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| POLAND_SPUD |
 Posted: 07/07/2008 13:26 PM Post subject: |
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Brigadier General
Joined: 13 Oct 2007
Posts: 985
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@gippeto to be honest I am not sure myself as it's only a programme and I have no idea how accurate it is... though D_Hall ( who helped to write it several years ago) said it's relatively accurate...
I've just found this -- > http://www.physicsforums.com/archive/index.php/t-190850.html they mention two ways of drag stabilizing - either fins or flared projectile base (+ of course center of mass moved to the front of ammo)
well anyway I hope that someone will clear things out
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| pocket |
| Posted: 07/07/2008 14:11 PM Post subject: |
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Sergeant
Joined: 05 Jul 2008
Posts: 94
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| frankrede wrote: | | on my last order on mcmaster it gave me the option of ordering from a non us country..... |
ya the num nut i was talking to didnt want to he said it wouldnt be worth it and his excuse made no sense *sigh* so i just hung up but there web site looked great
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| POLAND_SPUD |
| Posted: 07/11/2008 1:26 AM Post subject: |
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Brigadier General
Joined: 13 Oct 2007
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now I am not going to start a new thread to ask this question so would rather ask it here
Does a projectile need a flared base to be drag stabilized ? I recall JSR saying that all you need is the centre of mass moved forward but I've tried looking for some info on the internet and everytime flared base was mentioned
so is there anyone who can answer that question?
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| Gippeto |
| Posted: 07/11/2008 8:16 AM Post subject: |
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Colonel
Joined: 19 Jan 2008
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I won't suggest that I have the last word on it, but here goes.
The cg of the projectile will form the center of rotation.
As the rear of the projectile enters the airstream, it directs air away from the projectile.
For every action there is an equal and opposite reaction. Therefore, redirecting the air away from the skirt, pushes the skirt back into line.
This would create a series of small occilations, but would keep the projectile from actually tumbling.
If the cg is too far aft, then the forces would also act upon the front of the projectile, destabilizing it.
By skirting a projectile, we increase the angle of the deflection, and give our force the longest moment arm to act on the cg.
I do not believe that a skirt is absolutely necessary, but I have developed a new mold and swaging die to at least make some effort to find out.
http://www.spudfiles.com/forums/download,id,11122.html
I need to find time to play with that rocket modeling software. 
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| jackssmirkingrevenge |
| Posted: 07/11/2008 8:50 AM Post subject: |
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space monkey
Joined: 15 Mar 2007
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| Model rocketry provides some good explanations on the subject, here's some further reading that's relatively easy to understand and should prove helpful.
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| POLAND_SPUD |
| Posted: 07/11/2008 9:37 AM Post subject: |
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Brigadier General
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yeah JSR I knew about that before - it seems logical... and I would like that to be true...
however when I use rocksim I get different results when I either select 'rocksim' or 'barrowmans' stability method' for calculating stability... using 'cardboard cutout method' gives results that are in accordance with what you said
I'd add 2 pics ( in both pics the blue dot is the centre of gravity, the other one is the centre of pressure)
Eventhough what you said is ok to me I still don't know why rocksim gives results which are...... hmm contradictory? (well to some extent they are) it seems I have to wait for D_Hall to answer that question
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| D_Hall |
| Posted: 07/11/2008 17:20 PM Post subject: |
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| POLAND_SPUD wrote: | | Does a projectile need a flared base to be drag stabilized ? |
No.
Most famous example: The Apollo crew capsules as they descended through the atmosphere on their way home from the Moon.
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| jackssmirkingrevenge |
| Posted: 07/11/2008 17:28 PM Post subject: |
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space monkey
Joined: 15 Mar 2007
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| D_Hall wrote: | | Most famous example: The Apollo crew capsules as they descended through the atmosphere on their way home from the Moon. |
...but not the most recent 
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| D_Hall |
| Posted: 07/11/2008 17:30 PM Post subject: |
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| POLAND_SPUD wrote: | | however when I use rocksim I get different results when I either select 'rocksim' or 'barrowmans' stability method' for calculating stability... using 'cardboard cutout method' gives results that are in accordance with what you said |
1) The Barrowman equations are a set of equations that were developed by a guy named Barrowman (imagine that!) for predicting the stability of slender, finned objects (ie, rockets) back in the days before CFD was available. Your projectiles are neither slender nor finned. As such, any results that the Barrowman equations yield should be taken with a grain of salt.
2) The "cardboard cutout" method is really nothing more than than the calculation of mean projected surface area locations. It's a sloppy method but it's computationally trivial so it's been popular in the hobby world since the days of slide rules. Why anybody would use it today is beyond me (exception for people who are trying to get rough numbers on the back of an napkin while talking rocket stuff at the local pub).
3) The RockSim method... Not sure what that would be. They'd need to publish but I gather they don't feel like doing that. Most likely, it's probably a derrivative of Barrowman.
Suggestion? Get your hands on DATCOM. It's old. It's hard to use. But it's free and gives pretty damned good results. Note that it's an old code that the miltary developed in the days before CFD. It too relies heavily on Barrowman's work, but there was also a lot of artillery shell-based stuff in there as well.
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| POLAND_SPUD |
| Posted: 07/12/2008 3:18 AM Post subject: |
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Brigadier General
Joined: 13 Oct 2007
Posts: 985
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@D_Hall hey thanx
I am going to find DATCOM on the Internet
BTW - rosksim works fine (it gives expeted results) for designs resembleing a diablo aigun pellet or even the Appolo crew capsule but it has serious problems with designs that I posted
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| ajdevi92 |
| Posted: 07/12/2008 4:32 AM Post subject: |
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Major
Joined: 23 Nov 2007
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hi guys, spose this is the topicto post in now !
Im after some sort of barrel that will fit .22 pellets, seeing that you guys reckon that those pellets fly straight out of smooth bore barrels - i belive you ! so what size brass of steel piping will i need to fit .22 air gun pellets.
Thanks
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| jackssmirkingrevenge |
| Posted: 07/12/2008 5:39 AM Post subject: |
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space monkey
Joined: 15 Mar 2007
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| ajdevi92 wrote: | | so what size brass or steel piping will I need to fit .22 air gun pellets. |
22 pellets are normally around 5.6mm in diameter, easily swaged down to fit a 5.5mm tube if you manage to find some. Alternatively, you can use airsoft barrels
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| ajdevi92 |
| Posted: 07/12/2008 5:53 AM Post subject: |
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Major
Joined: 23 Nov 2007
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| would this work -- mcmaster part number 89965K39
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| jackssmirkingrevenge |
| Posted: 07/12/2008 5:58 AM Post subject: |
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space monkey
Joined: 15 Mar 2007
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| ajdevi92 wrote: | | would this work -- mcmaster part number 89965K39 |
Inside Dia. .21"
That means you'd have to roll down the pellets (in the same way you'd turn a ball of plasticine into a cylinder) if you want them to fit in the barrel.
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| Ragnarok |
| Posted: 07/12/2008 6:02 AM Post subject: |
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Lord of Karma
Joined: 19 Dec 2006
Posts: 2629
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| jackssmirkingrevenge wrote: | | 22 pellets are normally around 5.6mm in diameter, easily swaged down to fit a 5.5mm tube if you manage to find some. |
Well, actually, most brands of .22 pellet are about 5.5mm anyway.
The only one I can think of that is 5.6mm is the larger size of Eley Wasp intended for use in older air rifles, when they were actually .22, rather than the current metric 5.5mm.
Of course, .177" has an advantage here - old or new, the 4.5mm metric equivalent is almost an exact match.
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