New to the series? You might want to start with Part 1!

ULA’s ACES space tug concept art

Welcome back to the Cislunar Economy series! It’s been almost a month, but Part 6 is finally here, and we will talk about the coolest thing there is: spaceships! That is, the transportation network of the Cislunar Economy. An effective transportation network is essential, since it will take care of most of the distribution activities of any products produced within the economy. Having covered production activities before, we will jump straight into the transportation network.

As with any network, the way to define it is to look for nodes and links between them. We can do this by analyzing the flow of materials and finished products in cislunar space. Lucky for everyone reading this, I won’t have to go through all the analysis because I spoiled the result in Part 3 with the Cislunar Verticals. In this case, the Cislunar stations will form the nodes of the network, while lunar landers and in-space transportation will form the links between the nodes. Let’s see how this works.

Placing the nodes

Just as industrial activities on Earth tend to concentrate in industrial parks, they will concentrate at different stations in cislunar space, either crewed or automated. On Earth, this simplifies logistics, reduces costs, and simplifies operations in general for everyone involved. The same logic applies in space, with the added point that at the very early stages of development, stations will likely be too expensive for any single player to have one of their own, and they’ll be forced to share infrastructure in some way. These stations will be either orbital or surface stations on the Moon. However, I’ll dedicate a complete post to the Lunar base, and we’ll focus here on orbital stations.

Many orbits have been proposed for stations in cislunar space. In general, it’s a trade-off between the difficulty of getting to the orbit, the difficulty of staying there, and the distance to the Moon and low Earth orbit (note: when I say “difficulty” and “distance” I’m referring to the propellant requirements and travelling time). There are also other considerations, such as the frequency of launch windows to both Earth and Moon, and the difficulty of leaving or reaching that orbit from interplanetary space (this last one is especially interesting for asteroid missions).

However, that’s not all. Once we bring economy into play, we also need to look at which markets we want to serve, and how hard it is for us to get to those markets. All these requirements reduce the incredible spectrum of possibilities to a few key interesting positions for a station: the Lagrange points 1 and 2, the geostationary belt (GEO), and low Earth orbit (LEO).

The Lagrange points, or libration points, are positions of gravitational stability in cislunar space, meaning that you can place an object around the point and it will stay there (relatively stable, you still need fuel to maintain the orbit). These zones remain fixed relative to the Earth and the Moon , making them a great strategic position for an orbital station: you have a constant launch window for both the Earth and the Moon, you need relatively low amounts of fuel to stay there, and thanks to the magic of orbital mechanics, they are also easy to reach. There are five of these points in every two-body system, two stable ones (L4 and L5) and three unstable ones (L1, L2, and L3). In our case, the points of interest are L1 and L2 : they require the least fuel to get to from Earth or Moon, and the least to leave, and the difference more than offsets the extra fuel needed to stay around them. They also provide good access to the other orbits of interest and interplanetary space, and low transit times, which is key for human trips.

, making them a great strategic position for an orbital station: you have a constant launch window for both the Earth and the Moon, you need relatively low amounts of fuel to stay there, and thanks to the magic of orbital mechanics, they are also easy to reach. There are five of these points in every two-body system, two stable ones (L4 and L5) and three unstable ones (L1, L2, and L3). In our case, the points of interest are : they require the to get to from Earth or Moon, and the least to leave, and the difference more than offsets the extra fuel needed to stay around them. They also provide good access to the other orbits of interest and interplanetary space, and low transit times, which is key for human trips. The other two orbits of interest, GEO and LEO, are of interest due to their market potential. Current satellite activity is limited to LEO (easy to reach) and GEO (good for broadcasting and monitoring). If initial markets, as we discussed before, are propellant and satellite retrofit, we need to go where the satellites are.

The five Earth-Moon Lagrange points.

Stations in these key positions in cislunar space would act as staging points between launchers (LEO), lunar landers (L1, L2), space tugs (all), and interplanetary spacecraft (L1, L2), and act as hubs for the movement of goods such as propellants and human consumables. The stations would also concentrate other activities such as orbital hotels, microgravity research, and repair stations for satellites, as well as manufacturing activities.

However, a single station is unlikely to gather ALL the activities in our Cislunar Economy. There will be multiple stations exchanging goods, potentially competing with each other, but likely serving different markets altogether. They will also need to be connected to Earth and the Moon, and receive materials from interplanetary space. This is where the spaceships come into play.

Connecting the nodes

We’ve mentioned impedance matching before (in Part 3) as the idea of making the spacecraft fit for each operation. That is, avoid having a ship that does everything, and instead have a different ship for the launch, the space trek, and the landing. This provides a number of benefits for the spacecraft (simplified design and operations, reduced cost) and is the reason why it makes sense to divide the links in the network in two parts: in-space transportation (spaceships!), and lunar landers.

The In-space transportation vertical covers space tugs and propellant depots. These are grouped together for a number of reasons. Depots and tugs require a bunch of common technologies: long term storage of cryogenics, refueling capabilities, and a long-lifetime design. Some space tugs could even perform as temporary fuel depots, such as ULA’s ACES, and, with a few modifications, a space tug could be made to store propellant for prolonged periods of time. Once you develop a space tug, you’re just a few steps away from a depot, so you may as well deploy your own automated depot network for your tugging services, or partner with the depot developer to save development costs.

Tugs with chemical propulsion will require on-orbit refueling and will depend on propellant depots, but electric propulsion tugs could be supplied with propellant from Earth. These might be used for missions that can afford the long transfer times inherent to electric propulsion, and could be spun off for space debris removal or spacecraft servicing.

Space tugs can be used to deliver spacecraft to their operational orbit, allowing higher launch masses, since the spacecraft would only have to be launched to LEO. This would include satellites, station components, crew capsules, and other payloads, creating a market with a wide customer base. Space tugs would connect any orbital station with the rest of cislunar space, including other stations, but not the Moon. The Moon deserves some special treatment.

Lunar landers are special enough to justify defining the market as a different niche. Technology wise, they have many similarities with space tugs (ACES can readily be adapted to become the XEUS lunar lander), but that doesn’t make them part of the same market niche. In fact, you could consider lunar transportation a niche within space transportation that only takes care of the final trek to the lunar surface, starting from a station somewhere in L1 or L2. This changes the requirements from a generic space tug: high thrust to allow landing, ability to withstand lunar dust, potentially different thermal and power systems for long-duration surface stays, a different navigation system, and systems to operate in gravity (especially important for any life support systems). Following the principle of impedance matching, you’ll be better off with a completely new ship designed for the job, instead of jury-rigging a space tug to land on the Moon.

Initially, landers would address markets such as surface payload delivery for scientific missions and lunar base support. Once reusable landers come in play, we can start exporting resources from the surface. Crewed landers may be reusable from the beginning, since the crew will need to take off from the surface anyway. However, initial landers for short sorties to the surface may only be partially reusable, and leave behind a first stage in full Apollo style.

With lunar landers and space tugs, we can reach virtually any place in cislunar space. Coupled with stations acting as transportation nodes, we have a complete distribution network ready to fuel a dynamic Cislunar Economy. How far are we from seeing all this? Turns out, a bunch of people are already working on it.

Creating the network

I said before that all the market verticals of the Cislunar Economy have companies working on them right now. Transportation is no exception.

In space tugs, we already mentioned ULA’s ACES, which is potentially the most exciting of all the projects. We also have people working on ships that could act as electric tugs, such as Skycorp, Orbital ATK, and Effective Space. Work on depots is also moving forward, with Altius working on cryogenic propellant feed systems and other robotics needed, and ULA working on short-duration depots for distributed launch.

Lunar landers present even more movement, thanks to the Google Lunar XPrize. All the competitors are working on technologies for lunar landers or lunar surface operations. ULA is working on XEUS, Astrobotic is moving forward with Griffin, and companies like ispace are developing tech for surface operations.

Finally, stations don’t have as many competitors, but the technology is more developed. Bigelow Aerospace is already testing their BEAM module at ISS, and will be sending their first BA-330 by 2020. On the other hand, Axiom Space plans to deploy a traditional module by 2020 to ISS, and setup their own space station in LEO.

As you can see, things are moving forward on all fronts, pretty much at a similar pace. This is key to the appearance of the Cislunar Economy, as I mentioned in Part 1: good timing is essential for the different pieces of the puzzle to come online and create a value chain that can sustain the initial stages of the economy.

Once we’re past that initial stage, things get easier. Risk perception goes down, investors relax and put more money in, and the initial capabilities allow for other businesses to flourish. In the In Part 7 we’ll talk about the business that will profit the most from initial capabilities: the lunar base.