Frequent viewers of orbital rocket launches would have noticed that rockets do not travel straight up to Space but follow a curved trajectory. This path is made possible by a maneuver known as a gravity turn.
A gravity turn, also known as a zero-lift turn, is a maneuver using the Earth’s gravity during a launch to put a rocket on the correct trajectory to establish an orbit around the planet. It has the added advantages of saving fuel and allowing the rocket to have a zero angle of attack during launch.
Most readers are well aware of the fact that all orbital launch vehicles, even the space shuttle, stand and launch vertically from their respective launch platforms. However, shortly after launch, they might perform a slight roll before tilting over in a specific direction.
As they gain altitude & speed, they continue to turn more horizontally until, by the time they enter the lower parts of space, they are basically flying parallel to the surface of the planet. This maneuver is called a gravity turn and is a direct result of Earth’s gravitational pull.
This raises the question as to why orbital rockets simply don’t fly straight up to reach space as quickly as possible, or as in the case of the Space Shuttle Program, why the orbiters didn’t just take off like a conventional aircraft from a runway.
This brings us to the necessity and advantages of a gravity turn and why it is performed by all orbital launch vehicles, which will be discussed later in this article. However, one first needs to understand exactly how a gravity turn is executed.
How A Gravity Turn Works During Launch
As mentioned during the introduction, all orbital rockets take off vertically from a launchpad. This is so that they can get through the thickest (and most dangerous part) of the atmosphere as quickly as possible while continuing to accelerate into space.
Shortly after liftoff, the spacecraft gimbals its engines to rotate the vehicle to align itself with its launch azimuth and then pitches the spacecraft slightly off-center to allow it to pitch over in the direction of the rocket’s intended trajectory.
(This maneuver is known as an orbital launch vehicle’s roll and/or pitch program. To learn why a cylindrical rocket rolls shortly after launch, you can read the full article here.)
After the initial maneuver, the spacecraft gimbals its engine nozzles back so that the thrusters themselves propel the rocket straight forward, using all of the available thrust to accelerate the vehicle, and no additional fuel/energy is spent on turning the vehicle.
At this point, Earth’s gravity takes over and tries to pull the rocket back to the surface, causing the vehicle to continue turning towards a more horizontal orientation parallel to the planet’s surface.
As the rocket accelerates well past the speed of sound, the drag and lift provided by the air in the upper atmosphere combined with the thrust provided by its first stage boosters allow it to continue gaining altitude while the Earth’s gravity continues to pull it more horizontally.
The rocket’s guidance system is programmed so that the amount of pitch and acceleration use the planet’s gravitational pull in such a manner that the vehicle is already in the lower parts of space by the time it is traveling parallel to the surface.
Why Rockets Perform A Gravity Turn After A Launch
The goal of an orbital rocket is to reach space and establish an orbit around the Earth as quickly and efficiently as possible. To do this, they do only need to gain altitude and escape Earth’s atmosphere but also move at a very high velocity.
(For a spacecraft to reach and stay in orbit, it needs to travel at a speed of 28 000 km/h or 17 500 mph. This equates to nearly 5 miles per second a rocket needs to travel to reach space and achieve Low Earth Orbit.)
A gravity turn makes it possible for a rocket to reach orbit in the fastest and most fuel-efficient way while reducing the amount of stress on the vehicle during its ascent. Making use of this maneuver has three important benefits:
- Zero-lift Turn
- Crucial Fuel And Energy Saving
- Little To No Angle Of Attack
Each advantage can be explained by looking at it in more detail:
1) Zero-lift Turn
A rocket needs to launch vertically to gain altitude as quickly as possible to clear the thickest part of the atmosphere before too much stress is put on the vehicle by the increasing drag from the air in the lower atmosphere.
(To learn more about why a rocket always needs to launch vertically to reach orbit, you can read the full article here.)
At the same time, it needs to start turning more horizontally to allow the vehicle to gain enough speed to obtain velocity. If it continues to travel straight up, it will eventually run out of fuel and just fall back to the planet’s surface.
For example, sounding rockets can reach far into space, but because they don’t achieve the required horizontal velocity, they simply follow a parabolic trajectory and eventually fall back to the surface.
Conventional aircraft need to travel fast enough on a runway to provide enough lift under their wings to allow them to become airborne and maneuver. But since rockets launch vertically, they don’t have the speed or structure to use any lift to maneuver or turn.
Using the Earth’s gravity by slightly gimballing its engines allows a launch vehicle to enter into a gravity turn, which forces the rocket to keep “turning back towards the surface” as it accelerates into Space.
As the following sections will illustrate, this not only eliminates the need for a rocket to travel fast enough to use air resistance or lift to change direction or gain altitude but also provides two other significant benefits.
2) Crucial Fuel And Energy Saving
During launch, more than 85% of a rocket’s mass consists of fuel. The vast majority of this fuel is used just to get the craft into space. All this energy is necessary to accelerate to orbital velocity while fighting the Earth’s gravity and the planet’s thick atmosphere.
If a rocket had to use its thrusters to steer a rocket into a turn and keep it at the right angle by continuously gimballing and adjusting its nozzles, a large amount of additional fuel would have been required to place the vehicle in orbit.
However, by making use of the planet’s gravity to do the “heavy lifting” and forcing the vehicle to turn by pulling it more horizontally as it accelerates into space, it saves a tremendous amount of fuel that would have been needed to turn the craft manually.
Little To No Angle Of Attack
Within a minute after launch, a rocket is already traveling at a speed of approximately 1 600 km/h (1 000 mph). It continues to accelerate to an orbital velocity of 28 000 km/h (17 500 mph) in the upper atmosphere and space.
Performing a maneuver like trying to turn at this speed puts a tremendous amount of stress on the vehicle. Fighter aircraft experience this when performing sharp turns at high speeds.
In layman’s terms, a vehicle’s angle of attack refers to how aggressively it turns while flying through the air. If it only makes minor adjustments to change direction, it has a small angle of attack. However, a big, aggressive turn requires a much bigger angle of attack.
The bigger the angle of attack, the more air resistance the whole body of a vehicle experiences, which puts it under a large amount of stress. The amount of stress increases as the vehicle travels faster, which can compromise a rocket traveling at hypersonic speeds.
By making use of a gravity turn, a rocket has a zero angle of attack while launching vertically & only needs a minimal angle when pitching over in the direction of its launch trajectory. By using gravity to turn the vehicle gradually, the stress on its structure is significantly reduced.
Conclusion
Getting a rocket into space is a delicate balancing act. It has to get to orbit as quickly as possible by gaining speed and altitude fast, using as little fuel as possible, while also trying to keep the stress on the vehicle’s structure as small as possible.
As this article illustrated, performing a gravity turn helps to make this possible by allowing (or forcing) the vehicle to turn toward its launch trajectory as it accelerates to space. In the process, it also helps to save fuel and reduce stress on the vehicle’s structure after liftoff.
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.
