SkyHook, LightCraft, Mass Drivers, and Space Launch Systems

The Skyhook

The basic idea for a rotating skyhook is to have a long cable with a counterweight on one end and a hookup connector, which could be magnetic, on the other. The hook would rotate such that the motion at the lower end moves in the opposite direction of the center of mass around which it rotates. The longer hookup end would then appear to slow down as it dips into the upper atmosphere, tracing a circular cycloidal pattern as it orbits the Earth. A skyplane would then meet the lowering tip and attach a payload. The skyhook would subsequently lift this payload and fling it at any tangent of the tether, optimally at the top of its rotation, where it would possess both the center of mass velocity and the rotational velocity.

A description of the rotating skyhook path: The green line represents the path of the hookup line, the blue line indicates the counterweight, and the red line depicts the path of the center of mass of the skyhook. The green line moves in the same direction as the center of mass at the top of its rotation and in the opposite direction at the bottom of its rotation. The space plane would approach the pickup location while moving in the same direction as the center of mass. In this way, the counter-rotational velocity, the speed of the approaching space plane, and Earth’s rotational velocity at the equator would all subtract from the orbital velocity of the center of mass of the skyhook. See the section below for a concrete example.

Lifting the cargo and then flinging it into space would incur an equivalent amount of loss of momentum that would need to be returned to the system before it could be reused. Therefore, the center of mass would likely need to have a thruster unit through which the tether pivots, and the counterweight would likely also be a thruster unit, both of which would be used to quickly recover the lost momentum.

The lower section of the hookup line will need to be replaced regularly since it will sustain damage every time it dips into the upper atmosphere. This would involve partially spooling up the hookup line to allow an automated system to detach the lower section, flinging it to burn up in the upper atmosphere, and attach a new lower section with the hookup. The counterweight would also need to be reeled in proportionally to maintain a consistent center of mass. Once completed, the orbit of the center of mass would need to be stabilized, and the lost momentum would need to be restored.

The rotating skyhook is viewed from above. It is not to scale, as the counterweight is likely situated 50 to 300 km (30 to 185 miles) above the center of mass, while the cargo hookup is probably located 300 to 1,000 km (185 to 620 miles) below the center of mass. The cargo (in blue) is lifted up to and synchronized with the skyhook using a space plane. A space plane is likely required since the skyhook’s hookup connector would only be able to reach into the upper atmosphere, and it would still be moving at a substantial speed relative to the ground at that location. We will examine this further in a separate chapter devoted to skyhooks.

The space plane is synchronizing with the skyhook hookup. From the perspective of the space plane, the hookup will appear to drop straight down out of the sky, stop, and then shoot straight back up. The skyhook will pick the cargo (blue) out of the space plane’s cargo hold and fling it into space. The intercept velocity is the orbital speed of the center of mass of the skyhook minus the rotational velocity of the hookup. The cargo will then be flung out in any tangential direction to the hookup’s rotation, with a maximum velocity equal to the orbital velocity plus the rotational velocity of the hookup. The altitude rendezvous will involve a trade-off between allowable wear on the hookup connector and the bottom 100 kilometers or so of the hookup cable. The hookup line does not require power.

History of the Skyhook

The earliest ideas resembling a skyhook emerged from concepts of space tethers and momentum exchange systems. In the 19th century, Russian scientist Konstantin Tsiolkovsky envisioned space elevators, proposing a tower that would extend from Earth to geostationary orbit.

While his ideas were more closely aligned with what we now recognize as space elevators, they laid the groundwork for future tether-based concepts.

Konstantin Tsiolkovsky’s ideas were inspired by the Eiffel Tower, as he imagined a structure that could rise indefinitely if built tall enough to reach orbit. Although impractical at the time, his work sparked future exploration of tether-based launch systems.

The modern skyhook concept began to take shape in the mid-20th century with advancements in orbital mechanics and satellite technology. In 1960, American engineer Jerome Pearson expanded on the concept of space tethers, proposing rotating tethers in Low Earth Orbit (LEO) that could “catch” payloads from the Earth’s surface and accelerate them to higher orbits through momentum exchange.

Pearson’s work explored the potential for tethered systems to transport payloads between orbital planes or to capture suborbital vehicles, thereby reducing the energy required for launch.

The term “skyhook” gained prominence in the 1980s through science fiction and aerospace proposals. The concept was popularized by researchers such as Hans Moravec and Keith Lofstrom, who proposed dynamic tether systems capable of transferring payloads to orbit without traditional rockets. In 1988, Pearson formalized the idea of a rotating skyhook in collaboration with NASA and the U.S. Air Force. His studies highlighted the feasibility of using tethers for momentum exchange and the potential for reducing launch costs. Around the same time, NASA’s Marshall Space Flight Center investigated skyhook systems as part of their broader research into space tether technologies and electrodynamic tethers.

In the early 2000s, skyhook research changed from just theory to real studies, backed by organizations like NASA and the European Space Agency. The focus grew to include rotating skyhooks, fixed tethers, and partial tethers that reach into the lower part of space. Advances in material science, especially new materials like carbon nanotubes and graphene, sparked renewed interest in skyhook ideas. These materials are very strong and could potentially be used to make tethers that can handle the tough conditions of space.

Skyhooks have featured prominently in science fiction literature, films, and video games. Works like Arthur C. Clarke‘s “The Fountains of Paradise” (1979) and Kim Stanley Robinson‘s “Red Mars” (1992) explore tether-based transport systems, often depicting them as essential infrastructure for space colonization. In films like “Interstellar” and “Elysium“, skyhook-like systems are envisioned as means to transport people and goods between Earth and orbit.

While no skyhook has been constructed, space tether experiments (such as the Tethered Satellite System deployed by the Space Shuttle) have successfully demonstrated the principles behind tethered transport in space. Research continues into space tethers and momentum exchange tethers as potential precursors to operational skyhook systems. Skyhooks remain an active area of speculation and engineering, representing a promising pathway to reducing launch costs, increasing space accessibility, and expanding humanity’s reach into orbit and beyond.

The discovery of carbon nanotubes in 1991 by Sumio Iijima sparked renewed interest in skyhooks and space elevators. Carbon nanotubes are both super strong and lightweight, which are crucial characteristics for the designs of both skyhooks and space elevators. Even though this was a game-changing discovery, the challenge of producing them in large, defect-free quantities remains a major hurdle to overcome.

In 2003, NASA’s Institute for Advanced Concepts (NIAC) funded studies into the feasibility of skyhooks and space elevators, focusing on material science, power systems, and stability dynamics. The research concluded that while the concept is sound, material limitations remain the largest barrier.

In 2004, the Spaceward Foundation launched the “Elevator:2010” competition, encouraging innovation in space tether and climber technologies. The competitions offered financial rewards for developing strong, lightweight tether prototypes.

In 2012, the Obayashi Corporation announced plans to build a space elevator by 2050. Their design envisions a tether made from carbon nanotubes, stretching 96,000 km into space, far beyond geostationary orbit.

There are many other skyhook designs, and we will be devoting an entire chapter to the topic.

Conclusion (SkyHook)

Not only is the skyhook one of the more feasible concepts presented so far, but NASA has also actively worked on this concept.

The concept of a skyhook has its roots in early 20th-century space exploration theories and has evolved through advancements in orbital mechanics and aerospace engineering. While the term “skyhook” itself gained traction in the latter half of the 20th century, the underlying ideas can be traced back to earlier speculative engineering proposals.

In a separate chapter dedicated to skyhooks, we will examine the various types of skyhook designs, along with some realistic numbers and their feasibility.

Mass Drivers

Mass drivers are a class of space launch systems that include space guns, spin launch and the Slingatron.

One of the big issues with this type of launch system is the atmosphere. Hence, although these systems may not be useful on Earth, they could be very useful for sending materials up from the surface of the moon or other bodies that do not have a dense atmosphere.

Mass drivers are electromagnetic catapult systems designed to accelerate payloads to high velocities without the need for chemical rockets. The concept is rooted in the use of linear motors or railgun technology, where strong magnetic fields propel an object along a track, gradually building speed until it reaches the desired velocity. 

Mass drivers offer a potentially more efficient and cost-effective way to launch materials into space, particularly for operations on the Moon, Mars, or asteroids where escape velocities are lower than on Earth.

The technology was popularized in science fiction by writers like Arthur C. Clarke and Robert Heinlein, but the underlying principles are firmly grounded in physics and engineering. A mass driver could be constructed as a long track, possibly stretching for hundreds’ of kilometers, positioned on the surface of a celestial body. By leveraging the lack of atmosphere and low gravity, materials mined from the Moon or asteroids could be launched directly into space, reducing reliance on fuel-intensive rockets. This could revolutionize space industries by facilitating the construction of large orbital structures, such as space stations or habitats, using resources extracted in-situ.

While still in the experimental and theoretical phase, mass driver technology holds promise for the future of space exploration and industry. Projects like NASA’s Artemis program and asteroid mining ventures could eventually adopt mass drivers as part of their operational toolkit, contributing to humanity’s long-term expansion into the solar system.

By harnessing local resources and launching them directly into space, the novel explores how the Moon can serve as a critical staging ground for deeper space exploration.

Suarez delves into the engineering and logistical challenges of building a mass driver on the Moon, addressing the construction of durable tracks, the need to mitigate recoil forces, and the alignment necessary to send payloads to specific orbits or transfer points.

The mass driver becomes a key component in a larger effort to establish energy and resource independence in space, reflecting the geopolitical and economic shifts driven by humanity’s increasing reliance on extraterrestrial resources.

Through Critical Mass, Suarez uses the mass driver not only as a technological marvel but also as a symbol of the transformative potential of space exploration. His portrayal highlights the practical steps needed to make space-based industries viable, underscoring the importance of collaboration and innovation in shaping humanity’s future beyond Earth.

SpinLaunch

SpinLaunch is an aerospace company developing a method to launch payloads into space using a kinetic energy-based system rather than traditional chemical rockets.

The core concept involves a massive, ground-based centrifuge that spins a payload at extremely high speeds before releasing it, propelling the payload through the atmosphere and into space. The idea is to dramatically reduce the cost, complexity, and environmental impact of space launches.

The system consists of a large vacuum-sealed chamber housing a rotating arm that accelerates payloads to speeds of up to 8,000 km/h (5,000 mph). Once the payload reaches the desired velocity, it is released through a launch tube, exiting the centrifuge and continuing its ascent. A small rocket or thruster may provide the final push to achieve orbital insertion.

The concept relies primarily on kinetic energy, which SpinLaunch claims would significantly reduce the need for traditional rocket fuel, thereby addressing both cost and emissions concerns. They assert that this approach could potentially reduce launch costs by up to 70% compared to conventional rockets, making space more accessible for smaller satellites and payloads. SpinLaunch envisions conducting multiple launches per day to support the growing demand for satellite deployment.

However, many challenges are associated with this approach when launching payloads from Earth. Payloads must withstand the extreme G-forces experienced during acceleration of up to 10,000 g. It is important to note that a typical human can only withstand 3 to 4 g, while a trained fighter pilot may endure up to 10 g (1 g = 9.81 m/s²). This limitation restricts the types of cargo that can be launched, primarily favoring ruggedized satellites and components.

Moreover, the payload must survive the intense heat and aerodynamic forces during its initial passage through the atmosphere. Furthermore, although 8,000 km/h sounds like a substantial speed, orbital velocity is at least three times that. Additionally, without a booster rocket, friction with the atmosphere and the loss of momentum due to gravity would probably prevent the projectile from even reaching the Kármán line, the official boundary between the upper atmosphere and space, located 100 km above sea level. For a low Earth orbit with an altitude of 300 km, the required velocity is 28,080 km/h (17,500 mph). Earth’s escape velocity is 40,270 km/h (25,025 mph).

Although the SpinLaunch device is largely ineffective for launching payloads from Earth, it could work well on the Moon, where there is both less gravity and no atmosphere.

Slingatron

A Slingatron is very similar to the SpinLaunch as described, with a few minor differences. It operates by accelerating a projectile along a spiral or circular track that continuously increases in diameter, much like a slingshot or a whip effect.

Conclusion (Mass Drivers)

Mass drivers will be very useful devices in the future. One application of a mass driver that will be discussed in detail in later chapters is a mass driver located on top of an orbital ring. Such a mass driver would have a very long track, potentially the entire length of the orbital ring, allowing for a gradual buildup of the velocity required to launch payloads, including human payloads, without the need to apply excessive g-forces. The potential exists to launch directly from the orbital ring to Mars or beyond, requiring speeds in excess of the 40,270 km/h escape velocity needed to leave Earth’s gravitational influence.

There will be many applications requiring various types of mass drivers in a space-based economy, including launching payloads from the Moon, as described by Daniel Suarez in his book Critical Mass. Smaller mass drivers or spin-launch systems will also be very useful for asteroid mining operations, among other uses.

One thing mass drivers will not be used for is launching payloads from Earth’s surface into space.

Lightcraft

A lightcraft is a conceptual spacecraft or aerial vehicle that uses directed energy, typically from a ground-based laser or microwave beam, to achieve propulsion. Instead of carrying heavy onboard fuel, the lightcraft relies on external energy sources to generate thrust, making it a highly efficient and lightweight system for potential space launches or atmospheric flight.

The concept behind a lightcraft is to use a directed energy source, such as a powerful laser or microwave beam, projected from the ground or an orbital platform, targeting the base of the lightcraft.

The energy heats the air beneath the craft (or, in space, an onboard propellant), creating a superheated plasma that rapidly expands and generates thrust.

This process can occur through ablation (vaporizing part of the lightcraft’s surface) or by ionizing the surrounding air. As the beam tracks the craft, it continues to propel it upward, potentially into orbit or beyond, without the need for chemical rockets.

Lightcraft designs can support small satellites or larger payloads, depending on the energy capacity of the laser or microwave system.

Atmospheric turbulence, weather conditions, and beam diffraction can reduce the effectiveness and accuracy of lasers, limiting the reliability of launches. Maintaining precise targeting of the lightcraft at high altitudes and speeds requires advanced tracking and adaptive optics.

The laser or microwave systems must generate extremely high power (in the megawatt range) to achieve the necessary thrust for space launches. Current lightcraft concepts are best suited for small payloads, such as microsatellites, due to energy and structural constraints.

NASA and private entities have conducted small-scale lightcraft experiments. One notable project, led by Dr. Leik Myrabo in the 1990s and early 2000s, successfully demonstrated laser propulsion for small test vehicles. It has been speculated that lightcraft technology could be used for rapid satellite deployment, hypersonic transport, or as part of a space launch infrastructure that complements existing rocket systems.

Although still in the experimental phase, lightcraft technology represents a promising step toward more efficient and cost-effective methods of reaching space, with potential long-term applications in aerospace and defense.

The lightcraft is an innovative concept that utilizes laser propulsion. This system relies on powerful ground-based lasers, which focus their energy on the craft. As the light hits the craft, it heats a specific material that produces thrust. This method allows for lighter structures since the propulsion does not depend on onboard fuel.

To visualize how this works, imagine a toy glider. The glider stays light and relies on a gentle push to fly. Similarly, if we can push a lightcraft with energy from the ground, it can ascend into the sky with minimal resources. This opens up possibilities for efficient space travel at a fraction of the cost of traditional rocket systems. Furthermore, since it uses energy from lasers, the environmental impact is significantly lower than conventional rocket launches.

Understanding the lightcraft involves recognizing its operational stages. First, it must align itself with the laser beam. After it absorbs energy, a control system optimizes the angle to ensure maximum thrust. Engineers must integrate several safety measures to protect the craft from overheating and to manage precise control during ascent. Failure to manage these factors could lead to a crash. Thus, while the lightcraft offers exciting possibilities, it also presents numerous technical challenges that must be overcome.

Conventional Launch Systems

Conventional launch systems have long been the backbone of space exploration, evolving from expendable rockets to partially reusable systems that reduce costs and improve reliability. These systems, driven by chemical propulsion, remain the primary method for placing payloads into orbit and beyond at this time. As space exploration enters a new era of commercialization and innovation, a diverse array of vehicles and technologies is emerging to meet the growing demand for satellite deployment, crewed missions, and interplanetary travel. Here is an overview:

SpaceX Starship

One of the most ambitious projects in the realm of conventional launch systems is SpaceX’s Starship. Designed as a fully reusable spacecraft, Starship aims to transport large payloads and crews to low Earth orbit (LEO), the Moon, Mars, and beyond. Powered by SpaceX’s Raptor engines, Starship is capable of carrying over 100 tons to orbit, making it the most powerful launch system ever developed. Its reusability and scalability are poised to revolutionize the economics of space travel, potentially lowering launch costs to unprecedented levels.

SpaceX’s Starship is designed to be a fully reusable spacecraft capable of carrying substantial payloads to various destinations, including low Earth orbit (LEO), the Moon, and Mars. In its fully reusable configuration, Starship has a payload capacity ranging from 100 to 150 metric tons to LEO. This capacity allows it to transport large satellites, space station modules, or significant quantities of cargo for interplanetary missions.

In an expendable configuration, where the vehicle is not recovered for reuse, Starship’s payload capacity increases to approximately 250 metric tons to LEO. This enhanced capacity is achieved by utilizing all available propellant for ascent, without reserving any for recovery maneuvers.

For missions beyond LEO, such as to geostationary transfer orbit (GTO) or interplanetary destinations, the payload capacity decreases due to the higher energy requirements. For instance, to GTO, Starship can deliver approximately 21 metric tons in a fully reusable configuration. (Wikipedia)

SpaceX’s Starship is designed with in-orbit refueling capabilities, a crucial feature that enables deep space missions to the Moon, Mars, and beyond. This refueling process involves multiple Starship launches, where a tanker variant of the vehicle delivers propellant to a Starship already in orbit.

NASA Space Launch System (SLS)

NASA’s Space Launch System (SLS) is the backbone of the Artemis program, which aims to return humans to the Moon and eventually send astronauts to Mars. The SLS is the most powerful rocket built since the Saturn V, capable of carrying heavy payloads to deep space. It features a combination of solid rocket boosters and liquid-fuel engines, drawing on legacy technology from the Space Shuttle program. The SLS is designed to carry the Orion spacecraft, which will transport crews beyond low Earth orbit (LEO).

While the SLS program has faced scrutiny over its costs and development timeline, it remains a cornerstone of NASA’s strategy for human space exploration. The agency is actively working to address challenges and enhance the system’s efficiency for future missions. As of January 1, 2025, the SLS is active and continues to play a central role in NASA’s deep space exploration initiatives. The first uncrewed test flight, Artemis I, was on November 16, 2022.

Electron by Rocket Lab

Targeting the growing small satellite market, Rocket Lab’s Electron is a lightweight, partially reusable rocket that provides dedicated launches for small payloads. With a payload capacity of around 300 kg to LEO, Electron is tailored for rapid and cost-effective deployment of CubeSats and small satellites. Rocket Lab’s innovative approach to rocket recovery, utilizing parachutes and mid-air helicopter retrieval, underscores the industry’s push toward reusability even at smaller scales.

Blue Origin’s New Glenn and New Shepard

Blue Origin, founded by Jeff Bezos, is developing the New Glenn rocket, a heavy-lift vehicle designed for orbital missions, capable of carrying 45 tons to LEO. New Glenn‘s reusable first stage is intended to be flown up to 25 times, reflecting Blue Origin‘s commitment to sustainable spaceflight. Additionally, Blue Origin’s New Shepard suborbital rocket has already demonstrated human spaceflight capabilities, offering commercial space tourism and microgravity research opportunities.

Space Shuttle (Retired)

NASA’s Space Shuttle program, which operated from 1981 to 2011, marked a significant leap forward in space technology by introducing the first partially reusable spacecraft. The Shuttle consisted of three main components: the Orbiter, a large external fuel tank, and two solid rocket boosters (SRBs). Unlike traditional expendable rockets, the Orbiter and SRBs were designed to be recovered and reused for multiple missions, reducing launch costs over time. The Shuttle was instrumental in constructing and servicing the International Space Station (ISS), deploying satellites, and conducting scientific experiments in low Earth orbit. It also enabled the retrieval and repair of satellites, including missions to service the Hubble Space Telescope.

Despite its many successes, the Space Shuttle program faced significant challenges, including two tragic accidents — the Challenger disaster in 1986 and the Columbia disaster in 2003. These events highlighted the risks associated with human spaceflight and led to design and procedural overhauls. Ultimately, the program was retired in 2011 due to high operational costs and the desire to pursue new space exploration strategies.

The Space Shuttle’s legacy lives on, influencing the development of modern reusable spacecraft such as SpaceX’s Falcon 9, Falcon Heavy, and Starship. Its contributions to space station construction, satellite deployment, and scientific discovery remain a cornerstone of NASA’s storied history in space exploration.

Space Planes

Space planes, blending elements of aircraft and spacecraft, represent a unique branch of space launch systems. Virgin Galactic’s VSS Unity and VSS Imagine offer suborbital space tourism flights, carrying passengers to the edge of space for brief periods of weightlessness. These vehicles are air-launched from a mothership, minimizing fuel requirements and enabling frequent reusability. Virgin Galactic’s long-term goal includes developing larger space planes capable of point-to-point hypersonic travel, expanding their role beyond tourism into high-speed transportation.

Sierra Space’s Dream Chaser is an orbital spaceplane designed to carry cargo and crew to the ISS. Dream Chaser draws inspiration from NASA’s earlier lifting body designs and offers a versatile platform capable of runway landings at conventional airports. This flexibility not only simplifies recovery but also opens the possibility for rapid refurbishment and relaunch. Future variants of Dream Chaser are expected to be crew-rated, making them integral to NASA’s Commercial Crew Program.

Space planes are expected to play a crucial role in the future implementation of skyhooks. Skyhooks could dramatically reduce the energy required to reach orbit by acting as a transfer point between Earth-based launch systems and orbital infrastructure. Space planes, capable of high-speed atmospheric flight, would be ideal vehicles for docking with the lower end of a skyhook, enabling efficient and cost-effective delivery of payloads and crew to space.

Space planes represent a promising avenue for expanding space access by combining the best aspects of aircraft and spacecraft, enabling more frequent, cost-effective missions with smoother re-entry processes.

Future Launch Systems

In the realm of space exploration and travel, various launch systems have been proposed and discussed. Among them, three systems emerge as the most promising: the lightcraft, the skyhook, and the orbital ring. As observed in this chapter, each of these systems possesses unique features that make them worth considering for future space missions.

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