Launch Video: Does anyone know why my P-29 COBRA crashed?

[jeffgeraci]

After watching the launch video, does anyone have an idea what went wrong (smart alec answers are always welcome of course, but i'd also like to know the real reason). Maybe after watching the video, you will have some idea and you can share it, cause i'd sure like to know. Thanks for your help!

Meanwhile, here's a few pics of the P-29 COBRA on the launch pad at NSL 2011, Lucerne Valley, CA. The name COBRA is an acronym for "Conceptual Open-Body Rocket Assemblage" and the P-29 is for the 29-Palms Marine Corpse Base in Southern California.

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[jeffgeraci]

Here are the pics of the aftermath

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[jeffgeraci]

Here it is, the video:

https://www.youtube.com/watch?v=8JWvzGvNZv8

 

[GregGleason]

My guess is that the CP/CG relationship was off. The nose looks pretty draggy, and I'm thinking as it got faster it made it swap ends. I'm thinking that with a more conventional nose cone wouldn't have been a problem. A bunch of nose weight would probably help.

Greg

 

[cjl]

It looks like a pretty cut and dry case of instability to me. The forward wings and relatively small tail would pull the CP pretty far forward, so I'm guessing you'd need a rather immense amount of noseweight in order to make it stable.

 

[jeffgeraci]

It looks like a pretty cut and dry case of instability to me. The forward wings and relatively small tail would pull the CP pretty far forward, so I'm guessing you'd need a rather immense amount of noseweight in order to make it stable.

The CP was my guess too, which I posted in the video description. But do you know why it flew straight for the first 100 feet or so? Would the increase in velocity create more instability with regard to the CP, or would greater velocity result in more stability? My other thought was, could one of the 2 motors have reached a higher thrust point causing the rocket to tumble?

 

[sandman]

The CP was my guess too, which I posted in the video description. But do you know why it flew straight for the first 100 feet or so? Would the increase in velocity create more instability with regard to the CP, or would greater velocity result in more stability? My other thought was, could one of the 2 motors have reached a higher thrust point causing the rocket to tumble?

My guess is it was just lucky.

Even if a rocket is unstable as long as it's going straight it will keep going straight. If it veers from a straight flight path just a hair the instability will not let it return to a straight flight path.

In other words it was just lucky for the first hundred feet.

 

[GregGleason]

The CP was my guess too, which I posted in the video description. But do you know why it flew straight for the first 100 feet or so? Would the increase in velocity create more instability with regard to the CP, or would greater velocity result in more stability? My other thought was, could one of the 2 motors have reached a higher thrust point causing the rocket to tumble?

The CP can change with angle of attack. That is a complex-shaped nose and, while it is very cool, my guess is that once the angle of attack gets much off zero, that those surfaces at the nose are beginning to impart lift and torque.

https://www.grc.nasa.gov/WWW/k-12/airplane/cp.html

Drag increases with the square of speed. At the lower velocity, things seem fine. The drag on that nose got squared when it went from 30 fps to 60 fps.

https://microgravity.grc.nasa.gov/education/rocket/termvr.html

I think the design can work, but you likely need to add nose weight and/or add more fins or fin area on the back.

This is my "gut feel" for what's going on, but I will defer to the aerodynamicists on this.

Greg

 

[jeffgeraci]

The CP can change with angle of attack. That is a complex-shaped nose and, while it is very cool, my guess is that once the angle of attack gets much off zero, that those surfaces at the nose are beginning to impart lift and torque.

https://www.grc.nasa.gov/WWW/k-12/airplane/cp.html

Drag increases with the square of speed. At the lower velocity, things seem fine. The drag on that nose got squared when it went from 30 fps to 60 fps.

https://microgravity.grc.nasa.gov/education/rocket/termvr.html

I think the design can work, but you likely need to add nose weight and/or add more fins or fin area on the back.

This is my "gut feel" for what's going on, but I will defer to the aerodynamicists on this.

Greg

I think you and sandman and Greg are right about the need to add weight to the nose cone. I made a huge mistake by not checking the CG with the MOTORS installed, duh! The CG wass already pretty far back to begin with. OK, i'm convinced. I'm going to add weight to the nose, make the needed repairs, and launch it next month. Thanks guys!!

 

[bill2654]

From your description on youtube, I dont think you understand the relationship between CP and CG. And Rocksim or OpenRocket ARE great tools

 

[jeffgeraci]

From your description on youtube, I dont think you understand the relationship between CP and CG. And Rocksim or OpenRocket ARE great tools

I'm not a CP/CG specialist, but I do know that my center mass was probably not as far ahead of the CP as it needed to be. If you have a clarification to offer, or if I missed something, we'd all like to hear your thoughts. I'm just here to learn and have fun

 

[RoyAtl]

Here it is, the video:

https://www.youtube.com/watch?v=8JWvzGvNZv8

Very simple. That is your basically unstable rocket. It will go in the direction the motor pushes it until something (wind, thrust imbalance, etc) knocks it off. Then, it has no way of correcting itself. You can see that as it falls from the sky. It doesn't know which way to point.

 

[RoyAtl]

Very simple. That is your basically unstable rocket. It will go in the direction the motor pushes it until something (wind, thrust imbalance, etc) knocks it off. Then, it has no way of correcting itself. You can see that as it falls from the sky. It doesn't know which way to point.

And simply adding weight to the nose may not be the proper answer. The amount of error allowed before going unstable may increase, and it may increase enough for it to get by, but once something changes in the air, it may not have enough force for correction.

 

[bill2654]

The "rule of thumb" as I have heard it is the CG needs to be at least 1 caliber (body diameter) ahead of the CP. Example: If your airframe size is 4 inch, then the center of gravity needs to be 4 inches forward of the center of pressure. If your not sure where the CP is, a sim program will get you real close. An easy way to find the CG is balance the rocket on you finger. Be sure it's fully loaded with motor and recovery gear, just like it's gonna fly. Hope that helps. Thats a really awsome design and I hope it get repaired and flys again.

 

[jeffgeraci]

Very simple. That is your basically unstable rocket. It will go in the direction the motor pushes it until something (wind, thrust imbalance, etc) knocks it off. Then, it has no way of correcting itself. You can see that as it falls from the sky. It doesn't know which way to point.

Great observation! I see in the video that the rocket tumbles at first, and then it pivots a couple times from front to back, undecidedly as you noted, before it finally hits... aft first.

Why do you think adding weight to the front won't work? If the added weight places the CP far enough behind the CG, do you think it might work? BTW: It launched on twin H-250's- Should I go with slightly smaller motors, or would that make it worse? (rocket is 60" long, about 6" x 6" wide, and less than 8lbs.)

 

[GregGleason]

Here is what I would do (and this is just me).

I would make a down-scale boilerplate (same externals, but no paint, etc) of the rocket, and test it with a B and C motor. Maybe about a third or quarter size of your current rocket.

Run a couple of sims to get a rough idea of where the CP is.

Add about 2 ounces of weight at the nose and see how she goes. If it goes unstable, you know you need to add more. For insurance, add an ounce to increase your margin. Now you know where your CG needs to be for your current rocket.

Also, you may want to take into account the thrust curves and the thrust to weight ratios of the down-scale and make sure that they approximate what you have with your current rocket. Add motors to the down-scale that approximate the thrust curves of the ones that the current rocket will likely see.

Once you get the down-scale stable, you add weight to your current rocket until it matches the CG of the down-scale. Then you should have a fairly high confidence level that you should have a stable flight, once the LCO begins the countdown.

Greg

 

[cjl]

Adding weight to the front will work, but I think you'll need to add a rather impressive amount to make it stable. I think its largest problem is that the tailfins in the same plane as the wings are very small. The tailfins perpendicular to the wings are nice and large, but the ones parallel to the wings are small. This means that the CP in that axis will be up in the area of the wings, not back by the tailfins where you want it. I think it would help quite a bit to increase the size of the tailfins parallel to the wings. Noseweight wouldn't hurt either.

Personally, whenever I build something that I'm not completely sure of though, I'll always start at a smaller scale. A B or C powered version could tell a lot about the stability without the cost, risk, or effort of the full scale version.

 

[RoyAtl]

Great observation! I see in the video that the rocket tumbles at first, and then it pivots a couple times from front to back, undecidedly as you noted, before it finally hits... aft first.

Why do you think adding weight to the front won't work? If the added weight places the CP far enough behind the CG, do you think it might work? BTW: It launched on twin H-250's- Should I go with slightly smaller motors, or would that make it worse? (rocket is 60" long, about 6" x 6" wide, and less than 8lbs.)

I explained that in the next message :

And simply adding weight to the nose may not be the proper answer. The amount of error allowed before going unstable may increase, and it may increase enough for it to get by, but once something changes in the air, it may not have enough force for correction.

You have motors spaced far apart, off the center-line axis. That is a recipe for increasing the angle of attack. Either the thrust vectors will be slightly off, one motor ignites slightly after the other (or worse), or wind will push the rocket in one direction or another. In any case, the inertia of the larger mass in front will require a higher opposite force to correct the rotation, and the fins may be too small (or too close to the CP) to exert enough force to correct it. If you're lucky, you'll get 'coning', and at worse you'll get pretty much what you got on this flight. Though it might get higher before it does.

I've had this problem with a stock-built Quest X-15. First time I flew it on a C6-5, it went unstable right off the launch rod. Added three pennies weight to the nose cone, second flight was perfect. Third flight with three pennies, but higher wind -- unstable again! Fourth flight stable, but some coning.

 

[jeffgeraci]

This means that the CP in that axis will be up in the area of the wings, not back by the tailfins where you want it. I think it would help quite a bit to increase the size of the tailfins parallel to the wings.
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That's an excellent point; I considered enlarging the side fins for just that reason, but I didn't want to detract from the wings- doesn't look like I have much of a choice though.

I think if I add the weight to the front like everyone says, and also use larger side fins on the tail like you say, it ought to be way more stable. Thanks, i'll do it. I hope to launch it with the modifications next month.

 

[edwinshap1]

somehow i thought it'd be safe to put this to the away pad, i wonder what was i thinking lol. did look cool that far away though. :horse:

 

[jeffgeraci]

somehow i thought it'd be safe to put this to the away pad, i wonder what was i thinking lol. did look cool that far away though. :horse:

I gotta say, you guys did a great job at "mission control" for NSL :clap:

 

[T-Rex]

First let me say that is a way cool design! I would like to see it straighten up and fly right!

Nose weight may help, however it seemed to fly like a spool. In other words, it was only stable while under boost, then went wonky.

Floating back down as it did, shows it was definitely unstable after the motors burned out. If it was stable it would have arced over and come in ballistic.

Just observations....and yes, I know, I am a master of the obvious

 

[jeffgeraci]

First let me say that is a way cool design! I would like to see it straighten up and fly right!

Nose weight may help, however it seemed to fly like a spool. In other words, it was only stable while under boost, then went wonky.

Floating back down as it did, shows it was definitely unstable after the motors burned out. If it was stable it would have arced over and come in ballistic.

Just observations....and yes, I know, I am a master of the obvious

EEEEEEEEKs!!! How did this thread get ressurected???? I thought it had died long ago...

This rocket needed to be about 6-inches longer for stability. It's somewhere in a landfill now, but I salvaged the eyes and the fins. RIP

 

[edwinshap1]

And that face will haunt me for the rest of my days. Thanks Jeff

I love watching that video, it was my first important judgement call as pad manager