How Spacecraft Move in Orbit

BEFORE getting to the business of discussing the orbital aspects of modern spacecraft missions later in this chapter, there are a few fundamentals about the orbital motion of a spacecraft that we need to discuss, and a few popular misconceptions about it that need to be put to rest. The first of these fundamentals is how a spacecraft remains in orbit around Earth, effectively forever, without having to fire rockets to sustain the motion. The answer lies in understanding that the spacecraft,...

Nuclear Electric Propulsion NEP

Electric propulsion systems have been developed over many years and have already flown on unmanned spacecraft. There is a fundamental difference between chemical and electrical propulsion systems. With a chemical system the energy required to accelerate the propellant out of the rocket nozzle is obtained from burning the fuel oxidizer combination. The speed we can impart to the vehicle is fundamentally limited by the amount of energy contained in the chemical propellants. However, an electric...

Orbit Transfer

Some spacecraft can be placed directly into their mission orbit by the launch vehicle, and thus have no need to perform orbit transfers. However, other spacecraft have to transfer between orbits to reach their final destination. This process of orbit transfer usually involves the use of a large rocket engine onboard the spacecraft, and such a system is referred to as primary G. Swinerd, How Spacecraft Fly Spaceflight Without Formulae, DOI 10.1007 978-0-387-76572-3_9, Praxis Publishing, Ltd....

Launch Vehicle Environment and Its Effects on Spacecraft Design

Another attribute of the launch process, which has a major effect on the spacecraft design, is the launch vehicle environment. Usually the structural design of the spacecraft (see Chapter 9) is not governed by its lifetime in orbit, which could be 10 years, but rather by the few minutes it spends climbing to orbit on the launch vehicle. We have already seen how much energy has to be released in a controlled manner during this relatively short period of time to achieve an orbital state for the...

What Did Einstein Do for Us

Einstein's contribution was fundamental and profound, a revolution in the way we think about the physics of motion, and in particular the motion of bodies in a gravitational field. This revolution began with the publication of Einstein's special theory of relativity in 1905, when Newtonian physics was well established, and most scientists believed their understanding of the physical laws of nature was complete. After all, newtonian physics had reigned supreme for something like 220 years This...

Earth Orbit Perturbations

For Earth-orbiting spacecraft, there are four main sources of orbit perturbation that can have a significant effect on the orbital motion In what follows we summarize how these work, and discuss in what way they affect the characteristics of the orbit. There are many other perturbation forces of lesser magnitude that influence the motion in fact, the list of possible perturbations is long. For a particular spacecraft, the choice of which perturbations to include in controlling and operating the...

The Spacecraft Design Process

How is this methodology of spacecraft design played out in an industrial setting The process is very people-intensive, and as such some might observe that it is not quite as objective as you might expect, particularly in the early stages when feasibility and preliminary design issues are addressed. However, we will come back to this perhaps slightly contentious statement later. It is important to set the design method that we have discussed so far in the context of the overall spacecraft...

Orbits Around Small Irregularly Shaped Bodies

Recently there has been a lot of interest in spacecraft missions that visit the smaller objects in the solar system, such as asteroids and comets. These types of objects are essentially debris scattered across interplanetary space, which are fragments left over from the processes that formed the Sun and planets. And therein lies their attraction as targets of scientific interest for spacecraft probes. Asteroids, sometimes called minor planets, are usually solid bodies which vary in size from...

Control Torquers

We have already briefly mentioned control torquers in our walk around the control loop in Figure 8.1. These are items of ACS hardware that are essentially the muscles of the ACS, converting the virtual commands produced by the on-board computer into physical torques that rotate the spacecraft. Perhaps the most obvious way of producing a control torque is to fire two thrusters in opposite directions, as illustrated in Figure 8.4a. These small rocket engines are set up in groups, called thruster...

Gravity Anomalies

Precession Rig

If the Earth were perfectly spherical, and its internal density distribution had a particularly simple form, then the gravity field of Earth would be a perfect inverse square law, as described by Isaac Newton. However, Earth is not perfectly spherical, nor does it have a simple internal mass distribution. There are topographical features that spoil that perfection, such as mountains that are 8 km (5 miles) high and ocean trenches that are 11 km (7 miles) deep. In addition, Earth's shape is...

Popular Operational Orbits

Now that we have a grasp of the three principal distinguishing characteristics of orbits shape, size and orbital inclination we can begin to look at the Earth orbit types that are most commonly used by spacecraft operators. Obviously, if we allow all possible variations in these three characteristics, then there is an infinite number of resulting Earth orbits to choose from The popular orbits that we are about to introduce, therefore, are a small subset of this vast number of possibilities, and...

Trapped Particle Radiation

Charged Particle Magnetic Field

As we saw earlier, some of the solar wind particles emanating from the Sun, generally charged particles such as electrons, protons, and atomic nuclei stripped of their attendant electrons, penetrate the protective shield of Earth's magnetic field. Some are focused by the field into the atmosphere above the north and south polar regions, causing auroral displays. Others are trapped by the magnetic field, producing radiation belts that pose a hazard to people and spacecraft alike. These belts are...

Communications Frequencies

Information on a satellite communications link is carried by electromagnetic (EM) waves Figure 6.2 in Chapter 6 illustrated the various parts of the EM spectrum. As a consequence, the speed of communication is the speed of light, which is around 300,000 km per sec (186,000 miles per second), so that communication with spacecraft in LEO is effectively instantaneous. However, for a communication satellite in GEO, the altitude of the satellite is around 38,000 km, so that EM waves take just over a...

SwingBy Trajectories

If a spacecraft passes close by a planet, as it journeys through interplanetary space, then the path it takes is described as a swing-by trajectory. It may be worth recalling some of the background we discussed in Chapter 1 about this type of trajectory, and indeed you may wish to reread the text associated with Figures 1.9 and 1.10 to refresh your memory. The shape of the curve describing the spacecraft's path is called a hyperbola, and it is one of the four basic conic section shapes found by...

Weightlessness

Weightlessness The Weight Body

Now that we have a good feel for the nature of orbital motion essentially a spacecraft is in a state of free-fall under gravity we can also achieve a similarly good understanding of the phenomenon of weightlessness. Weightlessness is something we see routinely on news coverage of manned space missions. In this book I use the phrase manned space missions to mean flights involving people both men and women. I know that the phrase may not be quite politically correct, but I dislike the other...