Distance setting in TDC useless?

[Hylarion]

Hi all,

I am still wondering what's the use of setting the distance target in the TDC (I play 100% realism btw)?
From what i understand, it has no effect at all on the solution (for math addicts, it is based on Thales theorem , for others, just change the distance setting, you'll see it has no influence at all on the gyroangle solution).
Would it be somehow related to the max time the torpedo runs before it triggers automatically its pistol? So far, the best use i've found of this is to get an estimate of the expected time to impact (the red needle on the chronograph)

Just wondering if someone has a clue...

[Canovaro]

If you are firing on a 90 degree angle towards the target, then the range does not matter because it is a triangle with a straight corner.

[Grayson02sept1980]

well the distance is important... as it indicates how much the target will move until the torpedo reaches the calculated point of impact... and vixse versa: To get the correct point of impact end all the angles you need to know the angle of the bow and the distance combined with the speed...
distance can not be ignored

[joegrundman]

To develop Canovaro's post.

If you fire on a 000 gyro angle such that the you will hit the target at an AOB of 90, then range is irrelevant, however the further you are from this ideal situation, the greater the significance range has.

This is because the periscope view and the torpedo tubes are not at the same place.

Suppose you wish to shoot at a target that is on a periscope bearing of 90, and the target is running parallel to you.

When you fire, first of all the mouth of the tube is say 30m ahead of you. Then the torpedo runs straight ahead until it has cleared the submarine, say another 20 m, and then it starts to turn. the turn circle has a radius, so after all this, in order to meet the point you were aiming at, it has to head at quiite a different angle from the direct line of sight, possible even with a gyro larger than the bearing to target.

At short ranges this will obviously be more noticeable than at long ranges

[Hitman]

http://www.subsim.com/radioroom/showpost.php?p=639851&postcount=1

:know:

[Hylarion]

Indeed, there is a notable adjustment in the drawn trajectory on F6 screen. I never saw it since playing without any map update kind of nullifies the use of this screen (for me at least).
That explains why i missed some "easy" shoots i guess...

Thanks a lot for the help.

[Tomcattwo]

If you are perfect at matching bearing rate, then knowing the correct range is irrelevant (other than to know that the target is within weapons range at time of fire). However, for the REST of us mortals, the range to target at time of fire is VERY relevant. Here's why:

To hit a target, you must generate a torpedo solution that results in the torpedo closing the target on a constant bearing. You do this by matching the target's beraing rate (the rate of change of bearing to the target from the torpedo with respect to time) with a closing range rate (the rate of change of range between torpedo and target with respect to time).

You can match the bearing rate of the target by matching the speed across the line of sight of the target with torpedo speed across the LOS at time of fire. As long as the bearing rate is matched, the range will not matter - it just takes longer for the torpedo to hit the target at a longer range.

HOWEVER, if there is the least little bit of error in your bearing rate match ( because AOB is off slightly meaning target course is incorrect, incorrect target speed input (those are the two generally biggest errors), tube parallax, gyro inaccuracy, timing, torpedo speed inconsistencies, seastate effects on torpedo speed, and tens of other factors), having the range be off MAGNIFIES the effects of your bearing rate error. Let's say, for instance that you shoot and you have a 0.3 degree per minute bearing rate error at time of fire. At 500 yards' range, and the speed your torpedo is going (let's say 30 knots, where it'll cover that 500 yards in just 30 seconds) that bearing rate error is inconsequential. However at 3000 yards, (where it takes the torpedo 3 minutes to get to the intercept point) that bearing error rate may be enough to make you miss (ahead or astearn depending upon the direction of the bearing rate error).

A 1 degree arc subtends 33.3 yards (100 feet)(approximately) at 1 NM (2000 yards). That means at a mile and a half range, your 0.3 degree per minute error in 3 minutes yields an arc error of about 1 degree, or 1.5 x 33.3 yards = 50 yards or 150 feet. Depending on the length of the vessel and where you aimed (let's assume it's a 200 foot long vessel and you aimed amidships and its AOB is broad) you just missed! (in our example, 50 feet ahead or astearn depending on the direction of the bearing rate error)

Moreover, your fire control solution depends on all three factors being accounted for when the bearing rate determination is made. For instance, lets assume we are 2000 yards from a target and we are at all stop - so we are contributing nothing to the bearing rate of the target - all bearing rate change is a direct result of the target's motion. If the target is at a 90 degree angle on the bow, all of the target's speed is across the line of sight (xDMHt = Tgt Speed (DMHt) x sin (90) = DMHt x 1 = DMHt. If the target is going 10 knots at 2000 yards, his bearing rate will be:

Bearing rate (DBy) = (approx) xDMHt x 2000/ Range (Rh) =(approx) 10 x 2000 / 2000 = 10 degrees/min

However, at 6000 yards range, what is the target's speed across the LOS for the same 10 deg/min beraing rate?

10 deg/min x 6000 / 2000 = xDMHt = 30 knots!

(By the way, this is an excellent thumbrule to keep in your head: if you are not moving or are contributing no speed across the LOS, then DBy (Bearing Rate) = 1 degree per minute at 1 knot target speed across the LOS (xDMHt) at 1 NM (2000 yards) Range (Rh) - it's called a Single Leg Range)

Point is that fire control geometry is a result of the complex geometry associated with ownship motion, target motion AND the range between target and ownship. It's about angles and distances and times. That is why machines were made to compute this complex geometry. If you have an incorrect range in the machine, and your course and speed are perfect, your bearing rate cannot be perfect, and therefore your match to bearing rate WILL of necessity be incorrect (how incorrect it is depends on the magnitude of your combined errors and in which direction they lie). Since the fire control computer uses bearing rate to match torpedo geometry against, it will negatively affect your firing solution. Inaccurate range entries WILL cause you to miss the target if there are any errors at all in your course and speed.

In fact, your entire purpose in tracking target motion is to generate a solution with the least amount of error in these three variables in order to provide the best chance to properly match bearing rate and put your torpedo on the target. This is a real acquired artform when shooting dumb (i.e., non-acoustic) torpedoes such as
those used in WW II. You really have to be pretty close in all three variables to get a hit, and you minimize the effect of compunded errors by reducing the time of travel from torpedo to target (by shooting from close range) and shooting with as small a gyro angle as possible (to minimize tube parallax and time-to-turn errors).

See these links for some more info:

http://www.subsim.com/radioroom/archive/index.php/t-82367.html

http://www.haptonstahl.org/ppt/tma_files/frame.htm

Oh and also what Hitman said above :)

R/
TC2

[Zantham]

I think I need more fingers and toes to figure all that out :oops:

[Hylarion]

Thanks for all those inputs.
I felt this precision in measurements issue quite rapidely while playing although i didn't spend time in the math calculation of the impact of measurement errors (and now, since you nicely did it for me, thank you for saving my time).

In order to be as precise as possible in my firing solutions, I try to self-discipline and follow this routine:
I shadow my target for at least 1/2 hour IRL in order to get the most accurate bearing and speed measurements achievable (specially in rough weathers where the boat can move sidewise due to the roughness of the sea and set apparent bearing way off if the measurements are taken too close from each other).
I use a chronograph for my time measurements (too much pain in using the F3 screen chronometer and switching back and forth F3 F5), then the included nomograph for speed calculations (20 mns of time IRL gives a pretty good speed accuracy)
Then the classical outflanking with regular hydrophone checks vs theorical position estimations to detect course changes, and 90° approach against my calculated target bearing.

And then, i never try to fire at a gyroangle above +/-15° (usually 10° gyro max).

Using this method, I get quite a good accuracy in my hits up to 2500 meters (still in june 41, so no fancy torpedoes yet) :arrgh!:.