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Old 03-26-2017, 02:11 PM
luke strawwalker's Avatar
luke strawwalker luke strawwalker is offline
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Join Date: Dec 2007
Location: Needville and Shiner, TX
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Quote:
Originally Posted by Jerry Irvine
Technically on any rocket you want one short lug at the CG and one short lug as far back as possible. My prior comment notwithstanding.


I agree with the principle of two short lugs... but I put them about as far apart as I realistically can. One does NOT need to be at the CG with two lugs-- they will experience the least about of "leverage" (aka "binding" force) on the launch rod when placed as far from each other as possible. It IS best to have the second one as far back on the rocket as possible, to provide guidance for the rocket all the way to the end of the rod.

I would SERIOUSLY argue against placing a single launch lug at the CG. There's an argument for it, in that since ALL the mass force of the rocket is concentrated and acts through the Center of Gravity point, that this point will exert the least "torquing leverage" (again, read "binding force" on the launch lug and rod when the rocket is sitting inert on the pad. This is, to an extent, true (by that I mean that a single launch lug does indeed "bind" to some extent irregardless of placement, due to the fact that the rocket will, to some extent or other, tend to rotate slightly and take up the excess clearance between the lug and launch rod, so that one side of the rod is against the top edge of the lug on one side, and usually the bottom edge of the lug has the rod touching the opposite side of the lug. In effect, the lug is acting like two short lugs placed very close together, with the upper lug hanging off the rod to one side, and the lower lug pushing against the rod to the other side. By being very close together, and concentrated closely with the center of mass (CG), the 'leverage' forces on the lug are minimized (for the single lug case). Still, two SHORT lugs placed as far apart as possible will exert less sideways force on the rod than a single lug, and the contact area between the lug and rod will be virtually the entire length of the lugs, if two lugs are used, because they are far from the CG, giving them a longer "arm of inertia" (more leverage against the force exerted from the mass at the CG point).

NOW, all that above applies to a rocket sitting on the pad WITH NO WIND. When the wind is blowing, it is exerting force on the rocket. We've all seen a rocket "spin on the rod" and turn in the wind, usually shorting the ignitor clips to the rod, deflector, or each other, or pulling them off entirely, and having to make another trip out to the pad to correct the problem. When a rocket is sitting still on the pad in the wind, the fins are STILL exerting force on the rocket's body. Since the rocket has no VERTICAL velocity sitting on the pad, (since it's not yet in flight), and since the wind is coming from some direction (N, S, E, W, or some direction in between thereof), the force of the wind is coming from a 90 degree angle of attack (horizontal) to the rockets static pointed direction (vertical). Therefore, the wind is acting through the rocket's center of lateral area, (aka the "cardboard cutout" CP location), which is as far AFT as the CP can theoretically move during flight (the definition of the "center of lateral area" method of determining CP assumes the airflow acting on the rocket at a 90 degree angle of attack, which is precisely the way the wind is acting on the stationary vertical rocket on the pad, since the wind moves horizontally for all intents and purposes at Earth's surface (ignoring ground effects and turbulence from nearby buildings or other objects disrupting the wind's airflow and creating vortices/drafts). ANYWAY, by having a SINGLE lug at the CENTER OF GRAVITY POINT on the rocket, the "binding force" created on the lug can be MUCH GREATER due to the leverage distance between the lug's location (assumed to be straddling the CG) and the actual "center of lateral area" (rearmost Center of Pressure) due to wind torquing the rocket around this CP. The stronger the wind blows, the more pronounced this effect... So therefore the "best" location for the lug, especially in windy conditions, would 'apparently' be at this "center of lateral area" CP point, rather than the actual CG point. I say "apparently' because it really isn't, but I'll discuss why shortly...

Now, as the rocket ignites and lifts off, the "apparent wind direction" changes... this concept can be difficult to visualize, but think of a car sitting at a red light in the rain... if it's windy and the rain is coming from behind the car, the windshield may stay almost dry, because most of the rain is hitting the back glass and roof of the car, and on the hood, due to the angle of the rain falling in the wind coming from behind the car. The light changes and the car accelerates away, the angle of the raindrops changes... at a point where the car's forward speed matches the wind and falling rain, the rain will appear to fall "straight down" onto the car, and as the car accelerates further, the rain will appear to be falling "toward" the windshield, because the car is now driving forward through the rain, which to a stationary observer is STILL falling from behind the car to toward the front... As the car accelerates more and more, the angle of the raindrops hitting the windshield gets flatter and flatter to the ground (more and more "horizontal") as the car is 'racing' through the rain... (this effect is more pronounced and easier to see driving through falling snow...) The same is true with a rocket and the wind... As the rocket sits on the pad, the wind's "apparent" direction is 90 degrees to the vertical rocket, thus the rearmost CP location. As the rocket accelerates, at the point at which it is moving forward at the exact same velocity as the windspeed, the apparent angle of the wind has shallowed to 45 degrees from the vertical direction of flight (actually the angle of the launch rod, but we'll assume "vertical" for simplicity). As the speed increases, this apparent wind angle shallows more and more, proportionally, but never actually approaches "Zero" (except in "zero wind" conditions). So, the maximum "torque force" a single lug at the CG of the rocket places on the rod is at zero velocity. As speed increases and the "apparent angle of the wind" decreases, the CP moves FORWARD TOWARD the CG. Of course this is ALSO proportional to the forward speed compared to the wind speed, which is why rockets experience "weathercocking" in windy conditions-- the rocket does not achieve maximum speed until usually about a second or two (sometimes more depending on the rocket and motor combination) or usually 40-~200 or so feet in the air, and the "minimum apparent wind direction" off the rocket's flight axis does not occur until then, (assuming a rocket traveling exactly along the path directed by the rod, which it does not do in actuality once it leaves the rod). The rocket, once free of the rod's restraining influence, will rotate about the CG due to the aerodynamic force of the wind acting on the fins through the apparent wind direction exerting force on the fins through the CP, turning the rocket until the "apparent" wind direction of air moving past the rocket is aligned with the rocket's velocity vector, IE, at "zero" angle of attack... (of course we know that due to inertia it actually continues PAST the "zero point" until force builds to arrest this rotation and reverse it back towards the zero point AGAIN, creating an "oscillation".) Since the rocket IS in flight at this point and velocity is increasing, the apparent direction of the wind is aligning more and more with the flight direction as velocity increases in proportion to the wind speed, and the rocket turns "into the wind" around the CG due to the rotational force exerted on it by the fins through the CP, this is a highly dynamic process...

TBC...

OL J R
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