So many of you have probably seen the plug Sploid did for Aerion’s AS2 supersonic bizjet, the latest tri-engined evolution from their original SBJ concept. Your reaction probably fell within one of two camps: this is incredibly awesome! or more likely nice vaporware. Then again, it has the backing of both Airbus and FlexJets so maybe it’ll become a reality. Either way, it’s kind of stupid. Here’s why.
There’s a reason why airlines gobbled up Boeing 747s instead of Concordes. Hell, there’s a reason why they’re gobbling up 777s and A330s and now 787s and A350s instead of A380s (and 747s). It actually turns out flying just below the speed of sound is pretty dang fast as it is. As much as Aerion wants to sell you on the idea of taking 8 hours to get to London being slow, think about that. In the time it takes for you to get through The Force Awakens spoilers and trailers and get paid for it instead of doing actual work, you would’ve covered the distance that before the age of the airplane would’ve taken a few days by ship. Airlines are pretty satisfied with how fast that is. Not only that, but aircraft manufacturers have gotten very skilled at making airplanes sip fuel (at relative rates) in order to bring you that speed, which is twin twin-engine airliners now dominate.
But let’s revisit the whole speed and fuel economy thing later. Let’s look at the Aerion AS2 from just a structural engineering standpoint. It’s very long - see those windows and how they’re stuck on to only a tiny portion of the plane? That’s the entire passenger compartment. The rest of the plane is all fuel, engine, and structural dead space necessary to make the aerodynamics works (kind of like Kris Jenner’s face at this point). That’s a big part of the reason why this plane can only carry 8 passengers, as opposed to something similarly sized - it’s hard for me to judge the scale exactly, but I’d wager it’s about the same size as a Bombardier Global Express which can be configured to be a flying party bus if you want.
This is incidentally the same problem the Boeing 2707 ran into. Because Boeing wanted to make what was essentially a Mach 3 767 (before the 767 was even designed) they had to adopt a lot of weird-ass engineering solutions and prioritize aerodynamic engineering over structural engineering to the point where they came out with a ridiculously long aircraft testing the state of aeronautical metallurgy at the time. So strained, in fact, that the project simply proved too complicated as airlines began to change their minds over paying for supersonic fuel bills and the project was canceled.
Image by Adrian Pingstone via Wikipedia
The secret to Concorde’s formula is that it was simple exactly where it needed to be. It only carried about the same number of passengers as an early 707 so it didn’t need to be any bigger or excruciatingly long (and on that note, do note that it’s still quite a long aircraft). It was designed to fly “only” at Mach 2.2 instead of close to Mach 3, so it didn’t need to be made out of super-exotic materials that were just being researched at the time. It used engines that were based on well-understood principles instead of trying to pull fancy tricks to make the impossible possible. The Concorde is solid Anglo-French engineering through and through, and it still couldn’t overcome the problematics of supersonic flight.
The Mach 2.2 cruise speed of the Concorde wasn’t just some arbitrary number the designers picked because it sounded cool. The near Mach 3 speed of the Boeing 2707 wasn’t picked just because it sounded even cooler and because it was faster than the Concorde (well, actually it kind of was but...). The Anglo-French team actually conducted extensive studies to find a best “sweet spot” cruise speed that would make the most sense and present the greatest justification for the engineering effort and fuel bills while still being technically feasible. They had done all of this research and came to this conclusion back when the Concorde was still the Super Caravelle and the British said “hey we’re doing the same thing let’s team up.” That conclusion being that Mach 2 was the minimum cruise speed necessary to truly justify an SST - and that naturally things become more efficient the faster you go assuming only minimal increases in fuel usage and engineering expenditures. The Mach 2.2 cruise speed of Concorde allowed it to reach London from Paris in 3 and a half hours - just fast enough to leave by morning and arrive by noon to conduct business. In comparison, the 5 and a half hours claimed by the Aerion AS2 would be fast enough to leave during the morning and...arrive just in time for supper. It might not seem like much of a difference, but it’s enough of a difference to question whether or not that extra speed is really worth it if it’s only saving you 2 to 2 1/2 hours - or just under the old Concorde’s total flight time (when factoring in its fastest record of under three hours).
Remember that Bombardier Global Express I mentioned? Using this as a reference, and factoring in the Global Express’ claimed cruise speed of Mach .85 (slightly faster than what the airlines fly) it will take just under five and a half hours. If you really need to hoof it, you can crank up the Global Express to its max speed of Mach .88 and get there in just a tad over 5 hours, or take a Gulfstream G650 to its max speed of Mach .925 and get there in under five hours. Using the same math and data the Aerion AS2’s Mach 1.5 will cover the same distance in close enough to exactly three hours, consistent with their claims compared to the baseline of airliner speeds at Mach .8. On the other hand, I’m not convinced saving the 1 hour 53 minutes is going to result in improved productivity compared to a G650, or even just booking a ticket on Expedia. In the digital age we live in you can either commute at the speed of light through Skype if you absolutely must “meet” a person that critically, or the economic situation will probably justify having you stay for the layover and just resume the next day. For reference, Concorde can cover the same distance in just a teeny tad over two hours, saving over three and a half hours compared to a Boeing 767 and one hour over the Aerion AS2. I’ll leave it up to you to decide whether that’s worth it or not.
The Aerion AS2 will apparently use three of a highly modified version of the good old Pratt & Whitney JT8D-215, a medium-high-bypass turbofan engine that was popular on the McDonnell Douglas MD-80 airliners and was considered very efficient and cutting-edge back in the late 80s through the 90s. That “medium-high-bypass” descriptor is a very important one in this case, because it means it has extra-large fanblades sticking out the front of the engine outside of the actual compression and combustion stages. The reasoning for this is multi-fold - for starters it pulls air around the actual engine itself (hence “bypass”) which helps aid in making it quieter (probably the biggest single reason why modern airliners are so relatively quiet now). It’s also akin to sticking a giant, multi-blade propeller in front of the engine, kind of like a turboprop - and like a turboprop, it also helps directly with propulsion, which in turn helps directly with efficiency. Unlike a turboprop it also aids directly with compressing the air that gets sent directly to the engine - it’s more or less literally like adding a turbocharger in front of your turbocharger (or for the more technically knowledgeable, exactly like multi-stage turbochargers, pretty much same exact principles in play).
All of that’s great for subsonic flight and it’s why jet engines are some of the most efficient means to move stuff around from a pure technical standpoint (to the point of putting them in ships now). For supersonic flight, not so much. Those bypass stages end up just getting in the way - the larger fanblades just block supersonic air coming in, create more drag, create a harder work environment for the engine, and in the end just become counterproductive by forcing it to suck down more fuel in order to overcome those aerodynamic hindrances. This is why supersonic military aircraft to this day overwhelmingly use low-bypass engines up to the F-22 Raptor and F-35 Lightning II - they represent a best compromise for great subsonic efficiency (where the plane will spend most of its flying existence anyway) and raw supersonic speed. It’s also a contributing factor why until recently (again with the F-22 primarily) supersonic combat aircraft were really only capable of supersonic dashes - sustained supersonic flight either burned up too much gas to make it anything but short or the strain of the operational conditions during supersonic flight would actually trash the engines.
The Aerion AS2, however, will spend most of its flying life in the supersonic regime, going subsonic only to take-off and land - that’s the whole point of it in the first place. In this case, it’s worth trashing the fan bypass stages altogether and do what the Concorde did, go with straight, pure and simple axial-flow turbojet engines just like what first-generation supersonic aircraft like the F-104 Starfighter had.
...which is why I’m confused as to their decision to use civilian-derived turbofan engines with a significant bypass section (unless they plan to yank the bypass section off).
It’s also worth mentioning that Concorde had planned updates to its engine that would’ve improved range and speed and essentially make it into a “half-turbofan” - but instead of adding a true bypass section, it would’ve increased the raw diameter of the existing compressor turbines and added additional turbines to the exit/combustion stage of the engine.
A few years ago a company called SAI floated around their Quiet Supersonic Transport, or QSST, that had official backing from Lockheed Martin (including having actually be designed by them). SAI, like Aerion, said all the right words and made all the right speeches to convince at least a few people they were legit, that this was going to be the return of supersonic greatness. I believe they even had a few orders signed on, maybe even from FlexJet again.
But then...nothing. They disappeared back into the vaporware ether, just like the bulk of the Very Light Jet market.
15 orders for jets is almost minuscule-small and might make only a small dent in terms of recouping R&D costs - in order to do that, Aerion will need to make hundreds of confirmed orders and sustain that for several years. 15 jets is a start, and it’s a launch all right, but it’s not exactly a guarantee that you’ll be able to whisk yourself over the Atlantic at Mach 1.5 so you can arrive at your destination 2 hours sooner than otherwise.