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Ramjet Sabot

A place to ask general spud cannon related questions.
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Unread postAuthor: PaperNinja » Fri Dec 14, 2012 8:40 pm

That's a great paper! Thank you for the find.
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Unread postAuthor: ferret_guy » Sat Dec 15, 2012 12:14 am

ramses wrote:As I remember, Autodesk CFD doesn't simulate compressible fluids.


CFD can do compressible fluids and supersonic flow along with tones of other stuff however it cannot do 2 essential things
Temperature increase due to gas compression
Burning/Ignited gas or fluid
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Unread postAuthor: Alanstone » Sat Dec 15, 2012 5:31 am

Have you looked at N2O as an oxidizer? At the pressure ranges you are contemplating I think you may consider it.
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Unread postAuthor: PaperNinja » Sat Dec 15, 2012 9:34 am

Would an ETG be a suitable injector? Or does it need to be something more like a LGG or hybrid (because of the flame)?
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Unread postAuthor: D_Hall » Sat Dec 15, 2012 9:09 pm

ferret_guy wrote:After several different destines this is the one that did the best

You should be able to get rid of the forward....uh.... strakes or whatever you want to call the canard-like features that are no doubt intended to keep the projectile centered in the bore. You may even be able to get rid of the aft ones too but I don't know what that would do for stability.

Keep in mind you're in the barrel for a VERY short period of time and already moving quickly. After you've injected the projectile into the RAM acceleration tube, the timelines are short enough (for any project that isn't government funded, at least), that bullet "drop" is negligable, thus inertia should suffice to keep the projectile centered.
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Unread postAuthor: PaperNinja » Sat Dec 15, 2012 9:44 pm

D_Hall wrote:
ferret_guy wrote:After several different destines this is the one that did the best

You should be able to get rid of the forward....uh.... strakes or whatever you want to call the canard-like features that are no doubt intended to keep the projectile centered in the bore. You may even be able to get rid of the aft ones too but I don't know what that would do for stability.

Keep in mind you're in the barrel for a VERY short period of time and already moving quickly. After you've injected the projectile into the RAM acceleration tube, the timelines are short enough (for any project that isn't government funded, at least), that bullet "drop" is negligable, thus inertia should suffice to keep the projectile centered.


By the way, we had been calling them guides :)

I think it would be a good idea to keep the rear guides, if only because they can double as fins. I do see what you're saying about bullet drop, but it still would be nice to have a LITTLE support.

I WISH this were government funded... I'm 14 though so that's just a LITTLE unlikely :roll: (though my dad does have a secret clearance)
I will have a high budget for this however (multiple thousands---hopefully)

btw, I'd been talking to a friend of mine, and he's gonna help me build this, right now we're working on the power source for the ETG that injects into the accelerator.
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Unread postAuthor: DYI » Sat Dec 15, 2012 10:25 pm

Let me first say that it's quite refreshing to see someone using modeling tools around here. I'm quite busy with school at the moment, but I'd be interested in discussing this project with you over the break (starting a week from now).

I'm the resident ETG builder here. I love designing them and have reached a bit upwards of 3000m/s with them, but they're a tremendous money sink, the extant literature on them is pretty slim, the capacitors are huge and hard to come by, and modeling them adequately is beyond even the prodigious capabilities of CFX or Fluent. Fun hobby projects, not so much for utility.

Just to get you on the right track: don't use an ETG for the injector. "Multiple thousands" will vanish and you may not even have a working injector to show for it. Just build a suitably high pressure hybrid, possibly with a low density buffer gas to improve the sound speed. It's quite practical to push them up to 600m/s, and not overly difficult, if you can afford the machining and high pressure plumbing, to reach more than twice that (three or four times even, if you're really determined, but I digress...).
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Unread postAuthor: PaperNinja » Sat Dec 15, 2012 11:37 pm

DYI wrote:I'm the resident ETG builder here.

I noticed :P

DYI wrote:but they're a tremendous money sink

DYI wrote:"Multiple thousands" will vanish

That's unfortunate, but if I spread the project over a large time period, I'll probably be more likely to find cheaper capacitors and such. Also, my parents will be more willing to fund it if it's X (but many X for a big project like this) dollars over two years, instead of over a few months.

DYI wrote:the extant literature on them is pretty slim

Hopefully, your thread(s), larda's thread, and a few others should teach me everything I need to know about ETGs

DYI wrote:the capacitors are huge and hard to come by

hopefully not so hard if I wait long enough.

DYI wrote:Fun hobby projects, not so much for utility.

I, of course, would also be shooting the ETG without the accelerator, it would be awesome to have one in general. The ram accelerator is honestly just ("just" :roll:) an accessory, albeit an awesome one.


DYI wrote:Just to get you on the right track: don't use an ETG for the injector. and you may not even have a working injector to show for it. Just build a suitably high pressure hybrid, possibly with a low density buffer gas to improve the sound speed. It's quite practical to push them up to 600m/s, and not overly difficult, if you can afford the machining and high pressure plumbing, to reach more than twice that (three or four times even, if you're really determined, but I digress...).


I'm building a hybrid right now, started and planned before I knew I was going to be doing this. It'd be kind of a letdown to build two hybrids in a row... especially seeing as I want to build and have and use an ETG (as you mentioned, like a hobby project, not really as a utility, though it will make a great utility if it works). Also, one of the reasons my parents pay for this stuff is the stuff I learn in the building process, so they might not quite be excited to pay for another hybrid, given that I've already built one---unless it were significantly different.
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Unread postAuthor: ferret_guy » Sun Dec 16, 2012 1:23 am

This is the latest design based on papers and my own influence and results (i will try the non stabilised one next because right right the projectile is in there for something like .00789 seconds with a 8ft tube (w/o acceleration due to combustion)

Edit:
With open air flight (it too was an influence in this design)
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Unread postAuthor: PaperNinja » Sun Dec 16, 2012 1:25 am

I don't exactly understand where the combustion happens in that model, given how protruding the fins are.

Also, as the barrel likely will be split into multiple pieces anyway, a much longer barrel is certainly feasible.
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Highlighted areas with combustion

Unread postAuthor: ferret_guy » Sun Dec 16, 2012 1:40 am

Highlighted areas with combustion
This is of course uper rough

Edit: finished some preliminary math from some papers I read and found out that we are talking about exit velocities hovering around mach 10 (3,402.9m/s)
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Unread postAuthor: D_Hall » Sun Dec 16, 2012 2:04 pm

You're gonna need to keep the fins on that one. No way is it stable without 'em.
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Unread postAuthor: PaperNinja » Sun Dec 16, 2012 9:30 pm

Keeping the fins shouldn't be a big deal, I'll just need a salt in the injector.

Mach 10 is great, higher than I was expecting :)

By the way, my dad said he'd get me to the desert for awesome shots if I ever finish it. Also, we have a really high speed camera
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Unread postAuthor: D_Hall » Mon Dec 17, 2012 12:12 am

Unless you're very very wealthy, I guarantee your dad's camera isn't up to the task if you're actually pushing Mach 10. At the office we play with things in the Mach 8 category and our cameras are barely up to the task.... And our cameras run about $80,000 each.
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Unread postAuthor: PaperNinja » Mon Dec 17, 2012 12:43 am

D_Hall wrote:Unless you're very very wealthy, I guarantee your dad's camera isn't up to the task if you're actually pushing Mach 10. At the office we play with things in the Mach 8 category and our cameras are barely up to the task.... And our cameras run about $80,000 each.

Oh well nvm :lol:
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