Scramjet

Engines · Jet · Demo

A ramjet that burns supersonically — the key to hypersonic flight.

ENGINEER MODE

Flight condition

Flight Mach (M₀)0.85

Altitude11000 m

ISA at 11.0 km: 217 K, 23 kPa, a = 295 m/s. Design envelope: Mach 0–3.

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In design envelopeTurbojet operating near its intended regime (Mach 0–3).

Specific thrust898 N·s/kg

TSFC28.5 g/kNs

Exhaust velocity1148 m/s

Flight speed251 m/s

Fuel–air ratio (f)0.0256

Turbine exit (Tt5)1344 K

Efficiency chain

Thermal

57%

Propulsive

36%

Overall

20%

Overall = thermal × propulsive. Thermal efficiency comes from pressure ratio; propulsive efficiency from not throwing the exhaust away faster than needed.

In plain terms

Hypersonic test vehicle — engine slung under the belly

A turbojet is basically a metal tube that sucks in air, burns fuel in it, and shoots the hot gas out the back — the push from that jet is what moves the plane. It is the original jet engine: the kind that powered the first jet fighters and Concorde. Today you mostly find them in older military jets and missiles.

More detail

How the turbojet works

A turbojet is the purest jet engine: an inlet feeds a compressor that squeezes the air, fuel burns in the combustor, and the hot gas rushes out through a turbine (which drives the compressor) and a nozzle. All of the thrust comes from the momentum of that single, fast exhaust jet.

Because the whole airflow is accelerated to very high speed, a turbojet is efficient when the aircraft itself is fast — but wasteful and loud at low speed, where a lot of energy is thrown away as jet kinetic energy. That is exactly what the propulsive-efficiency term shows.

The exhaust is accelerated to $V_9$ ; thrust per unit air is the momentum gain, and thermal efficiency is the ideal Brayton result set by the total pressure ratio $\tau_r\tau_c$ :

\frac{F}{\dot m} = V_9 - V_0, \qquad \eta_{th} = 1 - \frac{1}{\tau_r\,\tau_c}

Idealised model. This is a cold-air-standard ideal cycle: one calorically-perfect gas ( $\gamma=1.4$ ), no component losses, and exhaust perfectly expanded to ambient pressure. It captures the trends that decide what each engine is for — it is not a station-by-station performance deck.

Use cases & real engines

Where it's used

Early jet airliners and fighters (1950s–70s)
Supersonic flight where high jet velocity is an asset
Missiles and drones needing a simple, compact engine

Real-world examples

GE J79 (F-4 Phantom)
Rolls-Royce Olympus (Concorde)
Pratt & Whitney J57