This episode of Primal Space is supported by Skillshare.
You are currently riding onboard a Falcon 9 rocket as it descends through the atmosphere
at over 500km/h.
What happens next is truly incredible.
At the last possible second, it fires up its engines and lands perfectly in the middle
of the droneship.
But there’s nothing new about this, SpaceX have been landing rockets for years.
But what is impressive is that we actually saw it live!
Ever since they started landing rockets, SpaceX have had a problem actually showing us.
It’s arguably the most exciting part of each mission, but the cameras at the landing
zone are well known for cutting out, just as the rocket comes into frame.
Of course, some people will claim that this is where SpaceX ran out of their special effects budget.
But actually, the problem is much more complex.
You have to remember that each droneship landing usually takes place hundreds of kilometers
off-shore.
When the rocket starts to head out over the Atlantic, it sends telemetry and a video signal
down to a ground station in Florida.
As the rocket gets further away from Florida, there is a point where it loses contact with
the ground station and switches over to the droneship.
From this point onwards, the rocket's only communication is with the droneship.
The video signal (along with other important telemetry) is sent from the booster to the
droneship using an S-band transmitter.
These transmitters can be really small and they are less affected by clouds and weather,
making them perfect for things like communications satellites, weather radars and airplane WiFi.
Both stages of the Falcon 9 are equipped with two S-band transmitters, which send the telemetry
and video individually.
An important part of every landing is the booster’s ability to communicate with the
droneship.
The droneship doesn’t communicate back with the rocket, but it constantly receives speed
and position data.
The droneship does its best to stay in the exact same position, providing the booster
with a stationary target to hit.
But the communication problems start before the rocket gets anywhere near the droneship.
As the rocket reenters the atmosphere, it’s travelling at hypersonic speeds.
The rocket heats up the air around it which breaks down the molecules, causing a layer
of plasma to form around the base of the rocket.
Plasma has a natural frequency between 2 and 20 MHz.
As radio waves get closer to this frequency, they get absorbed and have a harder time travelling
through the plasma.
This creates an enormous amount of noise in the signal and blocks it from reaching the
droneship in its entirety.
But in recent missions, the video signal has survived this portion of the landing, giving
us a great view of the rocket’s reentry.
SpaceX could be experimenting with their transmitters by placing them in different places on the
rocket.
Since the rocket re-enters at an angle, it’s likely that there is a thin window of space
where the transmitters can avoid most of the plasma.
In the past, it was almost guaranteed that the video feed would cut out during the rocket’s reentry.
But getting the video signal from the rocket to the droneship is only half of the problem.
If and when the video reaches the droneship, it has to be sent back to the broadcast team
in Hawthorne California.
Since the droneship is hundreds of miles out at sea, this can only be done via a satellite.
In the past, the droneship used a directional antenna to send the signal up to a satellite.
Although this offered a stronger signal, it had to be incredibly accurate or else the
signal wouldn’t be transmitted.
This is where the second problem comes in.
As the rocket comes in for a landing, it’s slowed down using either 1 or 3 of it’s
Merlin engines, each firing over 100,000lbs of thrust towards the surface of the droneship.
As the rocket gets closer, the thrust from the engines starts to shake the droneship
with such a high frequency that the video feed antenna loses lock with the satellite,
causing the video feed to cut out.
Once the rocket has landed and the vibrations have dissipated, the video feed would usually
reappear, showing a rocket standing tall in the middle of the droneship.
In our previous video about the droneship camera, many of you suggested having a secondary
ship, far away from the droneship which wouldn’t be affected by the vibrations.
Despite the need for an extremely large video cable to connect the two ships, this option could work.
But it comes down to cost vs benefit.
In order for SpaceX to do this, they would need to add an extra ship to their fleet.
This would add millions of dollars to the recovery cost, all for the sake of having
a live video feed.
So, SpaceX started to search for a more elegant solution to fix their video feed.
Luckily, SpaceX also happened to be working on an internet communications system called Starlink.
When fully operational, this massive constellation will provide high speed internet to almost
every part of the globe, including the most rural areas.
But it’s actually the satellite technology itself that proved to be very useful for SpaceX’s
droneship.
Starlink dishes have a phased array antenna, which allows the signal to be directed towards
a satellite without any moving parts.
It uses destructive and constructive interference to steer the radio wave in the right direction.
With more control over the direction of the signal and stronger resistance to the intense
vibrations, these Starlink dishes seem to provide a more stable connection than the
standard transmitters they were using before.
Sure enough, we started to see Starlink dishes appear on the droneships earlier this year.
Although this seems to have improved the overall video signal, it’s not as simple as it may
seem.
Since the Starlink satellites are still in an early design phase,
they don’t yet have the ability to communicate with each other.
For now, each satellite can only communicate with ground stations on Earth.
So when it comes to sending a video signal from the droneship, a satellite needs to be
in range of both the droneship and the ground station in Florida.
The majority of these satellites currently orbit at an altitude of around 550km and each
one can cover an area of 950km.
With most droneship landings taking place around 600km offshore, a single satellite
should be capable of receiving the signal and sending it down to the ground station.
However since each satellite moves across the sky very quickly, the Starlink dish on
the droneship will most likely have to switch between multiple different satellites as each
one goes out of range.
This should happen seamlessly just like your phone switching between data towers.
The result is a perfectly stable video signal from the time the rocket enters the atmosphere,
all the way through to it touching down on the droneship.
In the grand scheme of things, this was a small problem to fix - and one that most companies
wouldn’t have put any time or effort into.
But SpaceX understands how important it is for people to witness these incredible moments
of engineering.
When they landed their first rocket 6 years ago, it was hard to understand how something
so large could land with so much control.
The fact that landings like this have become so routine, shows how much SpaceX has revolutionized
the space industry.
With their ambitious plans to colonize Mars, they need more and more people to be interested
in the future of spaceflight - so it’s important that the technology is there for everyone
to see it happen.
Even a relatively non-important problem like this required a lot of clever engineering
from SpaceX’s brightest minds.
With everything that SpaceX does, they look for the smartest and most ambitious workers
with the right skills for the job.
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Thank you very much for watching and I’ll see you in the next video.