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  • 11/30/2007 (2:36 am)

    Induction Launchers & EM Rocketry by

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    Filed under: FM_Projects ::

    Induction Launchers (aka Mass Driver)

    A Ring Launcher is an inductive launcher. When the coil is energized, it creates a strong magnetic field around it. Since this coil is flat and a non-ferrous ring is placed direcly upon it, this magnetic field creates a huge rush of current in the ring itself. This current turns the ring into a strong magnet of opposing force which repels the ring away from the coil at amazing speed! I’ve been using aluminum rings exclusively as they are great conductors and easy to get – take any broken 3.5″ hard drive apart to get perfect rings for launching!



    The pic on the left shows the ring in place, ready to be launched. The ring shown is one I fabricated out of 5/16″ aluminum to the same dimensions as a hard disc platter. The bonus is a heavier ring which is also able to produce a stronger magnetic field. The result is more energy out. There will of course be a point where the weight of the ring is too much and efficiency will start dropping again. On the right is the coil after about 30 shots. It used to be round and not epoxied, but after 1 shot it formed ears!! So I epoxied it and it didn’t change much after that, though you can see the epoxy beginning to break apart.

    Preliminary Results

    Coil: 14awg magnet wire

    Ring – 3.5″ hard disc platter

    Weight – 22g

    Speed – 57.3288 m/s

    Energy – 36.16j

    Efficiency – ~2.1%

    Ring – 3.5″ 5/16 self cut aluminum disc

    Weight – 58g

    Speed – 60.68568 m/s

    Energy – 106.78j

    Efficiency – ~6.2%

    This is my ring former. I cut two 3.5″ circles out of plywood. One will be the coil backing and the other is used as a vice. I place a washer the thickness of the wire around the bolt to be the center. I then cover the block that will be part of the coil with double stick tape. Next I lightly clamp it using the nut and bolt and start wrapping the coil. Then I unclamp it. The tape holds the coil in place while I paint it with epoxy. Then I put an index card over the epoxy and reclamp it until it cures. Voila! “Perfect” coil. I didn’t develop this method until after the coil seen above was wrapped…

    Results for “perfect” coil:

    Coil – 14awg magnet wire

    Ring – 3.5″ 5/16 self cut aluminum disc

    Velocity – 67.60464 m/s (151mph)

    Weight – 58g

    Energy In – 1.7kj

    Energy Out – 132.54j

    Efficiency – 7.7%



    The effects of the new coil on a soda can

    This is my capacitor bank used for all of my electromagnetic guns. It consists of 8 capacitors. 4 are wired in parallel to form two banks, which are then wired in series, giving 700v. The power output is > 1.7kj. On the left you see a power switch – this powers the transformer in the lower right to produce 700v+. This is fed into the bridge rectifier to the right of the transformer, and then to the resistor seen in the middle of the caps. The resistor prevents high currents during the start of charging.. Finally the “resisted” 700v is fed to the cap bank to gently charge the caps in about 20 seconds. A voltmeter tells me when it’s fully charged, at which time I turn the switch off. When the fire button is pressed, 3 volts from the AAs seen bottom right are fed to the SCR triggers, with a resistor in front of each. The three SCRs are in parallel. When thr SCR gates close, all the power is dumped into the load connected to the terminal block. A large diode seen left of the SCRs is connected so that back EMF from the coils do not damage the SCRs.

    March 2006: I upgraded my capacitor bank to double its capacity – now > 3.5kj. It’s simply two of the smaller banks in series. The new coil is a dual layer coil, with each layer wired in parallel.

    This is a 1-2-3 of the test results for various wirings of the cap bank, different projectiles, and different coils.

    It’s best to match the coil to the projectile than the other way around…so I got some aluminum slugs and made plywood blocks the same diameter. These blocks hold the coil.

    First I paint on some poly glue and then I wrap the coil around the block by putting a spacer in the middle and another block on the other side, held together with a bolt. It goes into the vice for good measure while curing.

    After the glue sets, a finished coil. Repeat for second layer.

    Coil is done, wired and painted. Ready for testing in incremental voltages.

    Oops…after successful firing at 1000v (1.8kj in the bank), I figured why not see what it can handle…1200v and the sound of a shotgun later, and the solder has vaporized…That grey and black residue is solder. So at least it has a built in fuse…I’m going to rate this size coil at 1000v max.

    I launched my electromagnetic rockets “EMRockets” the other day. The footage shows induction launched rockets at progressively higher voltages…the higher the voltage, the more altitude, but also the more Gs, and therefore the rockets get destroyed. It’s back to the drawing board to build a rocket that can withstand more than one full power launch.

    Unfortunately I don’t know how high they’re going…I need an altimeter for accurate measurements, but a little math tells me that the highest launch reached about 47.1 meters…minus resistance of course so it’s a bit less.

    The launchpad wire is 14awg magnet wire…same stuff used in all my other CGs and launchers. The only difference is that this coil is only 2.5″ in diameter, instead of 3.5″….don’t know how many turns…I just wrapped it until it was the right diameter as I built the coil for the aluminum slugs I’m using, which are 2.5″ diameter and 3/8″ thick. It is a dual layer coil, though I think I’m going to make a 3 layer one as the current is vaporizing the solder before I can fire the bank at full capacity. My 3.5″ diameter DL coils had each layer in parallel…this smaller one has them in series.

    I picked those aluminum slugs only because that’s what I got my hands on…nothing else. Once I got the aluminum slugs, I designed everything else around them. From my other ILs, I did discover that thicker aluminum is definitely better….e.g. hard disk platters are nothing compared to 1/8″ al in terms of efficiency and energy output.




    And believe it or not, all those were all launched from a 12v 5ah battery!

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    13 Responses to “Induction Launchers & EM Rocketry”

    1. Itrees Says:
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      What’s with the low efficiencies on this experiment? I don’t understand where all the energy is being lost.

    2. Itrees Says:
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      Another question, would it be possible to launch a rocket using two electromagnets, one on the launchpad and one on the rocket?

    3. FastMHz Says:
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      Coil guns and induction launching are very inefficient for a few reasons, the largest being that it’s just not possible to convert all the stored energy to the motion of a projectile that fast without huge losses. It’s a limitation of physics. Induction launchers are more efficient than coil guns, however, and I have managed to create one of the most efficient in the hobbyist community so far.

    4. FastMHz Says:
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      Yes, in theory. But then your rocket would have to have a huge store of energy on board in capacitors, and the power to weight ratio of this storage method is terrible. The costs of added weight far outweight the gains from magnet-magnet repulsion.

    5. Itrees Says:
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      I guess what I’m wondering is more where all this wasted energy is going. I was wondering if all of 1.6k joules is going into waste heat, or if the capacitors are continuing to discharge after the rocket is too far away for the system to efficiently accelerate it.
      I was also wondering if some considerable energy might be stored in the magnetic field which could theoretically(using some sort of circuit breaker and diode system) be used to give the capacitors some of their initial charge back.

    6. FastMHz Says:
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      You’re pretty much right on….some is wasted heat in the wires, the coil is most likely still energized after the rocket is far away from it, and the coil does have a field that collapses and induces a reverse charge, hence the clamping diode that absorbs this energy to prevent SCR death.

    7. Sieg Says:
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      Hey, i have a doubt, i’m trying to build a capacitor bank just like yours, i have 16 capacitors of 3700uF @ 350v to be feed by 1086v dc
      But i don’t have the slightest idea of how to connect them so they could withstand the voltage
      Can you help me out?
      Thanks in advance

    8. FastMHz Says:
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      Hi – a bank of that size will be ~3.6kj of energy, a pretty nice bank. If you are determined to feed it 1086v, you’ll need to set up an array of 4 series, 4 parallel. So, make 4 strings of 4 in series, and then parallel those strings. The resulting bank will be capable of 1400v @ 3700uf.

    9. Sieg Says:
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      Well, i have put the bank that way, now i’m waiting for the SCRs and the big diode, thanks, and another doubt, i copied your schematic for the gauss gun, do you think that it will be able to manage the 3.6kj?

    10. FastMHz Says:
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      What your system can handle depends on a multitude of things, including SCR surge capability, wire sizes, coil design, etc. I blew many SCRs with my 3.5kj bank, so for yours, you’ll most likely need larger ones than I was using (SemiPack SKKH5720E). For my huge 24kj bank I’m using a triggered spark gap, because SCRs of that size are prohibitively expensive.

    11. Sieg Says:
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      Well i have one final question, and i think that it’s a tough one, half of my capacitors were damaged, that leaves me with 8 capacitors of 3700uF and one of 2500uF (@350v all of them) and we couldn’t find a large diode and we just have one big SCR, we were thinking about changing the power input to 600v to avoid overloading everything, any sugestions?

    12. FastMHz Says:
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      If you do not protect your SCR and capacitors with a large clamping diode, aka connected in reverse to the SCR, they will receive a surge when the coil’s magetic field collapses, wiping out the SCR and, eventually, causing additional capacitor damage.

    13. Sieg Says:
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      well i will stay with my actual 8 capacitors, plus we found some medium sized diodes, and we were thinking about puting 2 or 3 in parallel to avoid getting them burned

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