Would a portable railgun make a lot of noise?

[member 656954]

I'm using a portable rail gun in my novel - details in the paragraph below - but I wondered whether it would be as quiet as I'm suggesting. The slugs are coming out at Mach 5, which is a touch under 6,200 kmh. Obviously, they're breaking the sound barrier in the barrel, so would a tiny (or big) sonic boom follow each slug out?

"There was no plasma cannon, but there was a nifty short-barreled rifle that used some fancy electronics to shrink a linear accelerator into a portable package that fired tiny slugs of gadolinium at about five times the speed of sound with only a slight whine to give the attacker away."​

 

[Klystron]

Roughly 30 years ago I had a software engineering assignment not far from a projectile test installation located at sea level behind bunkers testing projectiles not unlike your description. The hypersonic projectiles created a distinctive warbling scream separate from the 'thump' of the rail gun. The impact on armor plates was spectacular, felt through the body even while wearing ear protection two buildings away.

"Slight whine" and SBR (short barrelled rifle) seem more consistent with subsonic projectiles, if your plot allows; else less dense atmosphere. To quote the original blurb for the film "Alien": "In space, no one can hear you scream."

 

[member 656954]

Thanks @Klystron, interesting observation on the noise, I like the idea of a "warbling scream", so that's going in, even though it's a major rewrite of an existing fight sequence

And I can change the barrel length, but not the atmosphere as the action happens mostly at sea-level, but that's okay, my novel has a fair bit of handwavium so a little more is not going to strain credulity...too much!

 

[essenmein]

Any supersonic projectile will make a lot of noise traveling in air even if the "report" is silenced.


Go to about 4:15 in, the first sound you hear is the bullet going by... I imagine faster = louder

 

[member 656954]

Any supersonic projectile will make a lot of noise traveling in air even if the "report" is silenced.

I don't know in what part of my brain I squelched the knowledge that breaking the speed of sound makes a lot of noise, no matter how small the bullet, but thanks for the reminder, @essenmein, and the video, you and @Klystron have saved me from an obvious writing mistake.

 

[Klystron]

I read an old crime/detective novel set in 1930's US; possibly written by Jim Thompson. A dull-witted hit man acquires a magnum revolver and goes to considerable difficulty to thread the barrel and attach a silencer (sound suppressor) not trusting the small caliber assassin handguns then in vogue.

Silent Dillon hides in a closet to surprise his archenemy. When he fires the large revolver the supersonic blast causes him to flinch, miss his target and destroys his hearing. Deaf Dillon learns the hard way that silencers require quiet sub-sonic ammunition to be effective.

 

[DaveE]

"...fancy electronics to shrink a linear accelerator into a portable package that fired tiny slugs of gadolinium at about five times the speed of sound..."

It's science fiction right? If you can make this gun, then you can also make a bullet that's quiet at Mach 5. If it's important to your story leave it in, if it isn't take it out so your readers can believe in whatever else you create.

Of course we all think it can't work, but that's because we aren't smart enough to shrink a linear accelerator (yet).

 

[hmmm27]

Of course we all think it can't work, but that's because we aren't smart enough to shrink a linear accelerator (yet).

Well, no ; we all think it can't work because supersonic projectiles make noise. Personally I'd leave it in the story : it's not that offensive.

 

[Stephen Tashi]

How much recoil will the portable rail gun have? Depends on $F \Delta T = M \Delta V$ for the $M$ and $V$ of the bullet - something to ponder if you intend to make the physics realistic.

 

[member 656954]

Thanks everyone for your thoughts, I've updated the fire sequences to include @Klystron's warble, it gave me a couple of other attributes to the scenes so ended up being a better read.

And @Stephen Tashi, recoil is similar to a .45 as the slugs are 5g each, so about 9 kg·m/s per slug.

 

[Stephen Tashi]

recoil is similar to a .45 as the slugs are 5g each, so about 9 kg·m/s per slug.

The recoil ,I assume, can be approximated by applying a force over a length to accelerate the projectile from rest to muzzle velocity. That implies the relevant $\Delta T$. Maybe your story be cited in introductory physics texts as a problem: Compare the recoil of the rail gun in the story... with the recoil of a .45 automatic that accelerates a 230 grain bullet to a velocity of 830 m/s along a 3.3 inch barrel.

For part B: Could a more refined scifi hand held weapon employ an internal counterweight that is propelled in the opposite direction of the bullet to reduce recoil to zero?

 

[zoki85]

Man, only UFOs can move supersonic through the atmosphere without generating sonic booms
The larger the object and higher the Mach number, the boom is louder.
Recently I've heard a noise of a decent meteor as it cut through the sky at about 50 Mach:



Despite I was aprox 100 km away , both sounds (sonic boom and final explosion) were distinct and quite loud!
Reminded me somehow of my old PF question:
https://www.physicsforums.com/threads/jet-sound-contribution.161030/

 

[member 656954]

Maybe your story be cited in introductory physics texts as a problem: Compare the recoil of the rail gun in the story...

That would be truly meta, but I'd wonder if physics students would recoil themselves at having to solve a sci-fi story problem

 

[GTOM]

I wonder, does the bullet has to be made of somekind of unobtanium, so it isn't burned quickly in air at Mach 5 speed?

 

[member 656954]

I wonder, does the bullet has to be made of somekind of unobtanium, so it isn't burned quickly in air at Mach 5 speed?

That's an aspect I've willfully ignored, @GTOM

(Hopefully readers just assume little slugs of gadolinium won't burn up!)

 

[Klystron]

That's an aspect I've willfully ignored,

(Hopefully readers just assume little slugs of gadolinium won't burn up!)

Idea: borrow a term from geology: spall. It actually refers to a rock splinter but the rubes, uh audience, will think it means splatter.

Our Hero groaned as the gadolinium spalled, destroying her left lower prostheses in a shower of deadly shards. She spun with the impact on her remaining leg bringing her shortened railgun to bear on her grinning target.

"Take that! You tool of the tyrant! For Home, Peace and Freedom!"

 

[member 656954]

Idea: borrow a term from geology: spall. It actually refers to a rock splinter but the rubes, uh audience, will think it means splatter.

I did think about that, as a spray of molten metal hitting you at hypervelocity would be destructive, but went for solid slugs as that makes the end sequence work.

I like your little story snippet, and esp. because my novel is called Tyranny, so there is a tyrant involved