Author Topic: weapon comparison (velocities) (Read 519 times)

niklas · « **on:** July 03, 2001, 04:10:00 AM »

Hi

I did a weapon test, basically to get the speed of the shell. I measured the time the shell needs to fly through several sectors. I used filmrecorder/replay in slow motion and a view like in the top left of the following picture. The trajectory can be seen in the 4 similar pictures, and at the end the result.
The times are of course not 100% exact, but the trend is obvious.

The result:
Already at the beginning, the shell of a hispano and .50 Browning seems to fly twice as fast as the shell of a mg151 or mg131. While the german shells slow down, the other keep their speed almost constant, so after ~700yards they fly 3 times as fast.
For a ~700y shot, the german shells need twice the time to reach the target (at least)

Pls have a look at the trajectory pictures. You can see that the 109 shoots, sitting on the runway, a more flat trajectory compared to the Hellcat (.50) or F4u-1c (Hispano). That means, because i took the time for the projection of the trajectory on the ground (like a shadow) that the way for the later ones was longer. So the difference if even a bit higher!

Something is definitly wrong with the speeds of the shells imo.

niklas

[ 07-03-2001: Message edited by: niklas ]

Tony Williams · « **Reply #1 on:** July 03, 2001, 07:07:00 AM »

I have been doing some research into this recently. Most information is available on the German guns, from which I have calculated the following:

MG 131 34g HEI loses 33% of muzzle velocity by 300m, remaining velocity 501 m/sec.

MG 151/20 92g M-Geschoss loses 38%, velocity 432 m/sec

MG 151/20 117g HET loses 23%, velocity 552 m/sec

I have no comparable figures for Allied ammo but can extrapolate:

Hispano 130g would be better than 117g, so say 18-20% loss, velocity 700-720 m/sec

.50" was better than Hispano because of superior aerodynamic shape, say 12-15% loss, velocity 750-775 m/sec.

Tony Williams
Author: "Rapid Fire: The development of automatic cannon, heavy machine guns and their ammunition for armies, navies and air forces"
Details on my military gun and ammunition website: http://www.users.globalnet.co.uk/~autogun/index.htm

[ 07-03-2001: Message edited by: Tony Williams ]

Toad · « **Reply #2 on:** July 03, 2001, 08:46:00 AM »

Tony,

By any chance do you have the Ballistic Coefficients for the various rounds?

If so, you can easily get time of flight and complete energy/trajectory/velocity data out of online ballistic computers. ( http://internet.cybermesa.com/~jbm/ballistics/calculations.html )

The exact BC's are pretty hard to find but if you have engineering drawings of the projectile there are also BC calculators.

Thanks!

[ 07-03-2001: Message edited by: Toad ]

niklas · « **Reply #3 on:** July 03, 2001, 11:37:00 AM »

Toad, i´ve seen a 0,457 number for the BC value of a hispano round.

If you use it in this calculator you will see that after 750yards it should have lost about 40% of the speed. But it doesn´t here in AH. And the initial velocity of the mg151 shell seems to be a bit to slow, but i can´t measure the muzzle velocity.

The hispano round should slow down much more. MUCH more! And something is wrong with the MG151 imo

niklas

minus · « **Reply #4 on:** July 03, 2001, 12:48:00 PM »

well ,seems the LW guns are more acurately modeled like others , so i em a Lw wenie whiner and not a arcade score potato

Hooligan · « **Reply #5 on:** July 03, 2001, 12:49:00 PM »

Niklas:

Ballistic coefficients are only valid for a given ballistic equation. So 2 rounds which have BCs of .5 and .6 for the G1 equation might have BCs of .2 and .3 for some other equation. Saying that a round has a particular BC is essentially meaningless unless the equation is also specified (G1, G2, etc...) Do you happen to have the source that specified the BC you reference? Does it give any more information? If you have trajectory data for the round in question I can easily calculate a G1-BC. FYI I do have trajectory curves for many German rounds and some .50 rounds. The G1-BCs for the 131 and 151 are generally in the range of .2 to .5, while the G1-BCs for .50 rounds are in the vicinity of .7. I have been told that Hispano rounds have similar (but slightly inferior) BCs to .50 rounds. Sadly I don't have the Hispano trajectory information that would allow me to calculate G1-BCs.

Hooligan

Pyro · « **Reply #6 on:** July 03, 2001, 03:11:00 PM »

Hi Tony,

I've been meaning to tell you that I picked up your book awhile back and really enjoyed it. It's definately a must-have book for those interested in the subject. Anyway, here's some data on the subject that you're looking for.

niklas · « **Reply #7 on:** July 03, 2001, 03:36:00 PM »

Hooligan - I tried it with every equation given there. Most of them produce almost identical results, 2 of them produced results which were even worse.

Anyway, if you look at Pyros charts then you can easily see that the .50 and 20mm Hispano looses REALLY approximate 40% of the muzzle velocity after 800 yards.

Pyro, when you have so charts, why don´t you model it this way??

I mean the current .50 and 20mm hispano shell has almost no tendency to loose speed. Something is really wrong here.

Maybe you find the time to have a look into it

thx
niklas

Toad · « **Reply #8 on:** July 03, 2001, 03:39:00 PM »

Pyro &/or Tony,

Would it be correct to assume these charts are at Sea Level or somewhere around 1000' Pressure Altitude?

I ask because I think it's important for all to remember that as you raise the altitude, atmosphere decreases in density. For example, at 15,000 feet atmospheric density is about 62-63% of Sea Level.

Or am I off target on this?

Range /Ballistics Calculator/ Charted

500 / 2228 / 2210 ~ Equal
600 / 2151 / 2100 ~ 2% increase @ 15k
700 / 2075 / 1970 ~ 5% "
800 / 2001 / 1875 ~ 7% "
900 / 1928 / 1745 ~ 9% "

It would seem that intially the lower air density doesn't affect it that much when comparing the ballistic calculator vs the chart but as range increases it becomes more of a factor.

Just curious.

BTW, Niklas, I ran the 20mm AP 75 numbers through the calculator and it shows the BC as follows for that bullet:

BC (Velocity) Output

--------------------------------------------------------------------------------

Input Data
Near Velocity: 2610.0 ft/sec
Far Velocity: 2310.0 ft/sec
Distance: 600.000 feet
Temperature: 59.6 °F
Barometric Pressure: 29.92 in Hg
Relative Humidity: 0.0 %
Altitude: 0.0 feet
Air Density: 100 % of Sea Level

--------------------------------------------------------------------------------

Output Data
G1 Ballistic Coefficient: 0.582
Time of Flight: 0.244 sec

[ 07-03-2001: Message edited by: Toad ]

Tony Williams · « **Reply #9 on:** July 04, 2001, 12:35:00 AM »

Thanks for publishing the charts, Pyro, I've been looking for that kind of data for ages! I'm pleased you like the book; don't forget to check my site from time to time for the Updates page.

It seems that my estimate for the .50" wasn't far out; the MV was slightly lower than I had assumed, the velocity at 300m looks to be just under 750 m/sec. The 20mm AP isn't strictly comparable, of course; it was significantly heavier than the HE and had a different nose shape.

I wouldn't worry too much about minor variations in ballistic performance. In real life, there were considerable differences depending on the ammo batch, the weather (ambient temperature affected the propellant) and of course the gun. I have a WW2 report of British firing tests of the MG-FF. The first gun they tried delivered only 1,600 fps instead of the 1,900+ they'd been expecting. It turned out that the barrel was worn. So they retested with a newer gun which did hit 1,900, but it didn't make any significant difference to the test results. Random factors such as exactly where the projectile struck had far more effect.

Tony Williams
Author: Rapid Fire - The Development of Automatic Cannon, Heavy Machine Guns and their Ammunition for Armies, Navies and Air Forces.
Details on my military gun and ammunition website: http://www.users.globalnet.co.uk/~autogun/

niklas · « **Reply #10 on:** July 04, 2001, 11:03:00 AM »

Toad, the real important thing is that the chart is for an AP round.

HE rounds have a higher muzzle velocity and are lighter. Furthermore the nose section is usually worse compared to an AP round for supersonic flight (fuze)

So for a HE shell we can say:
Due to the nonlinear characteristics of drag in relation to speed, a higher muzzle velocity means more drag

20% less mass means 20% higher tendency to slow down

Worse aerodynamics due to the different nose section.

Put all together and you can estimate that the HE round has a significant higher tendency to slow down than this AP shell.

And this is my only point. The AH-20mm Hispano shell doesn´t slow down enough, like the AH-.50. The muzzle velocity seems to be ok. But why do you think they´re considered to be lasers by so many people?

niklas

niklas · « **Reply #11 on:** July 04, 2001, 11:04:00 AM »

double post deleted

[ 07-04-2001: Message edited by: niklas ]

Pyro · « **Reply #12 on:** July 04, 2001, 11:26:00 AM »

Niklas,

Here is a screenshot taken in a debug version that is outputting hit information. This is an M3 halftrack so I'm firing the .50 M2HB with 45" barrel. The range to target is exactly 800 yards. Impact speed is 1920 fps. Tell me what's wrong.

F4UDOA · « **Reply #13 on:** July 04, 2001, 11:41:00 AM »

Pyro,

I think if you package your debug software and sell it as ballistics test software you could probably make a killing. I know I wouldn't mind tinkering with that program a little.

Just a though

niklas · « **Reply #14 on:** July 04, 2001, 02:00:00 PM »

Hi Pyro

nice debug info - i just tested the F6F against the M16 with the same method (i shot with an similar angle like the F6F does up in the sky)

The result: For the whole way (~800y) the times in slow motion were:
M16: 17
F6F: 13
The absolut times are not interesting, because it was slow motion, but nevertheless
the .50 of the F6F needs 25% less time (though it has the shorter barrels??). Pls do the same debug test with the aircraft .50
It was visible without stopping the time that the tracer of the F6F flew way faster. Is it only a problem with the tracers? I don´t know, maybe.

regards
niklas