Hello, I'm new to this forum, and I did a search but found neither this question nor, obviously, the answer. (And I'm positive this is the place to ask this question):
When viewing footage of spacecraft in flight using RCS (particularly Apollo LEMs maneuvering), the spacecrafts end every change in attitude looking (unnaturally) rock-solid. Wouldn't the inertia caused by using the RCS cause a little "overspin" that needed some, even a little (visible) correction, especially in the near vacuum of space?
I assume there must be a built-in "counter thrust" that eliminates this expected (to me) overspin, but I can't figure out how such a system might work, and work so precisely. The only thing I can think of is the RCS being somehow connected to an inertial sensor, but how could such a sensor "predict" the end of an astronaut's command so that the counter-thrust (if that's what it is) comes in at exactly the right moment (and power) to stop the translation cold?
This question has been bugging me for years.
Thanks.
It is the "auto pilot"
Here is the shuttle's modes
For rotations, the flight crew can select either automatic or manual control through the use of the DAP panel push button light indicators or by moving the RHC. In manual, the capability exists to rotate in any axis in a pulse mode, in which each RHC deflection results in a single burst of jet fire, or in a discrete rate mode, in which RHC deflection results in a specified rate being commanded in that axis for the entire time the RHC is deflected. It is also possible to go to a free drift mode, in which no RCS jets are fired, or to an attitude hold mode, in which the DAP sends commands to maintain the current attitude with null rates within premission-specified dead bands. Also, if the RHC is deflected beyond a certain point, continuous RCS jet firings will result.
DAP - Digital Auto Pilot
RHC - Rotational Hand Controller
Apollo also had an autopilot that allow for the start and stop of rotations based on RHC deflections.
Even Mercury had a rudimentary autopilot for attitude control. But it also had a mode with direct linkage to the thruster valves and the astronaut had to dampening all his motions.
I'll give this a try as well. The density of misconceptions in your question is high, but I think I've identified the base misconception: you don't fully understand inertia.
You seem to be under the impression that "the astronaut command" (or computer command) to use the RCS system consists of holding down a firing button or trigger or some sort with the button being released precisely at the moment the spacecraft has reached the final desired orientation. And then some mystery action you are inquiring about causes the craft to stop.
This is not how it works. In a simple example the RCS thrusters are pulsed once at start of movement. While the craft is rotating the thrusters are off. Then at the end of the planned rotation (usually controlled by a computer) the thrusters are pulsed again to stop the craft. It is true that inertial sensors and guidance hardware predict exactly the RCS pulse needed to come to a clean stop, among many other things.
I'll give this a try as well. The density of misconceptions in your question is high, but I think I've identified the base misconception: you don't fully understand inertia.
You seem to be under the impression that "the astronaut command" (or computer command) to use the RCS system consists of holding down a firing button or trigger or some sort with the button being released precisely at the moment the spacecraft has reached the final desired orientation. And then some mystery action you are inquiring about causes the craft to stop.
This is not how it works. In a simple example the RCS thrusters are pulsed once at start of movement. While the craft is rotating the thrusters are off. Then at the end of the planned rotation (usually controlled by a computer) the thrusters are pulsed again to stop the craft. It is true that inertial sensors and guidance hardware predict exactly the RCS pulse needed to come to a clean stop, among many other things.
Ahhh! Finally! (and the answer is so elegantly simple, too).
Thanks LegendCJS! (You're right. The "density" of my misconceptions was high!).
But I've always prefered appearing dense asking the questions (and getting the right answers) than continuing being dense!
Thanks a bunch!
Just sitting here thinking about it (the Reaction Control System)...
What amazing technology.
I'm sure many people will think rocket boosters, inertial guidance systems, fuel cells, heat shields, and space-suits are pinnacles of engeneering, and they'd be right. But just making the spacecraft point where you want it to in space...
Just one of the hundreds of engineering marvels achieved for space flight.