8 ms·

I believe most explosives detonate in the air anymore. Is there a speed threshold where the kinetic energy is enough to justify not doing that?

kurupt213 4y ago
There are tank fired kinetic anti tank rounds. Depleted uranium darts are devastating. But to answer your question, I believe the energy released by a kinetic impactor quadruples every time you increment a Mach number. So a kinetic impact at Mach 4 is 16x more energetic than a mach 2 impact.
djmips 4y ago
I think there might be a grammar problem in your statement. What did you mean?
- oneoff786 4y ago
  https://ygdp.yale.edu/phenomena/positive-anymore https://ygdp.yale.edu/phenomena/positive-anymore I picked it up from Pennsylvania unintentionally. Not a huge fan of it tbh.
- Buttons840 4y ago
  He means how fast does a missile need to go so that the kinetic energy is greater than the chemical/explosive energy.
- jstanley 4y ago
  It's relatively common to use "anymore" to mean "nowadays". I think it must have come from hearing things like "They don't do that any more" and not recognising "any" and "more" as separate words, but rather as a concept that just means "now", such that "They do that anymore" is equally valid.
  - djmips 4y ago
    Wow really? I missed out on that one. Thanks for the update.
zmgsabst 4y ago
Not really. Even for hypersonics, the efficiency of chemicals at storing energy and then releasing it is hard to beat. 1 kg of TNT produces 4.2MJ; to get the same energy in 1kg of projectile you’d need to travel at ~6500 miles per hour. (About mach 9 at sea level.) The role of hypersonics is in application: - sudden attacks, to prevent response - low altitude, high velocity attacks, to defeat intercept - penetration, coupled with a chemical warhead, to hit hardened targets Source: https://en.wikipedia.org/wiki/Orders_of_magnitude_(energy) https://en.wikipedia.org/wiki/Orders_of_magnitude_(energy)
- theptip 4y ago
  Appreciate the detail. I'm not sure I see the conclusion as obvious though; if a 2kg weapon is going Mach 20 (as the weapon in the OP apparently does), with 1kg of that being TNT, then the explosives only provided 20% of the energy (rounding to the nearest 10% to make the example clearer). If you just lobbed the same 1kg weapon at Mach 20 then you'd have halved the weight, and reduced the energy to 40%. Presumably lower weight means easier to maneuver (more agile for intercepting or avoiding countermeasures, depending on what you're trying to hit). It seems like around Mach 10-20 is where the tradeoff could become non-obvious, depending on how much you value weighing less?
  - zmgsabst 4y ago
    Per OP: > The U.S. Air Force said on Monday it had conducted a successful test of a hypersonic weapon, which flew at five times the speed of sound. But to your question: > around Mach 10-20 is where the tradeoff could become non-obvious, depending on how much you value weighing less? The same logic holds at mach 5-8 (ie current hypersonics), which is why they’re being developed: You want a weapon that can surprise, evade, or penetrate better — even if that’s less destructive overall.
  - adrian_b 4y ago
    When close to the ground, something flying at Mach 10-20 would either be deccelerated very quickly to much lower speeds, if flying passively, or require a very large mass of fuel to maintain the speed. Moreover, it would be hard to avoid being partially vaporized before reaching the target. So hitting something at Mach 10-20 seems much more difficult than filling the missile with explosives and detonating it. Weapons based on kinetic energy alone seem practical only against satellites or against targets that fly in the upper atmosphere.
michael1999 4y ago
Many anti-air missiles detonate at range and destroy with shrapnel. And shrapnel damage is just kinetic. But that's probably not what you mean. Yes. For lightly armoured targets, their own velocity might do it. See https://en.wikipedia.org/wiki/Brilliant_Pebbles#Brilliant_Pebbles https://en.wikipedia.org/wiki/Brilliant_Pebbles#Brilliant_Pe... And if you want to go further, de-orbiting tungsten rods make the explosives redundant. https://en.wikipedia.org/wiki/Kinetic_bombardment https://en.wikipedia.org/wiki/Kinetic_bombardment
- robonerd 4y ago
  > de-orbiting tungsten rods Over-hyped I think. Sure the kinetic energy is incredible, but what of the terminal ballistics? I think these would have substantial over-penetration against anything except deep bunkers; most of that kinetic energy would be spent driving that rod through bedrock, like an APFSDS dart shot into tank armor. Good for punching holes in things.
  - michael1999 4y ago
    Completely agree. Those systems solve a bunch of non-problems and completely destabilize MAD. There's a reason they were never developed. But I wouldn't discount to the ability of weapons designers to figure out the frangibility and deliver the bulk of the energy to a tuneable depth.
aidenn0 4y ago
About mach 8, but then you need to actually hit your target. For ground based targets that's probably fine (in fact for slow-moving, armored targets kinetic-energy penetrators are already a thing). For anti-aircraft fire the goal is to get "close enough" that the explosives will damage it.
hammock 4y ago
Look up "rods from God." https://en.wikipedia.org/wiki/Kinetic_bombardment https://en.wikipedia.org/wiki/Kinetic_bombardment https://www.businessinsider.com/air-force-rods-from-god-kinetic-weapon-hit-with-nuclear-weapon-force-2017-9#:~:text=The%20%22rods%20from%20god%22%20idea,bunkers%20or%20secret%20underground%20sites https://www.businessinsider.com/air-force-rods-from-god-kine....
- mywittyname 4y ago
  It always baffled me that these weren't named after Zeus.
  - xatlasm 4y ago
    Drop a deuce from Zeus?
- nradov 4y ago
  "Rods from God" is a silly impractical sci-fi concept. It would be too expensive to launch a large enough satellite constellation to provide persistent global coverage. And the satellites themselves wouldn't be survivable. The prompt global strike mission can be accomplished more cheaply and effectively by missiles launched from aircraft or ships.

kurupt213 4y ago

There are tank fired kinetic anti tank rounds. Depleted uranium darts are devastating. But to answer your question, I believe the energy released by a kinetic impactor quadruples every time you increment a Mach number. So a kinetic impact at Mach 4 is 16x more energetic than a mach 2 impact.

djmips 4y ago

I think there might be a grammar problem in your statement. What did you mean?

oneoff786 4y ago
https://ygdp.yale.edu/phenomena/positive-anymore https://ygdp.yale.edu/phenomena/positive-anymore I picked it up from Pennsylvania unintentionally. Not a huge fan of it tbh.
Buttons840 4y ago
He means how fast does a missile need to go so that the kinetic energy is greater than the chemical/explosive energy.
jstanley 4y ago
It's relatively common to use "anymore" to mean "nowadays". I think it must have come from hearing things like "They don't do that any more" and not recognising "any" and "more" as separate words, but rather as a concept that just means "now", such that "They do that anymore" is equally valid.
- djmips 4y ago
  Wow really? I missed out on that one. Thanks for the update.

zmgsabst 4y ago

Not really. Even for hypersonics, the efficiency of chemicals at storing energy and then releasing it is hard to beat. 1 kg of TNT produces 4.2MJ; to get the same energy in 1kg of projectile you’d need to travel at ~6500 miles per hour. (About mach 9 at sea level.) The role of hypersonics is in application: - sudden attacks, to prevent response - low altitude, high velocity attacks, to defeat intercept - penetration, coupled with a chemical warhead, to hit hardened targets Source: https://en.wikipedia.org/wiki/Orders_of_magnitude_(energy) https://en.wikipedia.org/wiki/Orders_of_magnitude_(energy)

theptip 4y ago
Appreciate the detail. I'm not sure I see the conclusion as obvious though; if a 2kg weapon is going Mach 20 (as the weapon in the OP apparently does), with 1kg of that being TNT, then the explosives only provided 20% of the energy (rounding to the nearest 10% to make the example clearer). If you just lobbed the same 1kg weapon at Mach 20 then you'd have halved the weight, and reduced the energy to 40%. Presumably lower weight means easier to maneuver (more agile for intercepting or avoiding countermeasures, depending on what you're trying to hit). It seems like around Mach 10-20 is where the tradeoff could become non-obvious, depending on how much you value weighing less?
- zmgsabst 4y ago
  Per OP: > The U.S. Air Force said on Monday it had conducted a successful test of a hypersonic weapon, which flew at five times the speed of sound. But to your question: > around Mach 10-20 is where the tradeoff could become non-obvious, depending on how much you value weighing less? The same logic holds at mach 5-8 (ie current hypersonics), which is why they’re being developed: You want a weapon that can surprise, evade, or penetrate better — even if that’s less destructive overall.
- adrian_b 4y ago
  When close to the ground, something flying at Mach 10-20 would either be deccelerated very quickly to much lower speeds, if flying passively, or require a very large mass of fuel to maintain the speed. Moreover, it would be hard to avoid being partially vaporized before reaching the target. So hitting something at Mach 10-20 seems much more difficult than filling the missile with explosives and detonating it. Weapons based on kinetic energy alone seem practical only against satellites or against targets that fly in the upper atmosphere.

michael1999 4y ago

Many anti-air missiles detonate at range and destroy with shrapnel. And shrapnel damage is just kinetic. But that's probably not what you mean. Yes. For lightly armoured targets, their own velocity might do it. See https://en.wikipedia.org/wiki/Brilliant_Pebbles#Brilliant_Pebbles https://en.wikipedia.org/wiki/Brilliant_Pebbles#Brilliant_Pe... And if you want to go further, de-orbiting tungsten rods make the explosives redundant. https://en.wikipedia.org/wiki/Kinetic_bombardment https://en.wikipedia.org/wiki/Kinetic_bombardment

robonerd 4y ago
> de-orbiting tungsten rods Over-hyped I think. Sure the kinetic energy is incredible, but what of the terminal ballistics? I think these would have substantial over-penetration against anything except deep bunkers; most of that kinetic energy would be spent driving that rod through bedrock, like an APFSDS dart shot into tank armor. Good for punching holes in things.
- michael1999 4y ago
  Completely agree. Those systems solve a bunch of non-problems and completely destabilize MAD. There's a reason they were never developed. But I wouldn't discount to the ability of weapons designers to figure out the frangibility and deliver the bulk of the energy to a tuneable depth.

aidenn0 4y ago

About mach 8, but then you need to actually hit your target. For ground based targets that's probably fine (in fact for slow-moving, armored targets kinetic-energy penetrators are already a thing). For anti-aircraft fire the goal is to get "close enough" that the explosives will damage it.

hammock 4y ago

Look up "rods from God." https://en.wikipedia.org/wiki/Kinetic_bombardment https://en.wikipedia.org/wiki/Kinetic_bombardment https://www.businessinsider.com/air-force-rods-from-god-kinetic-weapon-hit-with-nuclear-weapon-force-2017-9#:~:text=The%20%22rods%20from%20god%22%20idea,bunkers%20or%20secret%20underground%20sites https://www.businessinsider.com/air-force-rods-from-god-kine....

mywittyname 4y ago
It always baffled me that these weren't named after Zeus.
- xatlasm 4y ago
  Drop a deuce from Zeus?
nradov 4y ago
"Rods from God" is a silly impractical sci-fi concept. It would be too expensive to launch a large enough satellite constellation to provide persistent global coverage. And the satellites themselves wouldn't be survivable. The prompt global strike mission can be accomplished more cheaply and effectively by missiles launched from aircraft or ships.