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DerAlbi

Wed Mar 06 2019, 10:59PM

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

If you come anywhere close to the capacitors internal RC time-constant with your LC-circuit period time, you by definition, dont have much L at all. Therefore the coil of the "coil"gun is missing. Therefore you dont have a coilgun.
Energy extraction speed matters in induction launchers, not reluctance motors.

Signification

Fri Mar 08 2019, 04:20PM

Registered Member #54278 Joined: Sat Jan 17 2015, 04:42AM
Location: Amite, La.
Posts: 367

2Spoons wrote ...

If you run through the math, for a given quantity of any particular dielectric it doesn't matter if you go low voltage/high C or high voltage /low C the energy storage is the same in J/kg. The speed with which you can extract that energy will be different, however.

I think I get what you are saying before going through the actual relationships pf parameters such as E, C, Q, k, q, etc.
OK.....We use a given 'volume' of a particular dielectric. I am assuming that the energy of the charged capacitor is stored in the Electric field. Also, it would be convenient to have a rectangular (including square) flat dielectric of which the plates (identical) can fit close-contact, fully, and squarely. The dielectric can be made thicker for a low C / high V OR thinner for a high C / low V.
I am mainly thinking in terms of the equation C=(A/d)e*k. Where A is one plate area (same for both), and d is the plate separation. e is epsilon naught (the permittivity) and k is 'kappa' (the dielectric constant). Since 'e' and 'k' are constants, the ratio of A/d determines the 'hi /lo' C, V parameters (I will later check to see if this ratio acts as I suspect).
Since capacitance is, by definition, Q/V (charge per potential 'difference'), I will say (WRT your last sentence), that the capacitor with the low C / high V will be the one that tends to discharge (in general) the fastest.

DerAlbi

Fri Mar 08 2019, 07:19PM

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

Why is it important that the capacitor discharges fast. Looking at other design, you know already with certainty that electrolytics are very much sufficient devices. Lowering the ESR of the capacitors is only interesting if you can capitalize on that somehow - so why is it your priority?
All your other considerations are... without a clear destination. Capacitor technology performance is characterized, among other stuff, by energy density. There are 2 forms that are important: volumetric energy density (energy per capacitor size) and mass related energy density. (energy per kg of capacitor).
In both aspects electrolytic capacitors are superior over foils. (because AlO3 has higher EpsilonR).
Foils come into play when you have high dV/dt requirements (that is how you specify the current rating/capability of a capacitor technology independent of capacitance). However in a coilgun, if you really need foil capacitors, you have done something wrong in your design and will invest into badly utilized resources for little effect and end up with a bigger an heavier build than actually necessary..

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Fri Mar 08 2019, 09:23PM

Registered Member #54278 Joined: Sat Jan 17 2015, 04:42AM
Location: Amite, La.
Posts: 367

@Albi:
MAYBE NOT!!

DerAlbi

Sat Mar 09 2019, 12:55PM

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

Is this your best rational argument you can do? I mean.. could you elaborate?

Signification

Sat Mar 09 2019, 01:45PM

Registered Member #54278 Joined: Sat Jan 17 2015, 04:42AM
Location: Amite, La.
Posts: 367

@Albi: Whenever I refer to a video or a new idea on the subject, even if it works like I wanted, you seem to have already shot it down and criticized me for even referring to it. One example if that YAK coil gun that fires at 256fps. And, YES, I am mainly interested in velocity first, then the efficiency mods (you will say that I am doing this backward). In this case an energy cap charged to a high voltage seems to fire the projectile a good velocities. One improvement was to use large AWG wire so that the high voltage pulse can get a lot of high current through the coil fast! This type coil is larger wire and fewer layers than 'normal'. I have found that using the first stage as a 'standard' electrolytic-powered coil (maybe the first two) to start the acceleration for the fast coils. If I get something worthwhile, I will post it. But it does seem that the slower coil is necessary for the fast coils to function and add high acceleration.

Sulaiman

Sat Mar 09 2019, 03:04PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

just for an idea of orders of magnitude,
if you hope for Mach1 projectile speed, with say 66mm long projectiles,
coil (charge + discharge) time = 100 us
so the coil ON/charge time will be around 90 us and discharge time around 10 us.

I consider electrolytic capacitors suitable for 100 us pulses and marginal with 10 us pulses.

so for projectile velocities below the speed of sound, electrolytic capacitors should suffice,
... in my opinion.

DerAlbi

Sat Mar 09 2019, 07:42PM

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

For a multistage approach i am with you; fast pulse currents can do their job if the projectile is pre-magnetized preferably up to saturation.
The thread title did not really imply that you dont want efficiency first. If you want to maximize energy transfer no matter what, then also tolerate suckback. Any current waveform that maximizes energy transfer will have some suckback. It has to do with the force vs position curve and with the nature of current rise- and fall times in an acceleration coil and the velocity that is gained.

However the way you put it, i seriously ask, why you dont just build. If nothing matters except kinetic output, then throw money, capacitor mass and build volume at the problem and you will get "velocity first" with no problem. This russian guy who spammed his gun everywhere ("V2006") had the same approach and he build the strongest hobby builds i know. Why not. I personally see it as a massive waste of resources, but thats everyone's own business i guess.

What is often missing is a goal that you want to chase. Some kind of figure of merit that tells you objectively what is good and what is bad. A coilgun, seen as an electric motor, does provide engineering challenges. Problems that require the balancing of a lot of problems and benefits vs loss considerations.
If you only present ideas without context and without goal or figure of merit, any idea on its own may work, but you have to agree that it is always important to point out which trade-offs you will make. You can, of course, close your eyes and ears if you like.
In the end you are a grown man. If you want to implement your ideas, go ahead.

On my side, your sudden idea to use pulse capacitors sounds much more motivated in misleading marketing headlines since the word "pulse capacitor" fits well with the "current pulse" idea in a coilgun.
You have presented no data, no calculation no goal, not even an order of magnitude why you would need such special capacitors and no argument why electrolytics will not suffice.

Signification

Thu Mar 21 2019, 09:58PM

Registered Member #54278 Joined: Sat Jan 17 2015, 04:42AM
Location: Amite, La.
Posts: 367

One reason for the pulse energy caps is to allow a faster more 'critical-to-underdamped' pulse which relaxes the problem of capacitor reverse polarity, while allowing a higher current via higher voltage fed coil. The underdamped pulse allows about three (3) times more: e=2.72 (NOT PI = 3.14) than the damped / overshooting / capacitor damaging pulse.

DerAlbi

Mon Mar 25 2019, 01:25AM

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

Correct me if i am wrong, but arent pulse capacitors mainly foil capacitors? Why fo foils care about polarity?

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