How Elon Musk Plans to Take Humans to Mars and Beyond

A technical breakdown of the SpaceX Mars vehicle.

After much anticipation, entrepreneur and future Interplanetary Overlord Elon Musk has revealed the SpaceX plan to make human beings a multi-planetary species.

“History is going to bifurcate along two directions,” Musk said at a meeting of the International Astronautical Congress (IAC). “One path is that we stay on Earth forever, and there will be some eventual extinction event. The alternative is to become a space-faring civilization and a multi-planet species. I hope you would agree that is the right way to go.”

Of course, even if everyone on Earth agrees with Musk, going interplanetary is no easy task. The most optimistic estimates put the cost of getting to Mars at USD $10 billion per ton, according to Musk. The SpaceX goal is to reduce that cost by roughly four and a half orders of magnitude to around $200,000 per ton. If that were to happen, our children could find themselves debating whether to buy a house on Earth or pack up and move to Mars.

“It sounds virtually impossible, but I think there are ways to do it,” said Musk. “Most of the improvement would come from full reusability; somewhere between two and two and a half orders of magnitude. The other two orders of magnitude would come from refilling in orbit, propellant production on Mars and finding the right propellant.”

To see why these advancements are so crucial to the SpaceX plan, let’s take a closer look at the company’s proposed Mars vehicle.

The SpaceX Mars Vehicle

The SpaceX Mars vehicle consists of three components: the booster rocket, the spaceship itself and a fuel tanker. The idea is to launch the spaceship into orbit with the booster, which would then return to Earth. The tanker would then be launched by booster—possibly the same booster, if Musk’s estimate of a 20-minute return time is accurate—to rendezvous with the spaceship for orbital refueling. Once the fueling is complete, the spaceship would fire up its engines and be on its way to Mars.

Despite its lofty aspirations, Musk was initially modest about the Mars vehicle itself.

“In some ways it’s not that complicated, really,” he said. “It’s made primarily of an advanced carbon fiber. The carbon fiber part is tricky when dealing with deep cryogens and trying to achieve both liquid and gas impermeability and not have gaps occur due to cracking or leaking.”

According to Musk, the vehicle will use autogenous pressurizing, gasifying liquid fuel and oxygen through the heat exchangers on the engine, and then use the resulting gas to pressurize the tanks. “This is a much simpler system that what we have with Falcon 9, where we use helium for pressurization and nitrogen for gas thrusters,” he said.

“So you really only need two ingredients, compared to four for Falcon 9, or five if you count the liquid used to ignite the engines,” Musk added. “In this case, we’d use spark ignition.”

When it came to comparing the Mars vehicle to other spacecraft, Musk was more grandiose. The projected payload to low Earth orbit for the Mars vehicle is 550,000 kg, compared to 135,000 for the Saturn V.

“For most rockets, including ours, the performance bar is only a small percentage of the actual size of the rocket. But with the interplanetary system, we believe we have massively improved the design performance,” said Musk “It’s the first time a rocket’s performance bar will exceed the physical size of the rocket.”

The Raptor Engine

Musk stated that the Raptor engine is one of the most difficult elements of the spaceship’s design. “[It’s] going to be the highest chamber pressure engine of any kind ever built and probably the highest thrust-to-weight,” he said.

The engine uses subcooled liquid methane for fuel and subcooled oxygen as an oxidizer.

“When propellants are normally used, they’re used close to their boiling points in most rockets,” Musk explained. “In our case, we load the propellants close to their freezing point, which gives a density improvement of around 10-12 percent. It gets rid of any cavitation risk for the turbo pumps and it makes it easier to feed a high pressure turbo pump.”

According to Musk, the sea-level version of the Raptor will have an expansion ratio of 40, generating 3,050kN of thrust at a specific impulse of 334s. The vacuum version will have an expansion ratio of 200 and generate 3,5000kN of thrust at a specific impulse of 382s.

“One of the keys here is the vacuum version of Raptor having a 382-second Isp,” said Musk. “This is really quite critical to the whole Mars mission.”

The SpaceX Booster

“The rocket booster, in many ways, is a scaled up version of the Falcon 9 booster,” said Musk. “The big differences being that the primary structure is an advanced form of carbon fiber as opposed to aluminum lithium, and that we use autogenous pressurization.”

The booster stands at 77.5m tall with a 12m diameter. It is designed to incorporate 42 Raptor engines, only seven of which—the center cluster—will be fitted with gimbals. Musk stated that redundancy was a driving factor in this design, such that one or more engines can fail without crippling the booster.

The booster will use seven percent of its total propellant mass for its boostback burn and landing, though Musk noted that additional optimization could see that number reduced to six percent.

The Interplanetary Spaceship and Tanker

According to Musk, the most difficult engineering task for building an interplanetary spaceship is the liquid oxygen tank, which SpaceX plans to build from carbon fiber. “It’s only recently that we think carbon fiber technology has gotten to the point where we can do this without a liner inside the tanks, which would add mass and complexity,” he explained.

Like the booster, the spaceship has a series of stationary Raptor engines around its perimeter, with a small cluster of gimbaled engines in the middle.

The tanker has essentially the same design as the spaceship, though it uses the pressurized and unpressurized cargo areas for fuel. “They look almost identical, which will help lower development costs which, obviously, will not be small,” said Musk.

Full Reusability

The most important part of the SpaceX plan to reduce the travel cost to Mars explains why the company has been so intent on reusing its booster rockets for orbital missions. In his presentation, Musk used the analogy of air travel, noting the astronomical cost of a hypothetical flight on a Boeing 737 if the aircraft could only be used once.

At present, SpaceX is aiming to get 1,000 uses per booster rocket, 100 uses per tanker and 12 uses per ship. This is based on what Musk called “the rendezvous” between Earth and Mars that occurs roughly once every two (Earth) years, when the planets are as close to each other as possible.

“You get to use the spaceship part every two years, but you get to use the booster and the tanker as much as you’d like,” said Musk. “That’s why it really makes a lot of sense to load the spaceship into orbit with essentially tanks dry, and have very big tanks, and then use the booster and tanker to refill in orbit and maximize the payload of the spaceship so that when it goes to Mars, you have a large payload capability.”

Orbital Refueling

According to Musk, refueling the spaceship in orbit removes the need to build a 3-stage vehicle, as well as reducing its cost and size by five to ten times. By spreading the required lift capacity across multiple launches, the company is aiming to both reduce development costs and compress its launch schedule.

“[The propellant tanker] will go up multiple times, anywhere from three to five times to fill the spaceship in orbit,” explained Musk.

“Over time you’d have many spaceships, I think a thousand or more, waiting in orbit,” he added. “So the Mars colonial fleet would depart en masse. It makes sense to load the spaceships in orbit, since you have two years to do so, and then make frequent use of the booster and the tanker.”

Propellants on Mars

Finding the right propellant and producing that propellant on Mars are both essential parts of the SpaceX plan. In his presentation, Musk contrasted three candidates: kerosene (C₁₂H₂₂₄), hydrogen (H₂) and methane (CH₄).

Of these three, Musk favors methane, since kerosene cannot be produced on Mars easily without petroleum, and hydrogen is expensive and difficult to store. “When we looked at overall system optimization, it was clear to us that methane was the winner.”

This is because Mars has a CO₂ atmosphere and water ice in the soil, which is all you need to produce methane and oxygen. “The trickiest part is the energy source, but we think we can do that with a large field of solar panels,” said Musk.

I guess that explains Tesla’s interest in Solar City.

The SpaceX Mission to Mars

“I’m not the best at this sort of thing,” Musk quipped, in reference to the SpaceX timeline. Although he wisely avoiding giving any firm dates, he did project that the first development spaceship would be completed within the next four years and “if things go super-well,” the first SpaceX manned mission to Mars “might be in the ten-year time frame.”

Musk also made another commitment on behalf of the space company: “We’re going to try sending something to Mars during every Mars rendezvous from here on out. With every rendezvous, we’re going to send at least a Dragon to Mars.”

As it happens, the next Mars rendezvous isn’t until 2018, which gives SpaceX close to the maximum amount of time to make good on its founder’s promise. For his part, Musk emphasized his commitment to the goal of making humans a multi-planetary species.

“The main reason I’m personally accumulating assets is to fund this,” he said. “I really don’t have any other motivation for personally accumulating assets except to make the biggest contribution I can to making life multi-planetary.”

So what do you think? Will a SpaceX vehicle be the first to put human boots on the Martian ground? Comment below.