scratch built rocket failed on test flight

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RobPodlaski

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Not sure what went wrong. test flight launch went up but turned tail over nose. The rocket was balanced, test flight was on a c6-5. did not get much altitude (weight was a bit much). I was more concerned with stability, and needless to say, it failed the test.

I think it might be the fins were too small to hold it stable? really dont have any idea. Anyone help me out so i can correct the issue before going on to up-scaling it?

100_2852.jpg

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Download Open Rocket and check stability.

As said by others, rear fins prob. are too small, forward fins are bad for stability (unless made up for somewhat in nose weight and/or larger rear fins). Also, more nose weight may be needed.
 
455 Buick said:
Download Open Rocket and check stability.

As said by others, rear fins prob. are too small, forward fins are bad for stability (unless made up for somewhat in nose weight and/or larger rear fins). Also, more nose weight may be needed.
Click to expand...

have tried to use Open Rocket, but cant seem to get the hang of it. Even "youtube" tutorials on how to navigate the designs i did on paper into a CAD program, leave me frustrated. My local rocket club has yet to begin having meetings, so I am left to trial and error, but sometimes you learn more that way. I realize Open Rocket, or ROCKSIM would make designing this easier. Just need someone with more experience using it to assist.
 
RobPodlaski said:
Not sure what went wrong. test flight launch went up but turned tail over nose. The rocket was balanced,...
Click to expand...

Hi Rob, Nice scratch build of a sub!

Not sure what you mean by "balanced".

For stable flight the CG needs to be at least 1 caliber in front of the CP.

The C6-5 that you used can safely lift 4 oz (that's ready to fly weight including the motor)

I'm not seeing a launch lug anywhere on your rocket, you did have one didn't you?
 
scsager said:
Hi Rob, Nice scratch build of a sub!

Not sure what you mean by "balanced".

For stable flight the CG needs to be at least 1 caliber in front of the CP.

The C6-5 that you used can safely lift 4 oz (that's ready to fly weight including the motor)

I'm not seeing a launch lug anywhere on your rocket, you did have one didn't you?
Click to expand...

Yes, LL was the last thing I put on. Balanced the rocket by adding weight to the opposite side of the "conning tower" to avoid it "spinning". I then added weight to the aft end to counter the "conning tower" fin and added weight in the nose. From the mathematical center of the rocket, it was balanced horizontally.

I either miscalculated the weight distribution, because it initially launched true, then slowly turned over, or the fins are too small. But from what I read before the build, the more fins, the smaller they can be.

If I could understand how to input the design into O R, I would be ahead of the learning curve. And would be able to run simulations. I just dont know how to "build" this in O R
 
Too much rear weight. You should not have placed any in the rear.

If your math center balance point (center of gravity) was done without motor, then again, way too much rear weight, especially after a motor is added.

Think of it this way- if you were to throw the rocket by hand sideways or up in the air, which way would it orient itself before it hits the ground? You want nose weight, not balance or tail weight. You want it to fly like an arrow, or a lawn dart (AKA Jarts for those of us that grew up as the world was beginning to turn from its risk-taking history making endeavors like Apollo and Shuttle to this risk-averse politically jacked up committee mentality of non-decision making we have today...wow, sorry, I got a little carried away there...had a chance to hit delete, but screw it).
 
OR wants you to start with a nose cone, so you add a nose cone then click on 'edit' to change size and shape, from there you add the rest of the rocket. OR has a number of 'examples' (file, open example) that you can play with, it also has a parts database(which I admit can be a bit hard to find...).
Fins
I think one problem you have is that the rear fins do not extend outward far enough. fluids don't scale, so what works at full size may not work when scaled down.
Rex
 
Tailcones reduce stability. That's 3 anti-stability items (Nose cone, tailcone, forward fins/finlike objects) vs. the tiny rear fins. Having more (rear) fins doesn't avoid needing about the same total area, and in this case using many small fins put them down in the turbulence behind the body. Finally, even though it could be made stable simply by moving CG forward rather than CP aft, the small total aerodynamic area results in needing to get more speed off the launch rod than ordinary designs.
 
Not really sure how to figure CP on my own, I tinkered with Open Rocket, trying to get one of their examples to resemble, albeit not exact, the Sub by adding fins and reducing the size. It did in fact show CP ahead of CG, and adding weight to the nose brought the CG up. Not sure how to read the simulations, but will try to make the changes, and repair damage from first attempt and test again. Thanks for all the input. This was my first scratch build (you can see the initial thread here: https://www.rocketryforum.com/showt...attempt-at-an-upscale-scratch-build-need-help ) to get an idea how it looks inside.
 
Maybe there is a different solution to the problem. Since you are clearly going for a look, and may not want to change the proportions of the tower and aft fins to get you to stable. Have you considered a seperate fincan that could plug into the aft motor mount when you are ready to fly that includes some larger clear fins? You could paint the central motor tube blue (waterish) and use thin, clear polycarbonate (or whatever the transparent fins people use). It could probably secure to the motor tube by having a small ring that would engage the motor clip of the current mount, friction fit, or a combination of both. This way you could have a static display that was exactly what you wanted, and have a simple add on that makes it fly stabily...kinda like the Dr. Zooch flame fins.

Also, cool rocket. I like different. Good luck with version 2, or 1.1...whatever.
 
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Cl(VII) said:
Maybe there is a different solution to the problem. Since you are clearly going for a look, and may not want to change the proportions of the tower and aft fins to get you to stable. Have you considered a seperate fincan that could plug into the aft motor mount when you are ready to fly that includes some larger clear fins?

Also, cool rocket. I like different. Good luck with version 2, or 1.1...whatever.
Click to expand...

Thanks, I actually was considering just that possibility if I cant get the design to fly as it is now. I have never done a staged rocket, but this may be a viable solution -- Thanks!
 
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RobPodlaski said:
Thanks, I actually was considering just that possibility if I cant get the design to fly as it is now. I have never done a staged rocket, but this may be a viable solution -- Thanks!
Click to expand...

I didn't mean staged. As I've described it would still be single stage as it would have only the motor in the add on, and the add on would stay with it through recovery. I think if it were staged you would be good until staging, but then the sustainer would be just as unstable as the initial test. There is a chance that the higher velocity would improve the stability enough, but I fear with everything in the proportions it is you are a good ways from stable, and velocity alone will not get you there. That big tower is killing you I suspect. I certainly could be wrong as I'm only using eye-ball sim on this one.
 
RobPodlaski said:
Balanced the rocket by adding weight to the opposite side of the "conning tower" to avoid it "spinning". I then added weight to the aft end to counter the "conning tower" fin and added weight in the nose. From the mathematical center of the rocket, it was balanced horizontally.
Click to expand...

Ummm, what do you mean by the term "balanced?"
Rocket stability has nothing to do with the mathematical center of the rocket. In fact, I'm not sure the term "balance" is ever used in terms of stability.

Your design is really cool, but inherently unstable, especially with those forward accoutrements. I don't think you should proceed to staging until you REALLY undertstand the CG/CP concepts. You can however single stage your rocket AS built by adding a big fin unit (which will carry the single engine) just behind your existing rocket.

I did something like that when I modified my ExoSkell to fly on 24 mm engines.

https://www.rocketryforum.com/showthread.php?36956-Exo-Skell-build-thread&highlight=exoskell
see post 9.

You are correct that you can go with somewhat smaller fins if you go with more fins. However, if the fins are really small (and therefore don't extend very far away from the body tube) they don't have as much effect. In your case, the fins are pretty tiny, especially for such a large body tube rocket.

You've got a neat idea here, I hope to see more of it and a successful flight report.
 
Uncle Scary said:
I did a very quick, very rough simulation based off eye-balling your pictures.
View attachment 169026

View attachment 169027
Click to expand...

That's very interesting. The CP is so far forward, but I don't think it's right. Experimenting with removing the forward fins doesn't fix it. Trying increasing the nose and body tube diameter, I was able to make the CP move so far forward it was past the front of the rocket and went somewhere off the left side of the page! And at 180 degree angle of attack (going back to the original file), the CP moves very far aft so it doesn't want to go backwards either. I think OR (i.e. Barrowman) ignores any fin not sticking out beyond the diameter of the rocket or even a little further. The actual design is very streamlined and a long taper rear, and I would expect the rear fins to work somewhat, especially at low speeds and turbulence. I think it could work to make it a little less realistic and slightly more rocketlike, plus plenty of nose weight of course (maybe a bigger motor then and more nose weight). Will need wind or string testing for final test. Also try Open Rocket with angle of attack = 90 degrees, which is basically the same as the cardboard cutout method of calculating stability.

Incidentally, the actual rocket pictures show the wood grain running across the direction of greatest stress, apt to break. Usually the grain is put parallel to the leading edge of fins.
 
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Uncle Scary said:
I did a very quick, very rough simulation based off eye-balling your pictures.
View attachment 169026

View attachment 169027
Click to expand...

So, looking at this, it shows CP in front of CG, which would explain why it turned in flight tail over nose. I am going to try adding more weight to the nose, it will be trial and error to figure out how much, but will try to video it so I can troubleshoot the flight step by step. I would like to get this to fly, and eventually upgrade to a 4" tube, and then go even bigger possibly 10" diameter body tube. I have learned alot during the build. Just need some hands on assistance from more experienced rocketeers. My local club has yet to begin spring meetings, and emails to the officers seem to have gotten lost in cyberspace. so, unfortunately, this is the only place I can turn for guidance.

Any suggestions that you may have using the simulation would be appreciated!

Rob
 
Looks like there is something hinky with the cp/cg in OR. Best bet is to go old school . Make a foamboard cut out of the sub. The balance point will be really close to your cp which I'm willing to bet is a tick behind where OR shows your cg. If it were me I'd have my cg somewhere near the back of the sail. To get there you'll need to balance the model with all flight gear installed and add weight as needed--I think your gonna need a ton of it though. Might be too much for your selected motor choice..
 
The design is so inherently unstable that it is not practical to fly it as it is. Two different approaches could make it work. The first would be large rear clear plastic fins (similar to those used to fly Mercury Atlas or Gemini Titan models). The second would be to use a base drag approach by mounting a base plate at the rear of the sub to give it stability.
 
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Remember that a tailcone moves the CP forward. In this case, the tailcone probably changes the CP more than the fins. The fins are just way too small. They barely extend past the body tube.
 
RobPodlaski said:
. I am going to try adding more weight to the nose

Rob
Click to expand...

There are some things more nose weight can't fix. Remember the more nose weight you add, the slower your rocket is off the pad. The slower your rocket is off the pad, the less effective your fins are. The less effective your fins are, the more your CP moves forward. So you compensate by putting in a bigger engine, Which is heavier and moves your CG BACKWARD. It is a vicious cycle.

IMHO you need bigger fins. Either clear plastic for aesthetic value or an added on fin can.
 
BABAR said:
There are some things more nose weight can't fix. Remember the more nose weight you add, the slower your rocket is off the pad. The slower your rocket is off the pad, the less effective your fins are. The less effective your fins are, the more your CP moves forward. So you compensate by putting in a bigger engine, Which is heavier and moves your CG BACKWARD. It is a vicious cycle.

IMHO you need bigger fins. Either clear plastic for aesthetic value or an added on fin can.
Click to expand...

+1

Really like the rocket, but think the added fin can may be your best approach. Before lighting another motor in this thing, tie a long string around the rocket at the CG (Balance point) with motor, recovery, etc. in place, and swing it in a circle over your head. Unless the nose stays forward it is not stable. Please, for safety sake, do not try test flights unless your rocket can pass this simple stability test.:duck:
 
Mr Rocket said:
+1

Really like the rocket, but think the added fin can may be your best approach. Before lighting another motor in this thing, tie a long string around the rocket at the CG (Balance point) with motor, recovery, etc. in place, and swing it in a circle over your head. Unless the nose stays forward it is not stable. Please, for safety sake, do not try test flights unless your rocket can pass this simple stability test.:duck:
Click to expand...

Ok this explains the "string test" I have not heard about until this thread. Can you explain what you mean about nose staying forward? I would think centrifugal force from swinging it would keep it forward, unless I am missing something in the explanation.

Sorry if I seem clueless about this, as I have said this is my first scratch build. Most of my rockets have been kits, so all this is extremely new to me.
 
BABAR said:
There are some things more nose weight can't fix. Remember the more nose weight you add, the slower your rocket is off the pad. The slower your rocket is off the pad, the less effective your fins are. The less effective your fins are, the more your CP moves forward. So you compensate by putting in a bigger engine, Which is heavier and moves your CG BACKWARD. It is a vicious cycle.

IMHO you need bigger fins. Either clear plastic for aesthetic value or an added on fin can.
Click to expand...

The statement above just crushed my happy little world where more power and nose weight solve all problems.:sigh: How about ditching the traditional motor and clear fins in back and keep the hind end scale.:y: Man up and put two canted motors up front. More power with the cluster and the forward motors will also act as nose weight (move up that CG nice and high). Do it the way the first rockets were built with the motors up front and no fins. I like Chinese! The steely eyed rocket scientists will call it a glorified pop bottle rocket, but you will know it will fly. Hide the canted mounts as best as possible with the fore planes. Tell 'em the canted motors will produce virtual "flame fins" with the exhaust plumes. Tell them two finned rockets are NOT dynamically unstable. Sure they will laugh at you but say you have Mind simmed the flight, and that your Hopesim predicts less than a 50% chance of grievous bodily harm. No computer simulation used! More laughter will follow, especially when they see you have finished and painted your rocket before the test flight. You are crazy! Just another nutty odd roc flyer! Put the launch rod through the center of the sub for added outrageousness. Trail behind a dorsal sonar probe on a string. Watch their reaction as you take the blast deflector off the pad! Say "I don't want no stinking fins!" Tell 'em ya ain't had much book learnin' but ya feel real good about this one.

SORRY! All that jabber in the paragraph above is just crazy talk. Flying sport scale subs with out big clear plastic fins is just not possible. Stick with safe, sound and proven 4FNC rockets. Stay away from sport scale airplanes, subs or flying monster rockets that can't be safely computer simulated. For Pete's sake, listen to a voice of reason!
 
Daddyisabar said:
The statement above just crushed my happy little world where more power and nose weight solve all problems.:sigh: How about ditching the traditional motor and clear fins in back and keep the hind end scale.:y: Man up and put two canted motors up front. More power with the cluster and the forward motors will also act as nose weight (move up that CG nice and high). Do it the way the first rockets were built with the motors up front and no fins. I like Chinese! The steely eyed rocket scientists will call it a glorified pop bottle rocket, but you will know it will fly. Hide the canted mounts as best as possible with the fore planes. Tell 'em the canted motors will produce virtual "flame fins" with the exhaust plumes. Tell them two finned rockets are NOT dynamically unstable. Sure they will laugh at you but say you have Mind simmed the flight, and that your Hopesim predicts less than a 50% chance of grievous bodily harm. No computer simulation used! More laughter will follow, especially when they see you have finished and painted your rocket before the test flight. You are crazy! Just another nutty odd roc flyer! Put the launch rod through the center of the sub for added outrageousness. Trail behind a dorsal sonar probe on a string. Watch their reaction as you take the blast deflector off the pad! Say "I don't want no stinking fins!" Tell 'em ya ain't had much book learnin' but ya feel real good about this one.

SORRY! All that jabber in the paragraph above is just crazy talk. Flying sport scale subs with out big clear plastic fins is just not possible. Stick with safe, sound and proven 4FNC rockets. Stay away from sport scale airplanes, subs or flying monster rockets that can't be safely computer simulated. For Pete's sake, listen to a voice of reason!
Click to expand...

:clap: Well played, sir!
 
Daddyisabar said:
The statement above just crushed my happy little world where more power and nose weight solve all problems.:sigh: How about ditching the traditional motor and clear fins in back and keep the hind end scale.:y: Man up and put two canted motors up front. More power with the cluster and the forward motors will also act as nose weight (move up that CG nice and high). Do it the way the first rockets were built with the motors up front and no fins. I like Chinese! The steely eyed rocket scientists will call it a glorified pop bottle rocket, but you will know it will fly. Hide the canted mounts as best as possible with the fore planes. Tell 'em the canted motors will produce virtual "flame fins" with the exhaust plumes. Tell them two finned rockets are NOT dynamically unstable. Sure they will laugh at you but say you have Mind simmed the flight, and that your Hopesim predicts less than a 50% chance of grievous bodily harm. No computer simulation used! More laughter will follow, especially when they see you have finished and painted your rocket before the test flight. You are crazy! Just another nutty odd roc flyer! Put the launch rod through the center of the sub for added outrageousness. Trail behind a dorsal sonar probe on a string. Watch their reaction as you take the blast deflector off the pad! Say "I don't want no stinking fins!" Tell 'em ya ain't had much book learnin' but ya feel real good about this one.

SORRY! All that jabber in the paragraph above is just crazy talk. Flying sport scale subs with out big clear plastic fins is just not possible. Stick with safe, sound and proven 4FNC rockets. Stay away from sport scale airplanes, subs or flying monster rockets that can't be safely computer simulated. For Pete's sake, listen to a voice of reason!
Click to expand...

LOL, I stand corrected, as I have seen your rockets and indeed you make the non-flyable, well, fly-able. I do wonder however if you agree that simply adding nose weight to this design would not be the optimal solution.

As for playing it safe, I think you will find my rockets (Gyskelion, Whopper Flopper Chopper, Three Dog Night, Cerberus III, Soliton Sled, Buzzed Lightyear SR-73 Raven, DoubleRNuthin, TripleSec, etc.) while not near your caliber are not your typical safe, sound and proven 4FNC rockets either. I guess my "Lucky 7" 2FNC (two planar fins and a nose cone) wasn't exactly supposed to be stable either, but it was.
https://www.rocketryforum.com/showt...s-and-a-nose-cone-The-Lucky-7&highlight=lucky
BTW, none of these was designed or tested with OR or RockSim, I did use ThrustCurve for engine selection on the Buzzed Lightyear.

The canted motor idea of yours for this is obviously a good one, and has been done with amazing results (Fliskits Design of the year 2004)
https://www.fliskits.com/services/dom/doy2004.htm Looks a bit like RobP's design.

In any case, I wish the original poster RobP best of luck, as mentioned I think it is a neat design, but definitely needs tweaking of some sort.

Tom
 
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