0:08 scott manley here on saturday
morning my twitter was abuzz with
discussions about rocket engine cycles
involving such luminaries as Tory Brunel
as CEO of ula and you know I'll people
got particularly interested in the
expander cycle rocket and then of course
Saturday afternoon
the Dragons Scituate situation
metaphorically and literally exploited
but I do want to come back to the
expander engines because a lot of people
seem to have a lot of questions and a
lot of interest now I did mention
expander cycle engines in my more
general video on fuel cycles but to give
you a quick summary when you have a
high-performance rocket engine you need
to have a high power pump that takes the
fuel from the fuel tank and the oxidizer
tank and it forces them into the
combustion chamber at higher pressure
now you can do this in some cases say
with an electric pump driving or an
electric motor driving a pump that most
rockets do this with the turbine driving
a pump hence the term turbo pump know
the powder they exhaust the gas to power
that turbine in most rockets that comes
from something called the gas generator
that's where you're burning a small
amount of the fuel and oxidizer and
there's basically a miniature version of
the rocket engine blowing the exhaust
gasses over the turbine and driving the
main pump and then an output from that
main pump goes into the engine
but for expander cycle engines what they
instead do is they take the cryogenic
fuel and the flawed around the outside
of the combustion chamber and they take
that heat boil the fuel and then use the
vaporized fuel to drive the turbines so
of course most rockets use this
technique to cool the combustion chamber
but with the expander cycle engines this
is the primary way in which the power
the turbo pump this engine cycle was
actually used in one of the most
successful engines in history the RL 10
this design was developed of sixty years
ago and it first flew in 1963 on an
Atlas and since then has been used as an
upper stage on Atlas Titan Dell
it's even been used on this from one and
it'll also be used in future rockets in
the SLS in the Vulcan on the Omega and
it might also be used as landing engines
for a lunar landers all the expander
cycle engines that have flew and work
with liquid hydrogen as a fuel and
liquid oxygen as an oxidizer now the
boiling point of liquid hydrogen is
about 20 Kelvin and you know the
engineering challenges of working with
hydrogen frequently Revolver I'm
stopping it from boiling but of course
when you need it to it's great that it
boils at such a low temperature I mean
it turns out that starting up many of
these expander cycle engines is
literally a case of letting the gas flow
into the engine which will be at some
ambient temperature which is well above
the boiling point of hydrogen and from
there that gives them just enough
thermal energy to start the pumps
properly and start the fuel engine cycle
but once the engine starts properly it
gets his energy from the combustion
chamber in the rl10 the aunt hydrogen is
flowing through the walls of the
combustion chamber keeping it of course
it very low temperatures but inside of
the combustion temperature chamber the
temperatures are thousands of degrees
Celsius and of course will melt
practically anything you can think of in
the design of the rl10 the hot liquid
hydrogen comes into the engine through
its pump and from there it's driven
through the walls of the combustion
chamber and the nozzle where it absorbs
a lot of heat it boils it's temperature
rises by a couple of hundred to three
few hundred degrees Celsius and then
from there the expanded gas flows
through a turbine which drives the turbo
pump and then as the gas leaves the
turbo turbine it then is goes into the
main engine and of course meets the
oxygen now there's obviously also an
oxygen turbo pump in the case of the RL
10 the turbo pump for the oxygen is
driven by a gearbox hydrogen is very
bulky and you need to have pump a lot
more of it so you don't need nearly as
big a turtle of pump for the oxygen in
the case of the RL 10 also I believe
they use a two-stage pump for the
hydrogen and a single-stage pump for the
oxygen this is actually a very efficient
system in fact the rl10 that's used in
the Delta for upper stage is currently
the most efficient chemical rocket
flying has a specific impulse of 462
seconds although a lot of that is due to
its very large nozzle extension which I
love showing because of the way it fits
up inside the stage and then extends out
during stage separation
the only other expander cycle engine I
know is flowing is the Chinese YF 75 D
which is used in the upper stage of the
long march 5 Europe is developing an
engine along these lames it's called
DaVinci it's going to use separate turbo
pumps for the oxygen and the fuel Russia
has the rd 0 146 which in theory may one
day fly on their angora rocket
whenever that goes up and yeah there's a
few other things floating around anyway
for comparison let's look at another
hydrogen oxygen engine the rs.25 that's
the engine that powers the space shuttle
and will power the SLS this is a vastly
more complicated design using pre
burners and separate turbo pumps of the
fuel and the oxidizer and the stresses
on the rs.25 turbines are way higher
with the working fluid having
temperatures of you know four to five
hundred degrees Celsius and containing
energetic combustion products that just
want to react with anything meanwhile
the turbine on the RL 10 it runs on
neutral hydrogen gas in this temperature
is well in some cases basically room
temperature in exchange for all that
extra complexity the rs.25 does generate
a lot more thrust with much higher
combustion chamber pressures the RL 10
by comparison has only ever operated as
an upper stage and it never is used to
lift a spacecraft against Earth's
gravity expander cycle engines like the
RL 10 have problems generating high
thrust because the energy to drive the
expanding hydrogen comes from the heat
absorbed by the walls of the combustion
chamber and basic geometry says if
you are to scale up the rocket by
scaling up the combustion chamber that
the the area of the walls where the heat
is being absorbed will grow as the size
squared where is the volume of the fuel
combusting will grow as the size cubed
so as you scale it up eventually runs
out of enough heat flux to run this and
so you you basically run into this kind
of limit where you can't get any more
thrust from an expander cycle engine and
there's a few different tricks here and
there you know you can change the
geometry of the combustion chamber to
make it longer and thinner
but you know that causes its own trial
and problems but yet if you talk to
people it suggested that the peak thrust
or the you know top thrust you could get
from this is probably about a hundred
and fifty kilonewtons or fifteen tons of
thrust however there are more powerful
expander cycle engines which get higher
thrusts is more correct to call the RL
10 a closed expander cycle because all
of the propellants all of the liquid
oxygen all of the hydrogen end up
flowing through the combustion chamber
the output from that turbine has to flow
into the combustion chamber so that none
of it is wasted now since all the output
from the turbines has to then go through
the combustion chamber that means the
turbine output pressure has to be higher
than the pressure inside the combustion
chamber and that means the pump that's
powering everything has to have enough
pressure to push all the fuel through
the cooling channels drive the turbines
and then get into the combustion chamber
and the performance of that turbine is
driving the pump that actually depends
on the pressure drop across that turbine
you know lots of rocket engine design
boils down to just balancing the
pressures and the flaws so everything
goes in the correct direction and you're
maximizing amount of power you're
getting out the other end an alternate
design simply dumps the turbine exhaust
overboard into the vacuum of space and
this means that the pressure difference
can be a lot higher which means the
turbine can generate more power which
means the pump can pump more fuel which
means the combustion chamber can be
bigger and have higher pressures and you
might naturally call this an open
expander cycle engine but rocket
designers that have actually built these
things prefer the much cooler sounding
expander bleed cycle Japan has developed
the Ali 5 series of engines using this
cycle for the upper stage of their h1
and their h2 launch vehicles but there
are further forthcoming h3 it will be
powered by 2 or 3 Le nain engines
running on the same expander bleed cycle
so this will be an expander cycle engine
powering a booster a first stage by
moving to the bleed cycle the engines
are able to generate 1 point 5 mega
Newton's or about 150 tons of thrust and
that's more than ten times what you
could theoretically get from a closed
expander cycle as you can guess you know
you lose some efficiency you drop a bit
10% in terms of specific impulse but you
still get to keep those nice cool benign
turbine environments so you can you're
cheap out on the hardware there in the
US Blue Origin have also been testing an
expander bleed cycle variant of the B
III which has previously powered the new
Shepard the new variant is called the B
III you and two of them are supposed to
power the upper-stage of the new glenn
the blue origin website specifies a
combined thrust of 1,100 kilonewtons and
as about 550 kilonewtons per engine
which is a bit of a step down from the
stock B III but I am sure it gets better
specific impulse incidentally that B III
that powers the new Shepherd that's also
kind of a rocket science curio because
it uses something called the combustion
tap off cycle instead of having a
dedicated gas generator or a pre burner
it actually had it takes gases exhaust
gases from the combustion chamber and
uses that to drive the turbines it's the
only a rocket engine that's flown with
this design as far as I can tell so I've
just grade two basic types of expander
cycle engines but of course rocket
scientists are always inventing things
and there is a fantastic article and
expander cycle engines on the NASA
website by a guy called William green
and he worked on the j-2x which was
supposed to be a better upper stage for
the SLA
project is currently shut down but the
articles are still up there and they
make fantastic reading and I'm just
gonna give you a quick rundown of some
of these variants so now if you remember
that all these propellant floors require
pressure or differences between input
and output of each step and pushing all
that fuel through the cooling channels
in the combustion chamber that requires
extra pressure so one idea is called the
closed split expander cycle where you
have a single shaft that's driving two
separate turbo pumps in the fuel and
some part of the fuel goes directly to
the combustion chamber and the other
part goes through the cooling channels
and then drives the turbo pump and in
theory this might get you some better
performance but of course it increases
the engine complexity a second concept
is the closed dual expander cycle and
this uses your fuel expansion to drive
the fuel pump but it also uses the
oxidizer to cool the combustion chamber
and then uses that oxidizer to drive the
oxidizer pump now a significant
engineering problem in creating in
running rocket engines like this is
making sure that the fuel and the
oxidizer remain separate and if you have
a turbine connected to a pump and the
turbine is being driven by fuel and the
pump is pumping oxidizer it's possible
for the oxidizer to leak through the
your bearing between the two things so
you need a very complicated sealed
bearing what they will one of the things
the true or one of the tricks they use
is they pump helium into the bearing to
make sure the fuel and the oxidizer
never touch so by using oxidizer they
are liquid oxygen boiling and then
driving it to use it allowing it to
drive the turbine you keep this separate
until the very last minute so you can
have much simpler bearings of course
you're now having a lot more complicated
plumbing maybe this works maybe it's not
I have only ever seen designs on paper
for this there's also concerned about
having spinning metal in a pressurized
oxygen environment because of its
propensity to you will make metal burn
and of course if you're a rocket
scientist you can combine both these
tricks to
a closed dual split expander cycle
because why not a final idea
where's to augment the heating with a
gas generator and the heat exchanger so
you're burning some of the fuel and
oxidizer and you're using that to heat
the hydrogen even more and so you need
that gives you a bit more thrust you're
obviously dumping the grant gas
generator over but it's kind of a
halfway step between a fool open
expander and a fool closed expander I
actually know that saw that William
Greene has a patent on this so again
it's been designed there's a patent that
hasn't ever been flown so yeah that's my
rundown off expander cycle engines they
basically take the problem off the heat
and the exploit that to drive the engine
itself and it's a sound concept the rl10
has been around for almost 60 years and
it will be around for decades to come
because it is a fantastic engine and a
fantastic concept
I'm Scott Manley fly safe
[Music]