Skyhooks, Space Elevators, and Other Things We Can't Build Yet

Preksha Sanjay Madhva, Robotics Engineer

7 minute read

Space travel is hard. Our current methods of reaching Earth’s orbits are like riding a paper boat powered by a backpack of explosives upstream. This is very fuel-heavy, you can’t carry a lot of cargo, and you might possibly die. Because rockets are essentially fuel containers with limited room for a payload or cargo at the tiny tip, they’re not conducive to preparing for interplanetary travel. A rocket needs to reach approximately 40,000 km/h (~25,000 mph) to escape Earth’s gravitational pull. Even to just enter Low Earth Orbit (LEO) it needs to travel at a speed of about 27,300 km/h (~17,000 mph). To put it into perspective, your daily commute to work likely takes more time than it takes a rocket to cross the Karman line (3-4 minutes on average) and enter stable LEO (an additional 5-7 minutes); at the cost of nearly 2 million times the fuel consumed by your car.

There is a clear tradeoff between the time it takes to travel and the fuel consumed. However, is there a way for us to get to space with less fuel, and more payload, at the cost of a slower journey?

Launching from a higher altitude, like a mountain, may help reduce fuel costs, but would pose multiple logistical challenges and we don’t have time in this column to talk about all that! We could strap a rocket to a plane and deploy the rocket in mid-air. That is the concept behind ALTO (Air-Launch-To-Orbit) rockets. Midair deployment eliminates some of the initial distance the rocket needs to travel since it gets a boost from the plane, and it can reduce the fuel requirement and increase the room for payload (since the rocket doesn’t need to store the fuel it didn’t need for launch). This method would also be greatly impacted by weather conditions and it is technically complex to develop the level of coordination between the plane and the rocket, among many other disadvantageous factors like high development costs. We could also utilize a slingshot or centrifuge style mechanism to accelerate the rocket before launch but we don’t have time to talk about that in this column either!

So what about space elevators? Picture a giant elevator straight out of Willy Wonka and the Chocolate Factory, except this tower goes all the way to geostationary orbit (GEO). It would greatly reduce the fuel consumption, provide a higher payload capacity, and a safer, albeit slower, journey. Objects in orbit are faster than Earth’s surface, but objects attached to the Earth’s surface, through some type of anchor or foundation, have the same angular or rotational velocity as the Earth while in orbit. Tsiolkovsky’s hypothesis for a space elevator states that objects would only need enough energy to get to the top of the elevator which is much lesser than the energy required for rocket launches at the cost of being a slower process. Earth’s rotation, essentially, would provide free horizontal acceleration to the object traveling up the tower. Once released at the top of the tower, the payload or craft would have enough horizontal velocity to remain in geostationary orbit without falling back to Earth. An expansive structure like this would be affected by harsh weather conditions or hazards like earthquakes that would pose risks, but the greatest material and structural issues come from the height of the structure itself and the depth of foundations that would be required.

Some ideas to work around the construction issues of a compression solution, with the weight supported from below, involve a tensile solution with the weight supported from above and include a tether, counterweight, climber, and anchor. The counterweight in orbit holds the tether steady while the anchor keeps the elevator rotating with the earth’s surface and thus keeping the tether vertical. The climber will contain the payload and move along the tether towards the counterweight. There are a few options theorized on what the counterweight could be, at one point even a decommissioned ISS was considered as a suitable option.

Space Elevator (Interesting Engineering).

The tensile strength requirements of the material for the tether pose a complex challenge, along with actually fabricating such a tether. In 2000, Bradley C. Edwards suggested creating a 100,000 km long paper-thin ribbon, using a carbon nanotube composite material. He believed this solution would be more resistant against potential asteroid impacts but still faced issues with fabrication and tensile strength. However, there have been many promising developments in the manufacturing of macro-scale, single-crystal graphene — a material with higher strength than most nanotubes, that have improved the feasibility of space elevators, as detailed in the “Road to the Space Elevator Era” published by the IAA (International Academy of Astronautics).

The most promising development thus far, was when researchers at Japan’s Shizuoka University launched the STARS-EC space elevator experiment. They launched two CubeSats, connected by a tether, one of which had a mini-elevator onboard that traveled from one CubeSat to the other via the ribbon tether. The experiment was designed as a test to provide information for developing a larger structure. With the successful tether deployment and retrieval in orbit indicating promising results, the project is taking the next steps with “STARS-Me2”, a CubeSat deploying a 10m steel tape tether to attempt orbital altitude change.

The greatest challenge remains overcoming the high tensile strength requirement that is necessary due to the Earth’s gravity. One option is to also implement such a space elevator on the Moon or even Mars as they both have a lower gravitational pull than Earth which would reduce the required tensile strength of the tether.

What if we broke the problem down into smaller parts? We could put tethers that are hundreds or thousands of kilometers long so that they spin while orbiting, similar to a spinning bola. The spacecraft attaches to one end of the tether and uses it to build speed and climb to higher altitudes without much fuel consumption. This is the theory behind skyhooks. A counterweight holds the tether in place while it rotates, and the tip would slow down at the bottom relative to the ground and would speed up towards the top similar to a catapult. By utilizing the energy of the spinning tether you essentially get a significantly lower cost boost to the rocket at nearly twice the rotational velocity of the tether. As a bonus, you could use a second rocket as the counterweight that needs to descend, and the rocket would be slowed down by the tether, thus making the trip more-or-less free for both rockets. Thus utilizing the skyhook as a battery of orbital energy gives benefit to two rockets and reduces the fuel costs of both while also avoiding the tether crashing back to Earth due to a loss of momentum that would need to be rectified by thrusts from the counterweight - the more we use it, the cheaper it gets.

This sounds simple, but there are several problems and concerns. For instance, at its lowest point, the tether tip is racing along at speeds estimated at 12,000 km/h, meaning the lower we use the skyhook the higher the thermal issues from air friction, placing the minimal operation range around 80-150 km. Specialized spacecraft would need to be able to meet the tether tip at the lowest point and synchronize docking to a small object moving at Mach 10 speeds with a very small window of opportunity - potentially a few minutes or less.

The suborbital approach, capture, and boost to higher orbit of a spacecraft by a rotating Skyhook (NASA, Tethers Unlimited Inc).

Now a skyhook is just a single unit - closer to a road than an actual infrastructure system. However, we could utilize multiple skyhooks end-to-end to build a skyhook “ladder.” One of the problems with tethers is they can only be so long before they break, and similar to the space elevator, the longer the tether, the greater the mass of the system, which increases the required tensile strength requirements. So we can shorten the tether and chain multiple skyhooks together; a synchronized transfer between them would be a bit of an exercise in good timing but not the most problematic of challenges. The skyhook ladder can include as many units as needed, with each skyhook as long as the material will comfortably permit based on its tensile strength, the gravity at its position, and the centrifugal force from spinning. This sort of skyhook ladder, combined with some form of tether momentum conservation, ensures that we can get off any planet relatively cheaply, even ones more massive than Earth, and even if we never figure out how to mass manufacture super-strong materials like Graphene, good old Zylon or Kevlar could do the trick. There are so many other hypotheticals and concerns we could explore, but once again we don’t have time to talk about that!

While the concept of skyhooks and space elevators offers tantalizing possibilities for interplanetary travel and space exploration, the technological hurdles to their construction are still significant. The demand for materials with extraordinary strength-to-weight ratios with mass manufacturing capabilities presents a considerable engineering challenge. Ongoing research and advancements in materials science offer promising avenues for future innovation. As we continue to explore the frontiers of nanotechnology and advanced composites, we may one day witness the realization of these ambitious concepts. Another strong challenge for such infrastructure is the existence of space debris. With the increased number of satellites and rockets being launched every year, our orbits are becoming increasingly risky due to the greater potential for collisions. During the founding of Kall Morris Inc, our founders considered many of these space infrastructures as the focus of the company, however each idea had orbital debris as a high concern in implementing them. Eventually, fed up with all their cool ideas being shot down because of debris concerns, they decided to tackle space debris itself. Thus, our company’s goal to keep space clear for all was born of the hopes of making such technological marvels like skyhooks and space elevators more feasible.

The potential benefits of such infrastructure are immense, from revolutionizing space travel to facilitating sustainable resource extraction from asteroids. While the path ahead may be challenging, the pursuit of these audacious goals is a testament to human ingenuity and our relentless drive to explore the cosmos.

Recommended column to read next: REACCHing for the Stars