TO MARS AND BEYOND

CAN WE BECOME A MULTI-PLANET SPECIES?

Last June, NASA’s Curiosity rover detected surprisingly high amounts of methane in the Martian atmosphere. At 21 parts per billion of methane, it was nowhere close to the levels seen on Earth, but it was enough to have people ask the tantalizing question again. Is there life on Mars? On our planet, methane is produced by microbes called methanogens or ruminants like cows. However, it can also stem from geothermal reactions, which have nothing to do with biology or living organisms.

Adding to the mystery is the fact that this isn’t the first time methane has been detected on Mars. In 2013, the Curiosity rover detected a similar spike in methane levels, a finding corroborated by a new analysis of old readings from Mars Express, an orbiting spacecraft built by the European Space Agency. What’s interesting is that these surges in Martian methane appear to be seasonal, following the rhythm of the red planet’s seasons. Furthermore, it only takes a few centuries – a blink of an eye in geologic time – for methane to be broken down by sunlight or chemical reactions, which means that this methane is relatively new.

But as scientists scramble to analyze existing data and collect more precise measurements in order to determine the source of this methane, some of us on Earth are instead looking to take life to Mars itself. SpaceX CEO Elon Musk has gone on record as saying that an off -planet colony is humanity’s best hope of coming back from what he thinks is the high possibility of a nuclear war or some other cataclysmic event.

With NASA having fallen so far from the heady rush of the Apollo days and the Moon landing, it often seems like progress in space today is in the hands of billionaires and private space companies like SpaceX and Blue Origin.

Musk in particular is really impatient and eager for progress. He wants to be there on Mars in 2024, an entire decade ahead of NASA’s planned 2034 arrival. But is NASA just slow, or is Musk simply foolhardy?

A REALLY BIG STAINLESS STEEL ROCKET

Of course, there’s a giant rocket at the center of Musk’s plan. In September 2017, Musk unveiled the Big Falcon Rocket (BFR) at the 68th International Astronautical Congress in Adelaide, Australia. This was a two-stage design, featuring a huge booster powered by 31 Raptor engines and the spaceship itself. In fact, Musk thinks that we can build a sustainable colony on Mars by 2050.

Fast-forward to November 2018, and Musk was ready to christen the BFR with a new name, even though it didn’t – and still doesn’t – exist in any fully operational capacity. The part of the rocket that will actually carry people is now called Starship, while the rocket booster is referred to as Super Heavy. Starship may not be going to any stars in the near future, but ambitious as always, Musk has said that later versions will.

The current prototype of Starship is now built out of 301 stainless steel, a departure from the aluminum and carbon fiber that was originally proposed. Stainless steel doesn’t sound as advanced or even as light as carbon fiber, but it is also a lot cheaper and may even perform better. It also suits the purposes of the rocket quite well, since the type that SpaceX is using, which has a high chrome-nickel content, actually gets stronger in cryogenic conditions. It essentially has a high fracture toughness, so it can withstand small structural imperfections like cracks and prevent them from propagating.

But while you want your rocket to be able to withstand the cold in space, you also want it to be able to tolerate heat when it reenters the atmosphere. Stainless steel has a really high melting point, and it can go up to around 1,500°F.

The metal also allows SpaceX to work toward Musk’s goal of a regenerative heat shield. The rocket will need a heat shield anyway for reentry, but by replacing the tiles used on the current ship with something akin to a stainless steel sandwich, SpaceX can inject water into the space between the two layers. The water will absorb heat, and the shield will bleed water through micro-perforations to cool the windward side of the rocket, a process known as transpiration cooling.

Truth be told, the Starship prototype looks glorious, and its stainless steel hull looks like a vision straight out of 60s science fiction. If all goes according to plan, Starship will stand roughly 118m tall atop the Super Heavy booster. That’s taller than even the Statue of Liberty and would make for a really imposing sight.

JOURNEY TO MARS

Getting to Mars is a complex affair though. The Super Heavy booster will first need to power Starship and help it get to orbit, before returning to the launch site itself. Next, Starship needs to be refueled in orbit by multiple flights of a similarly sized tanker before it can head to Mars.

A BIT OF ROCKET SCIENCE

Starship’s ambitions also call for a completely new engine design. The Raptor engine is far more powerful than the existing Merlin units used in the Falcon 9 and Falcon Heavy. More importantly, while Merlin uses a mix of RP-1 propellant, a type of kerosene, and liquid oxygen, Raptor runs off  liquid methane, which would allow it to be refueled on Mars where methane is in abundance. Similarly, Blue Origin’s new BE-4 engine will also use methane.

The Raptor is built around a full-flow staged combustion cycle, which has only been used by two rocket engines so far, including the Soviet RD-270 and Aerojet and Rocketdyne’s integrated powerhead demonstrator (IPD) project. However, none of these have actually flown, so if all goes according to plan, the Raptor would be the first such engine to send a rocket into space.

Instead of having just a single preburner, the Raptor features one oxidizer-rich and one fuel-rich preburner. In modern rocket engines, small volumes of fuel and oxidizer are piped to the preburner, where the resulting reaction powers a turbine that in turn drives the pumps that send more fuel and oxidizer into the combustion chamber.

The reaction in the combustion chamber is what produces thrust, and it’s what enables the rocket to lift off . The full-flow staged combustion engine is generally considered to be the pinnacle of rocket design, where it allows for the most efficient use of its liquid propellants. To understand why, you first have to look at the history of rocket engine designs.

THE OPEN-CYCLE ENGINE

Some of the best known rockets, such as the Soyuz, Saturn V, and Delta IV, have used an open-cycle engine. The technology has been immensely successful, and it works. However, it had one major flaw. While the preburner is required to turn the turbine, you can’t use the same ratio of fuel and oxidizer as the engine, because the resulting exhaust would be too hot for the turbine.

Instead, you have to use a fuel-rich mixture, which brings its own set of problems like incomplete combustion, if you’re using a carbon-based fuel. This sooty exhaust cannot be recirculated through the engine, so it ends up being dumped overboard. That’s plenty of waste right there, and you may have even noticed this in pictures as a distinctive black plume next to the flery exhaust.

CLOSING THE CYCLE

That said, the last thing you want on a rocket is wastage, and Soviet and American scientists ended up solving two halves of the same problem. To get around the problem of incomplete combustion, the Soviets decided they would opt for an oxidizer-rich mixture instead, where all the oxidizer, but only some of the fuel, was shot at the preburner. Unfortunately, this created its own problem. The oxygen-rich gas produced by the preburner was now so hot that no metal turbine could survive it. Fortunately, they managed to solve this by creating a special alloy that could actually stand up against the heat. The cleaner exhaust produced by the oxidizer-rich mixture also allowed them to close the cycle and pipe the preburner exhaust – comprising hot gaseous oxygen – into the combustion chamber, where it reacts with the liquid fuel.

On the other hand, the Americans stuck with a fuel-rich mixture, but they swapped out carbon-based kerosene for hydrogen instead. However, the engine had to be adapted for hydrogen, which is significantly less dense than RP-1 or even liquid oxygen, requiring a larger pump to get the right amount to the combustion chamber.

The differing densities and pump sizes also meant that the engine now needed two preburners and shafts, one each for hydrogen and oxygen. The oxidizer still goes directly to the combustion chamber, where it meets the hydrogen fuel that has passed through the preburner.

Either way, wastage is minimized as the exhaust is recaptured and not simply thrown out.

PUTTING THEM TOGETHER

But what if you could combine the best elements of both designs? Put them together, and you get the full-flow staged combustion engine. You have the dualpreburner design that the Americans eventually put into the Space Shuttle’s RS-25 engine, except that there’s now independent oxidizer-rich and fuel-rich preburners. The oxidizer-rich preburner also means that a very strong metal alloy is needed to withstand the heat, and SpaceX developed something Musk calls the “SX500 superalloy”.

On the fuel-rich side, you can’t use RP-1 because of all that soot, which is where the liquid methane comes in.

You also have the freedom to pipe all the fuel and oxidizer you need through the preburners without worrying about wastage as in the open-cycle design.

In effect, the fuel is burned twice, once in the preburners and again at greater efficiency in the combustion chamber. Wastage is completely minimized and you end up making the most of the available fuel and oxidizer. Both of them also arrive in the combustion chamber as hot gas, which further improves combustion.

Finally, you have to worry less about the seals between the turbines and the pump, which was an area of concern on the RS-25. The last thing you wanted was for liquid oxygen to leak into hot fuel-rich gas, but there is little risk of liquid fuel leaking into already fuel-rich gas or liquid oxygen coming into contact with oxidizerrich gas, as is the case with the respective fuel-rich and oxidizer-rich preburners in a full-flow staged combustion engine. This improves the reliability of the Raptor design, which is crucial given that SpaceX expects to reuse the rocket.

PAINTING THE MOON

One of the first to set foot on Starship will be Japanese billionaire Yusaku Maezawa. Maezawa is also an avid collector of art, and as part of a project called Dear Moon, he will invite and pay for a group of six to eight artists to come with him to the Moon.

The artists will come from varying backgrounds, including music, film, painting, and fashion. He hopes that the face of Earth’s companion will inspire these artists to create unique works. “These artists will be asked to create something after they return to Earth, and these masterpieces will inspire the dreamer within all of us,” Maezawa said. That’s a lofty goal to be sure, and the billionaire was reportedly inspired by imagining what would have happened if his favorite artists had had the chance to visit the Moon.

The crew won’t be landing on the Moon though, and will instead fly around the satellite and return to Earth. The lunar flight is currently scheduled for 2023, although it remains to be seen whether SpaceX will meet that launch date or sail haplessly past it.