Now, you may be saying, "Dallas, I thought you were a Space Engineer, how is it possible that you've never built a rocket before?" Well, the answer is that my focus has always been on spacecraft, satellites, and other things in space. I hadn't really spent too much time thinking about how we get there. Rocketry is an important topic, and it has captured my imagination before, but it was never something toward which I really gave much thought.
My colleagues, students at the International Space University, in the Masters of Space Studies program, were asked to design, build, and launch a rocket. Part of a team-building exercise, the requirements were simple. The rocket was to be made out of materials donated by the faculty, the rocket was intended to reach an altitude of 90 metres after launch, and its design was to be done on RockSim and approved by the faculty.
RockSim was easy to use and helped out a lot. Recently, many people have been enjoying Kerbal Space Program, a game where you can make rocket ships and try to fly them around, and for anyone who has played this, RockSim was similar, but with more scientific accuracy.
Engineering and design is all about choices. You start with a problem, and you work toward a solution. Often, there are many ways of doing things and you have to make a choice. That choice should be defensible, and make the most sense given your restraints of time, money, and material.
The first choice we had was the type of nose cone, of which we had elliptical or conical. While the conical appears as if it would cut through the air easier, we released that our rocket wouldn't need this and that the precision of the conical nose cone would actually make our rocket less stable.
How does that work? Well, as you'll see later in the section about fins, sometimes your components can be too precise and can cause your design to basically "oversteer". The conical nose cone would have caused more problems with stability than it would have saved in terms of having less drag.
Moving on, we added a rocket body to our simulation, basically a long, hollow tube, and looked at fins and engines. We had two different types of engines available to us, the Estes B4-4, or the Estes C6-5 model rocket engine. The letter generally refers to some sort of naming system, in this case the C was larger and more powerful than the B engine. The first number refers to the maximum thrust in Newtons, and the second number indicates the time between motor burnout to ejection charge.
The above diagram gives you an idea of general rocket design. The theory behind the engine is that it will burn and push the rocket forward. Once the engine fuel is expended, it sends a wave of pressure up the rocket body toward the nose cone, popping the nose cone off. This allows the recovery system, used to safely recover the rocket, to deploy. The diagram above shows a parachute, but we were instructed to use streamers. The reason behind this was due to the wind; we didn't want parachutes to catch the wind and take our rockets far away from the launch site.
The rocket design was a competition and one of the things we were judged on was our budget. All the parts needed for the rocket were provided for us but they "cost" money. Whether it be the nose cone, the rocket body, engine, or balsa wood for the fins, each component cost some amount of euros and part of our design process involved balancing cost with performance.
The B4 engine was cheaper so we worked to make our rocket work with the smaller engine. We added the mass of our payload, a quail's egg which must be protected, and the altimeter to our design. The altimeter would record the height of our rocket after launch.
The longest part of the design process was in choosing the fins. We had cut down on the rocket body's length and used the smallest components in order to save mass and use the smaller engine, but we still needed to keep the rocket stable during its flight.
Reading up on fin design, and talking with our team member who had some experience with aerodynamic systems, it became clear that fin shape did not exactly matter on this small scale. The important factors to consider were the centre of pressure (CP), and centre of gravity (CG). It is important to note that the centre of mass is not always the centre of gravity, but when we're dealing with anything close to Earth's surface, it is okay to use these terms interchangeably.
Anyway, the centre of pressure is exactly that, it is the place where all the pressures sum together on a design, and the centre of gravity is essentially the balancing point. Fins are used not only for stability through the air, but also to ensure that the centre of pressure is not ahead of the centre of gravity, that is, the CG must be closer to the nose cone than the CP.
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| Our rocket design, the blue circle is the CG, the grey is the CP |
We also noticed that adding 4 smaller, triangular fins to the near-front of the rocket added more stability without adding that much more mass. We were not entirely convinced with the stability of our design, so we mounted the fins to the rocket using tape and performed a stability check.
Quite simply, a stability check is every mother's nightmare. You tie your rocket to a string, around the centre of gravity, and whirl the rocket around and around your head. If the rocket is stable, the fins will help cut through the air and keep the rocket moving nice and straight. It is a rather simple test but I think it was very useful! With one test, you could tell if your rocket would fly or falter. It is something I will remember for future rockets/paper airplanes.
For our payload deployment, one of our team members had the idea to use just the nose cone. While some groups kept the egg and altimeter in the rocket body, we realized we could keep them in the nose cone and just attach the streamers to the nose cone. Thus, when the engine popped the nose cone off halfway through the flight, it would fall to Earth without being dragged down harder by the rocket body. Getting these design choices approved, we began construction.
Construction was actually a lot easier than we thought. I have to thank my parents and a few of my friends here as, throughout the years, they've forced me to roll up my sleeves and build things. When I was younger, I preferred theory, but they helped me understand application. Once construction was complete, we painted the rocket, a process which left my hands covered in blue and gold spraypaint, and decorated it with our country's flags. We had quite the international team with members from: Canada, the United States, India, Nigeria, and Indonesia.
The teams were not informed exactly of the launch specifics until near the end of the construction phase and I wish we had been warned earlier. The launch ramp was simply a metal bar with a rail on one side. Mounting some sliders to our rockets, the rocket engine would be ignited and the bar/railing was to be used to keep the rocket straight for the first metre or so into its flight.
I wish we had been told earlier about this as no team was able to reach the 90 metre mark. The friction of the launch ramp caused all our rockets to fall short, the highest reaching 73 metres. Had we been warned of this earlier, we would have designed our rockets to fly to 100-110 metres so as to counteract the friction. However, the teams were not penalized too harshly for this, and in the end, everyone learned some valuable lessons.
The rocket launches were quite exciting to watch! All flew quite high and straight, however, payload recovery was a problem. For our team, it seems that some glue made it onto our egg thus adhering it to the cotton we had used for protection. When we tried to separate the two, the cotton was pulling on the egg shell. Thus, our payload was "not recovered", so had failed just the same as the teams whose eggs had broken during the flight. It was a little sad, but again, lesson learned.
I tried to include the launch on my blog, but the video was of such good quality that it wouldn't work. Here is a link, you may watch the launch of the Quailonizer here!
As I mentioned in the beginning, this was more about team work than rocket design. Our team worked well, everyone finding a role to play, and we had absolutely no problems throughout the entire process. We all learned quite a lot about rocket design and had fun doing it. My first assignment at the ISU has finished, and I hope it to be the first of many enjoyable experiences here!
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