SpudFiles

Lentamentalisk wrote:The issue as I see it is that you have a burst disk sitting out super vulnerable, with the rocket up to pressure, the entire time. Then, when you press the trigger, it punctures it, but many things could go wrong. If, for example you were to make a weaker burst disk than normal, for what ever reason, it could fire while you were filling, or if the rocket were to slide down the tube and tap the burst disk. Be super careful about never pointing it in the general direction of anything important, even if you are nowhere near the trigger.

These cautions apply to any triggered burst disk pneumatic.

Considered adding a nozzle and some kind of reaction mass to the projectile?

inonickname wrote:Considered adding a nozzle and some kind of reaction mass to the projectile?

Somehow I don't think either of those would help in any way. While a nozzle/reaction mass would be a great help once the 'rocket' has left the tube, IMHO both would serve to reduce the initial launch velocity of the projectile.

If you think about how the launcher works, the vast majority of the propellant gasses are within the projectile. Adding a nozzle will restrict flow from the chamber (the rocket) to the barrel. The rocket will have a more efficient, rocket-like thrust, but will be launched from the tube much slower.

A similar problem is posed by adding a reaction mass, such as water. In this case it will basically be being used as a water rocket, inside a semi-airtight tube with no extra air filling the space behind the rocket. It will have thrust, but won't have that initial boost from the launcher. The barrel will stop acting as a barrel, and merely a form of launch tube.

Wow. You used one pipe both as barrel and chamber. Nice!
Very compact launcher. Downside is that it requires special projectiles but hey, look what you get in return.

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I'd like to share a few thoughts for improvement:
For more power, you could add some volume BEHIND the projectile, so that the pressure drops slower while the projectile travels.

One way to do this in a compact way is to create a coaxial tube around the launch tube, and drill a few holes in the side at the rear of the launch tube.

edit: Mis calculations... lots of em. I gotta stop multitasking.
A very rough calculation: normally: pressure goes from 100% to 50% over the projectile distance. With doubled chamber volume it goes from 100% to 75%. Draw both in a graph with projectile distance on the x axis, and you will find a 50% increase in surface area under the 2nd line, which means 50% more kinetic energy.
In practice this will probably work out to something as 30% extra kinetic energy. It also makes a louder bang.

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Another idea:
What if you would make the QEV dump an even higher pressure into the volume instead of releasing it? The pressure raises, the BD breaks (more cleanly due to more pressure), and the volume of air vents out near instantly, allowing the projectile to go.

What do you think?


 

psycix wrote:A very rough calculation: normally: pressure goes from 100% to 50% over the projectile distance. With doubled chamber volume it goes from 100% to 75%. Draw both in a graph with projectile distance on the x axis, and you will find a 50% increase in surface area under the 2nd line, which means 50% more kinetic energy.

You have very funny geometry you're using there. Firstly 100% --> 75% would be indicative of 3:1 C:B ratio, not 2:1...

Anyway, using 3:1, you'd get (1+0.75)/(1+0.5) more energy based on the areas of simple trapeziums (i.e straight line graphs), or 16.7% more energy. It's a crude simplification, but right from the start, it shows your numbers are likely off.

To go more fully into it... (Warning: Science!)

Of course, air does not decompress in such a straight line, and decompression is actually related to the equation PV^1.4, making the pressure fall to 37.9% in a strict doubling of volume (i.e. like a 1:1 C:B ratio, assuming perfect flow and adiabatic conditions), and 66.8% in your other scenario (a 3:1 C:B ratio).

Anyway, integrating to find the areas under each of those curves, you find 35% more area under the 3:1 curve than the 1:1 curve. This still assumes perfect flow and an ideal gas (this is also assuming atmospheric pressure to be negligibly low), so it's only the theoretical ideal you can get from an increase from 1:1 to 3:1, but it gives you an idea.

And for your benefit, a little graph showing said integrals:

Horizontal axis is C:B ratio - so 3 is 3:1, 0.5 is 0.5:1 (pretty simple). Vertical axis is the integral of the pressure in the system (where 1 is an infinite C:B ratio with no pressure drop at all).

Looking at this, you can see the obvious diminishing returns from increasing C:B ratio. 4:1 for example only yields 14% more energy at the muzzle than 2:1 and at the price of twice as much chamber.

I could go further, but that's really a subject for another topic.

Nice launcher, i'm still not convinced of the performance... so damage pics will be needed to persuade me

Ugh did I make those calculations up there? Its like a
*Unreal tournament voice*
Multi-error..error.ror..ror.
*/UT voice*

Miscalculation spree... I blame the lag!
Luckily Ragnarok was right on time to do what he does best.

A quick thought;

How much would the performance change if a tube was flared and silver soldered into the burst disk nut to extend the barrel twice as far as it is now.

It would give the launcher an appearance of a chamber and barrel with a mid valve. The barrel would be deceiving as it would look short but would pack the power of a longer barrel. 8)

The above suggestion would also make the disk less vulnerable to being accidentally ruptured on contact with an exteral object.

Thanks for the feedback

psycix wrote:I'd like to share a few thoughts for improvement:
For more power, you could add some volume BEHIND the projectile, so that the pressure drops slower while the projectile travels.

One way to do this in a compact way is to create a coaxial tube around the launch tube, and drill a few holes in the side at the rear of the launch tube.

------------

Another idea:
What if you would make the QEV dump an even higher pressure into the volume instead of releasing it? The pressure raises, the BD breaks (more cleanly due to more pressure), and the volume of air vents out near instantly, allowing the projectile to go.

What do you think?

The design is meant to be úber compact, which essentially means that currently all space is being used twice in there. By having space just for chamber air it strays from the original concept.

Of course by all means it can be done but I feel the best way to increase performance is raising the pressure.


On the alternative triggering method: I'm not convinced that would help too much, if you can use higher pressure in the QEV trigger, why not fill the chamber with it instead?

Also, if it doesn't work it now means the only way its going to fire is manually bursting the burst disk. Whereas physically ramming the burst disk can never fail on you, at worst the burst disk can be too tough and restrain the projectile while the air escapes.

I have got a new theory trundling about which should help the burst disk blow out instead of being punched out, less resistance for the projectile if the disk is greatly weakened before its shoulders try to muscle through.

Technician1002 wrote:A quick thought;

How much would the performance change if a tube was flared and silver soldered into the burst disk nut to extend the barrel twice as far as it is now.

It would give the launcher an appearance of a chamber and barrel with a mid valve. The barrel would be deceiving as it would look short but would pack the power of a longer barrel. 8)

Again, this strays from the compact nature of it, as it is the projectile on launch almost doubles the length of the launcher before it finally lets go which is quite surprising to see...

The barrel would of course increase power but increasing barrel length will always increase power, in this case creating a dedicated barrel adds weight, length and added problems when fixing the burst disk due to a length of pipe now being on the clamp.

If on the other hand you increase the length of the main body, you increase the compressed air charge, the length of the "barrel" at the cost of a heavier projectile (the projectile has to extend the length of the tube).

Swings and roundabouts. I feel it's a reasonable tradeoff to have no barrel at all but have an extremely short launcher with an unbelievably large punch.

If you're wondering about efficiency by the way, consider how it fits together, the chamber volume is much less than the "barrel"volume which is literally the entire space of the projectile plus the empty space it leaves behind.

The projectile is ~57cm with an ID of 2cm, an OD of 2.5cm. Due to the way the nose was fitted actual internal is less than 57cm, perhaps 55cm.

I'm excluding the space between projectile and launch tube walls as I should really allow for blow-by as by nature it is not airtight.

So 55cm x 2cm diameter = 172.8cc

The barrel would be more or less 60cm and the ID of the launch tube is 2.6cm.

So 60cm x 2.6cm diameter = 318.6cc

PLUS the volume of the chamber

(until the projectile lets go that volume is still part of the system even though by conventional standards it looks insane as the volume being taken into account is now outside the boundaries of the pre-fired launcher)

So a total barrel volume of 491.4cc

Compare again to the 172.8cc of gas to start with.

Aroundabout a ratio of 10:28 between chamber volume and barrel volume.

Increased pressure and looking into ways of improving the projectile I feel are the ways forward. With over 1kpsi and still using the same 60cm long launcher tossing the 150g projectile massive range can be achieved.

All from a launcher which is lighter, shorter and simpler than a conventional one. Wahey

I feel it's a reasonable tradeoff to have no barrel at all but have an extremely short launcher with an unbelievably large punch.

One you display the effects of said punch, I think the forum will be a little more convinced

For it's size

Also, missed one:

inonickname wrote:Considered adding a nozzle and some kind of reaction mass to the projectile?

Although it looks somewhat rockety it's very far from it. The mass of gas it kicks out is extremely small compared to the 150g of the projectile and the moment it exits the launch tube you can basically write off the internal pressure. The resistance of atmospheric pressure to the compressed gas blasting out will be nothing to the force of the trapped gas pushing the projectile before it exits the launcher. A nozzle would make a more efficient jet but you need raw force to get it going in the short launch time.

sorry to bring up an old topic but damage pics????



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