Design Goals
The design goals or requirements for the Level 3 Certification Rocket Project are as follows:
- Complete all requirements of Tripoli Level 3 Certification.
- Be able launch for a reoccurring cost of less than $160.
- Minimize the use of glue/epoxy.
- The longest body segment shall be no longer than 3.3ft (1m).
- Have on-board video camera in enclosed bay.
- Have excess electronics bay room to enable adding additional features/components later.
Analysis of Goals
Complete all requirements of Tripoli Level 3 Certification.
This is the most critical goal out of them all. The full requirements for the Tripoli Level 3 Certification are published on the Tripoli Rocketry Association’s website. Please read the complete requirements at http://www.tripoli.org/Level3.
Be able launch for a reoccurring cost of less than $160.
This is the second most important goal but it will also have the largest impact throughout the (hopefully long) life of the airframe. It will also have a large impact on the design of the rocket.
To be able launch with a reoccurring cost of less than $160, this requires being able to fly on smaller motors. This then mandates that the weight of the airframe be kept low enough that smaller motors still provide adequate thrust to weight. According to the Tripoli Range Safety Officer Guidelines, the minimum allowable thrust to weight ratio is 5:1. In order to provide additional safety factor, for the purposes of this project, the thrust to weight ratio will be calculated by
(Average Thrust of Motor) / (Gross Lift-Off Weight)
Since I have used CTI (Cesaroni) rocket motors for my Level 1 and Level 2 certifications, I first considered the average thrust of the largest CTI motor reloads that are available under $160 and that have a burn duration of >2 seconds. Three options are the K780 Blue Streak, K660 Classic and K820 Blue Streak, available for $132.95, $148.95 and $152.95 respectively from Apogee Components.
The K660 provides an average thrust of 660N (148.4lbs) and a maximum of 1078.9N (242.5lbs). Dividing the average thrust by 5 yields a maximum lift-off weight of only 132N (29.68lbs). The motor and its casing weigh close to 21.5N, leaving 110.5N for the entire airframe.
The K780 provides an average thrust of 780N (175.4lbs) and a maximum of 1015.7N (228.3lbs). Dividing the average thrust by 5 yields a maximum lift-off weight of 156N (35.1lbs). The motor and its casing weigh close to 19.5N, leaving 136.5N for the entire airframe.
Finally, the K820 provides an average thrust of 820N (184.3lbs) and a maximum of 1470.5N (330.6lbs). Dividing the average thrust by 5 yields a maximum lift-off weight of 164N (36.9lbs). The motor and its casing weigh close to 21.7N, leaving 142.3N for the entire airframe.
Switching to Aerotech motors allows for a K1000NT Blue Thunder to be purchased from Wildman Rocketry for under the $160 limit. This provides an average thrust of 1066N (239.7N) and a maximum of 1674N (376.3lbs). Dividing the average thrust by 5 yields a maximum lift-off weight of 213.2N (47.9lbs). I have not found a listing for the weight of the motor or casing but I will assume approximately 50N as a worst case weight. This leaves 163.2N for the entire airframe.
Therefore, the maximum lift-off weight of the airframe should be between 110.5N and 163.2N.
In addition, the remaining goals are “stretch” goals that will be used to direct the project but are not as absolutely critical to the success of the project.
Minimize the use of glue/epoxy.
While I was in the process of completing my Level 1 certification, I had several occasions where my rocket, after a successful flight, would land on rocks. This caused one of the fins on my rocket to break along the epoxied joint along the root edge of the fin and the internal motor-mount tube. This happened on several occasions, even after my successful certification. This motivated me to find another way of attaching fins and other parts of the airframe together than the glue or epoxy that I had used up until that point. As a mechanical engineer, I wanted to come up with a new design that would still be strong and resilient enough for many flights but also would allow for ease of repairs.
However, when I started building my Level 2 certification rocket, I had competing design goals that were not compatible with this idea. For my Level 2 rocket, my goals were to:
- Build the airframe, minus the recovery equipment and electronics, for less than $150.
- Build the airframe light enough that it could be flown without a waiver (less than 1500g or 3.3lbs when ready to fly).
These two goals were accomplished with the small variant of my BigEZ rocket. Now that I am designing and building a new rocket, I would like to pursue using other mechanical means of assembly and not rely on glue or epoxy as the main source of mechanical attachment of parts.
The longest body segment shall be no longer than 3.3ft (1m).
I drive a small hatchback as my daily driver and my normal launch site is about 85 miles (137km) away from my house. Therefore, I will need to consider how I will be transporting the rocket to and from the launch site. The hatch area of my car is approximately 4ft (1.22m) wide, therefore it would be advantageous to try and keep the longest body segment or sub-assembly to 3.3ft (1m) or less.
Have on-board video camera in enclosed bay.
This is purely for fun as I have always wanted to have video of what it the flight is like from the rocket. In order to help protect the rocket though from wind speeds of upwards of 700mph (1127 kph) when using larger M, N and O motors, I would like the camera to be inside an enclosed bay. This will require a specially designed window that may contribute to large part of the airframe’s cost.
Have excess electronics bay room to enable adding additional features/components later.
This is another fun goal. I would like to be able fly other hardware in addition to my Missile Works RRC3 flight computers, especially DIY payloads powered by an Arduino, Raspberry Pi or similar. Therefore, a larger internal volume of the electronics bay would be very helpful.