Login    Register
User Information
Username:
Password:
We are a free and open
community, all are welcome.
Click here to Register
Sponsored
Who is online

In total there are 66 users online :: 4 registered, 0 hidden and 62 guests


Most users ever online was 155 on Mon Aug 15, 2016 1:40 am

Registered users: Bing [Bot], Google [Bot], Google Adsense [Bot], Yahoo [Bot] based on users active over the past 5 minutes

The Team
Administrators
Global Moderators
global_moderators.png CS

High Velocity Launch Systems

A place to ask general spud cannon related questions.
Sponsored 
  • Author
    Message

Unread postAuthor: Technician1002 » Sun Apr 11, 2010 9:28 pm

Air core pulse transformere often have less flux lines. For more energy transfer in a small form factor, powdered iron core is recommended. It is the industry standard in switch mode power supplies.

It is the power transformer in practically every laptop power supply manufactured. High power, small size, high effeciency, low heat, no fan.

Remember it is the rate of change in flux lines crossing a conductor that induces current. A large capacitor bank for "sustained" current is pointless in a transformer. High rate of current change is important.
  • 0

User avatar
Technician1002
Chief of Staff
Chief of Staff
 
Posts: 5190
Joined: Sat Apr 04, 2009 11:10 am
Reputation: 14

Unread postAuthor: DYI » Sun Apr 11, 2010 9:47 pm

@245Tommy: I recently developed and tested a prototype design which should be very effective on the "holding together" front. I'll try to get around to posting it this week. Far simpler than my old design, and seemingly sturdier. At any significant energy, plasma will be very effective at leaking out, and any insulator will be strongly inclined toward exploding or deforming. The real trick is to give the plasma no unwanted volume to expand into, and the insulator nowhere to go.

The discharge time required is not extremely low. Even on very large, high energy, high efficiency designs, voltages as low as 20kV are used. I saw excellent efficiency from a design using 16uF @ 7.5kV, which certainly isn't impractically high (and could probably be done at lower voltage without much efficiency loss). For very high speeds, lower capacitance for a given energy is preferable, though harder on the chamber.
  • 0

Spudfiles' resident expert on all things that sail through the air at improbable speeds, trailing an incandescent wake of ionized air, dissociated polymers and metal oxides.
User avatar
DYI
Lieutenant General
Lieutenant General
 
Posts: 2861
Joined: Sat Jul 07, 2007 8:18 pm
Location: The People's Republic of Canuckistan
Country: Turks and Caicos Islands (tc)
Reputation: 9

Unread postAuthor: 245Tommy » Sun Apr 11, 2010 10:12 pm

DYI wrote:@245Tommy: I recently developed and tested a prototype design which should be very effective on the "holding together" front. I'll try to get around to posting it this week. Far simpler than my old design, and seemingly sturdier. At any significant energy, plasma will be very effective at leaking out, and any insulator will be strongly inclined toward exploding or deforming. The real trick is to give the plasma no unwanted volume to expand into, and the insulator nowhere to go.

The discharge time required is not extremely low. Even on very large, high energy, high efficiency designs, voltages as low as 20kV are used. I saw excellent efficiency from a design using 16uF @ 7.5kV, which certainly isn't impractically high (and could probably be done at lower voltage without much efficiency loss). For very high speeds, lower capacitance for a given energy is preferable, though harder on the chamber.
I look forward to seeing it, how much energy are you going to use? I'm still using camera caps so pulse time is still a problem for me.
  • 0


245Tommy
Specialist
Specialist
 
Posts: 45
Joined: Fri Jan 15, 2010 12:53 am
Reputation: 0

Unread postAuthor: Ragnarok » Mon Apr 12, 2010 4:59 am

rp181 wrote:Using a second coil in a air cored pulse transformer setup may well work just as well

I'm not entirely convinced. Anyway, I'm looking at the possibility of ferrosheathing the coils.

and simplifies issues like semiconductor protection.

I'll be honest, electronics is not really one of my strong points. The idea was designed to drive the coil as effectively as possible, not because it's great electronically.
I may well get to sorting out the specifics of some of the electronics and have to change everything based on that, but not all of my ideas are exactly designed with maximum practicality in mind.

Technician1002 wrote:Remember it is the rate of change in flux lines crossing a conductor that induces current.

Actually, that's half the reason I'm looking at the idea. While the high current changes will induce insane currents in the projectile, they'll last only a very short time and will only cause the decelerative force on the projectile for a very short distance, minimising energy lost to inductance.

~~~~~

Anyway, really not the topic. If you want to discuss reluctance coilguns, I'll be happy to in another topic, but we're talking about pretty mundane velocities here.
  • 0

Does that thing kinda look like a big cat to you?
User avatar
Ragnarok
Chief of Staff
Chief of Staff
 
Posts: 5339
Joined: Tue Dec 19, 2006 8:23 am
Location: The UK
Reputation: 8

Unread postAuthor: Technician1002 » Mon Apr 12, 2010 5:24 am

Ragnarok wrote: Anyway, I'm looking at the possibility of ferrosheathing the coils.



Confining the flux lines does increase the flux density. High flux density provides high force. Rapid change in flux induces high current in the projectile. Matching your cap discharge time with inductance to limit the duration to just the interval where the projectile is within the driving coil's influence will transfer the maximum energy.

Too low of a voltage or too high an inductace will create longer but weaker flux change intervals. High voltage with low inductace provides less acceleration due to the crumple zone effect cars use to reduce impact forces in a wreck. When done intentionaly, this is the coin shrinkers and can crushers. Energy is lost deforming the projectile and exploding the work coil. A softer longer discharge with the same energy content can provide higher velocities.

The flash capacitors you are using are designed for discharges lasting about .1 ms to 1 ms. They do not provide the low inductance, impedance path to provide the very high pulse current of a proper pulse capacitor. They are low impedance, low inductance, and mechanicaly built to withstand high mechanical shock from high currents. Very high discharge currents on flash capacitors will blow out the connections to the foil inside. They are designed to work into the resistance of a flashtube, not higher pulse currents.

Electrolytic capacitors (flash caps) should not be permitted to reverse polarity from the resonant LC circuit created by the work coil and capacitor. Reverse polarity will damage them. Use a fast high current diode to prevent reverse polarity on the flash caps.
  • 0

User avatar
Technician1002
Chief of Staff
Chief of Staff
 
Posts: 5190
Joined: Sat Apr 04, 2009 11:10 am
Reputation: 14

Unread postAuthor: DYI » Mon Apr 12, 2010 8:40 pm

As I mentioned yesterday, here's the new ETG design. It is scalable to any energy/voltage combination desired, so far as I can tell (with slight modification).

Image

The heavy black lines are threads. The real trick is the seal between the capillary tube and the nozzle - if the tube is too short (or the chamber not threaded in tightly enough), it is not crushed in and doesn't seal, allowing plasma to escape into the space in between, ruining efficiency and probably the chamber. If the tube is made too long, or the rear electrode threaded in too far, the tube becomes so crushed that it necks down and closes off, resulting in partial discharge (sounds impossible, experience proves otherwise) and lower efficiency.

At higher muzzle speeds, the chamber pressure could rip off any practical thread engagement for the insulator, necessitating a bit of a modification - the threaded end of the insulator at the rear is turned down, and a hollow steel plug is made to thread in behind it. The core of the insulator, and the electrode inside pass through the hollowed center of the plug, while the outer edges prevent the electrode from escaping by any means other than actual extrusion through the hole. If you reach high enough chamber pressures that extrusion of the insulator is a major problem, I'm sure NASA would be willing to help you out.

High tolerances are necessary, because if there is ANY sloppiness around the insulator it will shatter and/or deform.

Barrel and chamber erosion problems are always present in high muzzle velocity ETGs which manage to avoid exploding. There are two obvious ways to counter them: dense, high melting point internal liners made of tungsten, and ablative internal liners made of plastic (or perhaps metals, at very high energies). Ablative liners will become the only option eventually, and contribute to gas production. They are also cheaper and easier to produce, and therefore the preferred choice. In the case of barrel throat erosion, tungsten may be beneficial in that it would cut gas blowby of the projectile.

Also an issue is nozzle erosion; with higher energies, the nozzle should not be part of the chamber, as it is destroyed very rapidly, possible in a single shot. Stainless steel sacrificial nozzles would likely be effective, and not particularly expensive. By this point, there should also be a sacrificial electrode tip. I'd recommend tungsten/copper alloy for the electrode and chamber at very high energies.

That is the design as it stands now. As you may imagine, I've designed much further ahead than I can currently build. Other theories on the mechanical design of high muzzle speed ETGs would be appreciated.
  • 0

Spudfiles' resident expert on all things that sail through the air at improbable speeds, trailing an incandescent wake of ionized air, dissociated polymers and metal oxides.
User avatar
DYI
Lieutenant General
Lieutenant General
 
Posts: 2861
Joined: Sat Jul 07, 2007 8:18 pm
Location: The People's Republic of Canuckistan
Country: Turks and Caicos Islands (tc)
Reputation: 9

Unread postAuthor: rp181 » Mon Apr 12, 2010 10:48 pm

Why did you make the capillary tube and insulator flush? Thats never a good idea.
Extend the capillary tube to overlap the chamber some. You can poke a hole in the capillary tube to get the fuse wire out.

It will also help with high voltage safety.
  • 0

User avatar
rp181
Brigadier General
Brigadier General
 
Posts: 1090
Joined: Thu Dec 13, 2007 8:47 pm
Reputation: 0

Sponsored

Sponsor
 


Unread postAuthor: 245Tommy » Mon Apr 12, 2010 10:55 pm

rp181 wrote:Why did you make the capillary tube and insulator flush? Thats never a good idea.
Extend the capillary tube to overlap the chamber some. You can poke a hole in the capillary tube to get the fuse wire out.

It will also help with high voltage safety.
You mean in the side of the tube? That would probably make it fail.
  • 0


245Tommy
Specialist
Specialist
 
Posts: 45
Joined: Fri Jan 15, 2010 12:53 am
Reputation: 0

Unread postAuthor: DYI » Tue Apr 13, 2010 6:47 pm

@rp181: To remove the capillary tube, one simply threads the rear electrode in farther, pushing it out from behind. As mentioned in my description, the capillary tube is NOT flush with the surface of the insulator, it extends out a small amount. It only becomes flush when it is crushed down by the chamber being threaded on.

As the fuse wire extends outside the capillary tube, there's no trouble getting it out. Just pull on the end, and it's out. Why one would want to get it out is another matter entirely.

And high voltage safety? This design has VASTLY less potential for unwanted internal arcing than any of my others. I would go so far as to say that this design will eliminate the potential for any such problems (although I am highly doubtful that they could occur in the first place).

Diagrams don't work too well when you don't read the accompanying descriptions, do they? :lol:

@245Tommy: The tubes are not reusable, but they are so tightly constrained in this design that they cannot catastrophically fail, other than by being completely ablated (which I have nowhere near sufficient energy to do). They are, however, single use, as they need to be deformed significantly to create a working seal with the nozzle (and tend to deform further during firing and subsequent cool-down).
  • 0

Spudfiles' resident expert on all things that sail through the air at improbable speeds, trailing an incandescent wake of ionized air, dissociated polymers and metal oxides.
User avatar
DYI
Lieutenant General
Lieutenant General
 
Posts: 2861
Joined: Sat Jul 07, 2007 8:18 pm
Location: The People's Republic of Canuckistan
Country: Turks and Caicos Islands (tc)
Reputation: 9

Unread postAuthor: rp181 » Tue Apr 13, 2010 7:00 pm

By bringing the fuse wire out, I meant attaching the wire to the cathode.

The capillary tube still becomes flush. If you had a delibrate overlap, the chamber pressure would work to your advantage.

Think of it like the solid rocket booster on the space shuttle. To seal the sections, they have large O-rings. These seal when it is passive, but as soon as the engine is lit, the case expands out, and the force of the reaction forces the O ring to seal. Your capillary tube can be made to do the same thing.

You are relying on deformation to make a seal right now, but that method still has a greater possibilty of failing, this is exactly what happened in my railgun. When the projectile impacted the rails, the force always pushed the teflon rails back, letting some gas escape through the rail exit holes. The teflon was compressed between the rails and the injector mateplate.
On version 2, I had the teflon overlap the rails (Like it should be), and their was no escape of gas. The fake rail did allow gas to pas once, but that was because of the nature of the origin of pressure. With the pressure originating behind the mating point, that is almost fool proof.

Diagrams don't work too well when you don't read the accompanying descriptions, do they?

I read it :wink:

As mentioned in my description, the capillary tube is NOT flush with the surface of the insulator, it extends out a small amount. It only becomes flush when it is crushed down by the chamber being threaded on.

It still becomes flush, who cares if its not flush before firing? The chamber also isn't broken before firing, but that could very easily change :lol:
  • 0

User avatar
rp181
Brigadier General
Brigadier General
 
Posts: 1090
Joined: Thu Dec 13, 2007 8:47 pm
Reputation: 0

Unread postAuthor: DYI » Tue Apr 13, 2010 7:50 pm

Ah, I see the point you're trying to make, and I'm well aware of the mechanism you describe. Your original statement didn't make that clear in my reading.

It may well be a worthwhile modification. I'll definitely attempt it when I redo the chamber. As it stands, I couldn't drill in that far without destroying the integrity of the nozzle. The chamber's sealing face with the capillary tube is already damaged due to stupidity on my part, so I'll be making a new one soon anyway. I doubt I'll need too much overlap to get a working seal.

This will, however, make the issue of length tolerances all the more important. Any other ideas for sealing improvement are, as always, appreciated. Tiny leaks always seem to be the death of my ETGs' efficiency.

Also, cathode and anode aren't very useful descriptors here. In my case, the anode is the rear electrode. It's not really important to attach the fuse wire solidly, so long as it makes contact to some degree.
  • 0

Spudfiles' resident expert on all things that sail through the air at improbable speeds, trailing an incandescent wake of ionized air, dissociated polymers and metal oxides.
User avatar
DYI
Lieutenant General
Lieutenant General
 
Posts: 2861
Joined: Sat Jul 07, 2007 8:18 pm
Location: The People's Republic of Canuckistan
Country: Turks and Caicos Islands (tc)
Reputation: 9

Unread postAuthor: rp181 » Tue Apr 13, 2010 10:18 pm

It's not really important to attach the fuse wire solidly, so long as it makes contact to some degree.

I disagree. When I messed around with wire exploding, a poor contact resulted in fragments of wires being torn off before it had a chance to vaporize. With firm contact, the whole wire sees almost the same of everything, causing everything to vaporize at once. When the wire fragment breaks, you have a arc that conducts power, taking away energy that could be delivered to the chamber.

I speak from experience here (yes, it was in a confined space).
  • 0

User avatar
rp181
Brigadier General
Brigadier General
 
Posts: 1090
Joined: Thu Dec 13, 2007 8:47 pm
Reputation: 0

Unread postAuthor: DYI » Wed Apr 14, 2010 9:59 pm

The entire point of the fuse wire is to start an arc. The only function of the aluminum is to make sure that the arc forms inside the capillary tube (and creates an arc longer than could otherwise occur at the firing voltage). I'm not interested in how evenly the fuse "wire" vaporizes, only that it does so quickly, and starts the arc.

Small conductive fragments could shorten the arc path, but only very little, for a minuscule time interval. I highly doubt that the performance difference, if any, could be detected by my feeble instruments. I don't doubt that there's some chance of performance improvement by securely fastening the fuse wire, just that it is significant. Using extremely low voltages, as you presumably were in your experiments, would definitely worsen the problem of poor contact.
  • 0

Spudfiles' resident expert on all things that sail through the air at improbable speeds, trailing an incandescent wake of ionized air, dissociated polymers and metal oxides.
User avatar
DYI
Lieutenant General
Lieutenant General
 
Posts: 2861
Joined: Sat Jul 07, 2007 8:18 pm
Location: The People's Republic of Canuckistan
Country: Turks and Caicos Islands (tc)
Reputation: 9

Unread postAuthor: rp181 » Wed Apr 14, 2010 10:28 pm

I did some tests with 6.5kV.
Look around your chamber with loose contact, you will find wire fragments. So when it breaks, it does not vaporize. Im not talking about the arc length, that becomes negligible in a very short amount of time. I am talking about the plasma density. Vaporizing more material gets a higher plasma density, and more energy is available in the shorter amount of time.
  • 0

User avatar
rp181
Brigadier General
Brigadier General
 
Posts: 1090
Joined: Thu Dec 13, 2007 8:47 pm
Reputation: 0

Unread postAuthor: DYI » Thu Apr 15, 2010 7:49 pm

I've never found any trace of aluminum fragments in the chamber, on the target, or anywhere else for that matter. Not to say that they don't exist, just that they are not very large. It would be interesting to do tests with and without a fuse wire, although it would require a modified design and rather high voltage. Could you direct me to the study that you're basing your argument on? I've never seen anything dealing with the fuse wire in particular as regards efficiency - it is usually just mentioned in passing.

To the rest of you: This is not just an ETG discussion. Read my first post in the thread. If you have anything to contribute - ideas, papers, designs, working models... - please do. Any posts dealing with high velocity launch (and within forum rules, of course) are permissible here.
  • 0

Spudfiles' resident expert on all things that sail through the air at improbable speeds, trailing an incandescent wake of ionized air, dissociated polymers and metal oxides.
User avatar
DYI
Lieutenant General
Lieutenant General
 
Posts: 2861
Joined: Sat Jul 07, 2007 8:18 pm
Location: The People's Republic of Canuckistan
Country: Turks and Caicos Islands (tc)
Reputation: 9

PreviousNext

Return to General Spud Cannon Related

Who is online

Registered users: Bing [Bot], Google [Bot], Google Adsense [Bot], Yahoo [Bot]

Reputation System ©'