- Hi, it's me, Tim Dodd, the Everyday Astronaut.
If you've ever seen footage of a SpaceX rocket launch,
perhaps you've noticed a strange phenomenon,
when the rocket leaves the launch pad, it's white,
but when it comes back down from space
all of a sudden it's half black and half white,
and it has these really well defined sections
of black and white.
So what the heck is happening?
What causes this?
Today I'm going to explain just what exactly causes
this phenomenon and then we're actually
going to try to recreate it ourselves.
And no, it's not just because I wanted to play with fire.
Anyway, let's get started.
- [Announcer] Three, two, one, zero.
We have lift-off. (upbeat music)
That's one small step for man...
- Isn't it crazy that it's been less than three years
since we first saw a Falcon 9 rocket land
for the first time back on Earth?
I still remember watching OG2 on December 21st, 2015
with my friends, and my cat.
(cheering)
When SpaceX landed their first Falcon 9 booster,
I couldn't wait to see what it actually looked like
when a rocket came back from space.
The next morning pictures started flowing in
from launch photographers and from SpaceX themselves.
There it was.
A Falcon 9 on the landing pad only a few kilometers
away from where it took off from
at Cape Canaveral Air Force Station in Florida.
Just as soon as the pictures came back,
the questions started rolling in.
Why is it now half black and half white
with a very distinct line between the two?
Are there a bunch of aliens at the edge of space
waiting for rockets to fly by so they can spray paint 'em?
Alright, so let's first start off by figuring out why
the Falcon 9 and Falcon Heavy boosters come back
with any black on it at all, and then we'll figure out
why there's such a distinct line
where it is and isn't black.
(uplifting music)
And the answer is, soot.
The reason the Falcon 9 and Falcon Heavy boosters
come back so dirty is literally the exact same reason
why your chimney is dirty or why the tailpipe of your car
might be dirty, well, unless you own an electric car
and you don't have a tail pipe.
The Falcon 9 and Falcon Heavy burn
a mixture of RP-1 and Liquid Oxygen.
RP-1 is a very common form of rocket fuel
and it's basically just highly refined jet fuel,
which itself is just a highly refined kerosene.
When kerosene or any other carbon based fuel is burned,
say gasoline, diesel, coal or even wood,
it produces airborne impure carbon particles.
It's actually the result of the process
of thermal decomposition of materials known as pyrolysis.
We've all seen the black smoke come out of a semi-truck
when they're under high loads,
or perhaps a little black smoke coming off of a candle.
Well the Falcon family of rockets
actually produce virtually the exact same thing.
And I know what you're thinking,
doesn't this go against Elon Musk's green initiatives?
Well, I'll get into that in a future video
where we look at the total carbon output of rockets
and what technology can help curb their pollution.
Okay, so, back to soot.
On ascent, as a rocket accelerates forward,
the exhaust goes the exact opposite direction,
literally throwing the soot far away from the vehicle.
This is why on ascent, the rockets
maintain their beautiful exteriors.
But that's only one half of the story
of a Falcon 9 or Falcon Heavy boosters.
After the booster performs the work of getting
the second stage well on its way into orbit,
the booster has to get ready for its next feat.
And this is no small feat.
Regardless of where the booster is landing,
whether it be a return to launch site landing
or if it's following its ballistic trajectory
and landing out in the ocean on the drone ship,
it will need to scrub off velocity
before it re-enters the atmosphere.
If it doesn't do this, the heat from re-entry
will literally rip it apart.
So in order to survive re-entry the Falcon 9
lights up three of its nine Merlin engines
for approximately 30 seconds or so
to cut its velocity up to 50%!
And here's where things get dirty.
The booster is re-entering through its own exhaust plume
since the rocket re-enters engines first.
So now, all that exhaust winds up depositing
a large amount of soot on the booster.
We can actually see a pretty drastic difference
from before re-entry to after re-entry.
As if that one burn wasn't enough, a few minutes later,
it actually performs one final burn
to perform its propulsive landing.
Sometimes this utilizes three engines again, but most
of the time only one engine performs the landing burn.
But nevertheless, it's again a pretty messy event.
Once the booster lands, and the smoke clears,
we can now get a pretty good glimpse
of just what exactly the booster went through.
And here's where our next question comes in.
What's with the sections that are white?
How can that be?
Pretty quickly, we might spot
why there's some white still at the bottom of the rocket.
See those nice V-shaped white sections,
those are where the landing legs were tucked up
against the fuselage during ascent
and the majority of descent.
The legs don't deploy until just a few seconds
before touchdown, at which point some pneumatic pushers
begin to unfold the legs, and then a collet locks
them in place once they're at their full extension.
Pre-Block 5 boosters, or any Falcon booster
flown before May, 2018, had white landing legs.
All Block 5 boosters that we see now start off black.
Back on the pre-Block 5 boosters, we could easily see
the outside legs changing from white to black,
leaving the fuselage underneath nice and clean!
Okay, so that explains that section,
but what about that horizontal line just above that?
Now here's where this gets fun!
Remember those two things the Falcon 9
and Falcon Heavy use as propellant?
Liquid oxygen and RP-1?
Well there's your next clue.
SpaceX super chills their propellants to maximize
their density and increase performance of the rockets.
This means they chill the RP-1 and the liquid oxygen
more than any other operational rocket.
So SpaceX took their RP-1 from basically room temperature
at 21 degrees celsius down to minus seven degrees celsius.
Ugh, brrr.
But that's actually nothing compared to the liquid oxygen.
In order for oxygen to exist as a liquid
as opposed to the gas we're more used to it being,
it already needs to be extremely cold
but SpaceX continues to chill it until it's almost a solid.
Oxygen becomes a liquid at minus 183 degrees celsius
and a solid at minus 219 degrees celsius.
SpaceX chills their liquid oxygen all the way down
to minus 207 degrees celsius.
Our next clue is actually the most visible as the vehicle
is being fueled up on the launch pad right before lift off.
Notice all the condensation rolling off the rocket?
That's water vapor in the air coming in contact
with the super cold aluminum skin
housing the cryogenic propellant below.
Just like how a cold soda can will quickly
get large droplets of water on the outside of it
in a humid environment, rockets go a step further.
All that water doesn't turn into droplets,
some of it comes in contact with the fuselage and freezes.
This creates a good amount of ice build up
on the outside of the vehicle.
Some rockets have insulation to help prevent the build up
of ice from occurring, but more importantly to prevent
the propellant inside from warming up and boiling off.
This is most commonly seen in rockets
that use liquid hydrogen as fuel, which is much cooler
than liquid oxygen and boils off much easier.
Therefore we see insulated tanks on rockets
like the the Delta IV, the external fuel tank
of the Space Shuttle, or even the Saturn V.
But my favorite solution for having insulation
to prevent boil off, that doesn't add extra weight
associated with foam insulation is on the Minotaur rocket
which is sometimes known as the banana rocket.
It has an insulation on it during fuel up
that peels off on ascent.
Yes, that's so ridiculous.
I just hope no one slips on it during pad clean up.
(drum beating)
Back to the Falcon 9 and Falcon Heavy.
It doesn't use spray on insulation or a banana peel.
Instead most of the ice falls off in sheets
right after lift off.
Due to the rocket being white,
it's hard to actually spot the ice build up.
Bit there is a rocket that's really easy to spot
where there's ice on it
and that's Rocket Lab's Electron rocket
which is entirely made out of black carbon fiber.
Despite it being an entirely black rocket,
once it's fully fueled it suddenly looks
like an entirely different rocket!
How crazy is that?
It's so easy to spot where the liquid oxygen tanks are
and where the RP-1 tanks are on the Electron.
So now finally, we have all the puzzle pieces
for our answer!
That same sheet of ice that covers
the liquid oxygen tanks during ascent
is still fairly prevalent on the rocket on descent.
It might not be as thick, but it does provide a surface
that's harder for the soot to stick to.
Therefore, we see little to no soot
on the base of the oxygen tank,
providing a crisp line between the oxygen tank
and the RP-1 tank.
Since the cold oxygen sits at the bottom of the tank
which is now almost empty,
we have the most ice at the bottom,
which explains why the booster
actually gets gray again up top.
Two fun notes here.
All the current generation Block 5 Falcon 9's
come back quite a bit less sooty due to a new coating
on the paint that provides more thermal protection
and is harder for the soot to stick to.
SpaceX also cleverly changed the position
of the SpaceX logo to the oxygen tank.
This little trick is amazing, because now you can actually
still see the logo after the booster lands,
unlike previously landed boosters whose logos were so hidden
in the soot you couldn't see it by the time it landed.
Okay, and now for something fun, we're actually going
to see if we can recreate this phenomenon
using some aluminum, some dry ice and some kerosene.
Okay, time to play with some fire.
So, I have here some aluminum that I painted white.
That's going to go over the top of this cup
in this little thing here, so that the ice will sit on top,
the dry ice will sit right on top of this.
And we'll have that nice clean separation between
the sections that are really cold
and the sections that are not,
just like the oxygen tanks and the liquid fuel tanks.
Then we have some dry ice that, although it's really cold,
it's quite a bit warmer than liquid oxygen.
It's minus 78 degrees celsius and liquid oxygen is actually,
SpaceX cools it down to minus 207 degrees celsius.
So this is quite a bit cooler but hopefully
it does the trick to at least,
help us get some kind of ice barrier around the outside.
We have a very shoddily made, homemade kerosene candle
that's going to produce a lot of soot.
That's it's only job is to make a lot of soot
because that's what we're trying to demonstrate here.
And this is a spray bottle with some water in it.
And we're going to spray the outside of this
a little bit to help add a little bit of extra frost
some kind of extra ice layer.
So, let's get started.
So, first things first, let's go ahead and stick
this aluminum over the top of this cup.
There we go, just like that.
Let's put on our official safety gloves.
These are NASA certified,
not-at-all-certified safety gloves.
So I need to get a bunch of this,
as much of this as I can inside of here.
Okay you can already see
there's a small amount of water vapor on the outside
that's condensing into a little bit of frost here.
You can just barely tell.
But we're gonna let that actually
get cold there for a second.
And actually maybe I'll start spraying
a little bit of water here, very lightly.
Just so maybe that sticks to the outside.
Okay, I think it's time we light this candle here
and we're going to see if we can get a bunch of soot
cover the aluminum and then hopefully,
we'll see where it sticks and where it doesn't stick.
Don't try this at home, this is a very shoddily made candle.
But, it's going to again do the job,
hopefully creating a lot of soot.
So now I think we've got a pretty good sense here
of where that would be.
I'm going to put this fire out here.
Just starve it of oxygen quick.
So what happens if we let this actually thaw,
will it take some of the soot with it?
So let's go ahead and try and remove
the aluminum from the dry ice.
And let's kind of wash it down quick
and see if we get that nice line.
Well, look at that.
That line looks pretty familiar to me.
So, it looks like where the ice barrier was
the soot didn't stick to it.
But, where there wasn't an ice barrier
the soot did stick to it.
That looks just about like the line.
And again, that'd be a lot, a lot, a lot stronger
if we were using liquid oxygen.
There'd be a much thicker ice barrier,
a lot more protective.
But that's actually pretty darn good.
Look at that!
Huh.
So how'd we do?
Let me know if you have any other questions
about why a Falcon 9 goes up white and comes back
black and white or any other questions
about Falcon 9s and Falcon Heavys
or just any other question about rockets in general.
Leave your thoughts in the comments below.
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Thanks everybody.
That's going to do it for me.
I'm Tim Dodd, the Everyday Astronaut,
bringing space down to earth for everyday people.
(upbeat music)