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Lowpuller

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Please share some guidance on rail guide or launch lug placement.

Specifically is there a maximum distance you can place the lower rail guide in relationship to the motor/nozzle?

Is there a minimum spacing between the two?

Is there a relationship between rail guide placement and CG or CP?

I am currently working on a scratch build that will result in the lower guide being above the fin can which holds the motor mount, should I be concerned?


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Worsaer

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Keep in mind that the effective length of your rail is from the top of the rail to your upper button. As soon as the top button leaves the rail it needs to be flying fast enough to be stable. That's why I prefer that they be on the lower portion of the rocket.
 

blackbrandt

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Please share some guidance on rail guide or launch lug placement.

Specifically is there a maximum distance you can place the lower rail guide in relationship to the motor/nozzle?

Is there a minimum spacing between the two?

Is there a relationship between rail guide placement and CG or CP?

I am currently working on a scratch build that will result in the lower guide being above the fin can which holds the motor mount, should I be concerned?


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I tend to place one at the average CG of the rocket, and one almost at the bottom.
 

chadrog

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In most circumstances I put one into the lower centering ring and one into the upper centering ring. I don't believe there's any reason to aim for the CG or any other specific point, I've had them above, below, and everywhere in between and never noticed any difference. Main consideration is that you have them solidly mounted.
 

Lowpuller

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The current rocket I am building or should I say rebuilding is about 8' long. I am concerned with the amount of moment therefore friction generated the closer the lugs are placed together. Very tall top heavy rocket with tiny fins. I'm concerned the friction of the rail will greatly reduce my speed off the rail. 8' of cartwheeling mass won't be a good time for anybody close by.


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JohnCoker

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I tend to place one at the average CG of the rocket, and one almost at the bottom.
That's the advice I got from Scott Bartel (who popularized rails in rocketry with the blacksky "ProRail"). It sounds reasonable, but I've never really had a reason to believe the location was critical.
 

UhClem

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That's the advice I got from Scott Bartel (who popularized rails in rocketry with the blacksky "ProRail"). It sounds reasonable, but I've never really had a reason to believe the location was critical.
The as far aft as you can get it rail button location is not great. If there is any wind it will cause the rocket to rotate during the time there is only one button in contact with the rail. This may or may not be a problem depending on conditions. But if apogee is expected to be high and the winds are high, you may be walking a lot further downwind that you expected. A case in point is the flight that won the 100K Carmac prize. In spite of simulations suggesting that it would land within three miles, it landed over twenty miles away.

None of the simulators deal with this tipoff effect at all.

Placing the aft rail button at the CP will minimize this effect while putting it at the aft end will maximize it.
 

CarVac

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The as far aft as you can get it rail button location is not great. If there is any wind it will cause the rocket to rotate during the time there is only one button in contact with the rail. This may or may not be a problem depending on conditions. But if apogee is expected to be high and the winds are high, you may be walking a lot further downwind that you expected. A case in point is the flight that won the 100K Carmac prize. In spite of simulations suggesting that it would land within three miles, it landed over twenty miles away.

None of the simulators deal with this tipoff effect at all.

Placing the aft rail button at the CP will minimize this effect while putting it at the aft end will maximize it.
That's not correct: the tip-over came from a rail guide falling off, not because of the aft positioning of the rear rail guide.
 

Airobert

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That's the advice I got from Scott Bartel (who popularized rails in rocketry with the blacksky "ProRail"). It sounds reasonable, but I've never really had a reason to believe the location was critical.
Makes sense to me.
We need at least two buttons. So...
The rocket rotates around its CG so a button there should have the least amount of force (Side load)on it. So button one goes at CG.
Then a second button as far away as possible would then have the most leverage and thus the least side load on it. It should not be towards the top of the rocket because it will leave the rail too early. So button two goes on the bottom.
That being said I usually screw them to centering rings for added attachment strength and I have put in an extra ring at CG to do this.
 

Lowpuller

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I'm thinking one at the CG and one at the CP.

Or I am thinking about placing them no more than about 6" apart so they both leave the rail fairly close to the same time.

I believe there is a scientific proper placement, I just have not yet stumbled on it.


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blackbrandt

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Actually, you want them as far apart as possible. The closer you have them, the more torque that they will have applied to them (AKA they will rip out with enough force).

Matt
 

CarVac

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Actually, you want them as far apart as possible. The closer you have them, the more torque that they will have applied to them (AKA they will rip out with enough force).

Matt
Too far and they shorten the effective rail length though.
 

blackbrandt

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So, you want 1 at the CG (highest effective point) and one at the very bottom. Of course, you could always use 3... :)
 

UhClem

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Actually, you want them as far apart as possible. The closer you have them, the more torque that they will have applied to them (AKA they will rip out with enough force).

Matt
What is the source of this torque?
 

blackbrandt

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The rocket will not travel 100% dead accurate on the direction of this rail. Centering rings aren't perfectly straight, nozzle may have a slight angle, etc. That is the origin of the torque.
 

Handeman

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That's the advice I got from Scott Bartel (who popularized rails in rocketry with the blacksky "ProRail"). It sounds reasonable, but I've never really had a reason to believe the location was critical.
I agree. I have one rocket with a button at or below CP and the second on the bottom. Another rocket has one 3-4 calibers above CG and the other at the bottom. Both fly just fine and neither have any issues leaving the rail in any conditions.
 

Lowpuller

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Actually the source of the torque I am speaking of is from the weight of the rocket itself. In my particular case with a very long rocket think of the rocket almost as a lever with the pivot point being the bottom button and the load being applied to the top button.

The greater the distance between the buttons the lower the moment, but the greater the likelihood of the buttons being effective. I considering placing the buttons close to each other and near the CG and just below it, but placing the rocket on the rail such that the top button has at least 6' or rail exposure. This reduces the moment but still takes full advantage of the rail.

Comments please?


So many rockets, so little time, and money.
 

UhClem

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Actually the source of the torque I am speaking of is from the weight of the rocket itself. In my particular case with a very long rocket think of the rocket almost as a lever with the pivot point being the bottom button and the load being applied to the top button.
If one button is at the CG the the weight of the rocket produces zero torque around it. This being an issue only for a non-vertical rail. When the rail is vertical then the forces are from friction between the rail and the buttons which should be very low.
 

UhClem

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The rocket will not travel 100% dead accurate on the direction of this rail. Centering rings aren't perfectly straight, nozzle may have a slight angle, etc. That is the origin of the torque.
If misalignment between the thrust and rocket CG generates enough torque so that ripping a rail button off is a concern, then it is large enough to create serious problems in flight.
 

les

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I think the torque question is a combination of installing the rocket (horizontal and more a weight vs. torque question) and raising the rocket. Once vertical you will get torque on the second button with one at the CG

I've also heard of people using three buttons, but you need to be careful lining them up. Then you can have one at the bottom, one "high up" to reduce torque, and one in between. The importance would be how fast the rocket is traveling when this middle one leaves the rail
 

NikeMikey

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If misalignment between the thrust and rocket CG generates enough torque so that ripping a rail button off is a concern, then it is large enough to create serious problems in flight.
If there is sufficient wind to cause the rocket to rotate during the time (miilliseconds?) there is only one button in contact with the rail, then you should not be launching it! Of course, you could always rotate the rail so that the rail buttons are NOT perpendicular to the direction of the wind -- though I have never heard of anyone being so concerned as to do this.

NikeMikey
 

Handeman

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If there is sufficient wind to cause the rocket to rotate during the time (miilliseconds?) there is only one button in contact with the rail, then you should not be launching it! Of course, you could always rotate the rail so that the rail buttons are NOT perpendicular to the direction of the wind -- though I have never heard of anyone being so concerned as to do this.

NikeMikey
+1

I've never known anyone concerned about rail buttons and wind. The only rocket I ever had that lost the rail guides in flight was my L1 rocket on an I1299N-P. It pulled 80G during the 0.3 sec burn and ripped the rail guides off. It still flew straight and actually went 200 ft higher then it did the first time it flew on the I1299N-P.
 

1tree

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As I began building for L1 I had a discussion with our club president. He pointed out that during the period that there is only one rail button in the rail it can do nothing good. Thus if you think about it, placement becomes a balancing act of location. The further apart the buttons are, the less torque. The closer they are the shorter the time the last one can only bring about bad.

Personally, I have chosen to place the rear one at CP, and my forward button 1.5 caliber forward of that. This makes one button also serve to mark CP. And I know to be double checking things should the CG fall below the second one.

Having said all this, it is very common practice to place the lower button at the bottom of the rocket. And I have heard to place the upper one at CG (though that location will move depending on what motor is loaded).


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Handeman

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I've only been watching rail launches since the club got their 1010 rails back in '07, but I've never seen the "bad" happen when only one button is in the rail. What exactly would that be?
 

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