# 3 x D CanSat Launcher - do I need an ejection charge baffle?

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#### OZ1SEJ

##### Educator
I'm going to build a CanSat Launch Vehicle. Requirements: It must be able to launch a payload the size and weight of a soda can (66 x 115 mm, max. 350 grams) to an altitude of 100 m.

The largest engines I can buy are D.

So I came up with this design for a 3 x D rocket. My question is: Do I need an ejection charge baffle? Pro: Yes, this is a rather large diameter rocket. Con: It would add a lot of weight aft, and 3 D-engines should be sufficient to deploy without a baffle.

Best regards,
Steen

View attachment CLV.ork

#### dhbarr

##### Amateur Professional
I see that you have a shelf / baffle holding your chute forward. The volume you're pressurizing should be fine for any one of the three motors' ejection charge.

#### OZ1SEJ

##### Educator
Thanks! Is there a rule of thumb, or can you calculate this?

#### Rex R

##### LV2
if you are using cellulose insulation for wadding, then I would mount the chute shelf 7cm further aft. if however you are using a piece of nomex (or other fire resistant cloth to protect the chutes, then you should be good to go.
Rex
*edit*
BP = C x D2 x L

Where:
BP= The amount of black powder (FFFFG) in grams
C= on of the following values &#9702;0.002 = 5 psi
&#9702;0.004 = 10 psi
&#9702;0.006 = 15 psi
&#9702;0.0072 = 18 psi
&#9702;0.008 = 20 psi

(Note: C = (psi * 0.0004)

D= The airframe diameter in inches
L= The length of the cavity to be pressurized in inches

using the above formula and 5psi gives me an answer of 0.23g of black powder, if memory serves...a D motor usually has at least 0.3g equivalent of black powder, so 3 D motors ought to get you 15 psi(assuming that all 3 pop at the same time).
R

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