Acceptable stability ranges for an L1 certification rocket

[rigbone]

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?

 

[Antares JS]

That does not sound correct in the slightest.

3-4 calibers of stability is useful for rockets 20-30 times as long as their diameter. 1.5-2 calibers is more than sufficient for a Zephyr.

 

[MClark]

Epoxy is a very expensive material for ballast

 

[Steve Shannon]

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?

We don’t really know what your mentor was suggesting. Is that large stability margin without a motor? That could make sense as a way to ensure a stability of at least 1 for any size of motor.

 

[wsume99]

There is a such a thing as a rocket being overstable and 3-4 calibers with the motor installed for the Zephyr sounds like you are venturing into that territory in my opinion.

I would suggest talking to the RSO at your club and ask what their requirements are for stability. More than likely it's 1 caliber but I'd check anyway. If it's a certification fight the RSO may want to see more than 1 caliber. Could possibly be as high as 1.5 for cert flights. (If you plan to use this as a cert rocket.)

Once you know the requirements of your field I'd recommend you simulate a variety of motors from the range of motors you intend to fly and find the one with the lowest stability. Then add enough nose weight to increase the stability to at least 1.3 or whatever the RSO minimum was if it's more than 1.3. Technically it only needs to be 1 per the NAR/TRA rules but some margin (not excessive margin) is a good idea.

I would not recommend your nose weight be all epoxy either. Get some BBs and then fill with epoxy to cover the BBs. Add in a few wooden sticks or metal rods passing through the NC to ensure the slug stays in place. Search the forum and you will find many options for how to add and properly secure nose weight. Epoxy typically releases heat when curing and a significant amount of epoxy curing in a plastic NC could cause it to deform. Using the least amount of epoxy possible is what I would advise.

 

[bobbyg23]

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?

You need a new mentor

 

[Jmhepworth]

This doesn’t make sense to me either. But rather than just throw out your mentor, when something doesn’t make sense to me it‘s almost always because there‘s something I didn’t understand. 1.5-2 calibers with the motor installed is plenty. The CG without the motor installed isn’t very meaningful. It would be helpful to figure out what you don’t know that’s making the advice seem so wrong.

 

[Lord Rory Gin]

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?

I modelled the Zephyr in G-J motors. Empty was 1.85 diameters, Gs gave me 1.38 dia., my H120 cert flight gave me 1.22 dia. while Js gave me 1.00 dia. Additional nose weight is not necessary and even the 14 oz. that you've added is unnecessary. The rule-of-thumb is 1 dia. for initial stability. Remember, as your propellant burns the rocket's CG moves forward as well.

BTW, there is such a thing as too much over-stability and too heavy of a rocket. Extra weight means a slower takeoff and a lower velocity as it leaves the launch rail which depending on the engine used might be a problem. If you were to use the same CTI H120 motor that I used with the 14 oz already added, you would be too slow off the rail (10.8 m/s vs 15.7 m/s) to be safe therefore you are going to need a more powerful motor like an H225.

 

[rigbone]

This doesn’t make sense to me either. But rather than just throw out your mentor, when something doesn’t make sense to me it‘s almost always because there‘s something I didn’t understand. 1.5-2 calibers with the motor installed is plenty. The CG without the motor installed isn’t very meaningful. It would be helpful to figure out what you don’t know that’s making the advice seem so wrong.

I think my mentor’s thinking (he’s not actually my mentor, he’s just someone that has given me some advice on an upcoming launch) was that I could then fly some L2 motors on my rocket for an L2 cert

 

[Scott_650]

I think my mentor’s thinking (he’s not actually my mentor, he’s just someone that has given me some advice on an upcoming launch) was that I could then fly some L2 motors on my rocket for an L2 cert

One way to think this through is you can always add weight later - keeping the weight down now gives you more L1 flexibility. If you decide to use your Zephyr for your L2 adding more nose weight later will be a fairly straightforward thing.

 

[Jmhepworth]

I think my mentor’s thinking (he’s not actually my mentor, he’s just someone that has given me some advice on an upcoming launch) was that I could then fly some L2 motors on my rocket for an L2 cert

My experience is that often the larger motors don’t need added nose weight because the longer motor itself puts weight ahead of the CP. But there’s no way to know for sure until the motor is in. Add weight then if you need to.

 

[bobbyg23]

One way to think this through is you can always add weight later - keeping the weight down now gives you more L1 flexibility. If you decide to use your Zephyr for your L2 adding more nose weight later will be a fairly straightforward thing.

There is an extremely good chance you won't need any nose weight with a j motor either.

 

[thzero]

I think my mentor’s thinking (he’s not actually my mentor, he’s just someone that has given me some advice on an upcoming launch) was that I could then fly some L2 motors on my rocket for an L2 cert

Terrible advice.

 

[cls]

I think my mentor’s thinking (he’s not actually my mentor, he’s just someone that has given me some advice on an upcoming launch) was that I could then fly some L2 motors on my rocket for an L2 cert

I hope you build a new rocket for your L2 cert. It's supposed to be a journey, not a participation badge.

 

[OzHybrid]

Cg only moves forward IF the mass of the combustable fuel/s starts behind the initial Cg. This is particularly true for long hybrid motors where the Cg may start well forward then move backward then move forward again.
This may also be true for some solid propellant motors in compact short minimum diameter rockets.
Generally however Cg does move forward as propellant burns but it's important to know that it's not always true.

 

[cbrarick]

why would you need to add ballast to a kit? It's suggested motors are G thru J when built as instructed.
Someone's on a power trip.

if you really want to learn about HPR read "Modern High Powered Rocketry 2." It's a good primer.
Get rocksim or Open rocket and see what is really gonna happen. Don't be afraid to use science rather then rumor

Ditch your "mentor" and find a real one.

I don't know how many zephyr's I've launched for people's L1, but as the LCO at a fairly large club, it's a lot. Never seen one go wonky