Advice Request! First Scratch Build (24 mm)

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HeavyBurdon

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Hey Team,

This will be my first bottom-up build for a 24 mm rocket! Some specs and components (just received from Apogee and Estes):
  • 2 Estes BT-60 tubes
  • Estes 24 mm engine mount
  • Apogee ejection baffle (will act as the coupler between the two tubes, screw eye inserted as a shock cord anchor)
  • 3 balsa Apogee Rising Star fins (planning to paper them)
  • 15 inch nylon Apogee chute
  • Dino chute protector
  • 2 Estes 3/8 inch launch lugs
  • Estes plastic nose cone
- Planned to fly on black powder D12-5 to 600 ft, and E12-6 to 900 feet (OpenRocket sim). Trying to keep height limited for a relatively small field (1000 ft by 500 ft).
- Total length about 108 cm.
- Stability approx 5.5 cal (depending on engine used, sim'd in OpenRocket).

Attached is the 3D rendering from OpenRocket.

Can you guys give me a feel for if I've missed anything, recommendations, or general building advice for this one? Planning on naming her "EX-24" (Experimental, 24 mm)...#NerdAlert.

Thanks All!
- C.J.

EX-24 Rocket Plan.PNG
 

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boatgeek

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Looks nice! 3/8" launch lugs are an odd size. I normally see 1/8, 3/16, and 1/4, then rail buttons for larger rockets.
 

neil_w

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Two quick comments:
1) Stability margin is *very* high. Those fins could be smaller. (edit: I see it's 5.5 for D12, but you're thinking of flying on E12. Adjust your stability for the E12, and then you'll have a little extra margin for the D12. What's the margin with the E12?)
2) 15" parachute seems very small for a rocket of that weight. What does OR give you as descent speed? (or see: https://www.rocketreviews.com/parachute-size-calculator.html)
 
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neil_w

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Looks nice! 3/8" launch lugs are an odd size. I normally see 1/8, 3/16, and 1/4, then rail buttons for larger rockets.
Estes doesn't make 3/8", I assume it's a typo, meant 3/16".
 

DaveW6DPS

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I would be concerned about the rocket being over-stable. With 5 calibers it would take very little to cause a flight more exciting than safe. I have found that more than three calibers can be problematic unless you have no wind whatsoever, and no rod whip.

I would reduce the fin size to get about two calibers of separation between CG and CP.
 

HeavyBurdon

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Two quick comments:
1) Stability margin is *very* high. Those fins could be smaller. (edit: I see it's 5.5 for D12, but you're thinking of flying on E12. Adjust your stability for the E12, and then you'll have a little extra margin for the D12. What's the margin with the E12?)
2) 15" parachute seems very small for a rocket of that weight. What does OR give you as descent speed? (or see: https://www.rocketreviews.com/parachute-size-calculator.html)
OR says 5 m/s
 

mbeels

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I'm not familiar with OR, is that 5 calibers of stability with motor, or without motor?

I'd say looks good overall, you'll have a "zipperless" design that way. You could consider adding rail buttons if you fly at a club and want to put it on the rails.
 

HeavyBurdon

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I'm not familiar with OR, is that 5 calibers of stability with motor, or without motor?

I'd say looks good overall, you'll have a "zipperless" design that way. You could consider adding rail buttons if you fly at a club and want to put it on the rails.
OpenRocket. 5 to 5.5 with a D engine. If I cut the upper tube in half to 9 inches, I can bring the stability down to about 3.25 with a D engine, 3 with an E engine.
 

neil_w

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OpenRocket. 5 to 5.5 with a D engine. If I cut the upper tube in half to 9 inches, I can bring the stability down to about 3.25 with a D engine, 3 with an E engine.
Even 3 is high, particularly if the rocket is shorter.

If we go by @Nytrunner 's 10% rule, then at its current length you'd want about 10 cm stability, or about 2.5 calibers. If it's short, then less.

I just noticed you've purchased some pre-made fins. Honestly they are too big for this rocket. Would you be willing to cut them down a bit? You could do so very easily without messing them up.

(All of the above advice IMHO)
 

mbeels

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If you're feeling slightly more ambitious, you could cut the tube and cut a fin tab for through the wall fins. It wouldn't be necessary for strength on a rocket this light, but it would be one way to reduce the effective area of your fins. Or trimming, as neil_w suggested, is easier.
 

mbeels

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BTW, have fun with the scratch build, it's mucho fun turning something from your mind into a real flying rocket!
 

OverTheTop

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Looks good to me. I am not particularly concerned about the 5 cal of stability. Fly it in between wind gusts or use a longer rail and you will be ok.
 

Back_at_it

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Over all it looks like a pretty nice build. The fins do look to be a tad on the large side but you can offset that somewhat by adding weigh to the rear. My first observation is the amount of body tube to that the ejection charge will need to fill pop the nose cone. It would be a good idea in my opinion to add a stuffer tube in the rear section. A piece of BT50 tube about or 14 inches long would help take up a bit of that space and add needed weight the rear.
 

neil_w

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Nothing wrong with a stuffer tube, but I’d be surprised if a 24mm motor had trouble ejecting through that body tube.
 

GlenP

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Awesome.

Why not go ahead and just call it the #NerdAlert

:)

 

Flyfalcons

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All looks good to me! Don't listen to the "overstable" guys, they've probably never flown a Mean Machine or Fletcher in their lives. You could probably move the launch lugs back a bit but it seems like you've thought through your design very well. Have fun!
 

HeavyBurdon

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Thanks for the suggestions. How about a solution like the image attached? Make the fins smaller and move them forward? Looks unique...
Stability if done this way is about 2 cal with an E12-6 and 2.7 on a D12-5. (Don’t worry about the lug placement...I’ll fix that).
96FC5954-534D-4831-9BDD-EA2758D23033.jpeg
 

OverTheTop

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Personally I think it looks better with the fins further forward, but I would not quite pull them that far forward. YMMV.

It also lowers the risk of them breaking on landing, which is why I like fins a little bit away, at least, from the rear.
 

rklapp

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Thanks for the suggestions. How about a solution like the image attached? Make the fins smaller and move them forward? Looks unique...
Stability if done this way is about 2 cal with an E12-6 and 2.7 on a D12-5. (Don’t worry about the lug placement...I’ll fix that). View attachment 422818
At this, might as well turn it into a 2 stage.
 

BABAR

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Won’t help that much, but you could invert your fins. Forward swept fins are a bit less efficient, might drop your overstability by 1/4 to 1/2 caliber.

Another option would be to add some small destabilizing canard fins up forward.

Both above modification I think add to the “cool” factor, gets you away from “just another three fin and a nose cone” rocket.
 

neil_w

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Personally I think it looks better with the fins further forward, but I would not quite pull them that far forward. YMMV.
I tend to agree, just from my own aesthetics perspective, I'd move them back a bit. You've got some room to play with, stability-wise.

My not-quite-finished pencil rocket has a similar fin placement (although not as radically far foward), due to unique considerations.
1593695432681.png
 

mbeels

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Also reminds me of my pink 3-stage rocket that I built long before I knew about rocket simulation software. It is very over stable, so I only fly it in calm conditions. I added more motors and and that helped by moving the CG back, stage 2 doesn't need any fins. It flies fine as a single stage, but will definitely weather cock in any wind.

The pink rocket here:

 

Back_at_it

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Nothing wrong with a stuffer tube, but I’d be surprised if a 24mm motor had trouble ejecting through that body tube.
If you're running a composit motor I would agree but the ejection charge from Estes motors are so hit or miss, I wouldn't chance it. I've had issues with a 48in BT60 scratch build. More than once it only popped the cone and didn't hang the laundry. I pulled the motor mount and added 16in of stuffer tube. Problem solved.

Also, don't get too hung up on the over stable issue. If you have ever sim'ed a Mean machine or equinox you know that they are in the 14 ot 16 cal range with a motor and fly perfect.
 
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neil_w

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If you're running a composit motor I would agree but the ejection charge from Estes motors are so hit or miss, I wouldn't chance it. I've had issues with a 48in BT60 scratch build. More than once it only popped the cone and didn't hang the laundry. I pulled the motor mount and added 16in of stuffer tube. Problem solved.
OK, good to know. I guess I haven't gotten quite that large on any of mine, and I usually (due to design issues) end up with some amount of stuffer tube and have never had a problem.
 

HeavyBurdon

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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...
 
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