3" Darkstar Stability

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markg

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I built a 3" Darkstar for my L2. I've flown it 3 times and all four times it's shown the same behaviour. It has a very pronounced 'wiggle' for the first 1000ft or so of flight and then it straightens out and flies perfectly. I've flown it on an Aerotech I500T, Aerotech J450DM, and a Loki K527. It's 10.1 pounds without a motor. The slowest it sims off the rail is on the J450DM at 45.5 ft/s, the I500 and K527 both sim at 58 ft/s. The wind varied on the flights, but it was not what I would consider excessive on any of them.

Here is a screenshot of OR. Stability is 6.15 cal with the I500 in it. Stability is 4.41 cal with the K527.

DarkStar OR.JPG

When I launched it at NYPower in May, I talked with another guy who had built a 3" Darkstar. He told me that he'd had the same wiggle on his and cured it by chopping about a foot off the overall length of the rocket and putting a lighter nosecone on it.

I ran simulations in OR to compare stability on the rocket flying on the J450 at it's normal 88" length and again with 12" removed. It shows the rocket is less stable with the 12" removed. Here are the two graphs :

Full
darkstar-full-stability-j450DM.jpg

Cut
darkstar-cut-stability-j450DM.jpg

I'll attach the OR file as well.

Can someone help me understand why this is happening and if chopping 12" off the rocket will help or hinder this problem?

cheers - mark
 

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timbucktoo

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Curious - how long is the rail you are using? On a Loki K527, mine is about 71 fps off the rail. Many flights and always straight as an arrow.
 

les

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I'm at work so I can't open the file. But I am confused. Typically to increase the stability to add weight to the nose cone to move the CG up. Another approach is to make the rocket longer to help separate the CG from the CP.

So reducing the length or reducing the nosecone weight should make it less stable, as your simulation shows. Doing both together just compounds the problem....

Could there be coning occurring? (Does either OR or RS calculate coning?)
 

dr wogz

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We've witnessed it as well, but on the DS Jr. We assumed the wiggle came from the camera shroud attached to the side of the airframe..
 

Bat-mite

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A stability margin of 6.17 cal. seems way overstable. I've had wiggle problems with long, narrow rockets in the past. I'd call Tim and see what he says.
 

djs

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It could also be slightly out line rail buttons?

Or the darkstar may just be trying to shake itself free from the bonds of earth.
 

markg

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Curious - how long is the rail you are using? On a Loki K527, mine is about 71 fps off the rail. Many flights and always straight as an arrow.
I have it set to 60" in the sim, but usually fly it off a longer rail. I just leave it a 60" as I figure that's the smallest I would ever have to fly it off.
 

mpitfield

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When I launched it at NYPower in May, I talked with another guy who had built a 3" Darkstar. He told me that he'd had the same wiggle on his and cured it by chopping about a foot off the overall length of the rocket and putting a lighter nosecone on it.
Hi Mark,

That "guy" at NYPOWER was me, nice to officially meet you. Your recollection of the modifications I made is almost correct. My 3" Darkstar originally came with the heavier FW/FG nosecone with the aluminium tip. Instead of using that I used the polycarbonate nosecone with the phenolic tip, which I purchased from Wildman and I believe is made by CTI https://www.wildmanrocketry.com/ProductDetail.aspx?product=3134 That decision was made well before my initial flight and was made as a matter of preference to remove anything that could interfere with trackers, even if just marginally. It just happened to remove some nose weight which at that time I was not as concerned with the over-stability…I wish I was paying attention.

On the maiden flight (AT K700) I noted what I interpreted to be coning, or what was described as undampened roll-pitch-coupling in this thread https://www.rocketryforum.com/showthread.php?128878-Does-this-look-like-coning-if-so-how-to-mitigate&highlight=coning What I ended up doing was chopping roughly 5" from the airframe in total, which according to the sims lowered the over-stability by roughly 1+ caliber. I also kept the existing out-board mobious camera shroud, however I moved it aft by roughly 3", can’t recall the number, but closer to the CG.

I had a chance to fly the adjusted and re-painted rocket this past URRF4 on a Loki L1040 and although the boost from the ground looked much more stable than the initial flight, the on-board camera showed the same high roll rate and you can clearly see the coning by paying attention to the trailing exhaust trail. I am still editing the video and will post back here once complete.

I realize I should have re-flown the AT K700, but that Loki L1040 was just too tempting (awesome motor). FWIW the difference in CG from the K700 to L1040 is about an additional 1.5” aft. I believe that there has been an improvement from my initial flight, but that is just anecdotal at best. I will be revisiting the camera shroud this winter and have procured a new payload tube as well as the LiquidFyre rocketry internal mount for the camera, https://www.liquidfyre-rocketry.com/Cat-Mobius-ActionCam-Shrouds and so I will rebuild that section. I need to mockup the tube with the internal camera mount and recovery gear first to ensure I have the room, ideally I can even slice a bit more length.

At the end of the day no one can dispute that the design, as-is, is over stable, however the affect that over-stability has on flight seems to be up for discussion. I will be following your thread and look forward to reviewing your results, maybe at NYPOWER 2018.

I hope this helps.

Maiden flight.

[video=youtube;N4L0TyW7xV8]https://www.youtube.com/watch?v=N4L0TyW7xV8[/video]
 

timbucktoo

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Funny, mine is built stock with the polycarbonate NC and has totally different flight characteristics. Also have my camera shroud located about 6" above vent band.

[video=youtube;iZwMpT3Q9lk]https://www.youtube.com/watch?v=iZwMpT3Q9lk[/video]

[video=youtube;ptX-dqSw9-w]https://www.youtube.com/watch?v=ptX-dqSw9-w&t=48s[/video]
 

DavidMcCann

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Lol, as I read this, I knew who "that guy" was :)

Ive also seen it coning, and believe it due to being overstable. Saw it on a K805G at URRF1, and not as much on a K1100T, but the K1100T flight was straight as an arrow up to 7K, and dropped 200 yards away, so I'm calling that zero wind.
 

Tonimus

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If you've verified the CG, why not add some weight to the back end and see if it really is over-stable coning?
 

mikec

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Mined coned horribly on its first flight (CTI J760.) The second flight was on a CTI K740 and was arrow-straight to 9400 feet and landed about 50 feet from the pad. Out of 8 flights so far only one has coned significantly. What was different the first time? No clue.
 

michigander

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I can vouch for K300 is a horrible load for a 3 inch darkstar , flew like a airplane looked like 45 * angle once 3000 feet in the air but sure it wasn't that drastic

rocket went out to sea in Lake Ickey
 

DavidMcCann

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Overstable? Hit it harder ;)

The K1100T kicks significantly harder, and was a much cleaner flight.


 
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markg

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Hi Mark,

On the maiden flight (AT K700) I noted what I interpreted to be coning, or what was described as undampened roll-pitch-coupling in this thread https://www.rocketryforum.com/showthread.php?128878-Does-this-look-like-coning-if-so-how-to-mitigate&highlight=coning What I ended up doing was chopping roughly 5" from the airframe in total, which according to the sims lowered the over-stability by roughly 1+ caliber. I also kept the existing out-board mobious camera shroud, however I moved it aft by roughly 3", can’t recall the number, but closer to the CG.

At the end of the day no one can dispute that the design, as-is, is over stable, however the affect that over-stability has on flight seems to be up for discussion. I will be following your thread and look forward to reviewing your results, maybe at NYPOWER 2018.
Thanks for your reply Michael. I haven't flown the Darkstar since NYPOWER but am planning on flying it at MWP, so I figured I should try and sort this little issue out before that launch. I don't have video to confirm it, but my photos and memory of my launches seem to match the behaviour you describe in your thread.

I'm going to adding some weight to the rear of the rocket to reduce it's stability and see how it flies. If that works out then I'll remove the weight, chop it down and lighten up the front end to move the CG back.

I'll definitely be back for NYPOWER 2018, hope to see you there. Hmm, I just checked the Loki 1040, sims to 15224ft, max 1449ft/s with my OR file, what did yours fly to?

cheers - mark
 

markg

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Overstable? Hit it harder ;)
The K1100T kicks significantly harder, and was a much cleaner flight.
I have a Loki K960 that'll do that, but I also like to fly it on the very economical and waiver friendly I500 or J450.

cheers - mark
 

mpitfield

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Overstable? Hit it harder ;)

The K1100T kicks significantly harder, and was a much cleaner flight.
lol I might just have to try that load and put your theory to the test :). Although the Loki L-1040 Red kicks pretty hard, but it kicks longer as well.

Thanks for your reply Michael. I haven't flown the Darkstar since NYPOWER but am planning on flying it at MWP, so I figured I should try and sort this little issue out before that launch. I don't have video to confirm it, but my photos and memory of my launches seem to match the behaviour you describe in your thread.

I'm going to adding some weight to the rear of the rocket to reduce it's stability and see how it flies. If that works out then I'll remove the weight, chop it down and lighten up the front end to move the CG back.

I'll definitely be back for NYPOWER 2018, hope to see you there. Hmm, I just checked the Loki 1040, sims to 15224ft, max 1449ft/s with my OR file, what did yours fly to?

cheers - mark
Hi Mark,

Sounds like you have a good game plan that is the least intrusive and should test out the over-stable theory, do you plan on using the same load?

Using my initial K700 flight as a baseline and adjusting the sim based on actual results, my sim for the L1040 was just under 14K, 13,808' and my actual was 13,868'. Velocity was predicted @ Mach 1.3 and realized was Mach 1.4.

 

DavidMcCann

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1630N vs 1250N. The kicker is the weight, 1500g vs 3000g

dammit, now I want a 1706 case again.
 

markg

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Sounds like you have a good game plan that is the least intrusive and should test out the over-stable theory, do you plan on using the same load?
I'm going to try and fly it back to back on the same load, with and without the additional weight, either I500 or J450, as those were the motors I flew at NYPOWER and saw the wiggle both times.

Using my initial K700 flight as a baseline and adjusting the sim based on actual results, my sim for the L1040 was just under 14K, 13,808' and my actual was 13,868'. Velocity was predicted @ Mach 1.3 and realized was Mach 1.4.
Nice, I gotta fly one of those once I get this wiggle straightened out!

cheers - mark
 

Nytrunner

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So is the darkstar deliberately overstable to allow big honkin motors without much modification?
 

mpitfield

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So is the darkstar deliberately overstable to allow big honkin motors without much modification?
Who knows. As I pick up more experience in the hobby I realize that even kits need a bit of personal judgement when it comes to sticking to the design. I now look at it as the manufacture provides a proven framework and as the flyer you make choices based on your personal goals, and go from there. When I initially built this I didn't give it much thought as it's such a popular rocket I just went with stock. In hindsight, or if I were to build it again, I would chop as much as I could off the air frame, not just for stability but overall performance.
 

markkoelsch

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The real answer to this is that you're not flying the darkstar "correctly" if it doesn't have a big honkin motor in it.
Glen, why does it have a motor mount at all? 😀🚀
 

djs

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Glen, why does it have a motor mount at all? 😀🚀
For wusses!

My 2.6" Darkstar has a 54mm motor mount in it. It's got leetle itty bitty centering rings...almost MD.
 

DavidMcCann

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It's likely worth noting that even crazy of all crazies, Crazy Jim chopped his booster in the most famous of all build threads.
 

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