically you have a normal z-pinch fusion system but surround the fusing hydrogen with U238. It absorbs the fast neutrons from the fusion, fissions, and creates more heat driving the fusion. There's some radioactive waste flying out the nozzle but if you're already in space that's not a huge problem.

On the upside, people aren't going to get nervous about nuclear weapons in space since you can't trigger the fuel pellets outside the engine. And you don't need a massive pusher plate, you just direct the charged plasma with magnetic fields. And that means you can wait to turn this on until you're in orbit and avoid creating any fallout on Earth.

https://www.nasa.gov/content/pulsed-fission-fusion-puff-prop...

What's the Isp of this method?

The first paper mentions 19,400 seconds at one point; versus 450 for hydrolox and 3100 for Dawn's ion drive. I did a blog posts on the theoretical limit on this sort of drive a while ago, using propellant velocity instead of ISP.

http://hopefullyintersting.blogspot.com/2015/04/rockets-vi-v...

Happy to see this getting some attention. It's seemed like one of the only viable propulsion methods for interstellar purposes, though nuclear is unfashionable at the moment.

I remember seeing a chart showing that it was possible to reach other stars using a technique like this. Unfortunately the problem is slowing down. You could get there, but you'd be going too fast to stop. And since the more fuel you carry, the harder it is to reach that speed, it ends up becoming completely impractical to carry enough fuel to both get there and slow down.

Maybe it'd be viable to stop by smashing into a planet, but it's hard for anything to survive that kind of impact force.

> nuclear is unfashionable at the moment

It's more than that - this isn't just hippies, it's every major power on the planet. To fly Orion you have to put hundreds of nuclear explosives in orbit and pinky-swear that you're not going to use them to obliterate your enemies. Nobody is going to accept that.

Orion's a fun paper rocket but will never be more. Engineers and they "yay nukes!" crowd will drag it out every couple of months for a repost, though.

> To fly Orion you have to put hundreds of nuclear explosives in orbit and pinky-swear that you're not going to use them to obliterate your enemies.

To reiterate berntb's point: Every nation who has the ability to do this already has ICBMs that can deliver nukes anywhere on the planet in 20 minutes. So what difference does this make?

True, but we still have a treaty that prohibits bombs in orbit, because they'd reduce the warning time enough to threaten deterrence.

Maybe one day we can launch Orion from Mars.

Drive it to Earth, have lots of bombs left.

From a strategic point of view, what's the difference between building on Mars and building it in orbit? Obviously there is a difference because the hostile faction has to seize control of it and move it to Earth, but once they do that, the calculation becomes the same. Do you commit to obliterating them the second they start moving it to Earth?

Empress Sato!!

Best case scenario, Mars is 55 million kilometres from earth. Even using a ridiculous engine going full blast the whole way (in which case your payload will probably disintegrate when it hits the atmosphere) you won't get the transfer time under a month. If you want to actually deliver your payload in a useful manner you'll need twice that at the very least. 2+ months is a long lead time, and the engine you'll need will make the craft impossible to miss.

What's the difference to building it in orbit? Building it in earth orbit you're dropping it from under 500km and your assembly makes 10~20 orbits a day, you can deliver it quickly at pretty much any time.

The former allows countermeasure, the latter not really.

I'm talking about just flying the entire Orion ship to Earth, loaded with propellant bombs. Sure, you can try to take it out during the journey, but that may be complicated by it being the highest g ship in existence for the Mars-Earth distance. Once/if it arrives, you have the same strategic problems you have if you build it in orbit.

> I'm talking about just flying the entire Orion ship to Earth, loaded with propellant bombs.

Er… yes?

> Sure, you can try to take it out during the journey, but that may be complicated by it being the highest g ship in existence for the Mars-Earth distance.

That's not really a complication, it's still taking months to make the trip, and while it might be "the highest g ship in existence" that's linear acceleration, it's not a nimble ship performing a tailside in space to avoid projectiles.

> Once/if it arrives, you have the same strategic problems you have if you build it in orbit.

With the pretty damn big difference that you've had several months head start knowing the think was incoming at high speed instead of it just dropping of your head more or less instantly.

Since Orion doesn't need to accelerate constantly, it's in principle quite possible to turn it and fire bombs in the direction of any interceptor, and the thick pusher plate makes it actually pretty durable from some aspects. What happens when two Orion ships duel, I don't know, but shooting it down isn't quite as simple as for most other spacecraft.

> Since Orion doesn't need to accelerate constantly

If it's coasting along it's significantly increasing transit time and thus available window to consider and prepare counter-measures.

> it's in principle quite possible to turn it and fire bombs in the direction of any interceptor

Sure but that means less payload to deliver and even more time for the target to prepare counter-measures.

> and the thick pusher plate makes it actually pretty durable from some aspects.

No matter how thick your pusher plates are they're not going to stand up to the kind of kinetic impact involved.

Er… yes?

Sorry, I didn't understand why you were talking about bothering to do that and then crashing it into the atmosphere.

If you're crashing it into the atmosphere you're not delivering anything, the ship and its payload will just disintegrate, possibly bouncing back in the process. Even if the payload does detonate on impact, the only purpose an explosion at atmospheric boundaries would have would be EMP.

Indeed, I didn't understand why you thought I would suggest something pointless. I wasn't suggesting it.

If you want to argue that crashing into the atmosphere would be an inevitable result of the trip, okay, but I don't understand what else it would have to do with my initial comment.

> If you want to argue that crashing into the atmosphere would be an inevitable result of the trip

That's not what I'm arguing FFS, I'm saying that'd be the result of going with the fastest possible trip, which would still leave the target significant time to set up countermeasures.

Here's the rest of the paragraph of my original answer since you apparently stopped at the first period:

> If you want to actually deliver your payload in a useful manner you'll need twice that at the very least. 2+ months is a long lead time, and the engine you'll need will make the craft impossible to miss.

The point is that no matter how you slice it, "the difference between building on Mars and building it in orbit" is the target gets a warning several months in advance in the former case, a few minutes or seconds in the latter. One gives ample opportunity to set up and deploy countermeasures, the other not so much.

So why not build the countermeasure before you build it in Earth orbit? what's the difference then.

[deleted some pointless back and forth]

Because there's no way you'll be able to detect the launch and deploy your countermeasures, even assuming said countermeasures actually work which is unlikely: it's doubtful we can counter ballistic missiles as-is, a payload starting from orbit can launch much closer to the target (rather than half a world away) and is not fighting gravity for half the trip.

Galileo safely aerobraked into Jupiter at 47km/s. It takes one hell of a heat shield, but it can be done.

> It takes one hell of a heat shield

That's quite the understatement, the galileo probe was half heatshield by mass. And it took a 230g hit to the face.

It's not just the war aspect that makes this un-viable. It's the failure modes, several of which include the destruction of life on earth.

If you read the book (by Freeman Dyson's son) or even the wikipedia page, you'd know the bombs were quite specialized. And it would certainly be possible to allow some verification (the trigger would not need inspection, that would probably be the really sensitive part, from a technology secrets point of view).

[Edit: The bombs would be directed, throwing light weight molecules up against the pusher plate.]

Also, there is no direct lack of ways to distribute nuclear devices to capitals today, for the countries able to build an Orion. Compare that with the possibility to launch enough material to e.g. start on getting an industrial infrastructure going outside the atmosphere.

The "directed" bomb designs used tungsten - in "The Curve of Binding Energy" I seem to remember there is a comment that they were spectacularly good at digging tunnels but I don't know if that was a theoretical prediction or an empirical result.

If you consider that it was contemporaneous with plowshare, that makes sense - likely a degree of shared research.

I meant that the material thrown at the plate were planned to be plasma of more lightweight materials, iirc. (That is different than how to make an atomic bomb directed.)

I haven't seen any real details of directed bomb design discussed. Thanks for the pointer to that book.

Any mechanism that can propel a starship to 0.13c is going to make your day unpleasant if it is pointed at things you have a particular fondness for. Favourite pets, cities, aircraft carriers, that sort of thing.

It is a theoretical "can". :-) It will be a long time before anyone use a fraction of C inside the solar system.

Interstellar flight is a problem after we have gotten to a reasonable cost/kg for both launching from the Earth and for interplanetary travel.

I doubt there will be a pressing need for getting close to even 100 km/s in the inner solar system for decades. (Yes yes, "640K is enough for anyone". :-) )

(Consider: An AU is 150 million km. 100 km/second means 8.64 million km a day; 2 AUs in 35 days! Then you have to lower the speed, even more energy.)

Launching an Orion from the ground is probably not realistic after the 1960s either. It would have the problem with bombs and EMP killing satellites. I've seen suggestions to coast through the sensitive zone without using bombs, but haven't seen any discussion about how many Gs it would mean...

"I doubt there will be a pressing need for getting close to even 100 km/s in the inner solar system for decades"

Quoting https://en.wikipedia.org/wiki/Solar_Probe_Plus :

> Solar Probe Plus or Solar Probe+, previously NASA Solar Probe, is a planned robotic spacecraft to probe the outer corona of the Sun. ... As the probe passes around the Sun, it will achieve a velocity of up to 200 km/s (120 mi/s) at that time making it the fastest manmade object ever, almost three times faster than the current record holder, Juno. ... Launch date: July 31, 2018 (planned).

He he, point. But a probe doing multiple gravity assists to get close to the sun is different from going from point A to point B -- and stopping at B. :-)

Anything that can accelerate a starship to a velocity of ~0.1c can have devastating consequences if pointed to earth. Just like planes can be used to destroy skyscrapers. Chicken littles notwithstanding, having a much faster mode of transport is worth the risk.

It may be worth the risk but that doesn't change the fact that what you refer to as "Chicken Littles" represent power players in an intractable political dynamic. The only way this kind of cooperation happens is when humans move past current notions of nation states and begin to think strategically as a species.

Not if an independent organization (think a Corp, not a gov) is the party responsable. They would have no incentive to do such a thing, in fact they would have a negative incentive.

Too bad nuclear tech is basically too regulated for any non governmental organization to experiment inexpensively, not to mention the other nontechnical issues.

On a side note, this is why Independence Day and other interstellar alien invasion movies don't make much sense. The amount of resources necessary to move between solar systems in an even slightly reasonable timeframe is orders of magnitude larger than what you need to obliterate all life on the planet in just a few seconds. The idea that you could meaningfully fight back against an interstellar traveler is silly.

Even worse: the EMP from an Orion launch would fry a lot of satellites. It's not a practical design for use anywhere near... anything.

You could launch one from the far side of the Moon, but if you have that kind of space presence there are better engine designs. Some have been posted elsewhere in this thread.

Stupid that satellite electronics don't have more radiation shielding. We can refrain from nuclear detonation in orbit all we want, but the sun could still EMP us at any moment. Maybe it's expensive to lift heavy shielding into orbit, but it's more expensive to deal with the consequences of satellite systems outages and repairing/replacing broken equipment.

Yes, Orion looked a lot more attractive back before civilization became reliant on huge quantities of sensitive electronics in near-Earth space.

I remember seeing a chart showing that it was possible to reach other stars using a technique like this. Unfortunately the problem is slowing down.

So this is like being concerned how you will spend all that money, once you make it :-)

There's a lot of discussion on interestellar travel, but "we" have almost frozen solar system travel. Yes, there are very nice probes out there doing important work, but manned vehicles are limited to keep ISS's lights on.

The "impossible" tech needed to reach the stars will most probably be discovered or refined while conquering the solar system.

Big sailing ships were developed along four centuries of sailing. The analogy may not be perfect, but I think it's eye opening.

I agree with you in innovation terms, but there's a big economic difference: sailing used free energy and therefore could be developed over the long term by many actors. There are huge energy cost barriers to solar system travel which require bigger bets on economic return to be viable

This was considered by project Orion.

Basically you turn the vehicle around and use the propulsion to slow you down. It would use a heap of fuel to do this but it was possible.

Edit, also, great find of this by the poster.

In order to have fuel to do that, you'd need to accelerate it to begin with. It ends up being an exponential increase in the quantity of fuel required, far more than could be carried.

The chart from the paper was pretty convincing. I'll see if I can dig it up. Making the explosions more efficient can only get you so far. I'd prefer to be optimistic about it, but the math was showing that even with an ideal ability to convert mass into energy, it would require too much fuel, to put it mildly.

According to the wikipedia page on Project Orion:

Later studies indicate that the top cruise velocity that can theoretically be achieved are a few percent of the speed of light (0.08-0.1c).[17] An atomic (fission) Orion can achieve perhaps 9%-11% of the speed of light. A nuclear pulse drive starship powered by Fusion-antimatter catalyzed nuclear pulse propulsion units would be similarly in the 10% range and pure Matter-antimatter annihilation rockets would be theoretically capable of obtaining a velocity between 50% to 80% of the speed of light. In each case saving fuel for slowing down halves the max. speed. The concept of using a magnetic sail to decelerate the spacecraft as it approaches its destination has been discussed as an alternative to using propellant, this would allow the ship to travel near the maximum theoretical velocity.[18]

https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propuls...

The referenced paper [18] is informative: http://www.space-nation.org/images/a/a1/Mini-Mag_Orion_and_s...

===

2.3. Deceleration: the canonical issue

While accelerating to a sizable fraction of the speed of light is a daunting task, deceleration is a yet more challenging task. The reason is that no realistic scenario to provide the gravy-train approach to either momentum transfer or pellet energy transfer at distances of 4+ light years seems within known physics. We are therefore challenged with either sending sufficient numbers of pellet-sheath elements so that the spacecraft can decelerate with on-board energy, or to come up with some other innovative concept.

2.3.1. Fueling at departure point

One option is to provide all necessary propellant at the departure location. If the specific power and specific impulse is sufficiently high, this is a potential approach. However, for our MMO concept, it is not particularly feasible. For example, our specific power is certainly high enough, but our specific impulse is dictated by the pellet-sheath combination, which limits the Isp to 128,955 s. While this may seem like a prodigious specific impulse, the ratio of spacecraft speed .1 c to g * Isp is 23. This means that the deceleration propellant must be e^23 times the spacecraft mass, which is an unworkable solution.

2.3.2. Mag Sail deceleration

In 2003 both Andrews and Lenard postulated using a large superconducting ring to intercept charge particles in interstellar space to slow the spacecraft down from high speeds. Additionally, the solar wind emanating from a star system provides an additional source of charged particles that can interact with the magnetic field. Deceleration can actually begin a sizable distance from the target star system. The following two charts indicate deceleration, velocity and time as a function of distance from the target star system. In this case the first phase of the deceleration starts at 21600 AU with a twoturn superconducting carbon nano tube reinforced loop. This loop captures the charged interstellar medium and deflects it to decelerate the spacecraft. This initial hoop size is 500 km in radius and carries 1,000,000 A of current. The spacecraft decelerates from .1 c to 6300 km/s by the time the spacecraft reaches 5000 AU. This will be quite a light show, so if there are any intelligent life forms with an observing system, they should be able to see the arrival.

But since you already used a bunch of your pulse propellant during acceleration, this is partly compensated for by the vehicle weighing less.

In "Death’s End", that’s a science fiction book by Cixin Liu, they use a variant of nuclear propulsion where the nukes are pre-distributed throughout the Solar System instead of being carried by the ship itself. This could free up even more mass and make room for an even bigger magnetic sail to be used for deceleration. Later in the same book, and much later in the fictional timeline, they build a circumsolar particle accelerator, which is easier than you would think [1], and this could be used to produce massive amounts of antimatter in space with a lot more propulsion power than conventional nukes. So I remain optimistic. It’s by the way a remarkable series of books that will freak out a lot of hackers, I recommend sticking with the series to the end.

[1] https://books.google.dk/books?id=A_1oCAAAQBAJ&pg=PA451&lpg=P...

Nuclear salt-water rockets! https://en.wikipedia.org/wiki/Nuclear_salt-water_rocket

Powerful, fast, intriguing and disgusting. :)

An issue with NSWR systems is testing. If you test on the ground, you either eject fissile material into the atmosphere or have to build a very expensive containment and scrubbing system.

These systems may be post orbital assembly technologies.

Project orion has the name recognition, but nuclear salt water rockets would just be a much better use of the same amount of fissiles. Maybe we should start reposting the wikipedia link every month until people remember mr. Zubrin's creations more than project Orion?

It's hard to say which is a more efficient use of fissiles. It's easier to burn up all your fuels in the context of an explosion as in Orion than in a saltwater rocket. On the other hand by bulking out the fuel with water to form a larger mass of propellant the saltwater uses it's energy more efficiently at the cost of lower ISP.

But really, if we're willing to leak radioactive byproducts I think an open cycle gas core might be the best bet in terms of not-yet-developed rockets.

http://www.projectrho.com/public_html/rocket/enginelist.php#...

The big advantage of Orion is that we're reasonably confident it would work, and could probably build one within the decade with a serious effort. Zubrin's NSW rocket, on the other hand, falls into the "hasn't yet been conclusively proved impossible" category. We're not sure if we can control the reaction, and we're not sure how much heat would be transferred to the rocket if we can.

Orion would have exhaust velocity ~1000-10000km/s. Compare that to NSWR's 66km/s. NSWR seems fast compared to today's chemical rockets but it's nothing compared to Orion.

What do you guys think of Laser sails & laser-based propulsion methods? The science exists now, seems like its only an engineering challenge.

Phil Lubin has a paper on it as well https://www.nasa.gov/sites/default/files/atoms/files/roadmap...

That's what Breakthrough Starshot[1] is about, although scaled down to very small probes. I like the probe concept. If we really don't want all our eggs in this one basket, it seems to make more sense to put effort towards tech that can spread our seeds (whatever that may be) rather than humans physically traveling.

[1] https://breakthroughinitiatives.org/

Exactly - i think there should be more companies & organizations working towards pushing this. To become space-faring, we need to get from 1ev per bond of molecule level of energies obtained from chemical to 1GeV levels atleast!

We are a long ways from becoming space faring. For the cost of sending one human to Mars, we could send ten rovers to Mars.

Not if Elon Musk gets his way. If the cost profile works out, sending a human will be cheaper then sending a robot.

Even then, we can still send rovers more cost effectively than people! You can throw away the life support mechanisms, and so on, and get 550 tons of pure mars rover in your interplanetary transport. At ~800kg for a curiosity-level rover that gives about 600 rovers, instead of 100 people.

What to do with your 600 curiosity rovers on Mars is left as an exercise...

The Curiosity rover has driven 9 miles in a little over four years on Mars.

The Lunar Rover that came with Apollo 17 drove 22 miles in a little over 4 HOURS.

As it turns out, humans are the best robots.

Another way to view it is that human rovers must go fast, because humans are impatient. We could build a faster, battery powered robotic rover if we wanted, but why?

If it's simply mileage you want, Curiosity's RTG means it can just keep going for years. With an average speed of 30 meters/hr, I'm not seeing much reason to think that it wasn't capable of covering ~1000km over those 4 years (the LRV, on the other hand, could never exceed 92km, total, before the batteries ran out). But mileage simply wasn't the goal.

Yes, but with my super-rover-transporter system, we can cover 5,400 miles in the same time, with the 600 rovers. I'm envisaging a sort of airbag deployment system (like pathfinder) coupled with a dispenser system that fires a rover out every few minutes as the ship orbits the planet, for optimal coverage.

But to what end? Bigger and better probes? Finer and finer data points?

We can send probes forever, but when do WE go?

My underlying point is that we have enough data to start making attempts at settling Mars, and once we're there, we'll do more science and exploration than any existing robot can. If you doubt that, then show me a robot that can perform an archeological dig on a fossil site without destroying half of the fossils.

His cost estimates depend on hundreds of thousands of people signing up, to diffuse R&D costs. That's dubious.

Historically, colonists are self-interested. Typically they want to become rich, or they want to escape poverty or persecution. (Typically they're also poorly informed about just how dangerous their new life will be, and how much wealth they can expect to find, but in this age we shouldn't expect that to last much beyond the first landing). There's no precedent that I'm aware of for people leaving comfortable, privileged lives in mass to colonize someplace that has no wealth.

Giving a generation of people who grew up with science fiction as part of their daily lives the opportunity to move to a different planet is also unprecedented.

For the cost of sending one human to Hawaii, we can set up 10 weather stations instead.

Cost of putting Curiosity on Mars: $2.5 billion

Apple and Google combined valuation: over $1000 billion

The trouble is there are no new users and new ad impressions to be found on Mars.

Plus, mass driver approach is not the only way to use lasers right? Light can remotely power systems, heat solid propellants to extremely high temperature thus getting better thrusts, etc. So the technology, if scaled up correctly, can be used for large scale transport systems too.

They could be useful for some specific applications, but lasers have very limited effective range as power delivery systems when you're looking at the interplanetary scale. Unless you have very large collectors that is.

For example, for the lunar laser ranging experiment they use telescopes to direct the beam at the moon, but the beam is still 6.5 km wide at the moon's surface.

https://en.wikipedia.org/wiki/Lunar_Laser_Ranging_experiment

Please read Freeman Dysons's "Disturbing The Universe".

This book is autobiographical and Dyson explains his arc of passion for nuclear propulsion and Orion.

His strongest statement in this book is some deep respect for a biological scientist who, after seeing declassified army training manuals on chemical and biological warfare, supposedly discouraged the entire western hemisphere from further develomepment.

This kind of nuclear research is, thankfully, over.

>This kind of nuclear research is, thankfully, over.

Nuclear pulse propulsion is the only currently viable technology that could be used to make humans an interstellar species. It would also allow us to practically ship up enough materials to build self-sustaining habitats in near space. It is extremely unfortunate that this kind of research is over.

Fun fact: the background radiation levels introduced by nuclear propulsion would actually have a very slight positive health effect on humans according to more accurate radiation hormesis models, rather than the very small negative effects suggested by more naive no threshold models.

Radiation hormesis models aren't "more accurate." The long-term effects of small amounts of radiation are simply not well understood. It could be good, it could be bad, it could be neutral.

The far bigger problem for launching Orion from Earth is the electromagnetic pulse frying every satellite above the horizon and a bunch of stuff on the ground.

I agree that nuclear pulse propulsion would be awesome to have. However, as far as I know radiation hormesis is a hypothesis that is still disputed.

On the other hand if we'd gone all-in on nuclear pulse launchers, there would have been no incentive for companies like SpaceX and Blue Origin to develop reusable conventional rockets which will hopefully achieve the same result at lower environmental and perhaps also financial cost.

Even if you had effective nuclear pulse propulsion, you'd still need reusable chemical launchers to get them off the surface of Earth. Nuclear pulse engines are generally not something that you start up inside an atmosphere, for both safety and efficiency reasons.

The post I was responding to strongly implied using these for launches.

> ..It would also allow us to practically ship up enough materials to build self-sustaining habitats in near space..

How can chemical rockets with ~3km/s exhaust velocity ever achieve the same result? Also if we had gone all in on this concept maybe today SpaceX and BO would be working with a much more promising technology.

It depends how reusable nuclear launchers would be. A less efficient launch system in terms of ISP that uses thousands of times cheaper fuel and e.g. is 10 times more reusable might be very competitive.

I would rather classify that as "unfortunate". Yes there are obviously biological harms in radiation exposure but that is what science & engineering is for. We know the power of nuclear energy so in order to harvest it, the right thinking should have been like "How can we safely extract it so that it does harm people in the mix?" rather than "Oh its harmful to humans so obviously we should give up and not use it".

Current fission startups are working on versions of reactors which eliminate most of the harmful byproducts in the process.

I'm not sure all of the opposition to nuclear research is about dangerous byproducts from the typical situation(s). It is about the improbable problematic situation which lead to exponential disaster that are scary, and, even if improbable, inevitable...

well you have rockets which blow up on launch sites. so..

Yay, chicken little!

There's also this great book about Orion: https://www.goodreads.com/book/show/21243.Project_Orion

I really love this stuff, this is a little more out there. (interstellar travel and all...) But I really believe that Nuclear power will be required to continue to explore our Solar System. Nuclear Thermal Rockets like Nerva[1] just seem like the only way to really colonize Mars in a sane way. I'm surprised that Elon Musk thinks it can happen with chemical rockets (and big tanks of LOX to keep cool and haul through the void).

[1] https://en.wikipedia.org/wiki/NERVA

Well chemical rockets are at a stage where all it needs is scaling up to get more power. But I do agree with you, the complexity and sizes needed makes you wonder why waste so much effort on some technology which has inherent physical limits in terms of energy derived. Why not spend that money and effort on accelerating other forms of propulsion techniques (like nuclear) within the same timescales?

We need another Manhattan Project for space tech.

"orion's death" (page 6-7) says it all, we might be able to see something like orion by today if there were no ban on the nuclear tests, and there wouldn't be any "ban" if there were no "nuclear attack in 1945".

Think of this as a startup case for 2016: an exciting idea on paper with a very improbable, cumbersome and practically impossible execution = non-viable, fail fast, goodbye.

Eli5: how does a nuclear explosion create a change in velocity for the spacecraft?

The explosion throws the products of the explosion - mostly plasma - into all directions. The spaceship is nearby, so some of the plasma hits the ship, which is hiding behind a strong plate. Plasma transfers momentum to the ship.

Sort of like a sailboat, except the wind is the vaporized material from the nuke. The nuke explodes, throwing a ton of mass and energy in every direction. The rocket uses a sail or pusher plate to catch part of that mass and energy, which carries it forward.

The mass is pretty tiny ( smaller than the ejected fuel pellet); I presume it's the fact it has huge velocity, hence huge momentum? What about photons; do they have the same effect as other particles, or is there a different effect?

You add a significant amount of mass to the bombs to act as propellant. I don't know offhand what fraction is typical for Orion designs.

If only we could produce anti matter. A tiny amount would provide a fuel source viable for deep space exploration.

Anti-matter is produced at CERN for study https://home.cern/topics/antimatter.

The problem is not production of it but the economy of producing it. It's still very costly to produce and then to build and engine around it which uses the energy efficiently is still at least 10-15 years. But do believe that the time to start actively looking into it is NOW.

Is it stable enough to survive a space launch?

don't think so. I am guessing you would need really strong magnetic fields to contain the anti-matter.

Magnetic fields aren't the only issue here.

You won't be able to store large amounts of it since quantum tunneling will become a statistical problem.

Could you explain? How would quantum tunneling of antimatter be a problem?

The probability of quantum tunneling is exponential so adding a little more separation or reducing the energy can change the probability from 10^-1 to 10^-100.

As a practical matter most of the byproducts of antimatter annihilation aren't amenable to being directed out the rear of a spacecraft by a nozzle, magnetic or otherwise. So the maximum theoretical efficiency is possibly better with advanced fusion drives than with antimatter.

When discussing antimatter for propulsion, it might be worth checking out Robert L Forward. I saw the expression that he took the "giggle factor" out of the subject. :-)

I found this overview, discussing a bit about the antimatter research:

http://www.enthea.org/docs/Forward-Emerging-Technologies-Fut...

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