| Type |
Description |
Advantages |
| Water jet |
Squirts water out the back
through a nozzle |
Can run in shallow water, powerful, less
harmful to wildlife, (indeed used by squid) |
| Thermojet |
Most primitive airbreathing
jet engine. Essentially a supercharged piston engine with
a jet exhaust. |
Higher exhaust velocity than a propeller,
offering better thrust at high speed |
| Turbojet |
Generic term for simple turbine
engine |
Simplicity of design, efficient at supersonic
speeds (~M2) |
| Turbofan |
Most common form of jet engine
in use today. Used in airliners like the Boeing 747 and
military jets, where an afterburner is often added for
supersonic flight. First stage compressor greatly enlarged
to provide bypass airflow around engine core. |
Quieter due to greater mass flow and lower
total exhaust speed, more efficient for a useful range
of subsonic airspeeds for same reason, cooler exhaust
temperature |
| Rocket |
Carries all propellants and
oxidents onboard, emits jet for propulsion |
Very few moving parts, Mach 0 to Mach 25+,
efficient at very high speed (> Mach 10.0 or so), thrust/weight
ratio over 100, no complex air inlet, high compression
ratio, very high speed (hypersonic) exhaust, good cost/thrust
ratio, fairly easy to test, works in a vacuum-indeed works
best exoatmospheric which is kinder on vehicle structure
at high speed, fairly small surface area to keep cool,
and no turbine in hot exhaust stream. |
| Ramjet |
Intake air is compressed entirely
by speed of oncoming air and duct shape (divergent) |
Very few moving parts, Mach 0.8 to Mach
5+, efficient at high speed (> Mach 2.0 or so), lightest
of all airbreathing jets (thrust/weight ratio up to 30
at optimum speed), cooling much easier than turbojets
as no turbine blades to cool. |
| Turboprop |
Strictly not a jet at all a
gas turbine engine is used as powerplant to drive propeller
shaft (or Rotor in the case of a Helicopter) |
High efficiency at lower subsonic airspeeds
(300 knots plus), high shaft power to weight |
Propfan/
Unducted Fan |
Turboprop engine drives one
or more propellers. Similar to a turbofan without the
fan cowling. |
Higher fuel efficiency, potentially less
noisy than turbofans, could lead to higher-speed commercial
aircraft, popular in the 1980s during fuel shortages |
| Pulsejet |
Air is compressed and combusted
intermittently instead of continuously. Some designs use
valves. |
Very simple design, commonly used on model
aircraft |
Pulse
detonation engine |
Similar to a pulsejet, but
combustion occurs as a detonation instead of a deflagration,
may or may not need valves |
Maximum theoretical engine efficiency |
|
Air-augmented
rocket
|
Essentially a ramjet where
intake air is compressed and burnt with the exhaust from
a rocket |
Mach 0 to Mach 4.5+ (can also run exoatmospheric),
good efficiency at Mach 2 to 4 |
| Scramjet |
Similar to a ramjet without
a diffuser; airflow through the entire engine remains
supersonic |
Few mechanical parts, can operate at very
high Mach numbers (Mach 8 to 15) with good efficiencies |
| Turborocket |
A turbojet where an additional
oxidizer such as oxygen is added to the airstream to increase
maximum altitude |
Very close to existing designs, operates
in very high altitude, wide range of altitude and airspeed |
| Precooled jets |
Intake air is chilled to very
low temperatures at inlet in a heat exchanger before passing
through a ramjet or turbojet engine. Can be combined with
a rocket engine for orbital insertion. |
Easily tested on ground. Very high thrust/weight
ratios are possible (~14) together with good fuel efficiency
over a wide range of airspeeds, mach 0-5.5+; this combination
of efficiencies may permit launching to orbit, single
stage, or very rapid, very long distance intercontinental
travel. |
