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ShieldExperiments

Tue Jun 12 2018, 01:59AM

Registered Member #61689 Joined: Mon Jul 10 2017, 02:46PM
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Posts: 14

@DerAlbi: Yes, I know about the issues here, but his other work had always made up for it in after the fact analysis, even with all the flaws, but I agree that his site is hardly the best. It was just a similar design which I had seen.
Additionally I seem to recall a 9 stage coilgun pistol on one of the old russian forums using this barrel design. If I recall correctly it was halfbridge, with something like 20% total eff. I don't remember which forum it was on, but it was beautifully implemented.
EDIT! Well, someone already beat me to the punch. The one I'm thinking of is the second one that Eugen linked above

DerAlbi

Tue Jun 12 2018, 07:29AM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

Concerning the Power-Labs guy: afaik also his conclusion were bullshit. The problem with a limited knowledge is that your conclusions are based on whats inside your horizon.. I know that i went over the site once and left in pure anger - sources like this hinder everyone in achieving great things. The most offensive is the Donate-Button however..
Anyway: a slotted barrel works, but the slot also removes a lot of strength. Not sure that it will be a good solution to mechanical problems.

I also know the work of that russian.
The first link is actually new to me (i cant search russian sites - limited language skills). The 100/s with 13J @ 15%eff sound reasonable however. His 300J cap bank sounds like a heavy overkill.
Before i started, the second link was the state-of-the-art build for what i remember.. but its "only" 14% efficiency for the little energy he pushes (~6J?) with his 1kg device.

Concerning the BB-gun... i know too little about the build, but i feel like that the half-bridge approach like in the Russian links may not be practical with such a short projectile. With only a few 10Âµs actionable time a half bridge will be challenging. His switching scheme would demand a switching frequency well above the capabilities of even very good IGBTs.

_Eugen_

Tue Jun 12 2018, 08:38PM

Registered Member #57984 Joined: Thu Nov 19 2015, 09:44AM
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Posts: 58

Let us calulcate something (cannot persist myself :). Assume coil length 4,5 mm (equal to the length of BB, longer coil would be ineffective). At speed 100 m/s progectile will pass it in 4,5E-3/100 = 45 Âµs.
It is not very high frequency even for most powerful "slow" IGBTs (althow there are special families which are faster but less powerful). Of course very well built driver must be ensured.
Another question arises.
Rpopeye says that MOSFETS are used it means the voltage doesn't exceed 100 V (higher voltages require IGBTs to constrain the cost of the scheme in a reasonable limit). So the circuit must supply (2/48) = 42 mJ in 45 Âµs (suggesting the energy increment is equal in all stages). This means about 1 kW "pure" power or about 1 kW/100V/1% = 1000 Amps current (according to 1 % efficiency which is reasonable for such "uncomfortable" projectile).

A huge value. There is inconsistency somewhere.

DerAlbi

Wed Jun 13 2018, 08:11AM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

..the russian original applied a constant current drive to the coils. I did the calculations and also got 45Âµs. However my further conclusion were different: to make a constant current actually reasonable constant you need multiple switching cycles within the 45Âµs time window. This might leave you with a period of 4.5Âµs or 9Âµs depending on how many cycles you want to have. And this is actually unreasonable for IGBTs since this is >100kHz.
In the context of your calculations it actually makes not much sense to speak of a frequency since you implied a single pulse without repetition.

I dont think you have much inconsistencies. I am not sure where the 2 comes from in 2/48 (you assumed 2 Jules?)
Even if just 2 Joules, a coil needs _more_ energy in it than the energy it transfers mechanically. So the actual value might be much larger. But the voltage might be higher and the current lower. You can easily get 600V Mosfets with less than 30mOhm on-resistance (or higher - i assume the Mosfets to be a major problem for the efficiency)... Its not far from doable, but far from practical [cost].
Hmmh. Its a bit of a mystery how he did it without selling his home to be honest. But that is actually fine with me ^_^
..and he is actually demonstrating outdoors... maybe not without reason

Not sure if this is actually a push forward in technology or just the same old design build to a size that no one ever thought necessary [but still cool enough; 100m/s is a dream i still have]

rpopeye

Wed Jun 13 2018, 07:46PM

Registered Member #62069 Joined: Tue Jan 02 2018, 08:21AM
Location: Sofia, Bulgaria
Posts: 13

sorry I didn't reply the last few days, I was a bit busy
to answer some of the questions:
- the coils have different parameters along the barrel, in general due to the smaller and smaller time the projectile spends inside a coil as it is being accelerated, coils further down the barrel have a smaller number of windings (shorter pulse requires less inductance to reach maximum current)
- the mosfets are indeed 100V (optimum in terms of price/performance ratio - $1.5 per mosfet) and the maximum current is about 350A
- efficiency from capacitor bank to kinetic energy is actually 2%, it's 1% when including the DC-DC converter
- the BOM is about $200
- I am not saying that a matrix design wouldn't work, I am saying that it wouldn't be able to pass as high currents as the simple one-NMOSFET-per-stage design

rpopeye

Wed Jun 13 2018, 07:55PM

Registered Member #62069 Joined: Tue Jan 02 2018, 08:21AM
Location: Sofia, Bulgaria
Posts: 13

I am highly skeptical about portable designs that state efficiencies on the order of 15% and high velocities, only unquestionable video demonstrations can convince me that such beasts exist. I mean DARPA got it only up to 25% with a 2m long mortar that used inductive drive and insane voltages and currents, and a Ghz radar to sense the position of the projectile.

_Eugen_

Wed Jun 13 2018, 09:00PM

Registered Member #57984 Joined: Thu Nov 19 2015, 09:44AM
Location:
Posts: 58

2 DerAlbi

..to make a constant current actually reasonable constant you need multiple switching cycles within the 45Âµs time window. This might leave you with a period of 4.5Âµs or 9Âµs depending on how many cycles you want to have..

Yes but if the coils are similar, you may have multiple pulses (ensuring approx. stable current) in the beginning of acceleration, and only one (nearly triangle) pulse in the last coil which nevertheless reaches the same current in its peak. Such a configuration lowers the efficiency and I argued with Dantist (an authour of the coilgun I linked above) about that matter, but he insists his scheme to be optimal (maybe because construction profits are more critical).

2 Rpopeye:
Thanks, little information is better than nothing. Now the values become more realistic. 2 % total efficiency means you may have 4 or 5 % efficiency on the last stages which convert 1000A to 200 A peak current - quite tolerable for 350 A MOSFETs you spoke about. Of course 100V initially stored in caps are lowered to maybe 70V till that moment and must be compensated by more current as DerAlbi points, but reserve still takes place.

and a Ghz radar to sense the position of the projectile

interesting solution :)

DerAlbi

Wed Jun 13 2018, 09:13PM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

I am highly skeptical about portable designs that state efficiencies on the order of 15% and high velocities, only unquestionable video demonstrations can convince me that such beasts exist. I mean DARPA got it only up to 25% with a 2m long mortar that used inductive drive and insane voltages and currents, and a Ghz radar to sense the position of the projectile.

Dont mix induction coilguns with reluctance coilguns. Different concepts, different problems, different limitations.
A 25%eff induction launcher does not speak against my 26%eff reluctance coilgun or the Russian 15%eff design.
Of course my 40m/s arent really high velocities but the concept should be good up to 100m/s until i reach semiconductor limits. Of course thats nothing against induction coil guns, but they are a completely different class of devices.

rpopeye

Thu Jun 14 2018, 06:08PM

Registered Member #62069 Joined: Tue Jan 02 2018, 08:21AM
Location: Sofia, Bulgaria
Posts: 13

Here is a link to the DARPA paper about the inductive mortar launcher

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