Advice Request! First Scratch Build (24 mm)

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Flyfalcons

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You can also make the break point of the rocket where the coupler is. Add a lite ply bulkhead and small eye bolt to attach the shock cord to. No question about ejection ability.
 

Back_at_it

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WRT to a stuffer tube...

There doesnt seem to be one on the Mean Machine, and it's over 6 ft tall on a BT-60 airframe, flying similar motors to the the ones I'm planning on using. Havent heard of any problems ejecting the parachute on that one...
Some get lucky and never have an issue with the ejection. Others aren't so lucky. I've been unlucky a few times and have learned to build around it. Most of us that build longer rockets like the mean machine have taken to building them to seperate in the middle or just below the upper tube. My last MM was built with a 30 inch stuffer, and a 4 inch baffle. It seperates just below the upper tube.

Doing a seperation in the middle allows for easier transportation and helps with ejection. Below is a post someone made on doing the mid-seperation.

 
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HeavyBurdon

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Some get lucky and never have an issue with the ejection. Others aren't so lucky. I've been unlucky a few times and have learned to build around it. Most of us that build longer rockets like the mean machine have taken to building them to seperate in the middle or just below the upper tube. My last MM was built with a 30 inch suffer, and a 4 inch baffle. It seperates just below the upper tube.

Doing a seperation in the middle allows for earier transpost and helps with ejection. Below is a post someone made on doing the mid-seperation.

Sweeet, thanks! That was going to be my next question!
 

OverTheTop

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Forward swept fins are a bit less efficient, might drop your overstability by 1/4 to 1/2 caliber.
IIRC forward-swept fins are actually more effecient, as they reduce energy transferred to the tip vortex at non-zero AoA. The main problem with them is fin flutter, caused by the airflow over an aeroelastic component. The forward sweep causes the oscillations. That's why most aircraft you see have rearward sweep.
 

BABAR

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IIRC forward-swept fins are actually more effecient, as they reduce energy transferred to the tip vortex at non-zero AoA. The main problem with them is fin flutter, caused by the airflow over an aeroelastic component. The forward sweep causes the oscillations. That's why most aircraft you see have rearward sweep.
I didn’t know that. I just used them in my Helis because otherwise they look Ike swastickas, and I don’t l like that.
 

HeavyBurdon

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Final design. The boys are excited! Papered the fins for the first time. Seems easier than wood filler for sure. Now to finish, paint, and rig up the recovery. Thinking of going with a 12 inch semi drogue and 15 inch main.
 
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mbeels

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Very nice! I think you modified the stability situation in a creative way, and now you've got a good looking rocket there.
 

HeavyBurdon

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Final design. The boys are excited! Now to paint and rig up the recovery system. Going back and forth between doing 2 parachutes (12 inch semi-drogue, 15 inch main), or one 18 or 24 inch chute. Should I go with a mid-body separation or nose cone?
IMG_0587.jpg
 

Joshua F Thomas

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Sorry to interrupt what looks like a great build, but can someone explain why excessive stability is a problem?
 

neil_w

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Sorry to interrupt what looks like a great build, but can someone explain why excessive stability is a problem?
Weathercocking. An overstable rocket will have an excessive tendency to point into the wind, which is made worse if the speed off the rod is insufficient. It's not as bad of a problem as understability, certainly, but it's pretty easy to avoid so why not. High-thrust motors that really get the rocket moving off the pad help mitigate the problem.

 

Joshua F Thomas

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Thanks for the simple answer. This has also been bugging me in my simulations with OpenRocket, but what is a “good enough” launch speed? I’ve seen either 13.8 m/s or 15 m/s given; but some of the kits I have use speeds as low as 10 m/s.
 

OverTheTop

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I would fly down to about 12 m/s (40fps) for small rockets, but I would be waiting till inbetween gusts. Larger (HPR) rockets I would be happy with 18 m/s off the rail, but again I would be waiting for a relatively quiet wind speed to push the button. Anything less than that needs a bigger motor or a longer rail. YMMV.
 

HeavyBurdon

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Update: The EX-24 is assembled and ready for prime/paint! For recovery, I decided on nose cone separation, a 9 ft kevlar cord (anchored to the coupler/ejection baffle mid-tube), with a Dino chute protector (cut to size to fit well in the BT-60), and a 24-inch Apogee nylon chute, secured about a foot from the nose cone.
 

mbeels

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Nice, do you have a paint scheme in mind yet?
 

HeavyBurdon

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Nice, do you have a paint scheme in mind yet?
Yup! I just figured it out:
Nose cone: white
Forward body section: navy blue
Aft body section: white
Vinyl decals: "EX-24" in black (aft section), "HeavyBURDON" in white (forward section)

Just finished painting the aft section white. Going to wait until it dries, then mask it off with painters tape and a plastic bag to pain the forward section blue.
 

mbeels

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Congrats! Nice photos as well. It's good to see this project culminate with a successful launch.
 

HeavyBurdon

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Second launch! So happy with how this gal flies! Straight as an arrow and perfect recoveries.
 
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