Building Musk’s path to Mars

Momentum is Growing

This path is supported by a mixture of pure determination, massive cooperation and support, and the solid mathematics of significantly improving designs and increasing production rate of vehicles. The numbers are like bacteria multiplying in a Petri dish: in a few days the individually invisible cells become a visible colony, overwhelming some others in sheer numbers.

To the public, the media and even some of the space community, the last year for SpaceX has not seemed to be progressing that fast. However, right before our eyes, what Musk calls “stage zero”, the complex launch tower and associated platform and tank farm, was being designed and built and is now almost ready to support launches. Steady progress has been made in the production of and design improvements to the two main space vehicles, the Starship stage and the Super Heavy booster. The capacity to build more vehicles at Boca Chica is shortly to be four times larger, as a much larger high bay building nears completion.

The Mega Bay and High Bay will contain at least 8 bays for Starship construction.

In his February 10 talk, Musk also gave clear answers for the lack of progress in some areas such as the ocean platforms, Cape area launch facilities, the Starship crew cabin design, and development of a set of cargo items for his first Mars flights. He fairly pointed out that his available work force (and his own personal time) is focused on current needs: design, development, and construction of rocket vehicles and the Boca Chica experimental launch complex.

Mostly out of sight of the media’s cameras, work has been going on at the Hawthorne, California, site and in McGregor, Texas, where a second assembly line for Raptor engines is being built. By March, SpaceX expects to be able to build about one Raptor engine a day. By 2023, this production rate could exceed 400 engines per year. It is also reasonable to expect that production of Starship and Super Heavy components, especially the standard two-meter-high stainless steel rings, will increase before the end of 2022, to allow production of at least one vehicle in every bay about every three months. The eight bays they will have would thus allow them to build 32 vehicles per year. However, in a year, expect another increase in production capacity of both components, and bays to assemble and stack them in, as more high bay structures will be built at site at Cape Canaveral.

Production

What does this translate into in terms of actual vehicle production? For each complete booster, they need 33 Raptors on the Super Heavy stage and 9 on the Starship stage: a total of 42 Raptors, with a few being the vacuum version with the extended nozzle. I will assume arbitrarily that the Super Heavy boosters will launch multiple Starship stages during a given period with production components apportioned appropriately for this use at roughly three Starship stages for each Super Heavy. At one Raptor per day, in one year they could soon produce enough parts for 20 Starship stages (with 180 engines) and about six Super Heavies (with 165 engines.) If at some point, construction of designated production vehicles begins, they would soon have at least that many production vehicles. This does not count any existing stages which are presumably all development vehicles.

Note that increases in the capabilities of Starship and Super Heavy are continuing, as occasional scrapping of outdated models demonstrates. Starship will be lengthened to enable it to carry larger payloads. This is made possible by steady increases in the capacity of the Raptor engines, which have almost staggeringly large thrusts and pressures. Each Raptor 1 has a thrust of 185 tons, while the new Raptor 2 is now reaching thrusts of 230 tons and is aiming for 250 tons, with each engine producing about one gigawatt of energy while it is firing. Thus, for a few minutes, the first stage would produce as much energy as one of our larger states. The chamber pressure is more than 300 bar or over two tons per square inch, with the exhaust velocity more than three kilometers per second.

In the near future, a Super Heavy stage with 33 engines could have a liftoff thrust of 8,250 tons. If we assume the liftoff mass will be about 5,100 tons with payload, the thrust to mass ratio would be about 1.6 compared to the Saturn V’s 1.2. Future changes in the Starship stage mass will probably change this value, but this booster will rise off the pad fast.

What can be accomplished with this enormously strong and still burgeoning capacity? Musk will, once the stages are flight proven, soon have the ability to allocate Starship vehicles for several different purposes, and will also have several different versions of them. Several could be produced without aerodynamic surfaces and a small crew cabin for the NASA-sponsored lunar flight, with the expectation of those stages not returning to Earth’s surface. Some will eventually be outfitted with an extensive passenger quarters for the trip to Mars, but even more would be needed to carry bulk cargo to both the Moon and Mars. Just as in any city, the total mass of buildings far outweighs the mass of the people living in them. This is even truer for Mars, where the buildings must be pressurized, so the ratio of cargo flights to passenger flights could be as high as about 10 to 1. SpaceX will also be able to deliver extensive cargo to any site on the Moon with this reusable stage, for pay.

A significant number of Starship stages will be turned into tankers, to deliver propellant to payload-carrying Starship stages already in Earth orbit. If a stage can hold 1,200 tons of propellant, and a tanker stage can deliver 150 tons per flight, it would take eight tanker flights to refill the empty tanks. The most recent SpaceX images show the refueling being done with the stages side by side, rather than end to end. How the cryogenic propellant will be transferred rapidly still seems to be a proprietary secret, as the problem of rapid transfer of such propellant has been difficult to solve.

First Flights to Mars

A lot of the decisions on first cargo flights to Mars will depend on the timing: when the first cargo flights can be flown, and where the first landing site is located. The best guess now would be during the 2024 Mars window. However, use of such an early window would also depend on having a set of cargo items built and ready to be launched in just two years.

Cargo flights can be flown on Hohmann transfer orbits, the optimal path to save propellant, since there would be no crew on board and the cargo is not very sensitive to cosmic radiation. Such orbits end (a) directly opposite the sun from the starting point, but also (b) in the orbit of Mars and (c) right where Mars is located at that time. Current plans call for direct entry of the vehicles, using the Mars atmosphere to get rid of the bulk of interplanetary velocity before the last of it is zeroed during the landing by a combination entry and landing burn, very similar to what is done by Falcon 9 rockets. Such direct entry methods may depend on the positioning of radio beacons at the desired landing location for best landing precision, and/or a simple Mars GPS system, to allow use of active trajectory management during entry, but this would require another two years of delay.

The Hohmann transfer orbit to Mars.

The landing site must be proven to have millions of tons of water ice accessible relatively close to the surface, either by direct excavation or by drilling and melting. Otherwise no vehicles would be able to return to Earth from that site. Concepts for landing sites have been changing more rapidly as new sensor technology is applied to detect water ice deposits. The site will almost certainly be either along the equator of Mars or south of the 40th parallel north. The approximate location of the accessible “permafrost” boundary seems to be at about 32 degrees north, but recent detection of hydrogen in the floor of equatorial Vallis Marineris indicates there could be vast amounts of water ice under the glacial debris on the floor.

However, the first cargo vehicles to land probably will not return to Earth since they will have to sit on Mars for several years before crews arrive. It is unlikely that robotics will have developed to the point of being able to unload a ship and set up a propellant plant by late 2024. Thus the first crews to arrive at Mars, about two years after the first cargo flights, would have as one of their first major tasks the construction of the propellant plant and water ice extraction system. This would need to include the ability to store the accumulated cryogenic propellant, which is not as great a burden in the cold Mars climate, with a near vacuum surrounding the storage tanks. The water can be obtained by either direct excavation of ice or drilling, melting in place and pumping.

In addition to water ice, the site should if possible be selected for the availability of iron and other minerals which could be of near-term use to the first Mars development base, focused on the construction of a settlement. Compact equipment can be used to make pure iron from iron ore, which can be alloyed to produce steel on Mars. Mars rocks in some areas are essentially iron ore. NASA should make it a priority to try to find such sites with both water ice and useful minerals with its orbiting satellites and possibly dedicated landers equipped with real, rotary sampling drills.

The First Mars Bases

The first crew vehicles to land on Mars will also probably not return to Earth since they may be used as initial crew quarters. Crew flights to Mars will probably be at higher speeds to reduce the in-space transit time and thus the space radiation dose. The Mars atmosphere will easily absorb the extra entry velocity with appropriate thermal shielding. Both cargo and crew flights would need to land at a site at a safe distance of several kilometers away from the Mars base and be very careful to not overfly the base area on approach in case of a landing accident.

More cargo vehicles would probably be launched during the same window but well before the crew vehicles, and would be able to carry a large amount of basic equipment, along with food supplies for several years. Distributing the different kinds of supplies among several vehicles raises the chances of at least one landing successfully, enabling the crew to get right to work when it arrives. By 2026, a fleet of ten cargo vehicles carrying over 1,000 tons to Mars would be quite likely.

Shielding the crew from the cosmic radiation flux at the surface should also be a high priority as it would slowly degrade health and increase cancer risk if shielding is not provided. One concept for such shielding would be to use vacant space provided above and around the crew cabin areas inside the Starship stages after cargo is removed. Regolith—soil, sand, and rocks—is plentiful and free on Mars. It can be excavated, sifted, and poured into bags by an automatic system, and could also be lifted to the top of a Starship using the ship’s own elevator. The crew could manually move this regolith in bags into the space, providing the needed dense shielding. This is much easier in 38% gravity! Alternatively, this space could be filled with pure, filtered Mars water, but a much larger volume of water would be needed for the same amount of protection.

Once a larger crew has arrived, probably after 2030, underground or buried habitats can be installed, with just a few meters of regolith able to completely protect the crew from the space radiation. We would want to reduce the exposure from the expected surface dose of about 250 millisieverts per Earth year down to less than 50. (Mars itself and its thin air blocks about 60% of the slowly varying interplanetary dose of 500 to 700 millisieverts.) The surface dose might be even less if the landing site is in Vallis Marineris, due to the high canyon walls blocking some of the sky.

As later crews arrive, some of the early crews can return to Earth on Starship stages filled with Mars-derived propellant. Part of the rationale for many people to go to (or stay at) a developing Mars base and settlement is the attractive dynamics of participating in building a steadily expanding living space, rather than living on Earth in a more static social environment. Part of the planning would be in determining (as a ratio) how much larger the next crew, with a new crew arriving every 26 months, could be. Each existing crew would, in addition to other tasks, prepare living quarters and growth areas large enough to house, feed and support the next crew. This ratio could be called the ramp-up factor, but it would probably vary from year to year as the base starts to become more self-sufficient.

Partial Self-Sufficiency

Partial self-sufficiency depends heavily on two issues: energy production and food production, which itself depends on energy production. In addition, both depend on the ability to build industrial facilities to make fuel and materials, and to construct pressurized habitats to house crew and provide growing areas for food plants.

Much space will be needed to grow food crops on Mars.

Most people greatly underestimate the effort it will take to build growing areas and grow food crops on Mars or in space. On Earth, a one-square-kilometer (247-acre) farm gets a maximum of about a one gigawatt of sunlight on a clear day, at noon in midsummer. Much less than this gets to the plants due to clouds, etc., and the plants only use about 1% of what they get to make plant tissue, only part of which is actually edible food. To create one square kilometer of pressurized growing space will require a huge amount of structural materials, and most of that will need to be made locally. Even so, Elon Musk estimates that he will need to transport one million tons of cargo to Mars before a settlement is relatively self-sufficient.

It is important to realize how large the SpaceX cargo capacity to an operating Mars development base will be. Most NASA concepts envision barely enough mass—typically a few tens of tons—to support a crew for one short mission. The high SpaceX mass transport capacity will allow a large amount of industrial equipment to be sent. This would include equipment designed to smelt Mars minerals into metals, alloy them, and then to turn the structural metals into pressurized habitats, drill rigs, and other kinds of equipment. Large amounts of other artificial materials, such as plastics and polymers, will also be produced. Tunnel boring and lining equipment would also be included. Operations will be limited more by manpower than by lack of equipment and supplies.

Musk has a goal of building the large fleet of Starships needed to carry the required amount of equipment and supplies to get a settlement going. If an advanced Starship stage can carry 200 tons of cargo to the surface of Mars, 5,000 trips of such vehicles to Mars would be able to carry the one million tons. Ignoring the prior build-up phase, if he had 500 Starship stages with the tankers to support them, he would be able to transport that much during just ten Mars launch windows or in about 22 years. In actuality, the number of flights would be increasing from year to year, as the 500 stages could carry 100,000 tons during each window, and the existing crew would not be able to handle such a large volume of materials without a carefully planned ramp-up sequence.

In addition, the 500 stages would need a total of something like 500,000 tons of cryogenic propellant, stored and ready in tanks, to be able to take off from Mars and leave on a rapid return to Earth to be ready for the next launch window. To handle this, mass production of large fuel storage tanks from Mars metals will be required. If the fuel plant ran steadily for 25 months before each Earth return window opened, it would need to produce 20,000 tons per month, or about 67 tons per day. Some power will also be needed for cryo-coolers, even on Mars.

A Mars development base showing a fuel production area with direct excavation of shallow water ice and conversion to propellants. Cryogenic tanks are inside a covered depot for shade. (credit: Anna Nesterova)

The amount of propellant needed to get the cargo on its way to Mars from Earth is also very large. For each Mars window with a planned 500 payloads, several thousand tons is needed to launch each payload into Earth orbit, and perhaps 25,000 tons is needed via Starship-derived tankers to refill each cargo Starship. This is needed for the cargo stage to leave Earth orbit and enter the transfer orbit to Mars. The total launch propellant needed per window could be about 15 million tons. Settling a planet is a large-scale operation. Switching to lunar derived propellant accumulated at an L1 depot between launch windows is one way to massively reduce propellant use.

Although there are a wide array of different opinions on required areas for food production in space, estimates seem to center around 200 square meters of growing area with 100 kilowatts of power per person. That would mean a construction crew of 100 people would need 10 megawatts of power with 20,000 square meters of growing area for food production. If the growing areas are trays on four levels, the pressurized building area might need to be only 5,000 square meters, comparable to a football field. The amount of structure to keep this area pressurized, lit and at the right temperature, will be in the many thousands of tons, so production of structural metals and plastics from local Mars materials will be required rather quickly as the base and settlement grows. Thus the one-square-kilometer farm transferred to Mars (250 by 250 meters with four growing levels), could possibly support 5,000 vegetarians or about 2,500 people with some fish and meat production, needing about 500 megawatts of power. A large amount of power will also be needed for propellant production. The power would need to come from fission, fusion, or space solar arrays, as there are no fossil fuels on Mars, and erecting huge ground solar arrays in Mars weaker sunlight would be very difficult.

A Huge Benefit

If NASA is willing to cooperate with the SpaceX-led Mars effort, it can reap a huge scientific benefit, since direct human access to even one location on Mars would be of immense use to Mars science. Geologists would be able to investigate a huge area around the base or settlement with pressurized rovers, possibly discovering mineral site deposits of great use to the settlement. As Mars is a naturally cold-accreted planet, with ground water and volcanism through most of its geological history, it will have most of the common rock-forming minerals that Earth does, but not as many concentrated ore deposits and these may be hard to find.

As the settlement(s) grow, Mars will slowly grow in importance in our minds as a second abode of life beyond the Earth, and if the Mars settlers are persistent, they will be able to turn the planet into a second living world, even if much cooler and dryer than the Earth is. The settlers would have a very good reason to terraform Mars, as their descendants could then truly go outdoors without even wearing an oxygen mask. As Musk points out, space settlement is one way of improving the persistence of consciousness (and even intelligence) in the universe. Life on Earth is of immeasurable value, so a good backup location, outdoors on Mars, is certainly a grand goal for humanity to achieve.