Hullo it's scott manley here one of the questions I frequently get asked especially
after i posted an episode of deep space update is What the heck is falling off those chinese rockets
and the answer to this is actually quite simple they are foam
panels attached to the side of the rocket and they provide thermal insulation and they're
designed to fall off as the rocket launches China isn't the only country that does this you
can see these on Ariane rockets from Europe and some Indian rockets but beyond this simple answer
i think there's actually a much larger story about insulation that might be worth talking about
if you go back to some of the most epic rocket footage ever you've got the Saturn 5
right the apollo missions lifting off and all those amazing slow frame rate. I mean high frame
rate cameras giving you slow motion views of this epic rocket ascending off the pad and you can see
huge amounts of material cascading off the side this is ice that has formed
The saturn v was fueled by kerosene, liquid oxygen and liquid hydrogen and those extraordinarily
cold propellants will make the outer surface of the rocket below the freezing point of water
and that results in the air for your freezing out it's ambiguous water the humidity onto the side of
the rocket and then when the countdown hits zero those engines light up the vibrations the motion
the flexing of the vehicle causes that ice to fall off the side and cascade down and it is some of
the most glorious footage in rocketry but actually it's not a big deal the ice falling off the side
generally just fell down and you're smashed to the ground now the saturn v liquid oxygen tanks
like many other rockets the outer coating has no insulation on it's literally like a painted
sheet of metal with the liquid oxygen on one side in the atmosphere on the other and you just accept
the fact you're gonna get water freezing to the outside now the liquid hydrogen tank on the other
hand is significantly colder liquid hydrogen has a boiling point of about 20 kelvin it's so cold that
it isn't just the water from the atmosphere that freezes to the side of the tank you actually have
the air the nitrogen and the oxygen they would condense on the side of the tank and then they
would run down the side of it and that instead of forming like an insulating layer with the water
that would actually take heat and transfer it into the tank and therefore accelerate the boiling of
the hydrogen which is why they absolutely needed to have insulation on that tank so the saturn v
it had insulation. Early installation used was like a polyurethane foam with a fiberglass
honeycomb matrix and then an impermeable layer on top of that towards the end of the saturn V run
they switched this over to like a spray-on foam again with the impermeable layer.
The impermeable layer is important, there's a paper i found that pointed out if it's damaged and cracks form then
air can get into this then it starts to freeze and you start collecting liquid oxygen mixed with
polyurethane foam and if you hit that hard it can catch fire or even explode which is
'fun' when you've got hydrogen on the other side.
So anyway with the Saturn V taking off and all that ice falling down the side, that wasn't a big deal that was just a launch pad the ice was hitting
But after the Apollo program came the Space Shuttle program and for that if you had ice falling off
the side of your propellant tank you had the bottom of the Space Shuttle sitting right there
so they had to stop any ice freezing on that that's why the Space Shuttle tank absolutely
needed to be covered in a relatively thick one inch layer of polyurethane foam
So this foam they used was a spray-on type foam, they developed it for Apollo and has an impermeable layer on top of it.
Now we know as this is our familiar orange color which is also seen on the delta iv .
But actually when it's first applied it's much closer to a light yellow in color
and it sort of just turns that way over time as as it breaks down under sunlight
For the earliest versions of the Space Shuttle, the first two launches they actually covered
this with white paint to protect against the ultraviolet but after enough experience
They realized that this was just adding weight to the shuttle and not really helping
keep the foam intact in any more way. So that kind of foam has since been used on many
more rockets as i said the Delta iv it's used on SLS and it's also used on the centaur upper stage on the
Atlas 5 and will be used on the the the Vulcan as well of course the downside of this
is that while ice is very bad for the shuttle it turns out that once you get to supersonic
speeds the foam gets decelerated by the airflow if it breaks off and that can damage the tiles
and we saw that on one mission where they almost lost and we definitely saw a big chunk fall off
from the forward bipod and damaged the wing that led to the Columbia disaster so you know
the insulation is incredibly important. But unfortunately it also has been associated with
various failures. My solution, by the way is that they should have mounted the tank on the back of
it and got rid of that big tail plane and put like little tail planes on the side that would have
made far more sense but hey you know we've moved on to other things since then but anyway my point
about the foam falling off is that if it falls off once the vehicle is going very fast then it gets
entrained in these supersonic air streams and very quickly accelerates to high velocities
relative to the rocket or the this vehicle and damages it so this is why it's actually
good to have your foam fall off very early in the flight and for example Ariane 5 that does that
around the attachment points to its solid boosters there are foam coverings that cover the uh
the separation thrusters because you don't want to have the cold liquid hydrogen there causing any
condensation or blockages in those thrusters that could be a problem or having a chunk of ice build
up that when the thrusters fire gets knocked into the side of the rocket and causes a loss of that
so there's this glorious photo by Trevor Mahlman showing these panels falling off the side of the
Ariane 5 as it ascends so yeah you get the idea if you look at this you start to find these sort
of features in many many places.
But by this point many of you will say
"well wait a second the Long March rockets you're pointing at don't actually use cryogenic propellants"
the first long march one through four they all use nitrogen tetroxide and UDMH
these are hypergolic propellants these are storable propellants that are liquid
at room temperature so you might naively think that controlling the temperature of the propellant
isn't necessary but actually if you look at nitrogen tetroxide or dinitrogen tetroxide
as it's correctly called it actually has a very narrow range right i think it freezes at -11C
and it boils at 21C, you can put in some additives to extend this range a little
but beyond that it also disassociates or decomposes so dinitrogen tetroxide basically has a
pair of oxygen a sorry pair of nitrogen and four oxygens and sometimes as it warms up it splits
into nitrogen dioxide and by splitting up it reduces the density and it changes the reaction so
ideally you want to keep it at one temperature where your engines are all optimized so
you'll notice that for the Long March rockets they use these panels at the inland launch sites where
they have these old hypergolically fueled rockets and they will tend to only use them
in the winter and i think that's to keep it warm enough so that the stuff doesn't actually solidify
on the other hand the ariane one through four it also had a similar sort of shroud an insulation
shroud that was attached around the second stage again the first stage and the second stage of the
Ariane 1-4 use hypergolic storable propellants in the second stage they would have this uh
shroud and as the rocket lifts off it would get pulled off by wires and the rocket would continue
into space so i think in that case however that was used to keep the temperatures down to
a functionally useful temperatures given that the launch site they were working from was very
very close to the equator. The cryogenic third stage was also insulated but it actually kept
its insulation attached throughout the flight but now if we return back to historic american rockets
if you look at the early Centaurs they would have big foam panels attached around those to
control the temperature of the liquid hydrogen and oxygen and there's actually an incredibly famous
rocket disaster that many people have seen though who have no interest in rockets right
Atlas Centaur One was the very first test of the Centaur upper stage on atlas and
um the reason it's well known is it didn't work it exploded in this amazing fireball that
was captured from every single angle and many of those camera angles found their way into an
art movie called Koyaanisqatsi which basically takes a lot of stock footage and pairs it with
a soundtrack by Philip Glass and i think it's amazing but yeah you do have some confusion at
the end where a Saturn V turns into a an Atlas Centaur So anyway in this case what happened
was the aerodynamic forces on those foam panels were greater than anticipated and they
peeled off very early in the flight they were supposed to remain attached until it was above
80 kilometers altitude. So the upper stage hydrogen tank starts getting heated the hydrogen boils
temperature increases and the stage explodes and that ultimately led to the loss of the mission
and yeah one of the great scenes in art movie history as far as i'm concerned
finally i kind of want to give a shout out to the Minotaur rocket with its uh
'banana peel' that's the only way i can describe it so Minotaur is a launch system which is based on
the Minuteman missile you basically take the first couple of stages of a Minuteman
and put some other rocket stages on top of it and you have a very cheap launch vehicle
so it's wrapped in this big yellow like fat banana fabric it's lit it looks like a banana and it
pulls off as the thing ascends very quickly so the reason for this is that it's a solid rocket motor
system that was designed to be deployed in missile silos with temperature & climate control
because you want to make sure that you coddle your nuclear weapons so that they are comfortable
and ready to go to war at the moment that they're needed and so when they took the
created the Minotaur rocket this is launched from a pad which is outdoors and the specifications for
the first two stages require that they keep the temperature within these ranges because
solid rocket motors if they get warmer the reaction rate actually increases and if they
get colder the reaction rate decreases so you get more thrust for less burn time or vice versa
this effect actually has to be taken account for the Space Shuttle solid rocket motors and if you
look at a lot of old launch plans some they have maybe multiple uh multiple plans depending upon
what they believe the SRB temperatures are because they have to model the performance of those solid
motors based upon what they think the ambient temperature the propellant is so yeah this is
you know you start with a simple question and you end up finding all these little individual bits
uh it turns out by the way that while all those panels falling off of Long March rockets
are generally not a big problem there is one case last year where a panel or foam fell off a rocket
and it then got stuck on a grid fin and as the vehicle moved through supersonic speed to actually
uh it caused a deflection of the grid fin which then caused the rocket to go off course and they
lost the rocket so yes even the foam uh even when you don't have a side attached payload it's still
something that can mess you up i'm Scott Manley, fly safe
you