Using time of flight to determine velocity

[dittybopper]

OK, so I've been playing with Lock'nLoad for a few days, trying to figure some at least reasonable ballpark figures for things like range, and it struck me that I really have no idea what the muzzle velocity of my mortar is. 


I'm not really all that excited about trying to convince my father that I need to borrow his chronograph in order to do it.   I don't feel like buying him a new one when I inevitably blow it to smithereens due to some small misalignment between it and the bore of the mortar.


My idea is that the next time I shoot it, I accurately measure the time from firing to impact.  Since I'll know the barrel elevation to the nearest degree, and I know the weight of the projectile (7000 grains, +/-), and I know the shape and dimensions of the projectile (blunt cylinder, diameter 2.9"), I can calculate the BC fairly accurately.  I won't really care about the range at first, all I'm interested in is time to impact.  That should allow me to use Lock'nLoad to find a velocity, given the known parameters, that matches the time of flight that I observed.  I'd just keep adjusting the velocity until the time of flight matches.  Given that, I can then set about generating a reasonably accurate set of elevation X = range Y with charge Z tables


Of course, I'm going to have to average several shots with closely matching projectiles and powder charges, and I'll have to measure the temp, pressure, humidity, elevation above sea level, and latitude at the time of firing.


So, is my thinking off base here, or is this a sound method to get a reasonable estimate of muzzle velocity?

[GGaskill]

You may get order of magnitude values but unless your projectile files nose on, it will have a varying BC that will be hard to determine.  I would try a nearly horizontal trajectory and time from blast to first bounce and measure the distance from muzzle to first bounce, then divide the distance by the flight time.  I think that would be closer to reality than trying to do calculations on a high trajectory.

Another alternative would be to stand perpendicular to the line of flight with a digital camera and take a movie of the flight.  Set up a row of stakes parallel to the line of flight and see how far it goes between frames.  Divide the frame rate into the distance for velocity.  That will give a more instantaneous measurement of the velocity.

[dittybopper]

The projectiles do fly in a stable, nose-first attitude.  I've observed them doing that.  This is what they look like:





It's a standard 14 oz size tin can half-full of cement, and they weigh just about a pound.  I load them nose first, because when I loaded them tail first it sometimes "blew out" the skirt.  By direct observation, they are very stable in flight.   They must flip relatively quickly into a nose first position soon after firing, because for the entirety of the observable trajectory they are very stable, unlike similar cans when loaded tail first (tumbled and spun due to uneven blown-out skirt), or the same size cans full of cement (weight 2 lbs) which tumbled, even when a significant hank of rope was attached to a screw-eye imbedded in the cement.  I say "observable trajectory", because I have yet to see the projectile anywhere near the muzzle.  I usually acquire it right near the top of the trajectory, and I can follow it all the way to "splashdown" in the pond I'm using as my range. 


They are like huge metal and cement shuttlecocks, and they are very stable.  I'm pretty sure the BC will be accurate enough.   I might be off by a few fps, but I can't think of a more convenient way, given that I can't necessarily accurately measure the distance on a pond unless I have multiple observers taking compass bearings on the impact splash.


I have thought about improving the BC somewhat by taking a good solid piece of wood and carving a concavity in the same diameter as the cans, and by making a corresponding round nose wooden "hammer" to shape the bottom of the cans into a more aerodynamic shape.  This wouldn't work with the cans that have a separate bottom piece, but I think it would work well with the cans that are stamped out of a single piece of metal with a separate lid.   At this point, though, I want to know what it's doing with what I have now.

[keith44]

Ok, velocity is simply feet per second so 1,200 fps (a likely velocity of any black powder arm)


a stop watch is not accurate enough given the human operator, so something to start and stop the clock other than the human finger.  You might as well just count.


so feet measure off a distance...wait a minute you said "mortar" time of flight will not even get close due to the highly arched trajectory time of flight needs a nearly flat trajectory to get close to velocity calculations

[dittybopper]

Time of flight is something on the order of 7 to 9 seconds.  Taking the average of several shots should minimize any errors, but even if I'm off by half a second which seems like a lot, that would only change the muzzle velocity by about 20 fps.   Given my super-human reaction time, I should be able to time it (using a stopwatch, of course) within a tenth of a second.  That would equate to a velocity difference of less than 5 fps.


A more accurate way would be to simply video the firing and impact (I've been shooting into a pond, and the impact is an obvious splash), and base the timing on that.  I'd just set the video camera on a tripod behind the mortar, in line with the bore, and I could simply count frames from ignition to impact, which should be accurate enough:  Frame rates are something like a minimum of 24 frames per second, so that would more than double the accuracy of using a stopwatch.


In the end, though, all I'm looking for is a set of parameters that replicates the observed performance, then using them to extrapolate to other data, at least until I can find a range where I can safely shoot and accurately measure the distance at different elevations/powder charges.

[Double D]

I think if  you  search for all post by  member Parrott-cannon and read all 181 of them you will he as already done this and has even built a "ballistic calculator for cannons."  You might start here http://www.gboreloaded.com/forums/index.php/topic,209402.msg1099112159.html#msg1099112159


One of the first things you learn is the 1400 fps is way way of for cannons and mortars

[dittybopper]

I think if  you  search for all post by  member Parrott-cannon and read all 181 of them you will he as already done this and has even built a "ballistic calculator for cannons."  You might start here http://www.gboreloaded.com/forums/index.php/topic,209402.msg1099112159.html#msg1099112159


One of the first things you learn is the 1400 fps is way way of for cannons and mortars

Yeah, that's what I've been playing with, Lock'nLoad is the name of his software.


I've been using velocity values of 250 - 300 fps, which seem reasonable given my loads (~400 grains Fg, 1 lb projectile).  Don't know where the 1400 fps came from.