You may have noticed, reading this blog, that I don’t dive into hard science or serious calculations very often. That is because I am a squishy liberal arts major who dropped out of engineering school three years ago. My good friend Eamon Minges, however, has me covered—he has previously furnished Let’s Get Off This Rock Already! with some excellent, thorough, deliciously crunchy pieces, examining the mechanics of SSTO spaceplanes and orbital skyhooks, and today he will continue that theme of low-cost space launch, examining a truly mind-boggling piece of futuristic infrastructure: the orbital ring. I’ll let him take it from here!
Introduction:
As is often said in the aerospace industry, space is hard. Indeed, the first step to getting anywhere, whether it’s within the greater Solar System or cislunar space, is getting into orbit. The fundamental properties of our Earth make orbital flight using chemical-fueled rockets a serious feat of engineering—so serious, in fact, that nobody flew there for almost three centuries after Newton developed his laws of motion and gravity that predicted the physics of such vehicles. Given Earth’s deep gravity well and thick atmosphere compared to the other terrestrial planets of our solar system, Tsiolkovsky’s rocket equation does not take kindly to launches from its surface into a low orbit. In practice, it takes almost 9.4 km/sec of total delta-v to reach low Earth orbit from the ground. Currently, full and rapid reusability for orbital rockets (i.e., SpaceX’s Starship) is just beginning to come online, and while this is a crucial step for reducing the cost of access to space, there eventually comes a point were reducing costs further begins pushing up against the limits of the rocket equation.
With the exhaust velocities offered by chemical rocket systems (2,500
– 5,000 m/s), a two-stage rocket must be over 90% fuel by mass to get
any payload to orbit.
As an example, SpaceX’s Starship could, with full and rapid reuse combined with Tesla-style mass-production, reduce the cost per ton to orbit by one hundred fold. While I cannot emphasize any more just how revolutionary this will be in our current economic paradigm about spaceflight, even with a hundred-fold reduction in the cost per ton to low Earth orbit, the fundamental limits of the rocket equation insinuate that reusable rockets alone cannot be the sole means by which tens or hundreds of millions of people will leave Earth to colonize the Solar System. Starship and similar systems that follow it will likely be our first wagon train to space, doing the challenging work of bootstrapping transportation and industry into a place with relatively little established infrastructure. But if Starship is the wagon train, what stands for the ten-lane interstate freeway to space? That, dear reader, is the function of the orbital ring!
Orbital Rings
Though this may sound strange, a crude version of the orbital ring exists in nature—the rings of Saturn. While Saturn’s rings aren’t particularly useful, aside from raw materials and natural beauty, they do an excellent job illustrating the concept of an orbital ring. As the radius of a circular orbit increases, its orbital velocity decreases proportionally to the increase in radius. Two objects with the same orbital radius will thus have the same orbital velocity. Now, let’s apply this principle to the example of Saturn’s rings: say we focus on two chunks of ice orbiting at the same radius within a kilometer of each other. Because their relative velocity with respect to one another is effectively zero, it would be possible to build a bridge connecting them together—indeed, provided an astronaut is wearing a space suit with magnetic boots, they could easily walk between them. Now imagine that we built the same bridge, but this time to each successive iceberg, one ahead of the last, provided it has the same orbital radius as our staring ice block. Eventually, you would have a network of bridges encircling the entire planet. This circular structure orbiting the planet would be the basis for an orbital ring.
However, to make the orbital ring useful for getting megatons of people and payload off Earth and into space, there is a missing ingredient. The special sauce in this case is something called active support. Active support is the idea of using a constant stream of energy, rather than a material’s inherent compressive or tensile strength, to support its weight. A simple version of this principle can be shown by inverting a pie-pan and anchoring it to the ground with four pieces of string. If one were to shoot a stream of water with a pressure washer from ground level, it would be possible to cause the pie pan to hover in place, supported by the kinetic energy imparted onto the pan by the stream of water. The kinetic energy of the water stream is the substitute for the compressive strength of a truss or member that would otherwise be used to support the pie pan.
An orbital ring would use active support in a slightly different way from our pie pan analogy. Instead of relying on active support to allow an object to hover in place over a point on the ground, active support will be used to keep material (usually iron particulate) spinning around the ring in perpetuity to exchange enough outward momentum to counteract the effects of gravity. This outward momentum, caused by a stream of particles moving at faster-than-orbital velocities in a giant magnetically held circular track, would supply the outward force needed to allow an outwardly stationary ring structure to stay suspended over Earth’s surface, above the bulk of its atmosphere.
Here is an oversimplified diagram of its basic structure. The red arrows are the direction of the flow of material in magnetic confinement, and the two black vector arrows are, respectively, the force resulting from the centripetal acceleration of the particles and the force due to gravity. The force balance between F(ac) & F(mg) is what allows the ring to counteract the force of Earth’s gravity, and thus sit stationary with respect to the ground at a fixed altitude above it.
Going back to what I said earlier, Earth has two main problems when it comes to getting passengers and payloads into orbit: a thick atmosphere, and high gravity. The orbital ring structure can counteract the former explicitly and the latter implicitly. An orbital ring that sits at an altitude of over around 80 kilometers (about half the distance from Washington, D.C. to New York City) can clear the atmosphere and supply the structure that a mass driver would be built upon (thus ending the need for rockets). Indeed, an orbital ring would cut the need for rockets altogether in getting to Earth orbit. A working orbital ring around Earth would open immense possibilities.
Construction and Challenges
An orbital ring is totally within the realm of known physics. However, it is the engineering, as well as the necessary infrastructure and political cooperation, that limits the construction of one in the present day. Energy is currently a scarce resource; while there have been smaller-scale demonstrations of active support structures, anything even a tiny fraction of the scale of an orbital ring has yet to be built. This is mainly due to the massive amounts of energy needed to keep one standing. Indeed, before an orbital ring is ever constructed, smaller-scale demonstrators such as an actively supported tower or bridge would need to be built. With increasing investments being made into nuclear power, the possibility of active-support structures is visible on the horizon.
Beyond simply the engineering challenges, an orbital ring would have to be built using extraterrestrial (no, not alien) materials, such as from lunar mines or perhaps captured near-Earth asteroids. The construction of an orbital ring would require off-Earth infrastructure on a scale that most likely won’t exist for a century or more—however, the unseen benefit is that many of the technologies needed to build an orbital ring would be used in these infrastructure projects, which will supply valuable insight to the engineers who might one day construct the ring. Technologies such as on-orbit automated manufacturing, superconducting magnetic levitation, mass drivers (to deliver materials from the lunar surface to Earth orbit), and point defense (protection from debris) will all be developed, proven, and perfected before the orbital ring is ever built.
Outside of infrastructure, political concerns for such a structure would be serious and take a massive international effort to overcome. Currently the most plausible alignment for our first orbital ring is around Earth’s equator. If an orbital ring were to be built today it would pass over the land area of eleven sovereign countries. To successfully approve the construction of an orbital ring, various practical and safety concerns—as well as the special interests of these countries—would need to be considered and negotiated. While there would certainly be political opposition to such a project, the economic power that equatorial countries would gain in terms of becoming gatekeepers to access to space could perhaps be enough of an incentive to open them to the possibility.
In the end, while there are significant challenges to overcome, many estimates suggest that an orbital ring could be built around the equator for less than a trillion US dollars. One might wonder if such a project could spur international cooperation on a scale never before seen.
Orbital Ring Operations
To get an understanding of just how useful an orbital ring could be for high-volume ground-to-orbit transport, I’d like to walk through the voyage of a hypothetical traveler using an orbital ring to go from the surface of the Earth into low Earth orbit. Our traveler starts at the equator in Quito, Ecuador. After spending the night in a hotel, our traveler makes his way some 25 kilometers (about 15.53 mi) to San Antonio de Pichincha, where there is a ground station at the intersection of one of the hundreds of support cables attached to the ring and the surface of Earth. At the ground station, our traveler checks his bags and passes through security, after which he boards a sky tram.
The sky tram is a maglev train/elevator car hybrid that travels vertically up the side of the support cable. After taking his seat, our traveler feels a jolt as the sky tram begins accelerating upward. After about an hour of travel time, our traveler arrives at the orbital ring station “Alto Quito,” suspended some 300 kilometers (about 186.41 mi) above the actual city of Quito. At Alto Quito he and his luggage transfer over to a vehicle called an Orbital Shuttle.
The Orbital Shuttle is a large spacecraft with a very specialized purpose. The shuttle sits atop a mass driver carriage mounted on the exterior (space side) surface of the ring, and is accelerated by a magnetic track to a velocity of 7.8 kilometers per second. After being seated and strapped in, our traveler feels a much stronger jolt. As the shuttle begins accelerating at 1G (9.81 m/s^2) over its track, our passenger feels something strange beginning to happen—the downward-felt force of gravity begins to wane, until the only force left is the 1G horizontal acceleration toward our traveler’s back.
After only thirteen minutes of constant acceleration at 1G, our traveler and his shuttle detach from the mass driver’s carriage and begin moving slowly upward, now in weightlessness due to the shuttle’s orbital velocity. The shuttle uses monopropellant thrusters to circularize its orbit, some 50 km (about 62.14 mi) directly above the orbital ring. For the next three hours our traveler sits aboard the shuttle in orbit, all the while beginning rendezvous and docking maneuvers with an orbital transfer station. From the ring’s orbital transfer station (orbiting 50 km (about 31.07 mi) above the ring) or 350 km (about 217.48 mi) above the Earth, our traveler is now free to pick up his connecting flight to Aldrin Station on the Moon’s south pole.
In the end, our traveler paid no more out of pocket than for an intercontinental airline ticket to go from the surface of the Earth to an orbiting transfer station, all in some 4 and a half hours, and without a single rocket firing. All of this is made possible in terms of volume and cost by the orbital ring. When our traveler returns to Earth the process will reverse, since the shuttle is designed to reattach to a moving carriage using a harpoon. Below is a diagram of the traveler’s journey:
Conclusions and Implications
In accordance with the title of this blog, I think many of us aspire to a future where humans living on Earth are in the minority. Imagine billions of people living aboard O’Neill cylinders in cislunar space, on the Moon or Mars, and even in the asteroid belt and around the gas giant outer planets. To carry out this feat over the next few centuries, we are going to have to overcome the gravity shackling us to Earth.
Active support and the orbital ring that naturally follows supply a permanent bridge between the surface of our Earth and the orbital space above. This ten-lane interstate highway to space will supply the necessary capacity to move billions of tons of cargo and billions of people off Earth to create the interplanetary society, economy, and culture that may one day take our descendants to the planets—and perhaps, one day, to the nearby stars.
My thanks to Eamon for painting a picture of dirt-cheap orbital transport, and the glorious future it would entail. Part of me can’t imagine a fixed structure of that size, hovering hundreds of kilometers above the Earth, but the physics of it is hard to deny. It is possible that future generations will live in a world where skies near the equator are dominated by a tremendous arc of human-made metal, streaking from horizon to horizon. Yes—there may be marvels, yet.
Expect another post next Sunday—I’m planning a film review, on the classic 1959 sci-fi feature The Angry Red Planet. If surreal vistas and towering spider-bat monsters are your thing, you’re in for a treat.
Let's Get Off This Rock Already! 