controlling a spacecraft is a little bit
different than controlling an Earthly
vehicle like a car or a plane mainly
because outside Earth's atmosphere
there's not much to push off of but
aerospace engineers have some ingenious
ways to get rockets and satellites where
they're going you may have seen the
yo-yo trick called around the world but
you probably didn't know that the
spacecraft that actually do go around
the world have their own yo-yo trick hey
I'm Grady today on practical engineering
we're talking about spacecraft attitude
control and the yo-yo D
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spin how do you keep a rocket or a
satellite pointed in the right direction
you might think this is pretty simple
put an engine on the side you want to go
away from right but actually in the real
world of space fairing vehicles it's a
little bit more complicated than that
anytime you apply a force that's not in
line with the center of rotation that's
called a torque when you use a wrench
you're torquing a bolt by applying a
force offset from the axis of rotation a
torque is a product of angular
acceleration so anytime you apply one
you're going to induce some rotation
unless there's a reaction to
compensate naturally we want to keep our
spacecraft on the straight and narrow
but there's so many torqus that can
influence their rotation you can have
aerodynamic forces like strong winds or
atmospheric drag you can have
perturbations from variations in gravity
effects from Earth's magnetic field or
even solar radiation pressure you can
also have internal torqus from the
spacecraft itself for example if you've
got an engine maybe it's not mounted
perfectly in line with the center of
mass if you've got more than one engine
maybe one is a little bit stronger than
the others
there's a host of torqus that can induce
rotation of your spacecraft so Engineers
have come up with a host of ways to
counteract these torqus and keep things
headed in the right direction you can
use control surfaces to react against
the air rushing past the rocket just
like an
airplane you can gimbal your engine to
give you more control over the direction
of thrust you can use small directional
thrusters located off axis from your
spacecraft and you can also use fly
wheels to counter rotate against any
extraneous torqus all of these methods
have their own advantages and
disadvantages but the overarching
problem with all of them is that they're
complicated now you may think that
aeronautical Engineers isue the kiss
principle after all quote unquote rocket
science is the ubiquitous analogy for
something that's overly complex but
consider this if it's a moving part or
it creates incredibly hot and
fast-moving exhaust gases there's a
chance that it might might break and if
it breaks there's not a lot of
opportunity to get a technician into the
upper atmosphere or lower orbit for
maintenance or repairs so attitude
control Engineers developed a dead
simple widely used technique of attitude
control called spin stabilization and
you can probably guess how this works if
you've ever played with a gyroscope you
know that it resists changes in the
direction of its main axis it works the
same way with a rocket or a satellite if
you spin the spacecraft around the axis
it's moving along it will resist small
torqus just like a gyroscope as an added
benefit if there's some asymmetry in the
spacecraft instead of accumulating
torque in a single Direction now you're
averaging the asymmetry around the main
axis this is a really simple method of
stabilization because it usually doesn't
require any additional moving parts or
systems but it does have its own
disadvantage namely that your spacecraft
is wildly spinning in C CES at the end
of it sometimes that's okay but a lot of
the times your instruments or sensors or
cameras don't work if they're spinning
so after your engine burn you want to
dpin naturally there are several ways of
dealing with this as well but remember
we're trying to keep things
simple let's talk just for a minute on
what it actually takes to desin
something a spinning body has a certain
amount of angular momentum which is a
product of two values angular velocity
that's just the rate of rotation and
moment of inertia which is a measure of
how a body's mass is distributed around
its axis of
rotation a rotating Rod has a low moment
of inertia because its mass is
concentrated around the axis a rotating
tube of the same mass has a
comparatively High moment of inertia
because the majority of its mass is far
away from the axis now angular momentum
is preserved as long as there is no
outside forces applied that's just
Newton's third law so the angular
velocity or spin rate is on a seesaw
with moment of inertia if one goes up
the other goes down and vice versa this
is really cool because it says even
without any outside forces there's a way
that we can reduce the rotation of our
spinning spacecraft just increase the
moment of
inertia the classic example of this is
the figure skater and you could try this
yourself even if you don't skate spin
yourself in an office chair with a
couple of Weights in your hands if you
bring in your arms you decrease your
moment of inertia so your speed goes up
to
compensate the yo-yo D spin works
exactly the same way but in Reverse
except at the end you get your arms cut
off I put together a model to illustrate
this concept first I turned a disc of
MDF on the lathe and mounted it on an
aluminum tube with some bearing so it
can spin freely
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I've got a Servo on top which is
attached to a couple of quick release
mechanisms when the servo activates it
pulls these cords which release
whatever's attached there are two
tethers with fishing weights on each one
and these get wrapped around the disc
and clipped into each quick release and
this is powered by an Arduino so I can
push a button and activate the servo to
release the yo-yo masses let's see how
it works
when the yo-yo masses are released they
start to unwind since they're getting
further and further from the point of
rotation they're increasing the moment
of inertia of the system once the end of
the tethers is reached they slingshot
forward absorbing the rest of the
kinetic energy before they're released
the crucial parameters when you're
designing something like this are the
length of the tethers and the mass of
the two weights and actually if you're
going for a complete dpin the initial
angular velocity doesn't matter which is
convenient because I could just spend
the model by hand without worrying about
how fast it was going I designed this to
release the masses when they're exactly
radial but it's pretty tough to get this
perfect so sometimes the model was
actually spinning in the opposite
direction when the masses let
go in real spacecraft they have cool
release mechanisms like explosive bolts
but I wasn't able to get my hands on
these and as cool as a demonstration as
this is it probably wouldn't be much
good for a science fair or career day
unless you can come up with some way to
contain these flying
masses this is a problem in real
spacecraft as well and these masses can
contribute to the ever increasing amount
of space junk orbiting the Earth but
despite that the yo-yo dpin is a really
clever application of simple physics to
accomplish an important job and
especially with spacecraft sometimes
simple as
best I hope you like the video and if
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clicking the like button and subscrib to
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keep making cool stuff if you want more
details or you have a question or a
suggestion for a future topic I'd love
to hear from you in the comments below
thank you for watching and let me know
what you think
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