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