Why we don’t need an afterburner
What technological advancements will let the Boom passenger jet fly faster than the Concorde without afterburning?
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Concorde was designed 50 years ago with slide rules and wind tunnels; today we have the benefit of half a century of improvements in aerodynamics, materials, and propulsion. These technologies combine to enable an aircraft that is both faster and more efficient than Concorde. Today, we can make a supersonic aircraft efficient enough to match business-class pricing (not yet economy, but that’s where we’re headed over time).
The Boom engineering team made a brief video outlining the key improvements versus Concorde:
Concorde was originally spec’d for Mach 2.2 flight, but was reduced to Mach 2.0 for thermal reasons. The faster an aircraft flies, the higher the effective ambient temperature (see Stagnation temperature — Wikipedia). Temperatures increase quadratically — at Mach 2.0, the nose and leading edges can reach 242°F. At Mach 2.2, temperatures reach 307°F (and up to 350°F on an especially hot day) . At Mach 3, temperatures skyrocket to 632°F. The aluminum alloys available to Concorde’s designers couldn’t handle the high temperatures required at Mach 2.2, but today’s carbon-based material systems can handle temperatures well above those encountered at Mach 2.2. Speed is no longer limited by materials.
Additionally, improvements in aerodynamics and propulsion enable faster speed. Compared to Concorde, the Boom aircraft is much more dynamically shaped. For example, the fuselage is “area ruled,” meaning cross-section area is carefully controlled to reduce disturbances to the surrounding air. Concorde’s designers knew of this principle, but it was impractical to realize in aluminum. Since carbon composites are molded, it is possible to create a strong and lightweight structure in any dynamic shape desired.
As the question points out, Concorde used an afterburner to break through the high-drag transonic barrier. Besides being loud, afterburners are horribly inefficient. During takeoff, Concorde’s afterburners increased fuel consumption 78% while adding only 17% in extra thrust. Today, we have vastly improved turbofan engines, which can produce enough thrust for supersonic flight while also being quiet and friendly around airport communities.
You might wonder, why Mach 2.2? Why not faster, why not slower? We chose 2.2 because it is fast enough to make a massive difference in travel, yet slow enough to require only known, proven basic technologies.
The speed limiter at Mach 2.2 is related to propulsion and noise regulations: an aircraft needs to be fuel efficient at supersonic cruise yet quiet around airports. To be quiet, large-diameter engines with big fans (high bypass ratios, see Bypass ratio — Wikipedia) are desired. Yet, the faster one cruises, the lower the desired bypass ratio, for increased specific thrust and lower cross-section area. At Mach 2.2, the Boom aircraft can comply with noise regulations and be sufficiently efficient at cruise, thanks to a medium-bypass engine, adapted from the same technology that powers subsonic wide body aircraft. To achieve higher speeds without a significant increase in fuel consumption, a Variable cycle engine would be required — a technology too complex and unproven for a startup to adopt in its first product.
[EDIT: 11/20 — clarified that the Boom jet uses medium-bypass turbofans, not literally the same high-bypass turbofans on other aircraft. Also clarified that the tech is enough for business-class pricing, not yet for economy.]
— Blake Scholl (CEO/Founder)
Originally posted on Quora
