Endeavour Houston we see a nominal MECO Welcome to space. First things first
Reference frames. a reference frame can be thought of as an imaginary box which
defines directions, they could be up down left right forward and backwards. Or for
the more technically inclined among you XY and Z. On the surface of the earth the
reference frame as we think and are pretty obvious, the sky is up the ground
is down forward back left and right are defined by whatever direction we're
facing in. For satellites in orbit things are a bit different, let's say I
just chuck a satellite up in space and I don't touch it and I don't like anything
else touch it. It's orientation will stay fixed in space regardless of what
direction Earth's surface is in or the direction of travel for that satellite.
This is an example of being an inertial reference frame where our imaginary
direction box our reference frame has its orientation fixed relative to space
and not relative to earth. Now the International Space Station is a bit
different it has a trick, it spins once every orbit. This way it can keep one
side facing earth and another side facing the direction of travel. So with
the space station we can think of things in a different reference frame LVLH
that stands for local vertical local horizontal. In other words our reference
frame imaginary box is defined relative to earth's surface below us. If this were
a perfect easy world we could just set the space station spinning at the right
rate and since it's in space it would keep spinning forever and stay fixed in the
LVLH reference frame with no help from us. But as you've guessed there's a bit more
to it. Even though it's out in space the space station still has a whole bunch of
external factors pushing on it and I mean that quite literally. First is the
atmosphere even up at the 400 kilometer ish altitude of the International Space
Station there is a teensy tiny amount of atmosphere lingering around which
provides a small amount of air resistance on the space station
this is why it needs to be periodically reboost at every few months but it also
messes with our perfect spinning. The amount of air resistance the Space
Station experiences at any time largely depends on the orientation of its solar
arrays. If we look from the perspective of the oncoming air you can see that the
surface area for us to push against is much larger as the solar panels are
rotated into a more perpendicular orientation facing us. These solar panels
are designed to rotate in order to track the Sun throughout each orbit. This means
that the amount of air resistance changes throughout each orbit and
throughout each day. Since the center of air resistance and the center of mass
aren't quite in the same position this means that the air resistance causes some
twisting or a torque to be applied to the ISS, and that affects our perfect spin
rate that we need to stay in the LVLH reference frame. Now onto complication
number two and that's the gravity gradient torque. The ISS is getting pulled
on by Earth's gravity all the time, that's why it's in orbit. But not all
parts of the space station get pulled on equally. Gravitational pull between
objects gets weaker and weaker the further and further apart those two
objects get from each other, so parts of the space station that are closer to
Earth experienced a teensy tiny amount more gravitational pull than the parts
of the space station that are further away from Earth, and once again that can
mess without perfect once an orbits spin. Those two things gravity gradient torque
and that air resistance torque are the two main complications we have to contend
with. The simple answer to them is that we try and balance them out by putting
the space station in an orientation in the LVLH reference frame that means the
air resistance torque and the gravity gradient torque oppose each other this
orientation is called torque equilibrium attitude or TEA. These external torques
being applied to the space station are changing all the time so a torque
equilibrium attitude is planned as an average over a whole orbit. The job of
managing this falls to a controller in Houston called the ADCO
or attitude determination and control officer TEAs aren't always used, sometimes
the space station needs to be rotated into a different orientation such as
during docking of supply vehicles like Soyuz. Generally in these situations the
space station is oriented so that the docking port being used is facing along
the orbital path of the approaching vehicle
this makes docking a much simpler task. So the International Space Station
requires us to be able to actively control its attitude. This is done
through the use of for control moment gyroscopes or CMGs located in the Z1
truss and RCS thrusters located in the Russian segment. Before we dive into how
control moment gyros work let's have a look at their younger brother the
reaction wheel. Reaction wheels work on the principle of conservation of angular
momentum. You can see that if I use a motor to spin a wheel in the middle of a
satellite the satellite will spin in the opposite direction. The angular momentum
of the wheel and the satellite opposing each other and so add up to zero just
like they did before I started turning the motor. This is a pretty cool way of
steering a satellite without using thrusters and hence without using fuel
but it does have a limitation referred to as saturation. To understand what
saturation is I'm simply going to show it to you. I'll take our satellite that's
using its reaction wheel to spin, if we were to push on it from outside just
like air resistance or gravity gradient torque might do I can slow it down. But if I
were to switch off the reaction wheel now the satellite would now start
spinning in the opposite direction to which I had before. Now if we want the
satellite to stop spinning we need to have the reaction wheel spinning all the
time. If once again I push the satellite from the outside to stop the satellite
spinning the reaction wheel will have to go even faster. If I keep doing this
eventually the reaction wheel will be at its top speed and it won't be able to
stop the satellite from spinning from these external pushes any longer. This is
what saturation refers to. From here control moment gyroscopes are
pretty simple they work on the exact same principle of
conservation of angular momentum. the wheels in control moment gyroscopes
spin at a constant speed. We'll represent their angular momentum with an arrow
pointing through their axis of rotation. At their neutral position they're all
facing away from each other this means that the angular momentum arrows all
oppose each other and so the control moment gyros all add up to have a total
angular momentum of zero. The CMGs can then rotate independently so that their
angular momentums add up as we desire them. Then in order to conserve angular
momentum the whole ISS will have an opposing angular momentum an opposing
spin just like we saw when we span that reaction wheel on our satellite. It's all
very similar to the reaction wheel but instead of changing the speed of the
reaction wheel we're changing the orientation of the control moment gyros
to get the desired angular momentum. And just the same as the reaction wheels
CMGs can be saturated, this will be when all of the angular momentum arrows of
all four CMGs are pointed in the same direction
meaning they're at their maximum total angular momentum. In this situation the
CMG's need to be desaturated, this requires the use of reaction control
system thrusters on the Russian segment to maintain the space station's
orientation while the four CMGs return to their neutral position.
The use of a torque equilibrium attitude is most advantageous because it allows
the ISS to go for as long as possible without having to use thrusters to
desaturate the CMGs, as thrusters require fuel and can also mess with
delicate zero-g experiments. Generally most desaturations take place when the
ISS does large orientation changes like when it's preparing for a docking or
undocking. And that's how you steer the International Space Station, I sure hope
you found this as interesting as I have because it's pretty cool stuff.
I'd really appreciate it if you considered supporting Simply Space on Patreon and
I'll catch you in the next video