Principles of Mars urban planning (first pass)

Autarky Complete and indefinite self sufficiency, or breakout capability to reach such with trivial effort.

Regular readers will know that I occasionally discuss aspects of Mars settlement. This blog is inspired by some slides presented by SpaceX at IAC2017, which showed a SimCity base growing on Mars. Since Mars cities won't look like this, what will they look like?

The first city on Mars is oriented toward rapidly developing autarky, to minimize exposure to the period of time when the base is dependent on shipments from Earth. This differs a lot from a mostly static Antarctic station outpost, and thus determines a lot about how the city must be planned. No one knows for sure how many people are needed for autarky, but is likely at least a million and will thus require decades of blistering growth. The primary role of fixed infrastructure on Mars then, besides keeping death out, is enabling growth.

In Estimating Mars Settlement Rates, I attempt to estimate growth rates using ship construction and utilization estimates, combined with a population/self sufficiency relation. The Earth Mars launch window occurs every 2.2 years, and initial population growth targets are a factor of 4 per window, later dropping to a factor of 2 depending on ship production, capacity, and reuse.

No city in history has needed or managed to sustain growth this fast. On Mars, the primary task is building more city for impending arrivals, and the primary constraint is labor availability. To maximize production efficiency, construction will need to use mechanization, automation, and wherever possible, a shirtsleeves environment.

All this is fairly obvious. Can we now draw a map? Not really. I don't know what a self sustaining Mars city looks like and I probably will not live long enough to find out. Indeed, attempting to learn from experience that doesn't yet exist is a pointless endeavor. But I will wave my hands a bit about the first decade of growth.

In addition to enabling its own maximal growth, the Mars city will perform every other kind of function from life support, transport, recycling, and entertainment to privacy, education, mining, manufacturing, communication, and emergency management. Some of these functions will be distributed, others more centralized. To maximize the utility of limited living space, for instance, compact apartment geometries can be imported from Earth, while landing and launch operations, and other especially hazardous activities, will have to be separated from more vulnerable or less defensible areas. Practically speaking, all functions span a continuum from local to centralized. Somewhere in the middle of this continuum is a point of mandatory separation, and it is here around which individual pressure vessels, habs, vaults, arcades, tunnels, small domes, and vehicles will be divided from each other.

All of the more local functions (health, education, libraries, sport, food, recreation, spirituality, music, common space, food distribution, non-transformative recycling, life support, temperature control, atmospheric processing, grid stabilization, communications, data storage, residential, non-hazardous industrial live-work spaces, etc) are ideally collocated. Since these all take place in a climate controlled pressure vessel, each pod is self-contained and resilient enough to withstand substantial extrinsic challenges, while nominally meshing and sharing capacity with adjacent systems. Vacuum ops are required only for initial construction and exterior maintenance. Everything else is done in shirtsleeves at minimal marginal labor cost.

Opinions vary on ideal structure design and material, and methods will no doubt continue to evolve drastically during deployment. My personal preference is for hangar-like structures. A cylindrical roof spreads pressure, requires no internal support, can be shielded with dirt, and unlike spherical domes, has simple curvature and decouples ideal volume from geotechnical concerns in the foundation. With few or no windows, the interior could be somewhere between a modern submarine or a Vegas casino - both structures quite comfortable despite uninhabitable exterior environments. I envision structures ranging in size from Quonset huts to Hangar One at Moffett field and larger.

Arched structures of various scales (lrtb). Quonset huts, Project Iceworm, South Pole logistics archways, some building in Hawthorne, Hangar One, Atlantic City Convention Hall.

They may be connected by sealable bulkhead doors, while the roof can support solar panels or farms, especially the equatorward face of east-west oriented pods. They also have good volume to material/labor ratios. Building materials can range from curved prefab panels to locally produced concrete or brick. Brick vaults may be assembled robotically without formwork using a variety of techniques. Brick and concrete structures are compressional so need preloading before pressurization with several meters of dirt. Numerous other materials and methods are possible, including inflatables.

A growing Mars base, then, could be a densely packed crosscutting network of arched pods, with outskirts being built out at ever more ambitious scale. Manufacturing, chemical work and other non residential activities can be confined to dedicated pods, which can be repurposed (loft conversion!) over time as demand shifts.

Primary demand for water, (nuclear or solar) power, fuel synthesis and storage is associated with launch, so it makes sense to collocate much of this capacity outside the city. Pads with retractable hoses and robot arms can handle ship surface operations at each pad. If the city outgrows the spaceport, construction of new pads and pipes is much less labor intensive than demolishing and/or rebuilding pressurized pods. Spaceports will ideally be located 5-10km north and/or south of the city to keep east-west approach/departure paths clear. On Mars this is well over the horizon!

While it may be possible to house a million people under a square km of high rise roofs, and contain industrial processes under 10sqkm, farming on Mars, while not an immediate priority, will eventually require the bulk of covered land, perhaps 100sqkm. It makes sense to operate at the same pressure as the base, rovers, and suits (perhaps 340mbar) but with enriched CO2 for plant growth, and every other trick worked out by decades of dedicated research that hasn't happened yet! The primary difference between farming and habitation structures is that farms need transparent roofs, though ideally still with a layer of water, ice, or glass to mitigate radiation. I like the idea of transparent inflatables sealed to the ground at the periphery and anchored with cables at regular intervals to spread the pressure load into the ground. There's no reason why residential pods couldn't be interspersed between greenhouses. These structures, similar in concept to an air mattress, would look a bit like this.

One final consideration is scaling and congestion. Pod bulkhead doors are natural choke points. While clever neighbourhood designs will keep the base walkable for most activities (food, hygiene, schooling, training, recreation), movement of large equipment or lots of people may require progressively larger thoroughfares. This is a great problem to have, since that many people on Mars implies many other problems have already been solved! I think use of tunnel boring machines for subgrade roads and repurposing of legacy structures will prevent problematic congestion.

This is my first pass at Mars urban planning. I have no doubt overlooked many details and obvious issues. I'm interested in developing sensible system design axioms from which any given city plan can be more-or-less trivially derived. I don't know of much other work done with such an aggressive focus on growth, and I'm curious to get a better understanding.

Visit to Ohio and Australia

Last week, C and I had back-to-back weddings and excitement. 

Photos: https://photos.app.goo.gl/1gD4PDoqNNXiaNa02

It started late Friday evening. We found our way to a conspicuously ultra-baseline economy flight, settled in behind an actual dog, and took the red eye direct to Columbus, Ohio. There, we met C's mother's new cat and celebrated the delightful marriage of E and G, whose subsequent honeymoon was, we hear, rather exciting!

I ate a belated birthday cake and then we flew back west once more, passing over spectacular canyon scenery, various faults, a Hyperloop prototype, and the Ivanpah solar thermal plant on the California-Nevada border. Back in LA the sky was a bruised colour from numerous fires, and we found our way to a lounge in the international terminal. 

Later that evening, we boarded a flight to Sydney. I read a few books, watched the latest Pirates of the Caribbean film, and tried to understand system properties of urban planning in space. On the ground, we were unexpectedly collected at the airport by my parents B&A, and whisked off to our rental apartment. We had planned relatively little for the first few days so that essential tasks like finding suits could be taken care of. I squeezed myself into a sharp blue number, while finding time for a few hikes, avocado toasts, and admiring the luxury cat home my parents transformed their flat into.

All too soon it was time to dress up and help my brother M into a matrimonial state. The wedding went off without a hitch. Or rather, only one hitch! We had readings from Song of Solomon and sang Jerusalem, and I didn't burst into flames. Then out to the harbour foreshore for photos, then the golf club for a fabulous dinner reception. Once again I was required to engage in some gentle brotherly ribbing as I gave a mercifully short speech welcoming my new favourite sister (sorry A) into the family.

C and my wedding in August was conducted on a remote island, so it was fortuitous that M was able to assemble the entire family in one place for C and my convenience so soon after. I enjoyed catching up with all the rellies, recharging my accent, and doing some sneaky research into my family's more mysterious origins on the continent. This will be the subject of a future blog post!

Well, we breathed a huge sigh of relief, borrowed dad's car, and set off on a road trip to see a bit more of Australia than we had on prior trips. Australian roads are good but the speed limits are horribly low and the drive thus extremely boring. We did see a good variety of wildlife though. 

First stop was the NSW central coast, where we gatecrashed my grandmother's choir rehearsal, investigated the giant pelicans, went for a hike, and attempted to avoid being dive bombed by seagulls while rowing around the bay. C and I cooked a huge dinner for my grandparents which was well received. One of the parts were potato latkes, which seem to me to be a lot of work to get out something which is basically a baked potato. 

After a couple of days we set out once more, traveling via Norah Head Lighthouse, where we got the best Australian accent lesson ever, to Buttai, a remote corner of the Hunter Valley where my great grandfather used to "go off the leash" with his brothers in semi retirement, reliving their incredibly poverty stricken childhood. Sooner or later the entire area will be strip mined, so good to check it out while I have the chance. 

We continued up the road, turning off on the Bylong Valley Way, a picturesque route through the Wollemi National Park, and also soon to be strip mined. At the town, we enjoyed a quick snack in the general store and explored a nearby graveyard. Almost all the graves dated from around Australia's regional grazing boom (1870-1930) but there were two fresh graves from 2015 in which a 97 year old couple had been (post mortem) interred.

Nearly there. We arrived in the late afternoon at my aunt G's farm outside Rylstone, finding noone but a lot of dogs and a half-cooked dinner. Perfect for exploration, so we found the new house site, the folly (a whimsical shed to contain us) and, eventually, living humans. 

There was much excitement in town because it was the start of the semi-annual international chainsaw large scale wood sculpture symposium. Later, we met a local who was involved in making a horror film documentary, so all things considered it was an ideal time to spend a few days sleeping in an isolated shed with no electricity, running water, phone service, or much but trees and kangaroos around. 

We enjoyed meeting the sculptors and seeing the incredible art being installed everywhere. We found a lot of large spiders, not all of them still alive. The biggest by far was Mr Tiny, a 4" wide huntsman spider who kept a few eyes on us while we took a shower. Later he refused to stand still so we took him outside. 

Overnight it began to rain, so the next day we suited up and went for a quiet walk down the main ridge of the property. This part of the world has some incredible "beehive" sedimentary rock formations. I have long had a secret ambition to hollow one of them out and build a cozy house inside. Once, I stayed in hollow rocks in central Turkey. It might be easier to build the house and then clad it in rock-like material. Getting useful windows that are invisible from outside would also require some finesse. 

All too soon it was time to head back to the city. We drove south and east via the Three Sisters near Katoomba, then spent a few hours at my old school talking to students about careers in STEM fields. That evening we gathered a few friends and gatecrashed my sister's house for an amazing dinner of purple risotto and music. The following day we relaxed with family, hung out with my old neighbours whose house is full of Antarctic art, and then flew back to the US. About an hour after taking off I looked out the window and saw Lord Howe Island cruising by! It's also a pretty cool place to visit, some time.

Recently it has seemed as though I make it to Australia about once a year. It's odd to see evidence of how much time has passed, but I'm sure the experience is similar for Australian residents who rarely see me! I have a few more years before my grey hairs become overwhelmingly obvious, I think.

SpaceX update at IAC 2017

Late last Thursday evening I enjoyed watching Elon Musk deliver his second update on SpaceX's plans for Mars, or Making Humans Multiplanetary. If you haven't seen it, watching this video will make the rest of this blog much less confusing. I've written a bit about Mars over the years, and I'm always excited to hear what SpaceX has been up to.

A year ago, I wrote a blog about the plan as then presented, and I'm thrilled to see its evolution and write a bit about my new thoughts. I'm going to split this blog into three parts. The first will deal with the major development - money. The second will discuss the mission profile. The last will deal with Mars urban planning.

Money

In 2016, it wasn't immediately obvious how to pay to develop the Mars rocket let alone run the program. A couple of months ago, I wrote a blog on this topic. And I'm thrilled that not only did I not guess what SpaceX was planning (though I was closish), what they have proposed is a better idea than anything I wrote about there.

SpaceX plans to retire the Falcon and build only BFRs for every mission it can think of. It will redirect all its engineering know how to building the new system, and thus finally find a method to freeze design of the Falcon 9 at Block 5 without further meddling! Why is Falcon a dead end? It is not fully reusable.

SpaceX can build and sell the Falcon 9 for about $65m for a single expendable launch. While their recovery of the first stage can help their business both by decreasing their fixed costs by a factor of perhaps three and increasing the number of boosters available, they can do a lot more. Even if they could refly the first stage hundreds of times with marginal cost per launch, the fixed cost of the expendable second stage, which must be near $10m, means that they can't revolutionize launch completely.

Here comes the realization. At $65m a launch they're already crushing the competition. With the landing of the booster SpaceX lacks credible competition for at least a decade. They could lower their launch cost to maybe $20m a launch and nourish the microsatellite market, but the overall demand for launches is likely to remain fairly static in terms of mass to orbit. This is because thousands of cube sats don't weigh much compared to the really big communications and military satellites.

Enter the BFR. The BFR might cost $500m to build. But if SpaceX charges $65m a launch - the same as it is doing today - and the BFR is entirely reusable, then it can recoup that cost within the first year of launches and then some. The BFR only has to fly about 10 times to close the business case, and there's no reason it couldn't eventually fly hundreds of times. Given that the cost of fuel is somewhere around a million dollars per launch, SpaceX has a huge advantage, because customers have no ability to force the price down against other competition who already charges between four and ten times as much as SpaceX.

The BFR is hugely over specced for any launch currently manifested. With a nominal capacity of 150T to LEO and perhaps 40T to GTO, there isn't a satellite it can't launch. Hell, it could probably launch any existing satellite into the sun. Well, not quite - it turns out that's really difficult. Does it matter that there's no real demand for 150T to LEO launches? Does it matter that the BFR will thus fly 5% full for most launches? Not if those launches are profitable. The airline industry's costs are about 13% for fuel, so a single BFR launch may cost as much as $5m, but SpaceX won't have many of the airlines fixed costs, like hiring pilots. So while the BFR's capacity could enable huge space telescopes or probes to Saturn or gigantic space stations, it can also fly mostly empty, or perhaps carry SpaceX's own cargo (such as fuel or internet satellites) with its excess capacity.

Here's another way to think about it. Given the current cost of launch is 100x the fuel cost, a fully reusable rocket could be 100x too big and still make economic sense. Why 150T, then? SpaceX wants these rockets to be big, for moving stuff to Mars. But really large rockets are harder to build and transport. Elon tweeted that the BFR was sized to fit through a door in an assembly facility. The door size constraint also set the size of the Saturn V.

Since last year's presentation, the performance of the Raptor engine has deteriorated slightly, which probably reflects its development path. It's worth pointing out that while a heavier BFR with a less exquisite Raptor might not be very useful for flying back from Mars, it is still capable of flying there, and more than capable of launching stuff into orbit around the Earth. Big dumb rockets suffer a design constraint which is that small changes in structural efficiency have big (and bad) effects on overall system performance. A good illustration of this is the evolution of the Falcon 9 rocket. When it first flew, it could only just loft 9.5T into orbit. Today, it is rated at 22.8T, even though the underlying plan remains the same. What changed? The engines got a bit better, and the rocket got a bit lighter. Imagine if the Mars ship, designed to lift 228T to Mars, turned out for the first few years to only lift 95T? The mission would be over. Elon briefly addressed this in the talk, when explaining a benefit of on-orbit refilling of fuel and oxidizer. Even if the booster turns out to suck, the spaceship can still be fully fueled in orbit, retiring that developmental risk.

What about the possible use case for high speed transport on Earth? Very roughly, the numbers check out here too. Oxygen and methane cost about $200/tonne, and each rocket needs about 4000T of fuel. So taking the airline numbers again, each flight could cost $5m. If each rocket can carry 150T of payload, that's about 1500 passengers, so the per head cost might come to $3000, which is comparable to current long haul costs. Interestingly, over shorter distances, the spaceship alone (without the booster) could make a flight. So this seems to address current usage patterns and cost structures, though is much more marginal than the launch business. Finally, although Elon would no doubt love a launch pad near every city, they are noisy places and town planners generally did not provision for the 4-6 mile exclusion zone they require. My eyebrows remain raised!

So what are we to make of the other efforts to bring about industrial capture of disruptable industries? Even though SpaceX's plan is to capture the launch market with a fully reusable rocket ("shuttle done right") it has a few other plays in internet satellites (Starlink) and tunnel boring (The Boring Company). I think these efforts are developing technology which is important for the Mars project and may eventually become huge sources of revenue themselves.

Mission Profile

Elon Musk provided a few updates on the mission profile. As an example, the spaceship has enough ΔV to fly to the Moon and back without lunar refueling if refilled in an elliptical Earth orbit. This would take about three times as many tanker flights, first to fill up a tanker completely in LEO, then fly that to an elliptical orbit several times to refill the spaceship. A Mars ship launched like this could also take much more cargo to Mars, though its Mars entry would be proportionately more difficult.

I have heard some discussion about distribution of the engines and their uses. It is important to remember that the rocket is ten times heavier when fueled up, and the bigger engine bells work much better in space. Therefore, while one Raptor engine is adequate to land a nearly empty spaceship, all 31 on the first stage are needed to lift the whole stack off the Earth. Similarly, a fully fueled ship lifting off from Mars needs four high efficiency vacuum engines, while two sea level engines are adequate for landing.

I was fascinated to see the section on the Mars entry profile, as I wrote a somewhat less sophisticated model to study this problem some time ago. Landing on Mars is very difficult for all kinds of reasons, but attentive watchers may have noticed that the spaceship enters the atmosphere upside down. Why is that? It turns out that entering the Mars atmosphere at 8500m/s, as SpaceX plans to do, is easily fast enough to skip off and escape the planet entirely. The spaceship is a lifting body and uses its lift not to fight gravity, but to help gravity pull the vehicle closer to the ground as it tries to skip off the atmosphere. In my model I computed that the spaceship would have to fly below about 40km to be able to beat centrifugal force. In this way, the entering vehicle curves around the planet. When its speed drops below Mars orbital velocity (~3700m/s) then it gradually rolls to a nose up attitude, where its lift continues to dissipate speed until, falling straight down it lights the engine and performs a flawless landing on the surface. The 2017 spaceship has really pointy legs, so I hope they pick a really hard flat surface to land on.

The LEO refilling concept has also been simplified. Rather than two spaceships flying close along side like mating whales, they now back up to each other. A small thruster creates enough force for fuel to drain from the tanker to the ship. Alternatively, if the parking orbit is low enough, residual atmospheric drag could provide some ullage force.

Although this was not explained, I presume that the landing pads build on Mars have hatches beneath which coiled hoses can be lifted to the spaceship for autonomous refueling. For this and related reasons, I think the spaceship might need a robot arm. Similarly, while a crane on the ship can lower cargo to the surface, an established Mars base would have to have mobile gantries that can be rolled alongside like siege machinery to facilitate rapid unloading and loading of cargo.  

Mars urban planning

Finally, Elon had a couple of slides on how the Mars base would grow. I like the images. I like all space-related concept art. www.humanmars.net has some of the best!

But these images are very Sim City. Will a Mars base actually look like that? What will a Mars base look like? Why? I talked a bit about Hab design principles in my Mars book. But Mars urban design is not an established field, and I think Elon was trolling us with this design. Indeed, I think SpaceX's main goal at present is to systematically derisk a human Mars mission to entice NASA and Congress to bite. Designing Mars bases is a few steps ahead. Moreover, SpaceX would like other titans of industry to join in and collaborate on these issues.

In particular, all those glass domes are just 200m from the landing pad! How far from the landing pads does the city have to be? What are its primary functions? What are its design principles? All these questions are good topics for a future blog. But clearly balancing primary needs for transport, power, water, fuel, and above all growth, are non-trivial!

Conclusion

I can't wait to see what comes out over the next year. This, the settling of another planet, remains the single most challenging, exciting, and worthy problem for all of humanity. I firmly believe that if my generation doesn't achieve this, the eventual extinction of humanity is all but certain. For more in this vein, check out these epic Wait But Why posts.