Throughout spaceflight history, orbital rockets able to reach Low Earth Orbit and beyond were launched from an upright position. It may seem odd to some, but there is a good reason why rockets launch vertically.
Rockets primarily launch vertically to travel as quickly as possible through the thickest part of the atmosphere, where they encounter the most air resistance while performing a gravity turn to orientate the vehicle more horizontally to continue accelerating to reach orbital or escape velocity.
Most readers are familiar with air travel, and many also make frequent use of commercial aircraft to travel for business or leisure. One can observe and experience the speed with which a modern airliner is able to accelerate and achieve high altitudes.
In contrast, when observing a rocket launch, it may seem like an eternity before a rocket lifts off and start to gain altitude & speed at a painfully slow rate. Compared to conventional aircraft, this kind of approach may seem very impractical and inefficient.
However, there are two primary reasons why rockets use this seemingly inefficient way of getting airborne compared to conventional aircraft:
- A rocket’s goal/purpose is to escape Earth’s atmosphere and reach Earth Orbit as quickly and efficiently as possible, while an aircraft operates within the atmosphere and also uses the air in it to stay airborne.
- As a result, there are fundamental structural and operational differences between rockets and aircraft that determine the way in which they are able to get and stay airborne.
By taking a closer look at each explanation in more detail, it will quickly become clear why rockets need to launch vertically and how they differ from conventional aircraft as a result.
Why Rockets Launch Vertically To Escape Earth’s Atmosphere As Quickly As Possible
As previously mentioned, the goal of a 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.
In fact, for a spacecraft to reach and stay in orbit, it needs to travel at a speed of 28 000 km/h (17 500 mph). This equates to nearly 5 miles per second a rocket needs to travel to reach space and achieve Low Earth Orbit.
Close to the Earth’s surface, though, the air is so thick that it would be impossible to generate such a high velocity without spending an incredible amount of energy and fuel, which will make it impossible for a rocket to escape Earth’s atmosphere.
(If you hold your hand out of a car window while on a highway, one can gain a good understanding of just how much air resistance is created as a result of the high air density close to the ground. And it is just a fraction of the speed at which a rocket needs to travel.)
As a result, it is crucial for a rocket to be launched in a vertical position to clear the thickest part of the atmosphere as quickly as possible before reorientating towards a more horizontal position from where it can gain the required velocity.
Although a rocket starts to pitch over towards a more tangential orientation to the Earth’s surface within a few kilometers after liftoff, by the time it accelerates in a more horizontal direction, it is so high in the atmosphere that air resistance is practically non-existent.
(At 10 km or 33 000 feet, the height at which commercial airliners travel, the air density is already only ⅓ of the density at sea level. At 30 km or 98 000 feet, when a rocket accelerates in a more horizontal direction, the air density is only 1.5% of sea-level density.)
Shortly after liftoff, while gaining altitude and speed, a rocket uses its gimballed thrusters to pitch over (turn away from its vertical position) slowly towards a more horizontal position through a maneuver called a gravity turn.
A gravity turn is a maneuvering procedure through which a rocket uses its thrusters to tilt the vehicle away from its vertical position and allow the Earth’s gravity to pull the rocket towards a more horizontal orientation while gaining altitude & accelerating to orbital velocity.
(You can learn more about a gravity turn, why it is performed, as well as its various advantages in this article.)
After pitching over, a rocket continues its ascent on a parabolic trajectory while accelerating to reach orbital velocity at 28 000 km (17 500 mph). It also needs to obtain a minimum altitude of 150 to 200 kilometers (93-124 miles) to reach Low Earth orbit.
(It takes a rocket several minutes to reach Low Earth Orbit, but it can take much longer to get to higher orbits like a Geostationary Orbit or completely break free from Earth’s gravity.)
Why Rockets And Aircraft Takes Off Differently
Rockets and conventional aircraft both spend a certain amount of time in Earth’s atmosphere. However, airplanes spend all their time in the atmosphere, while rockets only spend a brief period in the atmosphere before entering the vacuum of space.
As a result, the two types of craft differ significantly in both the way they are designed and the way they operate:
Since aircraft operate exclusively in the atmosphere, they use its dense air to fly and stay airborne. Therefore, they need wings with sufficient surface area to create enough lift to keep the vehicle in the air and ailerons and rudders to turn and maneuver.
Aircraft engines are also air-breathing machines that need oxygen to operate. However, since there is more than enough oxygen in the atmosphere at lower altitudes, they burn the oxygen already present in the air as they fly.
This explains why aircraft need to travel horizontally fast enough to create sufficient lift under their wings for taking off and staying in the air while moving forward at sufficient speed to help supply oxygen to their main engines.
Rockets, however, only spend a fraction of their time in the atmosphere before entering space and Earth Orbit. During this period, they spend all their energy to create enough thrust to escape the thick atmospheric air near the surface to reach orbital velocity.
By the time they travel fast enough to use any air resistance for lift, they are at an altitude where very little air is present. Rockets also need to stay as light as possible to gain altitude and reach space, so wings or other aerodynamic structures add unnecessary weight.
The familiar long cylindrical shape of a rocket not only helps it to be more aerodynamic and cut through the thick atmosphere near the surface, but it also provides space for the liquid oxygen needed to burn the internal fuel supply.
Rockets primarily operate in space (and briefly in the upper atmosphere), where very little oxygen is present. As a result, they can not make use of the air in the atmosphere to burn fuel and have to carry all their oxygen internally.
This section makes it very clear that a vertical position is the optimal launch position for a rocket to reach space and its intended orbit as quickly and efficiently as possible. It also explains why it can’t take off/launch horizontally like a conventional aircraft.
Conclusion
Although a rocket lifting off slowly in a vertical position may seem very clumsy and impractical, it is the quickest and most efficient way to reach space and get into orbit, as this article clearly illustrated.
As it also explained, there is a good reason why rockets don’t take off and fly horizontally like conventional aircraft, which is mainly due to them serving completely different purposes and having very different designs as a result.
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.
