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Ballistic Seal Design

Ballistic Seal Design

(OP)
thread1-175890: Supersonic aerodynamic drag in a barrel - Help!

Following this thread provides some links, but I need some information of the design of ballistic seals. I have a hybrid design application which combines a "high-low" system similar to a pilot's ejection system with a hydraulic subsystem. It looks like conventional projectiles use copper for the ballistic seal, but I cannot find any information on seal design criteria. The anticipated breach pressure is below 30,000 PSI. The smooth bore is 8 inches. There is no muzzle velocity to be considered due to the damping of the hydraulic subsystem. The approach at present is to use metal piston ring design criteria.

RE: Ballistic Seal Design

The pressure you cite is comparable to many weapon systems; 30,000 psi = 207MPa.  Fore reference, a simulation of a 50 cal. m8 round has maximum breech pressure of 369 MPa.

To seal against that level of pressure, as discussed in the thread you cited, the copper rotating band is compressed and deformed in the process.  The "drag" that you're looking for will be a combination of drag and friction of copper sliding on steel, at high normal force.  This is not something that would be readily calculable.  There are probably interior ballistics programs that might do this.  

The only remotely similar program that I'm familiar with is PRODAS, which allows you to construct a physically meaningful model of the barrel and round and propellant charge.  The program outputs things like bearing stress, torque, angular acceleration, etc.  The specific module is described here: http://www.prodas.com/XQ/ASP/P.40604/QX/webPageXML4.htm

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RE: Ballistic Seal Design

Some cannon rounds use obturator bands from reinforced aramide plastics to spin with the rifling... yet allow the projectile [rocket-like] to emerge from the barrel with virtually no spin.

Might want to check-out AMCP 706-xxx series Engineering Design Handbooks [http://www.dtic.mil/dtic/]

AMCP 706-247 ENGINEERING DESIGN HANDBOOK - AMMUNITION SERIES - SECTION 4, DESIGN FOR PROJECTION

AMCP 706-270 ENGINEERING DESIGN HANDBOOK - PROPELLANT ACTUATED DEVICES

AMCP 706-445 ENGINEERING DESIGN HANDBOOK - SABOT TECHNOLOGY

etc...

I think AIAA also sells a text on "interior ballistics".

Regards, Wil Taylor

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RE: Ballistic Seal Design

interesting, but sort of defeats the purpose of rifling groves, but then maybe that's the intention ... to allow firing a smooth bore muntion in a rifled barrel ?

RE: Ballistic Seal Design

Kenat...

You nailed it. When velocity [kinetic energy] is stepped-up, spin-up can add a nasty increment of forces to the SABOT and in-flight instability... especially for the high L/D APFSDS penetrators.

 

Regards, Wil Taylor

Trust - But Verify!

We believe to be true what we prefer to be true.

For those who believe, no proof is required; for those who cannot believe, no proof is possible.

RE: Ballistic Seal Design

(OP)
Thank you all for the feedback.
For this application, the bore has no rifling.
The obturator's main function is to protect some additional non-metallic seals from the propellant gases. The obturator has no alignment function and the generation of spin is of no concern.
This is definitely a different "animal"

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