Reference / Paper · 2003
Peak of Flight #104: How to design and build engine mounts
Apogee Components' Peak of Flight newsletter, issue 104 (2003).
- peak-of-flight
- apogee-components
- model-rocketry
Reference / Paper · 2003
Apogee Components' Peak of Flight newsletter, issue 104 (2003).
Apogee Components' Peak of Flight newsletter, issue 104 (2003).
N I S S U E 1 0 4 , E S W L J U N E E 1 , 2 0 0 3 T T E R ROCKSIM 7.0 How To Design & Build Engine Mounts 1130 Elkton Drive, Suite A Colorado Springs, CO 80907 USA www.ApogeeRockets.com orders @ApogeeRockets.com phone 719-535-9335 fax 719-534-9050 Page 1 P E A K O F F L I G H T How To Build Engine Mounts By Tim Van Milligan As you may have heard me mention in the past, many of my ideas for newsletter articles come from questions that people email me. Whenever I get the same question more than once, I figure that there must be other people that would like to know the same thing too. So instead of repeating myself over and over, I just turn the “answer” into an article for this e-zine. So when I get the same question again in the future, I’ll be able to say: ‘go look up the answer in the newsletter issue on the Apogee web site.’ This weeks article is an answer to the question: “how do I build my own engine mounts?” If you’ve been in rocketry for a while, the information in this article will be fairly old news. So please bear with me. And besides, sooner or later, you’re going to get the same question from a junior rocketeer; so you’ll now be able to point him/her to the information in this issue. I like to start these “basics-of-rocketry” articles by plugging my book: Model Rocket Design & Construction. This information, along with tons of other hints, tips, and ideas is found in that book. So if you don’t already own a copy of your own, you can get one from the Apogee web site. The direct link to the book is: http://www.ApogeeRockets.com/ design_book.asp What is an Engine Mount? The definition from my design book is that “an engine mount holds the rocket engine firmly in place inside the model and aligns it concentrically with the centerline of the model.” The engine mount is critical to successful operation of the rocket. If the engine were to come free it could fly through the rocket during its thrust phase and destroy the interior of the model, or it might be kicked out of the rear of the model at ejection. It the latter case, the recovery device wouldn’t be deployed, and the model would come back down to earth with the same velocity that it went up. Also, if the engine isn’t concentric with the centerline of the model, the off-axis thrust line will cause the model to do flip-flops during the thrust phase of the burn. This is shown in Good alignment Poor Construction Figure 1: The engine mount must be constructed so the thrust is straight with the centerline of the model. Otherwise the rocket will flip-flop across the sky. Canted thrust is a result of nonconcentric centering rings (right side). figure 1. Typically, if you building a rocket from a kit, the rings supplied with the model are concentric. It is only when you cut your own rings that you have to be very careful to make sure the rings are perfectly centered. See Figure 6. This article will be about building engine mounts that fit into larger diameter tubes. When the rocket motor just barely fits inside the tube, the rocket is called a minimum-diameter model. The topic of securing motors in minimum-diameter models was covered in three issues of this e-zine newsletter (#95, #96, & #97). Those issues can be downloaded from the Apogee web site at: http://www.ApogeeRockets.com/education/newsletter_archive.asp The Basic Engine Mount Figure 2 shows the construction of the most common type of engine mount. It consists of just four parts: 1. A paper tube (called the engine mount tube) into which the rocket motor is inserted. 2. Two centering rings that hold the engine mount tube inside the body of the rocket. (Continued on page 3) About this Newsletter You can subscribe “FREE” to receive this e-zine at the Apogee Components web site (www.ApogeeRockets.com), or sending an email to: [email protected] with "SUBSCRIBE" as the subject line of the message. A P O G E E Page 2 R O C K E T S PAGE 2 P E A K O F Engine Mounts Continued from page 2 Engine Block Engine Mount Tube Centering Ring Engine Hook Figure 2: the basic engine mount. 3. An engine block (a paper ring that is glued into the front end of the engine mount tube), that prevents the rocket motor from slipping through the inside the model. 4. Metal Engine Hook. The engine hook is just a bent piece of metal. They are F L I G H T popular because they allow you to quickly change out rocket motors between flights. Quest, Estes, and Aerotech all use flat strips of metal. But you can easily make your own from music wire (spring steel) that you can buy from most well-stocked hobby stores. To assemble the basic engine mount, first cut a slit in the tube at the furthest forward location of the engine. Make sure the engine will extend out the back end of the tube a short length (1/2 inch). Insert one end of the hook into the slit, and secure it temporarily with a piece of masking tape. The engine block should next be glued into the front end of the tube using wood glue. Position it so that it butts right up against that little portion of the engine hook that is inserted into the tube. Finally, the two external rings are glued onto the outside of the engine tube. I like to position the front ring so that its edge is even with the front of the engine tube. But this is optional. If you do decide to put it at the front of the tube, you will need to hold the engine hook in place with seveal wraps of masking tape (similar to what is shown in figure 5). The aft ring is not even with the back edge of the tube. It should be positioned about 1/2 inch forward of the end of the tube. Otherwise, you won’t be able to bend the engine hook far enough to insert the rocket motor. When all the glue on the rings is dry, you can now glue the whole assembly inside the back end of the rocket (Figure 3). Again, use wood glue. The distance the engine mount is inserted into the back end of the rocket will depend on the outer diameter of the tube. Why? Because you need room to bend the metal engine hook back to insert the motor. On small diameter rockets, you’ll need to allow the engine mount tube to stick out the back a little bit. On large diameter tubes, you can have the end of the engine mount tube even with the back end of the rocket. Again, the location of the engine mount depends on whether or not you can bend the engine hood back far enough to insert the rocket motor. (Continued on page 5) Long Centering Ring Figure 3: The distance the mount is inserted into the rocket depends on the engine hook. Make sure the hook can be bent far enough to insert the rocket motor. A P O G E E Figure 4: A lot of beginner kits use a single ring. It is heavier, but quicker to assemble. R O C K E T S Page 3 RockSim 7.0 — Software That Lets You Go Beyond 4FNC* Shapes Design rockets that are not possible with other computer programs - even RockSim v6! Yes! RockSim, the most trusted software for designing rockets, and finding out how they'll fly, is now even more accurate, and is packed with more advanced features. Here's the new ones: o-use. It is Easy-t Non-Symmetrical Fin Positions - Each fin can be placed on the rocket seperately. If you want, you can even create airplane-looking designs! New Realistic Atmospheric Conditions gives you greater simulation accuracy. For example, the wind speed can be set to vary during the flight so that you get a better landing zone footprint. It now also includes thermals and descending air (called "sink") to allow your to really determine if the parachute is too big or too small. NEW .0 Version 7 sed! a le e R st Ju ns Come Alive! ur Rocket Desig Sim 7: Makes Yo Rock With the new random atmospheric conditions feature, you can now simulate NAR type contests: Parachute and Streamer Duration (yes — you can drift the rockets away, just like in real life), Altitude events, and Spot Landing. This was so much fun, that we've added a competition feature that will let you run all types of virtual contests, including egglofting (don't crack your egg), and the "drag race" event. This finally turns RockSim into a entertaining game as well as being a scientific research tool. So if the weather outside is too bad to fly real rockets, you can still fly them on your computer. Go to the Apogee web site for further details or to order your own copy! 1130 Elkton Drive, Suite A Colorado Springs, Colorado 80907-8501 USA Realistic Atmospheric Conditions Tel: (719) 535-9335 email: [email protected] Web Site: www.ApogeeRockets.com *4FNC — "Four Fins & a Nose Cone." A generic term applied to everyday, ordinary looking rockets. You're not an ordinary person, so why should you be limited to designing ordinary rockets? Page 4 P E A K O F F L I G H T Engine Mounts Continued from page 3 Variations on the Basic Design Once you have a grasp of the basic design and assembly of an engine mount, you can start making modifications to the design. The first modification is to use a single long centering ring instead of two shorter ones. See Figure 4. A lot of the beginner level rocket kits from Estes use this modification that I call a four-piece engine mount. The advantage is that it makes production of the kit for the manufacturer easier. Why? Because the minimum-wage worker on the assembly line only has to put one part into the plastic bag instead of two pieces. You might not think this is a big deal, but it is to the manufacturer. If the worker forgets one ring, the cost of sending out a replacement ring to the customer eats up all the profit the original kit’s sale. Another advantage is that it makes assembly for the buyer easier. He only has to glue one ring onto the tube. The disadvantage of the larger centering ring is “weight.” One big/long centering ring weighs a lot more than two short ones. The extra weight makes the rocket fly slower and lower. But for someone just getting into rocketry, this doesn’t matter much. A more common modification to the basic design is to use paper disks instead of thick rings. See Figure 5. Figure 6: You can cut your own rings from cardboard some thick cardboard and cut them out as shown in Figure 6. Finding cardboard is probably the hardest step in the process. But you can find some around home - if you eat breakfast cereal. The cardboard boxes can be recycled into rings for your next rocket. To make a centering ring, accurately measure the inside diameter of the large tube and the outside diameter of the small tube. Take a drawing compass and create two concentric circles of the diameters you just measured (remember to divide the diameter by 2, because you will be using it to set the radius on (Continued on page 5) Airframe Tube 24/18 Length (inches) - The length of the tube as sold. Tells what the part's primary use is for Masking Tape Diameter (mm) - Specifically, it tells what size engine fits into this tube. In this example, a "D" or "E" size motor will fit into the tube. The description Apogee Components uses for its tubes makes it easy to select the correct size. Disk-type Centering Rings Figure 5: Rings made from cardboard disks are typically used in larger diameter rockets. There are two reasons to switch to cardboard disks. First, the disks cut the weight down tremendously. Second, for small engine mounts in large diameter rockets, you really have no choice but to use disks. Thick paper rings just don’t exist unless you want to make your own (see the book Model Rocket Design & Construction on how to do this). It is usually much easier to make your own disks. Making your own paper disk rings is pretty easy. Just get A P O G E E Tube Description Apogee Part No. I.D. (in.) O.D. (in.) Airframe Tube 10.5/18 10048 .422 .446 Airframe Tube 13/18 10062 .518 .544 Airframe Tube 18/18 10085 .710 .736 Airframe Tube 24/18 10099 .950 .976 Airframe Tube 29/13 10110 1.140 1.180 Use this chart to help you choose the correct size tubes and to help you create your own rings. R O C K E T S Page 5 P E A K O F F L I G H T Engine Mounts Continued from page 3 the compass). Make two identical rings and cut them out using a hobby knife as shown in Figure 6. Glue the rings to the engine tube as shown in Figure 5. You may need to make a notch for the engine hook if the rings are near the ends of the tube. If you don’t have a ring to hold the engine hook down, it can be adequately secured by wrapMasking Tape Masking Tape Disk-type Centering Rings Figure 8: You can eliminate the metal engine hook by wrapping tape around the tube where the motor protrudes. Figure 7: By putting a tube coupler (the green part) between the disk-type centering rings, you’ll find it is easier to insert the mount into the body tube. ping several layers of masking tape around the middle of the hook. If you have a tube coupler that will fit into the larger tube, you can glue your centering rings to it, as shown in Figure 7. This will make it easier to slide the rings into the larger tube. Bye-Bye Engine Hook When people come to the Apogee web site looking for parts, and they don’t find metal engine hooks, they seem to get a little frantic. Not to worry... you don’t really need one. If you leave off the engine hook, you can simply secure the motor into the tube by wrapping the end with tape. Does this work? Yes! It works great. Many high power rocket use A P O G E E Page 6 this technique all the time. So if it works with bigger motors, it will work just as well with small ones too. There are other ways to restrain the motor in the engine mount. But I don’t want to give everything away here in this article. You can find those other methods (including methods for large rocket motors) listed in the book Model Rocket Design & Construction. About the Author: Tim Van Milligan is the owner of Apogee Components (http://www.apogeerockets.com) and the curator of the rocketry education web site: http://www.apogeerockets.com/education. He is also the author of the books: Model Rocket Design and Construction, 69 Simple Science Fair Projects with Model Rockets: Aeronautics and publisher of the FREE e-zine newsletter about model rockets. You can subscribe to the ezine at the Apogee Components web site, or sending an email to: [email protected] with "SUBSCRIBE" as the subject line of the message. R O C K E T S