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timerover51 May 17th, 2019 04:44 AM

Explosives and Atmospheres
 
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 May 17th, 2019 07:27 AM

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 May 17th, 2019 08:04 AM

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 May 17th, 2019 01:27 PM

Quote:

Originally Posted by Epee (Post 601867)
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 May 18th, 2019 12:49 AM

Quote:

Originally Posted by timerover51 (Post 601871)
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. :cool:

BlackBat242 May 19th, 2019 04:09 AM

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 May 19th, 2019 04:28 PM

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 May 19th, 2019 06:03 PM

Quote:

Originally Posted by Straybow (Post 601880)
Which wouldn't matter, because there is no atmospheric drag to be increased by the sabot. :cool:

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 May 19th, 2019 06:11 PM

The formula for kinetic energy is:

E=0.5mv^2

nobby-w May 20th, 2019 06:45 AM

Quote:

Originally Posted by timerover51 (Post 601862)
[ . . . ]
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.


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