Honest John Instability

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Bkdoubleu

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I bought a 2.5" fiberglas kit from Apogee, it's from Madcow Rocketry. I used all the parts except the little wood "rotating rockets" They looked a little cheezy, so I left them off. I launched the rocket with an Econojet F20 Single use composit motor on a 1/4" rod/pad. Straight up 10 mph wind. The rocket flew 30' straight before turnigh left and then rihgt making a zig zag pattern before lawn darting into the soft hay field I was at. The rocket never tumbled but I imagine you can guess it's flight from what I described. On the instruction sheet it says the CG is 29.5" from nose cone tip. The pad weight is 2.35 #. After launching it I noticed the rod had build up some powder residue and perhaps a little paint from 3 previous launches. I did notice the rocket was very slow lifting off the pad. Like crazy slow. Given the engine I used and what I noticed from the launch rod is the flight descriptive of under powered launch?

No Sim, no video, and no I am not sure where CP is. That's why i am asking questions. I added some weight to the nose cone but have not tried to refly it. I was going to yesterday but wind was blowing 24 mph. So I scrubbed the launch. Any thoughts my fellow rocketeers?
 
My first opinion after looking at the motors thrust curve is not enough velocity off the rail. A F20 produces 40.3 newtons at ignition giving a 3.9:1 thrust to weight ratio, that coupled with a sticky rail is probably the issue. Increase the T:W to 5:1 and make sure the railbuttons slide freely and it most likely will fly beautifully. The F20 burns 2.3 seconds and the average thrust is 22.3 newtons, the initial thrust spike then dropoff didn't allow rapid enough acceleration after leaving the rod, also the 10mph wind was too much in this situation. My $.02
 
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from your description, I would say that you tried using a motor that was too low an impulse. from the instructions an F50 is the lowest suggested impulse motor. your rocket simply wasn't going fast enough for the fins to work. assuming that the cg was at or forward of the specified location you're good to go.
Rex
 
Thanks Guys. I was hoping I was under powered. I could have swore when i ordered it from Apogee and I clicked on "first flight" engines The F20 was what it said, but I can make mistakes. The only other engine I had with me was a G80 and there was no way I was using that one until I convinced myself otherwise. I gave this rocket the swing test and it was perfect. I appreciate the help. You guys are awesome!!
 
My two or three cents on this:

Knowing the CG and CP of your rocket and choosing a motor with enough thrust to fly safely is very important as you move up the size and impulse ladder.

Adding random nose weight without knowing these values is kind of a crap shoot.

An underpowered flight can result in alot worse than soft lawn dart; you were lucky.

The F20 is not listed in the instructions recommended motors. https://www.madcowrocketry.com/content/pdf/hojo3126.pdf

Thrustcurve.org has a 2.6 Hojo in it's database and the F20 falls in the "Motors that Fail" category. https://www.thrustcurve.org/motorguide.jsp?rocket=3809

Madcow includes a Rocksim file for every one of their kits on the website. Rocksim files can be opened with the free OpenRocket sim program; a good way to get familiar with the tool without have to build the file from scratch. https://www.madcowrocketry.com/2-6-thin-wall-fiberglass-honest-john/

For my builds, I almost always have to clean some paint buildup out of the inside of the launch lugs before the first flight. Cleaning the rod before, during, and after flight operations is a good idea as well.



Hope this helps.
 
The concerning thing to me is that the package tells you where the CG is (which cannot be known because you get a different CG with every motor), but doesn't tell you the CP (which is known because it is based on the rocket's shape). That makes no sense.
 
I believe they are telling you where the CG should be rtf....they aren't giving you the CP and stability margin...assuming they are giving a safe stability margin giving the CG is fine...and keep to a 5:1 thrust to weight at liftoff and/or sufficient rod/rail exit velocity. You just have to balast the model to that CG location with your heaviest motor...

The concerning thing to me is that the package tells you where the CG is (which cannot be known because you get a different CG with every motor), but doesn't tell you the CP (which is known because it is based on the rocket's shape). That makes no sense.
 
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The concerning thing to me is that the package tells you where the CG is (which cannot be known because you get a different CG with every motor), but doesn't tell you the CP (which is known because it is based on the rocket's shape). That makes no sense.

I wondered about that. Shouldn't the manufacturer be able to provide the CP then you build it and it's up to you to find the CG?
 
I believe they are telling you where the CG should be rtf....they aren't giving you the CP and stability margin...assuming they are giving a safe stability margin giving the CG is fine...and keep to a 5:1 thrust to weight at liftoff and/or sufficient rod/rail exit velocity. You just have to balast the model to that CG location with your heaviest motor...

So if they are telling me that the CG of 29.5" and assuming I build it correctly and I check the CG and it's where they have it listed, that means the CP/CG offset should be okay without me making my own paper cutout? Which when I applied a little more epoxy and shoved the G80 motor in it that's where the CG was. 29.5"
 
Okay, let's assume the CG/CP ratio is correct. As others have noted, the F20 in that rocket (according to sim data at ThrustCurve.org), gets you a launch velocity of 36 fps. That equates to 24 mph. If you say the wind speed was about 10 mph:

The tangent of the weathercocking angle = vertical velocity / wind speed. So tanb = 24 / 10, or 2.4. To find angle b, we need the inverse tangent of 2.4. The angle that results is 67.38 degrees.

You can see how severe the weathercocking was.

Do the same math with the G80 that you mentioned and you get an angle of about 76 degrees, which is much closer to the 90 deg. that you want.

Now, one of the smart guys will probably be along in a second or two to point out how badly I screwed up my math.
 
Great resources and equations. I will work on my math and thanks for explaining the weathercock and thrust to weight. Good stuff
 
I have the same FG HJ 2." rocket, which I used to certify level 1. Yet I have never had a flight like the one described by the OP. Those who have commented before me have touched on what I would have noted as the main issue - woefully under-sized motor for the HJ 2.6.

My RockSim model (revised to reflect as-built weights & dimensions) places the CG at 25.3" and the CP at 32.7" - base margin is 2.12 - build weight: 1010g. The CG could move even closer to the CP depending upon how much epoxy was used to mount & fillet the fins.

The decision to launch the HJ using a F20 doesn't come close to a 5:1 TWR and a 10 MPH wind did not help.

Thankfully no one was hurt...
 
The concerning thing to me is that the package tells you where the CG is (which cannot be known because you get a different CG with every motor), but doesn't tell you the CP (which is known because it is based on the rocket's shape). That makes no sense.

Makes no sense at all. This ass-backwards convention is found in many Madcow instructions.
 
Madcow lists the min CG on their instructions, one caliber forward of the CP. As long as your CG is at or forward of the CG they provide, you should be stable.
 
Now you have to update it with your parts measurements to be accurate.

1) Na, I don't, because I don't have the kit, but thanks.

2) The screenshot I posted is that of the exact same kit the OP is having an issue with, so I'm not sure what you are talking about when you mention I?the OP has to "update" something as he described his kit to be built according to the instructions.

3) The OP has a stability issue. The OP states he does not know what the CP is. Stability issues at the hobby level can generally be resolved with the knowledge and relationship of two key data points: CP and CG. CP information was stated in my post along with the manufacturers file from which that information came.

5) Based on the testimony of the OP, the OP had a pretty simple question for any L1 on up rocket guy to answer but I only saw a bunch of speculation so I jumped in and simply gave him exactly what he needed to know to fix his problem.

Get that sucker stable and let us know how it works out, OP!

Regards
 
Makes no sense at all. This ass-backwards convention is found in many Madcow instructions.

Stop killing trees! All the information for madcow kits are contained within .rkt files on there respective product pages!
 
1) Na, I don't, because I don't have the kit, but thanks.

2) The screenshot I posted is that of the exact same kit the OP is having an issue with, so I'm not sure what you are talking about when you mention I?the OP has to "update" something as he described his kit to be built according to the instructions.

3) The OP has a stability issue. The OP states he does not know what the CP is. Stability issues at the hobby level can generally be resolved with the knowledge and relationship of two key data points: CP and CG. CP information was stated in my post along with the manufacturers file from which that information came.

5) Based on the testimony of the OP, the OP had a pretty simple question for any L1 on up rocket guy to answer but I only saw a bunch of speculation so I jumped in and simply gave him exactly what he needed to know to fix his problem.

Get that sucker stable and let us know how it works out, OP!

Regards

What I mean is, iirc that the Madcow .rkt are based on their weights, none of us build exactly the same way as they did. Every .rkt for any rocket you build should be corrected or entered as YOU built it. Close isn't always enough.
 
Hello all - sorry, just saw this thread...

Yes an F20 is a pretty small motor for this rocket. I wouldn't fly on anything less than what is suggested in the instructions as this is a heavy 2.6" rocket.

On the CG/CP here is the scoop... On kits that had instruction sheets written many years ago, we listed the suggested CG figuring we would do the CP/CG relationship calculation for you. Turns out most people didn't like that and what to know where the CP is. So, on newer instruction sheets we started listing the CP and sometimes both the CP and suggested CG.

BTW, when we list a suggested CG it is either a) exactly 1 caliber in front of the calculated CP or b) a CG derived from experience on designs that are difficult to calculate the real CP (Seawolf for example). On the 'b' option, the instructions are usually a little ahead of what we experienced as stable :)

I imagine, we'll eventually get all the instruction sheets updated to include both.

Hope that helps explain!

Mike
 
MadCow .rkt files are NOTORIOUS for wrong weights and wrong locations. EVERY ONE needs updated to as built for an accurate sim.
 
madcow clearly expects people to READ and FOLLOW the instructions. listed as 'step - nose cone assembly', are instructions on how to balance the model (note; a G64 is heavier than a G80). also the OP has stated that his model has passed the swing test w/ a G80 installed, knowing that and the motor used for the attempted flight. the cause becomes clear, insufficient airspeed leaving the rod(rod whip may also be a factor).
Rex
 
madcow clearly expects people to READ and FOLLOW the instructions. listed as 'step - nose cone assembly', are instructions on how to balance the model (note; a G64 is heavier than a G80). also the OP has stated that his model has passed the swing test w/ a G80 installed, knowing that and the motor used for the attempted flight. the cause becomes clear, insufficient airspeed leaving the rod(rod whip may also be a factor).
Rex

+1, I concur, neither of us made any other conclusion on the cause of the issue as anything other than underpowered.
 
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I have the same kit, though, I built mine with a 38mm mmt. I added no noseweight, but I did add a 38mm phenolic tube into the nc coupler that extends 5-1/2" into the nosecone for an av bay (magnetic apogee detector). Most of that extra weight is centered in the nc coupler, as the MAD needs to be away from ferrous metal parts that mess with the detector.

Anyway, my rocket is 3.51 overstable before adding the motor, according to Rocksim. This is a very stable rocket is what I'm getting at, even without the av bay.

As others have said, the motor you used definitely did not have enough thrust for a stable launch. As for not adding the wood pieces because they look cheesy, I completely disagree. But that's your choice.
 
[video=youtube_share;0ijHmDbmHKA]https://youtu.be/0ijHmDbmHKA[/video]

HJ needs a rail and 20' more shock cord. Other than that,
Flies great thanks everyone!!!
 
HJ needs a rail and 20' more shock cord. Other than that,
Flies great thanks everyone!!!
It appears that you have the same HJ 2.6 kit that I have. While I would agree with your "needs a rail" comment (mine always had RB on standoffs), my SC is only 15' long and functions quite well. I should note that I replaced the kit nylon cord with a length of 1,000 LBF braided Kevlar cord - lighter, stronger, more flame resistant, and packs considerably better in the chute protector. YMMV... :wink:
 
Mine was built as stock, used nomex blanket etc, had a nominal at apogee ejection, strap broke on the first flight and had tumble recovery. I personally would not use any nylon strap on a rocket I cared about, ymmv and I'm sure a lot of people do use them, I just won't after that.

Frank
 
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