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Explaining the Delta Rocket Fireball - Kerbal Space Program Doesn't Teach.... - YouTube
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Hullo it's Scott Manley here
and after yesterdays delta launch and indeed
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after every delta IV launch
people ask me "is that fireball that happens
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at launch supposed to happen?"
and the answer is yes
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but the reason for it is of course why I'm
making the video
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the delta IV is essentially a completely different
rocket from the delta III
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it switched over to using hydrogen and oxygen
as fuel
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and uses RS-68 engines
these were the first new engines developed
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in the US in a very long time
they were designed to be more powerful and
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cheaper to build than the space shuttle main
engines
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it's a bit less efficient, it uses the gas
generator cycle instead of a closed cycle
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it has slightly lower chamber pressure
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but the RS-68 is the most powerful Hydrogen
fueled engine ever built
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now the reason for the big fireball at launch
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is that during engine startup, for the first
few seconds
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they're essentially just running hydrogen
through the system
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the reason is that they don't want to have
oxygen getting in during the early startup
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because an oxygen rich environment, inside
oxygen rich pockets
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could damage the engine
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hydrogen is a lot less chemically active and
a lot less likely to corrode critical parts
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of your engine at an inopportune moment
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so that hydrogen exhausts out of the back
of the rocket
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and then, being, hydrogen is rises up very
quickly
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and at some point, one of the sparks from
the ROFIs will set it on fire
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and you will have this ginormous fireball
which runs up the side of the rocket,
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typically changing its organge foam into a
much browner form
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this spectacular start was something that
was always expected
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it was designed into the system
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to make sure that the rocket could get off
the pad regardless of how toasty it got.
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you might be surprised to find out that all
hydrogen oxygen rocket engines run fuel rich
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and it actually makes them more efficient
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you might think that rockets would go for
the maximum amount of energy to make sure
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that all of the oxidizer and fuel reacted.
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that's what's called the stoichiometric mixture
ratio
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for hydrogen and oxygen this would be an 8:1
mixture ratio
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with 8 times the mass of oxygen to that of
hydrogen
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the efficiency of the engine goes up as the
temperature of the exhaust products gets higher
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but also what's important is the ability of
your exhaust molecules to convert thermal
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energy into kinetic energy
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and smaller, lighter, simpler molecules such
as molecular hydrogen are much better at converting
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this thermal energy into kinetic energy
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so by running a fuel rich mixture you end
up with a higher specific impulse.
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And it turns out that for a hydrolox engine
the ideal mixture ratio is about 4.7:1
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and this fuel rich mixture actually reduces
the temperature of the exhaust which actually
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simplifies some of your engineering considerations.
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but as I mentioned, hydrogen is bulky and
having to pump a lot more of it by volume
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into the engine becomes harder and it also
makes your tanks larger
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so the space shuttle main engine and the RS-68
both run at a 6:1 mixture ratio
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anyway, for the fireball there have been some
attempts to mitigate this especially on the
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delta IV heavy which is essentially 3 delta
cores strapped together
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Friday's launch of NROL-47 is notable because
it's going to be the last of a Delta IV Medium
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on the West Coast
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The non-heavy versions of the Delta IV are
being retired so we won't see any more single
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cores or with strap on solid boosters
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because those payloads can be effectively
handled by the Atlas V which is a lot cheaper
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liquid hydrogen may be the best rocket fuel
but it is also bulk and difficult to handle
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making it very expensive
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another cost saving feature in the RS-68 engine
is evident in the colour of its exhaust
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unlike the space shuttle engine which produces
a clear blue flame the exhaust from a Delta
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IV is red and the reason for this
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is that the nozzle is a lot cheaper, it's
not a full regeneratively cooled nozzle the
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outer sections of the nozzle are graphite
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designed to ablate away in the face of the
heat of the chemical combustion.
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and of course that carbon reacts with the
hot exhaust gasses to give the reddish flame
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the fuel rich fireball at the start was also
cited as a problem when they were looking
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at human rating the Delta IV
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making the Delta IV human rated would require
hundreds of changes and would make those engines
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a lot more expensive than they are now
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so while the first launch of an Orion test
capsule was done on board a Delta IV it's
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unlikely that a Delta IV will actually carry
and crew.
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instead the Atlas V is being Human rated designed
to carry Boeing's Starliner in future launch.
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it's also worth noting that the Delta IV is
currently the US's most capable launch vehicle
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able to put just over 23 tons into low earth
orbit, making it comparable to the Russian
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Proton, or the Chinese Long March 5.
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However all of these are likely to be exceeded
soon, assuming the first launch of the Falcon
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Heavy isn't spectacular in the wrong way.
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it should be capable of putting something
like 64 tons into low earth orbit in a fully
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expendable manner.
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Of course if it's used in a reusable manner
then it won't be able to carry quite as much.
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But a reusable Falcon Heavy should be a lot
cheaper than a Delta IV Heavy.
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So coming back to the original question: Yes
that giant fireball at launch is entirely,
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as they say, Nominal.
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And you shouldn't be tell ULA to check their
staging
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I'm Scott Manley, Fly Safe.
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