is there a rule of thumb on linear placement and spacing when using 1010 rail buttons? I am scratch building and my air frame tube is 3" in diameter and 30" long. Where would you place them? My first experence using a launch rail.
Steve
Steve
where to put rail buttons
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lakeroadster
When in doubt... build hell-for-stout!
There's a plethora of discussion and links in the following thread:
Typically, the aft button would be about 2-3 cm in from the aft end of the body tube between the fins. The forward button up around the CoG, but up towards the forward end of the booster tube. In your case, perhaps 10" from the top of the tube. It really depends on you rocket’s configuration. If you can position them near centering rings, that adds a little extra strength to the body tube near the attachment points.
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Scott Bartell told me long ago: "one at the aft end, the other halfway to the CG." I generally follow this advice. I'm not too picky about measuring the CG (which I won't usually know until the rocket is complete), but I keep them more aft than most do. (This also gives them more time on the rail.)
Be aware that once the forward button clears the rail, the button/rail system is no longer effective.
Therefore, the distance from the forward button to the top of the rail is the correct rail length to use for an accurate rail exit velocity in simulations.
That may influence your placement decision.
(edit: which John said more succinctly above…)
thanks for the information. I was thinking the buttons would be effective measured from the lowest one. when I think about what Tim said it makes since. So my speeds are not high so adjusting my simulations to the correct rail lenght becomes imporant. I will put one low and the other will be placed based on off the rod speed and hope they are far enough apart.
good piece of information, thanks again
Steve
good piece of information, thanks again
Steve
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Thinking it through, it seems slightly more complicated.
Think of the rail buttons, however many are engaged, as acting like a CG that can effectively resist any amount of cross wind that's relevant to the discussion. Any cross wind on the rocket is pushing on the nose cone and airframe above the buttons, tending to turn the rocket downwind, and also pushing on the fins, using whatever leverage they may have outside the span of the buttons, tending to turn the rocket upwind. When the upper button clears the rail, all forces instantaneously change to being reacted through the lower button. There is now more area above the button, or pivot point, being acted on by the wind, with a center of pressure of the portion of the rocket above the button farther from the button, giving it more leverage. The wind now is more effective in tending to turn the rocket downwind. As soon as the lower button clears the rail, the center of the rocket's resistance to motion instantaneously changes to the actual CG, and we are flying.
If that unpacking is correct, then theoretically, in order to minimize disturbance of the rocket before it gets off the rail, placing the rear button at the CP would eliminate any moment tending to rotate the rocket prior to the rear button clearing the rail. Two competing considerations apply to positioning the forward button. Placing the buttons farther apart increases their moment arm, minimizing the force and thus friction required to resist rotation of the rocket as it moves upward on the rail. However, placing the buttons closer together minimizes the time between the upper button clearing the rail, allowing rotation to occur, and the lower button clearing the rail. The latter consideration should be minimized if the rear button is at the CP, which suggests that putting the forward button farther ahead is probably a higher priority.
However, all this is complicated when we consider that the CP shifts forward when angle of attack (AOA) is more than negligible. There are at least a few white papers on this phenomenon. ("What Barrowman Left Out" is a good search string.) So as the rocket velocity is low, if there is a cross wind, the AOA that results moves the CP forward. That would affect the theoretical best location of the rear button. We can box in the part of this moving target we care about by ignoring the AOA prior to the forward button clearing the rail. The rocket velocity vs. wind velocity -> AOA -> CP analysis can begin at that point. Then recall that once the lower button clears the rail, the rocket will change to rotating about the CG, not the button. Instead of being turned downwind, it will be turned upwind, assuming it is aerodynamically stable. The CP generally is moving rearward rapidly as rocket speed increases, but at the same time, the AOA is being reduced correspondingly, reducing the tendency to weathercock. Obviously, we want rockets going up as straight as possible (or maybe into the wind a little to save on shoe rubber), so the goal is to net out the "turn downwind, then turn upwind" sequence as close to zero change in trajectory as we can manage.
(Of course, a key consideration in all this is that the rocket must be stable at rail exit (rear button) with either 1 caliber if you like that bent and prone to mildew rule of thumb or 10-15 percent of airframe length if you buy into that one with me, accounting for the forward shift of the CP with AOA induced by any wind that is present.)
Having worked through all that, I don't know that it really points to any formula for "the best" positioning. The perfect answer will depend on things like how much wind there is at the moment of launch, rail exit speed (between buttons and after the rear button), wind-caused forward shift of the CP (resulting from wind speed, rail exit speed, and also airframe geometry), and probably a few other things that are difficult to know and subject to change from launch to launch.
Somewhere along the way, I read a general suggestion to put the rear button a little behind the CP and the forward button a little ahead of the CG. I'm still comfortable with the rear button a little behind the CP. I will probably place forward buttons a little farther forward on the airframe, as that minimizes binding. Minimizing binding maximizes speed at rail clearance, which makes everything better. But I probably won't put the forward buttons all the way forward on the airframe. Somewhere in the middle seems good. Extending the 10-15 percent of airframe length stability rule to something only vaguely associated, maybe somewhere in the range of 30-45 percent of airframe length separating the buttons would be a good balance among the competing objectives.
But I would never advocate that as a hard and fast rule. It just makes sense to me right now, having thought it through as I just did publicly. There might be something I'm failing to consider that could turn out to be important.
Think of the rail buttons, however many are engaged, as acting like a CG that can effectively resist any amount of cross wind that's relevant to the discussion. Any cross wind on the rocket is pushing on the nose cone and airframe above the buttons, tending to turn the rocket downwind, and also pushing on the fins, using whatever leverage they may have outside the span of the buttons, tending to turn the rocket upwind. When the upper button clears the rail, all forces instantaneously change to being reacted through the lower button. There is now more area above the button, or pivot point, being acted on by the wind, with a center of pressure of the portion of the rocket above the button farther from the button, giving it more leverage. The wind now is more effective in tending to turn the rocket downwind. As soon as the lower button clears the rail, the center of the rocket's resistance to motion instantaneously changes to the actual CG, and we are flying.
If that unpacking is correct, then theoretically, in order to minimize disturbance of the rocket before it gets off the rail, placing the rear button at the CP would eliminate any moment tending to rotate the rocket prior to the rear button clearing the rail. Two competing considerations apply to positioning the forward button. Placing the buttons farther apart increases their moment arm, minimizing the force and thus friction required to resist rotation of the rocket as it moves upward on the rail. However, placing the buttons closer together minimizes the time between the upper button clearing the rail, allowing rotation to occur, and the lower button clearing the rail. The latter consideration should be minimized if the rear button is at the CP, which suggests that putting the forward button farther ahead is probably a higher priority.
However, all this is complicated when we consider that the CP shifts forward when angle of attack (AOA) is more than negligible. There are at least a few white papers on this phenomenon. ("What Barrowman Left Out" is a good search string.) So as the rocket velocity is low, if there is a cross wind, the AOA that results moves the CP forward. That would affect the theoretical best location of the rear button. We can box in the part of this moving target we care about by ignoring the AOA prior to the forward button clearing the rail. The rocket velocity vs. wind velocity -> AOA -> CP analysis can begin at that point. Then recall that once the lower button clears the rail, the rocket will change to rotating about the CG, not the button. Instead of being turned downwind, it will be turned upwind, assuming it is aerodynamically stable. The CP generally is moving rearward rapidly as rocket speed increases, but at the same time, the AOA is being reduced correspondingly, reducing the tendency to weathercock. Obviously, we want rockets going up as straight as possible (or maybe into the wind a little to save on shoe rubber), so the goal is to net out the "turn downwind, then turn upwind" sequence as close to zero change in trajectory as we can manage.
(Of course, a key consideration in all this is that the rocket must be stable at rail exit (rear button) with either 1 caliber if you like that bent and prone to mildew rule of thumb or 10-15 percent of airframe length if you buy into that one with me, accounting for the forward shift of the CP with AOA induced by any wind that is present.)
Having worked through all that, I don't know that it really points to any formula for "the best" positioning. The perfect answer will depend on things like how much wind there is at the moment of launch, rail exit speed (between buttons and after the rear button), wind-caused forward shift of the CP (resulting from wind speed, rail exit speed, and also airframe geometry), and probably a few other things that are difficult to know and subject to change from launch to launch.
Somewhere along the way, I read a general suggestion to put the rear button a little behind the CP and the forward button a little ahead of the CG. I'm still comfortable with the rear button a little behind the CP. I will probably place forward buttons a little farther forward on the airframe, as that minimizes binding. Minimizing binding maximizes speed at rail clearance, which makes everything better. But I probably won't put the forward buttons all the way forward on the airframe. Somewhere in the middle seems good. Extending the 10-15 percent of airframe length stability rule to something only vaguely associated, maybe somewhere in the range of 30-45 percent of airframe length separating the buttons would be a good balance among the competing objectives.
But I would never advocate that as a hard and fast rule. It just makes sense to me right now, having thought it through as I just did publicly. There might be something I'm failing to consider that could turn out to be important.
I've been back and forth on the question of button or guide placement a few times. The analyses around cg and cp make sense to me, but I've been convinced by other flyers that such concerns apply mainly to rockets with insufficient exit speeds. Ideally we should be going fast enough off the rail that the time between buttons being freed from the rail doesn't permit significant deviation from the guided direction.
Nowadays I place buttons/guides mostly for practical considerations. A few of my rules of thumb:
Nowadays I place buttons/guides mostly for practical considerations. A few of my rules of thumb:
- To minimize binding, I try to spread the buttons out. This goes doubly if using guides. I don't have a hard and fast rule of how far is enough, I just have subjective opinions based on the length and mass of the rocket and how far - if any - the cg lies ahead or behind the span of the buttons (not always an unusual situation if building with a zipperless coupler).
- Due to having seen a few pads where a button all the way aft can slip out of the end of the rail, I place the aft button a little ahead of the aft end of the body to make it far more obvious if I'm having the rear slip off the rail when setting up on the pad.
- If going through the tube wall, I try to keep the forward button aft of the fore end of the motor tube so as not to interfere with a piston should I choose to fly one.
- If building with a zipperless coupler, a forward button goes right behind the forward end of the lower body tube to maximize span, or alternatively I could use guides on the upper body instead. I've not done the latter yet, but would if I were building zipperless for an especially short motor compared to the length and mass of the rest of the rocket (one of my builds matched these criteria, and I should have done this).
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I agree. All the CP/CG concepts sound cool and all, but I think it simply boils down to the speed and wind once the first button leaves the rail.
Putting a button by the CG is important if the rail is not perfectly vertical, and you have a top-heavy rocket where the buttons may be in tension.
Many times, I use 3 buttons, with one on the av-bay switch band.
Good point. However, if the rail exit velocity is significantly greater than any crosswind component, regardless of button position, I’d still be opting for a separation to limit binding.
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My rule of thumb is to put the aft button as far aft as I can and the forward button one body tube diameter forward of the center of pressure. That makes the forward button a quick reference point for stability. But as everyone else has pointed out, it really doesn’t matter so long as they are far enough apart to keep things straight and not so far apart that the forward button leaves the rail before the fins take over.
after reading all the comments, I'm as confused as ever.
I like to put the buttons where my hands or pliers can reach to tighten the buttons on the inside of the BT. If you an find a smaller pair of hands in the house, the operating range is increased.
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Ships, bottles.
You've been at this long enough- place them by eyeball and fine tune by gut feeling. If in doubt, use a higher-thrusting motor.
Someday I need to look into what tools they use to do the rigging in those.
Whenever a technical debate drags on for any length of time without a resolution, it means the real answer is it doesn't really matter. This discussion is an example.
The ship is built outside the bottle and the mast and rigging are lifted into place after it is inserted as they are built to pivot.
best answer yet
There you go, killing my dreams of entirely new classes of micro-hemostat-like devices for intricate work my eyes aren't good enough for anyway.
I'll be thinking of this thread every time I start to mark the locations of guides.
NateB
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I was almost tempted to change the location of the buttons on my L2 project, but then I came to my senses.
John, 1/2 way to the CG from where? Aft end of rocket? Rear button? Tip of nose? Front of airframe?
Thanks!
One in the rear most CR. The other in the CR at the top of the mmt. Depending on rocket, I use 3 quite often. If it is rather heavy or of large dia.
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Yes, the aft end of the rocket. (It stops being effective once it leaves the rail.)
I don't know how he came up with that, but since he brought rails to hobby rocketry he gets some credit. I do agree with those who say this is another case of splitting hairs.
In your range box, in case someone needs a rail button.
Seriously, you should ingore most of this thread, and FARGS as well. Rail buttons, if used, should be on the pop lug or launch sled, and never leave the launch rail in use.
