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1. Ease of access to parts - e.g. is a certain diameter of tube commercially available?
2. Manufacturability - e.g. how complicated is it to make this part?
3. Purpose - e.g. what is the mission of this rocket?
4. K.I.S.S. - e.g. Keep It Simple, Stupid. Don't over-complicate things. This is already rocket engineering, don't make it any harder.

Where do I start?

If you're starting from scratch, it's hard to nail down exactly where to start. It is often easiest to look at your requirements to determine important parameters such as tube size, weight, height, motor, etc. Tube size is often a good starting point because it constrains several key variables, including your motor options, and available volume for payload/chutes/etc. Some goals could be, "I want to get to the highest altitude I can using an H250 motor." or, "I need to get a Level 3 certification while staying under 5000 feet AGL." or otherwise. For our example, we will be designing a rocket for Level 1 (L1) certification.

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A rocket generally has three physical body sections, often collectively referred to as the airframe:

1. Nose cone
   1. For aerodynamic purposes
   2. Usually empty or capped for smaller 3" rockets
2. Body
   1. To contain recovery, avionics, and payload, and to transmit loads and for aerodynamic purposes
   2. Usually subdivided into two or more tubes. Subdivision usually depends on the recovery system and payload.
   3. Usually a 3" body tube (2.6" and 4" are also common) for L1 flights
      1. The dimemsion 3" refers to the inner diameter of the body tube. 
3. Motor section
   1. To contain motor, provides stabilization via fins (usually)

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