A quick question about Nuclear explosions

[takemeout_totheblack]

The nuclear explosion calculator on the SD.net main site is really cool! Very informative.
However, I have one question in regards to shockwaves and such. Correct me if I'm wrong (I'm no expert here) but the shockwave is created by the explosive energy displacing the air, right? If so, exactly how fast does a nuke of, oh say, 1 megaton displace the air within its fireball radius? An appreciable percentage of c or just very fast?
Again, I make no claims as to my knowledge about such things, in truth I ask because I have absolutely no idea as to the mechanics or math behind it. Part of the reason I came to SD.net was to be able to ask such questions and possibly get an answer.

[Patrick Degan]

The nuclear explosion calculator on the SD.net main site is really cool! Very informative.
However, I have one question in regards to shockwaves and such. Correct me if I'm wrong (I'm no expert here) but the shockwave is created by the explosive energy displacing the air, right? If so, exactly how fast does a nuke of, oh say, 1 megaton displace the air within its fireball radius? An appreciable percentage of c or just very fast?
Again, I make no claims as to my knowledge about such things, in truth I ask because I have absolutely no idea as to the mechanics or math behind it. Part of the reason I came to SD.net was to be able to ask such questions and possibly get an answer.

Nuclear shockwaves are supersonic, not relativistic.

[takemeout_totheblack]

Cool! And the shockwave is the more damaging event once outside of the thermal damage range, right?
What would happen if the same amount of air was displaced at relativistic speeds? Food for thought.

[Simon_Jester]

The energy to displace air at relativistic speeds just isn't there; I'll explain the math if you like, but it's not all that difficult.

Even if you could get a small amount of material near the blast moving at those speeds, the relativistic-speed nuclei* would start slowing down very quickly in air. Their kinetic energy would be dispersed among a large number of much slower air molecules. In effect, you're talking about a shower of alpha particle-equivalents, and alpha particles have effectively zero range outside of a vacuum.

*(anything energetic enough to get atomic nuclei moving at relativistic speeds is energetic enough to break molecular bonds and strip the electrons off the nucleus)

[takemeout_totheblack]

That makes sense. I just got the idea of a sort of superluminal shockwave from that one episode of nBSG, where the Galactica uses her FTL to jump into a planets atmosphere. It just got me to thinking that a wormhole displacing that much air is such a short amount of time would have done more than just make a 'boom', since its a pretty big ship (600m I think). But your explanation makes sense, thanks for answering!

[Simon_Jester]

That makes sense. I just got the idea of a sort of superluminal shockwave...

Stop right there; I hereby place you under citizen's arrest for violating the laws of physics!

Seriously though, "relativistic" is not the same as "superluminal;" if any way to make objects move faster from A to B than a light beam covers the same distance exists, you can be quite sure that the energy of the moving objects isn't described by relativistic equations.

from that one episode of nBSG, where the Galactica uses her FTL to jump into a planets atmosphere. It just got me to thinking that a wormhole displacing that much air is such a short amount of time would have done more than just make a 'boom', since its a pretty big ship (600m I think). But your explanation makes sense, thanks for answering!

Does the wormhole forcibly push air aside, or does it get sucked through into the other side of the wormhole? Do the wormholes expand from a point? How long do they take to form?

[takemeout_totheblack]

Oh shit! My mistake, slip of the fingers!

[takemeout_totheblack]

Does the wormhole forcibly push air aside, or does it get sucked through into the other side of the wormhole? Do the wormholes expand from a point? How long do they take to form?

There was a 'boom' noise that made Tigh and the other resistance fighters look up, although in retrospect it could have been Galactica breaking the sound barrier as she fell. There are a few passing comments about the FTL exerting some sort of force when entering and exiting, since I remember a comment about if a ship exited jump too close it would have caused serious damage to Galactica. That and her 'spine' breaking in the final episode, but that's more a structural thing.

[takemeout_totheblack]

I remember a comment about if a ship exited jump too close it would have caused serious damage to Galactica. That and her 'spine' breaking in the final episode, but that's more a structural thing.

Again, my mistake! Looked it up, and it's entering that makes the distortion apparently. When Boomer jumped too close to Galactica, the hull was dented. Why it didn't do this in the atmosphere above New Caprica I don't know. Oh well, right?

[Sea Skimmer]

The nuclear explosion calculator on the SD.net main site is really cool! Very informative.
However, I have one question in regards to shockwaves and such. Correct me if I'm wrong (I'm no expert here) but the shockwave is created by the explosive energy displacing the air, right?

A nuclear device has little to no inherent explosive energy in terms of blast. When a conventional high explosive bomb goes off, the physical mass of the high explosive is being turned into hot gas, which rapidly expands creating a shockwave. The mass of the air around it compresses, and makes the shockwave stronger. That means even if you set a conventional explosive off in an vacuum like space, you’d still have some blast energy. The lack of air would mean the gas can easily expand though, so it wouldn’t be a very far reaching blast. But a blast all the same. You also get some heat, but no ionizing radiation.

Now when a nuke goes off, the nuclear reaction is primarily releasing radiation. The mass of the nuclear fuel is turned into that radiation, rather then forming a gas cloud (a little gas forms, but not enough to matter). Some of it is ionizing radiation like neutrons and gamma rays (ionizing radiation is also commonly called ‘hard’ radiation a lot on web forums like this, this is the kind of radiation that causes cancer). Some is also thermal radiation, also know as heat.

In space in a vacuum if you set off a nuke, all you’d get is a flash and then a huge pulse of various kinds of radiation. That radiation would be so intensive that it would make steel brittle and destroy electrical equipment besides killing people, but its not a blast wave.

Now in an atmosphere, you get a blast wave from a nuke. The reason for this is that when the radiation leaves the nuke, it hits the atmospheric gas mixture we call air. The air absorbs certain kinds of hard radiation to varying degrees and turns it into heat, and it also directly absorbs the heat from the thermal radiation the nuke throwing out. That heat causes the air to expand… really rapidly. The air expands so much so quickly it is converted into a supersonic shockwave.

However not all thermal radiation and hard radiation is absorbed, which is why you still have the three basic nuclear device damage mechanisms of thermal pulse, hard radiation and blast wave. Blast will do most of the damage, but the thermal pulse can ignite fires well beyond the blast radius. Because this effects scale differently with the yield of a nuclear device (this is releated to the way the air absorbs the radiation), on a very small a nuclear weapons lethal ionizing radiation effects can extend further then the blast effects.

This was the logic that led to development of small neutron bombs for battlefield use against tanks. For any larger nuke, like the 20-15kt nuclear devices dropped on Japan, the lethal radiation effects are dwarfed by the lethal radius of the thermal pulse and blast effects. So ionizing radiation is then primarily a concern in terms of it coming down as nuclear fallout afterwards.

Nuclear fallout is formed when small bits of nuclear fuel (really really small bits, molecular scale) which did not totally convert to radiation are mixed with material from the ground, irradiating that material. This gets blow in the air in the mushroom cloud, then rains back down.

If so, exactly how fast does a nuke of, oh say, 1 megaton displace the air within its fireball radius? An appreciable percentage of c or just very fast?

The radiation leaves the nuke at around the speed of light and keeps going that fast. The blast wave however varies with the yield of the nuclear device, and the altitude at which it is initiated, as air density changes with height. In addition, the blast wave may be strengthen and accelerated by reflections off the ground, which combined together to follow the earths surface. This can make the actual damage pattern of a nuke on the ground different then a bunch of radiuses on a map would suggest. But the true beahavior of blast waves is a whole subject on its own.

Now as for the actual speeds involved, the speed of the shockwave and the overpressure value are related. Essentially the blast wave is just a kind of wind. Really fast wind which also blows all kind of debris around which causes yet more damage. An overpressure of 200psi is around 2,000mph (that’s about mach 2.7 at sea level), 50psi is around 930mph. 5psi is 163mph.

Natural wind gusts on earth have been recorded to reach 253mph for comparison, and tornadoes have exceeded 300mph. But normally 50mph would be a very windy day! Homes are typically built to withstand 90mph wind gusts.

The near center of a nuclear burst however is much more intensive, even a 1kt device can generate over 10,000psi of overpressure at about 75 feet, and the very center of a bigger bomb can be a few tens of millions of psi. I can’t find speed values for these kinds of pressures, but it would be stupendously high. Nothing like the speed of light, but many tens of thousands of miles an hour. However the pressure drops so quickly that the blast covers very little distance at this kind of speed. So thousands of miles an hour is a good way to think of it.

[Shroom Man 777]

So, if I am getting this right, the nuclear bomb's radiation makes the air explode?

[Sea Skimmer]

So, if I am getting this right, the nuclear bomb's radiation makes the air explode?

It makes the air expand, superheat and exert wind speed equivalent to conventional bomb blast. Near the fireball the air will heat up to the point that the suspended water vapor splits into O2 and H and the resulting products combust, adding slightly to the blast pressure. You can call it air exploding if you want. Unfortunately no one ever seems to have setoff a nuke inside a rainstorm to see what would happen.

[hongi]

Any speculation as to what would happen if you set off a nuke in the eyewall of a hurricane?

[Sea Skimmer]

If you are thinking in terms of disrupting the hurricane, it wont work with any remotely sane size of nuclear weapon. The military studied that to see if its nuke happy world could expand to include weather control in the 1950s and 60s. The energy and mass and shear size involved in a hurricane is just too high. A single hurricane involves more power use then is currently used to power all of modern civilization after all, and vaporizes tens of millions of tons of water per hour.

Even if you set off the largest nuke humanity ever built, it wouldn’t do anything to change the difference in barometric pressure that’s creating the storm. The significant blast radius of even that epic 100 megaton bomb is only 60km or so IIRC, not nearly as big as a hurricane. What's more, you are mainly blowing air up and away... in other words if you did anything you would likely LOWER barometric pressure. That makes storms worse. But more likely air would just rush back into the epicenter, and nothing would have changed. Nor can the nuke disrupt the rotation of the earth which makes it swirl, nor does it change the prevailing winds which move the mass of low pressure along. As another way you might make the storm worse, the storm is heat powered via the ocean water and you just added a whole bunch of heat. Not that much but it might make a difference in a bad way.

As for what the short term effects would be, I do not know. I would imagine you’d kind of vaporize a chunk in all the swirling clouds and that this hole would circle around but be filled back in within a few hours. A huge carpet barrage of 100 megaton nukes might conceivably eliminate a tropical storm early in its formation… but you’d then also have a huge cloud of radioactive fallout blowing onto people instead. As in more fallout then all other nuclear tests put together (Tsar Bomba was estimated to be equal to a 25% increase in world fallout if it had been tested full scale, they tested it as 50 megatons instead) If you truly got a big enough nuke (many gigatons) you could start to think abject physically ejecting the hurricane air mass into orbit or even past escape velocity. How this would help anyone.. I dunno. It'd be cool though in a world shattering kind of way.

I only mentioned the rainstorm, because vaporizing so much extra water suspended in the air might create an interesting intensified shockwave effect on a localized basis. So basically what I’m thinking is the shockwave might be more intense, but not spread as far. But weather control it is not, unless the offending weather is a single raincloud.

[cosmicalstorm]

Thanks for the great explanations Sea Skimmer!
For someone like me who has an amateur interest in these subjects, but lacks the formal education to make sense of many of the equations that often arrive as replies in threads like these, it's awesome.

[takemeout_totheblack]

You make a good point, but still there's no harm in asking people who do know the math behind these things. How else am I supposed to get a definitive answer to my relatively simple question? Go to GoogleAnswers? At least here I get word back from people who aren't talking out of their ass.

[Simon_Jester]

A nuclear device has little to no inherent explosive energy in terms of blast. When a conventional high explosive bomb goes off, the physical mass of the high explosive is being turned into hot gas, which rapidly expands creating a shockwave. The mass of the air around it compresses, and makes the shockwave stronger. That means even if you set a conventional explosive off in an vacuum like space, you’d still have some blast energy. The lack of air would mean the gas can easily expand though, so it wouldn’t be a very far reaching blast. But a blast all the same. You also get some heat, but no ionizing radiation.

I'd think you'd still get a bit of blast in vacuum from a nuke, from the bomb casing being vaporized, but it would be pathetic compared to the more direct effects...

[Sky Captain]

I suppose if you set off a nuke in space close to hull of a spaceship it would suffer a blast like damage because outer layer of hull would suddenly vaporize and that gas would create something like steam explosion.

[Cykeisme]

I suppose if you set off a nuke in space close to hull of a spaceship it would suffer a blast like damage because outer layer of hull would suddenly vaporize and that gas would create something like steam explosion.

In hard sci-fi nukes would need to be set off so close to a hull, that in effect it would be a direct-impact weapon. There would be a negligible effective radius due to the inverse square law (besides perhaps blinding sensors and sensitive equipment that as a rule needs to be exposed outside the hull).

Without an atmosphere (to absorb radiation, get superheated and cause a supersonic blast shockwave), the destructive radius of a nuke is pretty pathetic against properly hardened targets, as you'd expect an interstellar ship to be (as a rule, even a non-military spaceship would need to withstand cosmic radiation).

I would imagine that smaller short-range ships, both with less hull mass and not designed for long-range travel, might have less hardening and would still have their equipment and/or crew destroyed by the radiation pulse at some distance, though.

As an aside, I don't remember exactly how the scenes involving nuclear weapons were portrayed in BSG; did they strike their targets directly?

[Spectre_nz]

If you haven't found it already, there's an impressively in-depth dissection of nuclear weapon effects here: http://www.globalsecurity.org/wmd/libra ... 9/1ch3.htm

Which happens to also mention the speed of the fireball for a 1mt weapon:

a. Because of the tremendous amounts of energy liberated per unit mass in a nuclear detonation, temperatures of several tens of million degrees centigrade develop in the immediate area of the detonation. This is in marked contrast to the few thousand degrees of a conventional explosion. At these very high temperatures the nonfissioned parts of the nuclear weapon are vaporized. The atoms do not release the energy as kinetic energy but release it in the form of large amounts of electromagnetic radiation. In an atmospheric detonation, this electromagnetic radiation, consisting chiefly of soft x-ray, is absorbed within a few meters of the point of detonation by the surrounding atmosphere, heating it to extremely high temperatures and forming a brilliantly hot sphere of air and gaseous weapon residues, the so-called fireball. Immediately upon formation, the fireball begins to grow rapidly and rise like a hot air balloon. Within a millisecond after detonation, the diameter of the fireball from a 1 megaton (Mt) air burst is 150 m. This increases to a maximum of 2200 m within 10 seconds, at which time the fireball is also rising at the rate of 100 m/sec. The initial rapid expansion of the fireball severely compresses the surrounding atmosphere, producing a powerful blast wave,

Also in that document, it gives the breakdown in energy from a nuclear blast in the atmosphere; 50% from blast effects, 35% as thermal radiation, 10% as residual nuclear radiation and 5% as ionizing radiation; in space, only those last two are going to be in effect - the thermal radiation is generated by the fireball. I presume this means you'd get more energy in ionizing radiation, as it's not being absorbed by air, but even then, you'd have to position your nuke right on top of the hull of your target if you wanted to generate a fireball - even in air, the fireball is only generated from material within a few meters of the weapon. Thanks to Inverse Square, pretty soon you've only got enough energy to boil or melt metal, rather than turn it into an expanding ball of plasma. From what I can calculate (using nitrogen as a proxy for air in general) it's really only a few kilograms of matter that generates the fireball in addition to the weapon casing - even though the weapon casing is much more massive, yet, in high altitude detonations, the casing alone produces no real fireball. (There are some impressive clips of high altitude detonations in the docco 'Trinity and Beyond') Although, my impression of this is probably distorted by the fact that there's no obvious way to judge scale in those high altitude blasts.

Project Rho has good information on the subject: http://www.projectrho.com/rocket/rocket3x1.html
There's also this calculator: http://www.5596.org/cgi-bin/nuke.php much better for nuke effects in space. Unless it's giving you 'impulse shock YES' you're ablating armour rather than obliterating compartments with blast effect, and this will depend on your space ship material. Against a titanium hull, a 1mt blast stops generating impuse shock at around 750m; much less fearsome than the same weapon in an atmosphere.

[Cykeisme]

Very nice resources, Spectre, thanks.

But can you (briefly) explain what is meant by the term "impulse shock"?

[Spectre_nz]

I'm just taking the description from the pages I referenced, but my understanding was; if you ablate a material fast enough, instead of just boiling away one layer of atoms after the other, you generate a shockwave in the material that moves ahead of what you're ablating off, much as you would if you'd hit the material with a kinetic impact, and you end up crushing underlying compartments, rather than just vaporizing away the overlying material.

I reasoned that inside of 'impulse shock' range you're delivering enough energy to a target in a vacuum to set generate a fireball at the surface of a target and create a localized blast efect similar to what you'd see in an atmospheric detonation.

[Spectre_nz]

Ahh, here we go: The clips from Trinity and beyond I was looking for: http://www.youtube.com/watch?v=uEILIf8V ... re=related
(the original movie had a better musical score, IMO) I'm not sure what shots those are. There were only 5 high altitude US nuke tests, and the fact that it put those three togeather makes me think they're the shots from operation fishbowl
http://en.wikipedia.org/wiki/Operation_Fishbowl.
http://en.wikipedia.org/wiki/High-altit ... _explosion
At a guess, the first detonation is starfish prime, 1.4Mt @ 400km altitude. That would make the other two 200-800kt, ish at 50 and 100km.

I can't rule out that those are actually shots from operation Hardtack - Trinity and beyond does mix up its shots. In that case, those would be 3.8Mt at 50-75km. Not exactly in a vacuum.
There's another lower quality look at what is aparantly Hardtack-Teak: http://www.youtube.com/watch?v=PBxpHNCDfZQ
(I love the ground shot where it turns night into day)

Edit:
Looking at the second clip I think the first clip is just a better quality view of Hardtack Teak and Orange respectivly.

Post Edit edit:
Ok, looks like the first clip could be fishbowl afterall;
http://www.youtube.com/watch?v=KFXlrn6-ypg

Not Starfish, but posibly Dominic. Having found a clip of Orange, it looks rather different: http://www.youtube.com/watch?v=jmTQVoAm1UY

[Sea Skimmer]

I'd think you'd still get a bit of blast in vacuum from a nuke, from the bomb casing being vaporized, but it would be pathetic compared to the more direct effects...

You’d get a little bit of blast from that sure, but it’d be very hard pressed to do damage even at a very close range, because blast waves try to follow the path of least resistance. In an atmosphere the ambient air pressure plus the ground has a tamping effect which makes the blast wave spread along the surface of the earth. Water has an even better tamping effect which is why under an keel torpedo shot can split ships in half. But in a vacuum, anything solid will have an easy time deflecting the blast. Its not going to be a relevant damage mechanism compared to the massive radiation effects

[loomer]

Here's a question for you Skimmer. What would be the result of a nudet in a nuclear plant? Not talking 'powerplants are bombs!', talking straight up 'take regular nuclear power plant. Place bomb inside. Detonate.' Just extra fallout? A potentiated blast thanks to the fuel? Weakened overall effect on the rest of the area thanks to the sturdy structure dissipating a little more of the energy than most det sites before going up in a big smoke?