One Up One Down

In Chapter 1 we saw how a momentum exchange tether works. When an orbiting spacecraft vertically deploys a long cable with a mass at the end, and that cable is subsequently cut, the spacecraft moves into a new orbit. This orbit has either a higher or a lower average altitude than before (it will be an elliptical orbit), depending on the direction in which the mass is initially pushed away. The deployed mass also enters a new orbit if the mother spacecraft goes up into a higher orbit, the mass...

Electric Propulsion

There is a more mass-efficient type of propulsion that requires much less propellant for the same spacecraft and mission. Rather than ejecting hot gases that are products of a combustion process, electric propulsion systems eject charged particles using electromagnetic forces. In an ion engine'' the atoms of an inert gas, usually xenon, are ionized and shot out at a much higher velocity than in a normal rocket engine while an engine burning liquid propellants may shoot out hot gases at 4.5...

Satellite Experiments

A relatively cheap way of conducting experiments in space is to put them on the upper stage of a rocket used to launch other satellites into orbit. An upper stage needs to achieve orbital velocity to deliver its cargo, so after deploying the payload satellites it is carrying into space, the stage itself becomes an independent and passive satellite'' (in fact, when people thought they were seeing tiny Sputnik-1 fly across the night sky in 1957, they were actually looking at the much larger upper...

Artificial Gravity

Microgravity, or weightlessness, exists inside an orbiting spacecraft because its contents (including any astronauts) are falling around Earth we call this free fall'' at the same speed as the spacecraft itself (and not, as is often believed, because there is no gravity in space). It is like being inside a falling elevator, but without the hard landing at the end. Astronauts inside the International Space Station (ISS) can simply float through its many modules, have dinner on a wall, and sleep...

Elevators on the Moon and Mars

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If space elevators can be deployed around Earth, they could in principle also be used on other planets and moons. Of course it would be more difficult to transport the necessary equipment that far out, but there are also important benefits. The lower a planet's gravity and the faster it rotates, the easier it is to build a space elevator. Theoretically, it will thus be easier to build a space elevator on the Moon than on Earth, as the gravity on its surface is only one sixth of what we feel...

Spacecraft Stabilization

As explained in Chapter 1 (see Momentum Exchange), the difference in gravity at different altitudes above Earth can be used to stabilize two tethered satellites into a vertical position. This was first demonstrated by the Gemini 12 mission and SEDS-2, although not without difficulty (see Chapter 4, sections Gemini 12 and Satellite Experiments). Such gravity-gradient stabilization does not necessarily require a tether all that is needed is a spread of mass over a sufficient length in other...

Space Shuttle Experiments

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One would have expected the next logical step, after the sounding rocket missions, would be to fly tethers on experimental, orbital satellites. The experiences with the Gemini flights and the various suborbital rockets showed that deploying tethers in space is not a straightforward thing, and that the dynamics involved were not yet fully understood. However, the next tether mission was rather large, complicated, and daring, and involved the Space Shuttle. In July and August 1992, shuttle...

Tether Propulsion

The MXER tethers use electrodynamic tether propulsion to move back to the right orbital altitude after slinging payloads into higher orbits. However, this novel means of propellantless propulsion can in principle be used on any spacecraft that spends at least part of its orbit below an altitude of 1000 km (600 miles) (Earth's magnetic field strength and the ionospheric density are too low higher up). Tethers Unlimited is developing what they call the Microsatellite Propellantless Electrodynamic...

Electrodynamic Tethers

Electrodynamic Tether

Electrodynamic tethers are thin cables made of an electricity-conducting material, typically a metal. If we deploy such a tether from a satellite in a low Earth orbit, it will tend to orientate itself vertically due to the gravity gradient explained earlier (see Momentum Exchange). It is not even really necessary to have another satellite on the other end of the tether the tether itself has a mass and therefore experiences the same forces as the second satellite in the momentum exchange tether...

Fiction and Fascination

While engineers were defining the mathematical basis for the design of space tethers and space elevators, science-fiction writers and artists brought the new technology to the attention of a broader public. A Russian volume of paintings by cosmonaut and artist Alexei Leonov and space artist Anatoly Sokolov published in 1967, The Stars Are Awaiting Us, contains a painting entitled Space Elevator.'' The description accompanying the illustration states that it shows an Earth-Satellite-Earth''...

From the Ground Up

As we said before, an Earth-orbiting rotovator reaching down to the ground is probably unfeasible due to the drag of the atmosphere. However, you could use a bolo that only swings down to the top of the atmosphere in combination with a launch vehicle bringing payloads up to high altitude. Instead of going all the way into orbit, this rocket vehicle could follow a ballistic trajectory that would take it into space for only a short while. A satellite released by such a launcher at the top of its...

Cleaning Up the Belts

The possibility of removing dangerous charged particles from the vicinity of Earth was mentioned in Chapter 1 (see Electrostatic Tethers). As stated, the energetic particles in Earth's Van Allen belts steadily degrade spacecraft electronics, sensitive optical equipment, solar arrays, and human body cells because they break the chemical bonds within the materials they consist of. They also cause localized charge effects that can disrupt and even damage electronic equipment. The high radiation...

Rocket Propulsion Limits and Limitations

Soyuz Spacecraft Propulsion

Current space missions are heavily dependent on chemical rocket propulsion for the launch into space, for changing orbits, and for attitude control. For the launch, rockets with chemical propulsion engines are used because there is simply no other way to put anything in orbit. Launcher technology has improved a great deal over the last 60 years, so that rockets can now transport their payload more precisely to the desired orbit, are able to place multiple satellites into different orbits, and...

Cable Catapults

The Cable Catapult System is an idea of the late Robert L. Forward he died in September 2002 of Tethers Unlimited, a company working on the development of advanced space tether concepts. Forward was also a famous science-fiction writer who featured space tethers in many of his books the cable catapult appears in his novel Camelot 30K 1993 , but it is more than just fantasy. The Cable Catapult System uses a long tether as a launch rail in combination with a so-called linear motor. A linear motor...

Electrostatic Tethers

Electrically conducting tethers can also be used for purposes other than changing orbits of satellites and electrical power production. One interesting concept is to use them to remove charged particles from the vicinity of Earth. The Sun sends out a continuous stream of electrons and ions. Ions are atoms that have an electrical charge because they contain too many or too few negatively charged electrons to compensate for the number of positively charged protons in the atom's nucleus. Earth's...

Cable Material

The major challenge for both the space elevator and the aerovator concepts is that the incredibly strong yet lightweight tether material that they require does not yet exist. We have already seen that for his space elevator, Bradley Edwards needs cable material with a tensile strength of 130 gigapascals 1,300,000 kilograms per square centimeter, or 18,500,000 pounds per square inch . An aerovator could be built with a lower strength material, but it still requires a tensile strength on the...

Step by Step

It makes sense to build technology prototypes and subscale space elevators before attempting to construct a real one. The LiftPort Group has published a roadmap that presents a stepwise approach, where the complexity of the technology and prototypes increases with time. This allows space elevator engineers to gradually build up experience and to solve the problems they encounter before moving on to the next major development Fig. 6.7 . The LiftPort roadmap starts with two balloon-lofted tethers...

Space Elevator Versus the Aerovator

The aerovator concept has both advantages and disadvantages in comparison to the space elevator. An important advantage is that the aerovator does not require a countermass in space. Moreover, the aerovator ribbon length would be on the order of a 1000 km 620 miles , while the space elevator needs to be at least 60 times longer and maybe much more, depending on the type of countermass used . The space elevator needs to be deployed from space, and thus requires that nearly all equipment involved...

The Aerovator Alternative

Figure 6.9 Schematic of the aerovator, seen from the side. km 620 miles and a total mass of 240 metric tons 529,000 pounds . The aerovator would make one rotation every 13 minutes and needs to be propelled by a 5 mega-newton push provided by jet engines. The part of the aerovator extending from the ground-based hub into the upper atmosphere is called the ascent section. This section is rotating through the atmosphere, and thus has considerable aerodynamic drag acting on it. The rotational speed...

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Figure 4.10 Artistic impression of what the ProSEDS experiment would have looked like in space. Courtesy of NASA. Figure 4.10 Artistic impression of what the ProSEDS experiment would have looked like in space. Courtesy of NASA. over a long time, and how it would survive in an area with man-made orbital debris space junk'' and natural micrometeoroids flying around at orbital velocities. The U.S. Naval Center for Space Technology designed, built and operated the experiment for the secretive...