Reference / Paper · 2023
Peak of Flight #615: Fin Flutter Analysis Revisited (Again)
Apogee Components' Peak of Flight newsletter, issue 615 (2023-12-19). Also in this issue: Triffid Rocket Plan.
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- apogee-components
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Reference / Paper · 2023
Apogee Components' Peak of Flight newsletter, issue 615 (2023-12-19). Also in this issue: Triffid Rocket Plan.
Apogee Components' Peak of Flight newsletter, issue 615 (2023-12-19). Also in this issue: Triffid Rocket Plan.
www.ApogeeRockets.com 4960 Northpark Dr, Colorado Springs CO 80918 Ph# 719-535-9335 Issue 615 / December 19st 2023 In This Issue: Fin Flutter Analysis Revisited (Again) Also In This Issue: Triffid Rocket Plan https://www.apogeerockets.com/Model-Rocket-Kits/Skill-Level-3-Model-Rocket-Kits/Hermes-HLV Issue 615 / December 19 th, 2023 Fin Flutter Analysis Revisited (Again) By John K. Bennett I n Peak of Flight Issues #291 (2011)1 and #411 (2016)2, authors Howard and Sahr respectively proposed methods to calculate the velocity beyond which catastrophic fin failure was probable due to aerodynamically induced undampened fin oscillation. This article provides additional insight into the underlying calculations, allowing us to configure the calculation to more accurately match specific fin geometries, and corrects an error present in both previous articles that caused the flutter velocity to be overestimated by a factor of 1.414 (the square root of two). p/p0 is the ratio [air pressure at the altitude where the speed of sound was determined / air pressure at sea level] (dimensionless), and the constant “39.3” (whose value actually depends upon fin geometry) has units of pressure and is calculated as described below. OpenRocket and RockSim use different nomenclature for the distance between the body tube and the tip chord. RockSim calls this distance “Semi-Span”; OpenRocket uses the more intuitive term “height.” The Howard and Sahr articles were both based on the much earlier work of Dennis J. Martin, who in 1958 synthesized a large body of previous analysis and experimental data into an empirical working tool for engineers3. As part of that work, Martin derived the following formula for bending-torsion flutter velocity (Vf ): The main take-away from Martin’s analysis is that Vf is equal to the speed of sound times a dimensionless factor calculated from the fin geometry and fin material physical properties, as well as relevant atmospheric conditions. As observed by Sahr, if we use systems of units consistently, we can perform this computation using either SI (meters/ kilograms) or Imperial (feet/pounds) units. where (using Martin’s notation), There are more sophisticated (and more accurate) techniques for determining flutter velocity. A review of these techniques can be found in the graduate thesis of Joseph Simmons4. However, Martin’s technique is great for amateur rocketry because it is based on numerical data that are reasonably accessible (and understandable), it doesn’t require special software or an engineering degree to use, and, in a pinch, it can be employed using only a simple calculator. Let’s drill down a bit and examine how Martin’s calculation is performed. Vf is the calculated fin flutter velocity, a is the speed of sound at the altitude (above sea level) of maximum rocket velocity, GE is the shear modulus of the fin material (in units of pressure), A is the fin “aspect ratio”, equal to [(fin semi-span length or height)2 / fin area] (dimensionless), t/c is the fin “thickness ratio”, equal to [fin thickness / root chord length] (dimensionless), Calculating the Speed of Sound The speed of sound in air (a) depends only upon temperature (because, if we consider air as an ideal gas, the altitude-related effects of decreasing density and decreasing pressure tend to cancel each another out). In that case, the following equations produce the speed of sound: λ (lambda) is the fin “taper ratio” equal to [tip chord length / root chord length] (dimensionless), 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: Martin Jay McKee Editor: Tim Van Milligan Layout: Ryan Conway Issue 615 / December 19 th, 2023 Fin Flutter Analysis Revisited (Again) By John K. Bennett To compute the correct value for the speed of sound, we need to know the air temperature at the altitude (above sea level) where the rocket will be at maximum velocity, which for typical engine thrust curves is the altitude just before motor burn out. This altitude (AGL) can be found easily using RockSim or OpenRocket. Then add the launch site altitude to this predicted flight altitude to get altitude above sea level. The Troposphere (up to about 12,000 meters / 37,000 ft) is heated by the Earth’s surface, so as we go up, air temperature decreases. This decrease is approximately linear in the Troposphere, so air temperature will decrease by about 0.0065 °C per meter, or about 0.00356 °F per foot. Using these relationships, and a default sea level air temperature of 15 °C, or 59 °F, we get the following equations (which are only valid in the Troposphere): These formulae are for dry air under “standard” atmospheric conditions. If the conditions on the ground are non-standard, we may want to make adjustments. For example, if our launch site is at 5000 ft., the temperature is 95°F on launch day, and the apogee of our rocket is 5000 ft. AGL, we might want to replace “59” with 95 in the second equation and use the AGL altitude of maximum velocity to compute TF. In addition, the speed of sound in humid air is slightly higher than in dry air. This difference is negligible in cold air, and less than ½ % in warm air. as the action of a pair of scissors (“shears”), and shear strain as the effect of the shear stress (paper is severed, leather is warped, steel may be unchanged). Thus, the shear modulus is a measure of the stiffness of a material in the presence of shear stress, i.e., how much pressure must be applied to deform the material (either temporarily or permanently). Rocket fins are made of many different types of material: balsa, plywood, fiberglass, carbon fiber, aluminum, and sometimes more exotic materials like titanium or magnesium. The shear modulus can vary widely across materials, or even different instances of the same material. In addition, composite materials like fiberglass and carbon fiber are anisotropic, meaning that the shear modulus (as well as other strength properties) will be different depending upon whether the shear stress is perpendicular to, or parallel to, the embedded glass of carbon cloth. Since we are interested in what happens to the fins when they are bending and in torsion due to aerodynamically induced oscillation, the shear modulus of interest is parallel to the embedded cloth. In this axis, the shear modulus will be much more dependent on the epoxy than on the cloth, so there is little difference between carbon fiber and fiberglass when determining the GE used to compute Vf. Epoxy composite manufacturers rarely report the shear modulus directly, but they often report the Young’s modulus (also called the modulus of elasticity) and Poisson’s ratio, from which the shear modulus can be estimated using the formula: Determining the Shear Modulus The shear modulus (GE) (also called the modulus of rigidity) of a material, measured in units of pressure, is the ratio of shear stress to shear strain. Think of shear stress where E is the Youngs Modulus and v (“nu”) is the Poisson’s ratio for the material in question. This formula 3 Issue 615 / December 19 th, 2023 Fin Flutter Analysis Revisited (Again) By John K. Bennett assumes the material is isotropic, which is not actually true for most composites used to make rocket fins. However, it can be used for anisotropic composites, if we remember that this equation will tend to produce higher than warranted values for the shear modulus (and therefore for Vf) for anisotropic materials. If we search the Internet for data pertaining to the shear modulus of G10 or G12 fiberglass, we will obtain values ranging from 425K to 1.7M psi (or the equivalent SI values). Experimental direct measurements report values near 775,000 psi (5,343,436 KPa). This number is also approximately the median of many reported and calculated values for both fiberglass and carbon fiber composites. Some of the references that provide these data can be found here 5,6,7,8,9,10,11,12,13. Allowing for margin, the following table contains suggested working values for GE for various materials: Calculating Fin Geometry Ratios The three ratios related to fin geometry are straight forward to calculate. Since these ratios are dimensionless, any unit system can be used, as long as the same units are used in the numerator and denominator of each ratio. Fin semi-span length (height), thickness, root chord length, and tip chord length come straight from the simulator values. 4 Calculation of fin area depends upon the actual fin shape, but basic geometry provides the tools we need for many traditional fin shapes. If the fin is trapezoidal, area is calculated as follows: If the fin is elliptical, fin area is calculated as follows (assuming the fin is one half of an ellipse): For elliptical fin shapes, there isn’t an actual tip chord. To compute an approximate λ, we need to find the “pseudo” tip chord length of a trapezoidal fin that gives the same area as the elliptical fin. We do this by calculating the area of the elliptical fin, setting this value equal to the area of a trapezoidal fin with the same root chord and semi-span length (height), and then solving for tip chord length. This process is depicted in the figure below. Trapezoid abcd has the same area as the ellipse shown, and the “pseudo” tip chord length is bc, calculated as shown. Issue 615 / December 19 th, 2023 Fin Flutter Analysis Revisited (Again) By John K. Bennett Calculating the Air Pressure Ratio This ratio is the air pressure at the same altitude for which we determined the speed of sound, divided by the air pressure at sea level. In the Standard Atmospheric Model, air pressure at sea level is 101.325 Kpa (SI) or 14.696 psi (Imperial). To calculate air pressure at altitude, we use the temperature calculated previously in one of the following equations. mass behind fin quarter-chord, expressed as a dimensionless fraction of the full chord (see explanation below); For a symmetric fin, ԑ is 0.25, κ (“kappa”) is the dimensionless ratio of specific heats (also known as the adiabatic index) for air, equal to 1.4, regardless of unit system, p0 is air pressure at sea level in psi (14.696), or in KPa (101.325), and π (“pi”) is 3.14159 (dimensionless). To obtain the air pressure ratio, we divide this value by the appropriate p0, which has the effect of simply removing the first term of the equation above. Note that these equations are only valid under about 36,000 ft. Calculating the Denominator Constant The denominator constant (DN) of “39.3” in Martin’s equation is unusual in that it has the units of pressure. It is calculated as follows: where (again using Martin’s notation), the value of 24 is a dimensionless product of the whole number constants 4 and 6 found in Martin’s derivation (Equations 9 and 10), ԑ (“epsilon”) is the distance of the fin center of Since all components of DN are dimensionless except p0, DN will have the units of pressure. The pressure units chosen must match those of the units chosen for GE. Using the default value of 0.25 for ԑ, DN = 39.294 (Imperial units) or 270.552 (SI units). Martin uses Imperial units throughout, and rounded DN up to 39.3. In their respective articles, both Howard and Sahr compute DN incorrectly. Howard’s PoF equation uses the constant 1.337, which is 39.3/14.696 (p0: sea level absolute pressure), divided by 2, thus combining all constants in the denominator of Martin’s original equation. However, Howard also included an additional factor of two in the denominator of his equation. This double division by two in the denominator results in the calculated value of Vf being overestimated by a factor of the square root of two (1.414). Sahr’s article uses Howard’s constant as given, thus propagating the error. Martin’s denominator constant of 39.3 assumes the value of ԑ is 0.25, but he notes that “for sections with the center of gravity far from the 50-percent chord position, a correction may be required.” Let’s see how this condition might arise. Consider the following fin shapes, where black dashed lines represent the quarter chord and half chord lines: 5 Issue 615 / December 19 th, 2023 Fin Flutter Analysis Revisited (Again) By John K. Bennett body tube, and designed the following fin shape (with through-the-wall fin mounting tabs shown): For symmetric fin shapes (including elliptical fins), ԑ is always 0.25. For non-symmetric fins, such as the middle fin shown above, ԑ is offset from the 50-percent chord by δ (delta). To determine ԑ in this case, we must first find Cx, which is the axial distance from the front of the fin to the fin centroid (shown as a red and yellow dot), or center of mass, or as Martin call it, the “center of gravity” of the fin. As shown above, the centroid is also offset toward the tip chord (by Cy), but we usually do not need to calculate this offset. To calculate Cx we use a formula from basic geometry for the centroid of a trapezoid: The data in red is taken directly from the simulator. We now want to know the flutter velocity for varying choices of fin thickness, so we can choose an appropriate value. For now, assume a fin thickness of 1/8” (0.125”). Summarizing the given fin geometry: Fin Thickness: 0.125” The value for m in this equation comes directly from the simulator as “sweep length.” Once we have Cx, we can calculate ԑ using the formula: Semi-Span (Height): 3” Tip Chord: 2.5” Root Chord: 7.5” Sweep Length (m): 4.285” Recall that ԑ is a measure of distance expressed as a fraction of the whole root chord. A Worked Example Suppose we are designing a rocket intended to be capable of safe transonic and low supersonic flight, and we have decided that all structural components will be G10 or G12 fiberglass. We want our rocket to support 54mm motors up to K or small L. We have decided to use Imperial units. After working with our simulator, we have selected a 3” 6 Let’s starting by calculating DN. From our formula for Cx, we find that Cx = 4.49”. From our formula for ԑ, we find that ԑ = 0.349. Therefore, DN = 54.88 (note that this is 40% more than Martin’s default value of 39.3). Now we determine the altitude of maximum velocity for the largest motor we intend to use. In our case, this is the Aerotech L1090W. Let’s assume we are launching from Issue 615 / December 19 th, 2023 Fin Flutter Analysis Revisited (Again) By John K. Bennett the Brothers site in Oregon (elevation 4500 ft) and that the temperature will be 65°F on a clear, relatively calm (wind speed in the range 2.5 – 7.5 mph) day. This is cool enough that we will use the standard atmospheric model without adjustment. When we simulate (using OpenRocket) the L1090W motor in our rocket under these conditions, we see the following results (excerpted here): Now let’s calculate the fin geometry ratios: Fin Area = 15 in2 t/c (thickness ratio) = 0.0167 λ (lambda) (taper ratio) = 0.3333 A (aspect ratio) = 0.6 We are now ready to choose GE. Since our fins are fiberglass epoxy, we select a GE of 600,000 psi from our table of suggested values. Finally, put everything together and compute Vf: Vf = 1425 fps Since our rocket’s maximum velocity is 1500 fps, this rocket is a likely candidate for failure due to fin flutter. And there is no safety margin. Based upon this simulation, our rocket will reach about 1500 fps (Mach 1.33) at an altitude of about 14,000 ft. AGL (18,500 ft ASL), just before motor burnout14. Now we can calculate the air temperature, air pressure, and speed of sound at that altitude using the standard atmosphere barometric model embodied in the equations above. At 18,500 ft.: TF = -6.86°F a (speed of sound) = 1043.36 fps p (air pressure) = 7.2 psi p/p0 = 0.49 Let’s try increasing the fin thickness to 5/32” (0.156”). Now, Vf = 1991.7 fps, giving us a 32% margin. For the flight conditions given, this is probably OK, but on a 95° day, Vf would be 1580.2 fps (only a 5% margin). That’s not good, so let’s try increasing the fin thickness to 3/16” (0.1875”). Now, Vf = 2618.1 fps (74% margin) under the conditions given, and 2077 fps (38% margin) on a 95° day. This is a safety factor likely appropriate for any launch conditions, so we choose 3/16” thick fins. OK, now that we know how to perform this calculation, wouldn’t it be great if we could automate some of the process? Your prize for reading this far is a link to a spreadsheet that you can use to analyze your own fin 7 Issue 615 / December 19 th, 2023 Fin Flutter Analysis Revisited (Again) By John K. Bennett designs for flutter risk. You can download this spreadsheet from GitHub at this URL: https://github.com/jkb-git/FinFlutter-Velocity-Calculator. An image of this spreadsheet in use is shown on page 10. Some Final Thoughts 1. It is important that the choice of unit system (Imperial or SI) be consistent at several points in the calculation. In addition to being consistent with each numerator/denominator pair, the choice of unit system for Vf, the speed of sound, GE, and air pressure must also be the same. 2. Martin’s work was only intended to serve as a “guide in the preliminary design of lifting surfaces on missiles.” Several potentially important details have been abstracted away. For this reason, a significant safety margin should be included if these calculations will be used in go/no-go flight decisions. I would consider a safety margin under 25% for actual flight conditions a poor risk, and one under 20% as potentially unsafe. 3. Martin’s work does not consider fins whose thickness tapers toward the ends. Using the average fin thickness value is one possibility. Another possibility is to use the fin thickness at the fin centroid or mid-height. The best choice will likely depend upon the linearity (or nonlinearity) of the taper. 4. Tip-to-tip reinforcing, a common practice for transonic and supersonic amateur rockets, is also not considered directly. If using tip-to-tip reinforcement, a reasonable approximation that takes this reinforcement into account would be to double GE. The accompanying spreadsheet does this. 8 5. The selection of GE itself has a fair bit of windage. Virtually none of the materials in common use for amateur rocket fins come with guaranteed manufacturing specifications. This is especially true for composite materials. The margins in the suggested GE values found here attempt to take this fact into account. 6. The equations as presented are only valid in the Troposphere (under 36,000 ft. or so). It would be relatively straight forward to use a more sophisticated atmospheric model that handles higher altitudes. A good starting point for this endeavor might be found here15 or here16. On the other hand, this effort is likely unwarranted. First, Martin’s analysis tells us that fin flutter is most likely to occur in dense air at transonic speeds. Second, Vf increases as we gain altitude because air temperature and air pressure decrease. For example, if we use NASA’s RocketModeler high altitude temperature and pressure models17, our example rocket (with 3/16” fin thickness) has a predicted Vf of 5025 ft/sec at 50,000 ft, 9157 ft/sec at 75,000, and 16,726 ft/sec at 100,000 ft. Unless we are well into the supersonic range (where many other issues are likely to control rocket design decisions), fin flutter is not likely to be a problem at high altitude. Finally, Martin’s methods were only validated to about Mach 1.5, so if we want to design a hypersonic rocket, we need to be using more sophisticated design tools. 7. This method for fin analysis does not address the need for strong fin attachment, but any attachment method will benefit from not being stressed by fin flutter. Issue 615 / December 19 th, 2023 Fin Flutter Analysis Revisited (Again) By John K. Bennett 1 https://www.apogeerockets.com/education/downloads/ Newsletter291.pdf 2 https://www.apogeerockets.com/education/downloads/ Newsletter411.pdf 3 NACA TN 4917 - Dennis J. Martin, Summary of Flutter Experiences as a Guide to The Preliminary Design of Lifting Surfaces on Missiles, February 1958. 4 Joseph R. Simmons III, Aeroelastic Optimization of Sounding Rocket Fins, Air Force Institute of Technology, 6-9-2009 5 A. Iliopoulosa, J. Steubenb, and J. Michopoulosb; Determination of Anisotropic Mechanical Properties of G-10 Composite via Direct Strain Imaging, 2005, https://www.sciencedirect.com/science/article/abs/ pii/S0142941815301094 6 https://pdf4pro.com/cdn/g-10-fr-4-g-11-glass-epoxy-dielectric-corp4d23a7.pdf 7 https://www.usplastic.com/knowledgebase/article. aspx?contentkey=563 8 M. B. Kasen, G. R. MacDonald, D. H. Beekman, Jr., and R. E. Schramm, Mechanical, Electrical, and Thermal Characterization of G-10CR and G-11CR Glass-Cloth/Epoxy Laminates Between Room Temperature and 4K, National Bureau of Standards, Boulder, Colorado 9 http://k-mac-plastics.com/data-sheets/fiberglass_technical_data. htm 10 David S. Steinberg, Vibration Analysis for Electronic Equipment, WileyInterscience; 3rd edition, January 15, 2000, p. 131 11 https://apps.dtic.mil/sti/pdfs/ADA470630.pdf 12 https://www.imeko.org/publications/ysesm-2014/IMEKOYSESM-2014-032.pdf 13 K. Ravi-Chandar and S. Satapathy, Mechanical Properties of G-10 Glass–Epoxy Composite, Institute for Advanced Technology, UT Austin, IAT.R 0466 14 RockSim predicts slightly lower apogee and Vmax for same rocket and launch conditions. 15 https://en.wikipedia.org/wiki/International_Standard_Atmosphere 16 https://en.wikipedia.org/wiki/U.S._Standard_Atmosphere About the Author John Bennett is an engineering Professor Emeritus at the University of Colorado Boulder, and a TRA and NAR member in OregonRocketry. Using Brinley’s book as a guide, he built and flew many Zn/S and KN03/sugar rockets as a teenager. In retirement, he is now rediscovering his love of amateur rocketry, this time with better supervision. 9 Issue 615 / December 19 th, 2023 Fin Flutter Analysis Revisited (Again) By John K. Bennett 10 Issue 615 / December 19 th, 2023 Triffid Rocket Plan By Steve Riegel Apogee Quantity Part # to Order 1 10063 Item AT-13x18” (BT-5) 10086 1 AT-18x18” (BT-20) 10100 10164 13028 1 1 1 13032 1 13052 14097 14263 19997 29124 30325 1 1 3 1 AT-24x18” (BT-50) AT-56x18” (BT-70) Centering Ring 13mm to 18mm Centering Ring 18mm to 24mm 1/8” launch lug 3/32” x 3” x 18” balsa 1/8” x 2-12” dowel VFNC-24B Cut Size & Quantity 3 x 1/4” rings cut in half 16” and 6 x 2-1/2” 7” 3 x 1” 1 needed 2 needed 2 x 1/2” 2” 1 needed 1 Plastic Parachute 12” 8 Kevlar cord (100 lb) 8’ 11 Issue 615 / December 19 th, 2023 Triffid Rocket Plan By Steve Riegel Miscellaneous Parts to Build: Laminated posterboard (3 layers bonded with white or wood glue) for forward fin set (see pattern piece) 1/4” wide segments of BT-5, cut into two half-rings and glued one inside the other for laser cannon details 3/32” balsa stock for laser fin set (see pattern piece) and launch lug standoffs (1/2” x 1/4”) 3 x 1/8” x 2” wooden dowels for laser cannons Cardstock strips, 1/2” wide, for “sleeve” on laser cannons Cardstock shroud – AT-18 (BT-20) to AT-24 (BT-50), 1” long (see RockSim file - download at https://www. ApogeeRockets.com/Peak-of-Flight-Rocket-Plans) Paper shock cord mount (basic trifold mount from bond paper) Construction: 1. Cut a 16” long piece of BT-20 tube. Glue one centering ring 1” from one end, the other flush with the same end of the tube. There should be approximately half an inch of space between the rings. 2. Fold one end of the Kevlar cord into the shock cord mount and glue it into the same end of the BT-20 tube as the centering rings. When the glue is dry, stuff the Kevlar cord into the tube. 3. Glue the centering rings into the 7” section of BT-50 so that the aft edge of the rear ring is flush with the end of the BT-50 back edge. 12 4. Roll the cardstock shroud and fit it into place at the joint between the BT-50 and BT-20 tubes. 5. Gently blow into the rear of the rocket to push the Kevlar cord out the front end. Affix the cord to the nose cone. Connect the parachute to the shock cord. 6. Apply glue inside the aft end of the BT-20, insert the motor block, and use an old motor to push it into place. Leave 1/4” of the motor casing exposed. Quickly remove the motor casing from the body. Let the motor block dry. 7. Use the marking guide to mark the aft end of the BT20 with six marks for the tube fins. Extend the lines 11” from the rear of the body. 8. Glue the six BT-20 tube fins 1/4” up from the rear of the BT-20 along the lines from the previous step. 9. Cut three 1” wide rings from the BT-70. Glue these flush with the front end of the BT-20 tube fins in a triangular arrangement at 1/3 intervals around the body. 10. Cut out the laser cannon fins from 3/32” balsa stock. Bevel the root edge of the fin to fit neatly into the joint between two of the BT-20 tube fins. Round the leading and trailing edges. Leave the tip square. 11. Cut the 1/8” dowel into three 2”-long pieces. Using 1/2” wide strips of cardstock, wind and glue a sleeve around the dowel between 1/2” and 1” from one end. Keep winding until the sleeve is about 3/16” wide. 12. Cut the three BT-5 rings then cut the rings in half. Issue 615 / December 19 th, 2023 Triffid Rocket Plan By Steve Riegel Glue the two halves together and drill a 1/8” hole in the center of each. Glue this half ring to the laser cannon dowel on the 1/2” segment of the dowel so that the part sits against the the end of the sleeve. 13. Glue the laser cannons to the fin tip so that the 1” section of the dowel is attached to the tip and the sleeve and half-round detail of the cannon extend forward of the long, straight leading edge of the fin. 14. Glue the laser fins into the joints between the BT-20 tube fins so the rear of the fin root is flush at the rear of the tube fins. You should now have alternating BT70 rings and laser fins. Ensure the laser fin is evenly spaced between the ring fins. 15. Cut out the forward fin set from laminated cardstock. The long edge is glued to the body and the short edge is glued inside the BT-70 ring. Add glue fillets to the root and inside the BT-70 rings. 16. Cut two launch lug standoffs 1/2” x 1/4” from the 3/32” balsa stock. Glue a 1/2” piece of 1/8” launch lug to the top of each standoff. Glue the standoffs to the body along a tube fin line. The forward standoff is 5-1/4” and the aft standoff is 10-1/2” below the base of the conical transition. Ensure the two launch lugs are aligned and straight. About the Author Steve Riegel is a high school chemistry teacher, retired Air Force officer, amateur astronomer, and NAR Level 2 certified rocketeer. When not teaching, he enjoys adding steampunk details to model rockets and turning masses of perfectly fine fabric into tiny snips and shreds on his basement floor. He is a member of the Colorado Springs Rocket Society (COSROCS), NAR Section 515, and the Southern Colorado Rocketeers (SCORE), NAR Section 632. He lives in Colorado Springs with his wife, Jennifer, and dogs Lyra and Loki. 13 Print patterns and decals at 100% Make fins from 3 layers of poster board, laminated together. Wood grain direction P tion ansi r T r ape 1” Overlap Tube Tube Tube Forward Fin Launch Lug Forward Fin AT-18 Triffid Marking Guide Tube Tube Tube 14 Forward Fin Make from 3/32” thick balsa wood 15 A A 1/8"x2" Dowel Cardstock Sleeve 16in [406mm] AT-13 Half-Ring Detail Shock cord mounting point 1in [25mm] SECTION A-A SCALE 1:1 2.5in [64mm] 2.5in [64mm] 1in 1in [25mm] 7in [178mm] 10.5in [267mm] 5.25in [133mm] 14in [356mm] 16