Lunar exploration architectures

Recent (March 2019) announcements of new programs to land humans on the moon by 2024 have generated all kinds of responses. This blog is not going to deal with whether it’s a good idea, political parts, organizational issues, funding issues, and so on. Instead, I intend to explore variations in mission architecture that this new enthusiasm has exposed. In particular, there are many different proposed ways to “go back to the moon” and it is my intent to understand and explain why the “how” of lunar exploration is not obvious.

It is very easy to claim that, of the perhaps a dozen proposed architectures, each represents a pet idea devoid of reason (except for mine, of course) and people on Twitter are happy to argue all day long without really getting any closer to consensus. My working assumption here is that all proposed architectures do make physical sense when derived from their (sometimes obscure) starting points, and so this blog will be a good faith exercise in empathy, rather than a perpetuation of parochialism.

Design convergence and divergence as a sign of product sector maturity

When considering mature engineered products we often take design convergence for granted. For example, most modern SUVs, smart phones, and jet planes look basically the same. This wasn’t always the case. Whenever a new kind of thing is invented, it typically takes a few iterations for the customer and engineer to even understand what exactly it is that they want.

An instructive example is in the emerging electric car space. Starting from the same set of axioms:
– Batteries are very expensive
– Electric car drive trains are underdeveloped
– Customers will want good value

A typical electric car c. 2005 looked like this:

The founders of Tesla Motors realized that exactly the same set of axioms could derive a product which, if anything, delivered better value and looked like this:

Crucially, only one of these visions represented a product which could grow the market segment!

Space exploration is still an immature area in terms of product development, so there is a natural degree of divergence in architecture and vision. I don’t have a time machine so I can’t predict exactly which architecture or vision will win out, but I can at least demonstrate a process that gives rise to each vision.

Typical space program requirements

The overarching requirement of any large scale, government funded space exploration mission is “Program success on budget and on schedule.” Of course, any project that costs more than a billion dollars, in any industry, has a 50-90% chance of never being delivered. For more in this vein, check out Edward Merrow’s tome on the subject.

To deliver program success on budget and schedule, project managers develop a common sense list of subsidiary requirements, such as:
– Minimize cost
– Minimize weight
– Minimize fuel
– Minimize risk to astronauts
– Minimize programmatic risk (e.g. getting canceled)
– Emplace infrastructure for a future series of missions
– Avoid technological dead ends (e.g. expendable vs reusable rockets)
– Avoid cost of new rocket (to avoid sticker shock, since rocket development is considered to be expensive)
– Placate constituencies (to avoid sniping)

Some of these requirements are easier to measure. Others are effectively beyond the project manager’s control. Still others give very different answers depending on the order of priority, such as the electric car example above. In a sentence, this varied order and weighting of common sense requirements is what gives rise to such different architectures in an immature product space.

Understanding delta V

(original diagram)

This diagram shows the “distance” between different points in the Earth-Moon system using the metric of delta V. Delta V, or change in velocity, can be translated to fuel mass using the rocket equation. The task of any mission designer is to determine how to achieve the mission while minimizing delta V and thus fuel use.

The architecture zoo

Next, I’m going to attempt a non-exhaustive list of broad, conflicting visions and how we got here. I’m going to classify and rank them by how many people end up living on the Moon during steady state, and what they are doing while there.

Apollo architecture: fewer than 10 people doing short term exploration

This is a “flags and footsteps” style Lunar visit, like Apollo. A short exploration mission that swiftly advances then retreats back to Earth, without using or building any space or lunar infrastructure.

The initial Apollo plan called for the Nova rocket, about twice the size of the Saturn V. This rocket size was needed to support a “lunar direct” architecture, wherein the astronauts would fly directly to the lunar surface and then back to Earth.

This differs from the historical architecture, which left the command and service module in lunar orbit. The command and service module had substantial weight due to the heat shield and fuel needed to get from lunar orbit back to Earth, and there was no reason to fly all that weight to the lunar surface only to fly it back again. This architecture change traded complexity (lunar orbit rendezvous) for reduced mass and enabled the launcher to be halved in size. It is thought that this reduced cost just enough to avoid early cancellation.

Coincidentally, this blog was posted on the centennial of the birth of John Houbolt, the champion of the lunar rendezvous architecture. His memorable question “Do we want to go to the moon or not?” is an apt reminder of the focus a lunar program requires.

Rocket technology has advanced since the 1960s, but if the mission calls for the temporary delivery and return of a small payload (a few people and some rocks) to the moon, then the Apollo architecture is still the most fuel-economical way to do it.

The Apollo architecture doesn’t extend readily to delivering much larger payloads to or from the Moon, and it involves disposal of nearly every part of the rocket along the way. This leaves room for other architectures.

Space Shuttle architecture: fewer than 10 people living intermittently in low Earth orbit

For historical completeness, it is worth recognizing that the excitement over reusable rockets is not new. Even in the 1960s, space exploration advocates realized that the cost of expendable rockets like the Saturn V would never permit wide access to space. A minority voice throughout Apollo, in the 1970s, the spaceplane advocates got their way and led development of the Space Shuttle, which first flew in 1981. Although the orbiter was reusable, the hoped-for cost reduction turned out to be impossible to deliver and the Shuttle turned out to be more expensive than expendable rockets of equivalent capacity.

The original vision of the shuttle was dozens of flights per year with a per pound cost of around $100. Actual costs were more like 1000x that value. The original vision called for the rapid and relatively inexpensive deployment of the ISS precursor Space Station Freedom, followed by a steady expansion of orbital infrastructure and, yes, further lunar missions with reusable tugs, landers, and orbital stations. Sound familiar?

Yes, NASA’s budget wasn’t overly generous for much of the time, but the Shuttle also turned out to be much more difficult to engineer than was originally assumed. It also remained a dangerous and inextensible mechanism for human space flight, eventually costing 14 lives before retiring with no replacement ready.

I include the Shuttle mostly an example of how even rigorous application of sensible requirements can lead a development program astray.

Lunar Orbital Platform-Gateway: fewer than 10 people traveling to near the Moon

The Lunar Orbital Platform-Gateway (LOP-G) is the current (2019) NASA reference design mission, and entails the construction of a modular (ISS-style) space station in a high lunar orbit. It has previously been called the “Deep Space Gateway”. In this mission, astronauts launched on the SLS and Orion would spend up to 30 days per year at this station. Over time, the LOP-G would evolve to support a crewed lander to shuttle astronauts to and from the surface of the Moon. This lander would be much like the Apollo lander, except that it would require three stages instead of two to enable the transition from the LOP-G’s high orbit to the lunar surface, particularly if the astronauts are landing at the pole.

The LOP-G has weathered a lot of criticism, I think mostly because it’s actually receiving funding and universal attention. The plan is criticized for being much more expensive and complex than the Apollo program, with similar if slightly worse lunar surface capability. The LOP-G orbit splits the difference between the capabilities of the SLS and a perceived necessity to avoid Earth-orbit rendezvous, instead requiring numerous dockings in lunar orbit. Further, the design, construction and operation of modular space stations is now known, after decades of operating the ISS, to be extremely expensive. If the goal is to build a lunar base, why build another space station at all?

So why is LOP-G the official plan? Without claiming to have special insight into the decision making process, I can suggest a plausible set of requirements and decisions that lead to this architecture.

First, the principle contractors Boeing and Lockheed work closely with NASA and are very aware of the political realities surrounding human space exploration. In particular, a new mission should use mostly flight heritage hardware and avoid any open ended, costly development programs. If NASA had the budget to develop a new lunar lander, that would be a different matter, but NASA’s main development budget is committed to the SLS and Orion, as it has been since President Bush started the Constellation Program in 2005.

It is an unspoken but universally recognized truth that SLS development, despite agonizingly slow progress, is politically invulnerable due to the control of Senator Shelby over the NASA budget. A post-ISS human space exploration program has to get closer to the moon, and it has to do it with the scraps left over from SLS. Even better, make the SLS an essential part of the program.

Once these constraints are accepted, it isn’t hard to project forward to LOP-G, a slightly smaller ISS 2.0. It features a collection of stakeholders focused on NASA, Europe, and commercial partners, and a de-emphasis on Russian involvement.

ARM: fewer than 10 people traveling to a captured asteroid near the Moon

The Asteroid Redirect Mission, or ARM, was proposed in 2013 and canceled in 2017. Like LOP-G, it was an attempt to answer the question: What can we do with and around the programmatic debacle of the SLS? It consisted of two principle parts intended to galvanize a wide set of stakeholders, though I think in the end the coalition wasn’t able to resolve their differences. The first part was the development of a powerful solar electric propelled asteroid rendezvous spacecraft that would grab a small near-Earth asteroid and bring it back to an orbit near the Moon, similar to the LOP-G orbit. This process would take at least a few years, during which time the SLS would prepare to launch two astronauts on Orion to the captured asteroid and take some samples.

The key constituencies that such a mission could satisfy included:
– Planetary scientists who want to study asteroids
– Advanced propulsion proponents and developers
– Planetary defense developers
– Asteroid mining developers
– Crewed spaceflight/SLS proponents

I personally liked this mission because both high power solar electric propulsion and asteroid redirection are very cool technologies to have. Asteroid redirection is one technology the dinosaurs would have really benefited from. For all our computers and opposable thumbs, in 2019 humans are just as vulnerable as the dinosaurs.

Asteroid mining is an article of faith for many human spaceflight proponents, who point out that asteroids are the cheapest (in terms of fuel) sources of matter anywhere in space.

Ultimately the mission downselected towards boulder capture rather than entire asteroid redirection, and the crewed aspect of the mission gravitated towards an early version of LOP-G.

I include this architecture because there is still a substantial constituency who believes that human exploration of near-Earth asteroids is both easier and cheaper than building a lunar base, and advocates for redirection of multiple asteroids into distant orbits of the Earth.

Moon Direct: between 10 and 100 people traveling to and from the Earth

The Moon Direct mission is a recent proposal by Dr Robert Zubrin, a prominent space exploration advocate. Zubrin is best known for advocating for Mars exploration instead of the Moon, but points out in his article that even the Moon is better than the LOP-G, which he calls the Lunar Tollbooth.

Moon Direct calls for aggressive use of SpaceX’s Falcon 9 and Falcon Heavy rockets, which have, by far, the lowest costs to low Earth orbit (LEO) of any currently operational rocket. In addition, Moon Direct needs a new reusable lunar lander that can fly from LEO to the lunar surface (delta V of 6.1km/s), or back, requiring refueling at either end. In LEO, the lander would be refueled from Earth. On the Moon, the lander would be refueled by a robotic propellant factory built to process volatiles near the lunar poles.

Moon Direct also uses the lunar lander as an exploration hopper to fly around on the Moon, or as a shuttle to low lunar orbit. This lunar lander could be based, perhaps, on the Centaur upper stage.

Although Moon Direct has several strong advantages over the Lunar Port architecture discussed below, it does share one critical weakness, namely the requirement to design, build, and deploy a robotically operated propellant mine on the Lunar surface before humans can visit.

Nevertheless, if one of the axioms is that robotic lunar mining and fuel production is straightforward, or already exists, then the Moon Direct architecture is both much cheaper, faster, and more valuable than the default LOP-G plan.

SpaceX vision: between 10 and 1000 people living on the Moon

SpaceX is in the business of designing and building new rockets. So rather than answering the question “What can we do with existing rockets and not too much fuss?”, SpaceX’s lunar architecture answers the question “If we were serious about building a big, awesome Lunar base, what would the rocket have to look like?”

The axioms here are reasonably obvious:
– Need at least 10s of tons per flight cargo capacity
– Needs to be able to operate with or without lunar fuel availability
– Needs to be fully and rapidly reusable

The result is the SpaceX Starship. This rocket is intended to enable human exploration of the Moon, Mars, and other destinations. Refueling in LEO, it can fly a substantial payload to the Moon and back to the surface of the Earth with no further refueling. If Lunar fuel is available, or a propellant depot in a high energy Earth orbit, it could transport even more cargo.

This graph illustrates the cargo capacity of the SpaceX Starship assuming 1200T of propellant capacity, for flights from the Earth to the Moon and back, in a single stage, with no refueling. It is entirely next level.

The nice thing about the SpaceX architecture is that the program can be focused on building what humans need to live on the Moon – a base with plenty of resupply capability, some cool science instruments, and perhaps a gradual build out of mining capability. This is in contrast to other Lunar resource-focused architectures that put high-efficiency high-reliability robotic mining, which is currently fictional technology, on the critical path.

Lunar Port/Spudis architecture: Lots of robots and a few humans living on or near the Moon

The Lunar Port architecture is, in many ways, a less minimal version of Moon Direct. Instead of producing enough fuel to fly the lunar shuttle to LEO, the Lunar Port, originally advocated by Paul Spudis, instead casts the lunar base as the new interplanetary gas station. Lunar-produced fuel is flown to LLO and thence to a series of space station depots throughout cis-Lunar space. Even LEO refueling would be conducted with Lunar-derived fuels. The propellant base would be built by robots on the Lunar south pole over 15-25 years and eventually scale production beyond a rate of 10,000 T/year.

Because the Lunar Port architecture is focused more on developing a robust cis-Lunar economy in resource trade focused on Lunar production, as a platform it’s much more flexible and inclusive than mission-focused exploration architectures.

The primary weakness of Lunar Port is that its proponents seek not to work around the SLS resource free-for-all, but to subsume it. That is, given a few decades of blank checks, it is possible that NASA could do with robots on the Moon what it took millions of humans hundreds of years to do on Earth since the industrial revolution.

In my opinion, it is probably impossible to sell Lunar-derived fuel more cheaply than Earth-sourced fuel in LEO, and probably everywhere else in cis-Lunar space with the possible exception of the Lunar surface itself. To be fair, proponents of Lunar Port are yet to produce a remotely realistic cost estimate. The most recent major publication in this area, while impressively detailed on a technical level, doesn’t provide bench marked cost estimates or side-by-side comparisons. It does suggest that the price to beat for Lunar Port is $4b, or roughly the cost of 1.5 Mars rovers, which seems like it must be a joke.

Nevertheless, Lunar Port is the correct architecture given these assumptions:
– Sustainable exploration requires economic development and profit in space
– Lunar resources have a positive net present value
– There exists a market for rocket fuel in cis-Lunar space
– Robotic industry is on the cusp of practicality

O’Neill/Blue Origin architecture: Millions of people, most industry in cis-Lunar space

If we take the Lunar Port architecture to its asymptotic extreme then the cost of transport within cis-Lunar space becomes trivial and we can build basically all of Earth’s industry in space. Originally expounded in the 1970s by Gerard O’Neill, this architecture envisions millions of people living and working in gigantic rotating space stations between LEO and the moon. These stations would be built from, and process, a constant stream of raw materials launched from the Moon or captured asteroids.

In addition to low margin commodities like fuel or raw materials, orbital factories would also produce high margin specialty goods for use elsewhere in space and on Earth.

In O’Neill’s original work, the fundamental value proposition revolved around space solar power, beamed to the Earth to alleviate Malthusian shortages that seemed inescapable in the 1970s. The consensus view today is that electricity is and will almost certainly always be far cheaper to produce on Earth than in space, so a robust space economy is still wanting for a foundational product it can trade for Earth-produced necessities.

The set of requirements and assumptions that give rise to the inevitability of this architecture require a bit of extrapolation, but look like:
– The Earth is finite in size and resources, while space is really big
– Human population and resource intensity continues to grow, arguably exponentially
– The Earth will reach a point where it cannot provide a good quality of life to all humans
– Space resource exploitation offers a pressure relief valve and a mechanism for continued growth
– Given that human destiny is comprehensive space industrialization, what are the first few steps?
– We definitely need much cheaper transport to orbit, hence Blue Origin and SpaceX
– We will almost certainly need a Lunar base to mine and launch raw materials

Wrap up

It is my hope that this blog has given the reader some insight into the competing sets of assumptions and visions that the recent Lunar exploration announcement brought back to the surface.

Of the various architectures, it is my view that the best “bang for buck” concept looks more like the SpaceX vision, since it focuses resources on the bottom line outcome – enabling lots of humans to live on the Moon. Not farming robots, not assembling space stations of questionable utility, not placating constituencies in key congressional districts, but in building actual infrastructure where it is needed. Is it the last word on space exploration? Not by any stretch of the imagination.

Let us go forward into the future in a spirit of constructive dialogue, technical competency and results-focused optimism.