MMT Length vs. Motor Length and CTI Ejection Removal

[Cory]

I am currently building a Formula 98 from Rocketry Warehouse for a L2 cert flight. I would eventually like to try a long burn motor such as the CTI K160. I am using Proline 4500 epoxy for fin mounting for its ability to withstand high temperatures, but the motor mount is 12" where a K160 is ~15". This brings about 2 questions. Is it okay for a motor to be longer than the motor mount, and if so by how much? Second, I am using ~20' of 1" Kevlar for the drogue shock cord with a 15" Fruity Chutes drogue, the ejection charge will be fired by the e-bay in the forward section of the drogue compartment. The question is can I load the shock cord and drogue chute with nomex protection of both sides (e-bay ejection charge and motor), or am I at high risk of an internal fire? Also, I have already purchased a CTI 54mm J295 for the cert flight. This motor is pre-loaded with a motor based ejection charge. What is the best way to remove the CTI factory motor based ejection charge?

Thanks
Cory
TRA# 14170 L1

 

[maxvelocity]

I'm not sure I understand your question about the nomex and a high risk of fire??? But as far as removing the motor ejection charge I just pop the paper cap off with a knife or small screwdriver then dump the black powder out and plug with estes wadding then a piece of tape. The motor will be fine sticking out of the mount.

 

[DizWolf]

You don't -need- a drogue chute, but if you do want to use one, just wrap it in the nomex like a burrito. Covered from both sides. Zero risk of internal fire.

I'd leave the motor ejection charge in set to the longest delay. Cheap backup in case something happens with the altimeter. Even if the altimeter goes black, using the motor to break the rocket in half will greatly reduce the chances of a ballistic recovery, and even coming down on drogue/drogueless that rocket will likely survive.

And half inch or 3/4 tubular kevlar will likely do it. 1" is pretty beefy.

 

[Cory]

I mean to ask, if the rear nomex fails to hold the drogue rigging in place, and it gathers around the motor casing during boost, will the casing temperature be high enough to cause melting or burning of the Kevlar shock cord? I would most likely fly drogueless to prevent melting the chute.

 

[Cory]

You don't -need- a drogue chute, but if you do want to use one, just wrap it in the nomex like a burrito. Covered from both sides. Zero risk of internal fire.

I'd leave the motor ejection charge in set to the longest delay. Cheap backup in case something happens with the altimeter. Even if the altimeter goes black, using the motor to break the rocket in half will greatly reduce the chances of a ballistic recovery, and even coming down on drogue/drogueless that rocket will likely survive.

And half inch or 3/4 tubular kevlar will likely do it. 1" is pretty beefy.

The reason for removing the motor based charge stems from a horror story heard at a local launch, where 2 seconds into an L1 cert flight the ejection charge fired. I am using a PF Stratologger as the main and a RRC3 as backup electronics for ejection, and was unsure if a third BP charge would only be adding stress on the shock cord. I agree the 1" Kevlar is probably overkill, but figured it may help add to the drag during drogueless decent

Thanks,
Cory
TRA 14170 L1

 

[DizWolf]

I've got a 1" sitting in my L2 bird not gonna kill anything going overboard. My prefect did ask if I was planning on towing a car with the rocket

Motor casings shouldn't get hot enough to melt kevlar under normal circumstances. Remember, we're using cardboard tubes to hold them and they do just fine.

Lots of people remove motor charges. As Max mentioned, just cut the cap, dump the BP and plug the hole with wadding. Personally I've seen many more altimeters fail than delays 'burn hot' and pop early, so I plan for that. CTI delays seem especially reliable. Let your own experiences guide you.

 

[watheyak]

All my recovery blankets are Kevlar, not nomex. They insulate better, but they are also thicker. You shouldn't have to protect Kevlar from any heat sources.
Scott w