The Interplanetary Transport System

Space has been on my brain a lot lately. One of the causes was the long-awaited presentation by Elon Musk at the International Astronautical Congress (IAC) last month. During the talk, he finally laid out the details of his “Interplanetary Transport System” (ITS). The architecture is designed to enable a massive number of flights to Mars for absurdly low costs, hopefully enabling the rapid and sustainable colonization of Mars. The motivation behind the plan is a good one: humanity needs to become a multi-planetary species. The sheer number of things that could take civilization down a few pegs or destroy it outright is frighteningly lengthy: engineered bio-weapons, nuclear bombs, asteroid strikes, and solar storms crippling our electrical infrastructure are some of the most obvious. Rampant AI, out-of-control self-replicating robots, and plain old nation-state collapse from war, disease, and famine are some other threats. In the face of all those horrifying things, what really keeps me up at night is the fact that if civilization collapses right now, we probably won’t get another shot. Ever. We’ve abused and exhausted the Earth’s resources so severely, we simply cannot reboot human civilization to its current state. This is the last and best chance we’ll ever get. If we don’t establish an independent, self-sufficient colony on Mars within 50 years, we’ll have solved the Fermi Paradox (so to speak).

But Musk’s Mars architecture, like most of his plans, is ambitious to the point of absurdity. It at once seems like both fanciful science fiction and impending reality. Because Musk works from first principles, his plans defy socio-political norms and cut straight to the heart of the matter and this lateral approach tends to rub the established thinkers of an industry the wrong way. But we’ve seen Musk prove people wrong again and again with SpaceX and Tesla. SpaceX has broken a lot of ground by being the first private company to achieve orbit (as well as return safely to Earth), to dock with the International Space Station, and to propulsively land a part of a rocket from an orbital launch. That last one is particularly important, since it was sheer engineering bravado that allowed them to stand in the face of ridicule from established aerospace figureheads. SpaceX is going to need that same sort of moxie in spades if they are going to succeed at building the ITS. Despite their track record, the ITS will be deceptively difficult to develop, and I wanted to explore the new and unsolved challenges that SpaceX will have to overcome if they want to follow through on Musk’s designs.

SpaceX ITS Diagram

The basics of the ITS architecture are simple enough: a large first stage launches a spaceship capable of carrying 100 people to orbit. More spaceships (outfitted as tankers) are launched to refill the craft with propellants before it departs for Mars during an open transfer window. After a 3 to 6 month flight to the Red Planet, the spaceship lands on Mars. It does so by at first bleeding off speed with a Space Shuttle-style belly-first descent, before flipping over and igniting its engines at supersonic speeds for a propulsive landing. After landing, the craft refill its tanks by processing water and carbon dioxide present in Mars’s environment and turning them into propellant for the trip back to Earth. Then the spaceship simply takes off from Mars, returns to Earth, and lands propulsively back at home.

Now, there are a lot of hidden challenges and unanswered questions present in this plan. The first stage is supposed to land back on the launch mount (instead of landing on a pad like the current Falcon 9 first stage), requiring centimeter-scale targeting precision. The spaceship needs to support 100 people during the flight over, and the psychology of a group that size in a confined space for 6 months is basically unstudied. Besides other concerns like storing highly cryogenic propellants for a months-long flight, radiation exposure during the flight, the difficulty of re-orienting 180 degrees during re-entry, and the feasibility of landing a multi-ton vehicle on soft Martian regolith using powerful rocket engines alone, there are the big questions of exactly how the colonists will live and what they will do when they get to Mars, where the colony infrastructure will come from, how easy it will be to mine water on Mars, and how the venture will become economically and technologically self-sufficient. Despite all of these roadblocks and question marks, the truly shocking thing about the proposal is the price tag. Musk wants the scalability of the ITS to eventually drive the per-person cost down to $200,000. While still high, this figure is a drop in the bucket compared to the per-capita cost of any other Mars architecture on the table. It’s well within the net-worth of the average American (although that figure is deceptive; the median American net-worth is only $45,000. As far as I can figure, somewhere between 30% and 40% of Americans would be able to afford the trip by liquidating most or all of their worldly assets). Can SpaceX actually achieve such a low operational cost?

Falcon 9 Production Floor

Remember that SpaceX was originally targeting a per-flight price of $27 million for the Falcon 9. Today, the price is more like $65 million. Granted, the cost to SpaceX might be more like $35 million per flight, and they haven’t even started re-using first stages. But it is not a guarantee that SpaceX can get the costs as low as they want. We have little data on the difficulty of re-using cores. Despite recovering several in various stages of post-flight damage, SpaceX has yet to re-fly one of them (hopefully that will change later this year or early next year).

That isn’t the whole story, though. The Falcon 9 was designed to have the lowest possible construction costs. The Merlin engines that power it use a well-studied engine design (gas generator), low chamber pressures, an easier propellant choice (RP-1 and LOX), and relatively simple fabrication techniques. The Falcon 9 uses aluminum tanks with a small diameter to enable easy transport. All of their design choices enabled SpaceX to undercut existing prices in the space launch industry.

But the ITS is going to be a whole other beast. They are using carbon fiber tanks to reduce weight, but have no experience in building large (12 meter diameter) carbon fiber tanks capable of holding extremely cryogenic liquids. The Raptor engine uses a hitherto unflown propellant combination (liquid methane and liquid oxygen). Its chamber pressure is going to be the highest of any engine ever built (30 MPa. The next highest is the RD-191 at 25 MPa). This means it will be very efficient, but also incredibly difficult to build and maintain. Since reliability and reusability are crucial for the ITS architecture, SpaceX is between a rock and a hard place with its proposed design. They need the efficiency to make the system feasible, but the high performance envelope means the system will suffer less abuse before needing repairs, reducing the reusability of the system and driving up costs. At the same time, reusability is crucial because the ITS will cost a lot to build, with its carbon fiber hull and exacting standards needed to survive re-entry at Mars and Earth many times over.

It’s almost like the ITS and Falcon 9 are on opposites. The Falcon 9 was designed to be cheap and easy to build, allowing it to be economical as an expendable launch vehicle, while still being able to function in a large performance envelope and take a beating before needing refurbishment. The ITS, on the other, needs all the performance gains it can get, uses exotic materials and construction techniques, and has to be used many times over to make it an economical vehicle.

All of these differences make me think that the timeline for the development of the ITS is, to put it mildly, optimistic. The Falcon 9 went from the drawing board to full-stack tests in 6 years, with a first flight a few years later. Although the SpaceX of 2004 is not the SpaceX of 2016, the ITS sure as hell isn’t the Falcon 9. A rocket using the some of the most traditional and well-worn engineering methods in the book took 6 years to design and build. A rocket of unprecedented scale, designed for an unprecedented mission profile, using cutting-edge construction techniques… is not going to take 6 years to design and build. Period. Given SpaceX’s endemic delays with the development of the Dragon 2 and the Falcon Heavy, which are a relatively normal sized spaceship and rocket, respectively, I suspect the development of a huge spaceship and rocket will take more like 10 years. Even when they do finally fly it, it will take years before the price of seat on a flight falls anywhere as low as $200,000.

Red Dragon over Mars

If SpaceX manages to launch their Red Dragon mission in time for the 2018 transfer window, then I will have a little more hope. The Red Dragon mission needs both a proven Falcon Heavy and a completely developed Dragon 2. It will also allow SpaceX to answer a variety of open questions about the mission profile of the ITS. How hard is it to land a multi-ton vehicle on Martian regolith using only a powered, propulsive descent? How difficult will it be to harvest water on Mars, and produce cryogenic propellants from in situ water and carbon dioxide? However, if SpaceX misses the launch window, I definitely won’t be holding my breath for humans on Mars by 2025.

Fetishizing Apollo

America has an unhealthy obsession with historic US space missions. This obsession is even more pronounced in the space-enthusiast community; it is no surprise that there are multitudes of mods for KSP that allow users to build and fly their very own Saturn V rocket. Really, America’s fixation on the 1960s and -70s era NASA programs has achieved a pornographic level (I use this word not in the sexual meaning, but in the same sense as in the pornography of violence).

It is an understandable attraction, I suppose — many of the iconic space photographs were taken by Apollo astronauts.

earthrise astronaut fullearth

Landing people on the Moon might be considered one of mankind’s greatest achievements, and was certainly the height of glory for the US space program.

But the level at which America has turned the moon missions into a fetish is astounding. Countless books, movies, rehashed TV series, photo remasters, articles, celebrations… it’s depressing.

We should appreciate Apollo for what it was: an antenna. Celebrating Apollo is like including the antenna mast in the height measurement for a really tall building. Yes, the fact that we stuck a tall pole on top of a tall building is cool, but it’s not really the pole that you’re interested in, is it?

People like thinking about Apollo because they like the idea of humans expanding into space, and in their mind Apollo is the farthest we’ve ever gotten towards that goal. It’s an understandable misconception, considering the Moon is literally “the farthest humans have ever gone”. But Apollo was unsustainable (even if the Apollo Applications Program had gone forwards, it still would have been a step in the wrong direction!). We are now much closer to accomplishing the goal of long-term human expansion into space than we ever were.

SLS, more like SMH

Granted, it won’t be painted the same way in real life.

This is why the SLS is so disappointing, I think. Right now we have highly advanced computing and robotics technologies, excellent ground support infrastructure for space missions, incredibly advanced materials knowledge, and a huge array of novel manufacturing techniques being developed. As a civilization, we are much more ready to colonize space than we were a half-century ago. Yet the government has decided the best way to start human expansion into space is to build a cargo cult around Apollo. The US is building a rocket that looks like the Saturn V, as if some sort of high-tech idolatry will bring back the glory of Apollo. They are resurrecting an architecture that was never a good idea to begin with!

The space program paradigm is outdated. Despite my most optimistic hopes, let’s be real: the next big driver in space travel will be high-power corporations following the profits of a few innovative companies that pioneer the market. It won’t be enthusiastic supporters than become the first space colonists, but employees doing their stint in the outer solar system before returning to Earth.

On the SLS

NASA has always built its house on political sand, not rock. While they can get lots of funding at the political high-tide, they can also get bogged down to failure. For instance, the ill-fated Space Shuttle program (or STS) was a result of shifting goals and influences on design from disparate sectors.

Similarly, the new Space Launching System (or SLS) is the brainchild of political forces. Its first stage is an extended Space Shuttle External Tank, and its first-stage engines are Space Shuttle Main Engines. Its solid boosters are STS-derived. This means that most of the STS tooling can be kept, and thus constituencies keep their jobs. I have to admit, this re-use reduces design times, but it is not the most efficient way to build a heavy launch vehicle, especially since the technology is more intricate than it needs to be (jacking up production costs).

The SLS is designed to launch the Orion capsule, which was designed as part of the under-funded Constellation program. The Orion spacecraft uses the Apollo architecture, and NASA helps this comparison by painting the SLS with white-and-black roll patterns reminiscent of the famous Saturn V rocket. Combined with the use of STS systems, the SLS threatens to become one big nostalgic mash. Unfortunately for NASA, this means its shortcomings will be overlooked in the future, much like the Space Shuttle program.

For instance, nobody is quite sure what to do with the damn thing after we’ve designed and built it. NASA has recently released that instead of a circum-lunar flyby, the first manned mission of the Orion/SLS will visit an asteroid that will be tugged into orbit around the Moon. After that, everyone seems to throw up their hands and say, “Mars? I guess?”

Criticisms aside, its good that we are developing any sort of heavy-lift manned capacity, because eventually it will enable deep-space missions. I’m just worried that the SLS program isn’t coherent enough to survive shifts in funding or vision. Not that NASA is particularly gifted in either of those departments. With their limited funding, I’m more interested in their robotic missions (or potential thereof — submarine to Europa, anyone?), because any manned program in the near future will consist of paddling around in the metaphorical kiddy pool.

Interstellar Colonization Will Never Happen

There really isn’t an economical explanation for why a civilization would engage in long-range interstellar colonization.

To begin with, though, let’s look at interplanetary colonization. Why, for example, would someone fund the establishment of a permanent colony on Mars with the intent for it to become eventually self-sustaining? It’s not to relieve population pressure. Stuff is so ridiculously expensive to get into space that you’d be better off (from a monetary perspective) paying the people to live in the Sahara. It’s not for resources; asteroid mining is almost certainly a feasible economic opportunity, but the cost of lifting resources into orbit is again the obvious barring factor. It could be scientific, but scientific missions wouldn’t need to be self-sustaining or long-term. Perhaps a stint of 20 years on the surface. It could be done by a separatist group (plenty of people want to go start small settlements in the wilderness), but even if the money was raised (which is unlikely), the colony will lie on the fringes of human society. They would probably be unable to arrange a return trip, even if they wanted to, and nobody else (except more fringe groups) would want to continue colonization.

There is one argument that seems reasonable: outposts could serve as refueling stations for outbound craft (asteroid mining operations, etc). However, it would make more sense to pull these resources from asteroids and place an automated fuel refinery in high orbit around Mars (or other suitable candidate).

Many of the reasons listed above carry over to interstellar missions. The only difference is that groups would have much more trouble raising money for the mission, and that now lifting stuff into orbit isn’t the only tough part, but also accelerating your spaceship to a speed which makes for a bearable trip length.

Here are some scenarios where we do send a colonizing mission: we discover evidence of alien life, or the ruins of an alien civilization. It would only make sense to send a colonizing mission. Sending a scientific detachment with a planned return trip would be so expensive that it wouldn’t be worth it. I mean, it would be worth it, but nobody would be able to raise the funds.

Another scenario in which most of the above arguments go out the window: we build a space elevator. That removes the gateway for getting into orbit. We could expect many more people accessing and living in orbit (because they feel like it and the price is low enough). Once the population already flying around the solar system reaches a critical mass, colonizing Mars becomes a trivial step.


Actually, it came to me after I wrote this post that there may be one reasonable explanation for colonizing Mars: if we fail to find an economical way to increase biomass production either on Earth or in space, we will need large tracts of arable land. Terraforming Mars would provide this. However, the cost of lifting and storing that biomass may make it less preferable to aerocultures in orbit.


Here is a story I recently wrote. It was written over the course of an afternoon, for a school assignment.


One of the first few expeditions found him. As they plunged off their boat into the icy Atlantic and slogged onto Snaeland at, as they dubbed it, Seydisfjordur, his hollow ravings echoed down from the foothills and caused much inquiry. A group of the Norsemen set out from the expedition’s shore-side camp, and hiked up towards the source of these cries. They found him living in the carcass of a busse; the ship looked like it had been washed into the mountains, its beams broken across the crevices of the hills, its oars splintered on the tall pines. The Norsemen, not a little confused, dragged the raving lunatic back to their camp and kept him in an uthu adjoining the longhouse. For the next two nights, the Norsemen argued over what to do. Some wanted to hang him as an offering to the All-Father, but most were willing to wait and hear the prisoner’s explanation before deciding. How could such a heavy ship be lifted inland as far as a day’s walk?

After a few days of food and drink, the man began to speak some sense. They found his name was Gormund. As soon as this news spread, everyone was intensely curious to hear his story. In slow strides of language, Gormund began a discourse. His clothes were ragged and his hair long and unkempt, but his tongue was as erudite as the best skald. Gormund held himself very calmly, but every word he spoke was laden with insanity. Over the course of two days Gormund disgorged an enthralling madman’s tale.

He detailed an expedition made from Volmong (after explaining that Volmong was a Norse settlement hidden away in the mountains of Iberia, which was met by much disbelief), which had raided a string of monasteries. As they found out, these monasteries housed adherents to the Societas Eruditorum. Soon they had Charlemagne breathing down their necks. Pressed, the settlement held a thing, in which they decided that Volmong was doomed. The settlement was the size of a hundred, and leaving was never really an option, though not for lack of trying. The Norseman’s place is on the sea, not in the highlands; the settlers of Volmong were folly for ignoring that. The slow caravan to the coast was cut down en masse by the underfed armies of the mainland conqueror. Only a handful of longships left the Iberian shore, and fewer navigated the Channel successfully.

On the third day of Gormund’s consciousness, his narrative was cut short when a lookout cried from the palisade. A Gaelic warship had appeared off the coast. The men, suddenly electrified by the chance of combat, began to arm themselves, and pushed off in one of their three longships, dragging Gormund along in spite of, as they found, his deathly fear of the sea. As their longship drew closer to the Gaelic craft, the Norsemen made out its shape: it had the contours, in the front, of a Roman bireme, maybe a trireme, but as it turned they saw that the back was rough and squarely built. When the ships were three thousand fot apart, the Norsemen saw why; a massive ballista was mounted on the head of the Roman warship.

Too late to reconsider tactics, and already heady from bloodlust, the Norse threw their backs into the oars and plowed towards the Gaelic ship at ramming speed. The Gaels loosed a flaming bolt from the ballista, which struck the drum beater. The weight of the shaft sheared his body in two, and the flaming oil spilled across the deck of the longship. With cadence broken, the oarsman made slower progress, but still they closed the gap between the ships. Another bolt was loosed, and it struck the side of the longship, shearing away many oars. Crippled, the Gaels lobbed flaming bales of hay onto the longship, and sat apart as the burning wreck sank into the cold Atlantic.

The Gaels dragged the two floating survivors aboard. One was conscious, and lashed out at his rescuers. They cut his throat and dumped him into the ocean. The other corpse was limp, but after some time of lying on the deck, he awoke, sputtering. The druid aboard hoisted the man up and pressed him against the forward mast, seeking fear in his prisoner’s eyes. But as the druid gazed, he saw a spark erupt within the Norseman’s eyes. Backing away in fear, he averted his gaze as the man’s face glowed an unearthly pallor. Gormund then spoke out in Gaelic, in an attempt to quell his captor’s fear. Of course, this was in his best interest; he didn’t want these Britons dumping his lifeless body in the cold Atlantic. He said, “I am Gormund, son of Bjiolnir. Bring me to Kaupang.” The Gaels could do nothing but obey him, and so they sailed to Kaupang. They left him on the ocean-shore of Outer Kaupang.

When he reached a fishing village on the bay-shore, the villagers took him in as a fellow Norseman. When beseeched to explain his business, he refused. When pressed, he warily recounted his passage from Seydisfjordur. The villagers, realizing he was a madman, locked him in the boathouse. The next morning they sent an envoy to Inner Kaupang to inform the herad-lord of a madman who claimed to hail from Snaeland. The herad-lord, on a whim, called for the man to be brought to his longhouse.

The next day, Gormund was brought before Þorhrafn, the herad-lord. “I am Þorhrafn, son of Harald, son of Refrbrandr, chieftain of Kaupang and contender for the throne of Skiringssal. Name yourself,” commanded Þor.

“I am Gormund, son of Bjiolnir. I hail from this town.”

“You say you live here, eh? You claim to have come from Snaeland.”

“I was with a landing at Seydisfjordur, when a Gaelic warship sunk the Karvi we launched. I convinced the Gaels to transport me here.”

“A Gaelic ship got you? Hah! And you convinced a ship of victorious Gaels to ferry you a hundred vei? How’d you manage that? Got an all-tongue, do you?” The chieftain guffawed.

“Yes, I have been granted such powers by the gods.” At this, the herad-lord let out a cry, and doubled over laughing.

“Qlfuss tunga! Hah!”

“I was at Volmong, where we raided Societas Eruditorum bastions. They have unlocked many secrets of the gods.”

“Bahahaha! What is Volmong? You’re a crazy ‘kilg’n!” The chieftain turned to his attendant warriors, “Give his life to the All-Father.”

Gormund spoke out again, his voice slightly modulated, “Do you want to know how I got to Snaeland?” The warriors paused. “As I was sailing home, to Kaupang, when the worst storm any of us had ever seen beset our ships. Waves like giants walked among us, and threatened to carry our busse from the sea and into the sky. One by one, we lost sight of the other boats, their calls spirited away on more powerful gales, their image divided from us by sheets of water, and their wake obliterated by the churning of the sea. The sky and water were the same color, and the water so enveloped us that there became no difference between air and water, sky and sea, light and dark. The same wet grayness surrounded us for what seemed like an eternity.

“I hope the others fared better than our lot, but since you seem to be unaware of any return, I can only assume they succumbed to the sea, their valiant defense of the ship falling to a crushing blow of water… such a fate would be better than our own.

“At one point, I became distinctly aware that a set of eyes besides our own were among us. Yet every time I would turn to face the intruder, I found nothing. But then I, and others, noticed a transient murky form beneath us. Moments passed, in which the sea seemed to calm. Then a crew member cried out; the largest wave I have ever seen, seeming to rise above the sky itself, towered above us. As I watched, two glowing eyes pierced through the gray veil of the water, followed by a hulking, coiled shadow beneath the surface. I made out two enormous wings, limbs, and a wrapping tail. A silence descended, in which only the rushing of water was heard. Somebody cried, “Jormungandr.” Then the world went dark.


“The next thing I recall is being found in Snaeland by the Seydisfjordur expedition. When I regained sanity, I remembered in a rush the campaign in Iberia. The Societas Eruditorum had given me things stolen from the vaults of the gods. Loki sent the World Serpent to apprehend me, to destroy my ship and return what was taken. Jormungandr’s storm carried our ship to Snaeland, and only I survived. I suspect that Jormungandr will return; he didn’t get what he wanted. My purpose in returning to Kaupang is to raise a force to fight him off.” As Gormund finished, the throne chamber echoed emptily. Þor considered these words, rolling his tongue around in his mouth as if appraising the taste of the tale. The only noise was the crackle of the fire. Then the herad-lord snorted.

“That is quite the tale. And one I’m not particularly inclined to believe. Even if I thought you weren’t a qlfuss, your story doesn’t make sense. Why wouldn’t the Serpent get you while in transit from Snaeland? Why not send a god to take back… whatever you stole, rather than a Loki-spawn, no less? Would not Jormungandr’s release of Midgard to attack you allow the oceans to spill over? And what was it that you stole?”

“The all-tongue, to name one.” Gormund paused, and was about to speak again but was interrupted by a courier, bursting into the chamber.

“The sea is boiling!” he cried. Everyone rushed outside. Gazing at the sea from atop the town-fort walls, they saw that indeed, the frothy waves had sheets of steam rising off them. A storm had come up, and dark clouds coated the Kaupang coast. Giant waves smashed the shore, breaking some of the boathouse piers. Then, from the water, a tower column of scales and flesh emerged, atop which sat a terrible head, with a maw lined with innumerable teeth. A horror fell upon the lord and his warriors.

Gormund muttered under his breath, “Jörmungandr.”

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