Acceptable stability ranges for an L1 certification rocket

[rigbone]

Hi all,

I'm modeling my Apogee Zephyr in OpenRocket and my mentor has recommended that I aim for a stability factor of 3-4 calibers. My experience with stability in LPR has been more around the 1.5-2 caliber range. I am only planning on flying H motors on this rocket and have my nose cone modeled with 14 oz of liquid epoxy in the nose cone for a total stability of 2.63 cal. My mentor says that I should add more epoxy to the nose cone to get the stability to 3-4 cal. Does this advice sound correct to you guys?

 

[Rob Campbell]

One caliber is generally good enough. My Zephyr flies fine on H motors with no added nose weight. My concern with an over stable rocket is it will tend to weathervane and possibly fly horizontally. This can result in a shredded parachute or a fast moving rocket hitting the ground.

 

[cls]

Answered in your other thread. Get a new mentor. Build the Zephyr according to the instructions and fly it. 1.5 is a good number.

 

[sriegel]

3-4 caliber is overkill and will be very sensitive to any crosswind at launch. 1.5 +/- is fine.

 

[OverTheTop]

At one I would be saying perhaps make it a bit better. 1.5 IMHO you would be good to go. Would be looking for above 2 possibly if you were going supersonic.

 

[Back_at_it]

As someone that is hoping to fly for my level 1 here in a week or so I've been doing a ton of research on the topic and talking to people that have flown the same rocket. My cert. flight will be with a LOC Warlock. It's the definition of short and fat so there is a ton of base drag. Using OR without doing the rear cone I adjusted mine using nose weight to get the rocket to .81cal. After speaking to others with this rocket they have all confirmed that this is more than stable enough for the Warlock.

As for the Zephyr. The largest motor I've been able to buy and fly in mine is the G80. With the G80 and all the recovery gear on board my CG is right at the 38.20" mark for a stability of 1.07cal. I've flown this motor three times without any issues. If I was doing a cert flight on an H motor I'd add weight to get me to the 1.4 - 1.5 range but I think that is about as high as I'd feel the need to go. Your mentor telling you that 3.0 - 4.0 is better is not accurate.

 

[ThirstyBarbarian]

Your mentor is wrong. I’m sure they mean well, but they sound misinformed. That rocket is designed to fly fine in stock form and is meant for new L1 fliers, so if the instructions don’t say to add weight, you don’t need it.

The more weight you add, the more you have to consider other modifications. By adding almost a pound already, do you have to consider increasing the size of the chute? At some point, if you add enough weight, the recommended motors on the lower-thrust end will not have enough thrust to safely get it off the pad, and you’ll have to pick higher thrust motors.

I’d recommend not adding any more weight. Finish the rocket, paint it, and then load in all the recovery gear and motor retainer, motor adapter if using one, etc. — everything that will be on the launch pad except the motor. Then weigh the rocket fully loaded. And balance it to find the center of gravity. Do an Open Rocket or RocSim file and add the weight and CG you measured as global overrides. With an accurate sim file with the measurements from your specific rocket added as overrides, you can run your simulations and pick a motor that will fly safely with your modified rocket.

You might want to check on that chute size too. Maybe you can call Apogee about that and give them the weight of your finished rocket and ask if the stock chute will bring that rocket down safely. Or, if you just want to upgrade the chute, I like Spherachutes. Add the weight of an expended motor to the weight you measured and call them up for a chute size recommendation.

Good luck on your certification flight!