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Comment by topspin | Hacker News Reader

Something like a .50 BMG round has a large report and the flight time is just under 2 seconds at a mile, so anything recording audio in the neighborhood would pick up the distinct crack of a rifle prior to the pad explosion. Some of the noise can be suppressed with a big enough suppression device but the sonic boom is unavoidable. Subsonic ammo is simply not workable at these ranges; that's called 'artillery' and it doesn't have sufficient precision to ensure hitting the rocket.

You can listen to the explosion here https://www.youtube.com/watch?v=I6isMuPfxcI

I hear birds chirping. No rifles.

update: yes, I know Elon mentioned the 'quieter bang' sound and it could well be related, but it doesn't sound anything like the crack of a rifle round.

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I don't know anything about fluid dynamics but I know something about shooting and being shot at. I agree with your comment. I would only add that rifle fire sound can be somewhat directional and depending on the temp, density, humidity, height above ground, and reflectivity of the ground much of the impulse can be dissipated or distorted.

Surely sensors would have picked up the impact a .50 makes. Although I don't know why one would need a .50. It's a heavy round with lots of drop and it's not like the rocket is armored. From what I've read of the relative fragility of rockets you could get by with a very small round in just the right spot.

But you will need accuracy to hit "just the right spot", and to get accuracy at long distance, you need a heavier bullet (https://en.m.wikipedia.org/wiki/Sniper_rifle#Maximum_effecti...: "The recent trend in specialized military sniper rifles is towards larger calibers that offer relatively favorable hit probabilities at greater range"), don't you?

True, but "heavier bullet" does not always mean "larger caliber". Also there's no telling at what distance a sniper team would have to be. Maybe they could be inside of 1km?

Also "just the right spot" could be huge on a rocket. The thing is as wide as a barn.

You're right of course; except for one minor nit, the crack you hear is the sound of the bullet. The sound you hear from the rifle is a thump sound. Indeed most infantry soldiers are trained to estimate distance to a shooter by the delay between the crack sound above your head and the thump that follows; 3 seconds is approximately 1km or 1100 yards.

Elon heard something: https://twitter.com/elonmusk/status/774153847371501569

Although I also think sabotage such a weak theory.

Yes, that sound is clearly audible in this better version with synchronized audio https://youtu.be/maRTEzlSBLk?t=1m8s

Musk: “Particularly trying to understand the quieter bang sound a few seconds before the fireball goes off… May come from rocket or something else.”

Try this synchronized sound version, which shifted the entire audio stream. (The one you linked to appeared to only start sync'd audio at the exact explosion moment.)

https://youtu.be/maRTEzlSBLk?t=1m8s

You can hear a distinct and odd metallic noise just before the explosion.

> You can hear a distinct and odd metallic noise just before the explosion.

Which would be very consistent with a bursting tank within the rocket.

.50 BMG is supersonic but drops in velocity rather quickly, like any other small caliber round. Depending on the position of the shooter (especially if opposite to the recording equipment relative to the LV, for example), chances are that the cameras with mikes recording the LV might have problems hearing it. I'm not sure at what distance it drops into subsonic region but the size of the target should allow for some pretty large distance. Powder load could also be tweaked. One has to assume a shooter with significant resources if a deliberate action against a multiple-$100M target is the premise.

> I hear birds chirping. No rifles.

There's a very audible boom-like sound at 0:06.

Suppressed 50. Cal sniper rifles is available I believe.

That doesn't do anything about the crack of the hypersonic bullet though unless you use subsonic rounds instead.

Which, due to the drag over several miles, would be an impossible shot.

I've not heard of subsonic rounds being used at distances over five hundred yards, and even that is a very difficult shot for that low velocity.

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