Another torpedo thread?

[Wheelerdealer]

Neat idea Kevin. You can

Neat idea Kevin. You can also have a second receiver in the Aux WTC with the same frequency xtal as your main Rx and hook up the servos in the Aux WTC to the appropriate channels.

[KevinMC]

Hi Ramesh,

You know I really

Hi Ramesh,

You know I really thought about going that way, but in the end I decided against it. When I weighed it out, I had "Just buy it and be done with it" on one side, vs. "Don't need 2x crystals for channel change" and "It's much cooler with the custom solution" on the other side... (At this point I defer to Chris's signature tag. )

[Wheelerdealer]

Hi Ed, PM'ed you some

Hi Ed, PM'ed you some info

[KevinMC]

Ed,

Hopefully you'll take this under

Ed,

Hopefully you'll take this under "better late than never". (I went to shoot this photo yesterday morning before leaving for work but discovered the batteries in my camera were dead!)



One thing to note is that I've based the dimensions of my torpedo on a 533mm x 11m fish, not the more popularly quoted 533mm x 9m.

[ramius-ii]

Thanks Kevin

Thanks Kevin!
This answers a lot of questions!

Best, Ed

[libord]

Torpedos by Ota

Hello,

friend of mine, Ota Gerza (www.rcsubs.sub-club.org) is finishing a development of an electric small torpedo in 1]http://www.rcsubs.sub-club.org/images/stories/torpedo/p1010022.jpg[/img]



The torpedo shown is still unfinished - already operating, but yet to be painted.

Regards,

[cstranc]

Libor,
That electric torpedo is

Libor,
That electric torpedo is a work of art. I wonder how he made the prop. At 1:34 there must be a large submarine behind it...

Kevin,
I have to ask. I know you're an electrical engineer, but how would you know how to setup IR communications between two servo systems??? Isn't this stuff complex?

Does the servo control lead just contain pulses that you multiplex (3 channels) then send across as IR pulses.

Even that sounds complicated.

[cstranc]

Well I managed to shoot

Well I managed to shoot my first gas propelled test torpedo the other night. That was a fun milestone. Here is the basic design.


Nothing to special there. Hollow torpedo. Currently it has a 1/16" aluminum tube as the pickup tube. That is mounted into a 1/8" tube because the ID of my o-rings is 1/8".

The valve to keep the Propell pressure is off torpedo for weight reduction and simplicity. The valve assembly looks like:


I'm really happy with the fill assembly. Low cost, low effort, easy maintenance. It is built around a Schrader valve and an 1/8" end cap. The Schrader screws into the end cap. I have a little piece of machined aluminum that positions and compresses two o-rings (one by the valve, the other by the end cap). This gives me a small "high pressure zone".

I have a 1/8" hole drilled through the endcap to allow the end of the torpedo into the high pressure area. One of the o-rings seals against the 1/8" shaft on the end of the torpedo.

Assembly & maintenance is simply a matter of tightening the schrader so that it generates the right compression on the o-rings to seal everything. If the o-rings get worn you open it up and replace them.

This is what it looks like assembled with a 1/8" rod installed for a pressure test.


I did my initial testing using 110psi air from a bike pump. Fill it, then drop it into water to look for bubbles...


No bubbles even better after 10 seconds...



The stopper came free and shot across the water. I played with this a few times, and took it as a good omen. I needed something more torpedoey. So out with an early mockup of a torpedo. It was quickly retrofitted to include a 1/8" tail so it would fit the filler.



Again I pressure tested it with air. It held nicely, but when it released the compressed air was not special...

So out came the can of Propell. This was my first time using Propell. ha ha. It got on my hand when I tried to fill the torpedo the first time. That froze my skin effectively... Second time I wore gloves....

So I filled the torpedo with propell, dropped it into the jacuzzi and sure enough it managed to do 4 laps of the tub before coming to rest

As it stood that torpedo had a short length of 1/8" brass in the tail, about 3" of 1/16" aluminum pickup tube. That's all the weight, and I think it would actually just sink after running...

So I am fabricating my real torpedoes to the correct 1:60 scale. That will have a far larger volume of air. I think it should float no-problem. But then it will have heavier cast resign nose and tail sections....

I have a couple of loose ends / un-answered questions (as usual).

It's a little annoying having the torpedo self launch. I am hoping that if I adjust the compression on the o-rings they can give me enough friction to reliably hold the fish in place. If not I will need to design a launching tube that both holds the torpedo back during normal sailing, then pushes it free when firing. Some testing should answer that question (it's just an excuse to fire to torpedo again....)

The other loose end is that pickup tube. I have heard people talk about the types of needle to use. For this test I used a 1/16" aluminum tube that had a 0.024" hole.

So this is what I used to think. The pickup tube is there to:
1. It prevents the Propell from coming out too fast.
2. Suck up the gas Propell instead of the liquid propell.

I'm not sure I beleive that's what it really does though. Please allow me to ramble....

I started thinking the obvious questions:
1. What is the right length of the pickup tube?
2. What is the right inner diameter?

By changing these we shall have a dramatic impact on the length of time the torpedo is under power (bigger diameter or shorter length shortens run time), and speed of the torpedo.

Hmmm. What governs the speed of the torpedo? Well that would be the "muzzle velocity" of the propell as it exits the pickup tube and how much gas is leaving.

So what governs the muzzle velocity? Well for a fixed diameter let's look at some possibilities. The gas is being driven by a pressure differential. While there is liquid propell in the torpedo the pressure will remain at 78psi, and the outside pressure will be 1 atmosphere (unless you are deeply submerged...).

If we had a really short tube (maybe 1/16" long) the propell could jump out of the tube before it fully expands. Then it would continue to expand while in the water, but by then the energy is lost to our little fish.

If we had a really long tube (no idea how long) the friction going down the length of the tube would waste the energy and slow the muzzle velocity.

For a given tube diameter there must be a sweet spot where the exit speed of the propell is maximized. I have no idea what it is though.

Then we have the tube diameter. The bigger the tube, the more propell will enter and exit over a period of time. Also a bigger tube will have less frictional losses. Sounds pretty simple but what about.
1. Crimping the inside end of the tube (reducing the area the propell can enter the tube through).
2. Crimping the exit end of the tube (or installing a smaller needle inside a larger pickup tube, etc).

What would happen then?

This is a TOTAL guess on my part but if we reduce the input area less propell can enter the tube, but when it's there it effectively has a bigger tube to expand and accellerate in. It could get a higher exit velocity without cranking up the volume (I guess, maybe, sound possible after a drink and some coffee).

Now having a choke point at the exit end of the tube does not sound like a good thing. choke... The propell enters the pickup tube, starts to expand and accellerate then bang it hits a bottleneck. It would not really work like that. Almost instantly the choke point would create a pressure that is higher that atmospheric and that would slow everything down. So here we are possibly wasting energy in the first part of the pickup tube, then doing the real accelleration in the final section.

It's pretty obvious I think the pickup tube controls the muzzle velocity (and those other things). It says you will go from 78psi to 1 atmosphere over the following length. I am totally curious that if you shortened the length of the tube and crimped the inside end would you use less gas, but have a higher velocity...

You know it makes me want to dig out my old fluid dynamics text books from 25 year ago. Maybe they will make sense now (ha ha).

I sure hope someone out there knows the anwers because I'm just a curious type and I can't quite think of a good way to test this (that does not involve a lot of work).

[libord]

Libor,
That electric torpedo is

[quote]Libor,
That electric torpedo is a work of art. I wonder how he made the prop. At 1]

Not that large, ir is type XXIII a small coastal submarine, so in 1:34 it is about a meter long. I am building one myself, I think I was a 1st costumer of this model. That was like 5 years back, but I didn't have chance to finish this model, as I travel a lot. I moved instead on to the USS Jimmy Carter, which is smaller.

As for torpedoes, as I said Ota tried to fabricate a 1:72 torpedo as well, but he had few problems, so he left it for time being (however I believe he will finish it one day ). Then, to create the 1:34 was "piece of cake" . It is glued from two halfs - hot stampings fabricated by himself. He made it from 0.8mm ABS plastic foil, could be done from 1.2mm thick foil as well. I didn't see it working - yet!

I see plenty of interesting designs here as well. Sadly, most of them require very good model-workshop, which I don't have while traveling and staying abroad .

Regards,

[libord]

Torpedos by Ota

Found photos of the sub and torpedo hatch design.






(click to enlarge)

"Cstrans", it's great to read through your comments and learn more about the design you're using!

Regards,

[KevinMC]

Hi Chris,

As far as my

Hi Chris,

As far as my WTC-Link is concerned, yes it can get complicated, but it's amazing what you can do with a little imagination and a couple of PIC microcontrollers.

With respect to your last post, I believe there is value in a torpedo system that includes some kind of positive restraint mechanism to prevent an uncommmanded firing, or cook-off as it were.

Although I cannot provide any hard supporting doccumentation/thoeries as to why, I don't agree that the appropriate location for the restriction in the pick-up tube is at the upstream (or "inside") end. The forward motion of the torpedo is derived by accelerating the gas through the restriction. The sidewalls of the tube after the restriction will reduce this exit velocity of the thrust-causing gas. (Parasite drag, if you will.) Consider the location of the restriction, or nozzle, on every water rocket you've ever seen. (Or Google it if you've never looked, there's lots out there.)

As you've alluded to, choosing the nozzle diameter is a balancing act- A larger diameter gives you more speed. A smaller diameter gives you more range. But speed is also limited by hydrodynamic drag of the torpedo, and the amount of spin you may or may not try to impart on the torpedo. So many little things to think about...

[scott t]

I'm curious if you loose

I'm curious if you loose the liquid propel that may exist at a level above
the pickup tube.

Would it be advantages to have a floating inlet tube attached to the
pickup tube so that gas is the only thing expelled?

Scott

[hakkikt]

From what I know of

From what I know of rocket engineering, the expansion nozzle at the end of the rocket (in everyday terms: the bell-shaped thing where the fire comes out) is a very important factor for making it efficient. A rocket could just have a tube sticking out of its rear, but we would not reach space that way.

The nozzle is there to let the propulsion medium (gas) expand its volume and lower its pressure. In an ideal engineering world, the inside pressure at the rear end of the nozzle should be just as high as the outside pressure - then the gas will have given all its energy to moving the rocket, rather than to further expansion in the outside air after it has left the nozzle.

I dont know much about fluid dynamics, but I see no reason why things should not be the same in water as in air. Rockets are rockets

If that is true, there might be some significance to where you place your "crimp" in the tube - the part rearwards of the crimp could be considered your expansion nozzle. And then it might add to speed & range to add some kind of conus to the end to let the propel expand properly.

[KevinMC]

It really is rocket science!!!

Scott- I'm not sure that having a floating pickup is really necessary- Consider that for at least the first part of the torpedoes' flight it will be accelerating. Under these conditions the propellant will slosh to the rear of the tank, so as long as the pickup opening is near the front of the tank you should only be blowing gas out the back. By the time the propellant starts to slosh forward again, it'll be close to or more than half gone, so you're still just blowing gas.

Hakkikt- Your analogy is right on the mark. Wether we admit it or not torpedes on a model scale really are a derivative of rocket science!

[cstranc]

I like the rocket analogy

I like the rocket analogy too.

A floating pickup will be difficult. The torpedo spins as it moves (for directional stability), so the pickup would need to move quite quickly, it also has to be light. A difficult thing to create and put inside a fish.

In another thread I read someone describe how Popell works. As long as there is liquid it gives us 78psi. So any liquid that makes it into the tube (past a crimp?) will give us 78psi and it can push against the tube.

I guess I am actually thinking a little opposite to normal. We normally think about "lets try to get the gas in the tube and save the liquid". What happens if we shorten the pickup tube to get the liquid, constrict the end so we get less flow of liquid, but make the tube slightly larger and shorter so it can expand more efficiently.

And that's where Hakkikt's & Kevin's comments are so good. This is rocket science. And probably the ideal expansion shape is a nozzle like you see on a rocket. There's no way I'm trying to make one of those though!!!

Of course, there comes a time where you say "that's nice" to all the theory, shove in the tube you've got and send your sub off to hunt ducks

Chris