Wednesday, May 18, 2016

Thrust Vector Control

I'm trying a new format for this post, sort of like a Q&A.

Q: How do rockets steer?

A: Well, in a car steering refers to both figuring out where to go (looking at the road) and the actual physical mechanism that steers the wheels and changes the direction.  Which one do you mean?

Q: The physical mechanism.

A: Since rockets spend much of their time in a vacuum, the mechanism for steering must not rely on an external force, like fins do.  There are three ways that spacecraft do this: 1) with small rockets pointing in different directions, called reaction control thrusters.  This is Apollo's reaction control thrusters:

Credit: National Air & Space Museum, Smithsonian Institution.

And here is the Apollo CSM with the thrusters installed:

By National Aeronautics and Space Administration (Apollo Lunar Surface Journal (direct link)) [Public domain], via Wikimedia Commons

These are good for moving large spacecraft, but do burn a limited supply of fuel.

2) With reaction wheels.  This is where large spinning flywheels are sped up and slowed down to rotate the spacecraft.  These are beyond the scope of this post.

3) With thrust vector control.  Thrust vector control is where you rotate the engine bell a couple of degrees, effectively making part of the engines thrust go towards rotation of the spacecraft.  That is the steering losses talked about previously on this blog.  Thrust Vector Control (TVC) is good for moving large spacecraft easily with minimal added weight.  However, it can only work while the engines are firing.

Almost all rockets use it during ascent, due to how much control authority is needed.  TVC is how seemingly unstable  rockets, like the Atlas 411 ( (Lockheed Martin))  or the Space Shuttle, both of which have a balance that changes a lot during flightl, are able to stay flying straight (of course, all Atlas variants are asymmetrical).  The Space Shuttle has an extremely large vector range, which you can see here:

The engine rotation mechanism is called a gimbal, and uses different ways of rotating the engine.  Basically, though, the engine is hinged in 2 axes above either the bell, or the entire engine.  The engine is then rotated with hydraulic or electromechanical actuators.  Now, you might notice that I only said 2 axes.  That is because you cannot provide roll control without some kind of off-center thrust.  Rockets either use multiple engines, gas generator exhaust vectoring (beyond the scope, again.), or just reaction control thrusters.

Until next time.