Sound of a hypersonic projectile from a rail gun

[TheDeviantSaint]

This is my first post, so let me know if I'm doing it wrong. In my world, there are railguns roughly the size of the M240 machine gun that fire a projectile about a meter long with a radius of about 4cm. I believe the sonic boom would break 150db, but what kind of sound would the projectile make after that? I'm assuming someone a mile away would still hear the boom as an echoing crack, but would they hear anything as the projectile approaches? would there be any way to make the sound quieter by shaping the projectile to give it a spin?

 

[Flyboy]

You would hear nothing as the round approaches. It’s moving 5+ times faster than any sound it makes.

You cannot make it any quieter on approach, and you cannot muffle the shock of it passing through the air. Maybe at low supersonic, but at hypersonic, you can’t really disperse that shockwave.

 

[jrmichler]

I'm assuming someone a mile away would still hear the boom as an echoing crack

A supersonic jet a few miles up makes a dull BOOM, while a 30 caliber bullet at about Mach 2 to 2.5 a few feet overhead makes a SNAP. I think your description is about right.

 

[TheDeviantSaint]

You would hear nothing as the round approaches. It’s moving 5+ times faster than any sound it makes.

You cannot make it any quieter on approach, and you cannot muffle the shock of it passing through the air. Maybe at low supersonic, but at hypersonic, you can’t really disperse that shockwave.

So it would be more accurate to describe it as just appearing followed by a blast of wind? Since it's faster than the sound, there wouldn't be any need to attempt to make it any quieter, and I assume any wound would probably be shaped like a bullet, with a smaller diameter hole at entry and a larger one at the exit. I'm also assuming the speed alone would prevent any structure of the body from stopping the projectile, even at extreme distances.

 

[berkeman]

I assume any wound would probably be shaped like a bullet, with a smaller diameter hole at entry and a larger one at the exit.

You should do a Google search on hypersonic projectile impact and click into the Images and Videos sections. That will give you a better idea of what happens upon impact...

 

[berkeman]

while a 30 caliber bullet at about Mach 2 to 2.5 a few feet overhead makes a SNAP

It sounds like you and I have each had a couple close calls on hunting trips...

 

[jrmichler]

It sounds like you and I have each had a couple close calls on hunting trips...

Nothing so exciting, I was just pulling targets at a 600 yard rifle match.

But there was the time that some idiots thought that shooting into a swamp was safe, and a .22 bullet bounced off the water under some moss, and hit the tree that I was leaning against. That one went overhead with a whine - it was subsonic and spinning.

 

[Flyboy]

So it would be more accurate to describe it as just appearing followed by a blast of wind? Since it's faster than the sound, there wouldn't be any need to attempt to make it any quieter, and I assume any wound would probably be shaped like a bullet, with a smaller diameter hole at entry and a larger one at the exit. I'm also assuming the speed alone would prevent any structure of the body from stopping the projectile, even at extreme distances.

Yes, yes, and oh hell no. The impact of something that large at those speeds won't leave an entry or exit wound in a body.

It's gonna leave an entry and exit wound on the building that said body is trying to use as cover. That's less anti-personnel and more anti-everything. Including assets in low orbit if you're tied into the right targeting suite.

In addition to the above overperformance of the round, given the specified size of the round, I doubt you can fit a railgun that can fire that round into a package the size of a modern GPMG like the M240. You're talking at least the size of an M242 Bushmaster cannon. Unless we're talking it as a crew-served weapon or the primary weapon of a large mech suit, that round and weapon is way too heavy for infantry to field.

If intended for infantry issue, I'd argue for a round much smaller than what you have suggested. Say, 5-6mm diameter, 20mm long. Puts you in the ballpark of modern high velocity military rounds like NATO 5.56x45mm. Back that up with double or more velocity, and now you're talking 4 or more times the energy. Which is still going to make a hell of a hole in whatever hard material it hits, like, say, concrete. If it hits a body... There's probably going to be not much left of the center of mass area.

 

[Rive]

roughly the size of the M240 machine gun that fire a projectile about a meter long with a radius of about 4cm.

???
That projectile is roughly about the size of your reference gun.
Did you thought about the recoil, BTW?
That acceleration at that length ... erm... pretty rough, I think

 

[TheDeviantSaint]

Yes, yes, and oh hell no. The impact of something that large at those speeds won't leave an entry or exit wound in a body.

It's gonna leave an entry and exit wound on the building that said body is trying to use as cover. That's less anti-personnel and more anti-everything. Including assets in low orbit if you're tied into the right targeting suite.

In addition to the above overperformance of the round, given the specified size of the round, I doubt you can fit a railgun that can fire that round into a package the size of a modern GPMG like the M240. You're talking at least the size of an M242 Bushmaster cannon. Unless we're talking it as a crew-served weapon or the primary weapon of a large mech suit, that round and weapon is way too heavy for infantry to field.

If intended for infantry issue, I'd argue for a round much smaller than what you have suggested. Say, 5-6mm diameter, 20mm long. Puts you in the ballpark of modern high velocity military rounds like NATO 5.56x45mm. Back that up with double or more velocity, and now you're talking 4 or more times the energy. Which is still going to make a hell of a hole in whatever hard material it hits, like, say, concrete. If it hits a body... There's probably going to be not much left of the center of mass area.

It looks like I am going to need to revisit the overall design... I was going for the image of a giant spike appearing out of nowhere next to the heroes, but not at the cost of even remote plausibility. Thanks for your insights!

 

[TheDeviantSaint]

???
That projectile is roughly about the size of your reference gun.
Did you thought about the recoil, BTW?
That acceleration at that length ... erm... pretty rough, I think

yeah, and Flyboy pointed out that the size of the projectile would cause way more damage than I was considering. As for the recoil, I was imagining anyone using the weapon would have to be able to anchor it before they could use it. I don't want it to be too overpowered that it would become a world breaking item.