Editing Ramjet (section)

==Construction==

===Diffuser===
The diffuser converts the high velocity of the air approaching the intake into high (static) pressure required for combustion. High combustion pressures minimise entropy rise during heat addition,<ref>{{cite book |last=Builder |first=C. |title=1st Annual Meeting |date=1964-06-29 |publisher=American Institute of Aeronautics and Astronautics |page=2 |language=en |chapter=On the thermodynamic spectrum of airbreathing propulsion |doi=10.2514/6.1964-243 |chapter-url=https://arc.aiaa.org/doi/10.2514/6.1964-243}}</ref> this minimising wasted thermal energy in the exhaust gases<ref>{{cite journal |title=Propulsive Efficiency from an Energy Utilization Standpoint |journal=Journal of Aircraft |volume=13 |issue=4 |date=April 1976 |publisher=American Institute of Aeronautics and Astronautics |via=Internet Archive |language=English |url=http://archive.org/details/sim_journal-of-aircraft_1976-04_13_4}}</ref>

Subsonic and low-supersonic ramjets use a [[pitot tube|pitot]]-type opening for the inlet. This is followed by a widening internal passage (subsonic diffuser) to achieve a lower subsonic velocity that is required at the combustor. At low supersonic speeds a normal (planar) shock wave forms in front of the inlet.

For higher supersonic speeds the pressure loss through the shock wave becomes prohibitive and a protruding spike or cone is used to produce oblique shock waves in front of a final normal shock that occurs at the inlet entrance lip. The diffuser in this case consists of two parts, the supersonic diffuser, with shock waves external to the inlet, followed by the internal subsonic diffuser.

At higher speeds still, part of the supersonic diffusion has to take place internally, requiring external and internal oblique shock waves. The final normal shock has to occur in the vicinity of a minimum flow area known as the throat, which is followed by the subsonic diffuser.

===Combustor===
As with other jet engines, the combustor raises the air temperature by burning fuel. This takes place with a small pressure loss. The air velocity entering the combustor has to be low enough such that continuous combustion can take place in sheltered zones provided by [[flame holder]]s.

A ramjet combustor can safely operate at [[stoichiometry|stoichiometric]] fuel:air ratios. This implies a combustor exit [[stagnation temperature]] of the order of {{convert|2400|K}} for [[kerosene]]. Normally, the combustor must be capable of operating over a wide range of throttle settings, matching flight speeds and altitudes. Usually, a sheltered pilot region enables combustion to continue when the vehicle intake undergoes high [[flight dynamics|yaw/pitch]] during turns. Other flame stabilization techniques make use of flame holders, which vary in design from combustor cans to flat plates, to shelter the flame and improve fuel mixing. Over-fuelling the combustor can cause the final (normal) shock in the diffuser to be pushed forward beyond the intake lip, resulting in a substantial drop in airflow and thrust.

===Nozzles===
The [[propelling nozzle]] is a critical part of a ramjet design, since it accelerates exhaust flow to produce thrust.

Subsonic ramjets accelerate exhaust flow with a [[nozzle]]. Supersonic flight typically requires a [[De Laval nozzle|convergent–divergent nozzle]].

[[File:Bloodhound thor arp 750pix.jpg|thumb|[[Bristol Thor]] ramjet modified for display purposes. Two Thor engines were used on the Bristol [[Bloodhound (missile)|Bloodhound missile]]]]

===Performance and control===
Although ramjets have been run as slow as {{convert|45|m/s|km/h mph}},<ref name="primer">[http://www.alt-accel.com/ramjet2.htm Ramjet Primer].</ref> below about {{convert|0.5|Mach|m/s km/h mph|sigfig=2}} they give little thrust and are highly inefficient due to their low pressure ratios.

Above this speed, given sufficient initial flight velocity, a ramjet is self-sustaining. Unless the vehicle [[drag (physics)|drag]] is extremely high, the engine/airframe combination tends to accelerate to higher and higher flight speeds, substantially increasing the air intake temperature. As this could damage the engine and/or airframe integrity, the fuel control system must reduce fuel flow to stabilize speed and, thereby, air intake temperature.

Due to the stoichiometric combustion temperature, efficiency is usually good at high speeds (around {{convert|2|-|3|Mach|m/s km/h mph|disp=comma|sigfig=2}}), whereas at low speeds the relatively low pressure means the ramjets are outperformed by [[turbojet]]s and [[rocket]]s.