Pressure relief hole!!!???

I launched my first 29mm motor ever yesterday with bad unexpected results. I scratch built a 29mm rocket and loaded it up with a G80-10 for it's launch. I was using a Quest plastic fin cam and was also using a Quest plastic nose cone so the rocket wouldn't shred. The rocket went up maybe 300 feet and started skywriting all over the place before returning to the ground. I could see that the nose cone had come off and the chute had stripped off in the air. The ejection charge went off on the ground. There was a wicked zipper to the body tube but everything other than the chute survived just fine. I just studied up on what might have happened and it must be that I didn't put a pressure relief hole in the body tube. The nose cone was nice and loose like I like for smaller type motor rockets. Does everybody use pressure relief holes for bigger motors? How did I miss this info? Do I need my nosecones to fit way tighter on 29mm and above (eventually) motors? I have another bigger project I don't want to attempt to launch unless I am confident it is fairly safe.

Where was the CG in relation to CP?

Could have popped it early, could have shook free after being unstable.


A tight Nose come is best, you should be able to hold the cone and the rocket not fall away.

Do you have any photos of it in it's current condition?

Yes.. A little more information. If you dont know the CP. How about diameter - length- weight - fin size and how many. how about the shoulder length of the cone. As DizWolf, I suspect instability, but possibly the Quest cone simply pushed over from the pressure of flight(not pressure buildup in the air frame, but does need considered). its all guess without specs.View attachment 186989 Have a blast!!
fly safe
ed

Welcome to the 'transition stage' of rocketry! 300 feet is a little low for air pressure differences to make any sort impact BUT the speed is a different story. I'm not saying you have to become a physics major but you DO have to realize the forces you are dealing with are MUCH higher than say, a C-6. A basic Cp/Cg test is required, but I suspect 'fin flutter' was your basic enemy. Check into using John Coker's excellent 'thrustcurve.org' online and enter your configuration in the sim programs for motors. You may see how quickly you exceeded the 'speed of plastic'! Congratulations on your first 'shred'! It's a common problem when stepping up into AP motors while still using basic LPR construction techniques. While your bird may have survived a "D", "E" or even and "F" motor, now you are running in to material strength and eventually hardcore aerodynamic problems. Are the fins foiled? If not,teh blunt (square edge) tends to 'hunt' trying to find the least resistance to the air going by and contributes mightily to flutter.
I'm going to say yes, a relief port may have been in order, as this combo most likely would have taken you way above the altitudes you are used to seeing with BP motors. Don't get me wrong, there are contest models out there doing amazing things, but they are really engineered on the cutting edge just for competition. Rebuild (sleeve with coupler to repair the damage) and tighten up your nose cone fit. Try a smaller motor and check your Cp/Cg (sounds like you could 'swingtest' it!) or get into the free Open Rocket program and crank your stuff into the program to forecast what you may experience on the flight line. Don't feel too bad if it surprises you with a relatively higher Mach percentage than you've had experience with. This is the exciting part of experimenting and pushing your own envelope.
Mouse around TRF and you'll get a better idea about transonic flights and fin construction. Ideally, you'll be inspired to carry on upward to higher altitudes with greater speeds and get your baby back. Don't let this discourage you, as a lot of us have exceeded the speed of cardboard, balsa, plastic, fiberglass, and even carbon fiber!
Straight smoke and good chutes,brother!

You also could look at using shear pins...

We definitely need more info about the Design to determine why the Failure occured.

Welcome to mid&high power, where it's always fun, whether successful flight or post-mortem of unsuccessful one

G80 has 1-sec high thrust which ends very abruptly. Best guess from your description is that due to abrupt change of forces, sustainer drag-separated from nose cone, rocket lost stability, and nose cone pulled the laundry out.


Was the connection between nose cone and sustainer firm or on the loose side?


-Alex

I will fly this rocket again after extending the body, adding weight to the nose cone, wrapping tape around the cone to make it very tight, and adding a relief hole vent near the top of the body tube to cover all my bases in case my stability tests were done incorrectly the first go around and the nose cone didn't just pop off during the thrust phase like it looked like happenned. I am all in on this rocket and only want to launch it with a G80. The cheap Quest fin cam has held up so far, so I still have faith in it. I never expected to see this rocket again after I hit the button the first time unless the body tube folded. My ridiculous 8 foot blue tube 29mm minimum diameter beast is finished other than painting and waiting for the chute to arrive. The build didn't involve much but it is strong as hell and basically a big spear. I feel like that thing could kill somebody if something went wrong and don't want that to happen. I want to successfully launch this first 29mm one before this other much heavier one.

View attachment 187065 I guess I will need to break my camera out, but here is the fin cam I used for this one. It is supposed to be 30mm, but it needs moderate filing down of the interior to fit over a standard 29mm tube. I needed to file off the interior ribs. I think with significant filing down I might get one to fit a heavy duty motor mount size 29mm tube.

I don't think the pressure relief hole--or lack thereof--was in any way a contributing factor here. I do suspect that the nosecone fit had a lot to do with it--drag separation. Flutter might be contributing as well--but it's hard to say without more information.

You say this was a scratch-build... was there a Rocksim or Open Rocket file used to sim this? That may be helpful...



Later!

--Coop

With fins that small, and a big old motor like a G80, and lacking other info, my best guess is unstable.

When you say 29mm, are we talking minimum diameter here? If so, we are talking about much more significant accelerations both positive and negative during boost and coast. If you did sim it in OR or RS, the files would tell us much more.

300ft won't see much pressure change, not enough to cause the nose cone to pop off from the pressure differential.
Sure sounds like drag separation to me.

-Hans

fyrwrxz said:

I suspect 'fin flutter' was your basic enemy.

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+1 If it was skywriting under thrust @300' it was not drag sep. Those little plastic fin cans flutter like crazy on G motor. The cone came off from the forces because it was too loose, IMO. The thrust exceeded the strength of components.

As this was scratch built, did you use rocksim or open rocket and was the design stable? I'd also suspect the plastic fin can, as those fins were probably dancing quite a bit (they really are meant for much smaller motors). How long was your rod/rail? If it's too short or if it was small and really whipping, the flight could be having problems as soon as it left the pad. I've also seen weird things when an igniter is not all the way into the grains and not at the front of the motor.

300' isn't high enough to really worry about using a vent.

Nantucketdink said:

View attachment 187065 I guess I will need to break my camera out, but here is the fin cam I used for this one. It is supposed to be 30mm, but it needs moderate filing down of the interior to fit over a standard 29mm tube. I needed to file off the interior ribs. I think with significant filing down I might get one to fit a heavy duty motor mount size 29mm tube.

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With a 29mm tube, that is the ID of the motor tube (actually, usually a touch over 29mm), so if the fincan has an inner diameter of 30mm, you must have filed off quite a but, which means the fins could flutter much more than usual, as the wall of the fin can gets much thinner and doesn't support them as well.

Yep.. It looks like Nantcketdink is lookin to fly high and get there fast.. Way back when,, I tried an F something in a min 29mm Estes rocket( Was supposed to fly on D power) It went straight for about 500' I guess and did the shred and skywriting . Cool sight to see. balsa fins fluttering down and other parts. The problem is "low power" Estes and Quest parts arent well suited for a G80 flight.** It can be done tho** and you are getting some great advise and information from all.
Continue to have a blast.!!!( big smile here)

Just returning to model rockets and yes I immediately want to do those things I only imagined as a child. The 110 Astrocam camera rocket was out of my price range as a kid and so were the big rockets so I built a Mean Machine and ordered a keychain camera as soon as I really got the urge to jump into the hobby. Obviously unleashing the biggest motor I could get my hands on comes/came next. Launching my daughter's My Little Ponies into space happens later this week. I am building multiple rockets everyday (or at least working on multiple projects) and want to do more than just assemble kits. Kits seem to always work well so far and I want to experiment too. I have started exploring Thrustcurve but haven't looked into Rocksim yet. I thought I had a basic understanding of determining where the CG and CP are. I am using a 1/4" launch rod that's like 5 feet long or so (PSII). I got ahold of the only 29 minimum diameter plastic fin cams I could find (Quest and Landrau on Etsy) as I got scared of ripping the fins off for these first 2 minimum diameter builds. The Landrau one is sweet and tougher than any balsa fin out there in my opinion so far (total rookie).

First, your nose cone should neither be too loose or tight. In short, your nose cone should remain in place if you turn the rocket upside dow and shake lightly. At the same time, you don't want a lot more force to send the cone off.

As for Rocksim, you might also look at OpenRocket which is free. To be sure, it is not as complete as Rocksim, but it is a good program and will help you know where your CP is.

My test for how tight the nose cone should be is when I pick up the flight ready rocket by the nose cone and give it a slight shake. The nose cone should stay in, but move slightly in the BT. I use this test for all size rockets.

Nantucketdink said:

I got ahold of the only 29 minimum diameter plastic fin cams I could find (Quest and Landrau on Etsy) as I got scared of ripping the fins off for these first 2 minimum diameter builds. The Landrau one is sweet and tougher than any balsa fin out there in my opinion so far (total rookie).

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Plywood, G10 plate or carbon fiber plate and some high temp epoxy is the way to go. (or metal bolt-on fin cans) For G motors, I think you can get away with hobbystore plywood and 5 minute epoxy.

DizWolf said:

Plywood, G10 plate or carbon fiber plate and some high temp epoxy is the way to go. (or metal bolt-on fin cans) For G motors, I think you can get away with hobbystore plywood and 5 minute epoxy.

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You do know that metal bolt on fin cans probably won't be allowed at NAR launches, right. That "non ductile metal" thing (read the NAR rules). For mid power, plywood is fine. Heck, my L3 flight had plywood fins! In fact, my L3 was a cardboard tube, plastic NC, and plywood fins. People today seem to way overbuild stuff. You need to work on your building skills if you can't do it, IMHO. Carbon Fiber on a MPR bird is simply silly, IMHO.

RecoRocket said:

You do know that metal bolt on fin cans probably won't be allowed at NAR launches, right. That "non ductile metal" thing (read the NAR rules).

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Aluminum is the 2nd most malleable metal, and the 6th most ductile. But agreed, silly for mid power. Stick with good aircraft ply IMO.

Mr Nantucketdink I have a min. dia. 24mm rocket called VB Extreme 24, A vaughn Brothers kit from the 90's. I still fly it(just last week). They also produced a kit called the VB Extreme29. A 29mm min dia. kit./ On a G80-10 it advertised over 3,800 ft. It was 40" long-used G-10 fins. (sanding the frame first and epoxied 1" forward of rear of airframe),Weight was 6oz. without motor*. These specs might give you an idea where you otta be for stability. Also the separation point was about 12" from nose. allowing for a payload. The Extreme 29 was stable with anything from a D to a G and beyond(with in reason)lol the fins were about 5"long with a long sweep to the rearX 3. Not sure if its a delta swept wing or not. Got this info from an old advertisement and memory. I had the Extreme 54 also RIP
Have a Blast--Ed

This is the other fin cam. It only weighs 1.7 ounces and yet is very strong. Piece of cake fins for 29mm minimum diameter builds.

Here's the zipper which happened by dragging the chute around before the evacuation discharge (wrong word choice). For this one I am cutting out the damage, significantly extending the body tube, and adding weight to the nose.

Here's "Surfroc." 8 foot plus 29mm minimum diameter Bluetube with 4 foot blue tube coupler inside and urethane cone. 3D printed fin cam. Homemade 3 foot chute out of heavy contractor bag. G80-7 going in this one. Should I rename it "Trouble?"

It's going up later today if I can get the chute and shock cord stuffed in it.

It looks like the Apogee Aspire is right up my alley (and still available today) and I think I should probably buy one to use as a starting point for future 29mm minimum diameter builds. Heck, I can duplicate it exactly if I have no faith in my own skills. I need to learn to paper fins and will try it out on my next balsa fins build. I already have a lot of experience using epoxies which is a nice carryover skill for this hobby, but I haven't fiberglassed anything before. Getting familiar with the characteristics of various different epoxies seems to only come with hands on experience. I am super excited about all of this stuff right now. I would join a club right away, but I live on an island and getting off island for regular rocket launches wouldn't be realistic.

My nose cone test, and others can correct me if I am being naive, to do two things. First, like others said, lift the assembled rocket by the nose cone and see if you can shake it out. If so, too loose. Add tape. Second, see how much effort it takes to pull it out once it's in all the way. If you can't pull it out using your two hands, too tight. Remove tape or sand down.

Nantucketdink said:

View attachment 187184Here's "Surfroc." 8 foot plus 29mm minimum diameter Bluetube with 4 foot blue tube coupler inside and urethane cone. 3D printed fin cam. Homemade 3 foot chute out of heavy contractor bag. G80-7 going in this one. Should I rename it "Trouble?"

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Hello again,, NIce pics and they sure clear up alot of questions. You are plenty experienced and now by the looks of things you should have no troubles. Stability doesnt look like the previous problem (fin shred or cone slipping off) That fin can looks great and fin shape is pretty close the Extreme rockets. Be careful using the plastic bags for chutes and be sure that you can install it without having to "stuff" it. My only other thought was the strength of your air frame and you have that done. Maybe a CG test? That 3' bag chute worries me a little. if it deploys and opens your going for a walk LOL
Have a Blast out there