4 in Hawk stability

[Dawgbert]

I bought myself a 4" Hawk for Christmas, been working on it as much as the wife would allow, planning to fly it for a Level 1 Cert. I picked it because it had Big Cool Fins, turns out that these are a lot more work!
I'm new to rocketry, but it seems there is a lot of gray area around stability when looking through threads and listening to podcasts. The issue I'm hung up on is the stability. The jest of what I've learned is, short fat rockets can have cal of .8, long skinny can be times 3 to 4, all rockets must be twice body tube diameter. I believe the answer is somewhere in between!
There's not much I could find on the 4" Hawk out there, don't know what qualifies as a short fat rocket, but this thing has a LOT of fin on it.
OR says I need an additional 500 grams of nose weight to fly a H550, with a 1 cal. Getting close to a 7 pound rocket!
Quality input would be greatly appreciated.

 

[David Schwantz]

Hi Dawg, welcome. I do not have a Hawk, but several Phoenix. The CP is at the front of the fins, not that yours is, just on the Phoenix. Do you have a sim program? Open Rocket is free, Rocksim is made by Apogee Components. Both work well. You load your rocket file or design into the program, it will tell you CP, CG static margin. You can load various motors to locate the CG with each. When I put a J250 into my 4" Phoenix I need 3lbs of lead up front. Standard is 1 cal for stability. You can have more, but is apt to weather cock in flight. I would not go with less than 1 cal on this rocket. Try the sim programs, they do help. Good luck.

 

[David Schwantz]

PS, rocket looks sweet! Who's kit?

 

[David Schwantz]

If you need decals, Sticker Shock makes some of the best. Mark is a stand up guy and will bend over backwards.

 

[Scott_650]

I’m assuming this is a LOC kit? If you built it per the instructions stability with an H motor for your L1 launch shouldn’t be a problem.

 

[Dawgbert]

Hi Dawg, welcome. I do not have a Hawk, but several Phoenix. The CP is at the front of the fins, not that yours is, just on the Phoenix. Do you have a sim program? Open Rocket is free, Rocksim is made by Apogee Components. Both work well. You load your rocket file or design into the program, it will tell you CP, CG static margin. You can load various motors to locate the CG with each. When I put a J250 into my 4" Phoenix I need 3lbs of lead up front. Standard is 1 cal for stability. You can have more, but is apt to weather cock in flight. I would not go with less than 1 cal on this rocket. Try the sim programs, they do help. Good luck.

Hi Dawg, welcome. I do not have a Hawk, but several Phoenix. The CP is at the front of the fins, not that yours is, just on the Phoenix. Do you have a sim program? Open Rocket is free, Rocksim is made by Apogee Components. Both work well. You load your rocket file or design into the program, it will tell you CP, CG static margin. You can load various motors to locate the CG with each. When I put a J250 into my 4" Phoenix I need 3lbs of lead up front. Standard is 1 cal for stability. You can have more, but is apt to weather cock in flight. I would not go with less than 1 cal on this rocket. Try the sim programs, they do help. Good luck.

I have it simmed in Openrocket, been playing with different motors, was looking at the I500 to start with, backed off to the H550. The lower impulse motors seemed like they were too slow off the rail.

 

[Dawgbert]

I’m assuming this is a LOC kit? If you built it per the instructions stability with an H motor for your L1 launch shouldn’t be a problem.

Yeah, it's a Loc, trying to make it look as advertised, got a couple US ARMY stickers, nothing fancy.

 

[David Schwantz]

My L1 was with LOC 4" Phoenix and AT H283ST. Wanted the extra thrust off the rail with all that fin area. Good luck with your flights.

And nobody will believe you if you don't post pics

 

[SnapRoll]

I feel your pain. I’m building a 8.0”, all FG MiM-23 that loaded will be almost 135 pounds. And 42.5lb of that is the nose cone and associated weight.

 

[Dawgbert]

My L1 was with LOC 4" Phoenix and AT H283ST. Wanted the extra thrust off the rail with all that fin area. Good luck with your flights.

And nobody will believe you if you don't post pics

Thanks for the advice, this is all new to me! Rockets, simulations, launches.
Heck, I'm trying to figure out forums!

 

[Dawgbert]

I feel your pain. I’m building a 8.0”, all FG MiM-23 that loaded will be almost 135 pounds. And 42.5lb of that is the nose cone and associated weight.

Dang, almost a third just to keep it stable.
What size motor will you fly?

 

[SnapRoll]

Dang, almost a third just to keep it stable.
What size motor will you fly?

I’d rather be heavy and stable than put something that size into the air and have it be dangerous. It’s first flight will be on an N3180 Red, and I’m setting it up/building it to take a AT O5280X. It sims to 6500 AGL on the N, and around 13K on the O.

My humble advice, if you can adjust the nose weight where the CG is just at, or ever so slightly forward of the fins, you should be OK. Can you configure the Nosecone where you can add weight if needed on a larger engine?

 

[mo2872]

Should have the LOC RNWS for changing weight slugs.....

 

[Dawgbert]

Oka

I’m assuming this is a LOC kit? If you built it per the instructions stability with an H motor for your L1 launch shouldn’t be a problem.

Instructions say sim it and put CG in front of CP with nose weight, no reference to adding a body diameter. I'm thinking some of this may be "understood" in the rocket world, and I just don't know!

 

[Dawgbert]

Great,

I’d rather be heavy and stable than put something that size into the air and have it be dangerous. It’s first flight will be on an N3180 Red, and I’m setting it up/building it to take a AT O5280X. It sims to 6500 AGL on the N, and around 13K on the O.

My humble advice, if you can adjust the nose weight where the CG is just at, or ever so slightly forward of the fins, you should be OK. Can you configure the Nosecone where you can add weight if needed on a larger engine?

 

[Dawgbert]

Great,

It has the nosecone with weight that can be added, looks like I'm going to dump 500 grains of shot in front of it permanently, then adjust heavier for larger motors.

 

[Dawgbert]

Should have the LOC RNWS for changing weight slugs.....

Yeah, that's right! It has a bunch of air space in front of it. Going to load it up first to run smaller motors, then add weight as needed. That's the plan anyway.

 

[SnapRoll]

It has the nosecone with weight that can be added, looks like I'm going to dump 500 grains of shot in front of it permanently, then adjust heavier for larger motors.

The other thing to consider aerodynamically is the amount of fin area versus wind. The wind can displace Hawk fins easily, hence, a touch of stability with a nice high impulse motor will help if yours flies on a windy day. Mine will have a fairly conservative crosswind limitation for sure.

 

[Dawgbert]

The other thing to consider aerodynamically is the amount of fin area versus wind. The wind can displace Hawk fins easily, hence, a touch of stability with a nice high impulse motor will help if yours flies on a windy day. Mine will have a fairly conservative crosswind limitation for sure.

Okay,.....
Gotta get my head wrapped around it a little better but it's coming.

 

[Dawgbert]

It has the nosecone with weight that can be added, looks like I'm going to dump 500 grains of shot in front of it permanently, then adjust heavier for larger motors.

GRAMS, not grains.

 

[SnapRoll]

Okay,.....
Gotta get my head wrapped around it a little better but it's coming.

You’ll nail it. No worries, have fun with it and ask as many questions as needed.

 

[David Schwantz]

Dawg, just so you know, when adding weight put it as far forward as you can.

 

[Dawgbert]

You’ll nail it. No worries, have fun with it and ask as many questions as needed.

Okay thanks

 

[Dawgbert]

Dawg, just so you know, when adding weight put it as far forward as you can.

Roger that, thanks for the advice!

 

[JimJarvis50]

It helps to think of stability in terms of the percentage of the length of the rocket rather than body diameters. So, for a given percentage of the length, short/fat rockets would be stable with fewer calibers than tall skinny ones. Your milage may vary, but I'd shoot for a stability of 1.2 calibers for that rocket. Just an educated guess.

Learn to use a simulation program to find the CP (if the manufacturer doesn't tell you where it is in the instruction). Then, you can figure out where the CG needs to be, accounting for the weight of the motor. If you use shot to adjust the weight, encapsulate it in epoxy. Do not fly loose shot! A simulation program also tells you the motor delay that you need.

If you end up with a heavy nose cone and a high thrust motor, you might also want to think about shear pins to keep the nose cone from separating at burnout.

Jim

 

[Dawgbert]

It helps to think of stability in terms of the percentage of the length of the rocket rather than body diameters. So, for a given percentage of the length, short/fat rockets would be stable with fewer calibers than tall skinny ones. Your milage may vary, but I'd shoot for a stability of 1.2 calibers for that rocket. Just an educated guess.

Learn to use a simulation program to find the CP (if the manufacturer doesn't tell you where it is in the instruction). Then, you can figure out where the CG needs to be, accounting for the weight of the motor. If you use shot to adjust the weight, encapsulate it in epoxy. Do not fly loose shot! A simulation program also tells you the motor delay that you need.

If you end up with a heavy nose cone and a high thrust motor, you might also want to think about shear pins to keep the nose cone from separating at burnout.

Jim

I appreciate the information. I used openrocket for the simulation, by removing nose weight moved the CG all the way back behind CP and the simulation flew, with higher apogee and longer delays as CG moved aft.Confusing!
With all the different sizes and combinations of rocket builds, the stability issue is understandable, I just don't understand it. (Yet)
I'd been trying to figure how much epoxy weight / shot ratio for a thick liquid. We can figure something out there.
But, it hadn't crossed my mind about when the motor burns out, may be like stabbing the brakes!

 

[SkyFire]

I built a 2.6" Madcow Hawk. Had to add about 8 oz. of weight to nosecone to get 1 cal of stability.

 

[JimJarvis50]

I appreciate the information. I used openrocket for the simulation, by removing nose weight moved the CG all the way back behind CP and the simulation flew, with higher apogee and longer delays as CG moved aft.Confusing!
With all the different sizes and combinations of rocket builds, the stability issue is understandable, I just don't understand it. (Yet)
I'd been trying to figure how much epoxy weight / shot ratio for a thick liquid. We can figure something out there.
But, it hadn't crossed my mind about when the motor burns out, may be like stabbing the brakes!

I have a slightly modified Big Daddy that flies on J motors. It has some weight in the cone, and I shear pin the cone. Seems like four draggy fins with a weighted cone, going quickly from high acceleration to negative acceleration, could cause drag separation, so I would say better safe than sorry. Maybe others can weigh in on that. If you decide to shear pin the cone (using #2 nylon screws for example), there is a lot of information here on how to do it.

The Big Daddy is set up for about 0.9 calibers. It could probably be lower than that, but it flies nicely at that stability. I also have tall skinny staged rockets set up at 3 calibers. The correct stability just depends on where a rocket falls between those extremes. If SkyFire's rocket flies at 1 caliber, that might be just fine.

I think it's good to be a little conservative on the stability. I flew a rocket recently that was around 1.1 calibers on paper, but I'm pretty sure it was initially unstable.

Jim

 

[Dawgbert]

I built a 2.6" Madcow Hawk. Had to add about 8 oz. of weight to nosecone to get 1 cal of stability.
View attachment 449353

Good looking rocket, I like the black fins, they contrast well.
Motorcycle not too shabby either .
Thanks for the info.

 

[Dawgbert]

I have a slightly modified Big Daddy that flies on J motors. It has some weight in the cone, and I shear pin the cone. Seems like four draggy fins with a weighted cone, going quickly from high acceleration to negative acceleration, could cause drag separation, so I would say better safe than sorry. Maybe others can weigh in on that. If you decide to shear pin the cone (using #2 nylon screws for example), there is a lot of information here on how to do it.

The Big Daddy is set up for about 0.9 calibers. It could probably be lower than that, but it flies nicely at that stability. I also have tall skinny staged rockets set up at 3 calibers. The correct stability just depends on where a rocket falls between those extremes. If SkyFire's rocket flies at 1 caliber, that might be just fine.

I think it's good to be a little conservative on the stability. I flew a rocket recently that was around 1.1 calibers on paper, but I'm pretty sure it was initially unstable.

Jim

Jim,
This cone is going to be in the neighborhood of 800 to 950 grams depending upon where the CG is set. That's with the H motor. LOC document says it will handle K's. Just have to keep adding weight as the motors get larger!
Thanks again