DARK STAR arching over..need input

Hello all-

I have a question...please see attched photos...I am looking for the person(s) who have seen this before or knows what/why and how to correct. I have a 4" Dark Star dual deploy. built stock with only the addition of an Aero-Pac tail cone. I have launched this three times on three different motors, a K695 at LDRS31, K805 and a K540. EACH time I launch this rocket (off a 8' 1515 rail on a good pad) it goes up about 50 feet and then archs over as much a 25 degrees and proceedes on a flight path that puts it almost a mile away. The rocket simulation listed it as over stable. on-board video shows that this rocket does not even spin in flight...it boosts very well except that it archs over badly. This is NOT weather cocking, as this has happened in little or no wind and sometimes in the wrong direction for weather cocking

Somebody told be I could try and making it shorter (and thus decreaseing the damping frequency for correction) and maybe that would correct things. BTW: I could cut 6" off the booster and it is still listed as overstable on ROC-SIM.

I really like this rocket and want to keep it. but I do not know why it goes straight for about 50 feet then archs over. If I cannot solve this I think I will be parting this bird out.

Any help or input would be great.

Thanks
Andrew K
Mennonite Aerospace Group-Geauga County

AKVP said:

This is NOT weather cocking, as this has happened in little or no wind...

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It's not that unusual to have a wind shear at some distance from the ground, so this could be wind-related despite what it's like at ground level.

What is your rocket's unloaded weight and where is the CG? (Real measurements, not Rocksim.) Where do you have the rail buttons?

This could be a fin alignment (unlikely), MMT alignment, mass imbalance, or rail button misalignment/misplacement problem, assuming the rail is in fact straight.

What is your CP/CG margin when fully loaded?

Thanks for the input!!!

ROC-SIM puts the Margin at 3.53 - over stable, when loaded with the biggest motor (K700). The top rail button is 12" in front of the CP and rite hear the CG. It is a bit over 17 pounds loaded with K540 (it went 4967 feet even though it arched over!). As far as fin aligment and button alegnment, I know they are spot on. I also pack eveyhting tight so nothing "shifts" at lift off. I hope this helps. THANK YOU!!!!

Im assuming 3.53 is inches CG fwd of CP. Something doesnt look right. 1 caliper is the dia. of body= 4".
Like mikec said use real CG, 17lbs is lite enough to do a CG string test.

Ok, went into rocsim, seems like 3.53 is 3.53 calipers. too much over stable.

3.5 calibers although not extreme, is significant, and could result in windcocking on moderately breezy days.

Try to see if the arching is related to wind direction, or a side of the rocket or pad, or random.

I had a similar flight with my Darkstar Extreme on a K1000. I have flown it mostly on 75mm L's and M's with no issues. I would try and experiment with a bit of nose weight or shorten it up a bit. I would trim the booster down, leaving enough room for the longest motor you intend to fly in it. If you part it out, call me first. I run a Darkstar rescue and would love to have it.

Good luck and post the results.

Wow-THANK YOU for all the replies! As far as the rail, wind etc. I have launched it on three different rails/pads, and on three different and powerful enough motors. I havelaunched it in calm and windy condidtions. with all the same result. The launch at LDRS 31 this year was I think the most pronounced arching and those pads were GREAT! That was on a K695 to over 5000 feet. Just think how high it would have gone if it went straight up! It is so wierd how it goes straight for awhile then arches over. This is not an issue in huge launch areas but in Ohio, we are limited on space...beside I like a good flight profile . I could try a smaller motor....but I dont want to!!! and the arching could be worse. This is more of an issue as I have to save up for these K motors and I kind of hope for a good straight flight after all the time and prep....THANK YOU AGAIN for all the input!

GRIFFIN said:

I would try and experiment with a bit of nose weight or shorten it up a bit.

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Adding nose weight would likely make it worse, not better. Shortening the rocket is in the right direction.

Most of the fiberglass designs are longer than they need to be, but in my experience (G3, Wildman Jr, Extreme Wildman, Darkstar 3) this only makes them a bit pitchy on boost, not systematically arching unless you have a wind shear. A higher-thrust motor may help, though the ones you've flown so far have been 10:1 or better.

mikec said:

Adding nose weight would likely make it worse, not better. Shortening the rocket is in the right direction.

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This.




Justin

I think it stems from being overally stable.

I have seen landshark from the rocket flying straight off the rod, then going unstable sideways and as then getting stable almost immediately as the propellant burns or burns out- for a nice straight flight away from you . . . The arch or rainbow for one flight might be speed off the rodn engine not igniting properly or choking a bit as it comes off thr rod. But multiple flights with different engines and thrust profiles with the same result seem the common denominator is the rocket itself.

With a built rocket it is a tough to remove nose weight or make the fins smaller. But if dd you can quicklyy fly motor deploy without the av bay section on a midrange engine like a j350w for a quick and fun confirmation the shorter less stable rocket flies.

Kenny

AKVP said:

As far as fin aligment and button alegnment, I know they are spot on. I also pack eveyhting tight so nothing "shifts" at lift off. I hope this helps. THANK YOU!!!!

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If you have the motors and field, you could launch a few times in a day to isolate whether the wind direction or a physical property of the rocket is causing the arch. It seems like shortening it to be less overstable would likely help, regardless of the cause, but it would be interesting to determine the cause before permanently modifying your rocket.

If there is even a small fin alignment variance, I would think the split fin design of the DS would exaggerate its effect. If you paint one set of fins a different color from the others, you could potentially see from photos whether it always arced toward the same side of the rocket. And if you launch it a few times on the same day, that should roughly control for wind direction, and if you launched from different sides of the rail each time you could tell whether it arced toward the wind or toward a particular set of fins.

To me it sounds like a great excuse for a full day of launching and calling it "rocket science."

Let us know what you conclude, or how you solve the issue.

Thank you!

Thanks to everybody who replied to my question. It was truly, fun to read all your input and ideas. THANK YOU. Well, the Dark Star (I call it Vitamin AP) as of an hour ago is 7" shorter. I do not mean any offense to Tim, but I think it looks a better a little shorter! I will take the opportunity to try out a new color scheme on the whole darn rocket! I will post the "New Star/ Vitamin AP" when it is done. I hope to take it out to a launch in Southern Ohio in the next few weeks and give it a go on a K550 if I can.

Thanks again for all your input and help folks!!!
Peace to you all
Andrew K
TRA#12272 L2

It doesn't make a lot of sense so I have to ask these dumb questions.

What is the weight of the rocket without the motor and have you measured CG of your rocket as built, or are you going by what rocsim says the CG should be?

Are the fore and aft fins pair verified to be parallel to each other with a straight edge, and paralled to the central axis of the rocket so a fin misalignment is not are turning the rocket.

Is the motor mount aligned with the centerline of the rocket so the thrust vector isn't kicking the rocket over?

Puzzling problem.

Bob

CCotner's Talon was having odd issues like this after a repair: boosting straight and then suddenly veering off at an odd angle and going straight that way.

But lately, on low-thrust motors it just fishtails, and its latest flight, an intermediate thrust motor, was a beautiful straight flight.

To this day we still do not know the reason why.

Something else you could try is to epoxy a small tab on each fin to induce spin. You can remove them later. If it still arcs over while spinning, it is definitely a CP/CG issue and weathercocking. The spin will counter any misalignment of the MMT, balance, etc.

Handeman said:

Something else you could try is to epoxy a small tab on each fin to induce spin. You can remove them later. If it still arcs over while spinning, it is definitely a CP/CG issue and weathercocking. The spin will counter any misalignment of the MMT, balance, etc.

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I'd look forward to telling the RSO about that one... =p

One Thing I see is your pad rail is not stright up. Is that the way it arched. When I have a rocket that weather cocks I point the rail with the wind a little.
Looked like a nice flight.
gp

The rail in the video photo is not the same as from the one in the photo at the beginning of the thread. I usually make sure the rail is straight up before I leave the the pad. But I see what you mean!

Thank you all for your time and input! THANK YOU so kindly!

Andrew

Judging by the picture; it is a wind sheer.
Wind direction can change at different altitudes.
I flew My extended Dark Star Extreme on an L and didn't have any issues with it.


It about 4" longer than the stock version.

JD

CCotner said:

Handeman said:

Something else you could try is to epoxy a small tab on each fin to induce spin. You can remove them later. If it still arcs over while spinning, it is definitely a CP/CG issue and weathercocking. The spin will counter any misalignment of the MMT, balance, etc.

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I'd look forward to telling the RSO about that one... =p

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You sound like a spinning rocket would have difficultly getting past the safety check. It certainly shouldn't. If it does, I would think the problem is with the RSO, not the rocket.

I would have to agree with Bob on this, I have seen this problem before and it was always do to fin alignment, I've also seen a few tubes at the slots work quite correct.
Shortening or lengthening is not the answer.