This post will attempt to formulate an infrastructure for transportation in the Earth-Moon system (called “cislunar”), which is based on launcher systems. What is a launcher system? It’s when you forgo rockets to just accelerate a payload along a track, and then… let it drift.
I offer this as a parallel to a blog post by a well-informed and creative blogger. In his blog post, he offers a cislunar transportation system based on a tether system. Your reaction is probably “like a space elevator, right?” And yes, an Earth space elevator is in the same genre. However, the author (going by “Hop”) has been involved in the subject for a long time and knows enough to describe a system that actually could happen based on real engineering understanding (vastly unlike an Earth-based space elevator). It is that quality that I would like to emulate here.
With full intellectual respect for that work, I want to describe a competing architecture. Doing so, I hope we can have a “most reasonable” picture based in both tethers and launchers. The question for the reader is then which technology is our future — tethers or launchers? Or both? Full disclosure: the two competing architectures fulfill slightly different roles, so they are not necessarily mutually exclusive, but since we live in a world a limited resources, I’m posing the question as one-or-the-other.
The objective of this design is to get commodity lunar resources into Low Earth Orbit (LEO). You can say that its indirect objective is to get spacecraft out of Earth’s gravity well to beyond-earth-orbit destinations because those resources will be used to make propellant, among other things. Compare to Hop’s design, which is to get things up and down Earth’s gravity well. A technical nuance of his design is that the up-mass and down-mass come in equal proportions. My design is asymmetric, only delivering lunar mass to LEO, technically agnostic to what happens after that.
Design Overview
This components are
- a Low Earth Orbit (LEO) station
- + plus a swarm of small robotic aerobraking ferries
- an Earth-Moon Lagrange point 1 EML-1 station
- and Low Lunar Orbit (LLO) station
- a Lunar Surface station (a moon base)
LLO, EML-1, and lunar surface stations have launcher systems. The 3 orbital stations are equipped with catcher systems, all of which intercept the payloads at much lower speeds than what the launchers throw them at.
The story of a payload goes as follows…
- A small mass dug up from the moon, possibly on the order of 1 kg or 10 kg is launched from a horizontal track/gun at the moon base at high g-force, reaching 1.7 km/s, timed such that it intentionally collides with the LLO station, and is caught.
- The captured mass is then launched by the LLO station either to collide with the EML-1 station, or into a graveyard trajectory to fall back to the moon as a means of station-keeping, at around 0.65 km/s. Both actions can be completed once per orbit, about once every 2 hours.
- The EML-1 station then engages in a special abbreviated assembly process where a minimalistic robotic spacecraft (I call an “aeroshell” drone) is loaded up with the payload. The spacecraft has no propellant. The EML-1 station launches the drone at about 0.65 km/s into an Earth-grazing trajectory.
- The drone loses some velocity, and then makes a large number of passes, slowing shedding velocity each time. The long time frame and its control surfaces allow it to make adjustments so that it literally hits the LEO station on-schedule, which would happen at around 128 m/s of relative velocity or possibly a good deal less.
- Cannibalizing a small amount propellant produced from lunar-ice delivered in the payload, a spacecraft launches from LEO to EML-1 with hundreds of the robotic aeroshell spacecrafts, to reload with more payload and “sail” back down to LEO.
This version is the “bootstrapped” version. Larger versions are possible, which is discussed in a later section.
Commentary on Individual Steps and Philosophy
LEO is The Big First Step into space, but even after getting there, we are in an incredibly material-poor location. We need to develop a factory-like supply chain of materials to LEO in order to build heavy industries in orbit, and as a stepping stone to become a multi-planetary species.
The Launchers
Your obvious question for the above steps might be “why not just launch from the lunar surface to EML-1?” Or, for that matter, why not straight from the lunar surface to an Earth-grazing trajectory? To state the obvious, energy requirements are non-linear, meaning that 2 smaller jumps require 1/2 the total energy as 1 large jump, or 1/4th the energy per launch. This relates to the scaling of capacitor banks of a railgun, for instance. Much of the reduction in pure energy consumption is made up by the additional payloads used for the LLO station-keeping.
The insight I was trying to apply here is that, from an economics perspective, the costs scale dramatically more steeply than quadratic with respect to muzzle velocity. While railguns are getting more attention for the use in battleships at even higher speeds than called for here, the degradation problems of the tracks still haven’t been solved for extreme speeds. As space development goes into the commercial realm, we can’t count on the application driving technological development like it did in the Apollo era. Architectures need to be firmly rooted in what’s possible now.
Nothing convinced me of the challenges of catching better than the patent for crashportation for station-keeping in LEO. Entertaining read, honestly.
The Catchers
The 2nd insight I was trying to apply was that real engineering constraints, catching payloads is dramatically more difficult than launching them at the same speed. Launching involves a track or a barrel — a deterministic trajectory. The trajectories in this design have a fairly special set of traits which I believe is relatively special to the EML-1 corridor.
- arrival velocity is very low compared with the launch velocities
- launch windows are very frequent
- muzzle velocities just barely teeter around the boundary of slow hypervelocities, and are probably right around the limits of material contact-survivability, so that the lunar station might keep its railgun or light-gas gun in-tact for a decade of service
- no atmosphere to complicate the launches
Catching Course Corrections
Since this is supposed to be a low-cost option, the payloads would be best to be dumb rocks. There’s no good way to equip them with tiny rocket engines unless you manufactured them on the moon, and that’s not happening for a long time. Thus, we need to take seriously the idea of leaving them on a fully ballistic trajectory. There will still be some error, so that seems to leave us no choice but to maneuver the catcher (within some limited window) in order to adjust. Ironically, I think this might be the ideal application of a tidally-stabilized tether (like in Hop’s design). To get good multi-axis maneuvering, I imagine something almost like a spider web superstructure network tethered to a 3-D tensegrity structure.
Lunar Station
A chance still remains of missing the station. If the projectile misses the LLO station, it will come back to approximately where it was launched. Thus, the lunar station will need to be located in front of a mountain in order to avoid Newton’s cannonball.
The challenges for this station are the high muzzle velocity for the gun, and the punishing mass-budget for equipment delivered there. All considered, this would be the most expensive station by far. However, use of lunar materials could possibly mitigate some cost. If people are mining lunar-ice, then it would even be possible to make it a light-gas gun instead of a railgun, possibly making it even more economic, although good recovery of the gases in the vacuum would be complicated.
LLO Station
By a good margin, it is cheaper to deliver mass to lunar orbit to the lunar surface. Combined with the low impact speed at the point of catching, and a launching muzzle velocity that is well within the range of conventional military munitions, this doesn’t look that difficult to manage. Getting good aim to the EML-1 station might be the greatest challenge.
EML-1 Station
A critical weakness of this design could be the large distance that must be traveled from the LLO station to the EML-1 station, while still not relying on propellant.
Also, the question must be asked how position will be maintained in this unstable point while launching and catching. These actions will impart momentum to the station to the side of the EML-1 saddle point. Thankfully, this is not along the unstable axis. It in effect, pushes it “up the hill”, but if the impulses are relatively constant, it could very well stay there.
As a side musing, the tether network used to move the station around to catch payload might also double as a means of assisting station-keeping of this station. It’s not obvious that the scale is sufficient for this, and it might need substantial counterweight mass… but the after the system is bootstrapped, these notions could rapidly become viable.
LEO Station
LEO is the destination of the payload, and it is also the most industrially intensive of the stations, by design. Inherently, it must be doing some processing of water-ice deliveries into bi-propellant rocket fuel, and it must also do refurbishment and packaging of the drones.
The Aeroshell Drones
The robotic spacecraft is an aerobraking shield with communications and control surfaces. I believe that the control surfaces are needed beyond just “fan in” or “fan out” because both perigee and apogee must be considered as part of the control problem, and forgoing propellant means that these both need to be dealt with during the time it is passing through atmosphere. Lift is entirely possible to obtain at hypersonic speeds, and lift assists for gravitational assists is a completely serious idea. Here is a brief sketch of some of the principles. An arrow-like design would do well to maintain stability, somewhat like lawn darts.
It is also nice that 100% reliability is not needed for this step (only product is lost, not people), and that they can be the size of cube-sats. Some things, like attitude control and heat management (particularly if carrying ice) are still going to be thorny engineering issues. But maybe not major ones.
Scaling Considerations
In order for an design to be a good idea, I argue that it must 1) be small enough to launch all pieces on soon-to-be-available heavy lift vehicles. 2) have a high payload-to-mass ratio, such that it easily pays for itself within a decade. And 3) can scale up beyond its original efficiency and purpose.
The greatest challenge to meeting these requirements is the lifting power necessary to get the original gun to the lunar surface station. Take, for an example, this random battleship gun as a reference point. It weighs 121 tons, has 3 guns, a muzzle velocity of 0.82 km/s, and fires shells weighing around 1 ton. Those are not bad numbers to start out with at all. We need to scale down to 1 gun, increase the velocity to 1.7 km/s, decrease the weight to something more like 20 tons, and do this by accepting a payload reduction of perhaps 100x. This isn’t unreasonable, I’m actually being rather conservative.
Let’s look at the payoff time period. The 2-hour orbit means that our hard limit of launches per year is 12 x 365 = 4,380 firings per year. That gives a fairly comfortable multiplier of the system mass, such that a 10 kg payload could yield up to 43 tons of mass per year, or 430 tons in a decade.
That number isn’t tremendously convincing, because to make this be really really lucrative, you’ll have to scale up the payload masses. That is perhaps the single most important reason to keep the muzzle velocities modest. It’s not unreasonable to imagine increasing that 10 kg payload to 100 kg after the system has proven its ability to operate. The catchers aren’t really a limiting issue because the relative velocities are quite manageable. Then, scaling up to 1,000 kg is only bringing it in-line with masses of shells typically used in military applications today, although perhaps not in hypervelocity applications, but the moon is still a fairly easy place to do this due to the vacuum.
Beyond a 1,000 kg design, the next objective of interest would be sending delicate payloads, like spacecrafts. This would be vastly more costly than anything else discussed in this post. The track length has to extend to many 10s of kilometers if you want to include humans as part of the payload (like this). The operation would absolutely require moon-based manufacturing and industry, and it would be orders of magnitude greater scale than the guns launching “dumb” payload of rock or ice. Until we have cities on the moon, it’s best to keep launch-assist systems limited to commodity items.
Tethers Versus Launchers
Let’s compare to the Trans Cislunar Railroad with tethers.
The launcher system has a faster schedule, but the shortcoming is that it can only send “dumb” payload at extreme g-forces. The tethers are floating in much longer-period orbits and don’t synchronize for a transfer every orbit, but they can move delicate payloads that have higher value than the stream of commodities in the launcher system. They also need counterweights, and I remain skeptical of how attitude management will happen.
I the biggest problem with the tether railroad as-is is that there’s no obvious down-mass. Assuming that the spaceships use propellant to some degree, then even a regular flow of missions leaving and returning to Earth’s gravity well, the up-mass will vastly overwhelm the down-mass. Both the moon and NEAs are targets for propellant mining, and the asteroids do have potential, but they require major propellant investment for every capture and the payback per investment has a steep drop-off.
Tethers also must span continent-scale distances and have the climbers move at incredibly fast speed if the throughput is to be anything worth a darn. I think Hop credited a speed of something like 500 km/hr. While launchers would not require continent-scale constructions, they would require continent-scale accuracies, as if launching a shell from New York City and hitting a football field in LA.
While I recognize that those accuracies are a weak point, there’s no industrial minimums to what you can get out of accuracy. As an information technology, it’s something that has nearly unlimited room for improvement and it’s suicide to bet against such technologies in business.
Mixing and Matching
To wrap up, we should consider the “both” argument. That is, what are the merits of a hybrid space transportation system using a combination of tethers and launchers?
The obvious combination is to use the launcher network to lift material from the lunar surface to EML-1, and then ditch the aerobraking drones. Instead, the payload will be launched at a substantially lower speed, which puts it in an elliptical high-Earth-orbit that can match trajectories with the tip of the “Super GEO Tether”. Then, the lunar materials becomes the down-mass which maintains the orbit of the tethers.
If you tried switching them out, how would the tethers do for lifting mass of the moon? And how would launchers do for lifting mass out of Earth’s gravity well? For several reasons, I don’t think the strengths match. Tethers are manufactured and very mass-sensitive. Launchers can be scaled down well for commodity payloads, which fits the moon situation very well. I was a fan of the Liftport lunar architecture, but the distances, climber speed, and payload all pretty much put any hope of break-even out of the picture. For launchers in Earth’s gravity well, the speeds are high, and I don’t see the same kind of tricks working to get very low catching speeds like what you can in the system described in this post.
Calcs and Background
When I first started the calculations, I did the EML-1 catching velocity wrong, and asked on Space Stack Exchange about it. After that interaction, I was pretty sure about the basic order-of-magnitude of the number.
If you want to pick about the numerical details, feel free to dive into my poorly organized Google sheet with the numbers.
