Help needed with unstable rocket

[delta1999]

Hello to everyone! I'm fairly new to model rocketry and I recently made couple of homemade rockets from scratch. However when I launched them, every single one of them went crazy immediately after leaving the launch ramp. I used OpenRocket software to design the rockets and the program is telling me that the rocket should be stable. On the last launch (video uploaded as zip) the rocket had a static margin of 1,18cal. On the previous launch I even added some weight to the nose to move the CG and to separate it from the CP. The static margin was 1,83cal. Unfortunately I got the same result as in the last launch. Here's also a screenshot from open rocket design and some photos from the last rocket and I can provide additional information if needed. (NOTE: all the photos bellow are related to the launch shown on the video) Can anyone tell me what may be the possible problem?

 

[smstachwick]

OpenRocket is only as good as the data you feed into it. Did you weigh the rocket’s components or otherwise get an idea of how accurate its CG calculation is?

If it’s not that, the only thing I can think of is rod/rail exit speed. What are your ground support equipment specs? What kind of condition is it in?

I’ll be sure to check out the video once I can get to my computer, I’ll need that to open the ZIP file.

 

[delta1999]

OpenRocket is only as good as the data you feed into it. Did you weigh the rocket’s components or otherwise get an idea of how accurate its CG calculation is?

If it’s not that, the only thing I can think of is rod/rail exit speed. What are your ground support equipment specs? What kind of condition is it in?

I’ll be sure to check out the video once I can get to my computer, I’ll need that to open the ZIP file.

Yes all the components were measured on weight scale and the input data is 100% correct. The same goes for the dimensions of all components. As for the ground support equipment there is a photo below. (ignore the rocket on the photo because it is from a previous launch after which I made some modifications). For ignition I use cable with kanthal wire inserted into the nozzle of the engine. As power supply I use car battery.

 

[BigMacDaddy]

Agree with previous comment -- looks like your fins are made of metal? OR is usually defaulting to weight of cardstock or balsa wood for fins.

It is easy enough to use OR with a scratch built rocket -- just figure out where the Center of Gravity CG is (balance the rocket on your finger, mark that point, and measure the distance to that point from the tip of the nose cone). Then weigh the rocket. Finally, click on the "Sustainer" stage in the top right of the build screen of OR and put in the actual weight and CG.

Usually, the CP calculation will be close enough if you get the dimensions of fins, nose cone, body tubes, etc... in correctly.

 

[BigMacDaddy]

You also need to use a longer rod (need to let your rocket sit lower on the rod). Usually want 3-5' of rod above the rocket so that it can get up to speed and fins can do their magic.

 

[BigMacDaddy]

FWIW -- building a rocket with metal parts is actually a violation of the model rocket safety standards. If that crazy flying rocket happened to hit someone it will do a ton more damage than one with balsa wood fins.

 

[Bat-mite]

Longer rod and higher thrust motor.

 

[delta1999]

FWIW -- building a rocket with metal parts is actually a violation of the model rocket safety standards. If that crazy flying rocket happened to hit someone it will do a ton more damage than one with balsa wood fins.

Yes the fins are made of metal but the rocket was tested in extremely remote area where there was no people or property nearby. (it is a former military test range) so no worries no one was put in danger

 

[SDramstad]

Yes the fins are made of metal but the rocket was tested in extremely remote area where there was no people or property nearby. (it is a former military test range) so no worries no one was put in danger

You were......

 

[delta1999]

Agree with previous comment -- looks like your fins are made of metal? OR is usually defaulting to weight of cardstock or balsa wood for fins.

It is easy enough to use OR with a scratch built rocket -- just figure out where the Center of Gravity CG is (balance the rocket on your finger, mark that point, and measure the distance to that point from the tip of the nose cone). Then weigh the rocket. Finally, click on the "Sustainer" stage in the top right of the build screen of OR and put in the actual weight and CG.

Usually, the CP calculation will be close enough if you get the dimensions of fins, nose cone, body tubes, etc... in correctly.

Yes the fins are metal-made but I weighted them on scale and override the mass in OR so it should be fine?

 

[Jmhepworth]

I didn’t see any reference to making sure the as built and as loaded rocket has the CG a caliber in front of the CP. The sim just gives you what to expect, but you still have to do the balance test before launcing. Open Rocket will give you a good CP regardless of the material used. But if you get the material wrong or built heavy, the balance test before launching will tell you if you have a stability problem.

 

[SDramstad]

What motor are you using?

 

[delta1999]

I didn’t see any reference to making sure the as built and as loaded rocket has the CG a caliber in front of the CP. The sim just gives you what to expect, but you still have to do the balance test before launcing. Open Rocket will give you a good CP regardless of the material used. But if you get the material wrong or built heavy, the balance test before launching will tell you if you have a stability problem.

I did physically checked the CG location before launch and it corresponded with the one given by OR. I just didn't do the swing test.

 

[delta1999]

What motor are you using?

It's a homemade sugar rocket motor.

 

[Jmhepworth]

I did physically checked the CG location before launch and it corresponded with the one given by OR. I just didn't do the swing test.

Then it sounds like not enough motor.

 

[cwbullet]

This post is fine, but please do not ask or post specifics on your motor. ie…….No formulas.

 

[delta1999]

Then it sounds like not enough motor.

Maybe but it looks like the motor has enough of thrust. Did you watch the video?

 

[lakeroadster]

Maybe but it looks like the motor has enough of thrust. Did you watch the video?

I watched the video.. it looks to me like the thrust of the motor isn't inline with the rocket c/l... it's shooting out at an angle.

And that would explain the non linear flight pattern.

Maybe try a commercially made motor... fly it as a heads up flight and see what results you get.

 

[delta1999]

FWIW -- building a rocket with metal parts is actually a violation of the model rocket safety standards. If that crazy flying rocket happened to hit someone it will do a ton more damage than one with balsa wood fins.

I wasn't familiar with that rule. I will remove the metal fins and replace them with balsa wood or cardboard ones.

 

[delta1999]

I watched the video.. it looks to me like the thrust of the motor isn't inline with the rocket c/l... it's shooting out at an angle.

And that would explain the non linear flight pattern.

Maybe try a commercially made motor... fly it as a heads up flight and see what results you get.

Hmm...I drilled the nozzle on static pole drill. It was inline with the rocket's longitudinal axis. What could move the thrust vector?

 

[lakeroadster]

I wasn't familiar with that rule. I will remove the metal fins and replace them with balsa wood or cardboard ones.

Hmm...I drilled the nozzle on static pole drill. It was inline with the rocket's longitudinal axis. What could move the thrust vector?

The motor is an issue also per NAR rule 2, the fins per rule 1.

https://www.nar.org/safety-information/model-rocket-safety-code/

 

[delta1999]

The motor is an issue also per NAR rule 2, the fins per rule 1.

https://www.nar.org/safety-information/model-rocket-safety-code/

Will use a commercially made motor next time

 

[Zeus-cat]

I assume you are not launching in the United States. The NAR safety rules probably don't apply to you, but they are very good rules and you should follow them for your own safety.

Your launch rod is definitely too short. Plus you have the rocket starting halfway up the rod so you lose a lot of effective length by doing that. The rocket should be moving at roughly 50 feet per second as it leaves the rod. In metric that is 15 meters per second. If possible you want a minimum of 1 meter of rod for the rocket to travel on before it is off the rod.

Your motor may not be giving you enough thrust at the start. Commercial motors like Estes and Quest have a nice spike at the start to get the rocket moving fast right away. We have no idea if your sugar motor is doing that. It appears to move slowly at the start and after it leaves the rod to move faster, but it all happens quickly, so it is difficult to say.

 

[tim cubbedge]

I assume you are not launching in the United States. The NAR safety rules probably don't apply to you, but they are very good rules and you should follow them for your own safety.

Correct & good advice.

 

[delta1999]

I assume you are not launching in the United States. The NAR safety rules probably don't apply to you, but they are very good rules and you should follow them for your own safety.

Your launch rod is definitely too short. Plus you have the rocket starting halfway up the rod so you lose a lot of effective length by doing that. The rocket should be moving at roughly 50 feet per second as it leaves the rod. In metric that is 15 meters per second. If possible you want a minimum of 1 meter of rod for the rocket to travel on before it is off the rod.

Your motor may not be giving you enough thrust at the start. Commercial motors like Estes and Quest have a nice spike at the start to get the rocket moving fast right away. We have no idea if your sugar motor is doing that. It appears to move slowly at the start and after it leaves the rod to move faster, but it all happens quickly, so it is difficult to say.

Thank you @Zeus-cat for the good advice. Yes, I'm not launching in the United States and the NAR safety rules don't apply to me but I will keep in mind the rules because safety always goes first. I'm new to rocketry and it doesn't even officially exist in my country as a hobby so I'm pretty much on my own and I rely on information found on the internet. I will try to apply every advice you gave me regarding safety and rocket building materials, motors etc.

 

[BigMacDaddy]

I can imagine it is really a pain when you live somewhere in which the cost to import commercial products is astronomically high...

Some thoughts on materials --

• Nose Cones & Other Parts - Looks like you are 3D printing the nose cone, that is great and is the same as I do for nearly every rocket I build these days. You can also 3D print boat tails, motor retainers, centering rings, etc... EDIT: As noted later, you can also 3D print the entire rocket which may be a good option if you cannot get body tubes or cheap thin plywood.
• Fins - 2mm plywood (birch or basswood) is really great for making fins. I buy 300mmx200mmx2mm sheets for $25 or so for 20 sheets on Amazon but not sure how to get shipped to you cheap. If you need to save weight then you can cut centering rings from plywood and friction fit engine (i.e., the more 3D printed parts you put in rear of rocket the more you need to add weight to nose cone usually).
• Body Tubes - this seems to be the most difficult piece to replace with what you can find lying around. People make rockets with wrapping paper tubes, paper towel tubes, and even toilet paper tubes but these tubes are usually not as strong as the commercial cardboard body tubes used for rockets and tend to bend (before or after a launch or two). Other options tend to be too heavy. If you can afford to order a bunch of body tubes from the US this might be well worth the money. Get BT-50, BT-55, BT-60, and BT-80 tubes. The couplers are also useful if you want to save weight (but you can 3D print couplers).
• Shock chords - Kevlar line seems to be another essential for scratch building. A roll of a few hundred feet will last a long time. You can also get braided elastic which is used for sewing elastic into clothing which works better than the stock stuff included in most kits anyway.
• Parachute - looks like you have this covered but can make from plastic, rip-stop nylon, etc...

 

[delta1999]

I can imagine it is really a pain where there the cost to import commercial products is astronomically high...

Some thoughts on materials --

• Nose Cones & Other Parts - Looks like you are 3D printing the nose cone, that is great and is the same that I do for nearly every rocket I build these days. You can also 3D print boat tails, motor retainers, centering rings, etc... If you need to save weight then you can cut centering rings from plywood and friction fit engine.
• Fins - 2mm plywood (birch or basswood) is really great for making fins. I buy 300mmx200mmx2mm sheets for $25 or so for 20 sheets on Amazon but not sure how to get shipped to you cheap. If you need to save weight then you can cut centering rings from plywood and friction fit engine (i.e., the more 3D printed parts you put in rear of rocket the more you need to add weight to nose cone usually).
• Body Tubes - this seems to be the most difficult piece to replace with what you can find lying around. People make rockets with wrapping paper tubes, paper towel tubes, and even toilet paper tubes but these tubes are usually not as strong as the commercial cardboard body tubes used for rockets and tend to bend (before or after a launch or two). Other options tend to be too heavy. If you can afford to order a bunch of body tubes from the US this might be well worth the money. Get BT-50, BT-55, BT-60, and BT-80 tubes. The couplers are also useful if you want to save weight (but you can 3D print couplers).
• Shock chords - Kevlar line seems to be another essential for scratch building. A roll of a few hundred feet will last a long time. You can also get braided elastic which is used for sewing elastic into clothing which works better than the stock stuff included in most kits anyway.
• Parachute - looks like you have this covered but can make from plastic, rip-stop nylon, etc...

Thank you very much @BigMacDaddy for the nice suggestions. I will consider this in building my future projects. You helped me a lot. And yes you spotted it well, the nose is 3d printed because I couldn't find anything similar to use it for that purpose. Also, in my country there are no commercial model rocketry parts available whatsoever so I improvise for what I can and for other things I have to order them from abroad.

 

[Bill S]

I wasn't familiar with that rule. I will remove the metal fins and replace them with balsa wood or cardboard ones.

With the lighter weight of cardboard or balsa fins vs metal ones, that should move the center of gravity forwards, which could help with stability.

I agree with the others that you need way more launch rod, at least a meter long, perhaps longer if the engines is a slow burning one. Can't comment on sugar motors as I know basically nothing about them.

If I may ask, what country are you in? Its possible that there are organizations either in your country or a nearby one that could be of some help. I don't know how much research you've done, so I don't mean to be rude or anything.

 

[cwbullet]

Clarification: The NAR and TRA rules vary and so do local laws and regulations. High power and model rules are different also. Metal fins and sugar motors are not forbidden. Formulas are forbidden to be discussed on the forum. There is room for discussion what minimal required metal is, but people fly rockets with metal fins at TRA launches where the local club allows them.

 

[David Schwantz]

Microsoft Word - Document1

This is Tripoli stand on metal in rockets. Please read the entire document if in doubt.