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Are transistions needed?

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RazorConcepts

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There are rockets with a large diameter tube for the payload bay, and a smaller diameter tube for the booster. To join these tubes you use a transition. What if you don't use a transition and just use centering rings?

See image below (I came up with the idea after making the rocksim w/ transition... to fix it so the booster tube is inside the payload I will have to remake the entire rocket. Boo rocksim :bangpan: )

rocket.JPG
 
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RangerStl

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I doubt it would make a huge difference aerodynamically. I'll bet frontal area is more important than the transition in the drag calculation.

There really is no need for the transition. I would account for the mass if you plan to put it in there anyway. That way you get an accurate approximation of the liftoff weight and, therefore, minimum stable velocity and where in the flight that occurs. (hopefully before you get to the end of the launch rod! ;) )

N
 

Chrisn

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Is that rocket designed for estes motors?
 

cjl

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It'll make a huge difference in drag, but as far as stability is concerned, you should be fine. I'd be slightly concerned with that design because it looks like the fins will mostly be within the turbulence coming off the larger tube, but it would probably work.
 

Micromeister

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Completely agree with cjl:
You'll be paying an enormous penelty in Drag. That said there have been many models flown with similar planforms if that's what you really want to do. Personally as mentioned I'd increase the fin size a bit, or add a cardstock or CA soaked paper transition to smooth the transition.
 

shreadvector

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It'll make a huge difference in drag, but as far as stability is concerned, you should be fine. I'd be slightly concerned with that design because it looks like the fins will mostly be within the turbulence coming off the larger tube, but it would probably work.
YES! That saved me some typing.

To be clear, if the fins are hidden in the turbulence of a bulbous upper section, they are not acting like "fins" until the rocket turns and they are exposed to the airstream whooshing past the rocket. As a result, the rocket may wiggle or "cone" on the way up and lose a massive amount of altitude because of the increase in drag caused by the wiggling and/or coning.

Fins need to extend out into the airstream. A prime example of this is the Estes Viking kit where you can use 3, 4 or 5 fins and glue them on in a variety of orientations. When only 3 are used and they are glued with the long end along the body tube, very little fin sticks out and most of it is in the turbulent boundary layer. Therefore they are OFTEN unstable with C motors installed. And the worst case is when they are unstable and corkscrew around and then when some of the propellant is burned up, the balance point moves forward and they become stable - with the rocket pointing in some random direction!

On your design, a good transition will help reduce the turbulent boundary layer and allow the fins to actually act like fins. otherwise you should seriously think about larger fins that extend out farther.
 

jorpet

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...

Fins need to extend out into the airstream. A prime example of this is the Estes Viking kit where you can use 3, 4 or 5 fins and glue them on in a variety of orientations. When only 3 are used and they are glued with the long end along the body tube, very little fin sticks out and most of it is in the turbulent boundary layer. Therefore they are OFTEN unstable with C motors installed. And the worst case is when they are unstable and corkscrew around and then when some of the propellant is burned up, the balance point moves forward and they become stable - with the rocket pointing in some random direction!
...
Wow, good thing I read these things first...

I have a Viking built with the 5 fins sticking out the maximum distance (like the face card) and another one waiting to be built. The first one is overly stable and weathercocks like mad. I was planning on building the next one with only three fins and the minimum fin width. Now I'm thinking I will try something different.
 

shreadvector

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Wow, good thing I read these things first...

I have a Viking built with the 5 fins sticking out the maximum distance (like the face card) and another one waiting to be built. The first one is overly stable and weathercocks like mad. I was planning on building the next one with only three fins and the minimum fin width. Now I'm thinking I will try something different.

It (or any rocket) will only weathercock if the speed when it leaves the launch rod is not MUCH greater than the cross wind speed.

If your rod is a good length the rocket will be able to build up more speed and not be affected by the wind as much.

If the wind is high (10 mph to 20 mph) you are going to see wind effects. A slow moving rocket that is very stable will wethercock. A neutrally stable of any speed will be pushed sideways by the wind.

A Viking weighs so little that is should reach a very high speed by the time it leaves the launch rod. If it is not flying straight, check the cardboard fins for bends or misalignment.

For a 1/8" rod I prefer 4 feet long.
 

jorpet

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It always takes off straight and then bends into the wind. I would guess that is has flown in 10 mph winds. It doesn't just snap into the wind, but tends to make a long graceful arc until it is heading up wind. I doubt it never gets off vertical by more than 15 degrees at the end.

This does have the advantage that it almost always lands within 30' of the launch pad even in high winds and using a C6-5 motor.

Wouldn't fins that where bent or misaligned cause it to turn whichever way rather than always turing into the wind? Or will the rocket rotate so that the misaligned fins turn it into the wind? This was one of my very first rockets, so fin alignment could be a bit iffy and it has flown a lot and the fins do need to be straightened on occasion, so anything is possible.
 

spacecadet

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I doubt the drag penalty will be significant if your thrust/weight ratio is nice and high, as it should be. But I would size the fins as if the whole fuselage was of the larger diameter, as suggested.
If 15deg. of weathercocking was a deal-breaker, I'd never get anything flown:rolleyes:
 

BsSmith

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Transitions are needed about as much as boat tails, they look nice (IMO), but they are not needed. Both will increase the stability a bit.

You may want to put bigger fins on it anyway, your CG and CP are extremely close. Anything under 2 calipers of stability will result in an off-vertical flight.
 

shreadvector

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BsSmith

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Because this is the "Beginners and Education" forum, I provide the following two links:
Yes, by Calipers is also known as the Stability Margin in Rocksim. A caliper is one body ube diameter. You want the CP and the CP 2 body diameters apart (with 2 different body diameters, it's the average, right?).

Wiki probably explains all of this and more, but my internet is slow today and I can't open wiki.
 

gpoehlein

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Yes, by Calipers is also known as the Stability Margin in Rocksim. A caliper is one body ube diameter. You want the CP and the CP 2 body diameters apart (with 2 different body diameters, it's the average, right?).

Wiki probably explains all of this and more, but my internet is slow today and I can't open wiki.
Nope - what Fred was pointing to wiki for was the difference between caliper and caliber. What you are describing is caliber - a caliper is an instrument used to measure the thickness of a material or the diameter of a tube.
 

BsSmith

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Nope - what Fred was pointing to wiki for was the difference between caliper and caliber. What you are describing is caliber - a caliper is an instrument used to measure the thickness of a material or the diameter of a tube.
Whoops... :blush:

I guess I read too fast sometimes to see small differences in spelling. :bangpan:
 

powderburner

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It'll make a huge difference in drag, but as far as stability is concerned, you should be fine. I'd be slightly concerned with that design because it looks like the fins will mostly be within the turbulence coming off the larger tube, but it would probably work.
Yes, it will increase drag.

Stability (for this design) might not be affected. If the large bluff base area had been added at the nose the increased drag would be de-stabilizing. If the large bluff base area had been added at the tail the increased drag would tend to increase stability. Since the large base area is very near the c.g. and c.p. (effectively, the large base area is ON the c.p. when you consider the overall size and length of the design) it should have negligible effect on pitch/yaw stability.

And yes, the shed vortices from the large bluff base area will indeed reduce the fin effectiveness.

Use a transition fairing. They look a lot better too.
 

JRThro

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I have a scratch-built rocket like that, using a BT-80BB (Fat Boy) nose cone, a 9" length of BT-80, and then a longer length of BT-70. It has 6 trapezoidal fins big enough that they extend out further than the diameter of the BT-80, and flies on 3 18 mm motors.

It does quite well and gets up nice and high, and leaves the launch rod at a good clip.

I'm sure there's a big drag penalty, but since it flies nicely anyway, that doesn't matter much to me.
 

dave carver

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The technical name for what you are doing is: frustrum

After a short time under boost it's like the air packs into the space and the air pressure itself forms a kind of transition. It's probably more like a rolling doughnut of air has slipped over your nosecone and gets stopped by the flat surface it encounters.




mmmmmmmmmmm doughnuts
 

RazorConcepts

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Thanks for the replies! I did not want to use a transition is because a bt60 to bt80 balsa transition is around 10 bucks :bangpan:

Looks like I will be increasing the fin size. This rocket will be a payload rocket so I was hoping the extra weight in the nose would help stabilize it beyond the rocksim file.

Mmmm strawberry filled frustrum
 

Chrisn

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You can build yourself a paper transition for almost free!

Search youtube for apogeecomponents. There is a couple of how to videos for some good techniques.
 

Micromeister

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Yeap!
A paper or cardstock transitions are very easy to make, and you control the length to suit your design. Vary Long or Short to fit your need keeping in mind the shorter then transition the more turbulance/drag is created. There are a number of free computer programs that will generate the pattern for you to printout on cardstock or paper. Template Widgets comes to mind, I'm sure someone can give you the exact link.

After assembly your transition can be hardened if you deem it necessary by wiping it down with Thin CA then sanding smooth. it's very possible to make the transition seam line go away completely. the very long transition on the SS-N2 Styx is plain file folder BT-80 to BT-50. Fins are butt on glued directly to the CA soaked transition. This model has about 20 flights on it. So you see Paper or Cardstock transitions are CHEAP, sound and very effective.
Hope this helps.

147b_SS-N-2  Styx_static&flight Pg_01-12-94.jpg
 

adrian

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You can build yourself a paper transition for almost free!

Search youtube for apogeecomponents. There is a couple of how to videos for some good techniques.
I was going to recommend VCP for this but I can't find it any more - the last known site, http://myweb.cableone.net/cjcrowell/vcpmain.htm, now returns "The requested URL /~cjcrowell/vcpmain.htm was not found on this server." Does anyone know where version 1.65 is now?

Version 1.64 appears still to be available here:
http://v-serv.com/vcp/

Though less advanced, 1.64 should still be able to do what is required here, which is take in the forward diameter, aft diameter and length of a transition, then print out a paper shroud.

Of my own rockets, the one which gave me my most experience of paper cones was Thunderbird 3. At that time I had no computer tools but it's not that difficult to calculate the inner and outer radius of the ring which, when cut and wrapped round, will form the transition.

But if fins are to be mounted, I prefer to cut slits in the transition, mount the fin all the way through to the underlying body tube, then seal the transition with fillets in the usual manner. That is how I put the fins on my A4, for example.

tb3.jpg


a4.jpg
 

shreadvector

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Yup. VCP is what I have used. It is on the NAR/TARC Educational CD-ROM (which is also on the web) and directs you to the server where it lives now (same link you have)
http://www.2020vertical.com/nar_edu_cd_dev/index.html

That web page has links to the software and to the sites that host the software for the latest updates.

[FONT=verdana, helvetica, sans-serif]*Note: Clicking on the program names will install the software to your computer.[/FONT]
[FONT=verdana, helvetica, sans-serif]For updates, latest versions, info, etc... use the 2nd link in each program description. [/FONT]


It looks like the software (".exe") links may not work on the website, but my CD-ROM has the 1.64 version on it.

We hand out lots of copies of the CD-ROM and DVD at our club launches. The DVD is on Youtube:

http://www.youtube.com/user/AerospaceIndustries


I was going to recommend VCP for this but I can't find it any more - the last known site, http://myweb.cableone.net/cjcrowell/vcpmain.htm, now returns "The requested URL /~cjcrowell/vcpmain.htm was not found on this server." Does anyone know where version 1.65 is now?

Version 1.64 appears still to be available here:
http://v-serv.com/vcp/

Though less advanced, 1.64 should still be able to do what is required here, which is take in the forward diameter, aft diameter and length of a transition, then print out a paper shroud.

Of my own rockets, the one which gave me my most experience of paper cones was Thunderbird 3. At that time I had no computer tools but it's not that difficult to calculate the inner and outer radius of the ring which, when cut and wrapped round, will form the transition.

But if fins are to be mounted, I prefer to cut slits in the transition, mount the fin all the way through to the underlying body tube, then seal the transition with fillets in the usual manner. That is how I put the fins on my A4, for example.
 
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Chrisn

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well if your fussy about what software you use to print the transition, dont use any software and just get out your compass, square and calculater :)
 

Peartree

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I always use the tool on EMRR (okay I downloaded it but that's where I got it).
 
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