SpaceLiner: It is possible to fly from London to Australia in 1.5 hours with minimum environmental impact?
I am on an airplane right now travelling between two continents for my yearly holidays and have finally found the motivation to write this article. About 3 billion people travel by airplane every year, and this trend is increasing rapidly with the introduction of low-cost airlines. In the past, the world was mainly connected through ships, so tourism was reserved for those from a high-income class. Today, middle-income workers are able to enjoy frequent holidays in a different country or even a different continent. The airplane industry now provides a revolutionary advantage by reducing travel time and cost significantly. So, we must ask ourselves, what is the next revolutionary form of travel and how would it affect the environment?
Companies like SpaceX, Virgin Galactic and Blue Origin are pushing the boundaries of space travel like never before. Reusable rockets have been proven to work, and this sheds a light at the end of the tunnel for commercial space travel. The closest commercial space travel vehicle is the space liner, designed by the DLR (German Space Agency), and would consist of an orbiter and a booster.
DLR (CC-BY 3.0)
The booster is a reusable rocket that would put the orbiter, carrying 100 passengers, into orbit at 90 km of altitude. At this altitude, the air is so thin that drag is almost zero. Similar to satellites, once in motion it requires very little thrust to maintain that motion. Thus, the orbiter would require one burn to provide the initial thrust and achieve escape velocity, where it would glide the whole way through to the destination [1]. Hence, the orbiter can travel to the other side of the world by just gliding towards the final destination.
Typically, aircraft cruise at Mach 0.82 (where one Mach is equivalent to the speed of sound) to reduce the drag as much as possible and thus reduce fuel consumption and make more money. This could be witnessed in the 1960s [2], where aircraft would cruise at a higher speed but emit more carbon dioxide into the atmosphere. On the other hand, the space liner would be able to travel at an incredible speed of Mach 20, and thus require only 90 minutes to travel from London to Sydney, compared to 23 hours for conventional aircraft.
The environmental impact is yesterday’s major concern and today’s priority. Within the United Nation climate change conference meeting held in Paris last year, major reforms were set to limit man-made emissions to avoid catastrophic events. Today, the airline industry contributes 2% of the global man-made carbon dioxide emissions. Introducing space travel will lead to changes in those numbers and therefore, must be taken into account.
There are two main differences when it comes to the emissions of airliners and space liners. The first one is the emitted gas, which, for the aircraft, is mainly Carbon Dioxide (CO2) with relatively low Nitrogen Oxide (NOx), Hydrocarbons (HC) and Carbon Monoxide (CO). The space liner’s emissions are water since the rocket uses liquid hydrogen and oxygen as a propellant.
The second difference is the emissions altitude. The aircraft would have most of its emission at cruise (11,000ft), whereas for the space liner, the emission is at the beginning of the mission and spreads from ground level to 90 kilometres altitude. One might think that water emissions are harmless, and this would be true if they were emitted at ground level. However, this is not the case for the high altitude emissions, where water vapour would deplete the ozone layer thus affecting the radiative forcing and leading to temperature rise.
To be able to compare the impact caused by both methods of travel, the quantity of the emitted gases must be obtained. Thus a forecast of the airliner and space liner demand has been carried for 2050. The airliner is expected to see an increase in demand of 246% from 2005. In comparison, the space liner’s demand is 4% of that of the airliner in 2050. This is due to the very high cost of travel estimated at approximately $16,000 for a one way ticket from London to Sydney in 2050.
A good way to compare the impact caused by the emission is to estimate the AGTP (Absolute Global Temperature Potential), which predicts the earth’s temperature rise due to the emission. For one year’s worth of travel in 2050 between London and Sydney, it is estimated that the maximum AGTP is 2.6 and 1.6 µK for the space liner and airliner respectively. The lifetime of water vapour in the lower stratosphere is estimated to be 5 year whereas the carbon dioxide lifetime can reach 90 years. Therefore, the AGTP response due to airliner emissions would surpass the one caused by space liners sixteen years after the day of the emission. If the space liner was to replace the airliner, the temperature response due to the emission would exceed the one from the airliner significantly up to 100 years later than the day of emission.
Determining which method is more environmentally friendly at this stage would be bad practice as the accumulation of the emissions for several years has not been taken into account. Additionally, the effect of the temperature rise on the environment must be compared. The space liner offers a good alternative for a green hypersonic travel in the future. The production of liquid hydrogen can be obtained using electricity from solar and wind farms, thus limiting the emissions. One must think of new ways to improve future travel; perhaps a space elevator would be able to carry the orbiter into space and eliminate a significant amount of emissions, and thus reduce the environmental impact caused by space travel to an all-time minimum.
By Kinan Al Zayat
References:
[1] A. v. Foreest, “Trajectory Analysis and Preliminary Design of future Spacecraft for intercontinental rocket powered passenger flight,” DLR, 2005.
[2] https://slice.mit.edu/2014/03/19/airtravel/
Extended reference list used in the contributor’s original thesis project is available upon request
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