This article goes through the basics of designing a mid- to high-power rocket.

We will be using OpenRocket for design purposes. It is free, open-source, and runs on Java.

Some things to keep in mind while you're designing:

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.

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.

Protip: Rocket team is full of people with many wonderful and diverse rocketry experiences. Talk to them about the projects they've tried in the past, or how to design a rocket. They'll happily help you get started.

The jist

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)

The rocket has components inside of the airframe as well:

1. Recovery system
   1. To land the rocket safely on the ground after apogee
   2. Usually just a single parachute for L1 flights
2. Avionics
   1. To monitor the flight (like apogee and accelerations) and control various events, including separation and parachute deployment
   2. Definitely not necessary for L1 flights; motor ejection charges can be used instead

For our example, we will be designing based on a single separation, single deploy (SSSD) system. For most flights under 3000 feet, and most Level 1 (L1) flights, SSSD is fine. We do not need to separate any of the three airframe sections above for a successful flight. Let's use a 3" body tube with a relatively arbitrary length of 15". The basis for a motor section is also a body tube, so let's add another one below it. For this rocket, we will also make the nose cone the same diameter as the body tube. It should look like this: