Electromagnetic Accelerator or "Coilgun" in RC warships.

I am currently looking into coilguns as a replacement for the common coldgas guns currently used on almost all boats.
Coilguns do not fall under the strict gun laws in my country whereas the construction of coldgas BB guns would violate multiple laws at once.

I thought I would share my journey of researching, developing, constructing and eventually deploying a coilgun for a RC warship.

The working principle:
Coilguns use electromagnetic fields generated by coils to accelerate ferromagnetic projectiles through a tube. (see atached animation)

I will update this thread as I go along. I already made some progress and will post my findings shortly.

- Fritz

 

In the atached GIF you can see my second try at a magnetic accelerator. (There are sadly no videos left of my very first coilgun)
The accelerator is made from 4 Stages. Each stage is comprised of an electrical source, a sensor that detects the oncoming projectile, a mosfet board and a coil.
Each stage accelerates the projectile further.

This design is very crude and used a lot of energy (+20,000 kw) to achieve maybe 1J of kinetic energy.
Obviously this would never work on a 1:144 scale cruiser because it is too big and too heavy but I was proud of it nonetheless.

 

That's pretty fancy, but I'd hate to see what happens when such a (presumably) high voltage item gets wet.

I take it that in your area, "airguns" are considered weapons, right?

How about spring loaded, or motorized (i.e. centrifugal)?

 

Hey, thanks for the reply! I opted for a low voltage / high current power supply (6s 100C lipo) so there is no risk to my life.
My guess is that this is even easier to waterproof than a powertrain because there are no moving parts (aside from the projectile).
Every PCB can be cast into resin and made waterproof, just like an ESC.

The gun-laws here include coldgas and spring loaded guns as weapons but make no mention of centrifugal- or my coilguns.
Howerver using S-AEG drives on an RC boat might be in a legal gray area. But I am an engineer so take my interpretation of the law with a grain of salt

 

After destroying countless mosfets, batteries and coils I stopped working on coilguns in 2019.
I nearly set my apartment on fire because one lipo got upset about a short circuit caused by a broken mosfet.

Now that I have a new purpose for my coilgun project I got the parts out and re-constructed it on my desk.
The atached GIF shows me fireing a steel projectile with just one stage connected to a 6S lipo battery.
The steel projectile has no difficulties penetrating the 3/32 balsa target and still had enough force to put a dent into my wall...

But this design is still too large for a ship so now I am on a quest to modify it to fit three accelerators in a turret.

 

Fast forward to the current day: I have wound a new, much smaller coil with a more appropiate size for 1:144 boats.
As you can see in the attached GIFs; the kinetic energy of the projectile has decreased a lot. It is barely enough to split 1/16" balsa wood.
Currently I have no way to know how fast exactly the projectile is going so I will either have to buy a chronograph or build my own speed measuring device.

I also tried a steel BB with the new coil but it just bounced off the balsa target. BBs are not ideal for coilguns because of their suboptimal use of space in the barrel. I will stay with cylindrical projectiles for now.

Here are my ideas on how to increase the velocity of the steel projectiles:

1) Ferromagnetic shell around the coil to increase the magnetic field strength.
2) Longer projectiles to take advantage of the coil's magnetic field lines which are extending beyond the length of the coil.
3) An initial push from a solenoid.
4) Higher voltage, maybe using an 8s lipo instead of a 6s.​

I will get back to you once I got results from testing these ideas.

- Fritz

 

Welcome to the hobby and thank you for sharing your R&D on the rail guns. This is fascinating!

 

Looks like this would potentially be a great development for TORPEDOES! YES!

 

@FritzVanDelen

tell us about the electrical design of this gun?

 

Gladly! My original plan was it to get into the specifics after constructing my new model but I guess there is no harm in doing it now instead.
I have attached an Image of my current setup.

Here is how my coilgun fires a steel projectile:

1) Projectile is inserted into barrel and passes in front of an IR-sensor that is located right in front of a coil.
2) The IR-sensor detects the projectile and sends a signal to the I/O board.
3) The I/O board is powered by two 5V power supplies. The I/O board uses the input from the IR-sensor to turn the mosfet drivers ON or OFF.
4) The mosfet drivers on the I/O board set the gates on the mosfet board high.
5) The mosfet board switches the coil ON and supplies the coil with the electrical energy from the attached LiPo (6s).
6) The projectile is pulled into the middle of the coil and passes by the IR-sensor.
7) The IR-sensor detects that no projectile is present and sends the signal to the I/O board.
8) The I/O board switches the mosfets off.

I will get into more technical details when asked or when I made more progress on my newest model (image attached. New coilgun on the bow of a Königsberg class cruiser).

 

Looks pretty interesting!

whats the plan for water proofing it? Potting it?

 

Thank you! Yes I will cast the PCB's into resin. I don't know how the solenoid will hold up though...

 

In order to fit all of the electronics into a 1:144 cruiser (admiral hipper class - see pic) I reduced the size of the I/O board by removing two stages and placing the parts closer together. (see pic)
I reduced the amount of stages on the new IO board because I will only need a maximum of two accelerators on one side of my ship.
The new board also features some improvements to allow faster switching of the coils on the accelerator. (Both outputs of the TC4429 are now connected & a 6.3V 470μF cap has been added to the output side)

The I/O board is split into two halfs along the optocouplers. This protects all electronics on the input side from any back EMF from the coils.
I also wanted to add an automatic switch-off on the input side to prevent an extendet on-period (that might burn up the coil & damage the battery) but I could not figure out how to do that yet.
I will look into it with V3 of my IO board.

Before I made the new IO board I tried to reduce the size of the MOSFET board but found out quickly (after the board burned up on my desk) that there is no way to reduce the size of that.
When using a coil with a resistance of 1.3 ohm and a 12S setup (44V) I would need to switch 33.8 A or 1489 watts of power. I calculated this to increase the temperature of a single IRFP3206 MOSFET to +177,5°C above ambient!!
Divided by six it is only a +29.5°C increase which is much more managable. (especially if I add a bit of external cooling).

So, my conclusion regarding the switching board is that I will stick with the current design of my MOSFET boards, which include six 60V 200A IRFP3206PBF MOSFETs.

My worst fear is that the electronics will fry when the ship is sunk, and one of the coils of an accelerator will be permanently turned on. This would result in damage to the 12S battery by drawing too much current, risking it to explode. On the bright side, the amount of gas released by an exploding battery inside the hull would likely force out enough water to resurface my ship.

That is it for now, I will be back with more updates in two weeks.

Thanks for reading.
-Fritz

 

Can you use a fuse to protect the batteries?

 

Here is a test shot from my new design:

Accelerator:

-1.3 ohm Coil
-V2 I/O Board
-8S LiPo main power supply (~30V)
- 100mm Barrel​

Projectile: 6mm x 14mm hardened steel.

Target:

-2mm Balsa wood
- mounted on a 100mm section of the hull of an Admiral Hipper class cruiser
-45cm distance between target and accelerator​

Even at this scale it has enough of power to penetrate the balsa with a thickness of 2mm.
I can increase the power even further by adding 4 more LiPo cells and increase the voltage from 29.4V to 44V.
I am very happy with this.

 

Two minds think alike! I actually thought about trying fuses as they can widthstand short bursts of high energy and will melt when put under continious high current.
This is definetly something I will try.

 

Any way to see what FPS you are achieving? IE, perhaps high speed video on your camera to measure the time to travel 1 foot?
- I would be trying to achieve parity with today's CO2 guns, then move to water testing ASAP.

If you look at @Kotori87's post in my Deutschland build thread, he's talking about FPS as well as penetration of the projectiles.
- Oddly enough, I'm going through a similar situation as I develop my all 3d-printed cannons.

https://rcwarshipcombat.com/threads/pocket-battleship-deutschland.447156/page-11#post-542379

Here is an excerpt:

 

Thank you very much for linking me these resources.
A chronograph is already in development as any further improvements to my coilgun require a means to verify their effectiveness by measuring the projectiles speed.
I will just build my own from a couple of IR sensors but a high speed cam would do the trick aswell.

To achieve a similar muzzle energy as the 1/4 rounds I would only have to accelerate my 6x14mm projectiles to 30% of the FPS because the projectiles are 3.2 times heavier.
Granted it won't fly as far but might have a better penetration chance below waterline because of the increased mass.

Take all of this with a grain of salt as this is meer speculation. I will conduct further tests after I have my chronograph assembled.

Also your thread is a great read! Good luck on your R&D, i will keep an eye on it.

 

I assumed you were going to scale the projectile down to the same weight as the current projectiles? Which ruleset are you planning to adopt?

Too bad you are in the EU; if you were in Texas I would recommend us to do some collaboration. NTXBG allows for R&D program; so your ship would be allowed to fight in our battles.

 

I tried to use BB's in one of my coilguns but found out that their spherical shape is suboptimal for use in coilguns so I am sticking with cylindrical projectiles for now.
You can find one example of a coilgun that uses BBs successfully (here) but it requires a lot of stages to accelerate the BB to 427FPS (see pic).
I will possibly give it another shot but I got a bit pessimistic about their use as a projectile in coilguns.

This is a bit of a tangent to the topic of this thread but:

My brother and I have adopted our own, very loose ruleset. (see pic) We want to develop our own rules through trial and error however I used this thread as a guideline.
The biggest change we made is to use watertight compartments instead of waterpumps. This makes for slower and more strategic battles (At least that's what we hope).
However none of our rules are set in stone and are not even battletested yet, so we might scrap the whole document after our first fight.​

That sounds great but yeah it kinda sucks to be in the EU. The amount of laws preventing us from persuing a hobby like this is ridiculous.