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Jet engine — action and reaction

Every second, an airliner's engine swallows hundreds of kilograms of air, squeezes it, heats it with a kerosene flame and throws it backward at high speed. The gas flies backward — the engine is pushed forward. Start the engine, move the throttle and see what happens to the air at every point of the cross-section.

Parts

Hover over (or tap) any part of the cross-section, or pick it from the list above.

Controls

Engine off. Press “Start engine”.

Careful: keep the throttle at idle during start-up — too much fuel will overheat the turbine.

Turbojet only, with throttle ≥ 80%.
Air mass flow rate
0 kg/s
Core jet velocity
0 m/s
Bypass jet velocity
0 m/s
Overall pressure ratio π
1.0
Turbine inlet temperature
15 °C
Specific fuel consumption
—
Fan share of thrust
—

Cockpit gauges

N1 · LP spool0.0%
N2 · HP spool0.0%
EGT15 °C
Fuel0 kg/h
Thrust0.0 kN
Flight conditions
Turbine engines are shown on a test stand (the aircraft is standing still, M = 0). You can change speed and altitude for the ramjet and the scramjet.

N1 and N2 are the rotational speeds of the two coaxial spools, as a percentage of their rated value. EGT is the gas temperature behind the high-pressure turbine; the red line is the limit above which the turbine blades start to get damaged. Notice that thrust lags behind the throttle: the rotors have to spin up first.

The Brayton cycle (T–s diagram)

Pressure ratio π1.0
Ideal efficiency0.0%
At π = 40—

Every turbine engine runs the same cycle: 1→2 compression, 2→3 combustion at constant pressure, 3→4 expansion in the turbine and nozzle, 4→1 heat rejected to the atmosphere. The area inside the loop is the work obtained from each kilogram of air. The efficiency of the ideal cycle depends only on the pressure ratio: η = 1 − π−(κ−1)/κ — which is why modern engines compress air as much as 40–60 times.

Why does it go forward?

Newton's third law: every action has a reaction equal in size and opposite in direction. The engine accelerates a mass of gas backward (action), so the gas pushes the engine — and the whole aircraft — forward (reaction). There is nothing to “push against”: thrust works just as well in a vacuum, like the recoil of a shotgun.

F ≈ ṁ · (vexit − vflight)thrust = mass of gas per second × its gain in speed

The same thrust can come from a small amount of gas thrown out very fast or a large amount thrown out slowly. But the kinetic energy of the jet grows with the square of its speed, so a fast jet wastes a lot of energy. Try it:

Exit v—
Propulsive efficiency—
Power lost in the jet—

Propulsive efficiency ηp = 2 / (1 + vexit/vflight). That is why airliners have huge fans: the slower but massive stream from the bypass duct provides most of the thrust while burning less fuel. Fighters, on the other hand, need a lot of thrust from a small, slim engine.

Ramjet and scramjet — no compressor

At several times the speed of sound, a compressor is no longer needed: a properly shaped inlet is enough. Air rushing in at flight speed slows down in shock waves, and its kinetic energy turns into pressure and temperature. The stagnation temperature is given by:

T₀ = T · (1 + (γ − 1)/2 · M²)T — air temperature at flight altitude, γ = 1.4 for air, M — Mach number

At 20 km altitude (T ≈ −57 °C) this gives about 330 °C at Mach 3, about 1,500 °C at Mach 6, and over 4,000 °C at Mach 10. In an ordinary ramjet the air slows to subsonic speed, so above about Mach 5–6 it enters the combustor so hot that the walls cannot take it, and instead of releasing heat the combustion products break apart (dissociation). Adding fuel then achieves little.

A scramjet (supersonic combustion ramjet) only partly slows the air: it flows through the combustor at several times the speed of sound, and combustion lasts about 1 ms. As a result the gas heats up less and the engine works even at Mach 10 and beyond — but keeping the flame lit is like lighting a match in a hurricane.

F ≈ ṁ · (vexit − vflight)    Isp = F / (ṁfuel · g)specific impulse: for how many seconds 1 kg of fuel can give a thrust equal to its own weight

Simplified model: ideal gas (γ = 1.4), standard atmosphere, two-dimensional shock waves, ideal nozzle, constant losses. Ramjet: kerosene, max. about 2,100 °C in the combustor; scramjet: hydrogen. The numbers are approximate.

Specific impulse vs. speed

Approximate values (curve shapes as in textbooks). Turbine engines and the ramjet burn kerosene, the scramjet hydrogen, the rocket oxygen + hydrogen. The vertical axis is logarithmic. The rocket has the lowest impulse because it also carries its own oxidizer — but it works at any speed and in a vacuum.

Challenge

  1. Cold startStart the engine and bring it to idle without the EGT crossing the red line.
  2. Fan powerGet at least 130 kN of thrust from the turbofan. Hint: the throttle alone isn't enough.
  3. Mach diamondsIn the turbojet, light the afterburner and exceed 1,000 m/s exhaust velocity.
  4. Hypersonic ignitionRun the scramjet within its operating range and get positive thrust.

A bit of history

The jet engine was invented almost simultaneously in two countries. Englishman Frank Whittle filed a patent as early as 1930, and German Hans von Ohain independently built his engine a few years later. Von Ohain won the race: his engine powered the Heinkel He 178, the first jet-powered aircraft.

  1. 1930Whittle files a patent for the turbojet engine.
  2. 1937Whittle's and von Ohain's engines first run on test stands.
  3. 1939August 27 — first flight of the He 178.
  4. 1941The Gloster E.28/39 with Whittle's engine takes to the air.
  5. 1966The unmanned Lockheed D-21 with a ramjet flies at over Mach 3.
  6. 1967The rocket-powered X-15A-2 reaches Mach 6.7 — the record for a piloted aircraft (William “Pete” Knight).
  7. 2004The scramjet-powered X-43A reaches Mach 9.6; the engine ran for about 10 s.
  8. 2013The X-51A Waverider flies about 210 s on scramjet power, up to about Mach 5.1.
Keyboard ↑ ↓ throttle S start / shut down A afterburner T engine type L labels Space pause