Boom Technologies

Boom technologies, headed by engineer/entrepreneur Blake Scholl, and backed financially by names such as Jeff Bezos (Amazon) and Richard Branson (Virgin), is shaping up to be quite simply the single greatest hope in decades to build a new civilian SST today.
For the various ‘other SST’ pages on this site, I intend to give a candid review of up-and-coming concepts with regard to their technical and financial feasibility, from the perspective of an engineer who was involved in the design of the last fastest civilian aircraft, the Citation X+.

To begin, the first thing you notice is that the planform of the Boom SST is a clear tribute to the classic delta wing silhouette(s) of Concorde, or Tu-144, the Delta Dagger, the B-58 Hustler, the Saab Draken… I could go on. The similarity is hardly accidental.
The delta shape is one which epitomises and screams ‘speed’, and yet, it is sometimes neglected that this is actually somewhat of a compromise. The arrowhead shape of the delta wing lends itself well to the supersonic domain because it shrinks frontal surface area by reducing wingspan, while maintaining wing surface area. It also ‘tucks’ the leading edge of the wing neatly behind the shockwave angle.
The effect of the delta wing on the flight envelope is well understood and well documented – the most noticeable effect by far is the very high angle of attack needed for a delta winged aircraft to fly at low speed. This creates enormous levels of drag, and is necessary because a delta wing utilises ‘plate lift’ and ‘vortice lift’ rather than bernoulli-effect lift like a typical straight/swept wing with a shaped aerofoil.
The net impact of the choice of the delta wing is that low-speed performance is compromised, mostly impacting the take-off and landing flight regimes, but this is a relatively small price to pay for the enhanced performance achieved once at high altitude high speed cruise – Boom are onto a good thing with this general scheme, but have evidently decided a ‘droop’ nose isn’t necessary. (The Fairey Delta 2, Sukhoi T-4, Tupolev Tu-144 and Concorde warn otherwise)

The next thing to discuss, are the engines. There are three of them – two underwing in Concorde-like pods, and one in the tail with dual NACA-style inlet ducts either side of the tailcone. At time of writing these engines haven’t been selected yet. (See my ‘powerplant’ page for the issues surrounding this difficult problem)
Blake Scholl is quoted as saying that the Boom SST will use non-afterburning medium-bypass turbofans.

Oh dear. *facepalm*

The problems with this assessment are manyfold. This has to do with the fact that ingesting, compressing and accelerating supersonic air has to be done in a single stream of gas to not result in unacceptable drag losses. Consequently, supercruise capable aircraft always use zero-bypass turbojets, typically afterburning for acceleration and usually also with variable geometry inlets.
The special exception perhaps, would be the GE J58 engines used on the SR-71 blackbird, which accomplish some core bypass functionality with its variable inlet spike to act partially as a ramjet – but no fan is involved.
 The large diameter fan of a typical turbofan engine is producing most of the thrust on a typical subsonic airliner or business jet – but if you feed that fan with a supersonic stream of gas, it might as well be a solid wall. Turbofans stall above Mach 1, because the fan itself blocks the ingestion of the supersonic gas stream – it’s simply the wrong tool for the job.
Blake will have to choose another engine.

Finally, the currently available technical literature on the Boom SST suggests that it will make extensive use of lightweight materials like the Boeing 787 or Airbus A350, notably composites.
The thermal duty cycle of an aircraft cruising above Mach 2 necessitates some resistance to heat – composites are known to soften and lose shape memory at high temperatures, and are not isotropically conductive – they conduct heat only in the direction of their weave pattern, leading to uneven expansion and cracking over time.
Concorde was aluminium, but it operated at the limit of what the material could thermally withstand, the next option was heavy but durable stainless steel as used on the Bristol Type 188 or XB-70 Valkyrie. The SR-71 with its Mach 3 top speed used Titanium. The Space shuttle used ceramic heat shields for the ultimate thermal extremes. To try to build a composite skinned aircraft for a 300 degree Celsius duty cycle, is folly! (Unless, a high temperature composite has already been discovered and they intend to use that – Boom’s website suggest they have high confidence in their composites surviving the high temperature duty cyle)
All that said, the Boom PR campaign looks to be very sucessful, the venture capital is flowing, and the concept art looks fantastic. Blake Scholl’s project will hopefully not be a Moller Skycar… a bottomless pit of money that never gets off the ground. Bezos and Branson together have both bankrolled massive leaps in aerospace technological progress, hopefully Boom will be the same, and any technical problems that they encounter such as those mentioned above… can always be solved with more money.