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 Post subject: about rocket ignition
 Post Posted: Wed Jul 23, 2003 11:00 pm 
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Originaly posted by taimaishu1103,
if you have ever seen the US army dragon anti-tank rocket system, this si how it fires:

You acquire the system through the sights and estimate range. Once you fire the rocket an extremely small charge pops the rocket out of the barrel, than after it is a few feet out, the rocket engine fires and take sthe rocket at a 45 degree andgle to a few hundred feets than it drops down on top of the target.

the rocket ignites outside of the barrel only because it is to reduce recoil. But, on every other rocket system the rocket ignites in the barrel. This is because it is much more accurate. The dragon requires very advanced stabilizing systems.

The rocet will be most stable if fired within the tube. this would also cut downthe cost as you wont need an electrical ignition, merely a pyrotechnic charge (potassium perchlorate) mounted on the back of the rocket that would ignite via the cannon combustion.


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 Post subject: Re: Rockets
 Post Posted: Thu Jul 24, 2003 11:00 am 
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Originaly posted by cchant,
When shooting for range, I wouldn't really care how accurate the shots were. Unless you mean the stabilizers are needed just to keep the rocket in flight. Anyway, my reasoning was to give the projectile some time to make use of it's own kinetic energy before using the rocket's. I guess either way, faster muzzle velocity or secondary "boost" along they way, they would still be out of sight. Also, I'd be weary of setting off a rocket inside a pipe. The cannon would have been CO2 powered with a Supah valve and 16' barrel (just enough to get near the speed of sound). I could see it damaging the valve or weakening the pipe with repeated shots, which is pretty risky at 100psi.

I imagine a mechnaical ignition would still be pretty simple. The switch could be activated from the huge acelleration and a small battery to fire the rocket.


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 Post Posted: Thu Jul 24, 2003 11:00 am 
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Originaly posted by Rex_R,
the engine is rated at 2.5 newton seconds. with something that small then a delayed ignition would be highly desirable but, since I'm planning on using a 2" combustion gun, thought I would just fuel the gun then fire the engine and hope it works.


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 Post subject: you need an ignitor
 Post Posted: Thu Jul 24, 2003 12:00 pm 
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Originaly posted by taimaishu1103,
those engines arent too touchy with heat. You need a good clean burn to ignite those engines, and the combustion will most likely not pprovide sufficient heat to ignite the engine. A combustion plug would be very easy to make with standard firework ingredients.


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 Post Posted: Thu Jul 24, 2003 2:00 pm 
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Originaly posted by cchant,
In the case of not having enough heat to ignite the rocket from the gun, why not ignite the rocket and have that fire off the gun?

The only other problem I see with having in-barrel ignition, is having the force of the rocket negate the force of the gasses. For example, the rocket ignites which in turn ignites the gas. But, the gas can only expand so fast and the rocket might already be accelerating faster than the gasses can push on it, or at least enough so that the increase is very small. This would seem that an onboard ignition in the rocket would make the most of both the combustion gasses and rocket fuel.

Let me explain a little about the ignition I had in mind. The switch would be very simple... just two separated metal contacts that would touch during acceleration. This would trigger a 555 based timer circuit powered by a button cell battery. That should only add a few grams to the projectile weight. Then the timer would trigger a regular rocket engine ignitor powered by a small 12V battery. There is a kind (used in garage door openers) that is about the same diameter but half the length of a AA. All in all, fairly simple I think, and light weight. Would be nice if it all fit inside something like a film canister that could be taken out and reused. Rex, you got any pics or info on the rockets you are making? Looking forward to those...


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 Post subject: well no pics yet
 Post Posted: Thu Jul 24, 2003 11:00 pm 
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Originaly posted by Rex_R,
still making fins (3ply, 1/32 balsa), spent yesterdays hobby time turning a nose cone. yes I plan on using an engine igniter and let the engine fire the gun(hopefully). with a bit of luck the whole rocket/sabot should weigh in at 2.5 oz or less. will try to remeber to take pics before testing (if nothing else to have something to remember it by)


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 Post Posted: Fri Jul 25, 2003 12:00 am 
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Originaly posted by taimaishu1103,
if you have the rocket ignite the combustion chamber, then the ignition will start at the muzzle. This is a BAD BAD thing for spudgun efficiency. Youll get a nice ig muzzle flash but nmt much power.

there are two types of combustion, theres the standard explosion like ina rifle round where the percussion cap in the BACK ignites the fuel. this creates a l;arge amoung of energy

theres the second type of combustion, i dont know the term but its when teh ignition is at the muzzle end.

if the ignition occurs at the muzzle end of the chamber, your wasting your time. youll gain very little speed form the combustion.


just to prove it, take a 2liter coke bottle, spray in your rightguard, and light it at the neck. Notice how teh flame goes very slowly down the bottle. This is whats gonna happen if the rocket ignites the chamber.

Now, poke a hole int eh bottome of the bottle, than spray and fire. Notice how much more powerful it is??? this is how a spudgun normally fires


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 Post subject: except
 Post Posted: Fri Jul 25, 2003 12:00 am 
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Originaly posted by Rex_R,
that there are a number of folks who swear by having the electrodes at the front of the chamber that it works better


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 Post subject: Ignition placement
 Post Posted: Fri Jul 25, 2003 1:00 am 
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Originaly posted by cchant,
Latke's tests showed a spark is a spark, no matter if it's at the front, back, or both, comes from a stun gun or BBQ starter... it all worked the same. I would have thought the multiple ignition thing would increase combustion a lot, but at the business end, it didn't make much difference.

I don't think Latke ever published this on his site but he posted the results in the forum once. Might still be there if you look on google.


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 Post subject: Source:
 Post Posted: Fri Jul 25, 2003 1:00 am 
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Originaly posted by cchant,
Latke's Bold Statement...

http://www.spudtech.com/forum/showpost.asp?cat=3&postid=3424


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