Why the next generation of rockets will be powered by methane

One of the trickiest things to get right in space flight is the chemical properties of the propellant, and there’s a dizzying array of recipes. First you start with a liquid fuel that reacts with oxygen – of which there are several choices. Some require a second fuel that acts as an oxidiser, and then you need to get the ratios right.

The desired mix depends on the properties and parameters around your craft and mission – different needs demand chemical fine-tuning in the substances that propel them. The Saturn V rocket that carried Armstrong, Aldrin and Collinsto the moon used over two million litres of kerosene (also known as RP-1) and oxidiser LOX (liquid oxygen) – almost enough to fill a 50m Olympic pool.

But after a century of rocket fuel research that has looked at everything from RP-1 to hydrogen to paraffin, the industry is turning to a surprising new source – methane natural gas. One of the most abundant chemicals on our planet, methane is finally enjoying the spotlight.

And it could take us to Mars.

The first completed BE-4 rocket engine by Blue Origin. Credit: Blue Origin

A more efficient engine

Rocket fuel performance is measured by a property called the specific impulse, which is essentially how much momentum can be produced for every unit of fuel – a space version of miles per gallon.

Jake Teufert from US-based rocket builder Tesseract Space, said a methane/LOX combination offers around a five percent performance increase over kerosene/LOX when burned at the same pressure. “However, with methane, engines can be designed to run at much higher, more efficient pressures. When you factor in the increased efficiency, the performance benefit is more like 20 percent over kerosene.”

Those efficiency savings mean significant cost savings, making space travel far cheaper. With a higher specific impulse, the quantity of methane required for lift off is less, meaning smaller fuel tanks. There is also easier storage of the fuel before launch, and simpler and lighter fuel pumps on the rocket itself.

Methane also pressurises itself in its tanks by a process called autogenous pressurisation, which means complex and heavy systems pressurisation systems can be dispensed with.

Easier and cheaper to produce

However, that’s not the only way that using methane can make space travel cheaper.

Producing methane as a rocket fuel is far easier than refining kerosene for RP-1, says Patrick Neumann from Neumann Space, an Australian rocket propulsion company. “Getting all the gunk out of jet fuel is hard enough, but RP-1 needs even more work than that,” he says.

Teufert agrees. “The RP-1 grade of kerosene rockets run on is made in small, high price batches where methane rockets can readily use industrial grade LNG (liquid natural gas), which there’s a broad industrial base for.”

One major downside of swapping to methane fuels is a painful initial cost for the launch facilities, as they’ll need expensive new equipment to be able to support methane-fuelled rockets.

But that’s likely to be a case of when, not if. New players like SpaceX and Blue Origin are trying to turn a profit as much as their high profile founders are trying to shepherd humankind into a new age, something that might prompt the field to adapt and change faster than behemoths like NASA ever could.

The SpaceX Raptor engine during testing. Credit: SpaceX

Powering towards Mars

While the next generation of rockets and spacecraft won’t look wildly different on the surface, the biggest change is that we’ll get them back, give them a (metaphorical) hose down, and put them right back on the pad for their next mission.

According to SpaceX founder Elon Musk, reusing rockets could bring launch costs down to $90m versus billions spent on conventional rocket launches. So it’s not surprising that the two companies pushing the boundaries of reusability of rockets are also both developing methane rocket engines – SpaceX’s Raptor and Blue Origin’s BE-4.

When RP-1 fuel burns, it generates a lot of carbon, coating engine components with coke and limiting the potential for re-use. Cleaner burning methane engines are expected to improve that turnaround time and reduce launch costs even further.

But methane rockets’ future goes beyond just an expected improvement in reusability. In 2013 when the Raptor was announced, it was as a “highly reusable methane staged-combustion engine that will power the next generation of SpaceX launch vehicles designed for the exploration and colonisation of Mars.”

Methane factories on Mars could refuel rockets

Once Martian colonists reach the red planet they’ll find an atmosphere that’s over 95 percent carbon dioxide and plenty of underground water – both key ingredients in making methane and oxygen.

With the right technology, a colony could potentially become self-sufficient in its own heating and power needs and – given enough volume – fuel its own return trip back to Earth, maybe even further into the solar system thanks to highly re-usable engines. Mars becomes not just a destination, but potentially a stopover.

Meanwhile, Neumann thinks methane will be a cornerstone of the effort to get us there. “I reckon hydrolox (hydrogen and liquid oxygen) will have its place for the sheer amount of efficiency it provides, but methalox (methane and liquid oxygen) looks like it will be a strong contender for the fuel mix of the coming decade, at least as far as surface-to-orbit goes.”

“If the re-usability factor lowers cost so far it’s cheaper to do in two launches what hydrolox could have managed in one, why wouldn’t you choose it?”

And it may be getting closer than we thought, with a recent methane-powered Raptor test reaching launch power.

Along with smaller, more thrust-efficient engines and the big dreams of a small handful of jaw-droppingly wealthy men, this humble hydrocarbon might fuel the next stage of our relationship with the cosmos.

This article was originally posted on Australia’s Science Channel here.