7 replies to this topic

[barnsley-mark]

I've been looking for the formula to calculate the height of a shell by timing it from firing and back to ground.
It looked easy at first but then I got lost with acceleration, deceleration, transition from upwards to falling and then terminal velocity.
That's without looking at shape and mass etc.
Is there an easy formula, and am I over complicating this?

[portfire]

Shellcalc

 

http://www.jpyro.com/wp/?p=1443

Edited by portfire, 16 January 2013 - 05:45 PM.

[barnsley-mark]

That's ok for a trajectory, but I'm looking at straight up and down so the shell actually changes from forward to reverse direction whereas a trajectory continues in one direction.
Plus I'm just looking for a simple timing to gauge my shell height, or any shell height.
Maybe I'm looking at this far too technically - what method does anyone else use .. Ie for testing purposes or for length of time fuse etc?

Edited by barnsley-mark, 17 January 2013 - 09:53 PM.

[barnsley-mark]

Thank you, that's on the lines I was thinking, I was trying to work it from firing, to save tracking its flight.
You have me thinking of a 3 way timer now to get some data and maybe come up with a simple graph for others to use ..........

[starseeker]

That's ok for a trajectory, but I'm looking at straight up and down so the shell actually changes from forward to reverse direction whereas a trajectory continues in one direction.
Plus I'm just looking for a simple timing to gauge my shell height, or any shell height.
Maybe I'm looking at this far too technically - what method does anyone else use .. Ie for testing purposes or for length of time fuse etc?

For a 75mm shell you would be looking at a 1.5" time fuse or there abouts,a quarter inch each end for cross matching and then a inch of delay for your approx three second delay to get you to your 300ft. A 100mm shell i would add another quarter inch to that.

[whoof]

this is whart i used when i used to test my bp for the first time. about
 
"If you drop an object and time its descent to the ground, the distance the object has fallen, in feet, is given by the equation, Distance = 16 x time x time (16 x time squared), when the time is measured in seconds.
For example, if I fire my baseball, and start a stopwatch when its flight peaks out at apogee, and then stop the stopwatch when the ball hits the ground, I'll be able to read the time it took the ball to fall to the ground from that peak. Let's say that my stopwatch indicates a time-of-fall of 4.18 seconds.
 
Timing the Fall of a Dummy Shell

To see how high the baseball was when it started to fall (at apogee), all I have to do is multiply 16 x 4.18 x 4.18 and I get a height of 279.55 feet. That's pretty close to my desired 300 feet. So I know that using the amount of lift powder that I used, or maybe just a tad more, would be a good quantity of that BP to use in the future for this size and weight shell."

It's a good approximation up to about 200 feet I think.
After about 2.secs it reaches 20 m/sec and after 3 ' 30 m/s.
However the terminal velocity of a baseball sized object is about 36 to 40 m/s so after approx 3.5 secs it will fall at a constant speed.

 

edited figures due to brain fade

 

Iv done somemore sums and the error becomes 15 % at around 450 feet so its not a bad approximation  after all unless you are firing monsters. 

Edited by whoof, 19 January 2013 - 11:31 PM.

[digger]

I have done this calculation before, to include drag etc etc etc. If you want a copy of the spreadsheet please pm me your email address. The sheet gives you height, time to apogee, muzzle velocity etc etc.

 

G

 

P.S. No it is not a simple v=u+0.5at^2 jobbie. It needs drag as a function of reynolds number etc. the best way to solve the differntial equation is by itterative methods.

Edited by digger, 23 January 2013 - 06:38 PM.