Shape for injector spikes that provide the best seal

Question

In the near-future story I'm working on the protagonist is prepared for a wide range of possibilities, including getting outnumbered and surrendered.

For that possibility i though about a Banshee Box that the protagonist wears in the back at nape-level that emits an ear-hurting sound for the baddies to stop pointing their weapons to cover their ears (the protagonist uses electronic ear protection to counter that), and then launches guided spikes steered by retractable fins and cannards to the targets marked with the AR contacts.

The spikes penetrate their necks and injects a flammable gas to expand the wound cavity, that is then ignited for a decapitating explosion.

They have a very limited range and can't do very sharp turns, besides being expensive to make so it's basically a last resort for giving the character a chance to escape.

My inspirations were Mando's Whistling Birds, Yondu's Arrow and injection knives.

But there's still one inspiration that i'm still deciding if i should include, that is tri-edged daggers like the Jagdkommando and the M84 Cyclone.

Now i know, i know, these are obviously mall-ninja weapons that if something, makes stabbing more difficult because you would need to stab and twist at the same time for it to be effective, it can still be effective if the blade rests on a bearing that enables it to spin freely and screw through the flesh. The point is that for spinning projectiles, having a twisted blade can help it penetrate and then twist inside bones, breaking them and making penetration easier.

I even thinkered with the idea of a ballistic tri-edged injection knife, that was later incorpored into the Banshee Box.

But I,m not sure about the tri-edged part, because the ideal would be for the gas to not escape through the entry wound and i'm not sure if a spike would seal it better, though i'm also thinkering with some sort of vedative piece at the base that could work for both designs, and then I'd go for the one that penetrates better and has the best aerodynamics, maybe a mix of the two, the gas has to be stored inside of it anyway.

I'm open to other options.

Edit: It has been shown to me how unncessarily overkill it would be for the spikes to be hypersonic at such a a close range, so i edited it out and now the answers can focus in the aerodynamics, penetration and sealing of the shapes.

Answer 1

You aren't going to be remotely guiding anything at hypersonic speeds over human engagement distances. Besides, do you even need to? What's going to evade or dodge you at that those speeds and ranges?

Not to mention the air itself provides a bigger obstacle than the target for a hypersonic projectile. You needn't worry about the penetration, damage potential, or explosive payloads. It will be plenty.

Worry about accelerating the projectile up to speed over the short distance you have without disintegrating it (complex mechanisms onboard the projectile like propulsion, guidance and explosive payloads aren't going to make this any easier), any resulting recoil, and getting it through the air to the target in one piece. Fancy, deadly looking shapes and spinning projectiles with drag producing protrusions aren't going to survive.

EDIT:

If we assume that that we are aimed directly at the center of mass without any lead, and treat the human as a spherical cow 0.3m in width (meaning it has to move 0.15m to get out of the way), and the human target can produce an acceleration of 1g to evade (I pulled this acceleration out of the air. To me it seems very generous but maybe it is not), then a projectile moving at Mach 5 would still hit it's target up to a range of 300m.

If the target was already in motion at a sprinting speed of 13km/h then without lead you would still hit the target at a range of 71m. This is without trying to evade. If you did lead and the target attempted to evade by decelerate at the instant you fired you would hit them at 300m (this is basically the same as the first scenario with an initial velocity offset).

But all the above examples assume the target reacts instantly the moment you fire. People can't do that. Unconscious human reflexes max out at 0.08s they won't even have time to react or know what happened until the projectile has already traveled 137m. So if your criteria for evasion includes consciously reacting to a shot rather than random motion that is not in response to anything then you can add 137m to the numbers above.

I have completely neglected the acceleration time required to get the projectile up to Mach 5 which still need to be performed by some unknown means on some unknown material because the human arm distances involved (and thus acceleration times) is so short compared to the ranges and flight times involved.