- Hi, it's me; Tim Dodd,
The Everyday Astronaut. Here's a fun question that
not only have I myself asked but I actually get asked this quite often. Why do we hear a call-out like roger roll or roll program complete, at which point we see the rocket rotate
or roll on its x-axis? - [Announcer] Roll commence, clear. Roll checks out. Roll program has commenced. - The best example of this,
I think was the Space Shuttle which had a really obvious and
really dramatic roll program. As soon as it cleared the tower, you can see it making a very
impressive and sometimes, scary-looking roll. Now, a maneuver like this makes sense when a vehicle's asymmetrical
like a space shuttle but why do cylindrical
rockets like Saturn V or Titan or Atlas or Delta IV, why do they even bother doing a roll and can't rockets just tip over and go in whatever
direction they need to go, do a little pitch here,
a little yaw there, as long as the pointy end is going in the direction it's intended to go, who cares which side of the
rocket is facing the earth and which side is facing space? So today, first we'll define the pitch, yaw, roll and their
corresponding axes on a rocket. Then we're gonna dive
into why a rocket rolls in the first place, take
a look at launch azimuths and the relationships to trajectories and then we'll look at
some unique solutions to orientations, including some rockets that don't roll on ascent to line with their trajectories at all. Let's get started. - [Announcer] Three, two, one, lift off. (upbeat music) - This is one of those topics I love, where at first, the reason
feels kind of perplexing then you hear one
explanation you're like oh, I guess that makes
sense but then you think of some other reasons and learn of all these weird little edge
cases and come to find out there's actually a lot to unpack here. And just to clarify things, we're specifically talking
about the roll program of rockets and not their gravity turn. These are two totally different things. We're focusing on this, (upbeat techno music) not this. This, (upbeat techno music) not this. So let's start off with a
quick overview of pitch, yaw and roll and how they
correspond on a rocket. You may have heard the
terms pitch, yaw and roll, especially when talking about airplanes. On an airplane, pitch is the
nose pulling up or diving down. Yaw's the nose going left or right and roll, you can think of the wingtips going up or down while the
nose stays in the same place. With airplanes, it's really
easy to define pitch, yaw and roll because airplanes have really obvious
characteristics like wings, landing gear, a cockpit
and a vertical stabilizer. And you might think how do
you define these dimensions on a cylindrical rocket? Although a rocket is pretty symmetrical, it's still vital to
define these dimensions. Otherwise, your rocket might go north instead of east or something. So let's take a jetliner
and just remove the wings and tail stabilizer. Hey, look, the fuselage kind of looks like a rocket, perfect. So now we still have
our pitch yaw and roll. We just stand this baby up
on its tail and let it rip. This was literally true when
cockpits were put on missiles which is basically, all
the Vostok, Mercury, Gemini, Soyuz programs were. So now with a rocket on the launch pad, we can look at the cockpit
for that same pitch, yaw and roll. When sitting in the cockpit,
your pitch or your nose up and down is rotating on the y-axis, yawing, left or right is
rotating on the z-axis and rolling left or
right is on the x-axis. Unlike an airplane, the pitch,
yaw and roll of a rocket, generally isn't controlled
by wings or fins but it's actually controlled
by the engine itself via a gimbal and perhaps
some auxiliary thrusters to help control roll. However, wings and fins are sometimes used for stability in the atmosphere. A single engine on the bottom of a rocket can only provide two axes of
control; that's pitch and yaw. And this is because the engine goes through the center of the rocket. Because of that, it can only
apply torque on two axes. So in order for most single-engine rockets to have roll control, you'll normally see auxiliary
thrusters stuck on the side or the outer perimeter of the rocket. These auxiliary thrusters
are called vernier thrusters and I think they're the most obvious on the original Atlas SM-65 A rocket and there's several vernier thrusters on the bottom of the Soyuz Rockets as well but some single-engine rockets get clever and control their roll via
the gas generator exhaust like the RS-68 on the
Delta IV, Delta IV Heavy. You can see the engineers cleverly point and steer the dual gas generator exhausts on each side of the
engine for roll control. Now if you need to brush
up on gas generator cycles and the open cycle, I recently
did a really in-depth rundown of a few common engine cycles in my Is SpaceX's Raptor Engine the
King of Rocket Engines video. Both rockets that have
at least two engines or at least two combustion chambers like the RD-180 on the Atlas V. You can point the engines
in opposite directions which will induce your x-axis roll. So now that we know how a
rocket can control its roll, now we can get into why a rocket
needs to control its roll. Well, to begin, a rocket
needs to remain stable throughout the flight so it doesn't spin so fast it tears itself apart. Okay, sure, that's the most basic reason of why the rocket needs
to control its roll but we still get to the question why do they intentionally roll once they get off the launch pad? - [Announcer] Roll complete and (mumbles) - So I'm gonna tell you the reason here then we're gonna dive in and I'm going to define a few more things. The rocket rolls to align
itself to its flight azimuth so its flight path becomes
a simple pitch program. (laughs) We have a lot to unpack in just that one sentence, huh? So first, let's talk about the azimuth. Now, depending on the
destination of the payload, rockets need to head to
a very specific orbit and a fun reminder here, I like to say to go to space, you go
up but to stay in space, you need to go sideways
really, really fast, which really, that's all orbit is and now, to get to your desired orbit. You wanna make sure that that
sideways part of your flight is pointing in a very, very
specific and accurate direction. Now if you were to launch a
rocket right on the equator, straight east, not only
would you take full advantage of the Earth's rotation which gives the rocket a nice little
boost but you'd also place your vehicle on a zero degree inclination. It's like a nice little belt
around the earth's equator or another fun example of inclination is the International Space Station which is on a 51.6 degree inclination. Now it's on this exact inclination so the Russians can participate and they can launch without
dropping boosters on China or without doing a costly dogleg maneuver. And just as a reference, if
you launched straight east, out of Kennedy Space Center, you'd be on a 28.6 degree inclination which you may notice is the exact latitude of the Space Center. So here's where we get
to what your azimuth is. The azimuth is basically
if you're holding a compass on the launch pad, which
direction do you want the rocket to go to get to your desired orbit? But we should pause here for a
second and clear up one thing because this definitely confused me a bit. Let's be sure and note the difference between the azimuth and the inclination. The azimuth is what's on the
nav ball inside a cockpit. North on the nav ball is zero degrees while East is 90, South is
180 degrees and west is 270. Now this does not line
up with the inclinations. A zero degree inclination
is due east on the equator while a polar orbit is
inclined 90 degrees but again, minimum inclination
depends on your latitude. So flying due east will only correspond to a zero degree inclination if you were launching on the equator. And another side note,
all pro-grade orbits or orbits that follow
the rotation of the earth are between zero and
90 degrees inclination. If the rocket is flying
south from the equator, it's still between zero and 90 degrees because inclination is really
just a measure in degrees how far off angle the
orbit is from the equator. And of course, it's not
quite just as simple as this. If you wanna go to 51.6 degrees and rendezvous with the
International Space Station, you don't actually point at 51.6 degrees. You actually point at about 45 degrees but now we're getting
into some kind of fun math that takes into account
the Earth's rotation and spherical trigonometry
which might be getting a little too far into
the weeds for this video. So now that we know that
rockets don't all follow the same path to get to space
into their destinations, we're starting to get
some of the puzzle pieces as to why they might intentionally roll. For our next clue, we
need to look no further than the launch pads themselves and since we've mentioned
rockets like the space shuttle and the Saturn V, let's take a look at one of the most famous
launch pads in the world; a launch pad that saw lots of launches from both these vehicles
and now, SpaceX's Falcon 9 and Falcon Heavy, of course, I'm talking about Launch Complex 39A at Kennedy Space Center. LC 39A is a great example because it's perfectly lined
north, south, east and west. Take a look here. We can see the flame trench and the crawler way perfectly
runs north and south. So let's start off with the Saturn V which first launched from 39A
on its inaugural test flight on November 9th, 1967
and last launched Skylab on May 14th, 1973. With vehicle crawled out on the pad, you'll see the launch umbilical
tower on the north side with its crew access
arm that swings around and connects to the
east side of the rocket. This is where the astronauts get in and once they're in, they're facing, with the top of their heads, due east and their feet, due west. So of course, along
with the command module, the rest of the vehicle
had certain features such as the fuel and electrical umbilicals that connected the rocket to
the launch umbilical tower, had some external raceways
which had some important wiring and all that kind of stuff
but most importantly, when talking about the
alignment of the rocket, was a thing called the IMU. The IMU or the instrument unit sat on top of the Saturn V's third stage which housed the rockets guidance systems. This included a digital computer, pretty big deal at the time, an analog flight control computer, accelerometers and some gyroscopes. So in the case of the Saturn
V heading to the moon, the launch azimuth was 72
degrees which is 18 degrees north of due east. So while on launch pad, the flight path and the belly of the
rocket were 18 degrees off from each other. And here's where we
get to the first reason for the roll program. Now instead of moving
the entire launch pad to just face the belly of the rocket at that 18 degree angle, the
rocket could simply perform a roll to basically
zero out the difference between the flight path and the
body's physical coordinates. It would have a value
that's a nice easy zero. Now all the rocket has
to do is pitch over. This made it so the computer really only had to calculate one set
of numbers instead of two, making the math and the
calculations much, much easier. Less variables equals a good thing. It's nice to keep it simple. Another physical consideration is a thing called gimbal lock. Now gimbals can freely rotate
on all three dimensions and align themselves to
a fixed position in space which can then tell the guidance computers where the vehicle is pointing. Now, by zeroing out one of those numbers, you're keeping the gimbal as far away from potential gimbal lock as possible. And a gimbal that locks up
can be a very, very bad thing. So in order to demonstrate why
zeroing out a vehicle's roll is a good thing, let's
just build a quick rocket in Kerbal Space Program. Now by default, when you build a rocket, it's aligned perfectly
north, south, east and west with pitch aligned north and south and yaw aligned east and west. So to head out on an equatorial zero degree inclination orbit, you need to press only a
single key the right amount and in this example, that is the D key which will yaw over due east, one finger flying, nice and easy. Now let's rotate the rocket
about 20 degrees or so away from being perfectly
aligned and still try and follow that perfectly zero
degree inclination due east. Now this can still easily be done when you're super, super
talented like me, obviously but all kidding aside, you're
only using two keys this time but it is noticeably harder. So why not just keep it simple? Well here's another example
that's a fun thought experiment. This is a map of Downtown Waterloo, Iowa. Notice that the streets run
from northeast to southwest and from Northwest to Southeast. And they're aligned to
the river and not aligned to true north. Now if you're walking around,
it's probably unlikely that you wouldn't just
redefine your own coordinates in your head and start
thinking of anything on this side of the river as north and anything on this side as south. It just makes navigating a lot easier than thinking about
northeast and southwest. So if the rocket and the launchpad are always in a fixed
position which, spoiler alert, they pretty much always
are, well, kind of, we'll talk about that more in a second; the easiest thing to do
is to program the rocket to do a quick roll to align
itself with its azimuth. This takes the navigation from being a three-dimensional equation to just a two-dimensional equation and removes a ton of
complexity and variables. I know it doesn't seem like
much but it definitely matters. Now of course whether the
vehicle pitches or yaws is a bit pedantic because
doesn't someone just define that. Well, there's still some
other important distinctions. Sticking with Apollo, the astronauts heads were pointing due east on the launchpad. They were actually on the
belly of the Saturn V. But here's a fun fact. Do you actually know the command module in the Saturn V had exactly
opposite y and z coordinates. I don't exactly know why but I think it's kind of interesting. But this meant when the
rocket pitched over, the commander could look
out the small port window in the blasts protective cover and get a visual reference
of their orientation. So by zeroing out the roll, the horizon would appear across the window which made it easy to use as a reference. This also made it so if the commander saw the ground suddenly coming
up or the horizon spinning, they may have considered
aborting or, at least, had a good visual reference on whether or not that'd be necessary. Another reason why there's
usually a defined belly of a rocket is to place the radio antennas and the receivers in the optimal place to have best contact with
the ground during ascent. This is especially true
with the space shuttle which if it had ascended with the orbiter on top of the external fuel tank, it would have had a much
worse line of sight. When we're talking
about the space shuttle, it's roll program was even more necessary due to its unique shape. Not only was it
structurally the best option for the wings and the struts
holding the external fuel tank but by flying with the orbiter in the wake of the external fuel tank, there was actually a 20%
increase in payload capacity. And although most rockets
look relatively symmetrical, they almost always have some
kind of protruding feature. Take a look at the Saturn V, it had very large bumps
and bulges on the outside that definitely aren't insignificant when factoring in the ascent profile. You'll see these areas where
additional piping or wiring is housed inside sections called raceways. You notice there are
two different raceways on each side of the Falcon
9 and Falcon Heavy cores. You can tell the two outer
cores of the Falcon Heavy are 180 degrees opposite each other because of those two different raceways but back to the space shuttle. The shuttle controlled its pitch and roll via gimbaling nozzles on
the solid rocket boosters. Yes, the main space shuttle
engines could gimbal too and gimbal a lot but
they primarily gimballed to maintain the center of thrust going through the center of mass. By using the solid rocket
boosters to control pitch, the gimbal vectors are
in line with each other, relative to the center of mass. This probably makes it easier to control. This is also relevant
to multi core rockets like the Falcon Heavy
and the Delta IV Heavy which both have a roll
program which again, aligns the cores kind of
perpendicular to the flight path. Now, this might not be a huge deal or not but let's just take a look
at a vehicle like this and if it were flying with its engines in a roll perpendicular to the horizon, the engines that are on
the top and the bottom would have a different amount of leverage over the vehicle compared to that center engine or center core. So I'm not entirely sure but I think this might be another reason why they normally fly pretty
parallel to the horizon but they also fly with these
rockets flat to the horizon for stage separation so the boosters have the lowest chances of
hitting the center core. So now while we're on the
topic of the Falcon Heavy and SpaceX, here's a fun little fact. The Falcon 9 does not perform a roll program to align to its azimuth and neither does the Electron rocket. (upbeat techno music) Both the Falcon 9 and
the Electron just pitch and yaw over however much is necessary and roll for aerodynamic considerations and a few other variables as well. But controlling a rocket in
a true 3D space like this is actually a lot harder than it sounds. It took a generation of grad students to actually solve the linear algebra and have access to
computers powerful enough on the rockets to do
this math in real time for this type of control. So if the Falcon 9 and
the Electron Rockets don't need to roll, why do they? Well, apparently, for fun. So I totally got trolled here by Elon because on June 12, 2019, SpaceX launched a trio of satellites from the Canadian Space Agency. Soon after liftoff, the Falcon 9 did a pretty substantial roll. Now, again, rockets aren't
actually symmetrical and although the Falcon 9
can navigate along both axes, it's likely that this particular launch had a roll like this due to
some payload considerations. Customers might have certain constraints and with this particular launch
having an offset payload, perhaps they needed to
fly it in a certain way for the payload to best
handle the g-forces. The Falcon 9 is also perhaps little unique and that it for sure wants
to be oriented correctly at stage separation. So the first stage has both
of its nitrogen thrusters able to help do that flip maneuver. Since the Falcon 9 has only
two packs of cold gas thrusters that are 180 degrees
apart from each other, this means if the vehicle
rotated 90 degrees, only one set of thrusters
could help with the flip instead of two. Here's another fun story. Have you ever seen the
very first Falcon 9 launch? It unintentionally rolled
almost 45 degrees immediately after takeoff. This was due to the gas generator exhaust that has a slight angle to it. So just like how the Delta IV's RS-68 uses its gas generators to roll, the nine Merlin engines
had so much extra torque from the gas generators exhaust, it took a second for the engine gimbals to cancel the roll out. And one more reason why Rockets roll is for the fairing separation. Now I don't exactly know
what considerations go in to choosing whether
the fairing would split on its y-axis or its z
axis but it should be noted that this is definitely
taken into consideration. For instance, from what
I can tell, at least, SpaceX tends to ditch their
fairings on its y-axis or up and down while ULA
tends to ditch its fairings off to the sides on its z-axis. Why exactly each launch
provider chooses to ditch them in this manner, I'm not sure
but it's kind of fun to note. So a few 21st century
rockets finally took the roll to align to the azimuth program out but perhaps my favorite
rockets that didn't roll align were Soviet era rockets. Remember near the beginning, when I said it'd be too
hard to turn the rocket and/or the launch pad to
align with its trajectory, well that's actually exactly what the Soviet Union came up with for their R-7 family of
rockets like the Soyuz. That's right, the entire
launch pad of the Soyuz actually rotates to align the
rocket up with its azimuth. Now some downsides to this is your azimuth might change ever so slightly
throughout your launch window so by aligning the launch
pad to your azimuth, you might lose some flexibility
in the launch window and flight path. This is something the new
Soyuz too can do away with now that it has a digital flight computer and it can now align itself
on the correct azimuth. Although crewed missions
still use a Soyuz-FG which utilizes that
rotating table but lastly, there was still perhaps the most advanced, most ahead of its time rocket, the Soviet Union's N-1
rocket which was meant to, (coughs) never did, follow its flight path using both pitch and yaw. It had some roll control thrusters that were undersized for
the first three launches and then upgraded for the fourth launch but they weren't used
to align to the azimuth. They were just used for stability. (sighs) I still really
wish the N1 had worked out. It's such an awesome rocket. So to summarize, rockets
roll for a few reasons and like all rocket
science and engineering, there's actually some good
reasons but as for why, well, it's generally easier to roll to align the vehicle to its azimuth than it is to move the launchpad. It makes for easier calculations
for the guidance computer. Rockets roll for aerodynamic
and structural considerations. They roll for the
astronauts' vantage point and visual references. They roll for fairing
deployment orientation. They roll to align auxiliary
or control thrusters and they roll for best
line-of-sight for communications and down links. (sighs) So does this help
answer that question? It's another one of those fun things where you probably know
there's a good reason but it's just kind of hard to
find all those good reasons. Hopefully, this helps us appreciate just how many of these little but important decisions
engineers and scientists need to come up with every single day. There's always a reason for
all the strange little quirks. Let me know what other questions you have about roll programs or rockets or rocket science in general
in the comments below. I have a crazy long list I'm still trying to chew
a way out here for videos so stay tuned, there's still
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