Endeavour Houston we see a nominal MECO Welcome to space
Hey it's Ben here
Docking is an integral part of modern spaceflight. It is the physical
connection of two spacecraft in space, and to do this we are going to look at three main parts.
First is how we actually get that fine control over
the position and rotation of our spacecraft. The next is something called
station-keeping and then that approach for the docking. And then finally the
docking itself, actually physically connecting the two spacecraft together.
We control our spacecraft precisely using a Reaction Control Systems. What a
Reaction Control System is, is just a set of small thrusters around the spacecraft.
It is called a reaction control system because of Newton's third law of
motion, which is often phrased as every action has an equal and opposite
reaction. Here's the way I like to think of it, it's as if we've got two blocks on
the table here, squished a spring between them and then let go.
They are both pushed apart by the spring. So in a spacecraft this is how the
thruster works, it combusts fuel in the combustion chamber making pressure
releasing gas out of the back, the gas is one of the blocks, the spacecraft is the
other block and the pressure produced in the combustion chamber is that spring
pushing the two apart from each other. When we use our reaction control system
we'll see that there's actually no air resistance in the space because it's
well... space, so we'll keep going and if we want to slow down, then we'll have to
fire our reaction control system in the opposite direction to then stop that
speed that we've added in. We can also rotate our spacecraft using
a Reaction Control System by firing thrusters on opposite sides of the
spacecraft in opposite directions just like this, and again there's no air
resistance so to stop the rotation we'll have to fire in the opposite
direction again. Now the thing with Reaction Control System thrusters is
that they're made to be highly reliable and fire lots and lots of times
usually in quite short specific bursts. So to do this they often use hypergolic
propellants. What hypergolic propellants means is that they don't really need an
ignition source. Wou can just pump the fuel into the combustion chamber, which
is made of two chemicals, and when those two chemicals contact each other they'll
spontaneously combust producing that pressure and the gas going out the back.
That's how a Reaction Control System works, now let's look at station keeping and approach.
Station-keeping is essentially just firing our Reaction Control System every
now and then to make sure that we hold the same position relative to our target
we'll use the International Space Station since that's what's often being
docked to these days. So if we're in orbit we know that different altitudes
relate to different speeds of orbits, you'll know this if you saw my last video
about rendezvous. We might be near the space station like this but it
doesn't exactly match the Space Station's orbit, that'd be really hard to do
and as such we're gonna drift away because we'll be at a slightly different
speed relative to the space station. So in order to stay in the same relative position
we're just going to have to fire our reaction control thrusters every now and
then to stay in that fixed relative position until we're given a go to dock.
Once we given the go to dock we can fire our Reaction Control System to move us
toward the space station, and you'd think that might be all we have to do but
there's a bit more of a complication because we are actually in orbit.
let's think about this in the frame of the orbit here. When we fire our
thrusters towards the space station here, that's all well and good we're going
towards the space station but we're not necessarily going towards the space
station we're going towards the direction that the space station happens
to be at in that point in time if we go a quarter of the way around the orbit we
can see that direction is actually pointed upwards relative to the space
station so when we fired it here we'll find ourselves going towards the space
station but once we get a quarter of the way around we'll find ourselves going up
because of this as we go around the orbit while we're docking we actually
have to make sure that we're firing our reaction control system to account for
this to keep our approach speed in the direction of the international space
station until we get close enough that we're actually going to have our docking
port physically contact with a docking port on the international space station
let's talk about that. there are three main docking systems used on the
International Space Station right now one of them technically isn't a docking
system it's actually a berthing system, a little more on that later
so the first one that we're going to look at is a Russian one and it's on the
Russian segment of the International Space Station there's quite a few of
these docking ports and they used for docking the Russian Soyuz and progress
spacecraft they are a docking port of a design call this is SSVP which stands for
Система стыковки и внутреннего перехода that is Russian and it
translates to system for docking and internal transfer. it works using a probe
and a drogue. the probe is on the arriving spacecraft in the drogue is on
a space station. the drogue is just a cone really with this area in the middle
for the probe to latch into. when the reaction control system on the
spacecraft is firing it's pushing the spacecraft in that direction of the
drogue. the probe contacts the drogue and it is then because of the cone shape
pushed and to the center there, allowing for a margin of error. once the
probe reaches the center of that drogue, it can latch in, and once it latches in, the
probe will start to retract pulling the spacecraft in to a proper fit at which
point the proper locks latch into place, holding the spacecraft
to the space station at which point the probe and drogue together open up as a
hatchway for astronauts to go through
the next docking system is called the international docking standard, and it's
not really a system because standard is the key word there. It's actually an
"instruction booklet" on how to build a compatible spacecraft that will be able
to dock at the International Space Station's international docking adapters.
There are two of these and they're located on the harmony module in the
US segment. the advantage of having this standard, is that it means that
different companies and different countries can build spacecraft that are
able to dock there, without having to pay expensive fees to any specific company
or government. Now, there are two spacecraft in development right now
that are going to be using this docking adaptor in 2019. They're the SpaceX Dragon and
the Boeing Starliner. It works by having this target ring on the international
space station, and then a ring on the arriving spacecraft. But that ring on the
arriving spacecraft is held with these actuators
that can move it about and rotate it slightly, that way there's a margin for
error since the reaction control system can't get a precise fit of those rings.
When the spacecraft approaches, those actuator arms will line the two rings up
and then they will push together, at which point those rings will lock
together and then the arms can retract, pulling the spacecraft in at which
point a seal will be formed, some checks will be done and then the astronauts can
open the hatches and go through into the space station.
The next system is called the Common Berthing Mechanism, it's technically not a
docking port because a docking port is something you can attach to while using
your own reaction control system to maneuver you into position. The Common
Berthing Mechanism just requires way too much precision for that, because it's
literally just these two rings that end up getting locked together. As such, we
have to use the space stations robotic Canadarm to perform that precise
fitting. The advantage with a Common Berthing Mechanism is that it has quite
a large diameter, so it's great for cargo supply craft like the SpaceX Dragon and
the Northrop Grumman Cygnus. It's also great because it's used to actually
attach all the modules on the US segment of the space station together, because
again, that large ring allows them to transport equipment and experiments
through those Common Berthing Mechanisms between the modules. So that's docking
and also Berthing, it's Simply Space