Environmental performance of emerging supersonic transport aircraft
Weak supersonic regulation for me, but not for thee?
Recent years have been heady for aspiring supersonic manufacturers. What started as a clever play on words urging the nascent Trump Administration to Make America Boom Again morphed into Congressional support for reviving supersonic transport (SST) aircraft in 2018. That translated into a stalled push for international environmental standards to support their development, and, last March, an FAA proposal for a domestic landing and takeoff (LTO) noise standard for supersonic aircraft.
The FAA’s Notice of Proposed Rulemaking (NPRM) is a result of the 2018 FAA Reauthorization Act, which required the agency to propose, but not necessarily finalize, an LTO noise standard by March 2020. In its NRPM, FAA concluded that it “needed to create a new category of noise certification in order to enable a new generation of supersonic airplanes” and proposed weaker SST noise limits than for new subsonic aircraft (Stage 5 in U.S. parlance). The proposed Supersonic Level 1 standard would cover new SSTs up to 150,000 lbs maximum takeoff weight (MTOW) and a design speed of Mach number 1.8 (MN 1.8), or 180% the speed of sound.
The comment period for FAA’s rule closed on July 13th, so we now have a window into what various groups think of FAA’s proposal. The responses show that Congress’s enthusiasm for SSTs is not widely shared. In public comments submitted, FAA’s proposal was heavily criticized by individuals and citizen groups impacted by airport noise, major airports, environmental groups, and European governments that oppose further SST development. Trade associations representing European (think Airbus) and Canadian (think Bombardier) manufacturers also oppose the U.S. go-it-alone approach. (ICCT also submitted comments, which can be found here.)
Collectively, these comments highlight that the purported “inevitable” supersonic renaissance is actually quite contentious, in part due to environmental concerns that FAA does not address in its proposal. These include evidence that emerging commercial SSTs may emit 5 to 7 times as much CO2 per passenger as comparable subsonic aircraft on common routes, and that unconstrained SST deployment could expose parts of Europe and North America to sonic boom as frequently as once every five minutes.
As a researcher, it was interesting to review the industry comments and attempt to deduce what individual manufacturer strategies might be. Due to commercial confidentiality, the major aerospace players have been tight-lipped about their SST designs. But FAA’s proposal required them to carefully spill some beans, for some pretty interesting reasons.
Airplane manufacturers require as much clarity as possible on what environmental standards a new aircraft will need to comply with before large investments can be made. Furthermore, an FAA rule under the sympathetic Trump administration provides an opportunity to lock in a weak regulatory touch. But aircraft manufacturing is a competitive business, so these same firms have an incentive to deny such advantages to their competitors. So weak regulation for me, but not for thee. Thanks to these motivations, we can get a ballpark sense of what future SSTs from manufacturers might look like based upon their comments on the proposal.
Aerion, which is furthest along in commercial development, is supportive of the rule, but proposes applying the standards to designs only up to MN 1.4 and increasing the weight limit to cover heavier designs. The proposed speed nicely fits Aerion’s designated cruise speed but the weight limit increase suggests that its design may be getting heavier in the design process (see below). Aerion’s potential competitor Gulfstream supports this lower speed limit but also FAA’s weight limit of 150,000 lbs, implying that, like Aerion, it is exploring relatively slower supersonic business jets and hopes to develop an aircraft around the weight of Aerion’s original design (133,000 lbs MTOW).
That leaves us with Boom Supersonic and heavyweight Boeing, which recently spun down its engineering collaboration with Aerion to reduce spending during the COVID-related economic downturn. Boom is the big loser in FAA’s proposal because its faster (MN 2.2) and heavier (170,000 lbs MTOW) Overture aircraft is not covered by the proposal. Therefore, Boom gets no regulatory certainty to justify further investments in its aircraft. Accordingly, Boom has asked to be regulated within 2 to 5 years with a second rule. Boeing voiced support for FAA’s proposed noise and MN thresholds but says that they are also evaluating heavier and faster potential SST designs.
By assembling all of these bits of information together, we can estimate the weight and speed of each manufacturer’s aircraft, which is shown in the figure below. This approach, while rough, was validated by Virgin Galactic, which after asking what standards would apply to aircraft faster than MN 1.8 suddenly revealed preliminary work on an MN 3.0 design. Boeing’s range is speculative but based on the logic that as a large commercial jet manufacturer it would pursue designs more like Boom’s and less like business jet manufacturers. Note that Lockheed Martin, which is developing the X-59 Low Boom Flight Demonstrator, doesn’t weigh in on the weight or MN coverage of the rule and so is not included in this graphic.
So, where do we go from here? The FAA will next decide whether to finalize this rule or to defer to the ICAO rulemaking, as a large majority of aviation stakeholders have urged it to do. Deferment seems likely given that an international standard will be needed to unlock most of the market; for example, Boom argues that 96% of its deliveries may be to overseas operators.
Meanwhile, the declared SST manufacturers, along with their engine partners—General Electric for Aerion and Rolls Royce for Boom and Virgin Galactic—continue their development work as the large U.S. airframers (Gulfstream, Lockheed Martin, and Boeing) remain coy regarding their plans. Notably, all declared manufacturers have announced their intention to develop SST airframes and engines that meet the stricter Stage 5 subsonic noise standards that FAA rejected in its proposal. (An aside: in a true head-scratcher, GE’s comments argued that that it’s not technologically or economically feasible for SSTs to meet those standards despite the fact it’s explicitly developing an engine that can. Go figure).
So, FAA’s proposal may end up being overtaken by international standard-setting and the ambitions of SST manufacturers themselves. And, it’s likely that ICAO’s rules will follow subsonic noise limits and therefore be more stringent than is outlined in the FAA’s NPRM. So, in the end, weak regulation for neither me nor thee?