Ramjet engine

A ramjet engine for aircraft incorporating means for automatically changing from a subsonic combustion ramjet geometry to a supersonic combustion ramjet geometry. This is accomplished by utilizing solid fuel for the ramjet during the subsonic combustion mode and tailoring the solid fuel such that, as it burns away, the remaining internal engine geometry changes to the internal engine geometry for supersonic combustion. At the desired time, liquid fuel can be injected into the combustion chamber and the ramjet will operate eventually on liquid fuel only as a supersonic combustion ramjet.

BACKGROUND OF THE INVENTION 
As is known, there are two fundamental ramjet concepts. These comprise 
subsonic combustion and supersonic combustion, both of which are 
applicable to vehicles flying at supersonic speeds. In the subsonic 
combustion ramjet, the fuel is burned in air which has been slowed down to 
subsonic speeds within the engine. In a supersonic combustion ramjet, the 
fuel is burned in air which remains at supersonic velocities throughout 
the engine. Examples of supersonic combustion ramjet engines are shown, 
for example, in U.S. Pat. Nos. 3,280,565 and 3,430,445. 
A subsonic combustion ramjet operates best at supersonic flight speeds and, 
therefore, must be boosted to ramjet ignition speed by a first stage, 
usually a solid rocket. Practical upper flight limits for a subsonic 
combustion ramjet are usually between 500 and 8000 feet per second. If 
higher flight speeds are desired with ramjet propulsion, it is necessary 
to change from a subsonic combustion ramjet to a supersonic combustion 
ramjet. However, using the same basic ramjet requires effective geometry 
changes to the engine. Past and current efforts to develop dual-mode 
(i.e., subsonic-supersonic combustion) ramjets have required complicated 
mechanical geometry changes and/or complicated fuel injection location 
control with compromised performance. The major problem is to convert the 
geometry of the engine from a double-throat to a single-throat 
configuration. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, a solid fuel ramjet and liquid 
fuel supersonic combustion ramjet are combined into an integral propulsion 
system which does not require any complicated mechanical geometry changes 
and/or fuel injection location control. Preferably, the engine also 
incorporates a third propulsion mode, namely a solid rocket, for boosting 
the vehicle to supersonic speeds prior to initiation of solid fuel ramjet 
operation. 
Specifically, the invention provides a subsonic-supersonic ramjet engine 
comprising a combustion chamber having an air inlet and an exhaust outlet. 
A generally cylindrical solid ramjet fuel element is carried on the inner 
periphery of the combustion chamber and has formed therein a consumable 
solid fuel nozzle at its aft end, this solid fuel nozzle limiting the gas 
passing through the interior of the fuel element to subsonic velocities 
until it is consumed along with the remainder of the fuel element. After 
the fuel element including the solid fuel nozzle is consumed, a liquid 
fuel is injected into the combustion chamber and burned with the gases 
passing through the combustion chamber at supersonic speeds. Ordinarily, 
and in the preferred embodiment of the invention, the interior of the 
solid fuel ramjet element is lined with a solid rocket propellant which 
burns initially, without air passing through the combustion chamber, to 
bring the vehicle up to supersonic speeds.

With reference now to the drawings, and particularly to FIG. 1, there is 
shown a missile 10 incorporating a tank containing a liquid fuel 12. 
Beneath the tank is the integral rocket, solid fuel and liquid fuel ramjet 
engine of the invention, generally indicated by the reference numeral 14. 
It comprises a flared, generally cylindrical combustion chamber 16 having 
an air inlet opening 18 and an exhaust opening 20. Carried within the 
combustion chamber 16 is a generally tubular solid fuel element 22 having 
at its aft end a ramjet nozzle or restricted orifice 24, also formed from 
solid fuel. Disposed on the inner periphery of the solid fuel element 22 
is a second rocket propellant or fuel element 26 having an ejectable 
rocket nozzle 28 at its aft end which fits within the solid fuel ramjet 
nozzle orifice 24. The solid rocket propellant element 26 thus forms a 
solid rocket combustion chamber 29. Initially, the solid rocket propellant 
element 26 is ignited with the hot gases resulting therefrom passing out 
from the combustion chamber 29 through the ejectable rocket nozzle 28 to 
bring the missile 10 to supersonic speed. During this time, no air flows 
through the combustion chamber 16 or the rocket combustion chamber 29. 
After the solid rocket propellant element 26 is consumed, the rocket 
nozzle 28 is ejected, thereby leaving the configuration shown in FIG. 2. 
At this stage of operation, air flows through the inlet 18 and through a 
combustion chamber 30 formed by the solid ramjet fuel element 22. However, 
because of the restricted orifice formed by the consumable solid fuel 
ramjet nozzle 24, the gases passing through the combustion chamber 30 do 
not reach supersonic speeds, notwithstanding the fact that the missile 10 
itself is traveling at supersonic speeds. This action continues until the 
solid fuel element 22 is consumed, whereupon the configuration shown in 
FIG. 3 results wherein only the outer combustion chamber 16 remains. Fuel 
is now injected into the combustion chamber 16 from the tank 12 and 
ignited. At this time, the air passing through the combustion chamber 16 
travels at supersonic speeds. The manner in which the fuel is injected 
into the combustion chamber 16 is unimportant as regards the present 
invention. It is, however, schematically illustrated as a valve 32 adapted 
to connect the tank 12 to the air inlet opening 18 leading to the 
combustion chamber. In actual practice, the fuel tank 12 will be 
pressurized and the valve will be controlled to permit variations of 
fuel/air mixtures in the combustor. Furthermore, proper distribution of 
the fuel may require injectors consisting of small holes spaced along the 
inlet throat region on both of the side walls and on connecting struts 
within the airstream itself. The pressurized fuel will be injected through 
these holes into the airstream followed by downstream mixing and 
combustion of the fuel/air mixture. 
Thus, the sequence of operations involves initial rocket propulsion 
followed by subsonic combustion of a solid fuel, and finally supersonic 
combustion of a liquid fuel. 
Although the invention has been shown in connection with a certain specific 
embodiment, it will be readily apparent to those skilled in the art that 
various changes in form and arrangement of parts may be made to suit 
requirements without departing from the spirit and scope of the invention.