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Bullets from a Jet Fighter Mach 3.2

  1. Jan 20, 2006 #1
    :confused: A gentlemen on another site, not related to physics stated the following:

    [moderator removed personal information of third: indeed something that is inappropriate without consent]

    Bullet speed is relative. A .22 long rifle compared to the .22-250 is a great example. The SR-71 Blackbird aircraft was surreptitiously as the YF-12A fighter airplane until they realized that, at speed, the YF-12A was flying faster than the machine gun bullets it fired and that they would actually explode inside the gun magazine if fired.

    This particular rocket, accelerated after it passes the hold of Earth's gravity can attain and maintain a speed of 47,000 miles per hour, yet light travels at a speed of 186,000 miles PER SECOND. Doesn't seem very fast when compared to the grand scheme of things, does it.

    My theory, using my very limited knowledge of physics is: If an aircraft is flying at Mach 3, the weapons in the aircraft, are also moving at Mach 3 until they are fired. When they are fired, the speed of the bullets or projectiles would be Mach 3 plus the speed of the bullet upon firing.

    Thanks in advance, Bill
     
    Last edited by a moderator: Jan 20, 2006
  2. jcsd
  3. Jan 20, 2006 #2

    Pengwuino

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    First of all, please remove personal information from your thread, especially if it's not your own information.

    Velocities add up at such low speeds << speed of light. The bullet out of the airplane would have been ejected at the speed of the aircraft in addition to the normal ejection speed of the bullet. What I SUSPECT is that a bullet fired at such a speed (Mach 3.2 + probably a few thousand FPS) directly into the atmosphere would create tremendous friction and possibly destroy the barrel as it is going down it. If you'll notice the space shuttle's re-entry into the atmosphere, you'll see the enermous frictional energy being created when that baby hits the atmosphere. The friction might have caused some sort of barrel deformation once it was fired and on its way down the barrel.
     
  4. Jan 20, 2006 #3

    russ_watters

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    There would be higher pressure in front of the bullet, but it would be negligible compared to the pressure behind it. For all intents and purposes, such a bullet would be unaffected by the speed of the aircraft until after it was fired.
     
  5. Jan 20, 2006 #4

    FredGarvin

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    I have never heard of a gun being tried on a YF-12. The only armament that I knew of tested on that airframe was a missile. The Falcon IIRC. The story reeks of folklore to me. Why anyone would think of trying to fire a cannon at M3+ is beyond me.
     
  6. Jan 20, 2006 #5

    HallsofIvy

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    Yes, that's correct. Since the bullet, and the air in the barrel of the gun already has the speed of the gun, how fast the airplane is flying has nothing to do with the bullet moving through the barrel of the gun. (And it certainly won't "explode inside the gun magazine"! A bullet can't even be fired while it still inside the magazine!)

    I have, long ago, heard stories about a jet fighter firing a bullet, speeding up, and getting hit by its own bullet but that might be also be "urban legend".
     
  7. Jan 20, 2006 #6

    Bystander

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    Nerp --- early '50s, gunnery exercise shooting in a climb at a drone, or towed sleeve, nose down, throttle open, and the poor sap flew right into his own 20mm as they dropped on the far side of the parabola.

    He made it, the plane didn't.
     
  8. Jan 20, 2006 #7

    DaveC426913

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    I recall the F-86 Sabres (the first planes to unofficially encounter the sound barrier) had this problem. They'd go into a dive, aiming their guns at ground targets, manage to outrun their own bullets in the dive, and then when they pulled out, get hit.

    I confess, I have no credible source, and to be honest can't be sure I am not subject to urban legend myself.
     
  9. Jan 20, 2006 #8

    berkeman

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    Aw, c'mon Fred. Mach3 dogfights are pretty common these days. They have to use full flaps to turn at 9g, of course, but going to guns at M3 has been a Top Gun tactic for a while now. :uhh:
     
  10. Jan 20, 2006 #9

    russ_watters

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    I was gonna let that part go - [for OSHP295:] the YF-12 wasn't going to be a fighter, it was going to be an interceptor. Interceptors fire missiles and take out targets at long range while fighters fire missiles or guns and engage in dogfights at shorter range. With a turning radius on the order of 100 miles at mach 3.2, the F-12 would have had somewhat limited dogfighting capability.... :uhh:

    Regardless, if you did want to fire a gun from it, it wouldn't be a big deal.
     
  11. Jan 20, 2006 #10

    FredGarvin

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    You're right. If Tom Cruise could do it...
     
  12. Jan 20, 2006 #11
    The Navy's F-14 Tomcat is listed as a Mach 2 aircraft, not Mach 3. The same applies to the Air Force F-15 Eagle. The old P-58 Hustler and XB-70 Valkyrie were Mach 3. But because of it's fixed intake geometry, the F-22 Raptor has a top speed of about Mach 2.4. I don't know of any currently commissioned or planned aircraft capable of Mach 3 flight. At least, not any that they'd let me hear about.

    I have two questions about cannon fire at such high speeds. In the case of the SR-71 Blackbird (same airframe as the YF-12), managing the shock waves was an important consideration. Would cannon fire generate additional shock waves that would give the aircraft problems to fly through? Secondly, would the unpowered bullets be subject to drastic reduction in velocity due to friction, so that the plane would fly (straight line) into them, basically overtaking them and possibly sucking then into the engines. I sould see somethig like that happenig and spoiling the pilot's day.
     
  13. Jan 20, 2006 #12
    Overtaking and getting hit by your own bullets is equivalent to the poodle in the microwave tale. Bar talk. You would need to go into a ballistic (zero g) trajectory to accomplish that, since the bullets drop down in relation to your own flight path (or above you, should you happen to push negative g, very very hypothetically)) However getting into your own ricochets is a rather high risk in air to ground strafing. So remember guys, pull nose up 4-5 g, maintain g and always break away when level.

    Ah I can still do it
     
    Last edited: Jan 20, 2006
  14. Jan 20, 2006 #13

    rcgldr

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    Not if the bullets are caught up to so quickly due to drag than they only fall a short distance. Also bullets do "glide", a bit, decreasing the rate of fall, once they start falling, there's an angle of attack, because the spin rate is enough to reduce the downwards pitch reaction of the bullet as it falls. (This also answers the question about which falls faster, a dropped or fired bullet, the dropped bullet hits the ground first unless the bullet was fired in a vacuum on a flat earth).

    Regarding high-g maneuvers, I don't think there's any human driven aircraft that can pull high-g turns while going supersonic. Either they use their missles (which is most of the time), or they go sub-sonic for dog-fighting manevers.

    The only 9-g aircraft so far is the F-16, which tilts it's seat backwards about 30 degrees to allow the pilot to do this without passing out. I'm pretty sure flaps aren't used to do this as it would add too much drag. I've personally seen high g turns in F-16's and F-18's from directly below during air shows,and the angle of attack to do this is huge, and requires full after-burners just to maintain speed to overcome the drag. (Note, it's also very loud, bring earplugs).

    Missles have no problem pulling very high g's though. The most awesome missles were/are the anti-ballistic missles that launched at very high g's in order to intercept an incoming missle as quick as possible.
     
    Last edited: Jan 20, 2006
  15. Jan 20, 2006 #14

    rcgldr

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    Highest g's of a land based vehicle during launch is a rocket sled, over 150 g's during the last stage of this one, the stop was even quicker (it ran into a solid barrier at 6000+mph). Took about 6 seconds to go from 0 to over 6000mph, a small fraction of a second to go back to 0mph.

    Info and video on this web page (this website is hard to find):

    Click on the picture, to see the video. Note how long and narrow the rocket flame gets near the end of the run.

    http://www.46tg.af.mil/world_record.htm

    Cool pic of a smaller rocket sled at 4800ft/sec.

    http://www.meggaflash.com/rocket%20photogr...0flashbulbs.htm

    One of many articles:

    itl_amrdec_roadrunner.html
     
    Last edited: Jan 20, 2006
  16. Jan 20, 2006 #15
    No the seat is tilted for design, to allow it to be fit in a smaller cockpit. the g part is a selling story. Actually you're not inclined to sit back at all during flying. Probably psychologic, you seem to loose eagerness and agility when leaning back. Moreover you cannot reach several controls on the front panel and you have to adjust the seat pretty high to look through the head up display.

    Wrong again. The flapperons and leading edge flaps are essential in getting the lift required to attin the g. There is no flap control in the cockpit though. All automatic - fly by wire computer work.
     
  17. Jan 20, 2006 #16

    pervect

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    I could use some better figures, but if the shell was made out of lead, was 6 inches long, and was fired at 3x the speed of sound at sea level, it could de-accelerate at around 25 g's as soon as it left the barrel.

    Unfortunately I think the actual cannon shells are HE so they probably aren't as dense as lead, and planes generally don't fly at sea level, and I really don't know how long the cannon shells are.

    Here are the formulas I used

    [tex]
    \mathrm{Drag}=.5 \rho_{air} A v^2 C_d
    [/tex]
    [tex]
    \mathrm{Mass of bullet}= \rho_{bullet} A L
    [/tex]
    [tex]
    a = \frac{\mathrm{Drag}}{\mathrm{mass}} = .5 C_d \left( \frac{v^2}{L} \right) \left( \frac{\rho_{air}}{\rho_{bullet}} \right)
    [/tex]

    The [itex]\rho[/itex] are the densities of the air and bullet, [itex]C_d[/itex] is the drag coefficient and should be about .7 or so for a hypersonic blunt cone, A is the cross section of the bullet and L is its length. The mass of the bullet is slightly overestimated by area*length because it's not a cylinder.
     
    Last edited: Jan 20, 2006
  18. Jan 20, 2006 #17

    FredGarvin

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    It is a B-58 Hustler. And yes I know about the other aircraft. We were joking around about top gun stuff. The point being that even if you could get the physics around firing a cannon at that high of speed to work out, it's not practical because of the close proximity that one has to be to use it. At M=3 you are not going to do much of any kind of turning and you would have absolutely no time since the rate of closure on a target would be huge.

    The cannon won't effect the aircraft in terms of shockwaves like that. The main thing that we have no idea on is the aerodynamics of the shells at that speed. The speed at the time of firing is relative, so at that point in time it doesn't matter if the aircraft is moving or going that fast. The point of interest is when that shell hits the free stream and loses any effects of being in the barrel. My take would be that the shell had better be supersonic itself (relative to the barrel) or the ram pressure alone would kill off any energy it had. The whole point is kind of moot considering the pilot wouldn't have enough time to squeeze the trigger, let alone realize that he should shoot.

    The only instance I know of where a cannon has any effect on the aircraft is the A-10. Because the size of the shell and the momentum imparted to the airframe when it fires, there are burst limits when firing the cannon. Theoretically, it can bring the aircraft to a complete stop.
     
    Last edited: Jan 20, 2006
  19. Jan 20, 2006 #18

    rcgldr

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    Well these are the people that make the seats, and they seem to differ. In most fighters, the limit is around 7gs, the F-16 seems to be in a class by itself at 9gs. The F-22 with seat tilt at 22 degrees may be close.

    http://www.ejectionsite.com/f16seat.htm

    The stock seat height is high, visiblity is good, but you're pretty exposed as most of your body is above the base of the canopy. I have a view of the old El Toro Jet base in California, (a townhome with a nice view), and a few pilots haved lived in our complex, and from what they've told me and what I've seen from actual cockpit videos, they pretty much don't move much at all during dog-fight or air show maneuvers. More info on the F16:

    http://www.voodoo.cz/falcon/info.html

    Big difference between flaps and flaperons, which are just ailerons that can move in the same as well as in opposite directions. The leading edge flaps just rotate up and down and don't extend forward like the ones used on commercial airliners. The wind is a semi-delta wing, the front leading edge is angled backwards quite a bit, allowing for a high AOA while still producing a lot of lift (this is because vortices flowing across the leading edge help provide lift that you don't get with non-delta wing designs).

    I have flaperons on a few of my radio control gliders, in addition to actual center flaps, with the ability to move the entire trailing edge as a single surface, mostly to adjust the camber to work well through a range of speeds, and sometimes just opposing elevator for aerobatics (less down elevator requried for inverted flight), the F16 uses it's leading edge flaps and trailing flaperons for both purposes as well, camber adjustment for air speed, and increased response to elevator input.
    In the case of gliders, both full scale and models, dropping the flaps fully (almost 90 degrees in most rc gliders) is mainly use used to act as an air brake.

    Sameple video using flaps for hand catch landings, common when you don't want to scuff up an expensive model:

    http://jeffareid.net/rc/jrartms.wmv

    Since rc gliders don't have pilots they can pull some fierce g forces. Some F3J models like the one in that video have snapped 200 lb test fishing line during a tow launch (two big guys and a pulley), and these are models that weigh around 4.5 to 5.5 lbs. The strongest models are the ones used for dynamic soaring, the last time they had a good santa ana wind blowing over a local ridge here in so cal, a new 301mph record (radar gun) was set. Videos of this and other dynamic soaring stuff at the link below. All of the contest gliders use hollow molded fiberglass / carbon fiber / kevlar fiber contruction, strength depending on thickness and choice of materials (all carbon / kevlar fiber is strongest). Along with the videos, is a good picture of an 8 inch hole in hard packed dirt made by a rc glider crashing into the ground at 180mph.

    http://www.slopeaddiction.com/thekids.html
     
    Last edited: Jan 21, 2006
  20. Jan 20, 2006 #19
    Quite a response on this thread!

    First of all, I would like to apologize about posting the personal information. It was late, I was tired, but no excuse. I am a pilot, CFI, single and multi-engine, instrument. I also have to many hobbies and too little time. I performed a lot of crash reconstruction in my 27 years as a State Trooper and yes, I received help from some of the local math gurus.

    Believe it or not, this thread started because of a comment on a taxidermy website. I like to hunt, fish and do my own taxidermy and I always drop in on the taxidermy forums to see the topic of the day. In this case, it involved the bullets blowing up in the magazine of YF-12A due to its speed. Well, it got into a peeing contest and so I searched and found this site or should I say your site. Now on the taxidermy website there is no registering or log in. The one old timer likes to use phrases such as, it's too bad birth control isn't retroactive or you are an idiot, dumba.., etc. So I had to jump in on this one and was quickly "shot down" with a "crash and burn."

    If you go to this website http://www.taxidermy.net/forums/ and click on the green currents events bar, just scroll down to "You think a bullets fast" (01-19-06) it makes for some enjoyable reading on a site that only the moderators friends can break the rules.

    Thanks again, The Red Baron:cool:
     
  21. Jan 21, 2006 #20

    russ_watters

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    Just one thing, Jeff - the g-forces that pilots can withstand is all about duratinon. F-4 and A-4 pilots in 'Nam pulled up to 12 for short periods (less than a second) to evade missiles, and 9 is the typical limit for several seconds. The F-16's seat was reclined more than usual to improve g-tolerance, but it helps only a little-bit. Still, Mercury astronauts were able to withstand 12 g's for relatively long periods of time (a minute or so, I think) in a fully-reclined position.
     
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