Shaped charge impact physics

[Digi5]

Greetings everyone.

There has been a question that's been nagging at my mind lately, and I was wondering if some of you could help me.

Orbital debris have velocities in the hypervelocity. When they impact with a spacecraft, they can generate high temperatures and pressures, resulting in the vaporization of the impacted region.

Shaped charges on the other hand, can produce a jet with a tip velocity similar to that of particles in orbit (over 10 km/s). Where they differ is that the jet does not have a uniform velocity throughout the length of the jet, which eventually results in its collapse due to stretching.

My question is whether shaped charges would generate the same high temperatures and pressures as space particles on impact, and thus vaporize what they hit. I'm thinking that the front portions of the jet would be able to vaporize common materials, while the trailing portions would lack the velocity to do so. Thus the total mass of solid matter being less after impact than before impact.

I apologize if the question is stupid or pointless, but it would help me understand or confirm some things I've been wondering about.

[Simon_Jester]

It's not so much that the question is stupid or pointless as that only about one person in ten thousand has any chance of knowing the answer- maybe less. Materials behave very strangely and counterintuitively under those kinds of forces. Metals flow like liquids, or peel in layers (the 'self-sharpening' behavior of a uranium penetrator). Processes that we expect to be relevant don't appear because the timescales are too short for them to happen, and so on.

The dynamics of a shaped charge jet are complicated (and important bits are probably classified). You'd have better luck contacting someone with a known publication record, or who is otherwise clearly an expert in the study of shaped charges.

Then again, you might get lucky here.


One thing I CAN tell you is that the typical hypervelocity impact into a uniform mass of material, even in vacuum, doesn't create the deep, narrow hole typical of a shaped charge. It makes a crater- like the ones on the moon. A shaped charge rocket the size of your fist, fired at a boulder, should punch a deep, irregular hole into the rock, possibly with cracks radiating outwards. A meteor the size of your fist, striking a (large enough) boulder, will dig a crater, almost perfectly circular in shape.

[Digi5]

I figured something along those lines. Still, can't hurt to ask.

I always figured that the difference in crater shape was a function of a shaped charge's high length to diameter ratio. I wonder if meteors would make similar craters if they were more cylindrical in shape.

I did find this little gem however: http://adsabs.harvard.edu/full/1997ESASP.393..405B
...where scientists did use shaped charges to simulate space debris collisions, which would imply some degree of similarity between the two. Which would imply that a shaped charge and its target do at least partially vaporize, although I'm hesitant to draw conclusions like that due to my limited background in science.

[Spectre_nz]

I'll offer my non-expert opinion on the subject;

I'm going to go with no, shaped charges are probably optimized so they don't vaporize portions of the target (or, as little as possible) as this is wasted energy that could otherwise be put to better use pushing fluidized target out of the way, rather than heating it further to vapor.
Vaporization at the point of impact (especially if the armor was designed to exploit the effect) and the resulting expanding cloud of vapour pushing its way back out of the hole you were digging would probably disrupt your penitrator jet faster than if there was no vaporization. In fact, I'm pretty sure that part of the ceramic armor defeat mechanism against shaped charges is the high velocity ejecta that gets launched back out from the point of impact disrupting the incoming metal jet, in addition to the whole 'ever increasing surface area fractal cracking' thing.

Given that hypervelocity impact tests tend to produce nice, hemispherical craters while shaped charge penetrator jets don't crater around the impact hole, I get the impression shaped charge jets are either too slow and/or deliberately optimized to vaporize as little as possible to achieve a better depth of penetration.

I dug up a couple of papers on the subject:
this one looked the most useful, on page two it gives an equation to work out when vaporization will occur; dependent on your impactor KE, the density of your target and the specific heat capacity of the target. It also mentions that target vaporization doesn't really happen until you're above 10k/sec, and meteorite impacts tend to be 30-50k/sec.

This document, aside from having some cool pictures of metal jet formation, makes the comment that the tip of the jets get up to 10k/sec, apparently only just into the range you'd get vaporization of a portion of the target (depending on what it is made of)

Another one makes it look like debris fragments, not vapor, is the predominant (and most damaging) ejecta, but I didn't actually see them explicitly describe what the ejecta material was.


But yeah, complex subject, no simple answer.
When I look at this I think yeah, looks like vaporization on the impact face, but then when I've seen details of RPG-7 impacts they tend to leave nice, neat holes with no cratering.

[Sea Skimmer]

You're thinking is correct. The jet tip of some shaped charges do vaporize on impact in some situations. The target can also vaporize, but is less likely to do so because of its (almost always) greater mass. Vaporization is a lot more relevant to spacecraft because we literally make them out of aluminum foil and even lighter gauges of material leaving them with little ability to absorb energy. In any case, most material should be subject to plastic deformation or converted into fragmenting spall rather then vaporized.

Most military shaped charges have velocities alot closer to 8km/s then 10km/s however. Below is a crappy wiki source but its too late for me to look up a better one; basically most explosives cannot produce detonation velocities as high as 10km/s. It’s very possible to make the shaped charges that pump higher then detonation velocity projectiles out but it’s more difficult to do so compared to just making a larger or tandem full caliber liner (not a little extra tandem warhead just to explode ERA) warhead to destroy the target. So military shaped charges don't normally work on extra high velocity that. High performance shaped charges in the modern day would typically use HMX or RDX based explosives; though the current tend is to shift to IM compliant explosives which typically have lower detonation velocities. So with those materials and normal designs you aren't likely to get 10km/s.
http://en.wikipedia.org/wiki/Table_of_e ... velocities

Now when you try to start getting crazy high velocities at 10km/s and up you also start running into new problems such as the jet material possibly exceeding its own speed of sound, which causes it to break up and is generally annoying. This requires a shift to less conventional materials like molybdenum. IIRC they’ve made that stuff go 12.5km/s successfully, and I'm sure if thick armor penetration was not the goal even higher speeds would be possible, certainly they've made non military devices explode at much higher speeds using physical compressors. So vaporizing targets is totally feasible, its just not a desired feature of current designs.

[Digi5]

Thank you all for all the information, it has proven most helpful, and has answered my questions completely.