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38mm down to 29mm H123 for Level 1 certification

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Ccolvin968

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Hey everyone.
New to the HPR game and this forum but have been launching low and mid power for years.
I'm wondering if I could get some advice?

My club has a business that sells motors and components on site during launch days.
I wanted to fly my formula 75 on an H123 for its maiden flight and my level 1 cert at the end of this month.
Problem is since Cesaroni had a fire back in March, they are out of stock for the 38mm 2, 3, and 4 grain, but have the 29mm reloads.
I'm using an Aero pack retainer, so I know I can step it down with an adapter.
I don't have my laptop with me, but what would the performance drawbacks be from stepping it down?
Lower flight? Slower flight?
I will run Sims on it once I get home, just a little nervous that this will push my level 1 until next year.
Thanks for your time!
 

Coop

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As far as performance, the only penalty will be the weight of the adapter and any (negligible) impact it will have on your CG/CP. the ejection charge is 1.2g on the 29, if memory serves, and 1.3 on the 38's. Otherwise, expect the same general performance for the size (Ns) motor you're flying. 29's will tend to have faster burns, so the average thrust will likely be higher for a given Ns range.

Get pics!


Later!

--Coop
 

Banzai88

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Rough OR numbers come in at a CG shift of about .5 inch aft with the adapter, 60ish fps off a 6 foot rail and 1550ish ft altitude.
 
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Bat-mite

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Best advice I can give is to simulate everything in OpenRocket. If you don't have the time or energy to learn that product, you can do rough sims at ThrustCurve.org.
 

NateLowrie

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Hey everyone.
New to the HPR game and this forum but have been launching low and mid power for years.
I'm wondering if I could get some advice?

My club has a business that sells motors and components on site during launch days.
I wanted to fly my formula 75 on an H123 for its maiden flight and my level 1 cert at the end of this month.
Problem is since Cesaroni had a fire back in March, they are out of stock for the 38mm 2, 3, and 4 grain, but have the 29mm reloads.
I'm using an Aero pack retainer, so I know I can step it down with an adapter.
I don't have my laptop with me, but what would the performance drawbacks be from stepping it down?
Lower flight? Slower flight?
I will run Sims on it once I get home, just a little nervous that this will push my level 1 until next year.
Thanks for your time!
I would call Animal Motor Works about CTI stock. They should have them available and can ship you a single motor for your cert attempt.
 

Ccolvin968

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Bat-mite, I'm using openrocket for all my Sims. Love that program!

Banzai, that sounds close to what my Sims are showing. Thanksgiving the cg shift note.

I'll look into AMW. Thanks Nate.
 

Ccolvin968

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14690914_10154135828510547_7804625590710940007_n.jpg
Here is the Formula 75. My first HP rocket!!
It looks slightly larger just because of the short shelves in the background.
Almost complete except for the guide buttons and just a wet sanding to finish her up to a nice shine! :D
 

Ccolvin968

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Rough OR numbers come in at a CG shift of about .5 inch aft with the adapter, 60ish fps off a 6 foot rail and 1550ish ft altitude.

My sims are showing something different...
My total sim Formula 75 rocket weight is 64.6oz (dead on what has been measured) CG @ 27.5" dry, etc.
My simulation output is Apogee is 915 ft, 45.4 fps off of 6 ft launch rod with the H123 SK 29mm motor.
Compared to the 38mm H123 motor 43.3 Fps and an apogee of 1400 ft.
The 29mm H123-SK has an impulse of 176, the 38mm H123 has an impulse of 235...
Do the letters after the H123 (SK) indicate something that the 38mm H123 does not have following it?
This is a lot more in depth than C, D, and E motors. haha.
I know you said rough OR numbers, but do you think our numbers are within acceptable ranges. +/-33% seems a bit extreme.
What rocket did you run the sim on?
 

ActingLikeAKid

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View attachment 303185
Here is the Formula 75. My first HP rocket!!
It looks slightly larger just because of the short shelves in the background.
Almost complete except for the guide buttons and just a wet sanding to finish her up to a nice shine! :D
WOW you didn't mess around with your first cert attempt. Gorgeous rocket!
As for the motor situation, yep, a 6g case with a 29mm adapter ought to meet your needs. And the adapter is about 40g, which in a 3lb rocket is negligible in terms of performance. (In a 1/2A mini LPR rocket, yeah, it would keep it on the ground. But when you're putting up a 3lb rocket on an H motor, the adapter weight will likely disappear).

Another alternative - if your vendor sells it - is an Aerotech single-use motor. Slightly more expensive than a similar reload, but you aren't committing to an Aerotech case. Something like https://www.apogeerockets.com/Rocke...0T-14A?zenid=58f1af309aa03fc88f979b12e289a9ee
 

Swissyhawk

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Looks like somebody is into reloading. Very nice looking rocket by the way.
 

noffie79

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Both CTI H123's are Skidmarks. They have the same average thrust, but one has a higher total impulse (the 38mm), and therefore, a longer burn time. The longer burn time will result in a higher altitude. Hope this helps and good luck!
 
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Ccolvin968

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Thanks for the compliments on the rocket!

Noffie, that summed it up perfectly. I get it now!
 

Flyfalcons

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I saw one of these at the last launch I went to. The owner of the rocket did his level 1 and level 2 with it. Really nice design. Good luck on the cert flight!
 

KennB

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Both CTI H123's are Skidmarks. They have the same average thrust, but one has a higher total impulse (the 38mm), and therefore, a longer burn time. The longer burn time will result in a higher altitude. Hope this helps and good luck!
Ccolvin, check with the club you'll be doing your flights with to be sure sparky motors are allowed. Some clubs don't allow them at all and others only at certain launches.
 

Ccolvin968

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I have already checked with the club president. I'm good to go for the skidmark.
Thanks for the heads up!
 
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