How muzzle velocity is affected by altitude??

[saturno_v]

If we have the same rifle and cartridge, one fired at sea level and the other at 15.000 feet of altitude, the different air density and temperature will affect muzzle velocity???

Usually altitude affects aircraft max speed performances.

For example, an F-16 at 30.000 feet can reach almost double the speed compared to sea level....is the same for bullets??

I know that air density affects trajectory but I could not find anything that mention the muzzle velocity.

Thanks!!!

 

[Ridgerunner665]

Muzzle velocity is not affected... Aerodynamic drag only affects the bullet after it leaves the barrel.

Since you mentioned an F-16...I always thought it was cool that a bullet fired from an aircraft has a muzzle velocity that is made faster by the speed of the aircraft....example...a 50 caliber Browning mounted on an old aircraft had a muzzle velocity of 2700 fps (<<< thats 1841 mph)...plus the 400 mph speed of the aircraft....for a total muzzle velocity of 3,287 fps.

 

[R.W.Dale]

I don't think internal ballistics and muzzle velocity would be affected. External ballistics on the other hand are an entirely different matter. Most ballistics software allows you to take altitude into account

 

[ArmedBear]

Internal ballistics would probably be affected, but so slightly it would hardly be noticeable. The ambient pressure would be lower, so the pressure in the barrel would be a bit lower. So, the powder's pressure would have a bit less to work against.

Now, we're talking about tens of thousands of PSI from the propellant vs. 14 or 15 psi for atmospheric pressure, so the difference is pretty small at the muzzle.

 

[Ridgerunner665]

A loaded round should be airtight...so the pressure inside the round wouldn't be affected...it would be the same as the pressure of the atmosphere at the time it was loaded.

I realize that some...maybe even most rounds may not be airtight...but they should be air/water tight.

 

[taliv]

he's talking about the air in the muzzle. the propellant has to push that air out of the muzzle for the bullet to pass through the barrel.

 

[Ridgerunner665]

he's talking about the air in the muzzle. the propellant has to push that air out of the muzzle for the bullet to pass through the barrel.

Ahh...I see what you mean...that wouldn't be a noticeable difference in MV though...barely measurable I would think...but I may be wrong.

 

[taliv]

yep, probably why he started that post with "would probably be affected, but so slightly it would hardly be noticeable."

 

[OFT]

I think that the temperature at a high altitude would slow the combustion of the powder somewhat. That's likely the only affect that you would notice.

 

[ArmedBear]

Sierra Bullets has a long treatise on variations in MV and chamber pressure.

The low temperatures incidental to high-altitude hunting vs. the high temperatures incidental to low desert shooting and plinking ranges make a significant difference in MV. This can also lead to dangerous situations, if you leave a maxed-out hot hunting load in a hot car in the sun, then shoot it. What's safe at STP becomes unsafe when heated up.

 

[Zak Smith]

I recommend using a powder that is temperature-stable, and then just not worrying about it. MV won't be appreciably affected by station pressure.

 

[Kind of Blued]

The fighter jet mention reminds me of an episode in 3rd grade in which I argued with a lad who was telling me that fighter jets use "special boolitz" so they don't run into their own gunfire.

 

[N1150X]

I think a 10 C change in temp would have a bigger impact on MV than a pressure change of 14 psi

A jet has a continuous supply of thrust while a bullet has an almost instatanious supply of thrust.

I think this is why the jet can fly so much faster higher and I think the bullet will be almost unaffected

 

[AndyC]

...an F-16 at 30.000 feet can reach almost double the speed compared to sea level....is the same for bullets??

The speed of sound is slower at a higher altitude, so it's easier to hit Mach 1, 2, etc at altitude. It's not because the aircraft can do double the speed (although I'm sure the aircraft itself can achieve a slightly higher speed because of less air-density at altitude).

 

[saturno_v]

The speed of sound is slower at a higher altitude, so it's easier to hit Mach 1, 2, etc at altitude. It's not because the aircraft can do double the speed (although I'm sure the aircraft itself can achieve a slightly higher speed because of less air-density at altitude).

Yes actually a jet aircraft can reach a significantly higher absolute speed at high altitude, indipendently from the Mach number which is a function of the air temperature as well, as you mentioned.

For example 2310 fps (1400 knots) are Mach 2 at sea level.
But at 30.000 feet the same speed is Mach 2.4

For example, an F-16 in "clean" configuration (no external loads) can reach 915 mph at sea level (Mach 1.2 at sea level) and 1500 mph at 30.000 feet (Mach 2+).

In this case the absolute maximum speed (indipendently from the mach number) increases significantly at altitude because of less air resistance and better efficiency envelope for the engine and the mach number almost double because Mach 1 at sea level is actually an absolute higher speed compared to Mach 1 at 30.000 feet.

 

[Ignition Override]

Yep, and one result is that many jets can not take off from higher airports such as Denver on a cold day without sometimes leaving passengers or a bunch of bags/cargo at the gate, due to the required fuel (weight), or last minute fuel increase ( wx etc).
The thinner air with higher density altitude makes it harder to comply with 2nd segment climb requirements if an engine fails: a minimum of a 2.4% climb gradient is always required. Not only is there less thrust produced at takeoff power, but less lift under the wings.
Many use 5*, instead of 15 degree takeoff flaps, creating much higher V1, Vr and V2 speeds.

Even at Kansas City, MOT or RAP on a hot day. And Kansas City is just 1026' MSL (above sea level).

 

[saturno_v]

Ignition

At high altitude airports the problem is worse with hot temperature not cold...cold air is actually denser than hot air and it improves wing lift.

Infact this condition in aviation is called "hot and high" when you sum up less air density because of altitude and temperature...one of the worse airport in that regard is Johannesbourg in South Africa....very hot and very high....

So at Denver airport, an airplane take off in a typical January day is less challenging than a typical July or August day.

A bullet doesn't care about lift, it doesn't have wings.. so less dense air offers less aerodynamic drag....

So when air density decreases (because of heat or altitude or both), aerodynamic drag decreases too and wing lift efficiency (for airplanes) decreases as well...

 

[RecoilRob]

Many supersonic aircraft are limited at low altitude by airframe stress requirements and not by power. You cannot fly full throttle @ sea level without doing damage to the airframe.

I used to work where the SR-71 engines were built. At altitude and max speed, the nose of the aircraft was red hot back to the RSO canopy. Maintaining speed and dropping altitude (which it could do...plenty of power available) would have overheated things and caused failure.

Bullets don't have the thermal limitations of aircraft and can be driven really fast @ sea level.

 

[saturno_v]

Many supersonic aircraft are limited at low altitude by airframe stress requirements and not by power. You cannot fly full throttle @ sea level without doing damage to the airframe.

I used to work where the SR-71 engines were built. At altitude and max speed, the nose of the aircraft was red hot back to the RSO canopy. Maintaining speed and dropping altitude (which it could do...plenty of power available) would have overheated things and caused failure.

Bullets don't have the thermal limitations of aircraft and can be driven really fast @ sea level.

It is true, many extremely high performance aircraft at low altitude are limited by the frame stuctural limits rather than the power of their engines..the SR-71 is definitely one of them.... we should rather say the King of them

Actually, if I remember correctly, the SR-71 was never flown at full throttle even at high altitude.....full power was used only to accelerate to certain speed regimes then the engines were throttled back to 40% or so..

Some more "normal" jet fighters (for example the F-16 and F-15) can be pushed to full throttle at low altitude..their engines are not that powerful to compromise their frame structure at least in straight and level flight...

Bullet construction does not take in consideration aerodynamical thermal limits because of the extremely short flight time, however, in very long range shooting, when flight times are in the order of few seconds, materials thermal behaviour come into play for high quality match bullets.

 

[General Geoff]

Maintaining speed and dropping altitude (which it could do...plenty of power available)

 

[RecoilRob]

There was a shoulder patch seen in the shop that said "Habu....Mach 5+" and I'd heard talk that the plane could sprint that fast if needed. But, dealing with anything Military one needs to be cautious about published numbers as they rarely tell anything other than what the publisher intends them to. Not that they would lie, just not want to let all the cat out of the bag.

Another consideration regarding engine performance @ sea level is the ability of the inlet diffusion to slow the air down for consumption. Not much reason to go full-out at extremely low altitude.