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FT2/SS · 04-11-07, 06:03 PM

That article is interesting, lots of stuff well before my time hehe. But it seems to be in reference to firing multiple torpedo's down a common gyro angle. Lots of old school weapons employment but little of value for TMA purposes there. The modified 3 minute rule works though, and its pretty simple to do, even mentally:

(Difference in range in 3 minutes / 100 +/- cos(OSAoB)OSspeed) / cos(AoB) = Speed

Will explain with an example incase anyone has trouble understanding what I typed, or wants to try it.

Code:

not to scale :P (excuse the crappy ASCII drawing, just installed vista x64 and have yet to install Photoshop.)

Ok for above the bottom line represents your sub and the direction he's pointing, lets say the contact bears 030 relative, that makes your angle on the bow to him 30 degrees (simple enough, if he was 270 it would also be 30 obviously). Lets say he has an 70 degree angle on the bow (he's up top). Also lets say we've worked out a range of 10000 yards via periscope ranging. Also lets assume ownship's speed is 10 kts.

First take an observation with the stopwatch, then you clock 3 minutes with the stop watch immediately afterwards and take another observation.

Take the difference in range (lets say 1000 yards) and divide by 100 giving you 10 kts.

Now you take that 10 kts and subtract it by the cosine of our AoB and multiply by ownships speed, which is (10 * .8ish) or 8kts, so you have 2 knots remaining. So take those 2 kts and divide them by the cosine of 70 (about 1/3) to get 6kts, which is his speed.

Couple of things to note here though, you are better off taking a rough average of the angle on the bow calls between the first and second observation, as range rate is determined over a broad period of time. Also if your pointing away from him or he is pointing away from you you need to adjust so that it factors your speed out of the equation (your both pointing in the same direction, ie. both pointing towards the top or the bottom of the LOS diagram / TDC display, add instead of subtract). Also this will not work with a contact that has all his speed across the line of sight (Port or Stbd 090), loses accuracy as angle on the bow aproaches P/S 90, and will not work right for most zero bearing rate solutions (just set him to ownship course and speed and shoot on a zero bearing rate contact anyways, you'll hit).

Also if you want to be able to do sines real fast mentally there's an easy trick:

for 0 - 40 degrees
x / 60 = sin(x)

for 40 - 74
(x + 25) / 100 = sin(x)

For anything 75 or greater sin(x) is essentially 1

To find cosines just do sin(90-x)

Anything beyond the tenths value for this purpose is pretty much discardable, firing solutions are just a rough guestimation anyways.

04-11-07, 06:03 PM	#4
FT2/SS Swabbie Join Date: Apr 2007 Posts: 5 Downloads: 0 Uploads: 0	That article is interesting, lots of stuff well before my time hehe. But it seems to be in reference to firing multiple torpedo's down a common gyro angle. Lots of old school weapons employment but little of value for TMA purposes there. The modified 3 minute rule works though, and its pretty simple to do, even mentally: (Difference in range in 3 minutes / 100 +/- cos(OSAoB)OSspeed) / cos(AoB) = Speed Will explain with an example incase anyone has trouble understanding what I typed, or wants to try it. Code: His Target \| \|\ \| \ \| \| \| \ \| \\| \| Your Sub not to scale :P (excuse the crappy ASCII drawing, just installed vista x64 and have yet to install Photoshop.) Ok for above the bottom line represents your sub and the direction he's pointing, lets say the contact bears 030 relative, that makes your angle on the bow to him 30 degrees (simple enough, if he was 270 it would also be 30 obviously). Lets say he has an 70 degree angle on the bow (he's up top). Also lets say we've worked out a range of 10000 yards via periscope ranging. Also lets assume ownship's speed is 10 kts. First take an observation with the stopwatch, then you clock 3 minutes with the stop watch immediately afterwards and take another observation. Take the difference in range (lets say 1000 yards) and divide by 100 giving you 10 kts. Now you take that 10 kts and subtract it by the cosine of our AoB and multiply by ownships speed, which is (10 * .8ish) or 8kts, so you have 2 knots remaining. So take those 2 kts and divide them by the cosine of 70 (about 1/3) to get 6kts, which is his speed. Couple of things to note here though, you are better off taking a rough average of the angle on the bow calls between the first and second observation, as range rate is determined over a broad period of time. Also if your pointing away from him or he is pointing away from you you need to adjust so that it factors your speed out of the equation (your both pointing in the same direction, ie. both pointing towards the top or the bottom of the LOS diagram / TDC display, add instead of subtract). Also this will not work with a contact that has all his speed across the line of sight (Port or Stbd 090), loses accuracy as angle on the bow aproaches P/S 90, and will not work right for most zero bearing rate solutions (just set him to ownship course and speed and shoot on a zero bearing rate contact anyways, you'll hit). Also if you want to be able to do sines real fast mentally there's an easy trick: for 0 - 40 degrees x / 60 = sin(x) for 40 - 74 (x + 25) / 100 = sin(x) For anything 75 or greater sin(x) is essentially 1 To find cosines just do sin(90-x) Anything beyond the tenths value for this purpose is pretty much discardable, firing solutions are just a rough guestimation anyways. Last edited by FT2/SS; 04-11-07 at 06:35 PM.