Weather Cocking

I recently got my level 1 certification. (yay!) The flight went well, however the rocket did weathercock about 15-20 degrees into the wind off the launch pad, resulting in a bit of a walk to recover the rocket. I flew on a pretty low thrust motor (I150), and the liftoff velocity would have been right around 45-50 ft/s. Would this problem be corrected by using a motor with higher thrust? I may attempt level 2 on the same rocket, and just want to clarify this.

This was a very heavy rocket for level 1. 4" fiberglass, dry weight around 100oz and a 54mm mount. (Madcow kit)

Velocity would probably have reduced the weather cocking, but will not eliminate it.

A higher thrust motor might well be in order. I tend to start at a thrust to weight of 8:1 or so and go up from there.

Yeah, that makes sense. The thrust ratio I was at was definitely on the low side.

A higher thrust motor will help to minimize weather cocking, particularly with short fat rockets. However, rockets weather cock into the wind, so it can decrease the amount of walking as the rocket is blown back toward you under chute.

Is your rocket very "over-stable"? That tends to add to weather cocking also. The bigger heavier higher thrust L2 motor should help straighten it out. And as Terry said a little weather cock helps reduce your walk on a breezy day.

I was under the impression that a L1 rocket had to be less than 3.3 pounds. Is that not correct?

noffie79 said:

I was under the impression that a L1 rocket had to be less than 3.3 pounds. Is that not correct?

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The weight restriction is for FAR 101 rockets known as Class 1, Level 1 rockets are TRA/NAR certification levels.

Ok, so a fiberglassed LOC Expediter that weighs 75 oz would be ok for NAR L1?

noffie79 said:

Ok, so a fiberglassed LOC Expediter that weighs 75 oz would be ok for NAR L1?

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Any weight is ok, as long as you have a good thrust:weight ratio. However, you do need a waiver for any rocket over 1500 grams. Since most people certify at club launches with waivers, the limit usually isn't a problem (My L1 was around 65 oz with the motor).

SCrocketfan said:

Any weight is ok, as long as you have a good thrust:weight ratio. However, you do need a waiver for any rocket over 1500 grams. Since most people certify at club launches with waivers, the limit usually isn't a problem (My L1 was around 65 oz with the motor).

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Ok great. Thanks! As far as thrust to weight ratio, what's best? At minimum 5:1?

noffie79 said:

Ok great. Thanks! As far as thrust to weight ratio, that's best? At minimum 5:1?

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Probably 5:1, although as low as 3:1 can work depending on the rocket, length of rail, and conditions. For a cert flight you may want to fly a slightly higher thrust to weight like 8:1 just for safety.

noffie79 said:

I was under the impression that a L1 rocket had to be less than 3.3 pounds. Is that not correct?

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No. The L1 Cert rocket must be made by the individual and must contain 1 certified rocket motor with an H or I impulse level. that is 160.01 to 640.00 Ns.

Please read the rules before you start so you know what you have to do to cert.

https://www.nar.org/high-power-rocketry-info/level-1-hpr-certification/

https://www.nar.org/wp-content/uploads/2014/08/hpappl.pdf

or

https://www.tripoli.org/Level1

https://www.tripoli.org/Portals/1/Documents/Certification/UniversalCertificationForm.pdf

Hint FWIW: I strongly suggest using a motor that will provide an >=10:1 thrust to weight ratio to minimize weather-cocking. You simply want a quick up and down to get your cert. That's the license to play with the big boys.

Bob

Back to the OP, you also need to look at the thrust curve of the motor. The thrust number given is "average". The motor you used may have lower thrust off the pad and then gain thrust as it burns. Thrust off the pad is key to minimizing weather-cocking.

https://thrustcurve.org can sim any rocket on any motor and let you know what is unsafe off the pad.

Of course, even the perfect rocket/motor combination can fall victim to the wind if it is strong enough. Safety code say keep it under 20 mph, but really it depends on the size of the rocket.

Low speed off the rail with a good wind result in weathercocking.

Here's a good link

https://www.rocketryforum.com/showthread.php?123404-Velocity-leaving-the-rail