Trope-a-Day: Every Bullet is a Tracer

Every Bullet Is A Tracer: Justified for non-slugguns, inasmuch as modern slugthrowers kick their tiny dust-grain bullets up to such a high velocity – a respectable fraction of c – that they plasmate the air they hit on the way to their target.  The bullet doesn’t glow, but the resulting plasma bolus does.

(The disadvantage that this has, that firing gives away your position unless you turn the muzzle velocity way down, has been noted by the relevant tacticians.)

8 thoughts on “Trope-a-Day: Every Bullet is a Tracer

  1. Pingback: Fun with TV Tropes | Home Gun Training

  2. So this one keeps nagging at me. I freely admit that mass drivers are cool, and present all sorts of interesting possibilities as far as design and capabilities. I’m just not sure how the “projectiles the size of a grain of sand” would work.
    First I think of how very small caliber projectiles tend to cause little damage, just poking a tiny hole through the target without much more.
    Then I think “wait, if it’s moving fast enough to cause a plasma shockwave, though the bullet itself might still just poke through immediately, maybe the shockwave will cause plenty of damage to the target.”
    But then I think “wait, if the bullet is the size of a grain of sand and is hitting the atmosphere hard enough to cause a plasma shockwave, then it must be experiencing massive air resistance.”
    A bullet the size of a grain of sand can’t have that much mass (unless you’re making it out of rather costly exotic materials), and little mass means little inertia. I would think that kind of air resistance would quickly bring the projectile to a stop (think trying to launch a ping-pong ball vs. a golf ball), which is why long range shooters in our world tend to use heavier projectiles.
    I get how a mass driver firing “regular-sized” projectiles would work, but how do the grain of sand projectiles a. do damage and b. travel any useful distance through an atmosphere?

    • The short answer here is “you’re not thinking fast enough”.

      The longer one is: for comparison, “hypervelocity” small arms built using chemical propellants, here on Earth, top out around 1.7 km/sec (around, say, five times the speed of sound in standard sea level conditions). Even the US Navy’s railgun program isn’t looking at going beyond 2.5 km/sec (a little over Mach 7).

      An Eldraeverse slugthrower’s projectiles are leaving the barrel a couple of orders of magnitude faster than that, despite their much lower mass. So, yep, more’n enough to create a plasma shock (and that does bleed kinetic energy, but in this sort of super-high-energy physics regime, not enough to matter, especially since the cross-sectional area of the projectile is insignificant). The surface of the projectile pretty much melts in transit, but the whole thing stays integral because the heat doesn’t have time to transfer. I won’t bore you with the whole set of calculations, but the numbers I have here suggests that the KE on impact of an average pistol is going to be something on the order of 24 KJ, which is to say, it’s like being shot point-blank by a GAU-8 Avenger.

      As for penetration – well, the thing is that at the relatively slow speeds we fire regular small caliber projectiles, they tend to remain basically intact, or at most misshapen, on impact and pass, as you say, through the target. In this scenario, there’s too much KE around for that to happen – the projectile vaporizes on impact, liberating all that KE right there onto the target, much of it as heat:

      My simulation based off the above shot of 24 KJ suggests that, were you to fire this at an unarmored human center-mass, it would leave a large, deep charred crater on impact which would last for just long enough for the fragments and hydraulic shock to pulverize their internal organs and blow them out of an exit wound the size of their torso. Basically, like Fallout 3 with the Bloody Mess perk turned on.

      (Which is tremendous overkill, but any gun that can meaningfully impact someone wearing equiv-tech armor with kinetic barriers is going to be tremendous overkill when firing at anyone without them. If you want to go nonlethal against soft targets, you use an electrolaser.)

      • Gorram! So in other words “Sir Isaac Newton is the deadliest son of a bitch in space.”
        Thank you for the prompt response. You’ve probably answered this elsewhere, but how does someone who’s not heavily modified survive firing even a pistol like that? Is it all vector control?

        • Most of them do indeed come with some pretty powerful recoil damping systems. It’s not quite as bad as it might seem, though – worth bearing in mind that the kinetic energy increased with the square of the velocity, but the momentum increase is only linear with velocity, and also that the momentum conserved is mv on both sides, and the gun/firer has a lot more m than the tiny dust-grain bullet.

          …that being said, it’s not like there aren’t plenty of slugthrowers on the market which come complete with warning labels advising that firing them without having the proper sort of bone and muscle weaving implanted will shatter every bone in your arm, so.

  3. I used to be rather enamored of this idea – hypervelocity pellets blasting incandescent plasma trails through the air – until I actually ran some numbers. At speeds in excess of several kilometers per second, the ram pressure of the air in front of the projectile greatly exceeds any plausible physical strength of the projectile material. As a result, the projectile acts as a hydrodynamic jet moving through the fluid of the air. As it punches through, the front end of the projectile is continually stripped away, leaving a shorter and shorter projectile. This has a well-known solution (proof available upon request) – the distance the projectile penetrates is equal to its length times the square root of the ratio of its density to the density of the surrounding medium
    D = L sqrt(rho_p/rho_t).
    For example, take the projectile density to be 20 g/cm^3 (about the densest stuff you can find – platinum, tungsten, gold) and the density of air to be 0.0012 g/cm^3 (the density of air on earth). Then you find that the projectile will travel a maximum of 130 times its own length. If you launch a 1 cm long tungsten needle, it will go 1.3 meters before completely disintegrating. Making it go faster doesn’t help – it just makes a larger blast from the energy deposited in that 1.3 meters (the exception is when the projectile is going so fast that the mean free path of the individual particles making up the projectile go farther than the distance the projectile would normally travel. Now the projectile behaves like a particle beam rather than a material object, and you end up with a directed beam of radiation, along with a high radiation environment in the vicinity as radiation is scattered from the beam in all directions and additional radiation is produced from collisions between the projectile particles and the air. You can also end up with amusing beam instabilities which cause the projectile/beam to veer from its path).

    • Hrmf. Oops. That’s going to be an interesting retcon to make, but, well, facts are facts, so… 🙂

      I would appreciate that proof, if it wouldn’t be too much trouble, mostly for my own edification. (And if you have any thoughts on how I might salvage some of the basic concept, I’d be very much appreciative of that, too!)

      Thanks muchly for the detailed information, either way.

      • I think it can be salvaged.

        I’m no physicist, but I suspect that at the “respectable fraction of c” these things are said to be reaching, concepts like “ram pressure” (or, for that matter, “structural integrity”) don’t really apply: what you really have is, indeed, a bunch of particles flying in formation, but it’s going so fast that the air doesn’t really interact with it much.
        Under normal circumstances, air pressure (like most contact forces) is transmitted largely through electrostatic repulsion, which has a fairly large cross-section. But at sufficiently high energies, as long as the collision is off-centre, the electrostatic forces simply don’t have time to deflect the particle much before it’s gone past. So the collision cross-section gets smaller, and the mean free path gets longer.
        All of which is to say that any sufficiently powerful slugthrower becomes, when fired in atmosphere, a plasma lance.

        Of course, there is another way to deal with it, if you’re good enough at hypersonic aerodynamics, without going all the way to “respectable fractions of c“. By combining the concepts of the ram accelerator and the scramjet, I wonder if it might be possible to combust part of the mass of your pellet with the air going past it (or through it — baseline design for these things looks something like a venturi, though I can’t remember if it’s the converging-diverging kind or vice-versa; ram accelerator pellets are designed to operate inside a muzzle, so they’re a reverse venturi) in such a way as to produce a thrust that counteracts the ram pressure drag. You’re going much slower, because the projectile needs to not disintegrate at the distance given by lwcamp55 (and sharp as you make these things, there will be a stagnation point — well, ring — that needs to withstand the ram pressure and the compression heating), so you probably don’t get much plasmated air — but your shell is basically an airbreathing rocket motor burning itself for fuel, and that’s gonna leave a trace too, even if not quite as bright as the plasma bolus. And since you’re getting thrust, your range is good despite the lower muzzle velocity — it’s possible that the speed would continue to increase after leaving the gun, in which case the only limit on range is one of specific impulse, determining the point at which the pellet has burned too much of itself away.
        Naturally this is beyond our current level of hypersonic aerodynamics (we have yet to build a working scramjet or an in-muzzle ram accelerator), but those Eldrae are smart chaps, I’m sure someone at Eye-In-The-Flame figured out how to do it a long time ago.

        (There’s also some fun stuff you can get into with the “Busemann biplane” and other ‘shockless’ bodies — see for some discussion — which minimises wave drag and shock noise. Shockless sniper bullets have already been developed, as described in that link. These concepts would doubtless be important to any attempt at a scram bullet, since otherwise shock entropy when mixing the air and fuel would make it too hot for combustion to give useful thrust.)

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