• Post category:Rocket Science
  • Reading time:3 mins read

In this short article, we will understand, how are rockets able to change their directions after launch and in space.

Gimballed Engines

“Gimbal” is a pivoted support that allows the rotation of an object about a single axis.

Giimbal Animation
The above figure shows a set of three gimbals mounted together, each offers a degree of freedom: roll, pitch, and yaw.

In spacecraft propulsion, rocket engines (as in Saturn V and Falcon 9) make use of gimbaled thrust for navigation. In such a system, the exhaust nozzle of the rocket can be swiveled from side to side. This results in a change of thrust direction relative to the center of gravity of the rocket.

Rocket-Gimbal-Navigation
The above figures show gimballed thrust for three different gimbaled angles.

Case 1: The middle rocket shows the straight-line flight configuration, in which the direction of thrust is along the centerline of the rocket and through the center of gravity of the rocket.

Case 2: On the rocket at the left, the nozzle has been deflected to the left and the thrust line is now inclined to the rocket centerline at an angle called the gimbal angle. Since the thrust no longer passes through the center of gravity, a torque is generated about the center of gravity and the nose of the rocket turns to the left.

Case 3: On the rocket at the right, the nozzle has been deflected to the right and the nose is moved to the left. As in the above case, a torque is generated which makes the rocket’s nose turn right.

Now let’s try to understand gimballed thrust in the case of the Falcon 9 rocket. All Merlin 1-D engines are equipped with a gimbal joint.

The above image is a cutaway of the Merlin engine.

The encircled label (in blue) is Thrust Vectoring Control Actuator (TVC Actuator). This is the part that will swivel the assembly to a certain degree to obtain desired offset thrust vector also called as a gimbal. Out of the 9 nozzles, the center nozzle of the Falcon 9 has a wider gimbal range. Any guesses why?
 
Please check this video showing the Gimbal in action (in a test setup). This is not of Falcon 9, the actual motion is quite similar.

Gimbal engine in action.

References:

  • https://science.howstuffworks.com/gimbal2.htm
  • https://en.wikipedia.org/wiki/Gimbaled_thrust
  • https://en.wikipedia.org/wiki/Gimbal
  • https://qr.ae/TWpqtr
  • “Mechanics and Thermodynamics of Propulsion”, (ISBN 020152483, 1992, 2nd Edition) by Philip G Hill and Carl R Peterson, Addison Wesley.
  • “Space Flight Dynamics”, by William E. Wiesel, Mc-Graw Hill.


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