LOC Iris motor and CG questions

[richP]

I180-SK

• richP
• May 28, 2019

Getting ready to paint my 3" Iris (DD/extended) and had a few questions for the folks here with infinitely more experience than myself.
First would be CG. In the current configuration, with an I motor (most likely for L1), the CG is at 3.93 caliber. Prior to paint, I can potentially cut 3" of BT and get it to 3.22 with the same motor. Would it be worthwhile to do this? For L2, with a J motor, the CG falls to 2.8 with that 3" removed.
Secondly, below is the plotting for an I180 motor. to me it looks fine, but any concerns from the folks here?

 

[Nytrunner]

Over-stability is better than under. Your rocket will just be a bit more susceptible to crosswinds. Adjust your rail direction as needed (ask experienced flyers on the field) and fly the field accordingly (may have to bite down on the go-fever and fly something else if the winds are brisk).

And as you've already realized, bigger motors fit with plenty of margin to spare! (can any of your nose weight or forward mass be removed?)

 

[richP]

Thank-you for the reply.
I think I am already at the bare minimum as far as nose/front-end weight is concerned. Ultra-light chute, aluminum rods in the AV bay, etc..

 

[Nytrunner]

Ah, I just read that you have an extended model. How long is your rocket now?

The stock 3" IRIS is 60" long, that's an L/D ratio of ~20 possibly putting your rocket into Super-Roc territory. The "caliber" rule of thumb is less applicable.
Long-skinny rockets can experience greater CP shifts during flight than shorter rockets, and large stability margin is good for them.

In cases like these, I aim for a CP-CG separation of 8-15% rocket length. Depending on your rocket's length, you're not far from that already.

 

[swatkat]

agreed, leave it and you'll be fine! Good luck on the certs!

 

[richP]

Total length is 64". I added an AV bay as well as an additional 3" of payload length. The stock payload would have left me with 6.5" for chute/shock cord and nomex, so I stretched it 3" to keep things from getting packed too tight. I might be able to shave an inch from there too.

 

[Nytrunner]

64! Definitely a super-roc now. With that length of vehicle your rocket isn't that overstable at all anymore. Cutting it down is probably more trouble than worth

Watch the wind and fly!

 

[richP]

Well, I did finally decide to cut it 2" to a final length of 62". I was able to fly it this past weekend for my L1 using a CTI I170 and it went up and back perfectly. Now, I am working on L2, and have to pick a proper motor to run it with. At the moment, it looks like I will be going with a CTI J270.
I will be restricted to no more than 4500', and that combination will get me to approx. 4150'.
Any input on motor selection is very much welcomed and appreciated!