Put all this together, with plenty of epoxy and a handful of bolts, and you get Nordri! Challenges we faced building NordriĪfter building 6 different iterations of Nordri, we know a thing or two about putting it together! Some of our biggest issues were: Inaccuracies in our measurements and manufactured parts This is a removable part that keeps the motor and casing inside the rocket while it's not firing. These allow us to bolt on the last component - the motor retainer (pictured above), also machined on a lathe. Just beneath the thrust ring, inside the lower assembly between the laser cut centring rings, there are 2 nuts mounted. The ring is epoxied to the bottom of the lower assembly and the outer tube, so it can evenly transfer this force into the main body of the rocket. The motor casing (off-the-shelf enclosure for the motors we use) sits on this thrust ring and transfers all the force from the motor's combustion into the ring. Our custom laser cut centring rings slip over the end of the fins, which isn't a structural design feature, but really helps us line up all the components perfectly before they're epoxied togetherĪt the base of the assembly, we have an aluminium thrust ring (above) that we machined on a lathe. This means the fins go through the outer body tube and attach to both the inner motor tube and the outer tube with epoxy fillets. While some model rockets can get away with just glueing the fins to the outside of the tube, we went for integrally mounted fins. This is the part that holds the motor, is where the fins attach and is by far the most complex component. The really meaty part of the rocket is the lower assembly (shown above). In the early days, we added masking tape to or sanded down these shoulders to get them to the perfect size, but after enough trial and improvement, we've managed to start printing them perfectly to tolerance so they can immediately slot in and fly. This has to be a very clean fit, as not only does the nose cone need to stay in the tube during flight (not too difficult with the air resistance of the launch), but it also needs to be able to pop out when the ejection charge goes off so that the parachute can deploy. Nordri's Nose cone is 3D printed from PLA plastic, with 2.5mm thick walls and a 50mm shoulder that slots securely into the body tube. While our rockets happily endure upwards of 60G in some of our launches, you may be surprised to learn that Nordri's body is made out of a plain old cardboard tube! We like to use postal tubes as they are cheap and easy for us to acquire, and are plenty strong enough for the job. Once you've satisfied these basic requirements, provided you stick to a fairly standard rocket shape, you're free to do what you want! In Nordri's case, we use quite powerful motors that require a certification to buy and fly, so we've gone to quite some lengths to ensure that she'll stay together under the intense forces of a launch with strong materials and meticulous reinforcement. The common wisdom is to stick somewhere from 1.5-2.5 calibres for a simple design, but of course, there are all sorts of weird and wacky rockets that don't stick to these principles, like this design called the "Foo Fighter". A Calibre is a relative unit, equal to the diameter of the main rocket body. In model rocketry, the distance between these two points is commonly referred to as the stability margin of the design and is measured in _calibres_. You'll see that the Centre of Mass is more towards the front of the rocket, and the Centre of Pressure is further back. Here's our design for Nordri in OpenRocket, with the Centre of Mass and Centre of Pressure labelled: This means the rocket can correct itself in flight if it swings out of alignment, forcing it to point in the direction of travel. In order to make a stable (and safe) rocket, it's important to keep the Centre of Pressure _behind_ the Centre of Mass. This is a measure of where aerodynamic forces act upon the rocket and is determined entirely by the shape of the rocket and its fins. The other key property is the Centre of Pressure. You're likely already familiar with the concept of Centre of Mass in our case, it's the point through which we model forces on the rocket - for example if we exert a force on the fins of the rocket while in flight, we would see it rotate around its Centre of Mass. When designing your own rocket, there are two fundamental properties that you need to be sure to get right - the Centre of Mass, and the Centre of Pressure. We'll also touch on how you can get started in rocketry yourself! Basic Design Today's blog post is all about how we built Nordri: our Mach-22 competition rocket, and the culmination of what we've learned over the last few months.
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