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.

The past few months have been an exciting time for renewable energy proponents in the US.  The first three commercial cellulosic ethanol facilities began operating in August and September, a major milestone for a technology jokingly thought to be “five years away” for almost three decades. Rather than converting corn grain to ethanol, these convert inedible corn stover – the stalk, husk, and cob. Biorefineries in Emmetsburg and Nevada, Iowa and Hugoton Kansas will have a combined capacity of 75 million gallons of cellulosic ethanol per year – more than a 300-fold increase from the total US production of 213,000 gallons in 2013. Although the US still lags behind the targets set by the 2007 Renewable Fuels Standard, which called for 100 million gallons in 2010 and 1.75 billion gallons in 2014, the Congressional Research Service reports that the EPA expects to meet this year’s proposed revised target of 17 million gallons of cellulosic biofuels.

Major news outlets have taken note of this new commercial technology, which allows biofuel production without competing with food crops.  The New York Times reports that most of the ethanol produced by Abengoa’s facility in Hugoton, KS may be destined for California, where more stringent fuel standards may generate demand.  Reporting on POET-DSM facility in Emmetsburg, IA, The Seattle Times emphasizes that the current production cost of cellulosic ethanol from these facilities ($3/gallon) is substantially higher than corn ethanol ($2/gal). The current production cost is also higher than predicted by a widely-cited report by the National Renewable Energy Laboratory (NREL) of Colorado, which anticipated a selling point of just over $2/gal based on a sophisticated process and economic model.

Though the cost of ethanol from these second-generation plants is expected to drop, the high price and saturated ethanol market in the US (nearly all gasoline already contains 10% ethanol – primarily produced domestically from corn grain – the maximum amount legally permitted to be blended for car fuel) have driven cellulosic ethanol manufacturers to seek additional markets. Abengoa is targeting beverage companies, who could use ethanol to produce a “green” alternative to plastic and reduce their carbon footprint. According to Bloomberg, DuPont’s 30 million gallon/yr facility in Nevada, IA may sell ethanol to Proctor & Gamble for blending into Tide detergent.  Both Abengoa and DuPont expect to license process design, enzymes, and other technologies for these facilities to other prospective cellulosic ethanol producers.

Abengoa’s biorefinery in Hugoton is the most capital-intensive of the new plants at $500 million.  Though it’s maximum capacity (25 million gal/yr) is not greater than POET-DSM ($275 million, 20 million gal/yr) or DuPont ($225 million, 30 million gal/yr), it features an on-site power plant to convert waste biomass to heat and power.  In addition to powering the biorefinery, the power plant is expected to generate a 10 megawatt surplus for sale to the regional electric grid.  The Hugoton facility will also incorporate new feedstocks as it becomes fully operational, including wheat straw and switchgrass.

Finally, I am pleased to note that concerns over biomass storage featured prominently in Abengoa’s decision to site their cellulosic biorefinery in Hugoton, where the climate is expected to be dry enough to store feedstock outdoors without cover, thereby reducing costs.