I recently promoted Robert Zubrin’s critique of the Space X plans for settling Mars. I am now convinced I was wrong.
Zubrin attacks the Space X plan in the video below and this article from New Atlantis. But after a fascinating conversation over at Reddit I am now convinced that Elon’s plan has the most robust mission parameters for the best price.
Zubrin sounds convincing. After all, he’s been in this game for decades, and has thought deeply about how to get some astronauts on the surface of Mars as cheaply as possible. At first I was taken in by some of his arguments. The main one concerns accommodation. From New Atlantis:
7. The sending of a large habitat on a roundtrip from Earth to Mars and back. This, too, is a very bad idea, because the habitat will get to be used only one way, once every four years. If we are building a Mars base or colonizing Mars, any large habitat sent to the planet’s surface should stay there so the colonists can use it for living quarters. Going to great expense to send a habitat to Mars only to return it to Earth empty makes no sense. Mars needs houses.
This initially won me over. Of course Mars needs houses! How stupid, flying houses all the way to Mars only to fly them all the way back to Earth again! But that image is too simplistic. Imagine workers on the early American railways building the first railway from New York to California, and they steam the first locomotive into California only to lift the expensive train carriages off the railway and then try and re-customise the carriages for permanent housing! Zubrin’s attack is that silly. The Space-X rocket, including the crew quarters, is over-engineered to be robust and re-used, many times, to bring the individual ticket price down. Imagine how slow movement to California would have been if passengers had to buy a new, single-use train carriage every time they moved there! Instead, we should load the train (or Elon’s great big rocket) with passengers and tools to build habitats out of the local materials once they get there. There are many plans for this. We can build better homes out of local materials without sacrificing the ship that got us there. Maybe the very first ship will be a completely unmanned supply rocket to dump tools and nuclear reactors and materials on the surface. Droids might drive around and film everything and confirm it is all operational, and an operational base could be installed before anyone even arrives! Or maybe they’ll have a few brave pioneers along to test the equipment and plug things together and get that first base up and running, and maybe even start growing their first salads and food before everyone else arrives on the next ship! They’ll have a few years before the first ship is due to return. That’s ample time to use the ship as a base until they’ve actually assembled their first habitat.
The second ship will be full of people and materials all ready to move into their new accommodations. Again, the second ship will have 2 years before it is due to return, and it can be used for accommodation during that time. A lot can be achieved in 2 years, given enough good tools and materials and motivation (like surviving there!)
However those first few ships work, the basic point is this. Once the first settlers are there with enough tools, it would be madness to turn an over-engineered, over-expensive rocket-ship’s crew habitat into permanent ground living. It would drive up the whole cost of getting to Mars in the first place, and could limit innovations in habitat design they might see as important. The first settlers will be constantly building and expanding their base. They’ll probably be desperate for new companions, let alone new seeds and foods and flavours that new company will bring with them! I mean, have you really counted everything you would need on Mars to make a really good pizza? 😉 When settling a new planet, you do not maroon the space-railroad that got you there. Instead, you drive the ticket price down and give the settlers all the tools they need. Mars needs houses, but it also needs people to build them! A first unmanned supply ship and second ship full of more tools and food and some settlers should do the job, and then the interplanetary ‘railway’ is established!
8. Quick trips to Mars. If we accept the optimistic estimates that Musk offered during his presentation, the SpaceX system would be capable of 115-day (average) one-way trips from Earth to Mars, a somewhat faster journey than other proposed mission architectures. But the speedier trips impose a great cost on payload capability. And they raise the price tag, thereby undermining the architecture’s professed purpose — colonizing Mars — since the primary requirement for colonization is to reduce cost sufficiently to make emigration affordable. Let’s do some back-of-the-envelope calculations. Following the example of colonial America, let’s pick as the affordability criterion the property liquidation of a middle-class household, or seven years’ pay for a working man (say about $300,000 in today’s equivalent terms), a criterion with which Musk roughly concurs. Most middle-class householders would prefer to get to Mars in six months at the cost equivalent to one house instead of getting to Mars in four months at a cost equivalent to three houses. For immigrants, who will spend the rest of their lives on Mars, or even explorers who would spend 2.5 years on a round trip, the advantage of reaching Mars one-way in four months instead of six months is negligible — and if shaving off two months would require a reduction in payload, meaning fewer provisions could be brought along, then the faster trip would be downright undesirable. Furthermore, the six-month transit is actually safer, because it is also the trajectory that loops back to Earth exactly two years after departure, so the Earth will be there to meet it. And trajectories involving faster flights to Mars will necessarily loop further out into space if the landing on Mars is aborted, and thus take longer than two years to get back to Earth’s orbit, making the free-return backup abort trajectory impossible. The claim that the SpaceX plan would be capable of 60-day (let alone 30-day) one-way transits to Mars is not credible.
Does it cost more fuel? Of course. Does that extra fuel cut into the available space for the crew? Sure, but
- We’re already talking about a craft designed to hold a maximum of somewhere between 100 and 300 people.
- A longer duration means more food. The average human eats about 2 kg of food per day. If we assume a crew size of 100 people, increasing the duration from 80 days to 180 means 20 000 kg of extra foodstuff alone is needed. That’s equivalent to the mass of over 300 people (assuming a 62 kg average body mass). If we assume the crew size was already at 300, we’re talking about 60 000 kg of extra food. Since we can’t put greenhouses in these ships, all food consumed on the voyage must be carried from launch.
- Based on the above figures, every extra person you add to the 180 day journey will cost at least 422 kg (their body mass + food). This pays no mind to the extra amenities (water processing, air processing, cabin structure,) required for each person, nor the extra cargo each will require for surviving on Mars. This also pays no mind to the extra volume occupied by all the extra cargo. It’s not just a matter of mass, its a matter of space too. And lets not get into the heat considerations for extra people.
- As I linked above, Zubrin is wrong about there being a fourfold (not three) gain in payload capacity. The current ITS ship design is capable of delivering 200 000 kg of payload at its maximum speed. If you reduce its Δv (and therefore transit speed) to the lowest possible amount for getting to Mars from Earth, its capacity increases by 2.25 times (to about 450 000 kg). If you want, you can consult SpaceX’s Δv chart directly. He may argue the ITS could carry somewhat more after its engine capacity is decreased, but SpaceX hasn’t given us enough data to confidently talk about that. Remember, we’re talking about about SpaceX’s stuff. They’re the ones with the authoritative figures.
- Okay, we can count on at least an extra 250 000 kg of carrying capacity if SpaceX decided to listen to Zubrin. That’s not as much as he said, but it’s still double. How far does that get us? It’s safe to assume at least 50 comes off the top, just to feed the original crew compliment for the extra 100 days. That leaves us with around 200 000 kg of extra capacity. Since each extra 2 people would cost about 1000 kg, we have a theoretical maximum of 400 extra people. But many other life support considerations will also increase mass requirement per person (thus lowering the number of extra crewmembers who can come). And, most importantly there’s the aforementioned space considerations. The ITS ship was originally designed to hold 100 people. SpaceX later decided it could hold as many as 300. Where exactly would hundreds of extra crewmembers go? A few dozen extra people could definitely fit, but there will come a point when we just have a lot of people packed into tin can like cattle.
- Getting people out of interplanetary space as quickly as possible isn’t a frivolous thing to spend fuel on. We don’t know how much protection the fuel tank will provide during a solar storm.
Long story short, you’re focusing on the tyranny of the rocket equation but ignoring the tyranny of the life support equation. The extra requirements for more people very quickly negates a ship’s ability to carry more people. SpaceX could redesign the ITS concept based on Zubrin’s suggestions, but they’d be going through a lot of trouble just to make the Earth to Mars voyage much less comfortable.
Eclipse back. Lastly, Zubrin attacks Elon for wanting to build a Big Friggin’ Rocket that looks like something out of retro-futurism? If it works… build it! If you build it, they will come. The settlers. The future Martians. And if it just so happens to look like something out of an old Buck Rogers sci-fi, all the better!
There are more great discussions about everything Mars over at the Mars Reddit. If interested, please join in!
After placing this image, it must be time to go and watch the Space X Mars promo again, see below.
Ahhh, that’s better!