Explosives and Atmospheres

[Timerover51]

I was looking at the weapon effects in the Cepheus Engine and then started to think about how conventional explosives cause damage, and how that can be modified by the atmosphere that they are in.

An explosive device causes damage in two ways. Either by blast effect, i.e. shock or pressure wave, or fragmentation, or both. Generally, one or the other is emphasized, although in aerial bombs, the aim is to get a reasonable medium for effects.

Now, when an explosion occurs in a Vacuum, say a demolition charge is set off against an object, something interesting happens. That portion of the explosive charge next to the object generates a shock wave in the material of the object. However, there is no atmosphere to support a shock ware elsewhere around the charge. All you have is a rapidly expanding ball of initially hot gases. Anyone standing a few feet away will not be effected by either the shock wave or the gaseous ball, and unless there is some form of fragmentation cover over the explosive, no fragment damage either. A Trace atmosphere would have a very similar effect, but you would get a minor shock wave that will dissipate very quickly, without a whole lot of effect.

Now, if you are dealing with say a fragmentation grenade or an artillery shell, then things get a bit more interesting. In a Vacuum, there is no atmosphere to slow the fragments down, so they retain their initial velocity and ability to cause damage. However, the effective radius of burst for casualties is not going to increase that much, as that radius is based on the probability of being hit by an effective fragment, and that will not change a lot. The problem is that those fragments are going to travel a lot farther then they would in a standard atmosphere, so the danger areas, that radius within which the possibility of being hit by a fragment is going to increase a lot. As artillery shells typically hit at an angle to the ground, some of those fragments are going to be given trajectories of 30 degrees of more, which means that they may travel several hundred meters to several kilometers, depending on the gravity of the planet. A combination of a vacuum and a low-gravity planet might make it a bit hazardous to use a lot of either hand grenades or artillery fire because of unwanted collateral damage.

Note, this would also hold true for bullets being fired. No atmosphere means no bullet is slowed down as it heads down range. If firing with any kind of a upward inclination, such as you would have if your weapon was sighted for a standard atmosphere, you are going to be firing high at targets a couple of hundred meters away, and your rounds are going to travel a LONG way. Your artillery and mortar ranges are going to increase as well, simply because of no atmospheric resistance. Oh, and fin-stabilized projectiles are not going to work too well either. With no atmosphere, the fins have noting to work on in stabilizing the round, so no Armor-Piercing Fin-Stabilized Discarding Sabot rounds. They may travel a long way, but they might be doing cartwheels while so doing, along with heading in directions that you might now want them to go. The discarded sabots are also going to be traveling a ways. Again, nothing to slow them down. If you are on a small, low-gravity planet, your tank guns might be firing a couple of thousand kilometers if the muzzle velocity is say 1500 meters per second.

Hmm, maybe the Zero-G combat skill implies a lot more than you think when combined with a Vacuum.

 

[Condottiere]

I would think spin would take care of bullet stabilization.

The trajectory of a slug fired from a smoothbore unrifled shotgun should be interesting. Or a musket.

 

[Epee]

Sabots may not discard either; as they generally depend on air resistance to strip them away from the projectile.

Thicker than standard atmospheres, and a higher than standard gravity field to a lesser extent, will retard projectile range and energy while enhancing shockwave effects.

 

[Timerover51]

Sabots may not discard either; as they generally depend on air resistance to strip them away from the projectile.

Thicker than standard atmospheres, and a higher than standard gravity field to a lesser extent, will retard projectile range and energy while enhancing shockwave effects.

I was thinking more of the sabots from a rifled gun, which would be tossed off that the action of the rifling. You are correct about smoothbore sabots though. They would just sail off somewhere with the round.

 

[Straybow]

I was thinking more of the sabots from a rifled gun, which would be tossed off that the action of the rifling. You are correct about smoothbore sabots though. They would just sail off somewhere with the round.

Which wouldn't matter, because there is no atmospheric drag to be increased by the sabot.

 

[BlackBat242]

And without gasses to drag on the fin-stabilized projectile, what would cause it to tumble?

And how would that tumbling affect its trajectory - by gyroscopic precession?

 

[Straybow]

Any differential in forces of friction and pressure as it emerges from the barrel would cause eccentric motion on a projectile, resulting in slow tumbling. Without atmospheric drag it wouldn't accelerate the tumble, but in a short distance it would no longer be aligned to deliver its KE effectively. If it is too long and narrow for spin stabilization then it would need some means of active stabilization. Tiny puffs of compressed gas, or chemical charges, would do. Quite within reach of present tech

 

[aramis]

Which wouldn't matter, because there is no atmospheric drag to be increased by the sabot.

Discarding sabot has two purposes -
1. to reduce air drag by firing a smaller diameter projectile than the barrel
2. to increase energy density on impact by a smaller diameter projectile

Lacking #1 doesn't reduce the effect of #2... the difference being that the sabot itself is likely to impact. GIven that E=MV²...
the sabot parts will be about 2% to 10% of total mass, and carry that same portion of the muzzle energy; on impact, they'll have much larger impact areas, especially if (as the one DS round I've ever seen in person does) the sabot is sprung so that it auto-opens once no longer barrel/case confined. (12ga shotgun APDS. I suspect it was smuggled off base.)

The sabot will, in a rifle, impart the spin on the penetrator. As long as the mass is symmetric about the spin axis, not going to tumble until impact.

 

[mike wightman]

The formula for kinetic energy is:

E=0.5mv^2

 

[nobby-w]

[ . . . ]
Now, if you are dealing with say a fragmentation grenade or an artillery shell, then things get a bit more interesting. In a Vacuum, there is no atmosphere to slow the fragments down, so they retain their initial velocity and ability to cause damage.

For reference, the figures I've seen for initial shell fragment velocity are 1.5km/sec for a Bofors 57mm pre-fragmented shell and 2km/sec for fragments from a 155mm shell. The escape velocity of the moon is about 2.4km/sec, so the fragments could travel a very long way on a small vacuum world. The high velocity (much faster than a rifle bullet) means shell fragments have a surprisingly large amount of kinetic energy.¹ Against a vacc suit this would probably still puncture the suit and inflict a serious wound on the occupant. Having said that, this is still an order of magnitude slower than a typical meteoroid. On a micrometeorite shielded building or vehicle the shielding would probably be effective against shell fragments, although they are not travelling fast enough for a Whipple shield to have any effect.
______________
1 - A tungsten sphere the size of a BB (approx. 0.25"/6.35mm across) weighs about 2.5g. At 2km/sec, this has around 5kj of kinetic energy.

 

[Straybow]

Discarding sabot has two purposes -
1. to reduce air drag by firing a smaller diameter projectile than the barrel
2. to increase energy density on impact by a smaller diameter projectile

The sabot does not change the energy density of the inner projectile. If, for some reason, the sabot didn't come off after leaving the gun it would pop off on impact.

 

[aramis]

The sabot does not change the energy density of the inner projectile. If, for some reason, the sabot didn't come off after leaving the gun it would pop off on impact.

It increases the cross-sectional area energy density, which is what generates penetration.

Remember, in a uniformly accelerated object (such as the sabot and penetrator, the energy distribution matches the mass fraction. That is, if the penetrator is 100g radius 1mm, and the sabot 10g and radius 3.5mm, and 220 J, the penetrator arrives with 200 J on 3.1416 mm² (63 J/mm²), and the sabot with 20 J on 38.48mm², or 0.51 J/mm².

Now, the energy denisty of a 7.62N is 10g and 3520 J on 45.6 mm², or 77.18 J/mm²

Note that we get the similar unit energy per mm² for 10% the energy... this means less damage, but similar penetration.

Now, if we take that same energy 3520 and put a 5g 1mm radius penetrator (probably DU) on a 7.62 round, with 2 g sabot... 5/7 of that energy is penetrator, 2514 J on 5g and 3.14159mm², for 800.7 J/mm² ;and 1005 J on 2g sabot on at least 45.6 mm², or no more than 22.05 J/mm²
We basically just penetrated about 3x as much armor for the same energy.

In vacuum, in both cases, all energy reaches target.
In atmosphere, we also get less air resistance, so more of that energy actually reaches target..

 

[Enoki]

I was looking at the weapon effects in the Cepheus Engine and then started to think about how conventional explosives cause damage, and how that can be modified by the atmosphere that they are in.

An explosive device causes damage in two ways. Either by blast effect, i.e. shock or pressure wave, or fragmentation, or both. Generally, one or the other is emphasized, although in aerial bombs, the aim is to get a reasonable medium for effects.

Now, when an explosion occurs in a Vacuum, say a demolition charge is set off against an object, something interesting happens. That portion of the explosive charge next to the object generates a shock wave in the material of the object. However, there is no atmosphere to support a shock ware elsewhere around the charge. All you have is a rapidly expanding ball of initially hot gases. Anyone standing a few feet away will not be effected by either the shock wave or the gaseous ball, and unless there is some form of fragmentation cover over the explosive, no fragment damage either. A Trace atmosphere would have a very similar effect, but you would get a minor shock wave that will dissipate very quickly, without a whole lot of effect.

Now, if you are dealing with say a fragmentation grenade or an artillery shell, then things get a bit more interesting. In a Vacuum, there is no atmosphere to slow the fragments down, so they retain their initial velocity and ability to cause damage. However, the effective radius of burst for casualties is not going to increase that much, as that radius is based on the probability of being hit by an effective fragment, and that will not change a lot. The problem is that those fragments are going to travel a lot farther then they would in a standard atmosphere, so the danger areas, that radius within which the possibility of being hit by a fragment is going to increase a lot. As artillery shells typically hit at an angle to the ground, some of those fragments are going to be given trajectories of 30 degrees of more, which means that they may travel several hundred meters to several kilometers, depending on the gravity of the planet. A combination of a vacuum and a low-gravity planet might make it a bit hazardous to use a lot of either hand grenades or artillery fire because of unwanted collateral damage.

Note, this would also hold true for bullets being fired. No atmosphere means no bullet is slowed down as it heads down range. If firing with any kind of a upward inclination, such as you would have if your weapon was sighted for a standard atmosphere, you are going to be firing high at targets a couple of hundred meters away, and your rounds are going to travel a LONG way. Your artillery and mortar ranges are going to increase as well, simply because of no atmospheric resistance. Oh, and fin-stabilized projectiles are not going to work too well either. With no atmosphere, the fins have noting to work on in stabilizing the round, so no Armor-Piercing Fin-Stabilized Discarding Sabot rounds. They may travel a long way, but they might be doing cartwheels while so doing, along with heading in directions that you might now want them to go. The discarded sabots are also going to be traveling a ways. Again, nothing to slow them down. If you are on a small, low-gravity planet, your tank guns might be firing a couple of thousand kilometers if the muzzle velocity is say 1500 meters per second.

Hmm, maybe the Zero-G combat skill implies a lot more than you think when combined with a Vacuum.

Blast is amplified in two ways: By air pressure and by reflection off of solid structures it can't penetrate. For example, if you were using explosives at the bottom of a deep crater on a vacuum world, the blast would be contained within the crate except for upwards. This would amplify the effect. Blast is most dangerous when it is contained within a limited space.
Outside, it will always fall off as a cube function. So, unless you are close to the explosion, blast is usually not too relevant.

Fragmentation on the other hand, is very lethal. On a vacuum world it would be devastating. When a shell, grenade, whatever explodes, it forms fragments in a reasonably predictable way.
A number of large fragments will form as the shell breaks up, generally following the shape of it and peeling open much like a banana peel would. These large fragments are very deadly but few in number, between say 10 and 50 depending on the size of the round. They will weigh several ounces to several pounds depending on the shell size. These are train wrecks if they hit you or even a vehicle. They can be effective to tens, even hundreds of yards.
The next group is fragments that form in the space between these large fragments. These will weigh about 1 ounce, or maybe several ounces but are large enough to really tear someone up. Think of these as bullets. There are typically about 100 to 500 of these. They are very dangerous close to the detonation out to a range of say 5 to 10 yards, maybe a bit more if the shell is large.
Then there are the tiny fragments and micro fragments that weigh well less than an ounce. These are little shards and flakes that form in those same gaps. There are anywhere between several hundred and several thousand of these depending on the shell size.
This last group is pretty much worthless in an atmosphere.

But, in a vacuum, you now have literally thousands, not tens or hundreds, of dangerous shell fragments. The small ones are dangerous because they have high velocities and little surface area. Think of them as tiny supersonic razor blades. Death of a thousand paper cuts.
If you're in a soft vac suit, it's going to get shredded. You need 100% coverage in some sort of armor protection to avoid dying. One puncture and...

So, it's still the fragments that are the killer and a vacuum only increases their effectiveness, and by exponential numbers.

 

[Straybow]

It increases the cross-sectional area energy density, which is what generates penetration.

Remember, in a uniformly accelerated object (such as the sabot and penetrator, the energy distribution matches the mass fraction. That is, if the penetrator is 100g radius 1mm, and the sabot 10g and radius 3.5mm, and 220 J, the penetrator arrives with 200 J on 3.1416 mm² (63 J/mm²), and the sabot with 20 J on 38.48mm², or 0.51 J/mm².

Now, the energy denisty of a 7.62N is 10g and 3520 J on 45.6 mm², or 77.18 J/mm²

Note that we get the similar unit energy per mm² for 10% the energy... this means less damage, but similar penetration.

Now, if we take that same energy 3520 and put a 5g 1mm radius penetrator (probably DU) on a 7.62 round, with 2 g sabot... 5/7 of that energy is penetrator, 2514 J on 5g and 3.14159mm², for 800.7 J/mm² ;and 1005 J on 2g sabot on at least 45.6 mm², or no more than 22.05 J/mm²
We basically just penetrated about 3x as much armor for the same energy.

In vacuum, in both cases, all energy reaches target.
In atmosphere, we also get less air resistance, so more of that energy actually reaches target..

I believe that is an accurate analysis, and it shows that the penetrator has exactly the same energy whether the sabot disengages or not. On impact, the penetrator will act as though the sabot were not present (which was my point). Even if mistakenly cemented to the penetrator by manufacturing error, the sabot is of such light and flimsy material that it will simply be stripped off at negligible energy cost.

In fact, bullets have been made as more or less traditional hollow points with a long penetrator embedded (the tail end hanging out the back of the lead slug). The lead slug deforms on impact and the penetrator is released to punch through walls or even armor. I believe they've been shown to penetrate ESAPI plates that stop standard AP rounds of the same calibre.

 

[Timerover51]

Blast is amplified in two ways: By air pressure and by reflection off of solid structures it can't penetrate. For example, if you were using explosives at the bottom of a deep crater on a vacuum world, the blast would be contained within the crate except for upwards. This would amplify the effect. Blast is most dangerous when it is contained within a limited space.
Outside, it will always fall off as a cube function. So, unless you are close to the explosion, blast is usually not too relevant.

Fragmentation on the other hand, is very lethal. On a vacuum world it would be devastating. When a shell, grenade, whatever explodes, it forms fragments in a reasonably predictable way.
A number of large fragments will form as the shell breaks up, generally following the shape of it and peeling open much like a banana peel would. These large fragments are very deadly but few in number, between say 10 and 50 depending on the size of the round. They will weigh several ounces to several pounds depending on the shell size. These are train wrecks if they hit you or even a vehicle. They can be effective to tens, even hundreds of yards.
The next group is fragments that form in the space between these large fragments. These will weigh about 1 ounce, or maybe several ounces but are large enough to really tear someone up. Think of these as bullets. There are typically about 100 to 500 of these. They are very dangerous close to the detonation out to a range of say 5 to 10 yards, maybe a bit more if the shell is large.
Then there are the tiny fragments and micro fragments that weigh well less than an ounce. These are little shards and flakes that form in those same gaps. There are anywhere between several hundred and several thousand of these depending on the shell size.
This last group is pretty much worthless in an atmosphere.

But, in a vacuum, you now have literally thousands, not tens or hundreds, of dangerous shell fragments. The small ones are dangerous because they have high velocities and little surface area. Think of them as tiny supersonic razor blades. Death of a thousand paper cuts.
If you're in a soft vac suit, it's going to get shredded. You need 100% coverage in some sort of armor protection to avoid dying. One puncture and...

So, it's still the fragments that are the killer and a vacuum only increases their effectiveness, and by exponential numbers.

I have a complete list of the average number of effective fragments for all U.S. Army artillery shells, Fragmentation and High Explosive Bombs, and 4.5 inch rockets from WW2. The fragmentation data for the artillery shells is still valid, as is the data for the fragmentation bombs. I did not see a purpose to posting that as it is about a hundred pages. Then there is the actual “butterfly” fragmentation pattern produced by a rotating shell. Mortar patterns are a bit different. The fragmentation pattern is by no means circular in shape.

At Gary Con last year, I gave Marc some of my ballistics data with respect to shell fragment patterns.

 

[Enoki]

In addition, the quality of the shell / container material and the velocity of the explosive used is important too.
As the toughness (a generalized term here) of the containment of the explosive goes up, the velocity of fragments will increase as the amount of energy necessary to break up the container will increase imparting a higher velocity on the resulting fragments.

The same is true for the explosive. A higher velocity explosive will impart more velocity on the fragments (but at a lower rate than having a higher quality case) and also increase the danger of the resulting blast.

So, going from say black powder (about 1500 fps) to RDX (about 7500 fps) you get a massive increase in the effectiveness of each round simply by changing the explosive. Making the shell casing from better steel, material, etc., increases this even more.

Interestingly from a historical note, this made the US 105mm shell in WW 2 about 50% more effective than a German 105mm shell due to the US using better explosives and much better quality steel in their shell.

So, in Traveller terms, I could see say a snub pistol HE round being really vicious when made from good, advanced materials and a very high velocity explosive of advanced design. Add in that the manufacturer designs the round to break up in a very specific way rather than randomly and that small round might be as effective as a much larger one of lower tech level.

 

[Diveguy]

Blast is amplified in two ways: By air pressure and by reflection off of solid structures it can't penetrate. For example, if you were using explosives at the bottom of a deep crater on a vacuum world, the blast would be contained within the crate except for upwards. This would amplify the effect. Blast is most dangerous when it is contained within a limited space.
Outside, it will always fall off as a cube function. So, unless you are close to the explosion, blast is usually not too relevant.

Fragmentation on the other hand, is very lethal. On a vacuum world it would be devastating. When a shell, grenade, whatever explodes, it forms fragments in a reasonably predictable way.
A number of large fragments will form as the shell breaks up, generally following the shape of it and peeling open much like a banana peel would. These large fragments are very deadly but few in number, between say 10 and 50 depending on the size of the round. They will weigh several ounces to several pounds depending on the shell size. These are train wrecks if they hit you or even a vehicle. They can be effective to tens, even hundreds of yards.
The next group is fragments that form in the space between these large fragments. These will weigh about 1 ounce, or maybe several ounces but are large enough to really tear someone up. Think of these as bullets. There are typically about 100 to 500 of these. They are very dangerous close to the detonation out to a range of say 5 to 10 yards, maybe a bit more if the shell is large.
Then there are the tiny fragments and micro fragments that weigh well less than an ounce. These are little shards and flakes that form in those same gaps. There are anywhere between several hundred and several thousand of these depending on the shell size.
This last group is pretty much worthless in an atmosphere.

But, in a vacuum, you now have literally thousands, not tens or hundreds, of dangerous shell fragments. The small ones are dangerous because they have high velocities and little surface area. Think of them as tiny supersonic razor blades. Death of a thousand paper cuts.
If you're in a soft vac suit, it's going to get shredded. You need 100% coverage in some sort of armor protection to avoid dying. One puncture and...

So, it's still the fragments that are the killer and a vacuum only increases their effectiveness, and by exponential numbers.

Having spent quite a bit of my life working with explosives and bomb disposal professionally, I wish to slightly qualify this - particularly based on the findings of the past 18 years conflicts:

Blast pressure is the hidden killer in explosions. You noted this in terms of the confinement issues, but the danger of even "minimal" reflective surfaces to bystanders is only now starting to be understood. Furthermore, we have now found that many of the "tried and true" formulas we believed in regarding blast pressure exposure and safe distances were effectively useless once you got into an urban area, much less indoors. Also, only in the past couple of decades have we really paid attention to the different injury patterns caused by explosions and fragmentation. Quite simply - at close distances it's pretty much a wash as to whether the blast overpressure or the fragmentation caused the lethal effects.

Given that we're dealing with a game involving a certain amount of hand-wavium, we can choose to leave out the growing research into long term traumatic brain injuries from even moderate blast overpressure exposure.

So, with respect. Yes, in a vacuum/low pressure atmosphere, your blast overpressure will fall off much more rapidly while fragmentation potentially travels much further. BUT, as many games are taking place in something close to "standard" atmosphere, or on board pressurized starships, if you're looking at the realism factor then you simply cannot ignore blast overpressure.

 

[Timerover51]

In addition, the quality of the shell / container material and the velocity of the explosive used is important too.
As the toughness (a generalized term here) of the containment of the explosive goes up, the velocity of fragments will increase as the amount of energy necessary to break up the container will increase imparting a higher velocity on the resulting fragments.

The same is true for the explosive. A higher velocity explosive will impart more velocity on the fragments (but at a lower rate than having a higher quality case) and also increase the danger of the resulting blast.

So, going from say black powder (about 1500 fps) to RDX (about 7500 fps) you get a massive increase in the effectiveness of each round simply by changing the explosive. Making the shell casing from better steel, material, etc., increases this even more.

Interestingly from a historical note, this made the US 105mm shell in WW 2 about 50% more effective than a German 105mm shell due to the US using better explosives and much better quality steel in their shell.

So, in Traveller terms, I could see say a snub pistol HE round being really vicious when made from good, advanced materials and a very high velocity explosive of advanced design. Add in that the manufacturer designs the round to break up in a very specific way rather than randomly and that small round might be as effective as a much larger one of lower tech level.

I hate to burst your bubble, but my data on actual WW2 tests is just the opposite when it comes to a thicker or stronger case. More energy goes into shattering the case, with no corresponding increase in either fragments or fragment velocity. Now the same case with a higher velocity explosive such as Composition B does somewhat increase the fragment velocity.

As for the snub pistol HE round, I view that as pure "handwavium". Then again, to each his own universe.

 

[mike wightman]

I don't see an issue with the snub pistol explosive rounds - with current technology we have the Neopup PAW-20 which is a 20mm grenade launcher.

The 10mm snub pistol firing explosive rounds is also best thought of as a mini-grenade launcher.

As to thick case/thin case - it's basic physics. You have to break bonds to make the case fly apart. The energy 'used' doing that is not available as kinetic energy for the fragments. If you want larger fragments then deliberately score points of weakness - hence the construction of the pineapple grenades.

 

[kilemall]

I don't see an issue with the snub pistol explosive rounds - with current technology we have the Neopup PAW-20 which is a 20mm grenade launcher.

The 10mm snub pistol firing explosive rounds is also best thought of as a mini-grenade launcher rather.

That's how I use the snub rounds often- as GL-RAM lite, suitable for attaching to say an SMG for extra options, or a semi-auto 2-3 round version for rifles.