SUBSIM Radio Room Forums - View Single Post - target speed: the eighty-ten method

Pisces · 08-10-10, 07:44 PM

I'm sure we are all very curious about your video. But I'm pretty sure we are not waiting to download 60 Gigabytes!!

for it. Surely there must be a way to compress it.

Your idea has merrit, however I think fixing your mind on a target AOB of 10d and a target speed of 11.3 knot if your sub is going 2 knots, well ... is just a too special case to be usefull in practise. If you manage to get the bearing to target steady over a long period, then you only know you are in pure intercept. However he could just as easily have had an AOB of 30d when moving at 4 knots. Or have 20d AOB when moving at 6 knots. And add to that the possibility of the target moving away, so the AOB could also be towards his rear.

Sure, you say, but the hydrophone operator can tell us what speed range he is in, or if he is moving away rather than closing to us. That helps narrowing it down. Well I really doubt if that could be done so quickly as in the game, the instant he hears it, or that he really could be so sensitive in real-life. What do I know, I have no real naval experience, I just find it hard to believe.

And I also would like to pose the question, how slow is constant? How long do you wait before you consider the bearing steady enough?

Let's generalize your idea. Because I certainly can see a use for it.

First of all, no need to fix your lead angle to 80. The sound on the 90 or 270 bearing is good enough, and get's that last percentage of use out of your speed. You can't expect the AOB to be near 10 degrees so why expect that you'll be crossing his bow perpendicular (which is what I expect you're expecting

)

So, now I assume you set course to beam the sound bearing, so sound on 90 or 270. Now make a record of the starting position. Very important!!! Now, set whatever speed you need to keep that bearing steady. That means accelerating and decellerating a bit as the bearing changes 1 degree forwards or backwards. Since you are perpendicular to the sound you automatically will do this with the least speed neccesary. Avoiding excessive noise. Do not turn your sub but keep it steady on course. Eventually you should still have the bearing on your 90 or 270 after a long time. Infact you could do this the entire time until you are about close enough to fire. Now you figure out your average speed during this time, which could be really accurate if you do it right until the end. You've moved a certain distance over a certain time and calculate with whatever tool the average speed(authentic with a nomograph

) Ok, you may ask, but then I need to measure with the Super-Duper-GPS-Map-localiser. Surely that is not realistic. No, you are right in that, .... but ships can have a very simple speed measurement system that counts the revolutions of a small propellor as it flows through the water, known as a speed-log. If it is connected to a tachometer then you can sum up all revolutions and get a distance. Give or take a bit current-drift, but let's assume that to be equal to both vessels. Atleast, that's how I make it credible to myself.

Ok we measured our average speed, with the bearing kept steady, thus approximately in pure intercept. Let's now assume he has an AOB of 90 to the sound bearing (for lack of anything better), so we think he is going parallel to us. He probably is not, as he is probably faster and closing, but the rest of his speed is now pointed along the bearing and we can't measure that. But we do not need to either. He may come as quickly as he wishes, infact the quicker the better.

We are in pure intercept, but we want a torpedo to be in pure intercept.

In comes the intercept formula:

sine(ownsub_leadangle)= sine(targetAOB)*targetspeed/ownspeed

or how we are going to use it as torpedo-equivalent:

sine(torpedo_lead)=sine (assumed_targetAOB=90)*assumed_targetspeed/torpedospeed

Now you have enough data to setup the shot. Or feed the TDC. Assuming he is on a imaginary course perpendicular to the given bearing, and so appearing to move at the same speed as we did. (that's as much as a Blind's Man intercept can tell) And eventually being on top of us, so just a matter of waiting not too long with firing because of arming distance.

I'm not saying this is accurate, because you have no idea how far he is, so if you fire too soon you're probably going to miss. (As objects in the distance look much smaller angularly than when they are near) Also, the gyroangle is probably significant and you need a range figure to correct for parallax aiming error. But if you let him get close enough his length makes him vulnerable anyhow. Anyway, the geometry of the speed triangle should be correct. What you are really interested in is matching the speed vector components that are perpendicular to the bearing, so inside the triangle, as in this image:

You just have to have patience when matching your speed to the steady bearing, and hope your batteries last that long.

08-10-10, 07:44 PM	#5
Pisces Silent Hunter Join Date: Dec 2004 Location: AN9771 Posts: 4,904 Downloads: 304 Uploads: 0	I'm sure we are all very curious about your video. But I'm pretty sure we are not waiting to download 60 Gigabytes!! for it. Surely there must be a way to compress it. Your idea has merrit, however I think fixing your mind on a target AOB of 10d and a target speed of 11.3 knot if your sub is going 2 knots, well ... is just a too special case to be usefull in practise. If you manage to get the bearing to target steady over a long period, then you only know you are in pure intercept. However he could just as easily have had an AOB of 30d when moving at 4 knots. Or have 20d AOB when moving at 6 knots. And add to that the possibility of the target moving away, so the AOB could also be towards his rear. Sure, you say, but the hydrophone operator can tell us what speed range he is in, or if he is moving away rather than closing to us. That helps narrowing it down. Well I really doubt if that could be done so quickly as in the game, the instant he hears it, or that he really could be so sensitive in real-life. What do I know, I have no real naval experience, I just find it hard to believe. And I also would like to pose the question, how slow is constant? How long do you wait before you consider the bearing steady enough? Let's generalize your idea. Because I certainly can see a use for it. First of all, no need to fix your lead angle to 80. The sound on the 90 or 270 bearing is good enough, and get's that last percentage of use out of your speed. You can't expect the AOB to be near 10 degrees so why expect that you'll be crossing his bow perpendicular (which is what I expect you're expecting ) So, now I assume you set course to beam the sound bearing, so sound on 90 or 270. Now make a record of the starting position. Very important!!! Now, set whatever speed you need to keep that bearing steady. That means accelerating and decellerating a bit as the bearing changes 1 degree forwards or backwards. Since you are perpendicular to the sound you automatically will do this with the least speed neccesary. Avoiding excessive noise. Do not turn your sub but keep it steady on course. Eventually you should still have the bearing on your 90 or 270 after a long time. Infact you could do this the entire time until you are about close enough to fire. Now you figure out your average speed during this time, which could be really accurate if you do it right until the end. You've moved a certain distance over a certain time and calculate with whatever tool the average speed(authentic with a nomograph ) Ok, you may ask, but then I need to measure with the Super-Duper-GPS-Map-localiser. Surely that is not realistic. No, you are right in that, .... but ships can have a very simple speed measurement system that counts the revolutions of a small propellor as it flows through the water, known as a speed-log. If it is connected to a tachometer then you can sum up all revolutions and get a distance. Give or take a bit current-drift, but let's assume that to be equal to both vessels. Atleast, that's how I make it credible to myself. Ok we measured our average speed, with the bearing kept steady, thus approximately in pure intercept. Let's now assume he has an AOB of 90 to the sound bearing (for lack of anything better), so we think he is going parallel to us. He probably is not, as he is probably faster and closing, but the rest of his speed is now pointed along the bearing and we can't measure that. But we do not need to either. He may come as quickly as he wishes, infact the quicker the better. We are in pure intercept, but we want a torpedo to be in pure intercept. In comes the intercept formula: sine(ownsub_leadangle)= sine(targetAOB)targetspeed/ownspeed or how we are going to use it as torpedo-equivalent: sine(torpedo_lead)=sine (assumed_targetAOB=90)assumed_targetspeed/torpedospeed Now you have enough data to setup the shot. Or feed the TDC. Assuming he is on a imaginary course perpendicular to the given bearing, and so appearing to move at the same speed as we did. (that's as much as a Blind's Man intercept can tell) And eventually being on top of us, so just a matter of waiting not too long with firing because of arming distance. I'm not saying this is accurate, because you have no idea how far he is, so if you fire too soon you're probably going to miss. (As objects in the distance look much smaller angularly than when they are near) Also, the gyroangle is probably significant and you need a range figure to correct for parallax aiming error. But if you let him get close enough his length makes him vulnerable anyhow. Anyway, the geometry of the speed triangle should be correct. What you are really interested in is matching the speed vector components that are perpendicular to the bearing, so inside the triangle, as in this image: You just have to have patience when matching your speed to the steady bearing, and hope your batteries last that long. Last edited by Pisces; 08-31-15 at 03:47 PM.