During the Space Shuttle Program era, observant viewers may remember the shuttle performing a rather dramatic roll shortly after liftoff. Like most other rockets, there is a good reason why launch vehicles roll after launch.
Rockets predominantly roll on their vertical axis during ascent to align themselves to their launch azimuth, the intended direction in which a spacecraft orbits Earth. The maneuver simplifies navigation and saves fuel and energy by putting the vehicle on the correct trajectory shortly after launch.
The Space Shuttle was not the only iconic launch vehicle that performed a roll shortly after liftoff. During the Apollo Program in the 1960s and 70s, the Saturn V rockets that sent the first humans to the Moon also performed a brief roll maneuver during their launch.
This roll maneuver is typically performed shortly before a launch vehicle starts to pitch over and follow a parabolic trajectory to its intended orbit. The latter maneuver is also referred to as a gravity turn.
(As the name suggests, the purpose of all orbital rockets is to establish an orbit around the Earth where it stays or travels further into the Solar System. Learn more about the different types of orbits spacecraft/satellites follow around celestial bodies in this article.)
It might make sense why an asymmetrical vehicle like a space shuttle (on top of a large fuel tank with solid rocket boosters) would rotate for aerodynamic and other practical reasons, but it doesn’t explain why a seemingly symmetrical rocket also needs to rotate.
However, as the following section will describe in more detail, it is actually critical for many rockets to perform some type of rotational maneuver after liftoff, and it’s all got to do with a rocket’s trajectory and its intended orbit.
Why Rockets Roll After Launch
In order to have a clear understanding of why rockets have roll programs in the first place, it is crucial to understand a rocket launch facility’s layout and orientation and how it relates to a launch vehicle’s flight azimuth and orbital inclination.
When looking at most major rocket launch sites throughout the world, it is noticeable that they are all situated either on or very close to the Equator. This is because the Earth reaches its fastest rotational velocity at the Equator.
Rockets launching from the Equator and traveling east get an instant speed boost (by roughly 1670 km/h or 1 000 mph) due to the Earth’s rotation. This explains why the vast majority of rocket launches and satellite orbits are in an easterly direction.
(Interested readers can take a closer look at the 22 most prominent space launch facilities in the world, their significance, as well as their location in this article.)
As a result, most launch complexes are built in a North-South and East-West orientation to make the best use of this speed assist. Unfortunately, not all spacecraft trajectories and orbits follow a strict easterly direction.
For example, for the Saturn V rockets to be placed in the optimal orbit to reach the moon, their flight path sat at an orbital inclination of 18 degrees, which means it had a launch azimuth of 72 degrees. (More on launch azimuth and orbital inclination shortly.)
Similarly, the International Space Station (ISS) sits at an orbital inclination of 51.6 degrees, which means it had a launch azimuth of 38.4 degrees. In both cases, launching a rocket east without any mid-flight course correction will make it impossible to reach these orbits.
First, a quick clarification between launch azimuth and orbital inclination is needed. The diagram below illustrates the difference between the two directional indicators.
The Difference Between Launch Azimuth And Orbital Inclination
The Launch Azimuth is what you find on the instrument panel of your craft, where North equates to 0 degrees, and East equates to 90 degrees. The orbital inclination, though, shows any deviation from East in degrees. In this case, East equates to 0 degrees, and North equates to 90 degrees. Subsequently, as this diagram illustrates, a Launch Azimuth of 60 degrees equates to an Orbital Inclination of 30 degrees.
As a result of a launch complex’s fixed north-south/east-west orientation, a launch vehicle either needs to cancel out the difference between the launch azimuth and launchpad’s orientation or use complex navigational calculations midflight to correct its course.
(A rocket uses the vast majority of its fuel and thrust to escape Earth’s atmosphere and insert the spacecraft into an orbit around the planet. Reducing the number of course corrections and orbital maneuvers it must perform during this period is absolutely critical.)
This is where a launch vehicle’s roll program comes into play. Rolling the rocket on its x-axis until it aligns the vehicle to its launch azimuth eliminates any additional navigational calculations & saves on the fuel/energy it would require for mid-flight course corrections.
After finishing its roll maneuver, a rocket simply needs to pitch over in the direction of its planned trajectory while following a parabolic path to its orbit. Together, the two maneuvers are known as a launch vehicle’s Roll And Pitch Program.
(Learn more about why rockets launch vertically before performing its roll and pitch program in this article.)
This still leaves the question as to why a seemingly symmetrical cylindrical rocket can’t just “point” its nose in the right direction and fire its thrusters, regardless of orientation. It is not that simple, though.
As this section already highlighted, a rocket’s guidance system is the brain of a rocket, without which a vehicle simply will not be able to function. It relies on the guidance system to stabilize the vehicle, control its thrust, guide its trajectory & also make course corrections.
However, it’s impossible for a rocket’s guidance system to function if it does not know the vehicle’s orientation in relation to the planet’s surface, whether it is pointing up & down or north & south. As a result, a rocket needs a defined belly to allow proper navigation.
The guidance system is not the only reason a rocket needs a defined belly (or floor and ceiling). Although they seem symmetrical, all rockets have multiple protrusions like pipes and other connections encased in structures called raceways running the length of the vehicle.
Alongside sensors and communication equipment situated on the surface of a launch vehicle, they are all influenced to some degree by the rocket’s orientation, which emphasizes the importance of having a clearly defined “floor and ceiling.”
But there are even more reasons rockets benefit from a defined belly. For example, launch vehicles like Spacex Falcon Heavy and Delta IV Heavy have multiple cores during launch.
During first-stage separation, it is crucial that the two side boosters are ejected safely away from the central core. Allowing the vehicle to fly perpendicular to the horizon (made possible by having a defined belly) helps to ensure a safe stage separation.
How Rockets Roll
Rockets which primarily operate in the Earth’s atmosphere, often use aerodynamic features such as tailfins that use air resistance to initiate a roll in a launch vehicle. This feature is typically found on ballistic missiles that form part of military defense systems.
Most modern orbital rockets, though, use their thrusters to initiate a roll maneuver on a spacecraft. Their engine nozzles can gimbal, and by directing different nozzles at opposing angles, they redirect the thrust in opposite directions, which enables a rocket to roll.
Not all rockets have multiple engine nozzles, though, and since at least two nozzles (pointed in opposite directions) are needed to roll the craft, they can only perform a pitch or yaw maneuver on their own. Launch vehicles with fixed nozzles face the same problem.
In both cases, smaller supplementary engines called vernier thrusters, typically placed on the side of a launch vehicle or offset to the sides of the main thrusters, are used to initiate a rocket’s roll program. (They are also often used for finer orbital adjustments.)
Examples include launch vehicles used during the early years of the US Space Program, like the Mercury-Atlas 5 rocket. More recent examples include the Soyuz family of rockets, which main thrusters don’t gimbal and rely on vernier thrusters for roll and directional control.
Engineers can also come up with novel solutions to enable rockets to perform roll maneuvers. For example, the Delta IV rocket had only one engine nozzle. By directing the two gas generator exhausts in opposite directions, though, the vehicle could perform a roll.
Conclusion
Like the Saturn V rocket that put the first humans on the moon and the Space Shuttle that pioneered the concept of reusable launch vehicles, most modern rockets perform some type of roll maneuver shortly after liftoff.
As this article illustrated, the primary purpose of this maneuver is to align a spacecraft to its launch azimuth. By simplifying navigation and avoiding unnecessary course corrections, the vehicle also uses less thrust and saves precious fuel in the process.
It also has a few other benefits, including allowing a rocket with additional boosters to fly perpendicular to the horizon, which assists with a safe and simplified stage separation. (In a similar fashion, this also helps with fairing separation when the vehicle’s cargo is deployed.)
In summary, no matter how symmetrical a cylindrical rocket looks, it needs a defined belly for the various reasons highlighted in this article. And with an orbital inclination not always in an easterly direction, the ability of a rocket to roll will always remain crucial to spaceflight.
This article was originally published on headedforspace.com. If it is now published on any other site, it was done without permission from the copyright owner.
