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Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Hello everyone
Seeing that little amateur work has been done lately in the railgun field, I decided to do some small scale research myself. In short, I decided to ignore many of the commonly established railgun concepts and pursue my own ways. The most important establishments I've made so far are:
- An amateur rail gun is obviously a low speed device. A tipycal case of 1cm wide projectile moving at speed of sound in 1 tesla magnetic field only produces about 3 volts of back EMF. Only the work done against this back EMF is the energy transferred to the projectile! Hence, the commonly used power supplies involving 300V+ electrolytic caps can be expected to have extremely poor efficiency at these low speeds.
- A solid metal projectile is clearly a no-go. Brushes are essential, though they still seem to be lossier than I desire. For an efficient low-speed railgun, the resistance of the whole circuit has to be well under a miliohm which is extremely hard to acheive. My last resort weapon are going to be liquid metal wetted brushes.
- In order to increase projectile EMF at low speeds, I used neodymium magnets for augmentation. The magnetic path is enclosed by an iron core to get about 1 Tesla field in the air gap. Their usefulness is questionable though, at high currents (10 000A+) their effect fades and may even get demagnetized at some point.
- Power supply is an extremely difficult part due to extremely low internal resistance required. I have still not decided which way to take, though it's clearly not going to be tens of kilojoules of 400V lytics. Efficient shooting requires large amount of charge (50Coulombs or so) at low voltages, implying farad range capacitances. One possibility would be to use many parallel car audio capacitors, which are in range of farads, up to 25V and about 2 miliohm internal resistance. Depending on brush resistance I acheive, it may pay out to use two in series (at cost of efficiency). Only a fraction of energy stored in the caps would be used per shot, making this an expensive approach unless I can find some cheap source of these caps.
Another possibility, which I will more likely go for, would be to use a transformer driven by IGBT bricks to convert 400V+ capacitor voltages to low voltages suitable for the railgun. For example, the supply would take 500V at 500A to produce 25V at 10 000A. Losses would be greatly reduced due to comparatively low currents that are now taken from capacitors. The transformer output would be rectified by a whole bunch of schottky diodes, and the secondary may have to be resonated in order to remove the negative effect of leakage inductance.
UPDATE
Obviously, I want a lot more destruction for this system, and a power supply with much better characteristics than a lead acid battery is required for this. After talking with Steve for a while, we reached an interesting idea: a fairly big iron transformer is used to drive the gun directly, with 400V electrolytics being discharged into primary by a big SCR. The transformer would have a third winding which would be used to presaturate the core in opposite direction before firing to get 2x the volt seconds in the pulse. Pics of that coming some time later - accepting all suggestions as well!
Marko
Raw materials:
The mess of drilling:
Ready to glue the magnets, paper insulation placed underneath the rails:
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Hi Steve
The brick is actually 6 parallel thyristor modules that I previously used for trans-cranial magnetic stimulation experiments. I didn't bother to disassemble them, even though a single one would probably do for this test.
My guessimation is that the current was less than 1000A, leading to projectile velocity of some 50m/s. BTW, the battery has a rating of 850A written on it - is this short circuit current, or some other value? If the rumor that starter batteries tend to have 10-20 miliohms internal resistance, I'd guess it may be short circuit current?
I have some 21000uF, 100V caps around, but am unsure whether to even bother trying using them. They store way too little charge and are likely to get discharged very early in the firing...
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
The 850A is usually 'cold cranking amps' or the likes, which means it can deliver 850A for 30s without the terminal voltage dropping below ~7.2v at 0° Fahrenheit
The short current will be much more than that, probably on the order of 2-10kA depending on charge, temperature, health, etc
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Marko wrote ...
Another possibility, which I will more likely go for, would be to use a transformer driven by IGBT bricks to convert 400V+ capacitor voltages to low voltages suitable for the railgun. For example, the supply would take 500V at 500A to produce 25V at 10 000A. Losses would be greatly reduced due to comparatively low currents that are now taken from capacitors. The transformer output would be rectified by a whole bunch of schottky diodes, and the secondary may have to be resonated in order to remove the negative effect of leakage inductance.
I have always believed this to be the solution, using banks of HV caps for a low energy transfer, has always seem to be the weak point in everyones attempts. (including really smart people like Sam Barros).
Ive alwayts wanted to try this with transformers, but i dont have the time or money, however the US Navy wants railguns in the next few decades to be practical.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Hi guys
Well, hmm, it'd be great if a lead acid battery can provide 10kA, though I'm somewhat skeptical about that...
The next thing I have to do is to set up some measurements. This involves
1.) Measuring the input current to the railgun with a suitably large shunt 2.) Measuring the voltage at the ends of tails 3.) Measure the projectile velocity 4.) Record everything on a microcontroller, fpga or oscilloscope and compare data
So far I don't have much idea what contributes the most to the resistance in my circuit, though I'm afraid it's still the projectile-rail interface. The projectile used was a piece of square wire bent into U shape, after which a piece of R>G-174 coax sheath was pulled over it. After shooting, signs of melting were visible and the braid has welded to the wire in some places. Obviously, significant heating still occured, despite it was far better than the solid projectile.
I would like to compare the current situation with the projectile brushes wetted with liquid metal. I'm certain there is going to be some improvement that way! Only then I can decide what power supply voltages and currents I actually need.
I have always believed this to be the solution, using banks of HV caps for a low energy transfer, has always seem to be the weak point in everyones attempts. (including really smart people like Sam Barros).
Ive alwayts wanted to try this with transformers, but i dont have the time or money
Well, I intend to use a setup pretty similar to a DRSSTC, except the output will be low voltage and rectified into DC. I think it would not be too ahrd to modify a driver of a suitably large brick DRSSTC for this application.
however the US Navy wants railguns in the next few decades to be practical.
Well, the navy railgun accellerates projectiles to many thousand m/s so the situation is somewhat different there... the EMF on their projectile is high enough to allow for use of high voltage caps (IIRC, they use 4-5kV).
What I'm more interested about is why they only attempted using a fairly simple armature system which does seem to erode rails greatly upon firing. Still, circumstances there are far different from an amateur model and fiber brushes or liquid metal may just not work at these extreme speeds.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Hi Patrick
My plan is to use this alloy, due to it's good balance of cost and melting point:
The whole railgun would be wrapped with a coil of wire and induction heated to some 50 deg C before firing, in order to keep this alloy in molten state.
A Gallium-indium-tin alloy would be superior because it's liquid at room temperature, but it's cost and shipping concerns make it a bit prohibitive for me at this time.
My primary concern though is possible amalgamation of gallium with copper, resulting in increase in melting point or even compromised mechanical properties of the copper rails. I know it does bad things to aluminum, but no idea yet for copper. Ideally, the alloy should wet both metals well and at same time keep it's low melting point.
It seems that I'll have to do an experiment to find out more about this!
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
Worth noting with French's metal is that it has cadmium and lead, so it may not be the best thing to be vaporizing. I was actually researching low melting point metals last week looking for some non-toxic solder that melts at 50-100c, and the only two that were free of cadmium, and mercury were Fields metal (mp 62c) and alloys of Indium/Tin (basically the same thing as fields metal, minus the bismuth ex mp 110c) I didn't really look into the gallium based alloys since they had a lower melting point than I was looking for, and like you said are expensive!
I am not sure if your application could handle the >100c melting point temperatures, but it is worth mentioning that bismuth has terrible electrical/thermal properties, being about 1/10th as conductive as indium and 1/80 as conductive as copper.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Oooo, keepig it near its mealting point upto the second of firing, then you really pulse the inductive heat. This kind of thinking may lead to a break through... youve clearly put a lot of thought into this... that Frenchs metal looks like a good starting point, its cheap enough, yet will stay solid when your device is not in use.
EDIT: mercury isnt so bad, just volatize some of that, felt hatters were likely nuts to begin with. EDIT 2: I was being facitous.
EDIT 3: Isnt there a U-boat missing of the norway coast? with 4000 liters of mecurcy in its keal?
Off topic but still interesting, Action of 9 Febuary, 1945. The first time a submerged ship shot another submerged target and killed it, and has never been done since. (65 tons of mercury on board)
This is one of those historic martial events like Major Phil Handly's 20mm auto-cannon kill of a MiG 19 from a F-4 at Mach 1.2, Using a high speed deflection shot-- the first and only time in history a supersonic gun kill was made.
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Permanent Magnet Rail Gun (PMRG)
