Nielsen Engineering & Research (nearinc.com) worked with Space Exploration Technologies Corp. (spacex.com) to develop steerable fins for the Falcon 9 first stage to help stabilize and guide the vehicle during re-entry.
NEAR used their in-house aerodynamic design software to evaluate a number of different options for fin geometry, size, and location on the vehicle.
Simulations were conducted as a function of Mach number, vehicle roll angle, and vehicle angle of attack.
Body wake extent and body-fin vortex interaction phenomena were modeled.
Based on the predictions generated, it was determined that the best option was a set of deployable grid fins mounted near the top of vehicle.
On June 17, 2014, the Falcon 9 Reusable (F9R) test vehicle was successfully flown with these grid fins installed.
A video of this flight is available at youtube.com/watch?v=DgLBIdVg3EM.
This design effort benefitted substantially from NEAR's previous experience in developing grid-fin-based stabilization systems for NASA's Orion Multi-Purpose Crew Vehicle (Ref. AIAA 2011-3018) and the Max Launch Abort System (MLAS).