Sustained hypersonic flight above Mach 5 has been a hard aerospace engineering problem for decades. You can strap a rocket to something and push it to hypersonic speeds, but rockets carry their oxidiser onboard. That limits both range and flight duration. Air-breathing engines solve this by compressing atmospheric oxygen, enabling extended hypersonic operations.
Scramjet engines—supersonic combustion ramjets—take this to the extreme. They work at Mach 5 and beyond, with no moving parts. Hydrogen fuel gives you energy density that kerosene can’t touch (142 MJ/kg versus 43 MJ/kg) and produces zero CO2 when it burns.
This article explores the technical foundations of scramjet propulsion as part of our broader analysis of deep tech and defense innovation.
Recent progress in materials science, 3D printing, and thermal management has pushed the envelope to Mach 12. Australian startup Hypersonix raised $46 million in Series A funding in October 2025 to advance their SPARTAN engine, which demonstrates hydrogen-powered scramjet technology reaching these speeds. The engine design draws on over 6,000 shock tunnel experiments conducted at the University of Queensland.
Understanding how these systems work—the physics, the materials choices, the manufacturing approaches—gives you insight into where aerospace innovation is heading.
What is a scramjet engine and how does it work?
A scramjet compresses incoming air at hypersonic speeds using only aerodynamic forces. No compressor blades, no turbines, no moving parts. The vehicle’s speed does the compression work.
Three sections make up the engine: an inlet that compresses air through shock waves, a combustion chamber where fuel burns in supersonic airflow, and a nozzle that accelerates the exhaust to produce thrust.
Air enters at Mach 5 to 12, stays supersonic—typically Mach 2 to 3—in the combustion chamber. Fuel injection and combustion happen in under a millisecond.
The key difference from other jet engines: scramjets only work at hypersonic speeds. They need a rocket booster or carrier aircraft to reach operational velocity.
Think of it as a flying stovepipe. Air enters, compresses, burns, exits.
A ramjet slows incoming air to subsonic speeds before combustion. That works from Mach 3 to 6, but beyond Mach 6 the pressure losses destroy efficiency. Scramjets avoid this by maintaining supersonic flow throughout.
Hypersonix’s SPARTAN engine demonstrates this in hardware. The Australian startup’s approach combines inlet geometry validated through extensive ground testing with combustion chamber design optimised for hydrogen fuel.
How does supersonic combustion differ from regular combustion?
Regular combustion in subsonic engines allows fuel and air to mix over seconds or minutes. Supersonic combustion completes in under a millisecond.
In a typical jet engine, air slows to near-zero velocity. Scramjets maintain Mach 2 to 3 airflow through the combustor. The supersonic flow prevents flame from travelling upstream—a phenomenon called flashback that would destroy the engine.
Fuel injector design becomes critical. Hydrogen must penetrate supersonic crossflow and mix rapidly. The extreme temperature and pressure causes autoignition—no spark plugs needed. But the short residence time limits fuel burning. Combustion efficiency typically hits 70 to 85 per cent, compared to 98 per cent in subsonic engines.
Thermal management adds complexity. Inlet temperatures reach 1,000 to 1,500 degrees Celsius. Peak flame temperatures hit 2,000 to 2,500 degrees.
NASA is developing cavity flame holder technology to reduce combustor length by 25 per cent. These are recessed pockets in the combustor wall that create subsonic recirculation regions where fuel can ignite, then spread to the main flow.
The physics here matter. Flame speed relative to flow velocity determines whether combustion is even possible. Hydrogen’s fast flame speed—2 to 3 metres per second versus 0.4 metres per second for kerosene—enables reliable ignition in millisecond timeframes.
Why is hydrogen used as fuel in scramjet engines?
Hydrogen gives you the highest specific energy of any chemical fuel: 142 MJ/kg versus 43 MJ/kg for kerosene. That’s more than three times the energy density by weight.
Faster flame speed matters at hypersonic residence times. Hydrogen combustion produces only water vapour—zero CO2, zero NOx at controlled temperatures, zero particulates. The lower molecular weight of the exhaust gases gives you higher specific impulse.
Liquid hydrogen at minus 253 degrees Celsius offers excellent cooling properties. It can absorb heat from engine components before combustion, implementing regenerative cooling that recovers waste heat for thrust.
Hydrogen also has a wider flammability range—4 to 75 per cent in air versus 0.6 to 5.5 per cent for kerosene. This improves combustion stability.
The challenges are real though. Cryogenic storage adds complexity. Hydrogen needs larger volume tanks. Hydrogen embrittlement affects material selection—hydrogen diffuses into metals, causing brittleness.
Unlike conventional scramjets that rely on kerosene, Hypersonix’s SPARTAN scramjets use hydrogen, producing zero carbon emissions. The DART AE demonstrator aims to achieve the first sustained hypersonic flight using green hydrogen.
Green hydrogen production via electrolysis using renewable electricity creates a carbon-neutral fuel cycle. This makes sustainable hypersonics feasible.
What temperature challenges does hypersonic flight create?
Aerodynamic heating at Mach 12 generates surface temperatures exceeding 1,800 degrees Celsius. Temperature rises with the square of velocity—the jump from Mach 5 to Mach 12 represents a 5.8-fold temperature increase.
The combustion chamber experiences combined heating: compressed air enters at 1,000 to 1,500 degrees, then combustion adds peak flame temperatures of 2,000 to 2,500 degrees. The surface might be at 1,800 degrees while internal structure must stay below material limits.
Engine components face cyclic thermal loads—heating during powered flight, cooling during coast phases.
At Mach 10, temperatures exceed 1,800 degrees Celsius, so you need advanced materials like ceramic matrix composites. Material temperature limits constrain design. Aluminium alloys max out at 300 degrees. Titanium handles 600 degrees. Nickel superalloys reach 1,000 degrees. Ceramics can withstand 1,800 degrees and beyond.
The design trade-offs are straightforward: add cooling mass, use higher-temperature materials, or limit flight duration. Each choice affects payload capacity, cost, and operational flexibility.
How does 3D printing enable scramjet manufacturing?
Additive manufacturing produces complex internal geometries that traditional machining cannot. Integral cooling channels, optimised flow paths, biomimetic designs—these are all possible with 3D printing.
Design-to-hardware time drops from 12 to 18 months to 4 to 6 weeks. Direct metal laser sintering works with high-temperature alloys like Inconel 718 and Inconel 625, which withstand 700 to 1,000 degrees.
Multiple machined components become a single printed assembly, cutting weight and potential failure points.
The SPARTAN engine’s 3D-printed design demonstrates specific capabilities: fuel injector struts with internal cooling channels, combustion chambers with integrated features.
The process: CAD design, CFD optimisation, DMLS printing, heat treatment, machining, inspection.
GE Aviation, SpaceX, and Relativity Space also use additive manufacturing for propulsion components. Hypersonix’s approach takes this further—vertical integration of design, printing, and testing enables rapid development cycles.
What materials can withstand hypersonic flight conditions?
Ceramic matrix composites operate at 1,400 to 1,800 degrees Celsius. Silicon carbide fibres in a silicon carbide matrix—SiC/SiC—go where metals can’t.
Refractory metals handle extreme zones. Tungsten alloys melt at 3,400 degrees. Molybdenum and niobium work for leading edges and combustor sections.
Nickel-based superalloys like Inconel cover the 700 to 1,200 degree range.
Carbon-carbon composites reach 2,000 degrees and beyond. The Space Shuttle used them.
Thermal barrier coatings provide 100 to 200 degrees of temperature reduction.
The trade-offs: Ceramics are brittle. Refractory metals are dense. Superalloys have temperature limits.
VISR will be built using high-temperature ceramic composites, demonstrating how these materials enable operational vehicles. CMCs and refractory metals run 10 to 100 times more expensive than titanium or aluminium. These material choices constrain where hypersonic technology can be economically deployed—which is why defense tech investment increasingly focuses on breakthrough propulsion technologies.
Frequently Asked Questions
What is the difference between a scramjet and a ramjet?
Ramjets slow incoming air to subsonic speeds—Mach 0.3 to 0.5—before combustion. This works efficiently from Mach 3 to 6.
Scramjets maintain supersonic airflow throughout. The combustion happens at Mach 2 to 3. This extends operational range to Mach 5 to 15 and beyond.
The transition happens because slowing Mach 6 and higher air to subsonic creates excessive pressure losses and temperatures. Scramjets avoid this by burning fuel in supersonic flow, accepting the combustion challenges this creates.
How does Hypersonix’s SPARTAN engine achieve Mach 12?
SPARTAN uses hydrogen fuel with 142 MJ/kg energy density and fast flame speed. The 3D-printed Inconel combustion chamber integrates cooling channels. Inlet geometry was validated through extensive shock tunnel experiments.
The design balances inlet compression, combustion efficiency, thermal management, and nozzle expansion. Development used the HYPERTWIN X virtual testing environment, combining CFD with shock tunnel data.
Can scramjets operate from standstill like jet engines?
No. Scramjets need hypersonic speeds—Mach 4 to 5 minimum—to generate inlet compression. Vehicles need rocket boosters or carrier aircraft launch to reach operational velocity.
The DART AE will launch aboard Rocket Lab’s HASTE booster from NASA’s Wallops Flight Facility. This limitation restricts scramjets to specific applications: hypersonic missiles, research vehicles, space launch upper stages, reconnaissance platforms.
What are the main engineering challenges preventing widespread hypersonic flight?
Thermal loads at Mach 12 require expensive ceramic materials. Supersonic combustion instabilities and low efficiency—70 to 85 per cent fuel burn—limit performance.
Hydrogen storage complexity adds operational burden. Cryogenic tanks, boil-off management, and safety protocols complicate operations. Materials degradation from oxidation, thermal fatigue, and hydrogen embrittlement affects service life.
Infrastructure doesn’t exist yet. Hypersonic test facilities are rare. Hydrogen refuelling at operational scale hasn’t been deployed.
Traditional full-up flight tests cost about $100 million per flight. Cost per flight runs thousands of times higher than subsonic aircraft.
How is 3D printing different for hypersonic engines versus rockets?
Both use similar metal additive processes. But hypersonic engines face sustained thermal exposure measured in minutes. Rockets see impulse heating measured in seconds.
Scramjets need complex internal flow paths for inlet compression and fuel mixing. Rockets optimise for throat and expansion geometry.
Hypersonic engines integrate cooling channels more extensively. Material choices overlap—Inconel, copper alloys—but scramjets use ceramic matrix composite inserts where rockets rely on ablative cooling.
What role does computational simulation play in scramjet development?
CFD simulates hypersonic airflow, shock interactions, and combustion. This predicts performance before expensive hardware testing.
Hypersonix’s HYPERTWIN X environment combines CFD with shock tunnel experimental data. Benefits: reduce physical test count, explore broader design spaces. Shock tunnel tests cost $10,000 to $50,000 each.
Limitations: Turbulence models remain imperfect at hypersonic conditions. Validation against experimental data is required.
How does hydrogen storage work for hypersonic aircraft?
Liquid hydrogen at minus 253 degrees Celsius needs cryogenic tanks with multi-layer vacuum insulation.
Challenges: continuous boil-off requiring venting or active cooling. Large volume requirements—hydrogen is four times less dense than kerosene. Hydrogen embrittlement as hydrogen diffuses into metals. Leaks create explosion risk.
VISR and DELTA VELOS integrate conformal tanks within the airframe. Green hydrogen production via renewable electrolysis enables sustainable operations.
What is specific impulse and why does it matter?
Specific impulse measures propulsion efficiency: seconds of thrust per unit weight of propellant. Higher specific impulse means greater range or payload capacity.
Hydrogen scramjets achieve 1,500 to 2,500 seconds at Mach 7 to 12. Kerosene ramjets reach 800 to 1,200 seconds. Rockets deliver 280 to 450 seconds.
The air-breathing advantage: not carrying oxidiser onboard improves efficiency. Scramjets excel at sustained hypersonic cruise. Rockets are better for acceleration and exo-atmospheric flight.
How long can a scramjet sustain hypersonic flight?
Current technology demonstrates 5 to 10 minutes continuous operation. The X-51 Waverider achieved 6 minutes at Mach 5.
Limitations: fuel capacity, thermal soak, engine durability, trajectory constraints. The SPARTAN engine is designed for multiple burn cycles on reusable vehicles.
Future goal: 30 to 60 minute hypersonic cruise. Thermal management and structural fatigue are the primary limiters.
What makes the University of Queensland significant to hypersonic development?
UQ operated shock tunnel facilities that conducted over 6,000 hypersonic experiments. These generated foundational data for SPARTAN engine design.
Shock tunnels create milliseconds of Mach 5 to 12 flow conditions. This enables ground testing of inlet performance, combustion ignition, and thermal loads.
Hypersonix’s founding team includes UQ researchers. Australia has been a global leader in hypersonic technology since 1989.
How does Mach 12 compare to orbital velocity?
Mach 12 equals 9,200 mph. Low Earth orbit velocity is 17,500 mph. Scramjets alone can’t reach orbital speed but reduce rocket propellant needed.
Two-stage-to-orbit: scramjet accelerates to Mach 10 to 12 at 100,000 feet. Rocket completes orbital insertion. Benefit: 30 to 40 per cent propellant mass reduction.
DELTA VELOS is designed for this profile, carrying small satellites to orbit. The challenge: air density at 100,000 feet becomes too low for scramjet thrust.
What are the environmental benefits of hydrogen-powered hypersonics?
Hydrogen combustion produces only water vapour: H2 plus O2 yields H2O. Zero CO2 emissions.
Scramjets produce some thermal NOx, but far less than kerosene. No particulates, no soot, no unburned hydrocarbons.
Green hydrogen pathway: electrolysis using renewable electricity creates a carbon-neutral fuel cycle.
Contrails from water vapour create climate impact. Ice crystals reflect sunlight. Lower flight paths can eliminate this.
The Strategic Context
Hydrogen-powered scramjet technology represents a convergence of materials science, propulsion physics, and manufacturing innovation. As development costs and complexity drive collaboration between startups, defense agencies, and investors, understanding these technical fundamentals becomes essential for evaluating opportunities in the broader deep tech and defense innovation landscape.
