The presented semester project was completed during the last academic semester (Fall 2020) by Théo La Marca, simulations engineer at the EPFL Rocket Team and 1st year master student in mechanical engineering at EPFL. Initially called « airbrakes characterisation », this semester project aims to provide an airbrakes characterization by CFD and design an innovative anti-turbulence “porous” airbrake.
In this article, Théo La Marca explains the procedure, purpose and importance of his project. He also mentions his expectations and results, as well as his next steps as a student and engineer at the EPFL Rocket Team.
Ansys Fluent was used for a complete CFD characterisation of the rocket AND the airbrakes (never done in the ERT) in the subsonic regime, in particular for computing aerodynamic drag. The EPFL Cluster “Helvetios” was used to benefit from very powerful parallel computing to perform a parametric study, i.e. change geometrical properties (such as deployment amplitude) and flight parameters (such as speed) and observe the evolution of the drag. The airbrakes deployment and rocket speed was changed by small increments to perform over 200 simulations (that would have taken a tremendous amount of time without the clusters), that allowed me to build a complete drag characteristic of the airbrakes for all standard flight configurations.
In a second part, I noticed some potentially disastrous phenomena associated with sudden deployment of braking surfaces at speeds approaching the transonic regime (such as Mach 0.8). Deployment at such speed is not a common and easy thing and needs careful analysis beforehand with knowledge of compressible aerodynamics and fluid structure interaction, to determine if we aren’t taking too much risk. In particular, phenomenons such as vortex shedding by the airbrakes’ perpendicular surface in a transonic flow could cause catastrophic velocity fluctuations downstream of the airbrakes and cause phenomena such as airbrakes lock in, fin flutter or fin buffeting, which puts the stability and structural integrity of the rocket at risk. It could be very well possible that the fluctuations are such that destructive vibration and disintegration of the rocket fin (or the entire body in the worst cases) could take place.
As a solution to this, an innovative mitigation technique inspired by WW2 bomber airbrakes or the new and famous SpaceX « grid fins » has been implemented, that consists of perforating the braking surface. This has never been done in the competition or at the ERT. Experimental and simulation data show that the perforation offers a better mixing of the flow downstream and disrupts the shear layers, which allows reduction of the velocity fluctuations or the shedding of vortices. This was done in cooperation with Simon Prêcheur on the airbrakes new design project. He was assigned to design the new airbrakes and I did the simulations in parallel, and advised him of the perforated design thanks to the results.
Purpose of the project
This was achieved in order to get an accurate active control of the rocket’s airbrakes (which are purposely designed for this task) during flight after the engine burnout, in order to achieve the target apogee of 3048 meters of the competition with the smallest overshoot. Without knowing the drag added by the airbrakes at all speeds and all apertures, it is impossible for the control algorithm to decide which aperture will lead us the closest possible to the target apogee.
Additionally, the new porous designs allow us to reduce the risks of vibrations and dangerous phenomenons associated with braking surfaces in transonic flows.
Why is it important for the team ?
Being the closest possible to the target apogee is one of the most important requirements of the competition, and airbrakes had never been analysed by CFD (the theoretical models are not reliable for such complex geometries and turbulent flows).
Additionally, the airbrakes have not been used yet in the rocket team and the next models are likely to reach even higher speeds (supersonic project), so the mitigation of risks associated to airbrakes is even more important for the future.
Expectations about the project
I expected to learn how CFD is applied to practical projects and how it can provide useful information.
In a way, the objectives of the project are more than fulfilled because I didn’t expect to be able to gather this much data using the clusters. Also, I didn’t expect CFD to highlight so clearly the dangerous phenomena associated to deploying airbrakes at high speeds, and that an affordable and innovative solution could be found and implemented without changing the core architecture of any subsystem.
The only regret is not not have been able to validate how the computational models deal with extreme situations in a wind tunnel because of the COVID19 situation, but the most common flight parameters have been validated with DATA gathered from the avionics in previous flights.
Finally, in my project report, I have written all the steps in a tutorial style, for new ERT students to be able to continue and improve my work or do a study on future new airbrakes or new shape designs of the rocket easily and fast by following my workflow, and using the files I put in the Drive !
A final word
It was very pleasing to be able to apply theoretical knowledge from the bachelor on a practical system and be part of a dynamic and motivating team such as the Rocket Team! I definitely learned more on how a complex project is managed in the industry, as well as precious group project skills.
The next step for this team and this project is to set up wind tunnel experiments during the 2nd term at Sauber. For me, it will be to apply and perfect this precious knowledge as the simulations Team leader in the new project ICARUS for thrust vector control !