polygons vs round tubes

[spacecowboy]

So far, both my triangular and square "scratch" or "odd rox" haven't worked so great. Beaut lifts, but 100 to 150 ft off the ground, a giant screw and they hit dirt before reco. . . .

There ain't no way to SIM these.

My guesses :
1) bunches more of nose cone weight.
2) slight imperfections in the body tube create a disturbance that just magnifies itself in air under thrust.
3) both of the above.

I have built and flown dozens of scratchers out of tubes, and beaut flights all (except for one that burned up)

Maybe this is why I went into computers vs aero engineering, huh

 

[Fore Check]

I'm gonna guess differently:

When looking down at the axis of the airframe, it is far more difficult to acheivet symmetrical airflow around the frame *unless* the airframe has an even number of sides (4, 6, 10, whatever) or it is circular. Mind you, no matter what geometric shape it is, it must be uniform. The fewer number of "sides" (gemoetrically speaking, a triangle would have 3 and a circle would approach infinite) makes any discrepancies more magnified in the affect of airflow over the frame.

Then there is the nose cone to account for. Once again, a conical or ogive cone on a circular airframe should give the most uniform flow over the aggregate surface of the airframe. Reducing the number of "sides" increases the negative affects of discrepancies in the non-uniformities of the "sides."

Ok, I'm no longer making sense...... :kill:

 

[spacecowboy]

No, you make sense.
what makes no sense is trying to build what I am doing.

I have a basic rule. Keep trying and have fun.
The 3 sided bird flew relatively well. It had real fins.
The 4 sided bird was a box, with a bigger box for fins. It didn't fly too great.

Perfect symmetry on the NC and BT, and some NC weight, I might be getting a good flight.

Up next is a Premium Cracker box, with a butter box for the top. Another 4 sided job. I just thought, a gapper, 2 stage.
C6-0 to get it out of here, A8-3 for fun.

 

[powderburner]

One or two data points isn't really enough to draw a conclusion. You might have had some surface irregularity (generate more nose force toward one side than the other?), or roughness in one spot (tripping the airflow to separate, causing flow and drag imbalances?), or any number of other little things gone wrong.
Build a couple more and pay close attention to fit, alignment, and surface smoothness/quality. I'll bet that you start getting better results.



originally posted by spacecowboy:
"they hit dirt before reco. . . . "

LOL on that one!

 

[adrian]

Originally posted by Fore Check
When looking down at the axis of the airframe, it is far more difficult to acheivet symmetrical airflow around the frame *unless* the airframe has an even number of sides (4, 6, 10, whatever) or it is circular. Mind you, no matter what geometric shape it is, it must be uniform.

So what you're saying is that if a rocket's airframe is assymetrical, it won't fly properly? I'm glad I didn't know that when I launched this.

Spacecowboy: I'd check that the airframes are rigid enough. In particular, are the fins just glued to the airframe, or extended through and glued to the motor mount? If the rocket starts off going straight and then becomes unstable later in the flight, something nasty is happening as the rocket goes faster.

More nose weight would probably help too, as it would make the rocket more stable. (The first flight of the Natter in the above photo went wrong - it went into a loop, flew horizontal, then crashed. That's because the wings made it unstable at launch, but stable when some propellant had burned out and the CG moved forward. After more nose weight had been added, the rocket has flown perfectly every time.)

I agree 100% with your "basic rule". Keep trying and have fun!

 

[spacecadet]

At the risk of repeating myself.... here's 'Out on its Own'. It did need extra noseweight though. (It's triangular).
Photo courtesy Adrian Hurt

 

[spacecadet]

Sorry..try this

 

[Fore Check]

Originally posted by adrian
So what you're saying is that if a rocket's airframe is assymetrical, it won't fly properly? I'm glad I didn't know that when I launched this.

No, no - that isn't what I meant. Powderburner's comments above are better examples of where I was going.
The non-symmetrical shaped airframes just make it more difficult to overcome imperfections without adding lots of nose weight (or bigger fins or whatever) to make it (otherwise) aerodynamically over stable.

 

[adrian]

Well, the Natter certainly has "more nose force toward one side than the other" due to the assymetrical nose cone. But the reason the Natter needs a lot of nose weight is the wings which move the CG forward. Other winged rockets have the same problem, as anyone who owns an Estes SpaceShip One knows.

 

[jflis]

I just got the chance to read through this thread and thought I would add some comments... (my 2 cents, as it were )

First, you can make *anything* fly... ...eventually

When learning odd shapes you want to reduce the number of *issues* that you need to take into account. The best way to do that in rocketry is to keep things as symetrical as possible.

If your BT shape is triangular, make sure it is an *equalateral* triangle.

If it is 4 sided, make it a square and not a rectangle

If it is *fat*, make it *long*

You not only want to be sure that your fins are straight and large enough, you want to be sure that they are strong enough and attached well enough that they won't flex or move during boost

Nose weight is your friend. If you are working on odd designs or with odd building materials you have pretty much already decided that *performance* isn't your goal... LOL

I have flown 3 sided (the CAUTION rocket is a great example), 4 and 5 sided without problem. Keep experimenting and exploring. That's what it is all about! Just be sure to keep us abreast of your progress!

jim

 

[adrian]

Originally posted by jflis
If it is 4 sided, make it a square and not a rectangle

If it is *fat*, make it *long*

Otherwise you end up with something like this.

It does actually fly. Just not very far. It did go higher than I expected, but that's because I expected it to barely clear the pad.

Nose weight is your friend. If you are working on odd designs or with odd building materials you have pretty much already decided that *performance* isn't your goal... LOL

Absolutely true for the above example.

 

[Micromeister]

Originally posted by spacecowboy
No, you make sense.
what makes no sense is trying to build what I am doing.

I have a basic rule. Keep trying and have fun.
The 3 sided bird flew relatively well. It had real fins.
The 4 sided bird was a box, with a bigger box for fins. It didn't fly too great.

Perfect symmetry on the NC and BT, and some NC weight, I might be getting a good flight.

Up next is a Premium Cracker box, with a butter box for the top. Another 4 sided job. I just thought, a gapper, 2 stage.
C6-0 to get it out of here, A8-3 for fun.

Don't give up on your Odd-Rocs, I have yet to see anything that can't be made to fly! Symmetry makes things easier but is not at all required for a safe, satable and fun flights. I love fling boxes and cartons, food containers and all manor of wierd off the wall configurations, Nose weight or Larger fins are...or even offset fins can be combined to get great flights out of the darnest looking things. Talk about Assymetrical: I've see a golf putter fly head up perfectly straight on a D12-3, a PMC M85 Tank on A10's and all kind of flat top boxes, triangles and other geometic shapes. They just take a bit more time to "Trim" out the wobbles How about a triangular body the is actually the rotor blades/fins of a Helicopeter recovery model that hinge to the rear. Looks like very kewl Traingle model going up and you should here the comments at transition.
Keep up the good work. Odd-Rocs are a Blast

 

[spacecowboy]

Flis, Micro, Adrian, Fore, Space, and Powder

EVERYBODY has good comments.
I think my biggest 2 problems with these birds are lack of BT stability and lack of NC weight.

I took a good post flight look at the failed BOX job, the BT crumpled rather badly. Hum? The fin can is rock solid.

No dang wonder I miss the alarm clock, thinking about the next project and how crazy I can make it.
Odd-rox is home for this scratcher/itcher.

 

[teflonrocketry1]

Originally posted by spacecowboy
So far, both my triangular and square "scratch" or "odd rox" haven't worked so great. Beaut lifts, but 100 to 150 ft off the ground, a giant screw and they hit dirt before reco. . . .

There ain't no way to SIM these.

I have a simulation that represents a model with a triangular tube airframe; it should be is pretty close to the real rocket since it has the same frontal surface area and weight! I actually made a RockSim version 7 simulation file for the FlisKits "Caution Rocket Launch In Progress".

Bruce S. Levison, NAR #69055

 

[adrian]

I've recently completed a small rocket with a hexagonal tube.

BT-5 core tube with bulkheads:

 

[adrian]

Cardboard wrap:

 

[adrian]

Fins, cut from balsa and laminated with paper. The slots go over the bulkheads, allowing the fins to be mounted TTW.

 

[adrian]

Body completed:

 

[adrian]

The finished article. (Folks who were at IRW 2005 might recognise it. )