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Next build: compromises and considerations.

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Senior Space Cadet

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Several days ago, I started work on one of my torpedo shaped rockets, but my next completed build will probably be something more conservative.
I've been concentrating on rockets powered by 18mm motors. Cheap to build, cheap to launch, and plenty powerful for the launch site I'm using.
I recently bought a Jolly Logic 2 altimeter. I tried to install it in a BT-50 rocket, that had a baffle, and there wasn't room for both the altimeter and the chute.
At this point, I'm unsure how long a body I need to get the room I need, in a rocket without a baffle. Could be fairly short, but I'm leaning toward longer bodies for the dynamic stability you get.
As you can see, the stability is pretty high, on this rocket. I could, theoretically go with smaller fins, but they are already pretty small. I'm afraid to go smaller.
Elliptical fins might be a good choice here, but clipped deltas are pretty efficient and easy to make.
I already have a similar rocket with swept clipped delta fins. After a few launches, I'll decide if I can go shorter.
Screenshot 2020-09-13 05.32.13.png
Screenshot 2020-09-13 05.33.34.png
 

jqavins

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The fins look well within the range of normal, and I wouldn't say the static margin is too high, even though higher than needed. So there's no need to shrink the fins if you think they look "right" as is. I like it fine.

I also like hoe the torpedo has shaped up. The fins are big, but that's the price of a big boat tail like that.

Have you considered going with 4 or 6 fins? They could then be a little smaller, if that would make you happier with it. And, I'm just thinking of this now, what about using the fins as pylons to also have a ring tail? The ring would do a lot for the CP, so the fins/pylons would be a lot smaller, again if that makes you happy.

None of that takes away from what I said first: I like it.
 

blackjack2564

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Kinda reminds me of one of my old designs......

abit more stylish and streamlined...flies great
Screen Shot 2020-09-14 at 10.54.32 PM.png
 

Senior Space Cadet

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The fins look well within the range of normal, and I wouldn't say the static margin is too high, even though higher than needed. So there's no need to shrink the fins if you think they look "right" as is. I like it fine.

I also like hoe the torpedo has shaped up. The fins are big, but that's the price of a big boat tail like that.

Have you considered going with 4 or 6 fins? They could then be a little smaller, if that would make you happier with it. And, I'm just thinking of this now, what about using the fins as pylons to also have a ring tail? The ring would do a lot for the CP, so the fins/pylons would be a lot smaller, again if that makes you happy.

None of that takes away from what I said first: I like it.
I think four fins might be a good move. I think it might look better with elliptical fins too.
 

Senior Space Cadet

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Kinda reminds me of one of my old designs......

abit more stylish and streamlined...flies greatView attachment 431734
I was going for lower subsonic drag, with the ellipsoid nose cone. I did a comparison between a long body with the fins on the body, like you did, and a short body with the fins on an extended body tube at the back and decided I'd get a higher apogee with the short body version. I'll either do some modifications to the fins or go elliptical. My finished rocket seldom looks exactly like my Open Rocket design. Ellipticals fins look good with elliptical nose cones. I didn't take my idea from anyone else, but after working on this design, I found photos of a superlight Russian model rocket and one made by the guy at Apogee, that look very similar. I see that you went with four fins. Yeah, I think that's the way I should go. Back to Open Rocket.
 

jqavins

Joseph Avins
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I'll either do some modifications to the fins or go elliptical.
And remember, as per a previous conversation, you can design it in OR with elliptical fins and then build it swept elliptical fins of the same surface area, if you want.
I didn't take my idea from anyone else, but after working on this design, I found photos of a superlight Russian model rocket and one made by the guy at Apogee, that look very similar.
Yup, nothing new under the sun. Or very little at any rate.
 

Senior Space Cadet

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I got as far as putting sandable primer on the small rocket yesterday, then realized I hadn't put the launch lugs on.
I just sanded a spot and glued them on. Hopefully they will be secure, when I'm done.
 

Senior Space Cadet

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Will you use a centering ring in the boat tail?
Probably two centering rings. I ordered a couple different sizes. I haven't done a trial fit yet to see if I have what I need. I was going to run the 24mm tube all the way to the front of the transition (a nose cone), but then I started thinking about room for the parachute, shock chord and altimeter. I'll either need to make the body section longer or only bring the tube up half way.
 

jqavins

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I got as far as putting sandable primer on the small rocket yesterday, then realized I hadn't put the launch lugs on.
I just sanded a spot and glued them on. Hopefully they will be secure, when I'm done.
Yeah, welcome to the club.
 

XrayLizard

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Not a good aspect ratio on those magical elliptical fins my fine feathered freind.
Do you know that elliptical fins are only theoretically better? That clipped delta actually work better in real life so to say
Can’t say I see the need for an altimeter..
Or even OR software for these Low power birds. I like OR for seeing how high it might go if I stuff a big motor in a little tube haha. Like 6k feet+ from a 19” bt50 rocket.
 

jqavins

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Xray: OR is very useful for people who are 1) fairly new without well developed mindsim skill, B) interested in getting every foot or foot per second even out of A-C engines, and/or III) designing unusual rockets that are hard even for experienced folks to mindsim. Designing in OR or RS is a great learning tool.

Senior: I agree with Xray that the aspect ratio on those fins is unusual. There's nothing inherently wrong with semicircular fins if that's the look you want. Semispan equals root chord or thereabouts is more common. It's quite possible you could shorten the root considerably without a critical effect on the CP, and correspondingly shorten the "tailpipe". But there's no reason you have to.
 

OverTheTop

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Do you know that elliptical fins are only theoretically better?
Correct. I think I remember reading about the design of the Spitfire aircraft back in WW2. They used the elliptical design as it was efficient but more importantly it was easy to model mathematically for analysis at the time. Modern computer simulations can cope with much more complex structures and still produce valid simulation outcomes. It is quite likely the computer simulations have found better solutions again.

A similar thing happens with the Haack or von Karmen NC profiles. Computer simulations have found shapes that produce remarkably similar performance with different profiles.
 

XrayLizard

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I admit I can only read the summaries of these types of technical articles etc,

I also do enjoy an occasional elliptical fin:)

I do think I have over the years read enough of them to decide most any planform is ok

one thing though when it comes to low drag etc. it is the sum of the parts.
For example the best nose cone shape is defeated ? if the fit is so bad that the edge of the body tube protrudes and presents an edge to the air. turbulence thing lol
I’ll also admit I just put together cookie cutter designs haha
Maybe one day I’ll become an actual space modeler !
 

Funkworks

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I think everyone does that at least once in their rocketry journey.
I’m on my 7th rocket this year and none of them have a launch lug yet. Most of them are waiting for rail guides, and especially for a rail to ensure alignment. I might sand before gluing but for some reason I’m not worried about any detaching.
 
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