Bullet drafting & supersonic turbulence

CharlesSmith · February 21, 2009, 12:13 AM

Question: Is the air immediately behind a supersonic round so turbulent it would be impossible to 'draft' another round behind it?
I have been looking at slow motion video of rounds going through water and it seems like there is a pocket of low density right behind the round.
Of course this is theoretical, please spare yourself the time of explaining the problems of placing a second round that close to another.
Have at it guys, tell me what you think.

THEZACHARIAS · February 21, 2009, 12:37 AM

Depends on the bullet. In theory you could do it, but I dont know what the application or benefit would be.

Two bullets that are that close together are not going to cause any more damage. The only thing I could think is you might get a slight increase in impact energy because of the increased mass. But considering the complexity in achieving this, it would be easier to just use a larger caliber.

Go Kiwi · February 21, 2009, 02:11 AM

If there was a significant aerodynamic advantage to the projectile behind, surely it would catch up to and meet the bullet in front (given the same initial shove etc) If they were anything other than perfectly in line the results would not be good??

The HK caseless round rifle - i forget the name of it is often said to fire a burst so fast there are "three rounds in the barrel at one time" no idea if this is true but perhaps that could set up the conditions you are thinking about?

Swampghost · February 21, 2009, 02:12 AM

I couldn't find the pic that I was looking for but this one will work. http://people.rit.edu/andpph/photofi...h-bullet-1.jpg

Notice how much the pressure waves resemble a boats wake? The difference here is that the turbulence will have rotational features due to the spinning of the bullet. This should have little inpact if the following bullet is rotating at the same rate as the forward one. What you are looking at is the turbiditys destabilizing influence vs. the bullets ability to remain stable by rotation.

The distance between the two is all important and is going to vary throughout as the second bullet catches the first.

You may want to contact the guy that built this thing. http://www.youtube.com/watch?v=xe-QP8s1ZJc

Jimro · February 21, 2009, 02:14 AM

The low pressure zone is not large enough to accommodate a second bullet of the same size. You might be able to draft a smaller projectile like a .177 BB round or pellet, but the rule is that small objects can draft behind larger objects. The "boat tail" gives a bullet a higher BC because the tail portion allows gas to flow back together faster than a flat based bullet. As such a flat based bullet will have a larger low pressure zone behind it than a boat tail bullet, however the low pressure area will not be larger than the object that created it.

When considering objects of the same size the two objects must be able to effectively close the gap to act as a single body aerodynamically (think drafting in NASCAR). The nose of a car and the tail of a car fit together a lot better than the ogive and tail of a bullet. The tip of the bullet would be inside a low pressure area but the bulk of the ogive would get the full force of the converging high pressure areas caused by the wake of the forward bullet.

If you could fire two flat based bullets, the first one forward and the second backward, then it would be possible to draft the second bullet. But otherwise the shape of the bullets will not allow bullet drafting.

Hope this helps.

Jimro

THEZACHARIAS · February 21, 2009, 02:23 AM

Sounds like the sort of thing that would have had to have been looked into when developing the metalstorm systems. Not sure if they have that kind of technical info up on the companys web site, though.

Jekyll · February 21, 2009, 05:03 PM

You make my head hurt.

CharlesSmith · March 18, 2009, 09:33 AM

Thanks gents!
You guys really know your stuff.

February 21, 2009, 12:13 AM	#1
CharlesSmith Junior Member Join Date: February 21, 2009 Posts: 5	Bullet drafting & supersonic turbulence Question: Is the air immediately behind a supersonic round so turbulent it would be impossible to 'draft' another round behind it? I have been looking at slow motion video of rounds going through water and it seems like there is a pocket of low density right behind the round. Of course this is theoretical, please spare yourself the time of explaining the problems of placing a second round that close to another. Have at it guys, tell me what you think.

February 21, 2009, 02:12 AM	#4
Swampghost Senior Member Join Date: April 18, 2008 Location: Florida, east coast Posts: 2,106	I couldn't find the pic that I was looking for but this one will work. http://people.rit.edu/andpph/photofi...h-bullet-1.jpg Notice how much the pressure waves resemble a boats wake? The difference here is that the turbulence will have rotational features due to the spinning of the bullet. This should have little inpact if the following bullet is rotating at the same rate as the forward one. What you are looking at is the turbiditys destabilizing influence vs. the bullets ability to remain stable by rotation. The distance between the two is all important and is going to vary throughout as the second bullet catches the first. You may want to contact the guy that built this thing. http://www.youtube.com/watch?v=xe-QP8s1ZJc __________________ NRA Patron Member

February 21, 2009, 02:14 AM	#5
Jimro Senior Member Join Date: October 18, 2006 Posts: 7,097	The low pressure zone is not large enough to accommodate a second bullet of the same size. You might be able to draft a smaller projectile like a .177 BB round or pellet, but the rule is that small objects can draft behind larger objects. The "boat tail" gives a bullet a higher BC because the tail portion allows gas to flow back together faster than a flat based bullet. As such a flat based bullet will have a larger low pressure zone behind it than a boat tail bullet, however the low pressure area will not be larger than the object that created it. When considering objects of the same size the two objects must be able to effectively close the gap to act as a single body aerodynamically (think drafting in NASCAR). The nose of a car and the tail of a car fit together a lot better than the ogive and tail of a bullet. The tip of the bullet would be inside a low pressure area but the bulk of the ogive would get the full force of the converging high pressure areas caused by the wake of the forward bullet. If you could fire two flat based bullets, the first one forward and the second backward, then it would be possible to draft the second bullet. But otherwise the shape of the bullets will not allow bullet drafting. Hope this helps. Jimro __________________ Machine guns are awesome until you have to carry one.

February 21, 2009, 12:37 AM	#2
THEZACHARIAS Senior Member Join Date: January 26, 2009 Location: Louisiana Posts: 537	Depends on the bullet. In theory you could do it, but I dont know what the application or benefit would be. Two bullets that are that close together are not going to cause any more damage. The only thing I could think is you might get a slight increase in impact energy because of the increased mass. But considering the complexity in achieving this, it would be easier to just use a larger caliber.

February 21, 2009, 02:11 AM	#3
Go Kiwi Member Join Date: October 11, 2007 Posts: 40	If there was a significant aerodynamic advantage to the projectile behind, surely it would catch up to and meet the bullet in front (given the same initial shove etc) If they were anything other than perfectly in line the results would not be good?? The HK caseless round rifle - i forget the name of it is often said to fire a burst so fast there are "three rounds in the barrel at one time" no idea if this is true but perhaps that could set up the conditions you are thinking about?

February 21, 2009, 02:23 AM	#6
THEZACHARIAS Senior Member Join Date: January 26, 2009 Location: Louisiana Posts: 537	Sounds like the sort of thing that would have had to have been looked into when developing the metalstorm systems. Not sure if they have that kind of technical info up on the companys web site, though.

February 21, 2009, 05:03 PM	#7
Jekyll Senior Member Join Date: January 27, 2009 Location: Easten PA Posts: 277	You make my head hurt.

March 18, 2009, 09:33 AM	#8
CharlesSmith Junior Member Join Date: February 21, 2009 Posts: 5	Thanks gents! You guys really know your stuff.