Grid fin

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Closeup of MOAB grid fins.

Conventional "planar" fins on a RIM-7 Sea Sparrow

A MOAB on display with grid fins deployed

MOAB tail assembly with fins in stowed position

Grid fins (or lattice fins) are a type of flight control surface used on missiles and bombs in place of more conventional control surfaces, such as planar fins. They 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, SS-25 Sickle, and the N-1 (the intended rocket for the Soviet moon program). In Russia, they are referred to as "Belotserkovskiy" grid fins^[1].

Grid fins have also been used on conventional missiles and bombs such as the Vympel R-77 air-to-air missile; the SS-N-27 ("Klub") family of cruise missiles; and the American Massive Ordnance Air Blast (MOAB) large-yield conventional bomb. They have also been used on specialized devices such as the Quick-MEDS delivery system and as emergency air brakes on the Soyuz TM-22 spacecraft.

[edit] Design characteristics

Conventional "planar" control fins are shaped like miniature wings. By contrast, grid fins are a lattice of smaller aerodynamic surfaces arranged within a box. Their appearance has sometimes led them to be compared to potato mashers or waffle irons.

Grid fins can be folded against the body of a missile more easily than planar fins^{[citation needed]}, allowing for more compact storage of the weapon; this is of importance for craft which store weapons in internal bays, such as stealth aircraft. Shortly after release, the fins are swiveled into place for use as control surfaces. In the case of the MOAB, grid fins allow the weapon to fit inside a C-130 cargo bay for deployment while the craft is in flight.

Grid fins have a much shorter "chord" (the distance between leading and trailing edge of the surface) than planar fins, as they are effectively a group of short fins mounted parallel to one another. Their reduced chord reduces the amount of torque exerted on the steering mechanism by high-speed airflow, allowing for the use of smaller fin actuators, and a smaller tail assembly overall. Their small chord also makes them less prone to stall at high angles of attack, allowing for tighter turns.

Grid fins perform very well at subsonic and supersonic speeds, but poorly at transonic speeds; the flow causes a normal shockwave to form within the lattice, causing much of the airflow to pass completely around the fin instead of through it and generating significant wave drag. However, at high Mach numbers, grid fins flow fully supersonic and can provide lower drag and greater maneuverability than planar fins.

[edit] Notes

^ Zaloga, Steve (2000). "The Scud and Other Russian Ballistic Missile Vehicles". New Territories, Hong Kong: Concord Publications Co. ISBN 962-361-675-9.