Why do cylindrical rockets roll?

- Hello It's Me; Tim Dodd, the everyday astronaut. Here's a fun question that I have not only asked myself, but am asked quite often. Why do we hear a prompt like roger

roll

or

roll

program complete, at which point we see the rocket rotate or roll on its x axis? - The shooting begins, of course. Roll checks. The roll program has started. - I think the best example of this was the space shuttle, which had a really obvious and dramatic roll program. As soon as he got over the tower, you can see him doing a very impressive and sometimes terrifying turn.

Now a maneuver like this makes sense when a vehicle is asymmetrical like a space shuttle, but why do

cylindrical

rockets

like the Saturn V, Titan, Atlas or Delta IV, why do they even bother to rotate and can't they just tip over and leave? in whatever direction they need to go, do a little pitch here, a little yaw there, as long as the pointed end is going in the intended direction, who cares which side of the rocket is facing the earth and which side is facing the space? So today, we will first define pitch, yaw, roll and their corresponding axes in a rocket.

Then we'll delve into why a rocket rolls in the first place, take a look at launch azimuths and relationships to trajectories, and then look at some unique solutions for orientations, including some

rockets

that don't roll during ascent. to align with their trajectories. Let us begin. - Three, two, one, take off. (upbeat music) - This is one of those songs I love, where at first the reason feels a little baffling, then you hear an explanation and say, oh, I guess that makes sense, but then you think of other reasons and you learn. of all these strange little edge cases and discovered that there's actually a lot to unpack here.

And just to clear things up, we're talking specifically about the rockets' spin program and not their gravitational spin. They 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 with a quick overview of pitch, yaw, and roll and how they correspond in a rocket. You may have heard the terms pitch, yaw, and roll, especially when talking about airplanes. In an airplane, pitch is the movement of the nose up or down. Yaw is the nose going left or right and turning, you can think of the wingtips going up or down while the nose stays in the same place.

With airplanes, it's very easy to define pitch, yaw, and roll because airplanes have really obvious features like wings, landing gear, cockpit, and vertical stabilizer. And could you think about how these dimensions are defined in a

cylindrical

rocket? Although a rocket is quite symmetrical, it is still vital to define these dimensions. Otherwise your rocket might go north instead of east or something. So let's take a passenger plane and simply remove the wings and tail stabilizer. Hey, look, the fuselage looks like a rocket, perfect. So now we still have our pitch, yaw and roll. We just put this baby on his tail and let him rip.

This was literally true when cockpits were put on missiles, which is basically what all the Vostok, Mercury, Gemini and Soyuz programs did. So now with a rocket on the launch pad, we can look at the cockpit to see the same pitch, yaw, and roll. When you are sitting in the cockpit, your pitch or nose up and down rotates on the y axis, yaw, left or right rotates on the z axis and left or right roll is on the X axis. Unlike an airplane, a rocket's pitch, yaw, and roll are generally not controlled by wings or fins, but are actually controlled by the engine via a gimbal and perhaps some auxiliary thrusters to help control the swing.

However, wings and fins are sometimes used to achieve stability in the atmosphere. A single motor at the bottom of a rocket can only provide two axes of control; That's pitch and yaw. And this is because the engine passes through the center of the rocket. Therefore, it can only apply torque to two axles. So for most single-engine rockets to have roll control, you'll typically see auxiliary boosters stuck on the side or outer perimeter of the rocket. These auxiliary thrusters are called vernier thrusters and I think they are the most obvious on the original Atlas SM-65 A rocket and there are also several vernier thrusters on the bottom of the Soyuz rockets, but some single-engine rockets get smart and control their wheel by through the gas generator exhaust like the RS-68 on the Delta IV, Delta IV Heavy.

You can see engineers skillfully aiming and directing the dual gas generator exhausts on either side of the engine to control sway. Now, if you need a refresher on gas generator cycles and the open cycle, I recently did a very in-depth overview of some common engine cycles in my video Is SpaceX's Raptor Engine the King of Rocket Engines? Both rockets have at least two engines or at least two combustion chambers like the Atlas V's RD-180. You can point the engines in opposite directions, which will induce x-axis spin. Now that we know how a rocket can control its roll, we can now understand why a rocket needs to control its roll.

Well, for starters, a rocket must remain stable throughout the flight so that it doesn't spin so fast that it breaks. Well, sure, that's the most basic reason why the rocket needs to control its spin, but we still get to the question of why they roll intentionally once they leave the launch pad. - Full roll and (mumbles) - So I'll tell you why here, then we'll go deeper and I'll define a few more things. The rocket rotates to align with its flight azimuth, so its flight path becomes a simple pitch program. (Laughs) We have a lot to unravel in a single sentence, eh?

First, let's talk about azimuth. Now, depending on the destination of the payload, the rockets must be headed to a very specific orbit and a fun reminder here, I like to say that to go to space, you need to go up, but to stay in space, you really need to go sideways. . very quickly, that really, that's all the orbit is and now, to get to the desired orbit. You want to make sure that that lateral part of your flight is pointing in a very, very specific and precise direction. Now, if you were to launch a rocket right at the equator, directly to the east, you would not only take full advantage of the Earth's rotation, which gives the rocket a small boost, but you would also place your vehicle at a zero degree tilt.

It's like a small belt around the Earth's equator or another fun example of tilt is the International Space Station, which has a tilt of 51.6 degrees. It is now exactly at this tilt so that the Russians can participate and can launch without launching boosters on China or without doing a costly dogleg maneuver. And just for reference, if it were launched directly east from the Kennedy Space Center, it would have a tilt of 28.6 degrees, which, as you may notice, is the exact latitude of the Space Center. This is where we get to what your azimuth is. Azimuth is basically, if you hold a compass on the launch pad, which direction do you want the rocket to go to reach the desired orbit?

But we should stop here for a second and clarify one thing because this definitely confused me a little. Let's make sure and look at the difference between azimuth and inclination. The azimuth is what appears on the navigation ball inside the cockpit. North on the trackball is zero degrees, while east is 90, south is 180 degrees, and west is 270. Now, this doesn't line up with the inclinations. An inclination of zero degrees occurs towards the east at the equator, while a polar orbit is inclined 90 degrees, but again, the minimum inclination depends on your latitude. Therefore, flying east will only correspond to a zero degree tilt if launched at the equator.

And another side note, all prograde orbits or orbits that follow the Earth's rotation have an inclination between zero and 90 degrees. If the rocket flies south from the equator, it is still between zero and 90 degrees because the inclination is really just a measurement in degrees of how far the orbit is from the equator. And, of course, it's not as simple as this. If you want to get to 51.6 degrees and meet the International Space Station, you don't actually aim for 51.6 degrees. You're actually aiming for 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 going too far for this video.

So now that we know that not all rockets follow the same path to get into space and reach their destinations, we're starting to understand some of the pieces of the puzzle of why they might roll intentionally. For our next clue, we need look no further than launch pads 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 many launches of both vehicles and now, SpaceX's Falcon 9 and Falcon Heavy, of course, I'm talking about Launch Complex 39A at the Kennedy Space Center.

LC 39A is a great example because it is perfectly delimited north, south, east and west. Check it out here. We can see the flame trench and the crawler path runs perfectly from north to south. So let's start with the Saturn V, which first launched from 39A on its maiden test flight on November 9, 1967 and last launched to Skylab on May 14, 1973. With the vehicle crawling on the pad, You'll see the launch umbilical tower on the north side with its crew access arm rotating and connecting to the east side of the rocket. This is where astronauts enter and once inside, they look, with the top of their heads, to the east and their feet to the west.

So of course, along with the command module, the rest of the vehicle had certain features, like the electrical and fuel umbilicals that connected the rocket to the launch umbilical tower, it had some external raceways that had important wiring and all that sort of stuff. of things. but most importantly, when talking about the alignment of the rocket, it was something called IMU. The IMU, or instrument unit, sat atop the Saturn V's third stage, which housed the rocket's guidance systems. This included a digital computer, quite important 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 east. So while on the launch pad, the flight path and belly of the rocket were 18 degrees from each other. And this is where we get to the first reason for the roll program. Now, instead of moving the entire launch pad to face the rocket's belly at that 18-degree angle, the rocket could simply perform a roll to basically zero out the difference between the flight path and the physical coordinates of the rocket. body. It would have a value that is a nice easy zero.

Now all the rocket has to do is tilt. This made it so that the computer really only had to calculate one set of numbers instead of two, which made the math and calculations much, much easier. Fewer variables equals a good thing. It's good to keep it simple. Another physical consideration is something called gimbal lock. Now the gimbals can rotate freely in all three dimensions and align in a fixed position in space that can then tell the guidance computers where the vehicle is pointing. Now, by zeroing one of those numbers, you keep the gimbal as far away from possible gimbal lock as possible.

And a gimbal that locks up can be a very, very bad thing. So to demonstrate why reducing a vehicle's roll to zero is a good thing, let's build a fast rocket in the Kerbal Space Program. Now, by default, when you build a rocket, it is perfectly aligned north, south, east and west with the pitch aligned to north and south and the orientation aligned to east and west. So to head into a zero degree inclined equatorial orbit you just need to press one key the right amount and in this example that's the D key which will move eastward, with one finger flying, nice and easy.

Now let's rotate the rocket about 20 degrees or so so that it's not perfectly aligned and continue trying to follow that perfect zero degree tilt to the east. Now this can still be done easily when you're super, super talented like me, obviously, but jokes aside, you're only using two keys this time, but it's noticeably harder. So why not make it simple? Well, here's another example that's a fun thought experiment. This is a map of downtown Waterloo, Iowa.Note that the streets run from northeast to southwest and northwest to southeast. And they are aligned with the river and not aligned with true north.

Now, if you're walking, it's probably unlikely that you wouldn't 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 so much easier than thinking about northeast and southwest. So if the rocket and launch pad are always in a fixed position, which, spoiler alert, they almost always are, well, more on that in a second; The easiest thing to do is to program the rocket to make a quick turn and align with your azimuth. This turns navigation from a three-dimensional equation to just a two-dimensional equation and eliminates a lot 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 no one simply defines that. Well, there are still some other important distinctions. Continuing with Apollo, the astronauts' heads pointed east on the launch pad. In reality, they were in the belly of the Saturn V. But here's a curious fact. Do you really know that the Saturn V command module had exactly opposite y and z coordinates? I don't know exactly why, but I think it's kind of interesting. But this meant that when the rocket was tilted, the commander could look out the small window in the blast shield and get a visual reference of its orientation.

Then, by zeroing the roll, the horizon would appear through the window, making it easier to use as a reference. This also meant that if the commander saw the ground suddenly rising or the horizon rotating, he could have considered aborting or at least having a good visual reference as to whether that would be necessary or not. Another reason there is usually a defined belly on a rocket is to place the radio antennas and receivers in the optimal location for best contact with the ground during ascent. This is especially true of the space shuttle, which if it had ascended with the orbiter atop the external fuel tank, it would have had a much worse line of sight.

When we talk about the space shuttle, its displacement program was even more necessary due to its unique shape. Not only was it structurally the best choice for the wings and struts holding the external fuel tank, but by flying the orbiter behind the external fuel tank, there was actually a 20% increase in payload capacity. And although most rockets appear relatively symmetrical, they almost always have some kind of standout feature. Take a look at the Saturn V. It had very large bumps and bulges on the exterior that are definitely not insignificant considering the climb profile. You will see these areas where additional pipes or wiring are housed within sections called raceways.

You'll notice that there are two different channels on either side of the Falcon 9 and Falcon Heavy cores. You can say that the two outer cores of the Falcon Heavy are 180 degrees in front of each other because of those two different raceways, but back to the space shuttle. The shuttle controlled its pitch and roll using stabilizing nozzles on the solid rocket boosters. Yes, the space shuttle's main engines could spin and spin a lot too, but they mainly spin to maintain the center of thrust through the center of mass. When using solid rocket thrusters to control pitch, the gimbal vectors are aligned with each other, relative to the center of mass.

This probably makes it easier to control. This is also relevant for multicore rockets like the Falcon Heavy and Delta IV Heavy, which have a roll program that, again, aligns the cores perpendicular to the flight path. Now this may or may not be a big deal, but let's take a look at a vehicle like this and if it were flying with its engines in a turn perpendicular to the horizon, the engines at the top and bottom would have a different amount of leverage over the vehicle compared to that central engine or central 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 these rockets flat towards the horizon to separate the stages, so that the boosters have the least chance. of impacting the central core. .

Now that 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 with its azimuth and neither does the Electron rocket. (upbeat techno music) Both the Falcon 9 and the Electron simply pitch and yaw as much as necessary and turn for aerodynamic considerations and a few other variables as well. But controlling a rocket in a real 3D space like this is actually much harder than it seems. It took a generation of graduate students to solve linear algebra and have access to powerful enough computers on the rockets to do these calculations in real time for this type of control.

So if the Falcon 9 and Electron Rockets don't need to taxi, why do they need to? Well, apparently, for fun. So Elon totally trolled me because on June 12, 2019, SpaceX launched a trio of Canadian Space Agency satellites. Shortly after takeoff, the Falcon 9 made a fairly substantial turn. Now again, rockets aren't actually symmetrical and while the Falcon 9 can navigate along both axes, this particular launch likely had a spin like this due to some payload considerations. Customers may have certain limitations and since this particular launch has an offset payload, perhaps they needed to fly it a certain way so that the payload would better handle the G-forces.

The Falcon 9 is perhaps also a bit unique and you will certainly want be correctly oriented in the separation of levels. So the first stage has both nitrogen boosters capable of helping to perform that turning maneuver. Since the Falcon 9 has only two cold gas booster packs that are 180 degrees apart from each other, this means that if the vehicle turns 90 degrees, only one set of boosters could help with the turn instead of two. Here's another funny story. Have you ever seen the first launch of the Falcon 9? He involuntarily turned almost 45 degrees immediately after takeoff. This was due to the gas generator exhaust having a slight angle.

So, just as the Delta IV's RS-68 uses its gas generators to taxi, the nine Merlin engines had so much extra torque due to exhaust from the gas generators that it took a second for the engine gimbals to abort deployment. And one more reason the Rockets roll is because of fairing separation. Now, I don't know exactly what considerations go into choosing whether the fairing would split along its y-axis or its z-axis, but it should be noted that this is definitely taken into account. For example, at least from what I can tell, SpaceX tends to ditch their fairings on their y-axis or up and down, while ULA tends to ditch their fairings sideways on their z-axis.

I'm not sure why exactly every launch vendor chooses to get rid of them this way, but it's fun to note. So some 21st century rockets eventually aligned with the azimuth program, but perhaps my favorite rockets that didn't align were the Soviet-era rockets. Remember near the beginning, when I said that it would be too difficult to rotate the rocket and/or launch pad to align with its trajectory, well, that's exactly what the Soviet Union came up with for its R-7 family of rockets like the Soyuz. That's right, the entire Soyuz launch pad rotates to align the rocket with its azimuth.

Now, one of the downsides to this is that your azimuth may change slightly throughout your launch window, so by aligning the launch pad with your azimuth, you may lose some flexibility in your launch window and trajectory. Of flight. This is something the new Soyuz can also eliminate now that it has a digital flight computer and can now align to the correct azimuth. Although manned missions still use a Soyuz-FG that uses that rotary table, but lastly, there was still perhaps the most advanced rocket and ahead of its time, the Soviet Union's N-1 rocket that was intended to (cough) He never did.

Follow your flight path using 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 were not used for azimuth alignment. They were only used for stability. (sighs) I still really wish the N1 had worked. It's such an impressive rocket. So, to summarize, rockets roll for various reasons and like all rocket science and engineering, there are actually some good reasons, but as to why, well, it's usually easier to roll to align the vehicle with its azimuth. have to move the launch pad.

Facilitates calculations for the guidance computer. Rockets roll for aerodynamic and structural considerations. They roll towards the astronauts' point of view and visual references. They rotate to guide the deployment of the fairing. They rotate to align the auxiliary or control thrusters and rotate to obtain the best line of sight for communications and downlinks. (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 a little hard to find all those good reasons. Hopefully this helps us appreciate how many of these small but important decisions engineers and scientists have to make every day.

There's always a reason for all the weird little quirks. Let me know what other questions you have about launch programs or rockets or space science in general in the comments below. I have an incredibly long list that I'm still trying to find a way to find videos of, so stay tuned, there are still a million things to learn. I owe a huge thank you to my Patreon supporters for literally being here on my Discord channel right now and still checking some things out in this actual video. So if you want to help fact-check or find fun little quirks and do all these other things like provide comments for videos and vote on upcoming videos and things like that, consider becoming a Patreon member where you'll get access to our exclusive Discord Channel , exclusive live streams, and our exclusive subreddit at patreon.com/everydayastronaut.

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