Thermonuclear Bomb-in-a-Bottle

[djolds1]

No, there's plenty of hydrogen bombs left.

No, there aren't.

Yes, there are.

http://en.wikipedia.org/wiki/List_of_nu ... ted_States

Current service:
http://en.wikipedia.org/wiki/W76
http://en.wikipedia.org/wiki/W78
http://en.wikipedia.org/wiki/W80_%28nuclear_warhead%29
http://en.wikipedia.org/wiki/W87
http://en.wikipedia.org/wiki/W88

[kunkmiester]

Regardless, they're not that hard to make provided you keep the infrastructure. And the current designs would allow a very low yield thermonuclear device, I'd imagine.

The stuff I've seen on Orion says that the bombs get bigger with time. You'd start out with .15 KT bombs, but you'd eventually be using several kilotons at least.

[Shubedobedubopbopbedo]

Ok. Don't know where I got the idea there weren't any fusion bombs in existence. Maybe because there aren't any multi-Megaton bombs in existence. But if I was wrong once, I could be wrong about that too.

Anyways, I think that any nuclear powered spacecraft would have to be constructed in orbit. As such, we could use a fusion reactor rather than bombs. An electric powered linear accelerator might be more efficient than a bomb blast for producing thrust.

A fusion reactor coupled to a steam turbine generator might achieve up to 60% efficiency, whereas a bomb cannot transfer greater than 50% of the blast energy to the vehicle, even if it is perfectly collimated.

Unproven. Speculative. Blah. Blah. Can't think of a reason why we'd need it anyway. Maybe to intercept an incoming comet.

[kunkmiester]

Getting more than 50% is the reason for developing the shaped charge. Part of the energy going the wrong way is reflected towards the space craft before the explosion rips the module apart. You might not get a lot more, but it might help.

A "fourth gen bomb" that is pure fusion would be ideal--half the advantage of Orion is that it keeps its advantages from the "ground up" so to speak. At the time it was looked at, they had decided that the fallout from a launch was pretty much irrelevant compared to all the nuclear testing. Bigger is better too, since larger bombs for larger ships would burn the plutonium more efficiently, leading to less long term fallout.

[IntLibber]

Ok. Don't know where I got the idea there weren't any fusion bombs in existence. Maybe because there aren't any multi-Megaton bombs in existence. But if I was wrong once, I could be wrong about that too.

Anyways, I think that any nuclear powered spacecraft would have to be constructed in orbit. As such, we could use a fusion reactor rather than bombs. An electric powered linear accelerator might be more efficient than a bomb blast for producing thrust.

A fusion reactor coupled to a steam turbine generator might achieve up to 60% efficiency, whereas a bomb cannot transfer greater than 50% of the blast energy to the vehicle, even if it is perfectly collimated.

Unproven. Speculative. Blah. Blah. Can't think of a reason why we'd need it anyway. Maybe to intercept an incoming comet.

Actually, a bomb can transfer more than 50% of the blast energy because its well established that you can make nuclear shaped charges. Such were specifically designed for the original Orion project. Beyond mere kinetic energy, neutron and other radiation striking the pusher plate is used to help enrich a layer of radionuclides on the pusherplate surface, so that the vehicle produces its own fuel for future use in transit.

Beyond this, it is more efficient to exert maximum thrust in as short a period of time as possible than to exert it over a longer period at a lower thrust level given the same mass to energy conversion efficiency. This is a matter of orbital mechanics.

[Shubedobedubopbopbedo]

Half the momentum of the explosion goes toward the pusher plate, half away from it. That's Newton's 3rd Law (action-reaction). In reality, it will be much less because the blast is not perfectly collimated, much of the blast will impinge on the pusher plate at a non-zero angle to the direction of travel. So, much less than 50% of the blast energy is used to impart delta-V to the spacecraft. This might be improved if the blast is fully contained by a combustion chamber/nozzle system like in a conventional rocket. If the nuke yield is tens of tons, forget that idea.

A steam-electric generator is limited by the Carnot efficiency. Modern industrial turbines can achieve a Carnot efficiency of 60%. However, if the heat initially comes from a fusion reactor, it might have a very low efficiency itself, unless the input heat energy can be added to the output heat energy to generate steam.

[kunkmiester]

Um, MORE than half the energy from a shaped charge explosion will go towards the pusher plate. Thus you CAN get more than 50% of the energy on the plate, collimation losses withstanding.

[Aero]

Results 1 - 30 of about 267,000 for shaped charge video.
This is from the first result.
http://www.youtube.com/watch?v=LudNqf56AFo

[Shubedobedubopbopbedo]

Action-reaction. The bomb-pusher-plate is much less than 50% efficient. Period. Shape charge or not. Newton's 3rd Law of Motion. Look it up. This is high school stuff.

[93143]

Depends what you mean.

Action-reaction only applies to momentum.

Energy can be directed. If most of the mass is present on one side of the bomb, the bulk of the energy release will happen in the other direction. Just like a rocket engine.

A nuclear shaped charge is not designed to increase the energy or momentum transfer beyond 50%. It's designed to (a) shadow shield most of the radiation from the blast, so the ship doesn't have to, and (b) slim down the fireball so that almost all of the forward lobe hits the pusher plate. So saying "much less than 50%" isn't accurate either.

Just a reminder, a thermal power plant would have to have enormous radiators, because that waste heat all ends up in the spacecraft's power generation system, instead of being blown into space like with an Orion. All-regenerative cooling doesn't work at high Isp because you end up needing too much hydrogen to cool the system at a plausible temperature. And the Stefan-Boltzmann law means that the minimum radiator mass for a given useful power output is achieved with a thermal efficiency of 25% or less, not 60%. Given that the radiator mass tends to dominate the system mass...

Besides, all this talk of energy efficiency misses the point (note that for a given thrust and engine efficiency, energy expenditure is directly proportional to Isp). The relevant measures are mass efficiency and power output; that is, Isp and thrust, and Orion starts at well over a thousand seconds with a vehicle T/W above one, with bombs two orders of magnitude weaker than the Little Boy, and thus far less mass-efficient - higher Isp available with less pulse unit mass, at the expense of thrust (unless you increase the yield, which helps Isp and thrust). Advanced systems were expected to hit 20,000-40,000 seconds, still with high thrust, and keep right on going - the 3.3% lightspeed starship design had an average Isp well over a million, and accelerated at one gee. Even a nuclear salt water rocket can't hold a candle to an advanced Orion.

[hanelyp]

...Even a nuclear salt water rocket can't hold a candle to an advanced Orion.

I remember starting an orion vs. NSWR frame war on sci.space.tech with a comment to that effect. Entirely unintended.

[kunkmiester]

I remember starting an orion vs. NSWR frame war on sci.space.tech with a comment to that effect. Entirely unintended.

Must have been fun. How'd it go?

[IntLibber]

Action-reaction. The bomb-pusher-plate is much less than 50% efficient. Period. Shape charge or not. Newton's 3rd Law of Motion. Look it up. This is high school stuff.

Do you even know what a shaped charge is? Sounds to me like you dont.

[93143]

Anyway, I'm assuming that the shock absorbers are actually shock absorbers. If they were what you describe, they would be called springs, not shock absorbers.

They're gasdynamic springs, as far as I can tell.

Hmm... Actually, from Figure 2.4 in the report, it looks like the dissipation in the secondary system is about 7% per half cycle. I can't see any dissipation in the primary system (Figure 2.3), so I'll assume 10% in total.

So double my estimate for heating. Or redesign the system - this is a conceptual study, after all...

Where does this leave us? Steady-state would be 1380 K, which looks a bit dubious, but we don't need to worry about steady-state. After firing 800 pulses, assuming no cooling, we're at 634 K, or 361ºC, or 681ºF.

Apparently it's still not a problem.

Larger vehicles would have to deal with longer burn times and more energy in the pusher system, but could hopefully be designed with more sophisticated and efficient shock absorbers, and perhaps some nominal cooling/radiator systems (nothing that sticks out past the pusher plate's shadow cone, of course)... For instance, I heard about a droplet radiator system that ejects the droplets forward from an accelerating starship, then collects them once the starship catches up...

Also, without actually running the numbers, I'm wondering if your estimate of vibration frequency is correct ~1 second? Don't you think this would shake the spacecraft to pieces?

Why? It's not the speed that matters; it's the induced loads (unless the vibration gets so fast that fatigue is a concern...) Ares I TO was significantly faster, at a higher gee loading, with structural resonance thrown in into the bargain, and the only hazard was to the astronauts.

Besides, do you really think they didn't take this into account? There's a vibroacoustic analysis in the report.

Judging from Figure 2.3, it's 1.10 seconds for half a cycle of the secondary system (the primary system is 9 times as fast). So 2.2 seconds for a full oscillation. (This also means my equilibrium power dissipation calculation is about 9% too high, all else being equal...)

The low-yield fission bombs are radation bombs, giving off up to 80% of their energy as neutrons and x-rays.

Hence the massive chunk of tungsten and beryllium oxide in between the physics package and the spacecraft.

But I'm not convinced that a low-yield bomb is the best choice.

It's not. It's just easier to build a spacecraft that can make use of it. Projections indicate that the effectiveness of the Orion design increases continuously with the yield of the pulse units, all the way up to megaton-range deuterium bombs.

I think the pusher plate design is inherently massive due to the compression members needed in the structure. There is a way to make the whole thing less massive, and hence faster.

Medusa?

It has its advantages and disadvantages with respect to a conventional Orion...

[Shubedobedubopbopbedo]

Medusa was an interesting idea, but ultimately impractical.