Building a battery operated railgun

[Kakarot]

can it be done? I am not engineer or anything and i was just wondering if it would be possible to build a railgun using batteries as the power source. my idea is to have around 25 electromagnets on two rails with an aluminum tube in the middle which houses the projectile. then each electromagnet is hooked up to its own 9v battery and powered up in sequence from back to front launching the metal. my question really is, would that be enough to launch a small projectile quickly? or is this setup going to launch it out 1 foot onto the ground lol

 

[Cliff_J]

You are thinking more along the lines of a linear motor where each magnet attract/repels in succession. A rail gun per se is just a pair of rails, here's a simple one using a 9V battery and some small magnets.

http://www.scitoys.com/scitoys/scitoys/electro/railgun/railgun.html

You can search the forum and find a few threads on rail guns and lots of hits on a search engine. As far as a linear motor is concerned, that is far more involved and would require some good circuitry skills to pull off successfully. And if you wanted a rail gun that could launch something more than a few inches, that is a matter of playing with lethal voltages and should not be attempted unless you know how to keep yourself safe.

 

[minger]

Yes, a railgun name comes from the look and design of it. On a railgun, you essentially have two conductive "rails" in which a projectile closes the circuit in between. I'm not sure about the exact physics (its been years since we covered those in physics), but when the current passes through the projectile, you use the right hand rule or something, and a large force is exerted perpendicular to the rails, shooting the projectile out.

here is a little information on railguns:
http://www.physics.northwestern.edu/classes/2001Fall/Phyx135-2/19/railgun.htm

I do believe in order for these to work though, you need quite large voltages. Even a car battery probably wouldn't be sufficient. Perhaps a battery with a few transformers would do the job...I'm not sure, I don't know too much about those sorts of things.

 

[Astronuc]

One needs high currents for the railgun. With low resistance, the voltage does not have to be so much.

I visited a center that built and tested railguns. The principal energy source was a homopolar generator which was used to charge up a set of huge inductors. The inductors stored the electrical energy, and when fully charged, the inductors where switched to the railgun.

The high speed switches were mechanical - and were opened and closed with 'explosives'!

 

[pallidin]

One needs high currents for the railgun. With low resistance, the voltage does not have to be so much.

I visited a center that built and tested railguns. The principal energy source was a homopolar generator which was used to charge up a set of huge inductors. The inductors stored the electrical energy, and when fully charged, the inductors where switched to the railgun.

The high speed switches were mechanical - and were opened and closed with 'explosives'!

Did you mean "huge capacitors" ?

 

[Ivan Seeking]

The high speed switches were mechanical - and were opened and closed with 'explosives'!

I have played with this quite a bit. And yes, the switch becomes one of the most difficult parts of the system. We found that for down and dirty testing, a sledge hammer, a cable, and a steel plate work fairly well. But ultimately, we had good luck using a low current, high voltage spark to create an ionized path, which acted as a switch.

Typically, high voltage is used even for the main circuit since the energy storage in the caps depend on it: 1/2CV². And as you mentioned, the internal resistance of the caps - really the ESR - is critical. Of course, as the caps get bigger, so does the ESR.

that is a matter of playing with lethal voltages and should not be attempted unless you know how to keep yourself safe.

And capacitors can explode; esp if they are not rated for the application. We had a big bank of oil filled caps that always scared me a bit.

 

[Astronuc]

Did you mean "huge capacitors" ?

No - they used huge inductors! Surprised me at first, but I saw them.

A battery may be OK for a low energy railgun, but one would probably blow a car battery on a high power gun.

The place where I witnessed the rail gun routinely hit speeds on the order of 3 km/s (yes that fast!) and fired down rather than laterally because they were using a several kg projectile with a spike (like the inlet spike of the SR-71) and they did not want to send this thing off-site.

 

[Kakarot]

hmm interesting. i figured it just wouldn't work with low voltage and I am too much of an electrical noob to work with high voltage stuff. and i guess it would be really hard to power up each electro magnet in a fast sequence and align the projectile right

 

[Astronuc]

How much power/energy do you want? What mass are you trying to accelerate, what acceleration, and over what distance?

From that figure the current, and from that the electrical energy source requirements.

 

[Kakarot]

well i thought maybe i could get a nail up to a few hundred feel per second. but after thinking about my idea i figure the nail would accelerate forward after being pulled by each electromagnet but lose a lot of energy unless the magnets fired off perfectly or else the nail would reach equilibrium points and slow down a lot. well the whole thing is just an idea for a fun project to do this summer hehe

 

[Danger]

i guess it would be really hard to power up each electro magnet in a fast sequence and align the projectile right

Rapid sequencing is no problem at all. In an ideal linear accelerator, the payload itself triggers its own propulsion. ie: the bucket or whatever trips photo detector circuits or magnetic switches that determine when a particular acceleration coil fires.

 

[KonradKorzenowski]

I have an idea. What if you have a ring-shaped magnet being pulled by another manget with a projectile in the middle. eventually the bullet would be forced thought the ring magnet at a high velocity, and pehaps the bullet could be steadied by several magnets within the barrel? Yes?No?

 

[karambit]

any idea if using a 1 FD cap from a car audio system would work

 

[Mech_Engineer]

The stored energy in a capacitor can be calculated using the following formula:

$$E_{Cap}=\frac{1}{2}*C*V^{2}$$

The capacitor you're talking about would operate at a nominal voltage of 12V, which would equate a total stored energy of 72 joules.

If we converted all of that energy (100% efficiency) to a kinetic projectile (say a bullet weighing 10 grams), it would be going 120 m/s (393 ft/s). Pretty fast and possibly dangerous, but probably no more dangerous than a powerful pellet gun. Also, this only provides one shot per capacitor charge, how would you recharge for shot #2-1000?

Upping the capacitor voltage can make a big difference in total stored energy, but it's still probably not enough to provide more than 10 shots or so before it needs a recharge, or possibly one lethal shot.

 

[DaveC426913]

any idea if using a 1 FD cap from a car audio system would work

Isn't a 1 farad cap, like, the size of a tennis ball cannister? People put those in their cars?

 

[Mech_Engineer]

Isn't a 1 farad cap, like, the size of a tennis ball cannister? People put those in their cars?

It's common practice because peopole think it helps smooth out voltage drop when their speakers hit really hard (especially high powered subs). From what I understand it doesn't actually do much for you though, it's better to have large high current batteries available.

 

[karambit]

is it more important to have a high voltage cap

 

[Sakha]

I build a railgun a few months ago. Talked to a guy at the pharmacy to collect the disposable cameras for me, they usually throw those away. Got plenty of good 330V capacitors, and had my own step-up converter to charge the cap bank.

 

[JoshuaPatrick]

I don't think so, the hightest volt battery is 12 volts. To a rail gun that is not a lot. And to my under standing the power needs to be distributed very quickly, like under one sec. A capacitor can hold, let's say 100s of volts to your 12 volt car battery, even a wall out lit is still only 120 volts. A perspon on YouTube used 330 volt capacitors, a 4 by6 rectangle and 120 farad. Believe me it did not look strong. That is my opinion.

 

[JoshuaPatrick]

But with that I don't know what I should max out. The voltes or farad? That would be very helpful to know if anyone knows that.

 

[Mech_Engineer]

The stored energy is linearly dependent on the capacitor's farad rating, but on the voltage SQUARED. So upping the voltage gives you a much larger inscrease in stored energy.

 

[JoshuaPatrick]

So I should weary about the volts?

 

[Mech_Engineer]

You should be careful with the total stored energy period. A hefty capacitor with a significant amount of stored energy can kill you through electrical discharge even without being in a gun. If you're not familiar with working around capacitor-based storage circuits, I would recommend DON'T.

 

[JoshuaPatrick]

I no know a lot in electricity, and it will only kill if it passes your heart. And I'm pretty safe. I did and do want to build a tesla coil

 

[JoshuaPatrick]

So I do know what I am getting my self into

 

[Mech_Engineer]

You know what you're getting yourself into, but you're not sure if you should "be weary about the volts"?