Choosing proper engine for Big Nuke

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Steven88

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Hello. I purchased the AT L850W as a motor for my Loc Big Nuke 3E rocket before building it and simming it, and now am wondering if it’s the best or even a good choice? I also lengthened the booster of the rocket by 14” to help with stability. RS is showing a velocity at launch guide departure of 47 MPH which according to some figures taken from Apogee’s website is only good for a 7mph wind or less on launch day. Would I be better off purchasing a higher thrust engine such as the AT L1520T engine to help prevent possible weather cocking provided this is only my second launch for this rocket, the first being a smaller high thrust J on the shorter version of the rocket? Don’t want any surprises...
 

dhbarr

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What happens to your sim if you increase the length of the launch rod?
 

Steven88

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It does help, I increased it to 96” which I believe is what the club has where I fly. But the 47mph launch guide departure was with the guide already set at 96”
 

afadeev

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Hello. I purchased the AT L850W as a motor for my Loc Big Nuke 3E rocket before building it and simming it, and now am wondering if it’s the best or even a good choice? I also lengthened the booster of the rocket by 14” to help with stability. RS is showing a velocity at launch guide departure of 47 MPH which according to some figures taken from Apogee’s website is only good for a 7mph wind or less on launch day.
I am not familiar with Apogee's analysis or the math and assumptions that drive it, but an often mentioned rule-of-thumb for safe, proven, stable off-rod speed is ~50 fps (34 mph).

Wind gusts vary in direction and speed across altitudes, so a fixed maximum wind speed value is an oversimplification. You want to achieve stable flight off-rod, since after that, the speeds and stability will only be increasing (speed goes up, weight and CG go down and shift forward). For stability purposes, the only wind speed that matters is that that prevails 8-50" off the ground (8" being the typical launch rail length).

Would I be better off purchasing a higher thrust engine such as the AT L1520T engine to help prevent possible weather cocking provided this is only my second launch for this rocket, the first being a smaller high thrust J on the shorter version of the rocket? Don’t want any surprises...
You can buy as many motors as you desire, but I think 47 mph (69 fps) is way more than adequate.
The real answer will be arrived by realistically modeling your rocket in OpenRocket, simulating flights at varying cross-wide speeds, and observing resulting angle of attack at lift-off.
Lucky for you, LOC is pretty good about publishing the .rkt files for its models, and you can easily run the sims with minimal complications.

Good luck, and have fun!

a
 

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