Beyond the Speed of Sound: The Future of Ultra-Fast Passenger Travel
Today, aerospace startups and giants alike chase ambitious dreams of passenger flights that exceed the speed of sound, connecting continents in mere hours or even minutes.
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The charm of supersonic passenger travel didn't end with Concorde's retirement in 2003. Today, aerospace startups and giants alike chase ambitious dreams of passenger flights that exceed the speed of sound, connecting continents in mere hours or even minutes. But what does this resurgence mean for venture investors?
By the way, it’s no coincidence aviation shows up in our spacetech blog — ultra-fast passenger travel is exactly where aero and space intersect in a meaningful way.
From Concorde’s Lessons to Modern Ambitions
Concorde, cruising at twice the speed of sound (Mach 2), ultimately failed economically due to excessive operating costs, poor fuel efficiency, and regulatory restrictions on overland supersonic flights. Today's aerospace startups argue that new materials, advanced propulsion, and sustainable fuels can overcome these historical challenges.
The speed of a vehicle in flight is usually expressed as a Mach number: the ratio between its true air speed and the local speed of sound (≈ 343 m/ s at sea level). Speeds between Mach 1 and Mach 5 define supersonic flight, while Mach 5 and above is considered hypersonic. Higher speeds generally require higher altitudes because thinner air reduces aerodynamic drag and heating.
At the extreme end, rockets avoid drag almost entirely once they are above ~50 km. Ballistic sub‑orbital (“Earth‑to‑Earth” or “Point‑to‑Point”) trajectories, like SpaceX’s Starship concept, momentarily reach ~ Mach 20 outside the sensible atmosphere before re‑entering on a purely ballistic arc. Including ballistic rockets as the “limit case” is therefore useful: they show what happens when you remove all cruise drag but pay a very large propellant cost during ascent and landing.
The Technology Challenge: Engines and Speeds
High-speed travel introduces significant technological challenges.
Modern high‑bypass turbofan engines used by commercial aircraft move a large mass of “slow” air for high propulsive efficiency. The bypass stream goes to the fan, and may be 10 kg of air for every 1 kg through the core, where it mixes with the fuel and burns. Above about Mach 0.95, that big and slow fan becomes a wall of drag, and the fan blades encounter supersonic flow. Therefore, engineers revert to much smaller, low‑bypass cores or entirely different cycles (ramjet/scramjet) that work only when the vehicle itself compresses the air thanks to its high speed:
The Economic Equation: Efficiency vs. Speed
In recent decades, aviation prioritized fuel efficiency over speed, transitioning to turbofan engines with increasingly higher bypass ratios. Higher bypass means engines use a larger amount of air relative to fuel combustion products, significantly improving fuel efficiency (specific impulse) at subsonic speeds but unsuitable for supersonic travel.
Consequently, both supersonic and hypersonic travel naturally require more fuel per passenger-mile compared to subsonic flights, leading to increased operating expenses and environmental impact. Quantitatively, the picture looks like this:
A Boeing 787‑10 in dense layout consumes 1.3 kg/100 km/passenger. The corresponding specific impulse Isp is ~ 12,000 s. Let us remind that Isp is inversely proportional to fuel used per unit of time. Compare it to a typical chemical rocket engine (Merlin) with its 350 s. The high specific impulse of Boeing 787 does not mean that it translates to 120,000 m/s (10x of Isp) exhaust speed, as it would be with a non-air-breathing rocket engine. It means that even with moderate exhaust speeds (417 m/s), thanks to the air trapped by the fan, fuel is a small fraction of the overall exhaust. Pushing extra mass of ambient air instead of hot exhaust cuts fuel consumption significantly;
Concorde managed ≈ 10.3 kg/100 km/passenger of fuel consumption. The corresponding specific impulse is 3,000 s;
Hypersonic aircraft have even lower specific impulse than Concorde, somewhere in the range between 1,000 and 2,000 s;
From the point of view of Isp, rockets perform the worst of all, about 10x worse than supersonic aircraft and 35x worse than subsonic aircraft. But rocket-based ballistic travel concepts like SpaceX's Starship differ economically. Though rockets have an order of magnitude smaller specific impulse and use substantial fuel at launch, they require no fuel during the ballistic trajectory, experiencing minimal air drag at extreme altitudes. So, which side of the tradeoff will prevail: the massive fuel consumption caused by engine inefficiency, or the potential fuel savings from smooth, low-resistance ballistic flight through space? Detailed analysis suggests Starship’s fuel efficiency per passenger-mile could be near 30 kg/100 km/passenger of propellant (methane and liquid oxygen combined), comparable with hypersonic jets, potentially making rocket travel economically viable within luxury and premium tourism segments.
Dramatic fall of fuel efficiency with increasing Mach number. Isp is inversely proportional to fuel used per unit of time. Image credit: Kashkhan.
More fuel per seat‑km means higher CO₂ if sustainable aviation fuel (SAF) is scarce. Supersonic NOₓ and water vapour are emitted directly into the lower stratosphere, affecting ozone and radiative forcing. Methalox rockets also inject large quantities of H₂O and NOₓ at >30 km.
As a result, economics forces very fast travel into high‑yield niches—premium business time‑savers, luxury tourism, and government/military missions, rather than mass economy travel.
Ambition vs. Reality: Market, Pricing, and Regulation
Boom Supersonic projects ticket prices for its Mach 1.7 "Overture" airliner around $4,000–$5,000, comparable to current transatlantic business-class fares (~6,000 km). However, initial pricing realistically could surpass $10,000 until operational economies of scale are achieved.
The global intercontinental aviation market is enormous—approximately 400 million passengers annually, generating around $600 billion in revenue. Yet, ultra-fast travel initially targets only 2–5% of this market: high-value business travelers and ultra-premium leisure travelers willing to pay premium fares.
Despite supersonic passenger travel could capture a $30–60 billion market by the 2030s, substantial funding gaps remain. Boom has secured roughly $700 million but requires billions more for aircraft certification and mass production.
Regulatory hurdles also remain significant. The current FAA ban on supersonic flights over land restricts market potential to transoceanic routes. NASA’s ongoing X-59 QueSST program aims to demonstrate quieter sonic booms, potentially changing regulations by the late 2020s and opening additional routes.
Comparative Snapshot of Key Companies
Investing in ultra-fast passenger travel involves substantial risks but significant potential rewards. Early success hinges on overcoming technological, regulatory, and environmental barriers. Realistically, commercialization remains at least a decade away; initial market niches will likely be premium business and luxury travelers. Dual-use strategies (civilian and military applications), as employed by Hermeus, may mitigate initial investment risks.
While breaking the sound barrier again captivates our imagination, prudent investors must be prepared for a long journey marked by innovation and considerable turbulence.
Great stuff. Keep up the good work.
You need to distinguish fuel and propellant more clearly. Liquid oxygen is very low in price; it's the second cheapest industrial liquid after water.