Jet damping or thrust damping is the effect of rocket exhaust removing energy from the transverse angular motion of a rocket. If a rocket has
pitch or
yaw motion then the exhaust must be accelerated laterally as it flows down the exhaust tube and nozzle. Once the exhaust leaves the nozzle this lateral momentum is lost to the vehicle and thus serves to
damp the lateral oscillations. The jet damping is stabilizing as long as the distance from the instantaneous spacecraft center of mass to the nozzle exit plane exceeds the instantaneous transverse
radius of gyration ''Radius of gyration'' or gyradius of a body about the axis of rotation is defined as the radial distance to a point which would have a moment of inertia the same as the body's actual distribution of mass, if the total mass of the body were concentr ...
. Most rocket or missile configurations meet this criteria and the jet damping has a dynamic stabilizing effect. The jet damping torque rotates at
nutation
Nutation () is a rocking, swaying, or nodding motion in the axis of rotation of a largely axially symmetric object, such as a gyroscope, planet, or bullet in flight, or as an intended behaviour of a mechanism. In an appropriate reference frame ...
frequency in the spacecraft frame.
The jet damping contributes to the pitch and yaw damping coefficients,
and
, where
is the rate of change of pitching moment with respect to pitch rate and
is the rate of change of the yawing moment with respect to yaw rate. For jet airplanes in cruise, the contribution of jet damping is usually negligible because the external aerodynamic damping is large relative to the jet damping. Rockets at lift-off, however, have practically zero external aerodynamic damping and the jet damping becomes significant.
References
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Rocket propulsion