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Aerotech J90W Question

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Rocket501

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Hi everyone. I have recently decided to work on my L2 certification. I was able to borrow a a 54mm/852 case from a friend, and was planning on flying a J90 reload in a EZI-65 that has been designed to work with dual deploy.

However, the casing I borrowed only has a standard forward enclosure, while the J90 needs an extended enclosure to handle to extra long delay grain. As I am already planning on using electronics, I was wondering if it would be legal under Tripoli rules to just leave out the delay grain and assemble it using the standard enclosure and fill the delay well with grease. I'm willing to spend the $60 if I have to in order to ensure the rocket is safe and legal, but I'd honestly rather just spend the money on more motors if I can.

Thanks in advance.
 

timbucktoo

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Keep the delay grain, also acts as tracking/smoke grain. Just don’t add the BP. Stuff some dog barf in as replacement and cover with masking tape. Perfectly acceptable with both TRA & NAR.
 

Rocket501

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Keep the delay grain, also acts as tracking/smoke grain. Just don’t add the BP. Stuff some dog barf in as replacement and cover with masking tape. Perfectly acceptable with both TRA & NAR.
The problem is that the current delay grain well I have it too short. The delay grain wouldn't fit. Modifying the delay grain wouldn't be legal (I think).

Grease will NOT hold pressure.
That's a good point. I forgot to mention it, but I was going to put a piece of cardboard in the front of the delay well to keep the grease from being forced out. I was only expecting the grease to act as an insulator.
 

GalantVR41062

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How about the AT J180T for that case? Have you built the rocket yet? Any ideas for final build weight?

A quick sim with thrust curve: 4" rocket with a build weight of only 44oz puts the rail exit velocity with the J90 @ 51 fps off a 86" rail.

Good luck on your L2 journey.

~John
 

Rocket501

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How about the AT J180T for that case? Have you built the rocket yet? Any ideas for final build weight?

A quick sim with thrust curve: 4" rocket with a build weight of only 44oz puts the rail exit velocity with the J90 @ 51 fps off a 86" rail.

Good luck on your L2 journey.

~John
I considered buying the J180T, but I'm worried it might go a bit fast for the rocket. I would rather not fiberglass the rocket, because the last time I did it, it was a PITA.

The final build weight with everything but the engine is 42oz. I'm not too concerned because my friend who I am borrowing the casing from flew the same type of rocket on a J90 too. He used to have an extended forward casing, but lost it when he loaned it to someone who's rocket was never recovered.

I have access to a machine shop and the schematic drawing of the extended closure. Do you know if it would be legal if I machined a custom closure and launched it at a Tripoli experimental launch?
 

rharshberger

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The J90's initial thrust at startup is 120newtons (27.27lbs), a 5:1 thrust to weight ratio as recommended would allow for the rocket to weigh up to 5.45lbs ready to fly. Still sims will be required to verify that the rail exit speed is high enough for the wind conditions being flown in and the effective rail length (good thing about the J90 is that its thrust increases for the first second or so after startup so a long rail will improve exit velocities) so rocket is stable.

As for the forward closure as others have stated the extend forward closure is what the motor is certified with so that is whats required to be used, they have also stated the downsides of the using the wrong closure or altering the assembly to allow a different one.Good luck on your cert.
 

Rocketjunkie

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A J90 works well in my EZI set up for dual deploy. It weighs 4.8 Lb. prepped less motor. I fly it off a 6 to 8 ft rail. The J90 is regressive. Look at the 1st 1/2 second of the thrust curve for thrust/weight calculation.
Onboard video
 

Rocket501

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The J90's initial thrust at startup is 120newtons (27.27lbs), a 5:1 thrust to weight ratio as recommended would allow for the rocket to weigh up to 5.45lbs ready to fly. Still sims will be required to verify that the rail exit speed is high enough for the wind conditions being flown in and the effective rail length (good thing about the J90 is that its thrust increases for the first second or so after startup so a long rail will improve exit velocities) so rocket is stable.

As for the forward closure as others have stated the extend forward closure is what the motor is certified with so that is whats required to be used, they have also stated the downsides of the using the wrong closure or altering the assembly to allow a different one.Good luck on your cert.
Thanks, yeah, I'll just buy the extended closure. It'll make a nice gift too, I guess.
 

Nytrunner

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considered buying the J180T, but I'm worried it might go a bit fast for the rocket. I would rather not fiberglass the rocket, because the last time I did it, it was a PITA.
4" Loc tubing should eat the J160 for breakfast. If there is a failure, it'd be at the fins
 

Rocket501

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Upon further investigation of NPS's rocket hardware, I think I may have found an extended forward closure. It has an internal well depth of 38mm (which another thread mentioned was the correct depth), but despite looking, I am having trouble finding confirmation of that in the Aerotech catalog. Do you guys know where I could find an official schematic?

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timbucktoo

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Upon further investigation of NPS's rocket hardware, I think I may have found an extended forward closure. It has an internal well depth of 38mm (which another thread mentioned was the correct depth), but despite looking, I am having trouble finding confirmation of that in the Aerotech catalog. Do you guys know where I could find an official schematic?​
Here ya go.
http://www.aerotech-rocketry.com/cu...ms_ext_dim_dwgs/54mm_hp_rms/hp_54-852_l-b.pdf
Also, I apologize for not clearly reading your initial post. I read it as your delay grain was too long time wise, not physical dimension.
 

BSNW

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I considered buying the J180T, but I'm worried it might go a bit fast for the rocket. I would rather not fiberglass the rocket, because the last time I did it, it was a PITA.

The final build weight with everything but the engine is 42oz. I'm not too concerned because my friend who I am borrowing the casing from flew the same type of rocket on a J90 too. He used to have an extended forward casing, but lost it when he loaned it to someone who's rocket was never recovered.

I have access to a machine shop and the schematic drawing of the extended closure. Do you know if it would be legal if I machined a custom closure and launched it at a Tripoli experimental launch?

My friend....you can put a K550 in your EZI and it would be okay. I have personally seen a guy launch his stock EZI set up for dual deploy on a CTI L motor (he converted motor mount to 75mm). No fiberglass just paper...wood and glue. I say try it on the J275W. Nice 3.5 sec burn and plenty of power. Enjoy your L2! Have fun!
Andrew
 

BSNW

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BTW...He did it twice with NO damage. See Tripoli-Mid Ohio launch reports. Good times
Andrew
 
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