3.9" scratch L2

[rhill]

Hello Everyone,


My L2 certification rocket is a 88" long 3.9" diameter Quantum Tube Airframe. I am doing dual deployment using motor ejection for the drogue, and a perfectflilte stratologger altimeter for the main. I am planning to fly an Aerotech J350 for this certification launch and some larger motors thereafter. The rocket weighs about 8 lbs and Rocksim puts it at ~3500ft for apogee.

My concern lies in stability. Right now, my stability margin fully loaded is about 2.7. Is this ok as far as static margin goes for a rocket this size??, or should I add some weight to the aft to pull the stability margin down a bit. I have heard before that as long as static margin is between 2 and 4 you should be fine. However, I have also seen multiple forums online that say to design for about 1.5 stability margin.

Just wondering if you all could help.

This is my first post so thank you very much to anyone who responds

[chadrog]

First off, welcome to the forum. Second, I think your stability margin is fine. Is the 2.7 caliber value from simulation or actual? Where are you going to be flying?

[G_T]

If it is from simulation, make sure you include everything in your model, and use actual measured weights. It is amazing how far off the results can be if one lets Rocksim (for example) pick the weights... But with real weights things are MUCH closer to reality.

It may be just me, but I'd prefer stability to be a bit less than that. I make some attempt at picking a stability margin to reduce how far I have to walk to recover a rocket. Not enough stability (ignoring the other issues) and one is going for a downwind walk. Too much stability, and that walk may be upwind.

The result you get for stability is going to depend on what motor you load. You want to look at the range of intended motor sizes, and at that range with a full fuel load and an empty fuel load. If you don't look at the range then you may get a surprise when you fly a different case than what you used in your simulation and design process. Of course, knowing where the CP is on your rocket allows for a quick field test of suitability of static stability margin just by checking the CG with the intended motor loaded.

Gerald

[grouch]

My concern would be the motor ejection. Depending on where you put your avbay, there is the potential to have more volume than can be pressurized on motor alone.

[cjl]

It's a little higher than it needs to be, but I wouldn't worry about it. Certainly don't add weight to the aft end. I often wonder why so many people are concerned about "overstability". There's nothing wrong with a large static margin - it just means that the rocket will tend to react more quickly to disturbances. Of course, if you fly it with a marginal thrust to weight ratio in high wind, it could be a problem, but I would contend that you shouldn't fly anything with a marginal thrust to weight ratio in high wind, so that's not really a problem of the rocket being overstable - it's a problem of poor motor choice (whenever it does happen). If anything, I find that so-called "overstable" rockets tend to fly better than ones that have only 1 caliber of stability.

As was mentioned before, one important thing you should consider is what the worst case CG location is. Simulate with a variety of motors, not just the certification motor, and design based on the worst-case motor (I'm guessing this has a 54? 54mm K motors are quite a bit heavier than 38mm J motors, so even if you have a large stability margin on the J, on a K you might have quite a bit less). Also, I agree about the concern of ejection volume - a 4 inch rocket can easily have enough volume that a 38mm motor won't have a sufficient ejection charge. Is there any reason not to just use the altimeter for apogee as well? Altimeters can usually detect apogee more accurately and reliably than you can guess (or simulate) a motor delay, so if you're going to bother with electronics in the first place, you might as well use them for both deployment events. If you are determined to use the motor however, you might consider adding extra powder when flying with a 38mm motor. With 54s, you should be OK, but 38s probably won't work reliably (depending on how large your drogue compartment is).

[DAllen]

It's a little higher than it needs to be, but I wouldn't worry about it. Certainly don't add weight to the aft end. I often wonder why so many people are concerned about "overstability". There's nothing wrong with a large static margin - it just means that the rocket will tend to react more quickly to disturbances. Of course, if you fly it with a marginal thrust to weight ratio in high wind, it could be a problem, but I would contend that you shouldn't fly anything with a marginal thrust to weight ratio in high wind, so that's not really a problem of the rocket being overstable - it's a problem of poor motor choice (whenever it does happen). If anything, I find that so-called "overstable" rockets tend to fly better than ones that have only 1 caliber of stability.

As was mentioned before, one important thing you should consider is what the worst case CG location is. Simulate with a variety of motors, not just the certification motor, and design based on the worst-case motor (I'm guessing this has a 54? 54mm K motors are quite a bit heavier than 38mm J motors, so even if you have a large stability margin on the J, on a K you might have quite a bit less). Also, I agree about the concern of ejection volume - a 4 inch rocket can easily have enough volume that a 38mm motor won't have a sufficient ejection charge. Is there any reason not to just use the altimeter for apogee as well? Altimeters can usually detect apogee more accurately and reliably than you can guess (or simulate) a motor delay, so if you're going to bother with electronics in the first place, you might as well use them for both deployment events. If you are determined to use the motor however, you might consider adding extra powder when flying with a 38mm motor. With 54s, you should be OK, but 38s probably won't work reliably (depending on how large your drogue compartment is).

This guy is very smart. Exactly what I would say.

[rhill]

Thanks for the great info everyone. Chadrog, I found the cp point by using rocksim and the barrowman equations. The cg, however, was found by completely loading the rocket (casing included but minus the motor) and then simulating the weight of the propellant. I will be flying in Bunnell, FL.

As far as the motor ejection and volume concerns, I am using a piston directly above the motor casing and I have already done ground testing. I am planning on bring my BP to the launch and adding to the standard motor ejection charge as necessary.

I should clarify my first post. The parachete that is going to come out at apogee (motor ejection) will essentially be a main parachute. Since it is my first time using electronics in rockets, I thought I would rely on the motor ejection to push my main chute out (merely to reduce the possibility for human error), and use the altimiter to push out a very small parachute at 700 ft. I just though this would be a good way to ensure that I was using the electronics correctly while still having a reliable recovery system.

I know the altimiters are very easy to use but I was just trying to safeguard myself.

[rhill]

After debating with myself I decided 2 things.

1. I'm still going to use the motor ejection mainly because I'm launching this weekend and already have the rocket set up for this configuration.
2. I'm scratching the idea of deploying the bigger parachute @ apogee. I'm going to deploy the 30 in chute I have at apogee. Descent rate should be about 50 ft/s on drogue and 15 ft/s after main comes out at 700 feet.

The BP charges are both 2g and I'll probably add a pinch more utilizing the "blow it out or blow it up" mindset. (I think that's how that goes).

If anyone sees any major problems with this please let me know. If not, wish me luck on Saturday!

[blackbrandt]

After debating with myself I decided 2 things.

1. I'm still going to use the motor ejection mainly because I'm launching this weekend and already have the rocket set up for this configuration.
2. I'm scratching the idea of deploying the bigger parachute @ apogee. I'm going to deploy the 30 in chute I have at apogee. Descent rate should be about 50 ft/s on drogue and 15 ft/s after main comes out at 700 feet.

The BP charges are both 2g and I'll probably add a pinch more utilizing the "blow it out or blow it up" mindset. (I think that's how that goes).

If anyone sees any major problems with this please let me know. If not, wish me luck on Saturday!

I always heard "Blow it apart or blow it up"

[Handeman]

After debating with myself I decided 2 things.

1. I'm still going to use the motor ejection mainly because I'm launching this weekend and already have the rocket set up for this configuration.
2. I'm scratching the idea of deploying the bigger parachute @ apogee. I'm going to deploy the 30 in chute I have at apogee. Descent rate should be about 50 ft/s on drogue and 15 ft/s after main comes out at 700 feet.

The BP charges are both 2g and I'll probably add a pinch more utilizing the "blow it out or blow it up" mindset. (I think that's how that goes).

If anyone sees any major problems with this please let me know. If not, wish me luck on Saturday!

Hope your launch went well.

Personally, I have a lot of problems with the "blow it out or blow it up" mindset. Didn't you ground test enough to actually know how much you need? The next issue is what the strength of the too large of charges do to your recovery system. Compensating for the large charges by lengthening the shock cords just causes other problems.

[rhill]

So I launched this past weekend and successfully moved up to L2 certification!

I ended up going with the J500 instead of the J350 for no reason other than the Mojave Green flame During prep, remembering all of the concerns that were talked about here doing motor ejection, I went ahead and loaded the main charge on the bottom of the avionics bay in addition to the motor ejection. THANK GOD I DID, THE AV-BAY CHARGE WAS THE REASON FOR PARACHUTE EJECTION!

I had ground tested before with 2g of bp for motor ejection, however when I asssembled the reload, I was unable to fit 2g of black powder into the well.

All in all, the launch went better than I could have ever hoped for. Liftoff was great with no roll and not a hint of overstability. The "B-Sting 3" (name of the rocket) soared to 2967 feet, all events happenned as planned, and landed only 100 feet away from the Range Officer tent.

In my brief history of launching high power rockets, I know that "perfect" launches are not something to expect or get used to. However I did take the time to enjoy it while I could!

Again, thanks to everyone for their help and for welcoming me to the forum.

Happy Flying!

[chadrog]

CONGRATULATIONS!

[DM1975]

Congrats.

[qquake2k]

Congratulations on your Level 2! No photos or videos?

[rhill]

Our member who maintains the association website gets great videos so I will link to them when they become available.

As for the photos, they will be up soon! Hopefully tonight after I get off work!

[Getmore]

Congrats on your L2 I like Bunnell its nice there, we will be going to the Tampa launch site this weekend. If you can make it to the Tampa Blast in September you should try and go it is a awsome place to launch.

[davdue]

Congrats on your L2. I plan to have mine by the end of Labor Day weekend at AirFest. I didn't see this post until today but I was going to say not to worry about being over stable. My L2 is a 3X scale of the Semroc Laser-X (Build thread here: http://www.rocketryforum.com/showthr...aser-X-Upscale. I test flew it on an AT I211W at Sweaty Balls and it flew flawlessly. Open Rocket has it at 3.1 caliber and the actual altitude was 1086 vs. 1111 simulated. Below is a compilation of onboard video (keychain cam), ground video, and pictures I made of that flight. I plan to put two keychain cameras on it for the cert flight (which will be on an AT J415).

[grouch]

Congrats! Can't wait to see what this thing looks like.

[rhill]

http://www.nefar.net/gallery/2012-08...-Hill-NTSC.wmv

Above is the link to the video of the launch. Here are some basic pictures. I will post some more component pictures when I get them uploaded.

By the way, it flew to 2967 feet! Only 33 ft away from estimated altitude!

[qquake2k]

Awww!

[MasonH]

Them be some mighty small fins. Great rocket and congrats!

[rhill]

Them be some mighty small fins. Great rocket and congrats!

Serious question about the fins...

Are there any specific problems that can arise with having small fins? Stability? Problems at a higher velocity?

I ask because I plan on launching with a J415 next time I launch. It is about the same maximum thrust as the J500 but with double the burn time. Is there anything I should be worried about?

[hardinlw]

Small fins give less stability than large fins all other things being equal. Since your previous posts indicate that you have adequate stability, don't worry about stability. If you have not already done this, mark the CP on the rocket and also mark the max aft CG one diameter (caliber) ahead of this. Stickershock sells the decals. If you want to worry, worry about finding the rocket after it zooms into the stratosphere. Well, maybe not that high. Seriously, dual deployment using a fairly small drogue significantly improves the odds of recovering the rocket without having to a) walk three miles, b) climb a tree, c) ford a small river, etc.

[DRAGON64]

The design looks good as is. Had there only been 3-fins, stability might have been an issue. But, a 4-fin design with small fins is quite stable, even with just a little more drag.