Monthly Archives: October 2017

Climate Change Impacts of Rocket Transportation

What would Elon Musk’s vision for flight-by-rocket mean for greenhouse gas emissions?

 

On September 29, Elon Musk unveiled another grand concept for the next generation of space travel and exploration, funded in part by a new ‘Earth to Earth’ rocket-travel industry. SpaceX’s vision is astounding. The idea of under-an-hour flights to anywhere in the world is almost as stunning as the reality that the technology may only be a few years away. But of course, professional carbon-counter that I am, I wondered about the greenhouse gas emissions of travel-by-rocket.

 

SpaceX’s plan uses one large multi-purpose, reusable rocket and booster system for all four phases of space travel and (eventually) colonization: Earth-to-Earth, Earth-to-orbit, orbit-to-Mars, and Mars-to-orbit. This “BFR” would be designed for up to 100 passengers and be capable of launching (and landing) a total payload of up to 100-150 tons. To do that, each launch would use up to 6500 tons of propellant – about 1400 tons of liquid oxygen and 5100 of methane. Once the rocket gets up to speed, it could easily reach anywhere in the world from near- or low-Earth orbit. Such as the 12,000 km direct flight from New York to Shanghai highlighted in SpaceX’s promotional video.

 

A very rough assessment shows that the climate change burden of manufacturing these propellants are about 0.15 metric tons of CO2 per ton of liquid oxygen, and 1.1 t CO2/t liquid methane. Combustion generates another 3 tCO2/t methane. Burning through the full capacity of the BFR, then, would release about 6500 tCO2 per trip. That corresponds to 65 tons per passenger, or 5.5 kg CO2 per passenger-kilometer for the New York to Shanghai flight. For comparison, a passenger on a full, 5500 km flight on a Boeing 737-800 would be attributed less than 0.1 kg CO2 per passenger-km. To look at it another way, a fully loaded 737 is about 25% fuel by weight, while a methane-oxygen fueled rocket must have at least 83% fuel by weight to reach orbit. Despite the shorter range of the 737, the sheer size of the BFR (or any rocket designed for a substantial payload) requires vastly higher fuel consumption by any metric. In fact, if a passenger on one of SpaceX’s flights wanted to make up for her carbon footprint by switching out her gasoline car for a Tesla, she would have to drive over 350,000 km – even assuming a relatively clean electric generation mix.

 

Now, as a multi-purpose launch vehicle, the BFR won’t be optimized for Earth-to-Earth travel. A smaller rocket might be much more efficient. What if we could build a 100-passenger rocket that weighed only 63 tons, as much as a 737? Assuming the same fuel mix as the BFR and the goal of reaching low Earth orbit, the launch vehicle needs to be 90% fuel. That gives us 565 tons of propellant per trip, way down from 6500. But our passengers are still on the hook for 6 tons of CO2 per trip, or 0.5 kg per passenger-km. That’s 2 to 5 times the emissions of an equivalent plane flight, and we haven’t even budgeted fuel for their landing!

 

It is possible to generate rocket fuel by capturing atmospheric carbon and using renewable electricity to liquefy oxygen. But while we’re inventing futuristic low-carbon technologies for space travel, it makes sense that we would use those same technologies for Earthbound transportation as well. Rockets are very, very cool, but they are not efficient.