Question about rail button spacing

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You're good to go Fred with your rocket into the wind, just don't piss, spit, or hunt that way.
 
My questions to Fred in post #45 are still open. But everyone that reads this thread can come to their own conclusions. But I agree, time to move along...

I guess I can't read but the only question I see is "why put your rocket upwind" which I didn't think was really a question.

My response is I like to put the rocket on the front of the pad facing launch control and the spectators....period.
If that's upwind, downwind or sideways I don't care.
I want to see and photograph the rocket taking off.

Your rocket should be strong enough to sit there in any wind and not excessively torque the railbuttons.
If you need to sit out a strong gust and wait for sub-20MPH winds then it needs to survive.

Is that a sufficient answer????

We put our rockets on the LCO side of the rail also. That way the rocket points away from people when we put it up or take it down. And the rail doesn't obscure the rocket.
If I felt my buttons could not hold my rocket in reasonable winds I would install larger buttons and use a larger rail and pad.
 
We put our rockets on the LCO side of the rail also. That way the rocket points away from people when we put it up or take it down. And the rail doesn't obscure the rocket.
If I felt my buttons could not hold my rocket in reasonable winds I would install larger buttons and use a larger rail and pad.

+1, we always position our pads so the rails lower pointing away from the crowd, that way if something weird happens before the rocket is vertical the rocket goes away from the spectators. The rockets are generally on the upwind side of the pad, no issues so far at any of our launches.
 
If that's upwind, downwind or sideways I don't care.

I've heard of a TARC team (it was either from my club or one of the Apogee articles - I'm fairly sure it was at one of the meetings and directed at a visiting TARC team but I'm not certain) that used a launch rail to try and mitigate the drawbacks of rod launching and was having issues, (Ok, It's almost definitely club) and when the person went to help, they found that they had the rocket oriented so that the side was facing into the wind, so when they launched, the rocket bound onto the rail. So I would say, based on my limited knowledge and hear-say, to either put the rocket up or downwind, but not sideways, especially for lighter rockets. For heavier rockets, the effect of wind is lessened. (Rocket and pad scale up but the wind speed stays constant.)

Of course, your experiences might vary. This essentially boils down to a pile of hot electronics- the smoldering remains of an internet server- and also whatever works for you. I think it's wrong to dismiss an idea because you haven't had success with it or don't think it'll work, but also that it's silly to worry so much about what little effect rail button placement has if there are bigger issues impacting performance, say, crooked fins. Just my 53 cents (adjusted for inflation.)
 
We put our rockets on the LCO side of the rail also. That way the rocket points away from people when we put it up or take it down. And the rail doesn't obscure the rocket.
If I felt my buttons could not hold my rocket in reasonable winds I would install larger buttons and use a larger rail and pad.
I just want to be clear as some of you seem to have misinterpreted this issue. This discussion had NOTHING to do with whether or not the buttons are strong enough to hold the rocket on the rail during launch. It is about the location of rail buttons and how they affect the launch.

I made the mistake of bringing up wind interference and my feelings that rockets should be downwind of the rail to reduce binding. Fred took exception to that idea and argued it's too much work to accommodate. That's it, no one ever mentioned the wind blowing a rocket free of the buttons. (Fred did mention that his rockets would tear the buttons off if loaded hanging from the rail, but that's a separate issue.)

I certainly understand the many rationales for how rockets are loaded onto rails. Certainly the ideas of the rail being lowered away from the flight line and rockets facing the LCO make complete sense. But I still stand by my reasoning that all things being equal a rocket downwind of the rail is less likely to be disturbed by the wind during launch. That was the sole point of my argument. You can agree or disagree. But somehow it has morphed into the rail buttons not being strong enough to survive the wind. That was never an issue in the discussion.


Tony
 
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Back to the original question, I'm not sure how to put the forward rail button at the CG. The CG on my rockets moves around depending on how heavy the motor is. So I like to put the forward button one body tube diameter forward of the center of pressure. Not because of physics, but because I find it to be a handy way to do a final check for stability. If the CG is forward of the forward button, stability shouldn't be a problem.

Joe
 
The forward rail button goes just above of the forward centering ring and the bottom rail button goes just aft of the aft centering ring.

What's the scientific reason you ask?

When I build a rocket, I get so excited about finishing and installing the motor mount, that I ALWAYS forget to think about rail button placement before hand. Hence, the rail buttons always end up just outside of all motor mount centering rings by default. I have a process!! :wink:
 
The forward rail button goes just above of the forward centering ring and the bottom rail button goes just aft of the aft centering ring.

What's the scientific reason you ask?

When I build a rocket, I get so excited about finishing and installing the motor mount, that I ALWAYS forget to think about rail button placement before hand. Hence, the rail buttons always end up just outside of all motor mount centering rings by default. I have a process!! :wink:

Not to one-up you, but I just discovered a DOOZY of a mistake...just finished my Rayzor 3, and because I thought it mattered, I marked as close to the CG as I could reasonably get and sunk my top button. I knew it was going to be close to the AV Bay, so I measured off the distance the bay would extend into the booster...gave myself a bit of wiggle room, and pop, there it is. Got ready for a ground test tonight after paint and everything else, and slid my AvBay in...it stopped about 1/8" shy of fully seating. Yup, you guessed it- I measured my coupler, but not with the bulkheads on...with the 1/4" rabbeted ply that MacPerformance uses, I had to remove my button.
I thought it would at least serve as a vent hole, but the AvBay bulkhead fully occludes it.

Oh well, nothing a little putty and paint can't fix! And lesson learned...buttons LAST (plan for them, sure, but don't drill and install!)
 
DOOZY of a mistake...

Shorten the coupler a bit....

I like the cut of your jib...not sure why it didn't occur to me, but that is a simple and elegant solution (especially since it's only a hair, and I'm using sheer pins in the booster)!
I'd like to think I'd have thought of that tomorrow while pondering it some more...but who knows.
 
Your explanation leads me to a slightly different conclusion: The forward guide should be placed far enough forward to support the rocket on the rail in a safe orientation. This does not necessarily mean CG/CP90, and is likely aft of that point. Moving the forward guide any further forward than necessary merely causes it to leave the rail sooner and expose the rocket for an increased amount of time to pitching/yawing moments from the forces you cite while only one guide is in the rail.
 
Your explanation leads me to a slightly different conclusion: The forward guide should be placed far enough forward to support the rocket on the rail in a safe orientation. This does not necessarily mean CG/CP90, and is likely aft of that point. Moving the forward guide any further forward than necessary merely causes it to leave the rail sooner and expose the rocket for an increased amount of time to pitching/yawing moments from the forces you cite while only one guide is in the rail.

The rear guide should theoretically be at CP90 to minimize any turning while the rocket is connected to the rail by only one button, the front should theoretically be as far forward as necessary to prevent the rocket from being able to jostle around too much but as far back as possible to minimize the time spent supported by one button.
 
The rear guide should theoretically be at CP90 to minimize any turning while the rocket is connected to the rail by only one button, the front should theoretically be as far forward as necessary to prevent the rocket from being able to jostle around too much but as far back as possible to minimize the time spent supported by one button.

Exactly
 
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