- 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. As always, I owe a huge thank
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earth for everyday people. (upbeat music)
