Achieving Stable Aerodynamic Flight (6)

(I try to link to the products I used so you can find them more easily. If you purchase them from these links I may receive compensation from affiliate programs. I am not employed or influenced by the manufacturers or distributors.)

Since I started working on this project I’ve had a lot of fun learning about aerodynamics. It’s something that only really matters once you put the parts together, but planning for an aerodynamically rocket is critical for success. There are a TON of great resources to help you learn about the science of aerodynamics. I’m going to include the videos that I found the most informative in here to help me illustrate the concepts that we will need to understand. After we cover the basics, I will show you how we can apply the concepts to our own designs using a rocket simulator as we plan to build the exact vehicle we need.

Basics of Aerodynamic Stability

Before we get started, we should establish the axes of a rocket, so we have a common understanding. The Z axis runs from the tip of the rocket to the tail, and then the X and Y axes run from side to side, sometimes in line with the fins (on a 4-fin model).

The three most important concepts in aerodynamics are the Center of Gravity, Center of Thrust, and Center of Pressure.

Center of Gravity is the point where the weight is evenly distributed on any axis you choose. This will change slightly over time as fuel is burned, which is something to consider in larger rockets with massive fuel tanks, but in smaller model rockets losing mass in the tail will only work to your benefit. On the Y and Z axes you want the CoG to be as close to the origin as possible. Moving the CoG along the Z axis will be how we achieve stability.

Center of Thrust is where the force of the motor(s) or thruster(s) is applied. You can change this using Thrust Vectoring or RCS thrusters to change the trajectory of the rocket. Typically your center of thrust will be at the rear, and optimally the direction of thrust is perfectly aligned with the X axis.

Center of Pressure is the center of the force applied on the body by the air flowing over it. This can be very dynamic, and there are a lot of ways to determine the CoP, some of which will be covered in this video.

Essentially the goal is to align the Center of Thrust with the X axis perfectly, then arrange the rocket body and payload in such a way that the CoG and CoP are at the origin of the Y and Z axes, and the CoG is forward of the CoP. This way, as the rocket flies the pressure forces acting on it will keep it pointing straight. This is usually achieved by adding fins toward the base of the rocket, which move the CoP backwards by giving added surface area to the body.

Unlike with engines (😂), bigger fins aren’t always better. There is a phenomena called Wind Cocking, which is caused when wind blowing along the Y or Z axes exerts more force on the tail of the rocket than the nose, tipping the body, and causing the engine to push the rocket into the wind. To avoid this you want to try to keep your fin size and shape just large enough to provide stability, but not much larger. Here’s a great video about estimating CoG and CoP, and properly balancing your rocket body so it is aerodynamically stable.

Rocket Stability – LabRat Scientific

Software Modeling for Aerodynamic Stability

Now that we understand how aerodynamic flight works, let’s talk about simulating the design. As I mentioned before, I have been using OpenRocket to do all of my virtual modeling so far. OpenRocket is a Java app, so you can run it on any computer, and it has some pretty simple components you can add and customize to suit your design goals.

For the first version of my rocket I had already done some shopping for parts, and settled on a clear payload bay for the 360º camera, a fiberglass engine compartment, and a pre-fabricated ogive nose cone. The fins that I used were made of balsa as well. With most of these components pre-determined, the two biggest variables would be payload placement/weight and fin size/shape.

As we discussed earlier the payload and engine weight and placement will both dramatically change the center of gravity. For the rocket’s maiden voyage I didn’t want to send up the camera, which only left the telemetry package, a mere total of about 60-80 grams, in the front and a large heavy motor in the back. This meant that the fins would have to be somewhat larger, to pull the center of pressure further back.

In the image below you can see what the rocket’s properties would be like with the motor I anticipated using and just the telemetry in the payload bay. I trimmed the tips of the fins to reduce the stability so the rocket would fly better if we encountered wind. Realistically, though, this was still a lot of fin in the face of any wind.

Version 1.0 digital model with an empty payload bay

When I add the 360º camera then the stability goes way up. This would result in a lot of wind-cocking, since the inertia of the heavy payload bay would exaggerate the tipping. This was about as far as I could trim the fins and still have a stable empty flight, though, so it would have to work.

Version 1.0 digital model with camera onboard

One of my favorite features of RocketSim is the ability to plot the model of the flight. There are a few considerations to remember here. The rocket takes off under power, and flies under power for a total of 3.5 seconds. After the motor burns out the rocket is in ballistic flight for about 6 seconds (the current model uses a 6-second delay) after which a charge detonates separating the engine compartment from the payload bay and ejecting the parachute. From this point the vehicle should gently descend back to the earth at a steady rate of ~-10m/s.

Plot of simulation altitude, vertical velocity, and vertical acceleration vs time

You can even adjust parameters like recovery chute delay and launch rod angle to accommodate for windy conditions and predict how far the vehicle will travel from the launchpad.

Plot of simulation altitude vs position

This last plot I found really informative, which shows stability over time. Note the first second or so, where stability is most critical. You don’t want the rocket to be unstable when it leaves the launch rod, because it may tip, and without enough wind passing over the fins it won’t correct itself, and may fly off uncontrollably.

Plot of simulation stability vs time

Finally, I’ll leave you with a couple of videos that I really enjoyed. The first one is very informative about rocketry on many levels, and the other illustrates how active aerodynamic controls works in the context of the F-22 fighter jet. In my next post, I will discuss parts, and the strategy I used that allowed me to assemble the rocket in just a couple of days.