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 57 users online :: 5 registered, 0 hidden and 52 guests


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

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

The Team
Administrators
Global Moderators
global_moderators.png CS

New guy here. Chamber design question (venturi design)

Post questions and info about combustion (flammable vapor) powered cannons here. This includes discussion about fuels, ratios, ignition systems, safety, and anything else relevant.
Sponsored 
  • Author
    Message

Unread postAuthor: jook13 » Tue Mar 24, 2009 11:23 pm

I agree that the shape of the chamber would have to do some pretty dramatic stuff to get these results with a spray cannon, especially as consistantly as he is claiming.

As to the results of starman v.s your cannon, I believe a good spin on his and a bad spin on yours could easily account for the difference. One thing I think people forget about combustions is there is no valve. Once it is ignited, there is no choke or time for any valve to open, it is fully open from the beginning. The burst disc only inhances the effect (if it breaks at the right pressure)
  • 0

I like to play blackjack. I'm not addicted to gambling, I'm addicted to sitting in a semi-circle.
User avatar
jook13
Lieutenant Colonel
Lieutenant Colonel
 
Posts: 434
Joined: Mon Jan 21, 2008 10:39 pm
Location: Prescott Arizona
Reputation: 0

Unread postAuthor: MrCrowley » Tue Mar 24, 2009 11:48 pm

jook13 wrote:As to the results of starman v.s your cannon, I believe a good spin on his and a bad spin on yours could easily account for the difference. One thing I think people forget about combustions is there is no valve. Once it is ignited, there is no choke or time for any valve to open, it is fully open from the beginning. The burst disc only inhances the effect (if it breaks at the right pressure)

I get backspin most of the time, the odd one will end up hooking right or left every now and then though. I'm fully aware of the valve also, no opening time, no flow constrictions. But that, coupled with hot gases, at a lower pressure, surely can't account for double the performance.

Sorry to somewhat hijack this thread...
  • 0

User avatar
MrCrowley
Moderator
Moderator
 
Posts: 10207
Joined: Fri Jun 23, 2006 10:42 pm
Location: Auckland, New Zealand
Country: New Zealand (nz)
Reputation: 4

Unread postAuthor: Ragnarok » Tue Mar 24, 2009 11:49 pm

Ugh, the backspin issue.
Never thought I'd need to do this, but give me a minute, I'm going to have to program lift and backspin into the range calculator.

*Programs lift and backspin into the range calculator*

Sorry Starman, unless you're going vastly supersonic from the out, you're not getting a half mile range.
Heck, it's hard getting much over a quarter mile subsonically, even with some backspin to help out.

The problem is, lift is dependent on the projectile having velocity in order to create that lift. Like drag alone squashes a perfect parabolic arc, increasing curve towards the end of the arc (as velocity falls), the projectile's lift suffers too suffers the same problem.

As drag slows the projectile, velocity falls, as does lift. Although the lift may allow the projectile to rise towards the start of the shot, the loss of velocity lessens it's effect. (That's not even getting into the loss of backspin speed, although for now, I'm considering that negligible for the effort of modelling it.)

Also, too much lift can alter the projectile's arc inefficiently (look at the linked graphs below), because it's not generated downwards, but perpendicular to the trajectory.

Essentially, what I'm saying is that it might seem like the projectile is still rising at a quarter mile, but I suspect there's a disjoint between what you think is a quarter mile in the air and what really is a quarter mile.
Most likely, your golfballs are all hitting the ground within about 500 metres.
Now, I accept my modelling may not be perfect, and I don't know the exact specifics of your cannon but I think it's more likely that you're misjudging how far away the balls appear to be, given that I was trying to give you every advantage in the simulation.

As a general rule, it's best never to make any assumptions of a projectile that's shot out of sight for so many reasons.
I automatically disqualify range claims of the order you're making unless it's either a calculated estimate using a proper ballistic model, or exceptional circumstances are at work.

~~~~~

All that said, you do get some very interesting trajectories from adding lift. Because a projectile has to trade velocity for height, you get less lift at the apogee than lower down (and also because the air is slightly thinner up there, so less lift from it).

Modelling the same projectile with increasing (and deliberately ridiculous) lift:
1500% lift
2500% lift
3000% lift

Each of the percentages represents the relationship between the projectile's weight and the perpendicular lift at the muzzle velocity. That is, at 100% lift, and if fired level, at the muzzle, there is no net vertical force on the projectile.

These graphs show that more lift is not always beneficial to range, as they're all the same projectile at the same initial velocity - they're also all fired parallel to the ground!

And yes, before anyone asks, with enough lift, it is possible to get it to land behind you - or even on you, for which the magic number is about 6750% in this case.
  • 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: starman » Wed Mar 25, 2009 12:19 am

Ragnarok wrote:Ugh, the backspin issue.
Never thought I'd need to do this, but give me a minute, I'm going to have to program lift and backspin into the range calculator.

*Programs lift and backspin into the range calculator*

Sorry Starman, unless you're going vastly supersonic from the out, you're not getting a half mile range.


Sorry Rag, but it is in that neighborhood. Like I said earlier, I haven't a way to get exact measurements, much less find the ball, so then I would estimate 800 yards +- 20%, depending on the shot and conditions. The best I can do is extrapolate based on known landmarks and visible flight characteristics. Let me just say, it's a lot closer to 1/2 mile than it is 1/4 mile.

I'm not one to exaggerate performance. I am using a longish barrel of 6.5', burst disk and a highly optimized chamber with MAPP gas.
  • 0

User avatar
starman
Donating Moderator
Donating Moderator
 
Posts: 3041
Joined: Wed Dec 12, 2007 12:45 am
Location: Simpsonville, SC
Reputation: 0

Unread postAuthor: Ragnarok » Wed Mar 25, 2009 1:47 am

starman wrote:Sorry Rag, but it is in that neighborhood.

I have the best eyesight of anyone I know by far. It's into the merest fraction of the top percent of people.
But even I wouldn't have a hope in hell of being able to see a golf ball at half a mile. It's the vague equivalent of being able to make out an ant at the other end of an Olympic swimming pool.

Until you can actually present me a better description of the method you're using to estimate it (and how you know how far your markers are), I can't really believe the results you're claiming.

The best I can do is extrapolate based on visible flight characteristics.

Extrapolation of trajectories, particularly with low ballistic coefficients and lift to consider, is very hard to do right. My work on the range calculators has told me that.
I know ballistics a great deal better than most people, and even then, I find it hard to make accurate guesses.

We'll try an experiment.
The image below is the first 200 metres of these three trajectories of golf balls.
A base one; one modelled with some lift; one modelled with a 10% reduction in drag coefficient over the other two and no lift - otherwise identical. I'll assume you can work out which is which.

Extrapolate them however you so choose, and give me your best guess where each is going to land. Try and be specific to within 10 metres so we don't have any confusion over the answer being too vague.
Image

The modelling I've used is pretty representative of real life, so there shouldn't be any issues with that.
I'll give you the full trajectories and the extra details for them after I have some responses. By the way, anyone is invited to guess, but guess before looking at other people's answers.
  • 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: starman » Wed Mar 25, 2009 2:09 am

Ragnarok wrote:I have the best eyesight of anyone I know by far. It's into the merest fraction of the top percent of people.
But even I wouldn't have a hope in hell of being able to see a golf ball at half a mile. It's the vague equivalent of being able to make out an ant at the other end of an Olympic swimming pool.


Yes, we know you are superlative at everything Rag... :roll: and I didn't claim to see the ball at 1/2 mile...did I..?..geeze...

You have the right to be skeptical if you want and burden of proof is on me...which I will eventually find a way to provide.

Again I repeat, I have nothing to gain by exaggerating performance claims. I can tell you that my shots are still rising way after the 200 m mark your graph seems to imply. However, I will hazzard a guess of around 600 m based on your trajectories in the graph...nothing calculated, just gut feel.
  • 0

User avatar
starman
Donating Moderator
Donating Moderator
 
Posts: 3041
Joined: Wed Dec 12, 2007 12:45 am
Location: Simpsonville, SC
Reputation: 0

Unread postAuthor: Ragnarok » Wed Mar 25, 2009 2:55 am

starman wrote:Yes, we know you are superlative at everything Rag...

I think that's unfair. There are many areas in which I'm truly rubbish, but I don't normally have reason nor desire to emphasise something at which I'm below average.

However, if you insist - I'm below average or rubbish at at most sports; anything but the most basic of history or geography, particularly where other countries are involved; foreign languages; keeping anything organised; time management; popular culture; personal musical talent; cleaning; Thursdays; making references anyone will recognise; remembering that I have other things to get on with; brain surgery; keeping track of what's going on in the world; and many many more...

I'm at best passable at programming; electronics; most computer games; card tricks/games; social situations; geology; astronomy; cooking; artistic talent; martial arts; etc; etc; etc;

However, what I am good at, I try to excel at. For example, I excel at being easily distracted, making a mess of my sleeping patterns, injuring myself, setting my hair on fire, getting splitting headaches, terrifying people by accident - amongst other things.

Now, is that enough, or do I have to keep criticising myself?

~~~~~

But regardless of that, the point stands. If the ball cannot be seen, at best, you must then be guessing.

Again I repeat, I have nothing to gain by exaggerating performance claims.

I'm not accusing you of exaggeration. I'm suggesting that it sounds like you're overestimating. That is to say, if there is a difference between reality and your claims, that difference is accidental, rather than deliberate.

I can tell you that my shots are still rising way after the 200 meter mark you graph seems to imply.

The graph isn't meant to be representative of your specific case.
It's a case that's meant to show that extrapolation of what you can see is not a reliable way of finding the actual answer as far as ballistics are concerned.

Reason I used golf balls was because that's what the modeller was set up for already. Reason I said it was golf balls was to give people an idea of what kind of projectile and drag they should be expecting.

If I get some guesses as to where each trajectory ends up, I'll share the entire graph.
  • 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

Sponsored

Sponsor
 


Unread postAuthor: starman » Wed Mar 25, 2009 3:10 am

Ragnarok wrote:If I get some guesses as to where each trajectory ends up, I'll share the entire graph.


Actually, I'll predict the 10% reduction-in-drag ball wins (I assume the green line) at a reference distance of 600 meters, the standard shot (blue line) at 570 meters and the hop up ball (red line) brings up the rear at 550 meters.

No, I wasn't insisting on you listing your weaknesses. In fact, I apologize for calling you out like that....you're right, it wasn't fair... :wink:
  • 0

User avatar
starman
Donating Moderator
Donating Moderator
 
Posts: 3041
Joined: Wed Dec 12, 2007 12:45 am
Location: Simpsonville, SC
Reputation: 0

Unread postAuthor: Ragnarok » Wed Mar 25, 2009 4:17 am

Well, you're right on the reduced drag ball (green) being the winner, but you're quite a way out on the actual numbers: Answers

Exact figures are as follows from the 30 degree launch angle...
Blue (Normal): 364m
Red (Hop-up): 369m
Green (Low Drag): 394m

They may still all be rising (although at different rates) at 200 metres, but they're all on the way down again before 260, and they've all hit the ground without reaching 400. Extrapolation of trajectory from what you can see isn't always a reliable way of finding the right answer.

Like I said, the drag of golf balls means the trajectories get very curved at the back end. The same thing applies even if you've got backspin, because without the airspeed to produce the lift, it can't make a difference.
As such, adding backspin doesn't have a huge effect on the maximum range possible with a golf-ball.

~~~~~

I do keep meaning to finish the range calculator and make it available to you lot (because I'm sure you'd find a myriad uses for it), but like I said above, I suck at remembering things I have to get on with.
And when I do remember I have to get on with it, I start adding extra features. I wasn't ever planning to add any feature for modelling lift, but it seems that it now it has one...
  • 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

Previous

Return to Combustion Cannon Discussion

Who is online

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

Reputation System ©'