Suborbital Spaceflight: Technical Aspects

Suborbital Spaceflight: Technical Aspects

Gerold F. L. Fuchs (Austro Control, Austria)
DOI: 10.4018/978-1-5225-7256-5.ch004
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One of the major historical milestones of spaceflight, suborbital flights played a vital role in the development of technologies and procedures for the operation of spacecraft. Today suborbital flights still play an important part in scientific and commercial fields, and in this chapter, the basic terms and values will be defined and also explained in the historical context. Small launching system for scientific applications called sounding rockets have a significant importance due to an unbeatable flexibility and cost effectiveness and the new and emerging field of commercial human flights to the edge of space is growing as an economically sound business model for private participants. Either of them will be detailed in the appropriate sub-chapters, followed by an outlook on the future possibilities of suborbital flights.
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Along with the old dream of flying and fuelled by the urge to explore new worlds and surroundings, many visionaries thought about the possibility not just to fly, but leave our planet and go forth into space. To reach the goals of leaving our planet for longer periods and even land on other celestial objects, researchers and engineers had to master one step after the other on the ladder of technological evolution. One important step were suborbital flights.

The definitions of the parameters in suborbital flights are sometimes a bit blurry, so they will have to be explained in the following sub-chapter. Whilst suborbital flights had a vital impact on the history of space flight there are two aspects that keep them interesting for future applications: Scientific applications and commercial human flights. Each of the applications will be detailed in the appropriate sub-chapters, followed by an outlook on the future possibilities of suborbital flights.


An orbit is, simply explained, the curved path of an object around a point in space, such as the path of Earth around our sun, or the path of a satellite around the Earth.

Figure 1.

Newton’s Cannon ball

Source: Author‘s Elaboration

As an example, Isaac Newton’s cannon ball depiction (Figure 1) can be used to illustrate the role of the force of gravity and speed in planetary motion. (Newton, 1728) The thought experiment depicts a cannon placed on top of a very high mountain with no atmosphere (and thus resulting drag).

If the velocity is very high, the cannon ball will leave Earth on a hyperbolic trajectory depicted by path (E). If the velocity is not enough to leave the gravitational sphere of influence of Earth, but equal to or greater than the so-called “orbital velocity”, the cannon ball will enter into an orbit around the Earth (C & D) following an elliptic or circular path, depending on the speed. If the cannon ball does not achieve orbital velocity, it will return back to Earth such as trajectories A & B illustrate.

Given a distance from a planet, a body needs to have the correct speed, so that the gravitational pull is matched by the centripetal force. In other, more simplistic words: an orbiting spacecraft is constantly falling towards the planet it orbits, but keeps missing it. To reach a stable orbit around Earth, a spacecraft has to be accelerated to the correct orbital velocity. For the negligible small mass ratio between a planet and a spacecraft, this velocity depends basically only on the mass of the planet and the desired orbital altitude. For the theoretical model of an orbit height along the surface of the earth (at altitude zero) this value would calculate to roughly 7,9 km/s.

Alas, “cruising on altitude zero” is not an option for a real planet especially one with the presence of a dense atmosphere for various reasons (drag, excessive heat through friction, collision with terrain or buildings, noise abatement, etc.)

Practical orbit altitudes around the earth usually start at around 200 km above its surface. At this altitude, the drag exerted by the few remaining air molecules upon a spacecraft is small enough to plan for extensive flight operations. However, this small remaining drag force leads to a constant degradation from the selected orbit altitude, and warrants the necessity compensation by using additional rocket thrust inputs, known as “re-boosting”.

The more precise definition of “suborbital” would mean that a body (spacecraft) is launched away from another body (planet) with a speed lower than necessary to enter a stable orbit. For practical operations on earth, the term “suborbital” excludes the craft which has already reached the mechanically correct orbit altitude and speed, but entered a degrading orbit due to drag. In the most simplistic definition, a suborbital space flight is a flight which flies into space, but will not be able to finish one orbit around the planet.

Key Terms in this Chapter

Spacecraft: A vehicle or machine designed to fly in outer space.

Sounding Rocket: A sounding rocket, sometimes called a research rocket, is an instrument-carrying rocket designed to take measurements and perform scientific experiments during its suborbital flight.

Ballistic: Behavior and effects regarding the motion of projectiles.

Apogee: For any satellite of Earth, including the Moon, the apogee is the point of greatest distance to the host planet.

Microgravity: The term is a synonym for weightlessness and zero-g, but indicates that g-forces are not quite zero, just very small.

Spaceplane: An aerospace vehicle that operates as an aircraft in Earth's atmosphere, as well as a spacecraft when it is in space.

Mach: Mach is a dimensionless quantity representing the ratio of flow velocity to the local speed of sound. An object traveling with Mach 1 has the same speed as the pressure (sound) waves that it is generating.

Hypersonic: Aerodynamic description of speed that is in a highly supersonic range, generally referred to speeds of Mach 5 and above.

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