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Surely that's like saying we've had nuclear submarines since Jules Verne wrote "20,000 leagues under the sea" insofar as Project Orion was little more than science fiction and didn't involve fusion drives ("atomic bomba" would be fission).

(Verne's Nautilus was powered by primitive batteries.)

Project Orion was much more than science fiction (http://www.scribd.com/doc/28668567/Nuclear-Pulse-Space-Vehic...) though it was much less than operational. Very big Orion spaceships would have used fusion explosions (http://galileo.phys.virginia.edu/classes/321.jvn.fall02/Dyso...), but not the relatively small ones that were subjected to detailed analysis.

You say "conceptual design" and I say little more than science fiction. Potato, potato -- some kind of solid fuel rocket is going to lift a shield strong enough to protect a crew with 90 days of "emergency" supplies from small stomic bombs going off... Right. And this is for a lunar mission!

There's more detailed stuff out on the warp drive NASA has funded study on, meanwhile the grandparent made it sound like someone had built one of these things (only with fusion bombs) in 1952 and it was rusting in a shed somewhere.

Then we've basically been on the moon for centuries now. Just because we can theoretically build something with known physics doesn't mean we have e.g. the material knowledge to actually construct the spaceship. Also, there's this issue of slowing down, and of how to get that spaceship with that much fuel out of the atmosphere, and how to get that spaceship back.... I would say we have a long way to go before claiming we "have" a fusion drive in a spaceship.

It's pretty much like rocket jumping.

> Arguably we've had "fusion drive" for spaceships ever since Project Orion. Very simply, build a ship with a big shield in the back, throw nuclear bombs out behind the ship, detonate, the shock and radiation hits the plate (some ablates, but it's still fairly study and safe), and some kind of shock absorber system pushes the spaceship forward. Repeat. You can feasibly get to 0.10c with this, using 1960s technology, and really big ships.

I never understood how the blast alone can propel the ship. Don't you also need to eject mass from a nozzle to move forward - in which case, it's just like a regular jet, you just replace part of the fuel with nukes?

Part of the mass in the fuel in the bomb is converted to photons; photons are massless but carry momentum.

I would never have thought about that. That just this is sufficient to push the ship forward is amazing.

There's also the fissile material involved, but I couldnt tell you what proportion of the energy is disappated through heat and what proportion is dissapated in light.

Everyone has seen the videos of the atomic bomb going off - that massive release of heat and energy that literally vaporizes mass that is close to the epicenter. Now imagine that reaction in space where the heat can't be disappated through atmosphere, water, or earth. All of that energy has to go somewhere, so it's all stored in whatever fissile material is left after the bomb goes off and dissapated through light and radiation.

I really wish I had some numbers for how hot that remaining material would get - even thinking about all the heat released in an atomic bomb concentrated into such a small amount of mass absolutely boggles the mind.

Nozzles are basically just a more efficient version of this, since they don't waste as much energy which would escape out the sides (it's instead contributing to internal pressure, which pushes everything out the open end more quickly). Burning rocket fuel expands it rapidly, which is essentially all a bomb does. The downside to a nozzle is that you have to keep the internal pressure low enough that it doesn't rupture. This is far less of a problem with an Orion-style ship.

Think of an explosion. They produce moving walls of compressed air that flings stuff around. Though this is just because it's in air, it's a lot of energy. The same amount of energy is still being released if you blow it up anywhere else. (ignoring bonus 'free' energy from e.g. burning the air itself)

Take the explosion out of air, and you still have a 'shock wave' from the material which made up the bomb, and electrons / photons / whatever else materialized from that much energy being released in a tiny space. And it's all moving very quickly.

When it hits your ship's back-end, it gives it a kick. Obviously far less than the total energy released (whatever % of the sphere of expanding energy which your ship covers), but still something. If the explosion is big enough, or close enough, you are still talking an immense 'kick'. And since you have no relatively-fragile nozzle (just an arbitrarily big, thick wall), the force can be many many times greater than a normal rocket.

So to make it survivable, you probably need a big ship and some kind of impact-absorber to take the shock out of it, and / or lots of small bombs that won't kill everyone due to acceleration. Or just get rid of the squishy humans and crank up the proximity / bomb size to whatever you feel like.

By the way, it's worth pointing out, to those who want to make a judgement on the reasons project Orion could not be implemented nowadays, that nuclear detonations are effectively a modern tabu.

This tabu is effective, has proven extremely useful (no detonations of nukes in a war since 1945) and thus cannot be considered the sign of a scientifically ignorant society.

Having said that, Orion would be quite cool.

Do you mean 'taboo', as in something that's considered off-limits due to cultural sensitivities? (Wondering out loud if the etymology is similar to voodoo and related suspicious stuff)

"Taboo" derives from the Polynesian (Māori or Tongan) "tabu" systems of social control. Wikipedia has a decent overview.

Yes, taboo. I suffer a bad case of EuroEnglish.

We've had hundreds (or thousands, if you count sub-design-yield) detonations since 1945, including several in space (Hardtack Teak and Orange, and USSR equivalents).

And lots of them were for purely posturing purposes, with little scientific or direct weapons-testing value.

The US probably could have pulled off something like Orion in the 1990s or maybe even today, since there's not a direct cold war threat. Domestic politics probably kill it, though, but if it were branded as "nuclear pulse drive" with sub-10kT pulses, it might be possible.

You can't launch an Orion spaceship without violating the https://en.wikipedia.org/wiki/Partial_Nuclear_Test_Ban_Treat.... Also given the estimated 0.1 to 1 human deaths from cancer per launch (I am using Freeman Dyson's estimate here), it is hardly a politically viable vehicle.

Didn't Dyson's son write in the book that part of the reason for the treaty's signing was internal politics to kill a competitor to NASA?

1. Those 0.1-1 dead is from a period of much worse cancer treatments than will be available a few decades after an Orion launch.

2. It is bad risk counting. The cost per life is a factor whenever e.g. roads are built and speed limits are set. ("We will get ~ X less deaths/decade if we build the motorway differently, but it would cost over our limit for $/life".) Also, just transporting materials/people when designing/building an Orion ought to be a lot more than one dead.

The cancer increase would be for launching from the ground/atmospheric use, which is a non-starter. I'm talking about use to run an orbital shuttle/tug/etc. between Earth and Mars. It would never enter atmosphere, and probably be constructed in space.

The US has enough weapons that it could comfortably withdraw from the treaty, or claim that these launches are not "tests" but rather some other use.

The cancer increase was from fallout. If you use it within Earth orbit, you still have significant fallout.

You don't really get fallout in the space.

The fallout is from neutron radiation of the ground, which doesn't happen in space. The amount of direct radiation from the uranium that is used is actually quite small.

And it would be trivial to point the exhaust away from earth. Space is full of radiation anyway, from the sun.

I would not have thought this, but you are right.

I would guess that the EMP would be dangerous to satellites, but I'm sure there are ways to mitigate that as well.

There's no EMP in space: https://news.ycombinator.com/item?id=5526333

The fallout is from neutron radiation of the ground, which doesn't happen in space. The amount of direct radiation from the uranium that is used is actually quite small.

Can you cite this? I think the major dose component of nuclear fallout is from fission products, not neutron activation products. For example, skimming through these appendices on dose calculations from historic nuclear weapons tests [1](e-h), most of the significant radioisotopes are either fission products, or transuranic activation products from the weapon (e.g. Am-241). The environmental activation products discussed, C-14, Mn-54, Fe-55, and Co-60, are much less significant.

[1] http://www.cdc.gov/nceh/radiation/fallout/

It's a matter of quantity - a ground burst produces enormous amounts of radioactive material. An air (or space) burst only has a few KG of radioactive material.

From: http://www.fas.org/nuke/intro/nuke/effects.htm

"Air Bursts. .... there is essentially no local fallout from an air burst."

"Surface Burst ..... In contrast with air bursts, local fallout can be a hazard over a much larger downwind area than that which is affected by blast and thermal radiation."

I think you're misunderstanding that reference. The amount of radioactive material is not that different; the distinction is that a ground burst localizes some of it at ground zero. It creates a small area which will be an acute radiation hazard for a short time. An air burst gives a very large, diffuse, atmospheric plume.

In the maps from the CDC link, large areas -- whole states (check out pages f29-f35) -- are contaminated with low levels of airborne radioactive fallout. From the isotope data, it's clear most of this is from the weapons, not environmental activation products. The doses are too low to be immediately dangerous (<1 mSv), but can have chronic health effects, as a slightly increased cancer risk.

You can tune your weapon to have MUCH lower fission products; essentially the smallest size fission primary you can (ideally, with boosters), and then a big fusion stage and no U-238 tamper. Conventional thermonuclear weapons are usually fission-fusion-fission since a U-238 casing for the third stage is a cheap and compact way to scale up the weapon, but that last fission stage is responsible for >95% of the fission products/long lived pollution. A "neutron bomb"/ERW/etc. is one application of the low-radioisotope weapon (and a "dirty bomb", a non-critical radiological weapon, is the other extreme).

I'm still hoping fission-free fusion weapons are not possible before I can live somewhere other than Earth, since all arms control essentially rests of preventing access to fissile materials. Once you eliminate that gate, it becomes much easier for a clandestine group to build a weapon. Pure fusion weapons would be amazingly destabilizing and, if they were approximately as hard to make as seems likely, would absolutely get used by some group.

That's not really how fallout works. Fallout occurs primarily when you detonate a bomb near the ground or the water, causing it to suck up a bunch of dirt/water, irradiate it, and distribute it throughout the atmosphere. With a fusion bomb, you could end up with some leftover plutonium from the fission stage, but that's not the primary source of fallout.

>given the estimated 0.1 to 1 human deaths from cancer per launch (I am using Freeman Dyson's estimate here), it is hardly a politically viable vehicle.

Considering that one coal plant statistically kill about 70 people per year due to air pollution, I think it's more a matter of insufficient political power than the product of rational policy.

We've had hundreds [...] detonations

None of them intended to kill other people, which is kind of the whole point of the taboo, not the detonation itself.

Plus, currently there is a kind of informal moratorium.

and you think that getting x nuclear bombs into orbit is safe?

and even when all bombs go into space without incidents, having them there until start is no risk neither?

i think something more conventional would be better, even when less economically ...

I don't think anyone thinks it's a particularly practical idea these days - for the reasons you identify.

However, it seems a bit of a shame that for all of the 2000+ nuclear weapons tests and tens of thousands of nuclear weapons built we never did anything as constructive as flying an Orion.

You can pack materials so that a failed launch will not create leaks.

The real problems for the Orion are political and frying of satellite electronics, iirc.

Mostly political. You could presumably do the detonations with the moon in between for a while, and it's not like space outside the Van Allen belts is particularly low energy, either, so GEO satellites are shielded.

i hate it when problems are "political" - or more correctly, that the general public's ignorance and fear retarding progress.

If I had ~10k nuclear weapons, I'd no longer have political problems.

I think you would suddenly have a great many political problems.

Well, combustion engine in my car is not very conventional either by XIX century standards. Maybe we should ride horses?

Why you make assumption that bombs must be delivered to space vs constructed there from resources mined in asteroids?

Plutonium ain't easy, so you'd probably want to ship pre-made pits into orbit (we've got plenty already made), preferably one at a time so we don't get a big critical mass of plutonium sitting in a desert in case the rocket crashes.

Plutonium is a synthetic element, requiring a nuclear reactor to create, then must be purified. Uranium is currently unknown in asteroids, and even if it were found would require refining with thousands of large centrifuges.

Converting uranium to uranium hexafluoride, centrifuging it over and over, then converting it back requires massive industrial facilities, currently impossible in space.

What about moon?

I'm pretty sure that smart people will figure out the solution. I just wanted to point out that most probably we can find solutions that are safe enough and the only limiting factor is our imagination (or lack of it specifically).

Why wouldn't it be safe? Nuclear bombs don't just explode by accident or when they are hit by something, they need to be deliberately triggered, so I wouldn't estimate the danger of this to be very big.

Furthermore, nuclear weapons have been sent to space before, both for regular long-range missiles and for EMP-like tests.

As others point out, nuclear bombs do not spontaneously go off. If they were that easy to accidentally trigger, building them would be no big deal and we wouldn't be talking about nuclear vs. non-nuclear countries to this day.

Furthermore, everything is space is already a weapon. The very act of being in space and not falling back to Earth means you are loaded up with enough kinetic energy to be going several miles per second. You don't need nuclear bombs to be a weapon in space, a bucket of sand is a deadly weapon. Militarization of space refers to things other than merely putting "big bombs" in space, where they are surprisingly useless. (Even the EMP is less useful than you'd think; space is already a sea of radiation so everything up there is already hardened against it.)

You won't have an EMP in space - the EMP is generated by the motion of intensely electrically charged pieces of atoms acting against the magnetic field of the earth, and by electrons that were launched from atoms by the gamma ray pulse.

There is a solar magnetic field, but it's much weaker, so wouldn't make much of an EMP, and there is no atmosphere is space so the gamma rays don't accelerate any electrons.

You won't have EMP in deep space. Earth orbit is still "space".

High-altitude nuclear explosions (which produce the most intense EMP) work better above the Kalman line: http://en.wikipedia.org/wiki/File:High_altitude_EMP.gif

There are also special EMP weapons that don't require nuclear explosions.

That graph tops out at 300km, but even LEO is 2,000 km. (Although you can orbit as low as 160km - why would you for this?)

And I've never heard of a Kalman line and google comes up with nothing.

LEO is below 2000 Km, and is typically 300-500 Km. http://en.wikipedia.org/wiki/Low_Earth_Orbit