Grid fin

Lore & Background

Grid fins were developed in the 1950s by a team led by Sergey Belotserkovskiy and have been used since the 1970s in various Soviet ballistic missile designs such as the SS-12 Scaleboard, SS-20 Saber, SS-21 Scarab, SS-23 Spider, and SS-25 Sickle, as well as the N-1 rocket intended for the Soviet moon program. In Russia, they are often referred to as Belotserkovskiy grid fins. They have also been used on conventional missiles and bombs including the Vympel R-77 air-to-air missile, the 3M-54 Klub cruise missile family, the American Massive Ordnance Air Blast (MOAB) bomb, the Quick-MEDS delivery system, and the launch escape system for the Soyuz spacecraft.

Design characteristics: Conventional planar control fins are shaped like miniature wings, whereas grid fins are a lattice of smaller aerodynamic surfaces arranged within a box, sometimes compared to potato mashers or waffle irons. Grid fins can be folded against the missile body for compact storage, important for tube-launched weapons or internal bays of stealth aircraft. They have a much shorter chord than planar fins, reducing torque on the steering mechanism and allowing smaller actuators. Grid fins perform well at subsonic and supersonic speeds but poorly at transonic speeds due to normal shockwave formation within the lattice; at high Mach numbers they provide lower drag and greater maneuverability than planar fins.

In 2014, SpaceX tested grid fins on a first-stage demonstration vehicle of its reusable Falcon 9 rocket, and on December 21, 2015 they were used during the high-velocity atmospheric portion of reentry to guide a commercial Falcon 9 first stage back to land for the first successful orbital booster landing. The first hypersonic flight test with grid fins was in February 2015. The Chinese company i-Space's Hyperbola-1 rocket appeared on July 25, 2019 to be equipped with steerable grid fins, and on the same date China launched a modified Long March 2C with grid fins atop the first stage for controlled re-entry.

Reader's Guide

Grid fins have become significant in the development of reusable launch vehicles, particularly for the SpaceX Falcon 9 rocket, where they provide increased precision and accuracy in controlling the landing location. The grid fin development effort is part of the SpaceX reusable launch system development program underway since 2012. After the first hypersonic flight test in February 2015, grid fins were used on all reusable Falcon 9 experimental test landings and, after December 2015, an increasing number of successful first stage landings and recoveries. Iteration on the design continued into 2017, with SpaceX CEO Elon Musk announcing a new version to improve reusability. The Falcon 9 Block 5 introduced new cast and cut titanium grid fins, replacing earlier aluminum fins that were coated with an ablative thermal protection system and had sometimes caught fire during entry and landing. The titanium fins enabled greater controllability and increased payload to orbit capability by allowing a higher angle of attack. These larger, unpainted titanium grid fins were first tested in June 2017 and have been used on all reusable Block 5 Falcon 9 first stages since late 2017. The legacy of grid fins thus extends from Cold War ballistic missiles to modern commercial reusable rocketry, demonstrating their adaptability across speed regimes and vehicle types.

Did You Know?

• They have been used on Soviet ballistic missiles such as the SS-12 Scaleboard and SS-20 Saber since the 1970s.
• SpaceX first used grid fins on a Falcon 9 first stage during a high-velocity reentry on December 21, 2015, achieving the first successful orbital booster landing.
• The Falcon 9 Block 5 introduced titanium grid fins in June 2017, replacing earlier aluminum fins that had sometimes caught fire.