New supersonic transport aircraft: Fuel burn parity, or environmental parody?

Posted Friday, 28 September 2018, 08:42

In 2016, Sir Richard Branson, president of Virgin Atlantic Airways, announced that the Virgin Group would assist aviation start-up Boom in developing a new supersonic transport (SST) aircraft.  Boom aims to develop a 55-seat commercial jet capable of operating at Mach 2.2 with a design range of 4,500 nautical miles (8,300 km). The Virgin Group holds options for the first ten aircraft.

Earlier this year, we released a preliminary assessment of the environmental performance of new commercial SSTs using Boom’s design as a reference point. The analysis suggests that a representative commercial SST could burn 5 to 7 times as much fuel per passenger as comparable subsonic aircraft on common routes. When broken down by seating class, the SST burned about 3 times as much fuel per passenger compared to new subsonic business class. 

In response, Boom Supersonic founder and chief executive Blake Scholl stated that the group’s planes “will see overall fuel burn at parity” with current business-class travel. Mr. Scholl declined to provide further information about this calculation, which, as we pointed out, is influenced by factors such as 1) the subsonic aircraft type used; 2) stage length; 3) payload assumptions (passenger and belly freight); and 4) the business-to-economy multiplier.

Boom knows its aircraft best. Ordinarily we’d defer to manufacturers on performance claims for their aircraft. But in this case, Boom’s design is still pretty far from finalization – crucially, it has yet to identify an engine partner for its design – so even our educated guess adds value to this conversation. So let’s dig in further using the fuel burn of current business-class travel on Virgin Atlantic’s flights between New York-JFK and London-Heathrow as the baseline.

In our most recent transatlantic airline fuel efficiency ranking, Virgin Atlantic had better than average fuel efficiency in 2017, with an average of 36 passenger-kilometers per liter of fuel (pax-km/L) on the New York-London Route. So about 136 kg of fuel per passenger, or about one-quarter of the 600 kg we modelled for our SST reference aircraft estimated from Boom’s design.  

The carrier used four different aircraft on the route: (1) Airbus A330-300; (2) Airbus A340-600; (3) Boeing 747-400; and (4) Boeing 787-9. Three seating classes are offered on each aircraft: (1) Upper [sic] Class; (2) Premium Economy; and (3) Economy. The number of seats offered in each seating class depends on the aircraft model (Table 1).

Aircraft Model Upper Class Premium Economy Economy
Seat Count Pitch [m] Width [m] Seat Count Pitch [m] Width [m] Seat Count Pitch [m] Width [m]
Airbus A330-300 31–33 2.02 0.56 48 0.97 0.53  185 0.79 0.45
Airbus A340-600 45 38 225 0.75 0.44
Boeing 747-400 14 66 375 0.79 0.45
Boeing 787-9 31 35 198 0.79 0.45

Table 1: Seating Configuration of Virgin Atlantic Airways Aircraft

Based on the seat pitch and width for each seating class and the number of flights flown with each aircraft model, an Upper Class seat occupied 3.2 times the floor area of an Economy seat on an average Virgin Atlantic transatlantic flight last year. This is consistent with other estimates, for example this World Bank report that the average business-class seat takes up an average of 2.28 times more floor space than a coach seat on a widebody aircraft, with a calculated maximum of 4 times the space. 

This is the starting point for building a business-to-economy class multiplier since most researchers assume that the fuel burn of the aircraft can be apportioned as a function of floor space covered by a seat. But that’s not the final answer. The other part of the equation is the relative weight of the Upper Class and Economy passengers. If the combined weight of Virgin’s Upper Class passenger, their luggage, and lie-flat seat equals exactly 3.2 times that of an Economy passenger, than we’re already done. But that seems to be a stretch since it implies 480 kg in total compared to only 150 kg for an Economy passenger (100 kg for a passenger and luggage, plus 50 kg for their seat and share of other furnishings).

So we need to revise the multiplier downward to reflect weight effects. This gets tricky because publicly available data on airline weights are closely guarded industry secrets. Taking into account both floor space and weight effects, the World Bank methodology suggests that moving one Upper Class passenger on Virgin consumes about 2.6 times as much fuel as an Economy Class passenger. This is a bit more than recommended by the International Civil Aviation Organization (ICAO) carbon emissions calculator, which assumes that the carbon intensity of premium seating is twice that of economy seating.

Assuming that an Upper Class passenger’s carbon footprint is 2.6 times greater than an economy passenger, Virgin Atlantic burned an estimated 266 kg of fuel to move one Upper Class passenger between New York and London last year. Recall that our reference commercial supersonic burned 600 kg of fuel per passenger, or more than double that amount, on the same route. So not what one would call parity.  

And that’s comparing a new supersonic aircraft not to other new aircraft in the market in 2025 but rather to an older subsonic aircraft is service today.  Virgin Atlantic’s average aircraft age on transatlantic flights last year was 8 years, so Mr. Scholl’s approach is implicitly comparing a new aircraft in 2025 to one first delivered circa 2009.  

Now, there does appear to be one existing flight which the fuel efficiency of a new commercial SST is comparable to: British Airways (BA) Flight 1. That’s a rather unique one: an all business class Airbus A318 configured with 32 lie-flat seats used for weekday service between New York-JFK Airport and London-City Airport. In the westbound direction, the flight stops in Shannon, Ireland to refuel and have passengers pass through US Customs Preclearance. It’s also the replacement flight for the original Concorde, which was retired from service in 2003.

Flying without having to interact with economy class passengers may have its advantages, but fuel efficiency is not one. We estimated that BA1 has the same fuel efficiency as our reference SST at about 7 pax-km/L, compared to the overall average of 34 pax-km/L last year on transatlantic flights (Figure 1). 

fuel efficiency of subsonic and supersonic transport jets

Figure 1: Fuel efficiency on 2017 transatlantic operations, subsonic aircraft plus SST estimate

But this is an example of the exception proving the rule. Boom being able to match the fuel efficiency of BA1 would prove small consolation. Fuel is an airline’s largest operational expense, and the economic viability of new designs is directly tied to their fuel efficiency. Boom has ambitions to sell over 2000 aircraft. Being as fuel-efficient as one unusual, all-business-class commercial flight is unlikely to convince more airlines to sign on the dotted line for a new supersonic airliner.  It also raises the question of why Sir Branson, who talks passionately about the need to address climate change and was crowned by Time Magazine as a “Hero of the Environment,” continues to support supersonic development.   

(Figure 1 revised on 10/18/2018 to correctly depict the maximum takeoff mass of the Boeing 757-200 and 757-300 in tonnes, rather than thousand pounds.)