Rail button (does it need to be above CM)

[David Schwantz]

You go to school to learn how it is suppose to be done. Then you find out in the real world how it IS done

 

[bjphoenix]

clearly we all agree you need some separation, so I don't know what you're driving at.

That was a "devil's advocate" type post. If one recommendation was to put the forward button in front of CG, and other recommendations are to put the buttons farther back to gain more effective rail length, then why not put both buttons close together near the bottom end of the rocket?

We don't know what forces act on guidance devices but I've had launch lugs torn off of rockets so there are some forces involved. And if there are some forces involved then I think we should place buttons to give some effectiveness in resisting such forces. If you think the forces are fictitious or very small then tell me why you wouldn't put the buttons 4" apart at the bottom end of the rocket.

 

[teepot]

mechanically, my method is just easy. clearly we all agree you need some separation, so I don't know what you're driving at. In the posted example the buttons are 22 inches apart. The discussion is where to put the buttons.

Teepot's design is a extreme example of giving up rail length, which is what i've been harping on. If he likes punchy motors, no problem but the less punchy motors, which require longer lengths to get to a stable speed, that could be a problem

Same rocket, Loki K250. Rocksim states it reaches stable speed at 50.3652 inches That would work for my rail button placement (70 inches of effective rail length) but fail for CG centered buttons (38.496 inches of effective rail button length). I won't even bother the exercise of calculating teepot's because it would just be worse.

this example clearly demonstrates lower rail buttons are more safe -- something to think about.

I use Thrust curve to determine the stable speed off the rail. Been good so far. The button placement depends on access to the inside of the BT. I use a long hemostat to position the T nut. It will only reach so far down the BT.

 

[cbrarick]

That was a "devil's advocate" type post. If one recommendation was to put the forward button in front of CG, and other recommendations are to put the buttons farther back to gain more effective rail length, then why not put both buttons close together near the bottom end of the rocket?

good luck on doing that with the example rocket. Your post is fairly ridiculous. Let's stick with the real world examples, not stuff you dream up in your head. My OP gave the reason why the upper button location was chosen.

then there's the fact that I agreed with the posted paper (and other posts) that there needs to be separation between buttons, so you're "devil's advocate" argument is invalid. The real question I posed was where do we locate the buttons, not how far apart

teepot: dowel rods and a bit of tape is your friend on longer rockets :> I'd suggest blue tape because you can't fly rockets without it, but of course i risk that we change this into a tape thread :>

 

[bjphoenix]

good luck on doing that with the example rocket. Your post is fairly ridiculous. Let's stick with the real world examples, not stuff you dream up in your head. My OP gave the reason why the upper button location was chosen.

then there's the fact that I agreed with the posted paper (and other posts) that there needs to be separation between buttons, so you're "devil's advocate" argument is invalid. The real question I posed was where do we locate the buttons, not how far apart

teepot: dowel rods and a bit of tape is your friend on longer rockets :> I'd suggest blue tape because you can't fly rockets without it, but of course i risk that we change this into a tape thread :>

If you need separation then how much do you need? Of course you need some separation, there would be no effective guidance if you didn't have at least some separation. If you don't know how much separation is needed then you can't really address the question of where to put them. Putting the buttons lower on the rocket provides longer guidance, that is easy to understand, but is there a reason to put a button closer to the CG? I was hoping someone would provide some insight into that. I understand the affect of forces at the CG and how they translate to forces at the buttons but what are the forces at the CG? I know not to think that a rocket slides smoothly up the rail, it's going to vibrate somewhat, the rod/rail will have its own dynamic response, there might be wind acting on the system. My guess is you put one button relatively close to the rear end, then you provide a reasonable amount of separation between the buttons. I think most rockets that would have buttons would have CG at least slightly behind mid point of the rocket so automatically the front button is probably close enough to the CG so it doesn't matter if it is right at CG or not.

 

[teepot]

good luck on doing that with the example rocket. Your post is fairly ridiculous. Let's stick with the real world examples, not stuff you dream up in your head. My OP gave the reason why the upper button location was chosen.

then there's the fact that I agreed with the posted paper (and other posts) that there needs to be separation between buttons, so you're "devil's advocate" argument is invalid. The real question I posed was where do we locate the buttons, not how far apart

teepot: dowel rods and a bit of tape is your friend on longer rockets :> I'd suggest blue tape because you can't fly rockets without it, but of course i risk that we change this into a tape thread :>

Tried the dowel and tape for a while. Gave up trying to get to get the T nut in the hole because it was very frustrating.

 

[cbrarick]

ok, so maybe I'm off my rocker but I've been having second thoughts about CG when the rocket is attached to the rail.

We all agree (I hope) that when the rocket is attached to the rail, there's not a whole lot of slop between the rail buttons and the rail.
We also agree that our rails are pointed vertically. (ish)

We currently think of CG as related to when the rocket is horizontal, and it's relationship to CP for stable flight.

I postulate that while the rocket is at rest (before launch) this is not correct.

That while the rocket is at rest (waiting for launch) the CG is in the midline of the rocket as it is oriented vertically
https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/center-of-gravity/
hmmm so for our example, a three fins and nose cone type of rocket, probably oriented in a line drawn from nose cone tip to the center of the aft end of the rocket

it therefore follows that the rail buttons secure the rocket to the pad to promote this relationship between the rocket and the pad
And can be supported by the observation that pad legs are not straight but generally at a 45 degree angle to promote the CG stability in the above reference.
Further seen in hydraulic pads which are equipt with some sort of outrigger to improve this CG relationship.

I also postulate that this relationship continues until the rocket is free of the rail.

If so, we can stop talking about the horizontal center of gravity and move onto discussing effective ways to promote the maintenance of the relationship between the rail and the rocket (does this push us closer to CP? after all, this is measure of the aerodynamic cross section of the rocket as presented to the wind when the rocket is vertical.?

Thoughts?


ps feel free to slap me down, this may be way off base.

 

[Hobie1dog]

The new rail buttons that have the well nuts are fantastic. Just drill a 5/16 hole in the airframe, push the well nut in, then use a cordless drill on low speed with a clutch setting of 1 or 2 and the rubber expands and provides a really secure installation. I have 2 of the 40 year old rockets I was gifted that had internal launch rail tubes ( which were worthless) and the well nut models saved the day.

 

[SolarYellow]

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.

Having thought this through further since that post last year, I wouldn't be critical of anyone putting the lower rail button as far rearward on the rocket as is structurally optimum. By the time the rocket is going ~45 ft/sec or better for rail exit speed, that time difference of a few inches doesn't seem likely to be significant and reducing the forces "binding" the rocket on the rail with friction by spreading the buttons farther apart is likely advantageous. I'll probably be putting buttons just ahead of the aft-most fin can ring and just behind the mid-body break junction (or into it if the construction allows).