Introduction to Rocket Design

2. Goals The first step in the process of designing a rocket is to clarify what our goal is. What do we want to achieve? Based on my own experiences, I am providing three example goals: Achieve a target altitude Launch a particular payload Propel a rocket with a specific motor Once we've clarified our primary goal, we must then decide if we have any secondary goals. If our primary goal is, for example, to achieve a target altitude, a secondary goal might be to loft a videocamera to record a video of the flight. Secondary goals can have a direct influence on our primary goal. For example, a videocamera (or other payload) might require a large enough (diameter) rocket body in order to fit the payload. This directly affects the rocket design. Another possible secondary goal relates to the motor. We may wish to achieve a certain altitude by restricting the size of the motor, for example, to G-class . To achieve our altitude goal, we may well want to accomplish this particular goal with a minimum diameter rocket. The reason for this will be explained later. If the objective is to launch a rocket with a specific motor (perhaps one that we have already developed), our secondary goal might be to achieve a target altitude, so we can see that all three of these goals are closely related. As such, the process of designing a rocket is much the same regardless of the primary goal. Nevertheless, it is important, moving forward, to clearly spell out all of our goals in advance. Knowing exactly what you want to accomplish will be incredibly helpful moving forward. Break down the journey into mini-goals and milestones. One of the essential elements for a project to be successful is to have a detailed plan, or timeline. A project timeline is a project management tool that helps you visualize how pieces of your plan all fit together. A project timeline can help you list out specific dependencies during the project planning process. A useful timeline can be simple to create, no need to use fancy project tracking software. Just make a list of tasks that need to be done to complete the project, and then assign dates to each, based on your best estimate of how long each task will take to complete, assuming no problems are encountered. Deciding how much time each task will take is the most challenging aspect of creating a timeline, especially if you are tackling things with which you have had little or no experience. It is wise to build buffers into the timeline. A buffer is one or more a blocks of time added to the timeline to provide a safety margin that helps manage the impact of uncertaintly and inevitable problems that arise during the course of a project. I first learned the value of project planning, and in particular creating a timeline, when I began my Cirrus project way back in 2000. I set out to build and launch a rocket that I would fly at the first-ever A-Rocket rocketry event that was to take place in New Mexico in April of 2001. I set my goals as launching a rocket that would reach an altitude of 10,000 feet (3000m.), achieve mach one velocity, and to recover the rocket safely. The greatest challenge was seen to be developing a rocket motor that would fit the bill. A K-class rocket motor was needed, which was substantially larger than the biggest I had made up to that point of time, my I-class C-400 rocket motor. As such, I began designing the motor nearly a year ahead of the planned launch date. The motor was ready for test firing in May of 2000. The result was a CATO. I was not too concerned at the time as I had lots of time to make changes and retest. The redesigned motor was ready two months later and successfully static fired. However, there were significant issues with the motor insulation and other features that needed to be addressed before I could consider the motor to be flight-worthy. As it turned out, I needed to test fire the motor two more times before I was satisfied and was confident that I had a reliable motor. It was now late November, and I had not even begun to create the rocket, which existed only on paper and consisted of little more than a general concept. Although I had built and flown many EX rockets, and was fairly confident that my Cirrus One rocket would be of similar concept, I was beginning to be concerned. Motor development had taken much longer than expected. What if I ran into problems with developing the rocket? After all, this rocket would be supersonic and would fly much higher than what I'd achieved with my earlier rockets. I could not draw solely upon my experience. Totally new features would be needed, such as a two-stage recovery system controlled by an electronic altimeter. At that point in time, I felt it would be wise to break down all the key tasks that were remaining to be achieved and set a timeline that would account for all tasks to be completed in the remaining four months. I was taken aback as I created the list, as there was a lot of work to be done. To the best of my ability, based on my experience and perception, I set dates for accomplishment of each of the tasks. I endeavoured to provide extra time for those items on the list that were untried. The resulting timeline was indeed sporty, meaning I had little time to dawdle. If everything went smoothly, I'd be ready in time. Needless to say, things did not all go smoothly. I struggled with development of the recovery system, even though it was, as much as possible, based on the tried-and-true system I had used on my earlier rockets. This unexpectedly ate up a lot of valuable time, and I had to revisit and rejig my timeline which necessitated having to spend a lot more of my "free" time on the project. It took about six weeks to develop the recovery system, which involved many failed ground tests that required redesign and retesting. This was double the stretch of time that I had originally budgeted, and involved a lot more hours spent during that stretch of time, on the effort than I thought was needed. Happily, I was ready on time and launched the Cirrus One rocket as planned. I was thrilled to have accomplished three of my project goals: 1) I designed, built and flew the rocket; 2) the rocket reached 10,000 feet; and 3) analysis confirmed the rocket achieved mach one velocity. I did not achieve my "safe recovery" goal. The recovery system anomaly was a result of inexperience with utilizing an electronic altimeter. A lesson learned, that paved the way for future successes. Another lesson learned was the indispensible need for planning and having a timeline, a lesson which I have gratefully exploited on subsequent endeavours. Setting goals and creating timelines to help achieve these goals is one thing. Actually accomplishing these goals is another thing. In the end, we may not accomplish all (or any) of our goals. This is ok. Rocketry, especially EX rocketry, presents many formidable challenges. As the adage goes "it's about the journey, not the destination". Having taken the journey, we can appreciate those challenges in a far more profound way then if the journey was never taken. And if we enthusiastically put in concerted effort, we will have, at the very least, broadened our horizons in a positive way, gaining the satisfaction that we have honestly tried. That achievement in itself is a very good thing. Next--  Aerodynamics and its Role in Experimental Rocket Design