Reference / Paper · 2019
Peak of Flight #496: What Size Parachute Should I Use?
Apogee Components' Peak of Flight newsletter, issue 496 (2019-05-28).
- peak-of-flight
- apogee-components
- model-rocketry
Reference / Paper · 2019
Apogee Components' Peak of Flight newsletter, issue 496 (2019-05-28).
Apogee Components' Peak of Flight newsletter, issue 496 (2019-05-28).
ISSUE 496 May 28th, 2019 IN THIS ISSUE What Size Parachute Should I Use? www.ApogeeRockets.com/Rocket-Kits/Skill-Level-3-Model-Rocket-Kits/Skonk-Wulf What Size Parachute Should I Use ? By Tim Van Milligan Assumption 2 - Standard Atmosphere Day This is a common question we’re asked here at Apogee Components. The answer depends on a lot of variables, which makes it difficult for us to give customers a specific answer when they ask what size parachute should I use. When NASA makes up the charts that give the density of the air, they have to assume some consistent weather conditions. For example, the temperature of the air is around 59°F. That is cool for a hot summer day, but warm if you’re flying your rockets in the winter time in Michigan. Weather conditions, like temperature, affect the flight by changing the air density. On cold days the air is denser, and it makes your rocket fall slower. On a hot day, the air density is reduced and the rocket will fall faster towards the ground. Assumption 3 - Launching at Sea Level Figure 1: Basic hexagon parachute We’ve written about this subject a number of times in this newsletter, going over the specific variables that have to be taken into account (see the references at the end of this article). But most modelers don’t know the conditions they are going to fly their rockets in, and are just looking for a simple answer. Because of that, in this article I’m going to plug in some simple assumptions on the conditions for the rocket, and then give you some look-up charts that will give you a rough guesstimate on the size of the parachute you’ll need for your rocket. The charts shown here are based on these simplifying assumptions: Assumption 1 - The Drag Coefficient of the Parachute is 0.75 The big variable for parachute performance, as you can tell by this assumption and the previous one, is about the density of the air. The launch elevation affects the density as well. At sea level, air is pretty thick. At higher elevations, like here in Colorado, the air is thinner and the parachute will fall faster. I did a quick calculation of the density of the air where I was launching rockets yesterday in Hartsel, Colorado. It is a launch site that is at 8,774 feet in elevation. With the actual weather conditions taken into account (53°F temperature, Relative Humidity of 21%, and a barometric pressure of 30.1 inches of Mercury), the density of the air was less than half that of sea level. That makes a big difference, and the rockets fall a lot faster to the ground. You can tell the air is thin when you’re in the mountains of Colorado. Especially when you start running after your rocket and you feel out of breath a lot sooner that you would if you were in Kansas. Assumption 4 - You have an average strength rocket How strong is your rocket? This is a question nobody knows the answer to, because the only way to find out how The drag coefficient is a dimensionless number that tell strong it is, is to drop it hard onto the ground until you break something off. us how efficient a parachute is at creating drag, which is Nobody wants to break their rocket, so they don’t do this the force that is trying to counteract gravity as the rocket test. Never have I had a person answer this question. is coming down. But it is probably the most important question you should Some parachute shapes are more efficient than others, answer. Basically, it is how high can you drop your rocket and hence they create more drag for the surface area of and not have anything snap off. canopy. But here we’re assuming that these are average The charts on pages 4-5 show the average descent flat sheet parachutes with lines attached at the corners of speeds: slow versus fast. You’ll have to pick one. the sheet. For slow descent, we’ll assume that your rocket can surIf you believe your parachute is more efficient, than vive a fall to the hard ground without breaking from a height you can use higher weights for your descent mass as of two feet. We’ll define a fast descent as being able to surshown in the charts on pages 4-5. vive a fall from three feet above the ground. About this Newsletter You can subscribe to receive this e-zine FREE at the Apogee Components website www.ApogeeComponents.com, or by clicking the link here Newsletter Sign-Up Page 2 Newsletter Staff Continued on page 3 Writer: Tim Van Milligan Layout/Cover Artist: Chris Duran Proofreader: Michelle Mason Issue 496 | May 28th, 2019 What Size Parachute Should I Use ? Continued from page 2 For example, imagine your rocket is sitting on your desk, and you knock it hard, and it flies off the surface and lands on the floor. If it survived without breaking off a fin, then it can withstand a “fast descent” (a speed of about 4.5 m/s). If you don’t think it could survive the fall from the table, then you should say that your rocket has to land with a slow descent speed. We would use the larger size, even though it will make the rocket descend slower. We can always reef the parachute or cut a spill hole into it later if we think it is coming down too slow. You can always use techniques like that to make a chute fall faster, but you’ll never be able to make it fall slower. So start with a chute that is slightly bigger than you think you need. Example on Choosing a Parachute How to use the charts • Step 1 - Find the chart that corresponds to the shape of your parachute. If you are using a hexagon shaped parachute, find the table that lists hexagon parachute. • Step 2 - Weigh your rocket. Use a gram scale. Ideally you want the descent weight, which is with the rocket motor installed, but all the propellant is consumed. Use a spent (empty) motor casing if you have it. If you don’t have an empty motor case, don’t worry about it. Use whatever weight you have. These charts are for making a first guess. It will probably work close enough for you to get your rocket back without too much damage. • Step 3 - Decide if your rocket is weak or strong. If it is a weak rocket (has big fins that are just surface mounted to the tube), then you should be selecting a slow descent speed. If it can survive a higher fall without breaking, then you can select a size where the descent speed is higher. • Step 4 - Once you’ve decided on a slow or a fast descent speed, you can go down the column and look for a weight that is close to what your rocket is. Obviously, it won’t be exact. You’ll probably have a choice between two sizes of parachutes. That is to be expected. • Step 5 - If you have to pick between two chute sizes, opt your decision toward the larger parachute. For example, say you have a rocket that weighs 118 grams with a spent motor installed. You look at what you have in your range box, and all the chutes are a hexagon shape. You assume your rocket is fairly fragile because it has really big fins that don’t have big fin fillets on them. You would then go to the hexagon chart and look at the “slow speed” column. Unfortunately, you don’t see a number that is exactly 118 grams in the “slow descent” column. You do see a 104 grams for the 18-inch parachute, and then below it, you see 184 grams for the 24-inch parachute. Which do you use? I would suggest the 18-inch parachute. Here is my reason: The 15-inch parachute can handle a 118 gram model, but the descent rate is close to the high speed. On the 18inch, a mass of exactly 104 would give you a slow descent speed. So 118 grams would probably come down slightly faster, but not even close to the fast descent speed. What would happen if you went to the next size up, which is the 24-inch parachute? Basically, it would be coming down at a slower speed and really drift a long way with the wind. The whole purpose of these charts is to get a safe landing speed without having to walk so far to retrieve it. Test Fly For most modelers, as long as the rocket wasn’t lost (drifted away) and it doesn’t have damage, we’d consider that a successful flight. That means you’ve picked a good size parachute. Continued on page 4 www.apogeerockets.com/Electronics-Payloads/Rocket-Locators/Simple-GPS-Tracker Page 3 Issue 496 | May 28th, 2019 What Size Parachute Should I Use ? Continued from page 3 From this point, you can decide to tweak the size of the parachute for the rocket if you want. Most people don’t though. And that is OK. If you got your rocket back in one piece, all is good. But if you decide you want to optimize the parachute size for the model rocket you’re flying, we have other articles on our web site that will guide you through the process of refining your choice. See the references at the end of this article. Circular Parachute Selection Chart Diameter Diameter (inches) Diameter (m) Area (m2 ) Mass (Slow Descent) Max. Mass - grams (Fast Descent) 8 0.20 0.0324 19 31 12 0.30 0.0730 41 69 15 0.38 0.1140 65 108 18 0.46 0.1642 94 156 24 0.61 0.2919 167 277 30 0.76 0.4560 262 432 36 0.91 0.6567 377 623 42 1.07 0.8938 512 848 48 1.22 1.1675 670 1107 58 1.47 1.7046 978 1616 72 1.83 2.6268 1506 2491 Figure 2: Circular parachute chart Hexagon Parachute Selection Chart Diameter (m) Area (m2 ) Mass (Slow Descent) Max. Mass - grams (Fast Descent) Page 4 Diameter Diameter (inches) Diameter (m) Area (m2 ) Mass (Slow Descent) Max. Mass - grams (Fast Descent) 8 0.20 0.0358 20 32 12 0.30 0.0805 44 73 15 0.38 0.1257 69 114 18 0.46 0.1810 99 164 24 0.61 0.3218 177 291 30 0.76 0.5029 275 456 36 0.91 0.7241 397 657 42 1.07 0.9856 541 894 48 1.22 1.2873 706 1168 58 1.47 1.8796 1031 1705 72 1.83 2.8964 1589 2627 Figure 4: Octagon parachute chart Continued on page 5 You get: (4) AT 29/13 (4) AT 41/18 (2) AT 56/18 (2) AT 66/18 (1) AC-56 (1) AC-66 Diameter Diameter (inches) 8 0.20 0.0358 21 34 12 0.30 0.0805 46 76 15 0.38 0.1257 72 119 18 0.46 0.1810 104 172 24 0.61 0.3218 184 305 30 0.76 0.5029 288 477 36 0.91 0.7241 415 687 42 1.07 0.9856 565 935 48 1.22 1.2873 738 1221 58 1.47 1.8796 1078 1782 72 1.83 2.8964 1662 2747 Figure 3: Hexagon parachute chart Octagon Parachute Selection Chart You get: (6) AT 13/18 (6) AT 18/18 (6) AT 24/18 (6) AT 33/18 www.ApogeeRockets.com/Building_Supplies/Body_Tubes www.ApogeeRockets.com/RockSim/RockSim_Information Issue 496 | May 28th, 2019 What Size Parachute Should I Use ? Continued from page 4 The Vortex Ring Decelerator In Peak-of-Flight Newsletter #492: Build Your Own Helicopter Parachute - Part 1, Dave Flanagan created plans for a new type of parachute called the Vortex Ring Decelerator. It is designed to rotate like a helicopter as it falls. The theory is that it produces more drag than a chute that doesn’t rotate. I really liked the design and the simplicity of the pattern. So I asked Pat Butler at Dino Chutes to make me one out of nylon cloth to test out. Vortex Ring Decelerator Parachute Selection Chart Diameter Diameter (inches) Diameter (m) Area (m2 ) Mass - grams (Slow Descent) Max. Mass - grams (Fast Descent) 8 0.20 0.020 25 41 12 0.30 0.042 52 86.6 15 0.38 0.066 82 136 18 0.46 0.095 118 196 24 0.61 0168 209 346 30 0.76 0.286 357 589 36 0.91 0.411 512 848 42 1.07 0.486 606 1002 48 1.22 0.634 790 1307 58 1.47 0.926 1154 1908 72 1.83 1.427 1779 2941 Figure 6: Vortex Ring Decelerator parachute chart Figure 5: Vortex Ring Decelerator parachute I’ve flown it four times, and every time it has worked and people have marveled at how cool it looks at it comes down. Based on that, I knew it would be a cool product, so I’ve ordered a whole bunch from Dino Chutes and we’ll be selling them in the ApogeeRockets.com web store. I also flew a Jolly Logic AltimeterThree (https://www. apogeerockets.com/Electronics-Payloads/Altimeters/JollyLogic-AltimeterThree) in the rocket with the parachutes to find the descent rate of the Vortex Ring Decelerator. I followed the procedure of determining the Cd of the parachute that was written about in Peak-of-Flight Newsletter #449: How to Determine the Cd of a Parachute. I was surprised to see that the Cd was pretty high. It came in at 1.63. This confirms what Dave Flanagan said, which is that it is about double that of a regular model rocket parachute. What this means is that the Vortex Ring Decelerator style parachute can carry a heavier load for the same descent speed! And the canopy has less surface area, so it can be put into a smaller space inside the rocket. While Vortex Ring Decelerator chutes are more efficient, I did find them to be tricky to prep for flight because they don’t fold as easily. Because of this, I don’t think regular parachutes will go away anytime soon. “Convenience” plays a big role in selecting a parachute too. References: Properly Sizing Your Rocket’s Parachute - https://www. apogeerockets.com/education/downloads/Newsletter149.pdf Everything You Wanted To Know About Parachutes - By Dave Virga https://www.apogeerockets.com/education/ downloads/Newsletter184.pdf Sport Parachuting Technology Applied to Rocketry - Annette Sostarich https://www.apogeerockets.com/education/ downloads/Newsletter279.pdf Continued on page 6 www.apogeerockets.com/Rocket-Kits/Skill-Level-2-Model-Rocket-Kits/SkyMetra Page 5 Issue 496 | May 28th, 2019 What Size Parachute Should I Use ? Continued from page 5 Simulating Cluster Parachutes in RockSim - https://www. apogeerockets.com/education/downloads/Newsletter354. pdf Selecting the Proper Size Drogue Parachute - https://www. apogeerockets.com/education/downloads/Newsletter361. pdf How to Determine the Cd of a Parachute - https://www. apogeerockets.com/education/downloads/Newsletter449. pdf Build the Vortex Ring Decelerator Parachute - https://www. apogeerockets.com/education/downloads/Newsletter492. pdf Air Density Calculator - https://www.engineersedge.com/ calculators/air-density.htm About The Author: Tim Van Milligan (a.k.a. “Mr. Rocket”) is a real rocket scientist who likes helping out other rocketeers. He is an avid rocketry competitor and is Level 3 high power certified. He is often asked what is the biggest rocket he’s ever launched. His answer is that before he started writing articles and books about rocketry, he worked on the Delta II rocket that launched satellites into orbit. He has a B.S. in Aeronautical Engineering from Embry-Riddle Aeronautical University in Daytona Beach, Florida, and has worked toward an M.S. in Space Technology from the Florida Institute of Technology in Melbourne, Florida. Currently, he is the owner of Apogee Components (www.apogeerockets.com) and also the author of the books: Model Rocket Design and Construction, 69 Simple Science Fair Projects with Model Rockets: Aeronautics and publisher of the “Peak-of-Flight” newsletter, a FREE ezine newsletter about model rockets. You can email him by using the contact form at https://www. apogeerockets.com/Contact. www.ApogeeRockets.com/Building_Supplies/Parachutes_Recovery_Equipment/Parachutes High Power Nose Cones • MONSTER Nose Cones from LOC-Precision • Durable Heavy-Duty Plastic • Fits Standard LOC Tube and Blue Tube • Get That Big Project Off The Ground • Affordable! www.ApogeeRockets.com/Building_Supplies/Nose_Cones/ Page 6 Issue 496 | May 28th, 2019