Resizing deployment charges based on motor size?

[Bat-mite]

Does anyone adjust deployment charge sizes based on the size of the motor? Obviously, a motor that sticks out of the MMT subtracts volume from the area to pressurize; and likewise, a motor that is shorter than the MMT adds volume. I never really thought about that before. Could the difference be enough to warrant adjustments to the amount of BP?

 

[Banzai88]

I get where you're going with this but empirically it seems to not matter much or this would probably already be a 'thing'.

 

[heada]

If the difference was on the order of cubic feet, it could matter for example a1G 98mm motor vs a 6G 98mm motor. Most of the time the difference in volume is not enough to worry about.

 

[Bat-mite]

If the difference was on the order of cubic feet, it could matter for example a1G 98mm motor vs a 6G 98mm motor. Most of the time the difference in volume is not enough to worry about.

I just may run into that scenario. I have only flown my Paranoid Android on a six foot long 98mm O motor. That will be rare, but I have a nice 76mm M to launch it on this winter. Volume of the motor will be significantly different.

 

[Banzai88]

I just may run into that scenario. I have only flown my Paranoid Android on a six foot long 98mm O motor. That will be rare, but I have a nice 76mm M to launch it on this winter. Volume of the motor will be significantly different.

I would run calculations on the two volumes, then plug it into a BP calculator and see what the delta is.

 

[Bat-mite]

I would run calculations on the two volumes, then plug it into a BP calculator and see what the delta is.

Subtracting a cylindrical volume from a cylindrical volume ... my head hurts already!

 

[blackjack2564]

Does anyone adjust deployment charge sizes based on the size of the motor?

Yes! on most of my build threads involving 3in and above been doing it for years. [recommending dif. charge sizes]

Take a standard 4in. with 75mm motor tube. Wildman fin cans are 60in.
When flying 54 motors like a 4 grain, use 2.25 g bp for apogee.
With a 6xl m in there, ain't much real estate left. drop down to 1.25.

Put the larger charge in and you risk stretching cord to limit and shaking out main..

For my 3in minimum diam. when sport flying on smaller fields with say a 2 grain 54 I use 1.75 for app.
At Argonia with a 6xl M there is only 6in of space between motor and bottom of av-bay, I drop down to .5 grams. After stuffing in shock cord and drogue there is virtually no room left needing to pressurize. I do use small long charge holders due to high altitude being obtained.

So yes I always adjust the apogee charge for motor size being flown.
To find what works best look at the negative velocity generated after downloading data.

U will find a spike, I try to keep it around 15 to 20 gees. When I first researched this was seeing spikes of 60-75 gees using the "one size fits all" ejection charge on large motors. That was back when I subscribed to " blow it out or blow it up" philosophy. Now I downsize to a much gentler deployment and all is well in recovery land.

PS ya don't need to bend your brain figuring this stuff out. Common sense and a bit of ground testing is all it takes. Just stuff empty motor hardware in , of sizes being flown. I keep log book of all builds and what charges sizes are, after awhile it's all second nature.

Of course build style, how tight parts fit, length of cordage, number and location of shear pins, weight of assemblies can vary for same kit, depending on who built it. So again there is no one size fits all....even if they look the same.