Reference / Paper · 2024
Peak of Flight #620: Launch Rod Whip and Solutions
Apogee Components' Peak of Flight newsletter, issue 620 (2024-02-27).
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- apogee-components
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Reference / Paper · 2024
Apogee Components' Peak of Flight newsletter, issue 620 (2024-02-27).
Apogee Components' Peak of Flight newsletter, issue 620 (2024-02-27).
Issue 620 / February 27th, 2024 In This Issue: Launch Rod Whip and Solutions https://www.apogeerockets.com/Rocket-Kits/Skill-Level-3-Model-Rocket-Kits/EggTosser Issue 620 / February 27 th, 2024 Launch Rod Whip and Solutions By Jack Haggerty T here is a part of model rocket flight that is often overlooked in rocket educational books, probably because it is over in less than a second (often much less), but it still has an influence over the entire flight. When you look at the dynamics of a rocket during launch [Fig. 1], the thrust of the motor is aimed directly up the centerline while the drag from the launch lug rubbing on the rod is offset to one side by half the body diameter. Even though these two forces are parallel, that offset is a problem. Analyzing how the forces interact [Fig. 2], we see Fig-2: Rod-whip Dynamics / credit: ARA Press that the portion of thrust from the motor that is Whether this causes a problem depends on several factors such as the overcoming the launch amount of launch rod drag, the diameter of the body (which controls rod drag works like a little how long the lever arm is), and the stiffness of the rod. lever. Two forces acting in opposition a distance In a small rocket, the thrust is low and the body is small so there isn’t apart is what engineers much force from the lever, and even a 1/8” launch rod is able to keep call a “couple” and it things going straight. If, however, you still need convincing of the reality causes a torque (twisting of rod whip, notice that even after the smallest rocket cleanly leaves the force). This torque does pad, you will see the tip of the launch rod wavering side-to-side. not cause rotation at If you're not careful, as you advance to higher thrust levels and larger the rocket’s center of rockets, things can get out of hand until you wind up with the situation gravity, like in free flight, seen in Fig. 3. but rather at the launch lug since the rocket is In this launch of Jim Bassham’s up-scaled Estes Alpha, the 3/16” rod being constrained by the was too small for the wide body combined with a higher powered “G” launch rod. In addition to motor, and the rod was severely bent over. This points out another stressing the glue joint aspect of rod whip other than just pointing the rocket off vertical. The holding the lug to the rocket, this torque will Fig-1: Launch Forces / Credit: ARA Press bending metal rod is a spring, storing the energy imparted to it by the rocket motor through the launch lug. You can see in this picture that bend the launch rod. About this Newsletter 2 You can subscribe to recieve this e-zine FREE at the Apogee Components website www.ApogeeComponents.com, or by clicking the link here Newsletter Sign-Up Newsletter Staff Writer: Jack Haggerty Editor: Tim Van Milligan Layout: Ryan Conway Issue 620 / February 27 th, 2024 Launch Rod Whip and Solutions By Jack Haggerty the bent rod is already imparting a torque to the launch pad itself, causing it to tip over to the left. Finally, once the launch lug clears the tip of the rod, it (the rod) is no longer constrained and the “spring” will start to straighten out. If you are unlucky enough, this fast-moving tip can “slap” the bottom edge of the rocket turning it completely sideways and causing it to fly in “cruise missile mode” or, even worse, down towards the ground. So, what can be done to Fig-3: Extreme Rod Whip credit: Jim Bassham minimize rod whip? Well, you have the three factors mentioned above to play with: the distance from the center of the rocket to the launch lug, the friction of the launch lug on the rod, and the stiffness of the launch rod. To minimize the distance to the rod, sometimes you can move the launch lugs from the outside of the rocket body to the inside, such as with this model of a Titan IIIB booster [Fig. 4], to get them closer to the centerline. While the outer body tube in this model is scaled to match the prototype booster, the working bits inside are a smaller diameter “stuffer tube” which has some launch lugs on it. The launch rod can be guided into and Fig-4: Internal Launch Lug / credit: ARA Press out of the inner launch lugs by some discreetly placed guides. Of course, the outside (for which you lose points in competition) with the reduced the main reason the launch lugs were placed inside this particular model rod whip being a side benefit. But what if you can’t run the lugs inside, was to eliminate the non-scale appearance of the lugs when placed on like with this PMC (Plastic Model Conversion) entry that Gary Miller1 3 Issue 620 / February 27 th, 2024 Launch Rod Whip and Solutions By Jack Haggerty entered in NARAM 53? [Fig. 5] This very wide model had no place to run the guides closer to the center, so Gary instead modified a different one of the variables: rod drag. Fig-5: Launch Rings / Credit: ARA Press Launch lugs on model rockets consist of a very thin-walled paper tube with a plastic coating. Compared to the metal launch rod, they are very soft and deform when pressed. As the thrust rotates the rocket into the launch rod due to the offset, the lug will deform slightly, causing it to conform to the rod at the points of contact as seen in Fig. 6, left side. This gives two large contact patches that rub on the rod causing a lot of drag which increases the couple making the rod bend. This can be greatly reduced by using launch rings rather than a cylindrical lug. Launch rings are generally made of heavy wire formed into loops and attached in pairs. Fig. 6, right side, shows the rings sliding up the same launch rod, and since both the rod and the ring are made of metal, the deformation is almost zero. Thanks to Pythagoras, we know that when two cylinders (the launch rod and the ring2) touch, they have to meet at a single point which reduces the drag to an absolute minimum. 4 Issue 620 / February 27 th, 2024 Launch Rod Whip and Solutions By Jack Haggerty Fig-6: Launch Lugs vs. Launch Rings / Credit: ARA Press You may ask why, if rings are so great, aren't they used on all model rockets? Well, they have their own issues dealing with craftsmanship. They can be difficult to form accurately, anchoring them to the body is not easy, and they must be carefully aligned to not impose their own rotation to the rocket as it rides up the rod. Gary Miller could do these things in his sleep, but they can be a challenge to ordinary modelers like you and me. Probably the easiest way to reduce rod friction is just to wipe your launch rod clean before every launch. The final variable, rod stiffness, is easier to deal with today, but the road getting here was not an easy one. Stiffness goes up with the area of the material, so to get a stiffer rod, all you need to do is go up in size. A 3/16" diameter rod is more than twice as stiff as a 1/8" diameter, so just put bigger launch lugs on your model and you're set. That said, though, the hobby today never uses rods more than 1/4" diameter since there's something better. Early on, Estes recognized the necessity for something stiffer than a small cylindrical rod to guide the ever-larger rockets hobbyists were building. In the 1968 catalog they introduced the "C Rail." This was a 5 Issue 620 / February 27 th, 2024 Launch Rod Whip and Solutions By Jack Haggerty 3/8" square, hollow aluminum extrusion that had a 1/8"wide slot in one The first was a regular launch face. The idea was that rather than the launch lug riding on the outside lug mounted on a thin piece of a small, flexible rod, it would ride inside a larger, much stiffer3 square of dowel to stand it off from guide. The part that wasn't completely thought-through was how to the body [Fig. 7a]. While make the guiding part attached to the rocket. Whatever it was, the part this had the added bonus inside the rail had to be larger than the slot so it wouldn't pull through of letting the rocket also be while guiding the rocket, but attached to the rocket by something launched from a standard smaller than the slot that wouldn't rub on the slot edges. As the blue launch rod, the problem is box in Fig. 7 shows, they had several suggestions for doing this, all of one of tolerances. If the little which had problems. standoff dowel was too small, the launch lug wouldn't fit into the slot far enough and would rub on the inside of the rail. Fig-7a: Standoff Launch Lug / Credit: Estes Industries If it were too big, the standoff itself would rub on the edges of the slot. Even if it were exactly right, the launch lug/standoff and the standoff/body adhesive contact patches would be very small, leading to weak joints. The second method was to use nylon screws twisted through the body tube and "trimmed and glued in place" [Fig. 7b]. With the standard body tube thickness being only 0.012" (0.3mm), there is only 1/2 thread (assuming the screw Fig-7b: Nylon Screws / Credit: Estes Industries is a 10-32 thread) holding the screw in place while it's being trimmed and glued. This gives very little support to the screws, even after they are glued. And if you built up a fillet around the screw, you're limited to 1/8" wide where it travels in the slot. The last method was to fabricate a "T" lug out of some pieces of wood harder than balsa [Fig. 7c]. While this is basically a good idea, it has the same issue of very narrow glue footprints on both the rocket body and at the "T" joint, plus the Fig-7: Estes C Rail / Credit: Estes Industries 6 craftsmanship needed to Fig-7c: "T" Lug / Credit: Estes Industries Issue 620 / February 27 th, 2024 Launch Rod Whip and Solutions By Jack Haggerty build the two lugs straight and square and keep them aligned. These last Despite these shortcomings, the C-Rail was a major step forward in two methods also had the problem of guiding using a flat surface rubbing reliable, solid launch guidance, and it remained in the Estes catalog on the inner flat surface of the rail, leading to greatly increased friction. for 11 years. While it was gone by the 1979 edition, the new plastic The final issue is with the rail itself. Estes sold launch rods in short, 18" lengths that plugged together to make the standard 36" rod. This was done so they would fit in the average high school rocketeer's range box. PortPad was introduced during this time and, to this day the rod clamp/ pivot assembly snaps into a 3/8" square hole that was put there to hold the C-Rail [Fig. 8]. The C-Rail was likewise sold in 18" lengths with an internal spring clip to This brings us up to current techniques. In the 1990's, companies hold the segments together. Being on the inside, your guide, whichever started developing systems of aluminum extrusion to allow the rapid you decided to use, could potentially snag on this clip and strip off. building of precision fixtures for industry. Led by Bosch in Germany, other companies world-wide have come up with their own versions, and the one embraced by the rocket hobby is made by the 80/20 Company in the US. Fig. 9 shows the fully engineered solution for guiding medium and large model rockets. This is a piece of 1010 extrusion4 Fig-8: PortPad C Rail Mount / Credit: ARA Press Fig-9: Launch Rail with Rail Button / Credit: Apogee Components 7 Issue 620 / February 27 th, 2024 Launch Rod Whip and Solutions By Jack Haggerty with a "rail button" riding in it. It is the precision manufacture of both the extruded rail and the rail buttons that make this system work. The simple, robust method of attaching the buttons to the rocket can be seen in Fig. 10. This particular image is from an Apogee Components training video (thanks, Tim!), but all manufacturers use a similar technique. A "T-nut" has its internally threaded shaft passed from the inside through a hole drilled in the body. The button mounts on this shaft and is retained by a flat-head screw. This type of screw not only anchors the button to the nut, but also presents a flush face to the air stream, reducing drag. Fig-10: Rail Button assembly / Credit: Apogee Components The buttons are always used in pairs, one placed in front of the rocket's CG and the other behind. Since the dimensions of both the extrusion and the buttons are held to better than 0.01" (0.25mm), the position of the rocket is similarly tightly controlled. The cylindrical center of the button fits in the rail slot and prevents movement side-to-side. The two conical faces control movement in and out. In Fig. 11 we see the dynamics of the buttons riding in the rail during guided flight. The thrust offset causes the rocket to rotate into the rail at the top, which is shown on the left side of Fig. 11. The rocket's inward travel is stopped by the outer conical face when it encounters the rounded outer edges of the rail slot (red arrows). This gives, theoretically, only two points of contact, although there will always be a tiny bit of deformation in the button. At the bottom end [Fig. 12, right side], the outward travel is arrested by the inner conical surface on the inner edge of the slot (red arrows again). On a typical large model, the off-vertical angles during launch are controlled to better than 1° both side-to-side and in-and-out. Additionally, 8 the very minimal contact area at the four points is between low-friction materials, so very little energy is lost due to friction. Issue 620 / February 27 th, 2024 Launch Rod Whip and Solutions By Jack Haggerty Fig-11: Rail Button Operation Under Thrust / Credit: ARA Press As for rod whip, you can put that concern to rest. A 1010 rail is some 3,300 times stiffer than a 1/8" rod! END NOTES 1. I had just picked this photo as an illustration and was about to write to Gary for permission to use it when I learned of his untimely death due to lung cancer. With his passing, the hobby lost a spectacular craftsman, not surprising with his "day job" being an ENT surgeon. In my more than two decades of judging Scale at NARAM this particular model was the only one to which I've given a perfect 300 point Craftsmanship score. RIP, Dr. Miller. You are missed. 2. Technically, the ring is a cylinder wrapped into a circle, which is called a torus. 3. A quick calculation shows the Estes C-Rail to be 55 times stiffer than a 1/8" diameter rod. 4. This is 80/20's name for an extrusion that is 1.0 inch on a side. Other sizes used in the hobby are 1515 (1.5 inches on side) and 2020 (2.0 inches per side). These latter two have different slot widths so would require different rail buttons when used on Level 2 and 3 projects. ABOUT THE AUTHOR Jack is a Mechanical Engineer working in the Medical Device and Automation/Robotics industries. He is the co-founder of LUNAR, the Livermore Unit of the National Association of Rocketry, the largest rocket club in the country (and, possibly, the world). 9