How would you arrive at proper ejection charge?

The Rocketry Forum

Help Support The Rocketry Forum:

gary7

Well-Known Member
Joined
Aug 2, 2009
Messages
701
Reaction score
82
Location
Mattoon, IL
I have a bay shaped and sized as shown. How would you arrive at the volume of 4F needed for ejection. There are 3 x 2/56 shear pins used in this design.
Do I calculate the two cylinders separately then add for a total? How much would you use to start your ground test? Doing so using BP Estimator – Rocketry Calculator gives me about 3.2 gr.

Thank you.
 

Attachments

rharshberger

Well-Known Member
Joined
Oct 13, 2014
Messages
10,189
Reaction score
2,136
Location
Pasco, WA
Total volume of both cylinders, then ground test, ground test, ground te.....did I mention GROUND TEST!
 

Bat-mite

Rocketeer in MD
Joined
Dec 5, 2013
Messages
11,664
Reaction score
2,411
Location
Maryland
So, volume of a cylinder is pi * r^2 * h. I'd calculate the volume of each "chunk' and add for total. Then, I would mess around with regular cylinder volumes that are very close and solve for the radius. Use the radius in a BP calculator.

So, 4 * 3.1416 * 17 = 213.6
And 3.75^2 * 3.1416 * 5 = 220.9
Total 434.5

434.5 / 3.1416 = 138.3
138.3 / 22 = 6.28
sq root of 6.28 = 2.5

So I'd plug 2.5 radius and 22 height into the calculator.
 

Kelly

Usually remembers to get the pointy end up
Joined
Apr 26, 2010
Messages
645
Reaction score
579
Location
Oregon
1) The pressure developed by the BP depends on the volume of the bay.
2) The force exerted by the BP pressure depends on the surface area of the element being ejected - the base of the nosecone, or the bay, for example. It's not clear from your diagram exactly where the separation point is.

To use the BP calculator, I would use the diameter of the element being ejected for the "body tube diameter", as one normally would. But, for the "Ejection charge pressure" row, I would override the formula there, replacing it with a formula that includes the manually calculated volume of the bay containing the charge. For example, replace "3.14159*(B4/2)^2*B5" with the actual volume. This should give you the correct values, in all other rows.

Then, whatever it tells you, ground test.
 

gary7

Well-Known Member
Joined
Aug 2, 2009
Messages
701
Reaction score
82
Location
Mattoon, IL
It's not clear from your diagram exactly where the separation point is.
See my avitar? That is a Loc 7.5 inch tube and nose cone. There is a 17 in long x 4 inch diameter parachute bay in that nose. There are only 5 inches above the forward centering ring to the top of air frame. In other words, the air frame volume is only about equeal to the amount of volume in the 7.5" Loc shoulder. The end of the shoulder was removed.

I flew this only once 2 years ago (a good flight btw on a K535). Before prepping for MWP last week, I decided to fly it again. I checked my notes and found I may have used 3.2 gr but didn't want to make the mistake of trusting an old man's notes and now, I can't remembe how I got that number. So I calculated again basde on what I thought might be the right way to do so. Whatever I did, the number seems to be close to what others are recommending above.
 

Wayco

Desert Rat Rocketeer
TRF Lifetime Supporter
Joined
Mar 23, 2011
Messages
3,753
Reaction score
851
Location
Goodyear, AZ
Ground test at 3.5 grams, done. :)
Yeah, that ought to do it. Then, do what rharshburger said....
Or you could be a rocket scientist and do all the calculations and still do what rharshburger said. :)
 

gary7

Well-Known Member
Joined
Aug 2, 2009
Messages
701
Reaction score
82
Location
Mattoon, IL
What are your thoughts on ground testing it?

Ducks for cover! :)
I am a proponent of ground testing. Was going with 3.5 gr to start. I usually ad a couple of tenths to start with. My original amount was calulated to be 3.2 gr.
 
Last edited:

David Schwantz

Well-Known Member
TRF Supporter
Joined
Jul 23, 2018
Messages
2,611
Reaction score
1,491
Location
MN
Ground test at 3.5 grams, done. :)
I just got 5 lbs of the stuff, I'd start with that :)
But in the reality, I have a 12" dia V 2, the drogue bay is 24" long and I use 3.0 on that and it blows off a 35 lb NC.
 

Kelly

Usually remembers to get the pointy end up
Joined
Apr 26, 2010
Messages
645
Reaction score
579
Location
Oregon
OK, so then it seems the volume being pressurized is the 4X17 cylinder plus the 7.5X5 cylinder, and the pressure acts upon the entire 7.5" diameter of the nosecone. So your diameter is 7.5", and the "equivalent length" of that 17" sections is 4.8", so you could just plug in 9.8" for total length.
Also, sorry, I didn't even look to see what calculator you were using, I assumed it was this one, but it wasn't, so my original reply didn't make complete sense.

However, using the xls calculator (or even the one you linked to) shows that 3.2g would lead to a pressure upwards of 14psi, which would lead to a force on the nosecone of close to 700lb, which is WAY more than you need to break 3 2-56 (nylon, I assume) screws. You could probably get by with 1g.

Last weekend I separated a 7.5" diameter nosecone, with about 7.5" of bay length, held on by 3 strips of masking tape (about the same as 3 nylon shear pins) with 1.1g of BP. That was, IMO, a bit of overkill, but I didn't have any redundancy and I wanted to make sure it came out. I had about 30' of nylon webbing, and rock-solid anchors, so I wasn't too worried about breaking anything. If there's any chance you might do ground testing :) I'd start with closer to 1g and work from there.
 

SDramstad

Well-Known Member
TRF Supporter
Joined
Jun 26, 2014
Messages
849
Reaction score
733
A lot depends on how you contain the powder. Generally the better contained the better the results
 

SDramstad

Well-Known Member
TRF Supporter
Joined
Jun 26, 2014
Messages
849
Reaction score
733
Also place the ematch at the top of the charge well not the bottom. Placed on the bottom a lot of the powder gets blown out with out burning. I put 4 grams of powder in a 4 inch rocket with no more the 24 inches of pressurizable space and it didnt eject the chute with the ematch on the bottom. Blew it way out when placed on top.
 

beeblebrox

8 C6-0, 12 D11-9, 20 D20-0, 20 E5-0, 3 Cinerocs
Joined
Mar 19, 2016
Messages
791
Reaction score
406
Location
West Chester, PA
I use a simple scoop... 1 scoop for 2.6", 2.5 scoops for 4", 4-5 scoops for 5.56" (Note: 4 scoops is 3.6 grams)

1 scoop = a 9mm shell case with a wire handle soldered to the bottom of it... (Note: Those who reload ammo use a device that measures by volume. Grams per unit volume is consistent when using commercial black powder, or smokeless powder. The loader for my cap and ball pistol is set by volume.)

No need to overthink. and... Ground Test anyway, the correct amount of powder will not work if there is an electronics failure...
 

FredA

Well-Known Member
Joined
Jan 19, 2009
Messages
2,359
Reaction score
511
Please DO overthink and don't swag or listen to people who did something similar.
LEARN how to do it right and why.

You're pressurizing a volume.
You need to increase the pressure so that the PSI on the object to be moved is sufficient to overcome friction and your retention strategy.
Some of the online calculators are very useful to help calculate the internal pressure rise - the rest is simple math.

Then ground test.
 

Latest posts

Top