When doing your BP Ejection charge calculations, should you do it on the total tube length or the length of the tube excluding the inclusions (ie, nose cone & coupler) ?
I've always thought the correct answer is, both. IIRC, most calculators use volume and BP quantity to give a value for pressure, and there's a Rule of Thumb about how much pressure you need. Then that RoT is adjusted for the diameter, less pressure for a larger diameter. What you're needing there is the force on the base of the nose cone, just the pressure times the base area. But how much force? Well, there are RoTs for how much force to break shear pins, etc. But do you really want just the force? When you ground test, what you're really looking for is a smart separation velocity, and that's going to depend upon the mass and action time/distance of the force. So, I've always thought your ejection charge calculator should calculate that separation velocity. With the shoulder length/diameter and masses of the nose cone and body, there is everything you need for that calculation (albeit with quite a few simplifying assumptions).
That's what the VCT ejection calculator does. What's more, it works forward and backwards; that is, you can use a BP quantity to calculate the separation velocity, or enter the desired velocity and it calculates the BP needed. But to do that, you have to enter the dimensions and masses, so it's a tiny bit more complicated to use than the other BP calculators. I've posted VCT a number of times, but there's never been much interest. I could probably dig it up again.
BTW, VCT also demonstrates that a ground test is not actually the same as an in-flight free body separation (but in most cases, it's pretty close).
