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60s the soviets developed an engine called the rd 270
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which never flew
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and in the early 2000s
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aerojet and rocketry worked on an integrated
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power demonstrator
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called wait for it
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the integrated overhead demonstrator
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which again never made it past the test stand
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and the third attempted developing a full flows
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stage combustion cycle engine is
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spacex's raptor engine
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ta da that's right
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the raptor engine is only the third attempt at
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making this crazy type of engine
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it's the first to ever do any type of work
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and leave a test stand
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and fingers crossed it'll be the first
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full flow stage combustion cycle engine to reach orbit
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well actually just about anything
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this engine does will be a 1st
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this means spacex had to tackle some
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crazy crazy problems
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I mean not only that same problem that plugs
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cite rich cycles
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like having to have a really
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really strong metal alloy
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they also have to learn how to control
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you know two different
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pres burners and two different cycles
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to create the highest pressures of
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any chamber pressure ever
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they just beat the rd 180s record of about 265 bar
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when they hit 270 bar
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they're not even done they're hoping for 300 bar
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inside the combustion chamber
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that's nuts and we'll talk more about that in a second
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but before we move on
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now that we've done a rundown on all these
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engine cycle types
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let's put them all up on screen and let them run
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for a bit
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you can watch each one and compare them side by side
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and for myself
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it helps a lot to see them all
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together on the same screen
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at the same time
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since the raptor engine can't run a fuel
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rich pre burner using rp1
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you'd think the next most logical choice would be
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hydrogen well
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space didn't opt for either rp1 or hydrogen
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they went with liquid methane
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so now we finally have another topic to touch on
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why did spacex 2's liquid methane for the raptor engine
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what are the qualities that make it advantageous over
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hydrogen or rp1
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today no liquid methane or
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otherwise known as methylock's engine
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has gone to orbit
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so what qualities does it have that make it desirable
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let's take a look at methane compared to
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rp1 and hydrogen
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let's put methane in between rp1 and hydrogen
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you'll see why here really quickly
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so let's start off with perhaps the biggest factor
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when designing your first stage
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the density of the propellant
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having a denser fuel means the tanks are
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smaller and lighter for a given massive fuel
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a smaller tank
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equals a lighter rocket
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so here's the density of these
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three fuels measure and grams per liter
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in other words
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how much does one leader of this stuffed way
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or really what's its mass starting off with rp1
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one liter is around 813 grams
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rp1 is 11 times more dense than hydrogen which is only
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70 grams per liter
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and methyls is right in the middle at 422
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grams per liter
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remember how
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airships or zeppelins used to be filled with hydrogen
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to make them
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lighter than air
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well that's because hydrogen is so much
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less dense than our atmosphere
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it makes for an excellent
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albeit really flammable gas for a balloon
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I mean we all remember the hindenburg right
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it should also be noted that 813
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grams per liter is an average for rp1
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space chills their rp1 and their falcon
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9 and falcon heavy
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for about a two to 4% increase in density
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but historically rp1's density is right around that 813
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grams per liter
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so in the case of density
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methane is kind of
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right in the middle of the two others
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but there's more to it than just density
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we also need to take
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into consideration the ratio of how much
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fuel is burned
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compared to how much oxidizer is burned
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this is the oxidizer to fuel ratio
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so here's where things get a little
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more interesting and the tables turn
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just a little bit
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rocket engineers have to take
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into account the mass of the fuel
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and the corresponding weight of the tanks
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so they don't actually burn propellant
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at the perfect stoichiometric combustion ratio
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they find the perfect happy medium that bounces
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tank size with thrust output and specific impulse
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let's look at the mass ratios for fuel and
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oxidizer that the
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engineers have come up with
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so for these numbers
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rp1 is burned at 2.7 grams of oxygen
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to one gram of rp1
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hydrogen burns at
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6 grams of oxygen to one gram of hydrogen
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and methane burns at
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3.7 grams of oxygen to one gram of methane
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these numbers can now help offset a little
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the massive difference in density
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so let's visualize this to help
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make it easier to digest
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liquid oxygen is 1 141 grams per liter
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it's a little more dense than rp1
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so burning locks and rp1 at a 2.7 to 1 ratio
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for every leader of locks
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you'd need a little over half a leader of rp1
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next up let's do hydrogen
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now with hydrogen being 11 times less dense than rp1
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you'd think it need a tank that's 11 times bigger
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but luckily
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engineers have found that it pays to burn locks and
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hydrogen at a
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6 to 1 ratio
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for a good compromise
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this means for each
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leader of locks you'd need 2.7 liters of hydrogen
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so your fuel tank needs to be approximately
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five times larger compared to rp1
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so yeah that helps
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that's why when we look at a hydrogen powered delta 4
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versus an rp1 powered falcon 9
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you can see the fuel tank is much
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smaller than the locks tank on the falcon 9
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but the delta 4 is about the opposite
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the locks tank is much smaller than its fuel tank
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so now let's take a look at methane
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and this one gets kind of interesting
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locks is 2.7 times more dense than liquid methane
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but the burn ratio is 3.7 grams of oxygen
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to one gram of methane
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so you'd need 0.73 liters of methane
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for every liter of locks
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in other words your fuel tank need to be about 40
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bigger for methyls than it would need to be for rp1
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despite rp1 actually being almost twice as dense
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and compared to hydrogen
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its fuel tank would be about 3.7 times smaller
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so the fuel to oxidize a ratio helps make
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