Hi it’s me Tim Dodd the Everyday Astronaut.
SpaceX is now better at landing their Falcon 9 rockets than I am at flying kites… but
one question comes up often “Why do they land on a barge in the ocean sometimes and
land on land other times?” and sometimes we see that SpaceX doesn’t even try to land
the first stage at all…
In order to answer these questions we’ll go through the variables that are at play
and show you some examples using the industry standard giraffestronaut in a convertible.
Wait what?
Yeah, this episode rules...
Watching a SpaceX rocket land is maybe the coolest thing to watch this day and age.
If you're unfamiliar with what I'm even talking about I suggest you check out my video “Why
Throw Rockets Away” or if you need an entry level vocabulary of rockets, I'd suggest you
start here with my series “A Beginner's Guide to Rocket Vocabulary”
Now we’ve seen SpaceX land their Falcon 9’s on both their Autonomous Spaceport Drone
Ship (ASDS) and back on land at Kennedy Space Center at their landing pad LZ-1.Their two
Drone Ships are the sizes of football fields and are cleverly named “Of Course I still
Love You” and “Just Read The Instructions” which are nods the science fiction book “The
Player of Games”
So what does determines where the Falcon 9 will land?
Well, I'm glad you asked because it’s really quite simple really -
First we take a Runge Kutta Integrator that will numerically integrate a system of different
equations from an initial condition state vector forward to an arbitrary moment in time.
Then we derive the equations of motion for your vehicle.
We need to take into account aerobraking with some translational equations that are highly-coupled,
nonlinear ordinary differential equations that require numerical routines.
I don't know what any of this means.
Whyyy!
My brain hurts just trying to read that sentence, let alone try to do any of the complex math.
So let’s start with something we can all relate to first, ROAD TRIPS!
And to be more specific road trips involving a tow truck pulling a car with 2 astronaut
giraffes, you know giraffestronauts.
Normal stuff.
Our 2 Giraffestronauts are on a road trip from SpaceX’s HQ in Hawthorne, CA and they
want to get to San Fransisco, some 400 miles away.
Now because they don’t have opposable thumbs, they can’t operate a gas pump, duh!
So they need to get there without filling up.
With two giraffestronauts, the car gets 30 mpg, (meaning it can go 30 miles on one gallon
of gas) and their car has a 10 gallon gas tank, giving them a range of 300 miles.
If they bring any other giraffestronaut friends along, it hurts the fuel economy by 2 miles
per gallon due to their long, gangly necks.
Unfortunately, the car can’t get them there on it’s own, so they get smart and say,
“I know, let’s tow our car part of the way and then we'll drive the rest of the way!!!!
HUZZAH!”
So they call up a tow truck who shows up with a massive 40 gallon fuel tank.
The tow truck on its own can get 10 miles per gallon with a total range of 400 miles,
but then they realize that when they put the car with its passengers on it, the gas mileage
drops to 4.5 mpg, with any additional passenger decreasing it another .5 mpg.
So now with the car and two giraffestronauts on board, the tow truck’s range drops to
just 180 miles.
The tow truck driver says, “Ok, I’ll take you 100 miles so you can get there and I can
still get home” In doing so, the truck will chew through 22.2 gallons to deliver them
the first 100 miles, leaving the truck with just 17.8 gallons to get home, and now that
it’s not towing the heavy car and goofy giraffestronauts, it gets 10 mpg and that's
178 miles of range, which is easily enough to get back home.
Ok, let’s add another giraffestronaut to the equation for a total of 3 in the car.
The car’s gas mileage will drop from 30 to 28 mpg for a range of 280 miles.
So the tow truck driver sighs and says, “ok, I’ll take you 120 miles, but now at only
4 mpg, I’ll use 30 gallons of gas, leaving me with only 10 gallons left to get home.”
He looks at the map and sees that’ll leave him hanging out somewhere near Beverly Hills.
Which isn't too bad of a place to hang out.
And he's got a cousin that lives there so he'll just hang out there until someone comes
and picks him up!
And lastly, what happens when we add one more giraffestronaut for a total of FOUR giraffestronauts
in the car, will they still make it to San Francisco?
The car's gas mileage will drop from 28 to 26 mpg for a range of only 260 miles.
The tow truck will only be getting 3.5 mpg dropping its range to only 140 miles, JUST
enough to get get all four giraffestronauts to their destination.
But in this case we’re going to leave the tow truck stranded way up near Bakersfield
and nobody makes it back from Bakersfield, am I right?!
Bakersfield!
So, do you see what we did here?
By simply adding a few more giraffestronauts, we quickly altered the trips of both vehicles,
drastically.
In all of these examples, we were able to get the giraffestronauts to San Fransisco,
but depending on how many of them there were, we were pushing the limits of how far the
tow truck could go and if it could return.
I may have lost a lot of sleep thinking about this...
Our parallels here are the first stage of the Falcon 9 is like the tow truck, the second
stage is like the car, and the giraffestronauts are the payloads.
So let’s take a look at three actual SpaceX missions to see what this looks like in real
life.
On February 19th, 2017, SpaceX launched one of their Dragon cargo vehicles on top of a
Falcon 9 to the International Space Station for mission CRS-10.
The dragon capsule weighed 4,200 kg.
This meant there was plenty of margins left over for the first stage to turn around and
return to Kennedy Space Center and land at their landing pad, LZ-1.
On March 30th, 2017, SpaceX launched an SES communications satellite to Geostationary
Transfer Orbit on a Falcon 9 for mission SES-10.
Now this baby weighs nearly 5,300 kg, so it only had enough margin to land on the autonomous
spaceport drone ship about 400 miles down range.
Lastly, On March 16th, 2017, SpaceX launched EchoStar 23 on top of a fully expendable Falcon
9 to geostationary orbit.
The satellite weighs 5,500 KG which pushes it just beyond what can be recovered.
So, just like rockets of the past, the first stage plummeted into the ocean after only
2:40 seconds of use.
To keep the price of the rocket down and to get the most performance out of the vehicle,
they strip it of its landing legs and grid fins, which looks just plain weird these days!
I call it a naked Falcon 9.
Much like with our giraffestronauts going to San Francisco, small changes in a payload
can mean big changes in how far each vehicle can travel.
Something to remember is the Falcon 9 1st stage weighs a whole lot less on its descent
without the 2nd stage and payload on top of it.
This means it requires a lot less fuel to land on its own than it would require if there
was all that additional weight on top of it.
People have also asked me, “Why don’t they land that first stage in Africa or Europe?”
Their thinking is the rocket is traveling east, doesn’t that mean the first stage
could end up in Africa or Europe?
Well, let’s take another look at that map to see how far it actually is.
Oh… we’re just barely off the coast of Florida still...
Notice that even an expendable first stage that burns all of its fuel still crashes only
a few hundred miles off the coast of Florida.
A very, very long ways away from Europe or Africa.
But most of all landing on a boat, although it seems crazy to us humans, has proven to
be a very achievable feat by a computer controlled rocket.
But all of this drills in why recovering and reusing a multi million dollar fine piece
of engineering is so important.
It’s a shame that the portion of the rocket that is the biggest and costs the most is
completely done after less than 3 minutes of use.
That's crazy!
That's really expensive.
And it only gets used for a little bit of time.
Then nothing happens!
*Mumbles*
So I think that does it for why SpaceX sometimes lands the Falcon 9 on land, sometimes the
ocean on the barge and other times it doesn't land at all.
If you have any other questions or comments about this, leave them in the comment section below.
Make sure and hit subscribe so you know when I make more fun, funny and factual content.
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If you want to help contribute, please visit Patreon.com/EverydayAstronaut
Thanks to Declan Murphy of the awesome website, flightclub.io for the amazing telemetry visuals.
Thanks everybody, that does it for me.
I’m Tim Dodd the Everyday Astronaut.
Bringing space down to earth for everyday people.