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neil_w

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There are, dare I say, about a hundred or more threads on doing epoxy fillets. :) You probably want small ones for that rocket, don't want to add too much weight in the back.
 

brockrwood

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There are, dare I say, about a hundred or more threads on doing epoxy fillets. :) You probably want small ones for that rocket, don't want to add too much weight in the back.
Gonna’ try the tongue depressor method next time.

But, because I build LPR tockets, I don’t really use epoxy much except when I need to glue plastic to paper, cardboard, or balsa. So next time may be a while.

Applying glue is like cooking a steak: You can always cook it some more (put on more glue), but you can’t un-cook it (take off the excessive amount of glue).
 

brockrwood

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Ok. Fin glued back on (really on there this time). Swing test performed. Swing test failed. The rocket swings tail first or swings sideways. I can’t get it to swing nose first. I am off to buy some modeling clay so I can put some weight in the nose and try again.

97E73100-F561-4299-8B62-DC6B892B1908.jpeg


34938931-D1CB-4212-B351-BBC22E6F8B76.jpeg
 

neil_w

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Ok. Fin glued back on (really on there this time). Swing test performed. Swing test failed. The rocket swings tail first or swings sideways. I can’t get it to swing nose first. I am off to buy some modeling clay so I can put some weight in the nose and try again.

View attachment 518932


View attachment 518934
Have you created an OR model yet? Would be very instructive.
 

brockrwood

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I always think things are simple. Clay is clay, right? Nope, there is more than one type of clay. The simplest things are complicated. Life is “materials science”. That is why they award Materials Science Bachelor of Science degrees at the Colorado School of Mines.
 

brockrwood

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Have you created an OR model yet? Would be very instructive.
Oh! OpenRocket simulation software! Sorry. Doh!

No, I was being lazy and just trying to do the stability testing the old-fashioned way. It takes a while to get all of the materials into OpenRocket and make sure the weights and densities are correct for each piece. Figured I could just swing test it faster than creating a computer simulation.

Plus, this gives me an excuse to go explore the hardware store.
 

neil_w

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OR? Sorry. I know not this “OR” model. What is that? I am such a newbie…
OpenRocket. No better time than the present to learn it. Creating a model for that rocket would be very easy. Folks are here to help if you have any problems.

IMHO it is highly preferable to doing trial and error with swing testing, especially given that the rocket you're dealing with is very simple.
 

brockrwood

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OpenRocket. No better time than the present to learn it. Creating a model for that rocket would be very easy. Folks are here to help if you have any problems.

IMHO it is highly preferable to doing trial and error with swing testing, especially given that the rocket you're dealing with is very simple.
But then I have no excuse to go to the legendary McGuckin Hardware in Boulder, Colorado, for modeling clay. McGuckin is a nerd candy land. :)
 

neil_w

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Oh! OpenRocket simulation software! Sorry. Doh!

No, I was being lazy and just trying to do the stability testing the old-fashioned way. It takes a while to get all of the materials into OpenRocket and make sure the weights and densities are correct for each piece. Figured I could just swing test it faster than creating a computer simulation.
You don't need to do any of that. All you need to do is get the airframe shape correct, and then override the mass and center of gravity of the entire rocket. You don't need to worry about mass and density of individual components. That's good for planning in advance, but largely irrelevant once the rocket is built.

Constructing a reasonably accurate model of that rocket would take about 5-10 minutes for an experienced OR user. Only one way to get that experience. :)

Also, you may need the clay for nose weight either way. But this way you'll know how much, and why.
 

brockrwood

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In this image, height = Semi-span
View attachment 518972
So, “height” is just the horizontal distance between a point on the root chord to a point on the tip chord, measured at a perfect right angle ftom the body tube.

A lay person would say, it is how far the fin “sticks out” ftom the body tube if you are looking straight down from the top of the rocket.
 

brockrwood

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So, “height” is just the horizontal distance between a point on the root chord to a point on the tip chord, measured at a perfect right angle ftom the body tube.

A lay person would say, it is how far the fin “sticks out” ftom the body tube if you are looking straight down from the top of the rocket.
Actually, to be technically precise, it is the horizontal distance from any point on the tip chord (as long as the tip chord is parallel to the body tube) to the body tube, measured using a perfect right angle (the line is perpendicular to the body tube and perpendicular to the tip chord).
 

Grant_Edwards

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So, this is how I map the fin dimensions in OpenRocket to a typical fin. I hope I sm getting this right.
The easiest way to learn how it works is to enter some values and than then just click on the up/down buttons and watch the fin shape change.

Sweep length is the red dimension arrow below. You can enter either sweep angle or sweep dimension. You can't really specify both.

foo.png
 

Grant_Edwards

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Note that sweep length is measured from the forward end of the root to the forward end of the tip.

In the diagram I posted, the corresponding measurement between the trailing ends of root/tip is the same because root chord == tip chord. If root and tip cords are different, that's not true.
 

neil_w

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Actually, to be technically precise, it is the horizontal distance from any point on the tip chord (as long as the tip chord is parallel to the body tube) to the body tube, measured using a perfect right angle (the line is perpendicular to the body tube and perpendicular to the tip chord).
In the context of OR (or Rocksim), which normally show the rocket in a horizontal orientation, the height is really a vertical dimension. In other words, you know, "height". :)

Maybe at some point we can get a little help dialog into the program to explain the parameters.
 

brockrwood

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The easiest way to learn how it works is to enter some values and than then just click on the up/down buttons and watch the fin shape change.

Sweep length is the red dimension arrow below. You can enter either sweep angle or sweep dimension. You can't really specify both.

View attachment 518981
I love OpenRocket but it assumes poor, hobbyist schmucks like me understand these technical terms of art. I need a glossary. Sigh.
 

brockrwood

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Ok. I know I am dumb, but not this dumb. I have my main rocket tube for the salvage loader with the fins and engine mount all created. Now I want to put a transition onto the front of the body tube. OR keeps putting it on the rear of the body tube (engine mount end). For the life of me I can’t figure out how to make OR put the transition on the front.

@neil_w You see why I just wanted to stick some clay in the nose and go swing test it again? 😉
 

neil_w

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I love OpenRocket but it assumes poor, hobbyist schmucks like me understand these technical terms of art. I need a glossary. Sigh.
If you're up to it, please compile a list of things you encounter that you don't understand. It would be very helpful to us.

There is a wiki with some documentation: http://wiki.openrocket.info/Main_Page. And as Grant says, sometimes the best way to understand a parameter is just mess around with it and see what happens.
 

neil_w

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Ok. I know I am dumb, but not this dumb. I have my main rocket tube for the salvage loader with the fins and engine mount all created. Now I want to put a transition onto the front of the body tube. OR keeps putting it on the rear of the body tube (engine mount end). For the life of me I can’t figure out how to make OR put the transition on the front.

@neil_w You see why I just wanted to stick some clay in the nose and go swing test it again? 😉
At some point you need to scale the initial learning curve.

To move a component up, select it and then either click the "Move up" button or drag the component to the desired location.

Normally it's a bit easier to enter the design from the front to the back, but reordering things is not hard.
 

neil_w

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To get ahead of this: when you've entered the complete rocket, double-click on "Sustainer", check both "override mass" and "override Center of Gravity" and enter your measured values for the entire rocket *without* motor.

Make sure you have added a motor mount to the rear of the rocket. Click on the lower body tube, then click on the "Inner Tube" button. Set the parameters appropriate for a motor tube (for whatever motor size you're using) and then go to the "Motor" tab. Check "this is a motor mount", and enter the rest of the parameters.

Then go to the "Motors and Configuration" tab, say "New Configuration", and... wait, are you using version 15.03 or the new beta? If 15.03, then after you create a new configuration, double-click on the "None" in the right hand column to open up the motor selector. Choose your motor. You can create as many different configurations as you want.

If you're using the new beta, the "Choose Motor" dialog will open automatically when you create the new configuration.

If you now go back to the "Rocket Design" tab, you should be able to select your new motor configuration under the "Flight Configuration" pulldown on the right. Now, the program will show you your total mass and stability margin for the rocket with that motor installed. If you have a good number there (1-2 calibers) then you can go to the Simulations tab and see how the whole flight is going to go.

At any point, please feel free to post your ORK file-in-progress so we can review.
 

RocketRev

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And because you are not certain of your OR skills, go ahead and do the swing test anyway. Besides, every simulation out there, is only as good as the information that gets input. And it is easy to forget things even if you're a regular user of your particular sim program. So, do the swing test right alongside OR, just to check up and see how sim meets reality.

Brad
 

brockrwood

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To get ahead of this: when you've entered the complete rocket, double-click on "Sustainer", check both "override mass" and "override Center of Gravity" and enter your measured values for the entire rocket *without* motor.

Make sure you have added a motor mount to the rear of the rocket. Click on the lower body tube, then click on the "Inner Tube" button. Set the parameters appropriate for a motor tube (for whatever motor size you're using) and then go to the "Motor" tab. Check "this is a motor mount", and enter the rest of the parameters.

Then go to the "Motors and Configuration" tab, say "New Configuration", and... wait, are you using version 15.03 or the new beta? If 15.03, then after you create a new configuration, double-click on the "None" in the right hand column to open up the motor selector. Choose your motor. You can create as many different configurations as you want.

If you're using the new beta, the "Choose Motor" dialog will open automatically when you create the new configuration.

If you now go back to the "Rocket Design" tab, you should be able to select your new motor configuration under the "Flight Configuration" pulldown on the right. Now, the program will show you your total mass and stability margin for the rocket with that motor installed. If you have a good number there (1-2 calibers) then you can go to the Simulations tab and see how the whole flight is going to go.

At any point, please feel free to post your ORK file-in-progress so we can review.
ORK file in progress now being uploaded!

As you can see, OpenRocket gives me a stability of 1.59 calibers with a C6-5 loaded into the motor mount. The center of gravity (no engine) and the weight (no engine) were measured and entered as "overrides" per @neil_w suggestion.

The swing test was failed but with 1.59 stability it should have passed the swing test. I will try again with a different string. If it still does not pass, then I will put weight in the nose.
 

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