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 when you're working:
- Ease of access to parts - e.g. is a certain diameter of tube commercially available?
- Something else
- Something else
Where do I start?
If you're starting from scratch, it's hard to nail down exactly what to put in first. It is often easiest to nail down what tube diameter you want first - this constrains several key variables, including your motor options, and available volume for payload/chutes/etc. Other 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 certification.
The jist
A rocket generally has three sections, which can be subdivided further:
- Nose cone (for aerodynamic purposes)
- Body (to contain chutes/payload, transmit loads)
- Motor section (contains motor, provides stabilization)
The subdivisions usually occur as a result of the recovery scheme you use. We will be designing based on a single separation, single deploy (SSSD) system. Other recovery CONOPS should be reviewed under Basic CONOPS, as we will discuss the design of rockets for three CONOPS.
The launch of a rocket is simple, as shown in the Concept of Operations (CONOPS) below:
Three recovery CONOPS --add a link to a conops article-- are as follows:
Single Separation, Single Deploy:
This CONOPS is useful for lower-altitude flights. It is as simple as possible: one event separates two sections of the rocket, and recovery webbing keeps the two pieces connected. A chute is on the cord, which inflates upon being released from the rocket. Typically, the event occurs near apogee because the rocket has no vertical velocity, decreasing the chance for crazy things to happen upon deployment. However, it is not as useful for high altitude flights because the safe speed at which rockets should fall under the parachute is relatively slow. For high-altitude flights, this leaves the rocket prone to drifting for up to several minutes, and possibly several miles.
For most flights under 3000 feet, and most L1 flights, SSSD is fine. We do not need to separate any of the three sections above to design this rocket. For an L1, a 3" diameter tube is commonly used. (2.6" and 4" are also common.) For our example, we add a body tube with an inner diameter (ID) of 3", outer diameter (OD) of 3.14", and a relatively arbitrary length of 15". The basis for a motor section is also a body tube, so we 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:
Dual Separation, Dual Deploy:
This CONOPS is more useful for high altitude flights because it decreases the amount of drift as compared to a single deployment. In this scenario, two events occur, one at or near apogee, and one much closer to the ground (usually 500 - 1000 feet AGL). The first event separates the rocket into two sections. A small drogue chute is typically deployed, although not always necessary. The rocket falls relatively quickly after this deployment until the second event, where the third section of the rocket separates to release the main parachute. Once the main chute is inflated, the rocket descends at a safe velocity. Just like before, cords hold the rocket sections together.
The drawback to DSDD is that electronics are required to detect the altitude at which the second event should occur, and initiate the second event.
Single Separation, Dual Deploy:
This is the most complex of the three CONOPS shown. It requires the drogue chute to pull out the main parachute at a target altitude. Like DSDD, this requires electronics. It also requires a restraint on the main chute which can be released at the target altitude. A COTS solution to this is the Tender Descender, which the Team has used in years past. The benefit to this system is that it only requires one body section to release two parachutes, which is beneficial if a large section of the body is needed for a payload.
This requires careful packing of all of the chutes and cords in the system, and it is usually not used for mid-power or certification rockets.
Optionally, the any section can be separated completely from the others, but chutes are necessary on all sections. Typically the rocket pieces are kept together to minimize the chance of multiple chutes tangling, and to save on structural mass, especially since larger chutes can take considerable volume.