So You Want To Fling Me Into Space?
By: Chloe Wang
Tags: Skyhooks, Space Exploration, Space

Skyhooks may be the first step to interplanetary travel.

Photograph by Suchart Kuathan

Humans have been stuck on Earth for about 7,000,000 years. It wasn’t until the last century that humans have finally felt the true touch of space, sending probes, rovers, and even humans off the Earth’s surface. We are still in our primitive space days, however. To really explore space, we need to cross one major barrier — becoming an interplanetary species. Of course, this is a far stretch, as we’re still yet to put humans on another terrestrial body other than the moon. To become an interplanetary species, we should begin with small steps.

Facing Transportation

The largest issue we currently face with traveling to other planets is simply transportation. The traditional use of chemical rockets is the most inefficient method of transportation when trying to become interplanetary.

A traditional chemical rocket has four major systems: the structural, propulsion, payload, and guidance system.

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The propulsion system, which is made of the rocket engine, fuel, and oxidizer, takes up about 80% of a standard chemical rocket. The diagram below represents a liquid-fuel rocket, used by both NASA and SpaceX. The fuel is something that burns, like liquid hydrogen, while the oxidizer is something that reacts with the fuel, like liquid oxygen. The fuel and oxidizer are stored in separate tanks pumped into a combustion chamber, where they react to form water vapor. The water vapor is then ejected at high speeds to produce thrust.

Liquid Rocket Propulsion System, by Aerospace Engineering

On the other hand, the payload system is merely the tip of the rocket. The payload system carries what is being sent into space, like a satellite or human being.

Payload System from Rockets 101, by National Geographic

Why is the payload system so much smaller than the propulsion system? It actually goes back to the Tsiolkovsky Rocket Equation, which describes the principle that, due to the law of conservation of momentum, a rocket can accelerate using thrust by releasing a part of its mass at a high velocity. The equation is used to relate the total mass of the rocket to the exhaust velocity of the rocket (the speed at which fuel is expelled).

Tsiolkovsky Rocket Equation, by Caleb Scharf

As we increase the payload and distance a rocket must travel, we must increase the fuel as well. However, when the amount of fuel is increased, that means the rocket’s mass is increased, so more fuel needs to be added to accommodate for that. As a result, we are stuck in a perilous cycle of trying to find the balance between the distance the rocket is traveling, the payload, and the amount of fuel needed. That’s why the payload system eventually became so small on traditional rockets. The limited size of the payload system is a huge problem since it means that we can’t send that much cargo with every rocket launch.

Additionally, this fuel only accommodates for escaping Earth’s gravity. After that, the rocket is free-floating. Trying to put the amount of fuel needed to continuously steer a rocket from the Earth to Mars is almost unfeasible.

Let’s not forget that the use of chemical rocket launches can also be unbelievably expensive. Up until the past few years, the cost of launching material into space could be up to $54,000/kg. That means that launching a single 70 kg person into space could cost you as much as $3,780,000. Developing a colony on Mars would require huge payloads, and if the costs of using chemical rocket launches remain the same, finances could be our limit to becoming interplanetary.

It isn’t reasonable to use chemical rockets to go to Mars and become an interplanetary species. It’s inefficient, costly, and limiting. So how do we overcome this? The Skyhook.

The Skyhook?

Skyhook, by Kurzgesagt (image not to scale)

Originally proposed by John Isaacs in 1966, the Skyhook is a theoretical space structure made of a tether and counterweight that orbits Earth near or at orbital velocity. Spacecraft could attach to the spokes of the Skyhook and be flung into space, allowing for the reduction of fuel and expenses while sending larger payloads.

How Does It Work?

The Skyhook acts as a large cable that rolls around Earth, similar to a wheel. When a spoke of the cable reaches a touchdown on Earth, a spacecraft can attach to it. Then the cable continues to rotate with the spacecraft until it is on the opposite side of the touchdown. The spacecraft is then released into space on its trajectory path, almost like a catapult or slingshot. High impulse engines or new spacecraft attaching to the spokes as the cable rotates can then return momentum on the Skyhook.

Model of the Rolling Skyhook, by the National Space Society

In one model, the Skyhook would be about 1/3 of the Earth’s diameter (8410 km). This small size would greatly minimize the Skyhook’s mass, helping it rotate smoother through the upper atmosphere. The spokes would touchdown about 6 times during each 2-hour orbital period.

The basic launching process, created by R. Hoyt

A spacecraft could then fly to the bottom spoke of the Skyhook and attach itself to it. As the Skyhook rotates, the spacecraft would gain enough speed to shoot out of the Earth’s atmosphere and escape the Earth’s gravity. When a spacecraft is coming back to Earth, it can attach to the top spoke of the Skyhook. As the Skyhook rotates, the spacecraft is slowed down and brought back below Earth’s atmosphere. In theory, these spacecraft would not have to travel much faster than an airplane to reach these spokes.

If The Skyhook Is So Perfect, Why Doesn’t It Exist Yet?

The concept of this Skyhook is drastically cheaper than using chemical rockets, highly efficient, and relatively simple. So if this magic piece of infrastructure exists, why are we yet to create it? The answer’s simple — we don’t have the materials to create the tether. The tether needed to create the Skyhook would not only need to be thousands of kilometers long, but also would also need to withstand space debris, radiation, weathering, and erosion. To add on, it would also need to be so light that it does not slow down the Skyhook in the space.

Interestingly, carbon nanotubes (CNT) are perfect for this position. CNTs are cylindrical molecules made of carbon formed in a similar structure to graphene. Usually, the diameter of a CNT is a few nanometers (a few billionths of a meter). However, a CNT can be hundreds of microns long, which is thousands of times its diameter. The high flexibility, tensile strength, light weight, and conductivity of CNTs would allow them to withstand the dangers of space debris, radiation, weathering, and erosion that a Skyhook tether would face. Unfortunately, CNT production is still in its developing stages. It is still extremely expensive to produce a few centimeters of CNTs, so using CNTs to produce a 6,000-kilometer-long tether is pretty far out of our reach.

There are other dangers too. If the Skyhook miscalculates when to release the spacecraft, the spacecraft could be flung in the completely wrong direction with no way of getting back to Earth. If the spacecraft miscalculates when to attach to the Skyhook when it is coming back, it could also crash into Earth.

Becoming Interplanetary

As space technology advances and the concept of the Skyhook becomes more popularized, researchers, engineers, and scientists may be able to address these issues. Thus, in the big scheme of things, Skyhooks could be the key to making us interplanetary. Imagine we had a self-sustaining colony on Mars. We could have a Skyhook on Earth and Mars that would allow for spacecraft to quickly travel between planets while remaining cost-efficient. Larger payloads could be sent while using less fuel and therefore reduced expenses. Spacecraft could transfer cargo, people, and more. The Mars Skyhook could even serve as a stop to send spacecraft even farther than Mars.

Skyhooks used for interplanetary space travel, by Kurzgesagt (image not to scale)

The Skyhook holds a bright future in space exploration. It would yield a completely new space travel system that is much more efficient, allowing us to carry much larger payloads with fewer costs and fuel. Throwing you into space might really be the next step for the human species.

Have questions? Send me an email at chloewang.lv@gmail.com and I’ll be happy to respond!