Thanks for hosting them Terry !

I've seen video footage of explosions with distortion shock waves like that. If I remember rightly, the distortion is only visible for the briefest moment with very high speed cameras. In most photos of nukes I've seen it's not visible at all. I wanted to depict what would actually be seen without fancy equipment... well, sort of. Actually it was made as a cover image for my 3rd year astrophysics project. So making a nice image was the overriding priority.

"....because the nuke blast is quite bright, one would not expect to see a very conspicuous shock front. Rather, one would expect to see a distorted or "smudged" view of the background near the edge of the shock."

Are you referring to the big white circular shockwave ? I only put that in to look nice. Makes for a more "sterotypical" nuke. Admittedly in photos of atomic explosions I don't see many circular shock waves (only in big fusion bombs I think), but then neither do I see any smudging or distortion effect. Besides, I would expect the shock front to be long gone at this distance by the time the fireball reaches maximum size.

Mind you, I don't know squat about the physics of explosions, so I might be totally wrong. Just going on images I've googled.

In any case by far the biggest error is the size of the fireball, at the time I had no idea how big a 0.1kt bomb's explosion would be, so I simply guessed a plausible size that would look nice. I have since learned it should actually be about the same diameter as the pusher plate.

I've been meaning to get around to making a more accurate version but exams have been in the way. They're over now, so I'm finishing animating the thing. Should be done in a week or two at most, unless my computer dies or rendering times are longer than expected, or I have a bout of laziness (won't animate a ground launch with bombs though, far too much work. I'll stick to chemical boosting unless someone offers me money ! )

.....on second thought, looking at your artwork again, I think that the groung surge is shown at about the right radius, relative to the Orion fireball -- its just that the shosk wave bubble is missing.

Rhysy wrote:

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Okay, that's what I get for not understanding nukes properly. Help me clear up my misunderstandings of what's going on and I'll try and make a more accurate image. The ship has to have travelled a reasonable distance, or there wouldn't be much point in making the image. About 1 second interval between bombs (of that order anyway), so what would the explosion look like at this point ? The archetypal "mushroom cloud" everybody is familar with, (what I was aiming for) or an earlier stage in the explosion ? If the mushroom cloud, I must have made it look un-mushroom cloud like. How can I correct this ? If an earlier stage, wouldn't that still be in the blinding flash that seems to accompany the explosions ?

No, definitely no mushroom cloud. That only starts to take its familiar form on the order of a fraction of a minute to a few minutes after the first pulse unit goes off (depending on explosive yield -- I suspect the Orion pulse units would be at the shorter end of that). But don't ask me how its shape will be affected by the other pulse units going off above it in the mean time -- I don't know of any instance of such closely-timed "stacked" nuke blasts ever having been tried during all the years of atmospheric testing. In any event, by that time the ship would have been lifted something like a mile off the ground, well above the position you're showing in your picture.

So the correct picture is that of an expanding fireball, but no mushroom. Also, the fireball is not entirely spherical, because the pulse units are specifically designed to direct thrust at the ship's pusher plate. I thought that's what you were trying to depict in your illustration, although I really have no idea of what the correct shape of a fireball from such a fancy device would look like at that particular moment. Some hints from the famous photos by Edgerton (particularly nuke explosions in or next to some heavy mass or equipment) suggest that in the earliest stages (before lift-off) the fireball would still appear spherical, with a possible dumbell shape, somewhat like your picture, appearing moments later -- particularly as the fireball sweeps past the pusher plate.

This "ground surge". I'm not at all clear what it actually is. Does it actually occur literally on the ground, or in the air ? Atmospheric detonations seem to show this effect, so I'm guessing it's in the air. What actually is it though ? Condensation of water vapour as the air is shocked ? Dust thrown up from the ground ?

The base surge is unclear because most photos are taken in situations where the effects are not plainly obvious. The effects become more obvious in a simpler situation, like that of an explosion over water. Here is an excellent example : http://www.cddc.vt.edu/host/atomic/nukeffct/enw77b11.jpg

The sharp-edged white oval or "slick" on the ocean surface is the churned-up water resulting from the passage of the spherical shock wave. The wave itself cannot be seen in this photo, due in part to the poor resolution of the photo, and due to the dark ocean background (as I said earlier, and as you can see in the MOAB blast photo, the shockwave has a relatively faint appearance.... shockwaves can be seen a lot clearer using Schlieren photography, but this is not the "natural" view you desire). The same thing happens on the ground, but you get dust instead of water foam (if the blast is very close to or in the ground, there will also be a shockwave travelling in the solid, creating a surface disturbance beyond the crater boundary). Incidentally, the mottled-appearing spherical white cloud in the photo, surounding the fireball in the center, is a water vapour condensation cloud, that forms a concentric spherical shell some distance behind the front of the shockwave -- at that location, the rapidly dropping pressure behind the shock front actually goes below atmospheric, and the humid air condenses, just like contrails behind a high-altitude jet. In your desert scene, you wouldn't have this, due to the dry air. But the shockwave should be visible, due to the day-lit background. It should probably be close to spherical, because its origin is from a very early stage of the blast, before a lot of the symmetry breaking occurs.

A simple spherical distortion effect, or even a whole host of them, is straightforward to do and poses very little difficulty. Interactions with the ground shouldn't be that hard either, as a sphere reflecting should just look like other spheres. But would these be visible during the mushroom cloud stage ?

No, forget the mushroom. As for the ground-reflected part of the shockwave, I suspect that in the view you have set up, it would form a torus maybe 15% to 20% the height of the spherical shockwave above. With the oblique view of the scene, and the dust ring, it would probably be almost imperceptible.

By the way, you don't seem to realise it, but the view you are portraying is one which no-one would have seen back in the days of atmospheric testing -- simply because all test participants were instructed to face away from the blast, until quite a few seconds later, when they could turn around to look, by which time the mushroom cloud was starting to form.

   Better software eh ? I use Blender. It's free. www.blender3d.org

Thanks for the tip, I'll have a look at it.

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Rhysy wrote:

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Thank you very much !

Glad to be of service !

Let me see if I have this right. The fireball is probably basically the right shape, though for completely the wrong reasons, and a bit big. On projectorion.org a formula R=110*Y**0.4 in feet was given as an approximation for the fireball size, so it should be no larger than the radius of the ship. (is it possible that due to the very small yield, 0.1kt, the mushroom would form faster than for ordinary, ~1 kt nukes ?) The ground surge is about the right size, but probably should actually be on the ground and not several metres in the air. The ground itself should be disrupted (and probably the buildings much more devasted by the blast).

Sounds about right up to here.

The dust ring shoud be accompanied by the shockwave itself, visible as a "warping" effect of the air, in a fairly flat (low to the ground) toroidal shape. I assume this torus would be lying flat on the ground ?

I'm not sure whether I understand you correctly here, or whether my "explanation" was not quite as clear as it could have been.

The dominant effect by far, as far as the shock waves, is the spherical "dome" part of it.

But please don't take my word for it -- that's another thing I should have done -- provided a reliable reference for you to read on your own.

In that respect, there is probably no more respected reference than Samuel Glasstone and Philip J. Dolan's "The Effects of Nuclear Weapons," posted at http://www.princeton.edu/~globsec/publications/effects/effects.shtml  .....go to CHAPTER III - Air Blast Phenomena in Air and Surface Bursts http://www.princeton.edu/~globsec/publications/effects/effects3.pdf  .....on page 125, you will find this little diagram : http://www.cns-snc.ca/branches/quebec/blast_wave_phenomena_Fig_3-81.JPG  ....where only the very bottom of the spherical shockwave, the part raking the ground, appears labeled as "incident blast wave." This is what should dominate the scene, arcing over the Orion spaceship like a big glass dome. Also, I think that the right hand side of the diagram, "LATE DEVELOPMENT," is the one that applies to your scenario -- the merging of the reflected blast wave with the main spherical shock front creates the "MACH STEM," which would appear as a kind of cyllindrical glass strip at the base of the sphere (just barely visible, of course).

   "By the way, you don't seem to realise it, but the view you are portraying is one which no-one would have seen back in the days of atmospheric testing -- simply because all test participants were instructed to face away from the blast, until quite a few seconds later, when they could turn around to look, by which time the mushroom cloud was starting to form." Yep - but then, cameras were focused on the explosions. Okay, these would be high-speed cameras, so the shockwave would definately be visible. I tend to take a very ad-hoc attitude to artistic license on stuff like that. Maybe too much.

Yeah, it seems a shame to put so much effort into creating a very realistic-appearing image, only to finish it off with a less-than-realistic depiction of the fireworks themselves. All the more so, when you consider the fact that these massive shock & fireball phenomena are the defining characteristics of the nuclear blasts that Orion uses. You may not be able to get it perfectly right, down to every detail, but I think its worth to at least try to show the main features -- if only for educational & debate purposes.

I'm sure I'm not the only one disapointed by sci-fi movies where $millions are spent on all sorts of fancy-looking gadgetry, only to be topped-off by phoney-looking fireworks.

Good luck ! (hope you get the funding you mentioned earlier)

Nice technical discussions here!

I'll toss in my two cents' worth.

The mushroom cloud results from convective cooling of the hot gasses within the fireball. The physics of it is hellishly complex to model, but the basic concept is as easy to understand as cigarette smoke.

Two basically sum up the effect of convection, you must understand that warm gas is less dense than cool gas. The hotter the gas, the less dense it is, so the more boyant it is. Once the fireball is established, the hottest part of the frieball (the core) will rise faster than the cooler envelope. This is why in typical atomic tests, it doesn't matter what yield if its in kiloton range, or megaton range they all have the same basic shape. The super hot core of the fireball (which is practically at a vacuum compared with air) shoots to the top of the cooler (incandescent) sphere we think of as the fireball. When it strikes the top of the bubble (where cooller, undisturbed air is) it spreads out is pushed aside by the hotter column beneath it. This is the origin of the torroidal convection column typical of the mushroom cloud. It forms a donut of hot, convecting material with a central column that is hottest. Sometimes the convection is so violent that even hotter material is trapped in a convection ring and cannot cool by mixxing--this is the origin of incandescent ring seen in some multi-kiloton shots!

But the point is, convection is the slowest method of cooling (radiation coupling is much faster) so it is the last to be established. The 'hang time' the time it takes to established convective cooling to form a mushroom cloud is approximately proportional to the cube root of the yield. Thus really big explosions can have 'hang times' of many seconds compared to low-kiloton explosions that seem to progress to the mushroom cloud phase pretty quickly. A 1 megaton airburst can generate a visible fireball a mile whide, and will have a hang time of perhaps twenty seconds before it visibly begins to flatten and convect upwards. 1 kiloton airburst will generate a fireball 0.1 miles (about 500 feet wide) and will begin convecting probably even before the fireball is completely expanded (say about 0.2 seconds or so.)

But now, with Orion, you now have the dynamics of multiple explosions in close proximity. So the 'hellishly complex' fluid mechanics is now compounded by the need to think of multiple shockwave trains and multiple flow columns.

However, one of the things in our favor is reflective shocks. Most reflective shocks form when the shockwave from an exploding nuclear device rebounds off the ground. This is the source of the "Mach Reflection"--that part of the spherical shockwave that is reinforced at the ground. One shockwave is the primary shock, and the second is a rebound shockwave from the ground. The locus is a ring that races away on the ground and momentarily doubles the amount of over pressure the primary shock delivers. (It's no wonder why airbusts are so destructive!)

Anyways, as Orion climes, this shockwave will be inverted, because the 'ground' is above each blast (the pusher plate points donward!) So, momentarily, shock driven flow (versus convective flow) will actually point down. This is what gives Orion it's 'kick' while in the atmosphere. I would expect that for a fraction of a second this will be the primary flow pattern. Natural convection takes over later, but by this time Orion has accelerated away from this blast before another one happens 1 second later.

So, while I would guess that eventually convection will cause the plume to disperse in a more or less conventional fashion, I would expect Orion to quickly outpace its out plume within the first three bursts or so. If Orion has an average vertical accleration of 2 g's (pilots would be pulling three g's,) then in the first 3 seconds, Orion would have moved vertically about 1/2*(64.4)*3^2=330 ft (say about 100 meters to the purist.) This seems reasonable. In the next second Orion would have about: 1/2*(64.4)*4^2=480 ft (about 155 meters) of altitude. It will very quickly outpace even the most energetic convective flow, despite intermittent shock flow directed downwards. In this sense, I would expect Orion to perform like a conventional rocket.

Its apperence might thus not be too different from a giant rocket with intermittent drive flares (think an Apollo launch with one second pulses of blindling flashes of light!)

Without doing complex fluid dynamic modelling (which is beyond my ability,) actually visuallzing the flow will have to await a flight test, or the educated guesses of really good artists!

By the way Rhysy, I love your visualizations. Keep up the great work!

Ty

Cool -- what BTW would be the launch altitude of Orion ? ....I mean there must surely be some space (elevated launch pad) between the ground and the pusher plate, for the first shot, no ? ....presumably a bit more than the distance typical of all subsequent shots - a few times the pusher plate diameter ? One would think that for ease of construction and ship rollout, the launch pad would be built on the side of a short cliff....

The way you describe it Ty, it sounds like in Rhysy's picture there would already have been two, possibly three pulse unit blasts.

One might think that with such close coupling, the spherical blast wave I talked about earlier would instead be a short chain of 2 or 3 spherical blast waves of decreasing size, appearing somewhat like stacked, intersecting soap bubbles. BUT the blast waves are quite fast, so with one-second pulse intervals they would in fact be contained within each other -- with slightly offset centers, and possibly not even appearing in Rhysy's picture at the same time (because the preceding one would have expanded out of view, by the time the next one appears....).

That means that the second and third blast waves would be interacting in a big way with the fireball from the previous pulse(s), before overtaking their expansion in the radial direction. In fact, the second and third blast waves would initially be entirely within the fireball of the preceding blasts, right ? (the pulse units must have pretty sturdy construction, to endure the environment for even a split second! ...guess its a bit like being in the eye of a huricane, huh ?). Wonder what the passage of a blast wave does to the appearance of a fireball.... any guesses ?

I found your point about the shock reflection from the pusher plate particularly interesting -- according to the Glasstone book, smaller objects like concrete bunkers seem to simply get passed by the main shock, re-form on their back side, and continue as if the thing wasn't there. I'm not surprised by this, since I remember from my gas dynamics class that shockwaves are quite good at going around corners & such, merging, etc. But I definitely agree that when it hits something of comparable size (i.e. pusher plate diameter not much smaller than the diameter of the shockwave when it hits it), then there would be a major reflection phenomenon. Sure, the shockwave passing the periphery of the pusher plate would continue on, but I think that the disturbance would be so big, that a nice spherical dome would probably *not* re-form above the Orion ship -- except perhaps much further out, by which time it would no longer be visible. Any thoughts on this ?

Also, have you seen any figures on the fraction of Orion propulsive thrust that comes from the impinging shockwave, versus the fireball and any other minor effects ? (pusher plate ablation, etc.)

Thnx!

GoogleNaut wrote:

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I have no figures on an initial height for launch, but building a tower the height of the diamater of the pusher plate might help. I'm thinking of a water tank with a million gallons of water in it. A small pulse unit centered on the bottom of this tank of water with a yield of say 0.1 to 0.5 kilotons ought to vaporize and atomize most of this water and throw a large percentage of it at the pusher plate. This ought to provide an initial shove to fling it clear of the launch tower, while simultaneously preserving most of the launch facility.

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Whoa ! ....using water to transmit the pulse unit's blast is one thing I would strongly recommend against. Water transmits shockwaves much more efficiently - with a stronger punch - than air. I forget the exact figures, but I think that nukes detonated under water can wreck submarines something like ten times farther out than ships are wrecked by an air burst. I think the Glasstone reference gives some good info on this in one of the chapters, but my copy of the book is at the office, and I don't like reading the stuff from a scanned pdf

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It might be better to use a large array of solid rocket boosters to just give Orion some initial altitude. Even a thousand feet would be a big help. This would decouple Orion's pulse units from the ground, so atleast you don't have reflected shocks from the ground to contend with!

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This sounds like a good option. I was thinking more of the kind of test platform shown in one of the illustrations in George Dyson's book -- which was apparently planned for actual testing of serial pulse unit detonations, in combination with prospective pusher plate designs, fixed to the structure.....

Fascinating, if very complicated, stuff.

GoogleNaut wrote :

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"A 1 megaton airburst can generate a visible fireball a mile whide, and will have a hang time of perhaps twenty seconds before it visibly begins to flatten and convect upwards. 1 kiloton airburst will generate a fireball 0.1 miles (about 500 feet wide) and will begin convecting probably even before the fireball is completely expanded (say about 0.2 seconds or so.)"

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This brings me back to the mushroom cloud. Would a 0.1 kt explosion be at the mushroom stage after 1 second, or still a searing-hot fireball ?

 

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"Its apperence might thus not be too different from a giant rocket with intermittent drive flares (think an Apollo launch with one second pulses of blindling flashes of light!)"

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If I understand you correctly, are you saying the spaceship ascends faster than the fireballs rising due to convection, so that each is essentially separate ? This sounds plausible. You'd need to get a nice distance away from each explosion, to avoid the next bomb burning in the fireball of the previous explosion. I would still expect the blast wave from each bomb to blow apart the cloud left by the previous one.

Whatever it would look like, it would certainly be a sight worth seeing.

 

It sounds like opinions on shockwaves are changing. What am I likely to see in terms of shockwaves, 1 second after an explosion ?

For a purely atomic ground launch (no boosters at all), perhaps a large launch tower could be constructed suspending the vehicle ~100m above the ground. This would make for a good excuse to put lots of burning debris flying radially outwards from the explosion.

I concur with Ty's description.

I would only check up on that blast wave velocity -- if I'm not mistaken (please check Glasstone or other references), its quite a bit faster than the speed of sound. As I understand it, that's actually the difference between a sound wave and a blast wave: strength and speed.

After a bit more searching, I found the following videos & pics that may help get a better idea of what the things look like..... Unfortunately, in most cases, the early phases of the blast wave are out-shined by the bright fireball. But the latter phases clearly show the circular slick on the water or the ground, as the wave expands radially..... 

Old video of a cannon shooting off an atomic bomb and showing the effects of the blast. File Size: 3942 KB  http://www.lookatentertainment.com/v/v-202.htm

Numerous video clips [i.e. hodgepodge] of nuclear tests done over the past 50 years. File Size: 8591 KB  http://www.lookatentertainment.com/v/v-517.htm

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http://www.afn.org/~scotsman/photos/uss.iowa.jpg

http://www.ifs.tohoku.ac.jp/IFC/images/img_e02/e02_c3b.jpg

Oh yes...no-one has yet complained that image still has cars in the car park !

can you clip some of the best shockwave frames from the DVD video ? .....I would be interested in seeing those (not too interested in the rest right now though). Maybe you could post 2 or 3 on the NuclearSpace web site, for reference ? (particularly for guys like Rhysy)

PS. a "gun-type device" need not involve gunpowder -- it can function in a reverse manner, whereby when the "gun" impacts the ground and stops, while the "projectile" flies into the "breech" to set off the bomb.... this is how earth-penetrating "bunker-buster" A-bombs work - simple & reliable - a real "wooden bomb".

BTW, the nuclear cannon shells were not gun-type -- they were linear implosion type, using a hollow HEU cyllinder (possibly with a tritium booster).

For the uploading part, its whoever has the server access that has to do it.

As for capturing video frames, one crude method that works quite well is to stop the video at the desired frame and then hit PrintScreen -- which you can then paste into any of a number of graphics screens -- including MS Paint.

Looking forward to the pictures -- Thanks !

Some Shuttle-related shock photography, for a change....  (note also the perfectly clear exhaust jet - prior to the shock diamond - in the last photo -- the same would happen with a nuke rocket engine, except in the vacuum of space there would be no shock diamonds....)

http://www.aviationnow.com/media/images/awst_images/large/AW_07_11_2005_205_L.jpg        Pressure change at Mach 1 around accelerating shuttle condenses water vapor into "shock collar" around fuselage and other streamers off the tail, main engines and right booster. This image was taken by a data camera during earlier liftoff.Credit: NASA PHOTOS

http://www.aviationnow.com/media/images/awst_images/large/AW_07_11_2005_239_L.jpg        Three Space Shuttle Main Engines generate shock diamonds in 6,000F exhaust flow exiting nozzles at 10,000 mph. to propel an earlier mission. One of two solids is also visible.Credit: BOEING/ROCKETDYNE

http://www.aviationnow.com/media/images/awst_images/large/AW_07_11_2005_209_L.jpg        SSME is fired in test stand at the Stennis Space Center, where static firings are being doubled to ensure post-Columbia quality control.Credit: NASA PHOTOS