Sectional view of the engine gimbal
about thrust vectoring
The thrust vector control system – or TVC for short – is the “steering wheel“ of our rocket. By redirecting the exhaust gases of the rocket engine, the rocket can steer itself by use of the resulting lateral forces in the air as well as in a vacuum. In its final form, our TVC will be a custom rocket engine nozzle suspended from a gimbal mount that allows the nozzle to move fluently in all directions. This gimbal is moved by multiple linear servos, which are controlled by our onboard computer.
The rocket engine choice
The maiden flight of our Atmoventus H will not be conducted with the largest engine we can fit. Instead, we will launch with far smaller engines, which we will also use in ground firing tests prior to a full launch. Because of this, we developed a gyroscopic gimbal that can be used for the test with smaller engines as well as modified for the use of larger ones.
Unlike the TVC system for a large rocket engine – which gimbals only the nozzle – the TVC system for small engines will move the whole combustion chamber including the solid rocket fuel. We are going to use two I55 engines from Cesaroni’s pro38 lineup. Compared to other 38 mm diameter engines, the I55 has a very long, 7,2 s burn time. This is advantageous for us, as the TVC will only work while the engine is running, so we try to maximize the time we can steer the rocket.
Our Gimbal setup
The gimbal consists of two stages which are pivotable on two perpendicular axes. The outer stage is mounted inside the rocket using two ball bearings and can be turned around the X-axis by a P16 Linear Actuator with a force of up to 300 N. The Y-axis is set at a right angle to it and is the turning point of the inner gimbal stage. Suspended by two more ball bearings, it is driven by at least two, but up to four PQ12-P Linear Actuators with up to 200 N relative to the outside ring. The inner stage is put together from multiple parts: The two pipes that hold the two 38 mm engines, an upper and a lower mounting ring, the first of which is responsible for transferring the lifting forces, and two side brackets to hold them together and act as a mounting point for the ball bearings of the Y-axis. This assembly enables us to vector our thrust at least 17° in every direction from the vertical Z-axis. The maximum movement of the engine is limited only by the length of the motor and the inner diameter of the rocket body.
As the construction material, we chose to use a high-tensile aluminum alloy. With it, we can keep the weight of our gimbal down to around 550 g while withstanding forces of up to 500 N. Also, the inner stage is designed in a way that we only have to replace a few parts in order to use it for a big 75 mm engine.
While we are still looking for a local company that can produce the gimbal parts for us at a low cost, we were able to 3D print a 1:2 model of our gimbal made from PLA plastic. We are also working on a functional 3D-printed demonstrator including working electronics as a proof of concept and first testing with our MCB, the electronics of which are already running (watch now on YouTube). Our goal for Summer 2019 was a running TVC Prototype, which we are now very close to successfully reaching.
Our TVC Gimbal assembly