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so let's start with the gas generator cycle
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known as the
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open cycle this is probably one of the most
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common types of liquid fueled rocket engine
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used on orbital rockets
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it's definitely more complicated than a pressure fed
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system but it's
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fairly simple
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well
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at least compared to their close cycle counterparts
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now I'm gonna
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way way over to simplify this
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so it's as easy to grasp as humanly possible
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in real life there's literally dozens of valves
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hive of wires and extra tiny little pipes everywhere
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helium to back pressure the tanks
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flowing through
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the nozzle and the combustion chamber to cool it
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and there's an ignition source for the pre burner
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and the combustion chamber
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but again for the purpose of
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making this as simple and as digestible as possible
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just know
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there's a lot of stuff missing from these diagrams
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but for now
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we're just gonna focus on the flow of these engines
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so we can grasp that concept 1st
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the gas generator cycle works by
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pumping the fuel and oxidizer
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into the combustion chamber using a turbo pump
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the turbo pump has a few main parts
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a mini rocket engine called the pre burner
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a turbine connected to a shaft and then a pump or two
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that pushed propellant into the combustion chamber
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now you might
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hear the turbo pump assembly called the power pack
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because it really
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is what powers the engine and the open cycle system
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the spent propellant from the pre
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burner is simply dumped overboard
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and does not contribute any significant thrust
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this makes it
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less efficient
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since the fuel and oxidizer used to spin the pumps
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is basically wasted now
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the funny thing
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about a turbo pump is that it kind of has a chicken in
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egg syndrome situation
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that makes it pretty difficult to start up
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since the pre burner
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that powers the turbo pump
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needs high pressure fuel and oxidizer to operate
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so the ore burner requires the turbo pump to spin
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before
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it can get up to full operational pressure itself
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but the turbo pumps need the pre
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burner to fire in order to spin the turbo pumps
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but the pre burner needs the turbo pumps to
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yeah you can see where this is going
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this makes starting a gas generator pretty tricky
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there's a few ways to do this
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but we don't need to get into all that in this video
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that sounds like a fun topic for future videos though
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so back to the turbo pumps remember
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pressure always flows from high to low
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so the turbo pumps need to be a higher
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pressure than the chamber pressure
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and this means the inlets leading to the pre burner
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is actually the highest
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pressure point in the entire rocket engine
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everything else downstream is lower pressure
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but notice something here
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take a look at spacex's merlin engine
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which runs on rp1 or
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rocket propellant
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1 and liquid oxygen
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notice how black
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the smoke is coming out of the pre burner exhaust
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why would it be so sooty
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compared to the main combustion chamber
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which leaves almost no visible exhaust
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well that's because rocket propellant can get super hot
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like thousands and thousands of degrees celsius
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so to make sure the temperature isn't so
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hot that it melts the turbine and the entire
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turbo pump assembly
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they need to make sure it's cool enough to continually
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operate
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running at the perfect fuel and eye oxidizer ratio
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is the most efficient and releases the most energy
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but it also produces a crazy amount of heat
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so in order to keep the temperatures low
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you can run the ore burner at a less than
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optimal ratio
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so either too much fuel known as fuel rich
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or too much oxidizer or oxygen rich
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an rp1 engine
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fuel rich means you'll see some
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unburnt fuel appearing as dark clouds of soot
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the highly pressurized unburnt carbon molecules
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bond and form polymers
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which is a process known as coking
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this sit starts to stick to everything
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it touches and can block injectors
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or even do damage to the turbine itself
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so what if you didn't want to waste all that highly
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pressurized propellant
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I mean
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after all since it's running cooler by being fuel rich
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doesn't that mean there's a bunch of unburned fuel
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literally being wasted
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what if you could just pipe that
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hot exhaust gas and put it into the combustion chamber
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huh welcome to the closed cycle
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the close cycle or stage combustion cycle increases
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engine efficiency by using what would normally be lost
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exhaust and connects it to the combustion chamber
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to help increase pressure and
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also increase efficiency
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so let's take the merlin engine and try
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closing the loop
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let's take the exhaust and just
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pipe it straight into the combustion chamber
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oh no
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we just put a bunch of unclogged all the injectors
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you do not go to space today my friend
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but there's a few solutions to this problem
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so let's see how the soviets solved it
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the 1st
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operational closed cycle engine they made was the nk
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15 designer for their n1 moon rocket
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they later upgraded it to the nk 33
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and then many versions from there stemmed out
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including the rd 180
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which is what is used on the atlas 5 today
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since the nk 15 and nk 33 runs on rp1 like the merlin
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you can't run your ore burners
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fuel rich because of the coking problem
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so if you want to create a closed cycle engine with rp1
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the answer is running the ore burner oxygen rich
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easy as that right
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well now you're blasting super heated
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highly pressurized gaseous oxygen
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which will turn just about anything into soup
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right at your precision machine
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crazy lower tolerance turbine blade
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doing so is
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actually considered impossible by the United States
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and they basically gave up on trying
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they didn't think a metal alloy existed
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that could withstand these crazy
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crazy conditions
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and they didn't believe the soviets had made
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such an efficient and powerful
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rp1 powered engine
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until after the collapse of the soviet union
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and the us
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engineers got to see them and test them out firsthand
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but the soviets had indeed worked their butts off
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and they had made a special alloy that can magically
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with science
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to stand the crazy conditions of an
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oxygen rich pre burner
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with a closed cycle engine
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you don't just use some fuel and some oxidizer
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and burn that in the pre burner to spin the turbine
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you actually shoot all all of the rich propellant
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through the turbine
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so with an oxygen rich cycle
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all of the oxygen actually goes through the ore burner
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and just
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the right amount of fuel goes to the pre burner
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you only need enough
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to give the turbine
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the right amount of energy
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to spin the pumps fast enough
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to get the right pressures for the pre burner
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and the combustion chamber
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to make the right amount of power
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to shoot the thing into space
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