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Author Topic: Does a Jet need to go sub-sonic before releasing a conventional gun burst ?  (Read 7236 times)

Offline tommya300

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Does a Jet need to go sub-sonic before  releasing a conventional gun burst?
If the muzzel velocity is slower or equal to the forward speed of the mounted gun, what happens to the velocity of the bullet? Can the vehicle run into its own fire or arch away??
« Last Edit: 06/06/2010 09:37:05 by tommya300 »


 

Offline graham.d

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Aircraft can run into their own bullets without being supersonic and it has happened. The bullets will slow and start to fall. An aircraft can accelerate and dive so the two paths can intersect. The bullets have air friction and slow more quickly the faster they are travelling so such a problem is much more likely the faster the aircraft. It doesn't matter, apart from this last point, whether the aircraft is supersonic or not.
 

Offline LeeE

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The M61 Vulcan cannon, which is one of the most widely used guns in western fighters, and which has been in service since 1959, has a muzzle velocity of ~1050 m/s (depending upon the ammunition being used).  The speed of sound in air varies between ~300 and ~350 m/s depending upon altitude and air temperature, so with typical top speeds for fighter aircraft being around 1.8-2.0 Mach the muzzle velocity will always be higher than the aircraft velocity.

However, while the speed of the aircraft should be added to the muzzle velocity, what graham.d says is absolutely true and the rate at which the round slows down increases with the initial velocity of the round, so if the aircraft is traveling at high speed there is a definite risk of running into its own fire.

In the end, if such a cannon round is fired at a supersonic target then unless it is fired at a very short range the target may be able to outrun the round.  Alternatively, if the target is not supersonic, or is stationary, there is likely to be insufficient time to fire the round and then maneuver to avoid a collision with the target, due to the high speed of the aircraft (no aircraft is highly maneuverable at supersonic speeds).

In summary then, guns are not very useful at supersonic speeds.
 

Offline SeanB

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Missiles are much faster and also have the very helpful ability to correct their position in relation to the target so as to actually hit it or explode close enough to destroy or damage it with shrapnel.

Another very good thing is that the missile self destructs after around 45 seconds if it cannot hit the target, unlike bullets which fall to the ground and can really ruin the day of whoever they hit on landing.
 

Offline tommya300

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Quote

However, while the speed of the aircraft should be added to the muzzle velocity, what graham.d says is absolutely true and the rate at which the round slows down increases with the initial velocity of the round, so if the aircraft is traveling at high speed there is a definite risk of running into its own fire.


Mmmm I have a problem  with this??? ???
So V1 the aircraft speed, V2 is the muzzle velocity,
 v1 + v2 = VT
But if V1 = 2 times V2 does this still hold true that V1 will overtake V2 and v2 in the sum is insignificant?


Yep, even though missiles is another subject with all weapons to the fighter's disposal and used in preference to the pilots judgement on the battle chosen at hand.
It is weird to hear the phrase "faster then a speeding bullet" is now a fact.

If the bullet is heading hypothetically straight onto the craft then I say add the 2 velocities.
Like 2 cars have a head on collision?
But if the cars going in the same direction and  the rear car hits the car in from isn't there a difference of the 2 velocities?

Let's say I'm from Missouri, Show me.
« Last Edit: 06/06/2010 15:17:32 by tommya300 »
 

Offline graham.d

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Bullets slow down quite quickly. A typical NATO rifle has a muzzle velocity of around 2400 m/s but the round loses 25% of its velocity within about 200 metres and are usually sub-sonic at 1000 metres. At 1000 metres if a bullet passes you don't hear the "crack" that you hear at up to 500 or 600 metres, but a "phut" sound because the round is subsonic. I had a weekend job in the butts on a rifle range as a lad, "marking" the targets.
 

Offline tommya300

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Since nobody corrected me I guess I was correct  ?
 

Offline LeeE

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tommya300:  this is best done in a swimming pool, but it does work with a wash basin or sink (you just make more of a mess if you try it at home).

Put your hand into the water and slowly sweep it around in an arc: you'll feel little resistance as you do so.  Next try moving your hand quickly through the same arc.  Apart from spraying some of the water everywhere (which is why it is best done in a swimming pool, rather than at home in a bash basin), you should notice that you feel much more resistance from the water when you try to move your hand quickly when compared with trying to move it slowly.

The air is much 'thinner' than water, so even when we run through it at our top speed we feel little resistance, but when we start to reach even a relatively small proportion of the speed of sound in air, as aircraft do, we start to feel the same sort of resistance that we feel when we try to move our hand quickly through water.

Ultimately, the faster you go, the more resistance you feel.
 

Offline tommya300

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Metaphorically or realistically it is all science. It seems to have all gone off in a tangent.
Thank for the low down LeeE, it is very helpful and answers allot of other questions I have had.
 

Offline Democritus

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The Lockheed SR71 Blackbird from Kelly Johnson's 'Skunkworks' still claims the record, held for more than 30 years, for the fastest piloted air-breathing aircraft. Mach 3 plus. Approx 3,500 kph or a little under a km per second. This is seriously fast.

If a stationary M61 cannon was fired at the tail of this passing SR71 the shells would impact the aircraft at a relative speed of approx 70 m/s. Or around 160 mph. (In the milliseconds of opportunity...)

On the other hand, the cannon shell fired head on at this approaching aircraft would impact at a relative speed of approx 2,000 m/s, or about 7,200 kph, or about 4,500 mph. I won't go into the prospect of two SR71s, each armed with an M61, each at record speed approaching each other head on firing canons. What the heck...should opposing shells collide, they would impact each other at a relative speed of approx 4,000 m/s, or 14,400 kph, or 9,000 mph.   

There's a pretty good treatment of this extraordinary aircraft here:
http://en.wikipedia.org/wiki/SR-71_Blackbird [nofollow]

Which includes this:
"A defensive feature of the aircraft was its high speed and operating altitude, whereby, if a surface-to-air missile launch were detected, standard evasive action was simply to accelerate..."
 

Offline Geezer

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Interesting plane the SR71. I saw the one they have at the Seattle air museum recently. Well worth poking around too if you happen to be near there.

They also have the Air Force 1 that was used by JFK. It even has a cabin temperature thermostat for the President to adjust if he was too hot, or too cold. Of course, the thermostat was never actually connected to anything!
 

Offline tommya300

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The Lockheed SR71 Blackbird from Kelly Johnson's 'Skunkworks' still claims the record, held for more than 30 years, for the fastest piloted air-breathing aircraft. Mach 3 plus. Approx 3,500 kph or a little under a km per second. This is seriously fast.

If a stationary M61 cannon was fired at the tail of this passing SR71 the shells would impact the aircraft at a relative speed of approx 70 m/s. Or around 160 mph. (In the milliseconds of opportunity...)

On the other hand, the cannon shell fired head on at this approaching aircraft would impact at a relative speed of approx 2,000 m/s, or about 7,200 kph, or about 4,500 mph. I won't go into the prospect of two SR71s, each armed with an M61, each at record speed approaching each other head on firing canons. What the heck...should opposing shells collide, they would impact each other at a relative speed of approx 4,000 m/s, or 14,400 kph, or 9,000 mph.   

There's a pretty good treatment of this extraordinary aircraft here:
http://en.wikipedia.org/wiki/SR-71_Blackbird

Which includes this:
"A defensive feature of the aircraft was its high speed and operating altitude, whereby, if a surface-to-air missile launch were detected, standard evasive action was simply to accelerate..."

So if the SR-71 was flying parallel to the ground, this M61 cannon where mounted on foreward section the jet, velocity of the craft is at mach 3, a round is released firing foreward.
From what you are saying that the round  will be coming out of the barrel will be traveling 70 m/s with respect to the plane? Is this an accurate deduction?
If it is a correct deduction, the bullet will only travel 8 meters in a forward arch before making its direct decent to the ground?
 
 

Offline LeeE

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So if the SR-71 was flying parallel to the ground, this M61 cannon where mounted on foreward section the jet, velocity of the craft is at mach 3, a round is released firing foreward.
From what you are saying that the round  will be coming out of the barrel will be traveling 70 m/s with respect to the plane? Is this an accurate deduction?
If it is a correct deduction, the bullet will only travel 8 meters in a forward arch before making its direct decent to the ground?

The round will leave the cannon at its muzzle velocity i.e. ~ 1050 m/s relative to the aircraft to which the cannon is fitted.  As mentioned earlier though, the muzzle velocity of the cannon should be added to the velocity of the aircraft to get the initial velocity relative to the ground, so in theory the initial velocity of an M61 projectile fired from an SR-71 would be ~2000 m/s relative to the ground.  Once again though, as mentioned earlier, it will lose this initial speed very quickly.

However, it's important to remember that this is just what would happen in theory, for the highspeed airflow around and aircraft traveling at Mach 3 will no doubt complicate matters somewhat.  Even more to the point, fitting a cannon to an SR-71 would be pointless because, iirc, its turning radius at Mach 3 is of the order of hundreds of kilometers; it simply wouldn't be able to bring its cannon to bear on anything without very quickly colliding with it.

An interceptor version of the A-12, from which the SR-71 was developed, was also trialled (designated YF-12) but it was, of course, missile armed only.
 

Offline tommya300

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LeeE
OK we are not on the same page.
Now I see why I have a problem with adding the 2 velocities.

I was looking at the bullet relative to the jet +------>
You were looking at the bullet relative to the ground -------+------>

The 8 meter is way out there that is not right...

Took a little time...
« Last Edit: 09/06/2010 03:26:26 by tommya300 »
 

Offline Democritus

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The 70 m/s example referred to is the speed, relative to the SR71, of the round fired from a stationary, yes I did say stationary, cannon toward the tail of the fleeing SR71. So relative to the ground, the aircraft is travelling at ~980 m/s while the cannon shell is chasing it in the same direction at ~1050 m/s. And catching up to it at the seriously slow (for a round of ammunition) rate of 70 m/s; but not for long, as explained by graham.d.

And of course, the SR71 was armed with nothing more offensive than cameras. Truly remarkable cameras, but that's another story... :) 
 

Offline tommya300

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Thanks Demo! It was understood the objects used was to make a point.
With me, you could of used a flying paint can a ken doll pilot and a plastic dart gun.
I kneaded to dough my brain! I was starting to think my mini stroke has more hidden entities then I physically know already.
I thinking, "not for long," means that the range of the gun is a factor, that was not needed to complicate the point.
I learned a bit more today in ballistics. It can apply to down hill high jump skiing, down hiss slide into a pool and more.
From the information given about muzzle velocity relative to range can be calculated min and max. and all between.
From .5 degree elevation to 45 degrees elevation.

I just wanted to know if it were true that a bullet fired, other than a multiple deflections, ricochet if you will, can come back on you in a certain scenario.

That is what I should of asked
« Last Edit: 09/06/2010 14:48:45 by tommya300 »
 

Offline tommya300

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70 m/s; but not for long, as explained by graham.d.

And of course, the SR71 was armed with nothing more offensive than cameras. Truly remarkable cameras, but that's another story... :) 
Pretty much remarkable stuff.
 Flying Tele-Microscopes in infrared and technocolor, panavision.
Now all they need is to do it with sound. The technology may exist already
 

Offline LeeE

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Just to add that as the SR-71 is no longer flying, the fastest currently flying aircraft are the MiG 25 and MiG 31, a MiG 25P being tracked by radar at Mach 3.2 whilst flying over Sinai in the Early 1970s.  It is believed that this caused irreparable damage to its engines though, which had to be replaced.

The MiG 25 has had a relatively successful combat history, managing to evade pursuers, shooting down a number of hostile aircraft and forcing others to abort their mission due to the threat they presented.

http://en.wikipedia.org/wiki/Mikoyan-Gurevich_MiG-25#Operational_history
 

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