My rail gun

[bobsmith1492]

This highly technical section is kinda dull.... here's my contribution


I've come up with a design for a cheap rail gun. This is a rough explanation; if I knew how to put a pic out on the web I'd try (hint hint!


Anyway, it works (or will work - someday - hopefully) by shooting a metal cylinder (or ball, not sure yet) out of a pipe which is slightly larger than the cylinder. As the cylinder travels, it hits a pair of contacts, one on each side. When it hits the contact, current travels through it and through a coil which runs around the outside of the pipe just above the cylinder and contacts. A large capacitor is discharged through the coil, which creates a magnetic field pulling the cylinder upward. It then hits another contact, and is pulled up by the next coil. Anyway, by throwing a bunch of caps on it, you should be able to launch the cylinder.

Now, how could you calculate the force on the cylinder? We just started magnetism in physics class (which is what inspired me). So far, all I really know is that force F=i*l x b for a wire of length l with a current flow i in a field of strength b. However, in this case, the cylinder isn't magnetically charged. I heard something about induced charges. I know a coil will attract unmagnetized steel/iron/nickel/etc. How could you calculate the force applied on the cylinder due to the current in the coil?


The next step would be: how do you find the total acceleration on the cylinder considering the current will be changing based on some RC circuit and the cylinder will be moving relative to the coil?

This is probably a tough order to throw out there.... if anyone would care to point me in the right direction I'd be much obliged.

Thanks

 

[Geniere]

Well that type of rail gun can be made to work but it?s not the way to go even for a beginner. There are 100?s of rail gun sites. Google is your friend.

 

[bobsmith1492]

The advantage of 'that type of rail gun' (I've found nothing similar on Google) is that it could be built easily and with readily available materials. Basically, I'm asking is if anyone knows how to find the force on a nonmagnetic steel/iron object due to a magnetic field.

Also to clarify, it isn't technically a rail gun as there are no rails. It is more of a 'multiple stage solonoid-based projectile launcher.'

 

[silverpig]

There are a lot of similar designs on the net. I've seen one, and seen videos of others. There are a lot of factors you need to worry about.

1. Your projectile must be very light in order to get a decent speed.
2. You must launch your projectile in through the back of your gun with compressed air or the like... if you don't it'll vapourize where it is.
3. You need a lot of capacitance. Like a LOT. This is the major cost factor here.

 

[imported_whatever]

no idea but...
your projectile is going to need to be moving DAMN fast to not weld itself onto the contacts. that, and you are going to need to use something that doesnt melt for the contacts.

 

[bobsmith1492]

Objections duly noted
I hadn't thought of the projectile welding to the contacts, but who knows. I've played a bit with highly charged caps (popping pop cans and such) with an array of caps out of tvs and power supplies charged to 160 volts and haven't welded anything yet.

I was looking at some caps and found some 4.7 F caps that, when wired in series, would amount to some 0.016F at 200 V; I can charge them to about 160 using line voltage (p-p builds up); this gives, from q=cv, 2.56 Coulombs of charge. So anyway, the question stands: how does this charge through a coil change into a force on a cylindrical or spherical metallic object?

 

[IEC]

Let me just say two things:

1. Massive amount of power
2. Friction

 

[CycloWizard]

Basically, you can determine this from a simple energy balance. The amount of power dissipated electrically will equal the kinetic energy of the particle, multiplied by some fractional efficiency (simplest way to account for frictional losses). It's been a LONG time since I took a circuits class, so I can't recall the electrical power equations other than P=IV or something. I know there should be some appropriate equations for this system, but I'm not aware of exactly what they would be. If you can provide those equations, I can help with the rest.

 

[silverpig]

Good rail gun site

I think you'll need more than 0.016F at 200V.

Higher voltage = lower electron charge collected = lower amperage = less heat = no vapourized projectile.

 

[bobsmith1492]

The simple energy balance won't work very well, as massive amounts of energy are dissipated as resistive heat through the wires and especially at the contact points. I'd guess more energy would be used there than would be converted to motion... by a factor of like 3 or 4 at least.

Also, I'm looking at some caps that would provide many times the power used in that experiment, although at lower voltages. This would also be just the charge provided at each stage. Multiple stages would greatly increase the power used.

Wow, that'd be a lot of current.... I think I'll need some inductors and mebbe higher voltage

 

[grant2]

To hijack this thread a bit, a friend once commented "railguns wouldn't have recoil; they exert no force on the platform"

To me this seems like total nonsense. Just because the projectile doesn't touch the rails doesn't mean it's not exerting a force... newton's laws & all that.

Who's right??

 

[bobsmith1492]

All right, I figured out the higher voltage... I've got a bunch of microwave oven transformers lying around. Those should do the trick (3300 V, anyone?)

 

[silverpig]

Originally posted by: bobsmith1492
The simple energy balance won't work very well, as massive amounts of energy are dissipated as resistive heat through the wires and especially at the contact points. I'd guess more energy would be used there than would be converted to motion... by a factor of like 3 or 4 at least.

Also, I'm looking at some caps that would provide many times the power used in that experiment, although at lower voltages. This would also be just the charge provided at each stage. Multiple stages would greatly increase the power used.

Wow, that'd be a lot of current.... I think I'll need some inductors and mebbe higher voltage

If you use lower voltages you'll need more current in order to get a decent amount of power. That translates into a lot of heat, and that means melted components. I don't think you'll get anything decent out of 200V.

 

[silverpig]

Originally posted by: grant2
To hijack this thread a bit, a friend once commented "railguns wouldn't have recoil; they exert no force on the platform"

To me this seems like total nonsense. Just because the projectile doesn't touch the rails doesn't mean it's not exerting a force... newton's laws & all that.

Who's right??

You are. There are videos on the site I linked

 

[bobsmith1492]

Anyway, I could use a microwave oven transformer and diode (or two or three) to up the voltage to around 3000 V rather than 160. That is exactly what was used on that rail gun page that was linked to. Now, would a single microwave diode in series with the output of a transformer create a DC voltage.... bottom half clipped, top gets through... .yeah, but it would be a knappy looking signal. I don't suppose they make 4000 V bridge rectifiers

I guess a pair of 2000V diodes in series would make an effective 4000V diode? So, four of those pairs could make a bridge rectifier for a decent output. That would have to be limited to 0.2 A though, so it would make for a slow charge.

 

[CycloWizard]

Originally posted by: bobsmith1492
The simple energy balance won't work very well, as massive amounts of energy are dissipated as resistive heat through the wires and especially at the contact points. I'd guess more energy would be used there than would be converted to motion... by a factor of like 3 or 4 at least.

Also, I'm looking at some caps that would provide many times the power used in that experiment, although at lower voltages. This would also be just the charge provided at each stage. Multiple stages would greatly increase the power used.

Wow, that'd be a lot of current.... I think I'll need some inductors and mebbe higher voltage

Well, there are more complex ways to calculate these things. Are you actually building this thing? If so, how accurately do you want/need to model it? I'd still be willing to help, but you'll have to give me a couple days to let my brain rest up from my transport phenomena test yesterday before I have access to that part of my brain again.

You can exactly calculate the energy losses due to heat and so on if you have the proper electrical equations. I know all about fluid dynamics, friction flows, heat and momentum transfer, et cetera, but I have virtually no experience using electronics as source terms for all these big equations, so I'd need some help there.

Originally posted by: grant2
To hijack this thread a bit, a friend once commented "railguns wouldn't have recoil; they exert no force on the platform"

To me this seems like total nonsense. Just because the projectile doesn't touch the rails doesn't mean it's not exerting a force... newton's laws & all that.

Who's right??

Kind of complicated to explain. Basically, magnetism exerts a body force (similar to gravity - no contact required) on the projectile. Since there is no contact between the projectile and the barrel, the projectile exerts no reactive force on the barrel itself. For this particular system, there is some small friction between the projectile and the barrel, so this will impart some recoil, though how much depends heavily on the specifics of the system. Ideally, you would impart all of the electrical energy on the projectile as kinetic energy and there would be no recoil. Obviously this isn't the case in reality.

If you don't like that explanation (I don't ), then maybe this works better. Newton's laws are upheld because there is a balance between electrical and mechanical forces. The electrical forces are counteracted by the imparting of their energy on the projectile.

 

[bobsmith1492]

Really what I want to model is the force exerted on a piece of metal by a circular electromagnet.

The coil of wire is here:
----
----- ------
-- --
----- ------
----

with the metal straight below it here:
____________
| |
| Force |
| /\ |
| | |
| | |
| |
| |
----------------

Ok that didn't work......

 

[CycloWizard]

Yeah, the forum software doesn't support spaces in text. You might be able to paste it as code, but I dunno what the relevant tag is for that.

 

[f95toli]

There are several problems with this approach but lets start with the most obvious: Where do you find a large capacitor that can withstand 3000V? It does exist exists but even standard 500V caps are VERY expensive. Another problem with large caps is that they are slow because of their high ESR.
I once saw a few large high-voltage caps used for fusion-research, even though the total energy was rather low (small reactor) the caps filled are large room.

Another problem is the energy, the total energy in 10*4700 uF Rifa caps (which would cost $1500 or so) would be enough to accelerate a 5g bullet to about 1400 m/s but that is assuming 100% efficiency, in reality it would probably be much less than 10% which would mean speeds of the order of 400 m/s.
And then you have the problem of friction...

 

[grant2]

Originally posted by: CycloWizard
Kind of complicated to explain. Basically, magnetism exerts a body force (similar to gravity - no contact required) on the projectile. Since there is no contact between the projectile and the barrel, the projectile exerts no reactive force on the barrel itself. For this particular system, there is some small friction between the projectile and the barrel, so this will impart some recoil, though how much depends heavily on the specifics of the system. Ideally, you would impart all of the electrical energy on the projectile as kinetic energy and there would be no recoil. Obviously this isn't the case in reality.

If you don't like that explanation (I don't ), then maybe this works better. Newton's laws are upheld because there is a balance between electrical and mechanical forces. The electrical forces are counteracted by the imparting of their energy on the projectile.

I don't like it either.

If you accelerate an object, using ANY force (mechanical, electrical, gravitational, magnetic), then an equal & opposite force must be applied to some other object(s).

In the case of a projectile launcher, there are 2 practical possibilities for what that opposite force is applied to: propellant, or the launching mechanism. Since a rail gun does not use propellants, the mechanism must receive the force. This is what I believe is called recoil.

At least that's my very basic understanding of physics.

 

[CycloWizard]

Originally posted by: grant2
I don't like it either.

If you accelerate an object, using ANY force (mechanical, electrical, gravitational, magnetic), then an equal & opposite force must be applied to some other object(s).

In the case of a projectile launcher, there are 2 practical possibilities for what that opposite force is applied to: propellant, or the launching mechanism. Since a rail gun does not use propellants, the mechanism must receive the force. This is what I believe is called recoil.

At least that's my very basic understanding of physics.

I was trying to explain that there are more than only mechanical forces. There are mechanical forces (the kind that you can easily see and understand) and electrical forces that are not really visible or tangible. As I said, the mechanical force imparted on the projectile counteracts the electromotive force. Thus, this interaction of forces will not result in mechanical force (i.e. recoil) being imparted on the gun itself except due to friction at the contacts.

 

[Gnosis]

ok - I don't want to be the the no-go here or so but...

why do you want to build a railgun in the first place?

if you want to "launch things" there are reasons as to
why todays guns use:

1. gun powder
2. compressed air

I suppose that if you want to learn about rail-guns - fine.
But it will be much more costly and labor intensive
than a compressed air / CO2 concept...

good luck though....

 

[FrankSchwab]

Originally posted by: CycloWizard

Thus, this interaction of forces will not result in mechanical force (i.e. recoil) being imparted on the gun itself except due to friction at the contacts.

And grant2 was trying to explain that you are wrong.

In your world, the gun would attempt to follow the projectile, because friction on the contacts will cause a force on the gun in the direction of travel of the projectile (it tries to drag it along). This has all kinds of interesting ramifications that are, umm, "not supported by experimental evidence".

/frank

 

[jmcoreymv]

I've built a rail gun before. Its not going to shoot anything hard and a multistage coil system is a PITA to setup.

 

[bobsmith1492]

Why build a rail gun? I don't care about launching things; heck you could build a catapault that would do the job more easily. I am building this only for the fun factor - catapaults don't use megajoules of electrical power and create tons of sparks while melting anything in its way!

I want to figure out the physics of the interaction between an electromagnet an a piece of metal; forget the 'rail gun' factor; I know there are difficulties involved, but they can be surmounted or dealt with. I'm looking for some math.