A Century of Ramjet Propulsion Technology Evolution - Faculty of ...

30 FRY
a) ATRJ
b) ATR
c) ERJ
Fig. 4 Schematics of generic hybrid (mixed cycle) ramjets that produce
static thrust.
number variations are limited. The air-ducted rocket (ADR), shown
in its IRR form in Fig. 3d, operates under the same engine cycle
principles. Here, a fuel-rich monopropellant is used to generate a
low-to-moderate pressure gaseous fuel supply for the subsonic combustor.
The choice of an ADR is generally a compromise between
the fuel supply simplicity of an SFIRR and the unlimited throttleability
of the LFIRR, GFRJ, or CRJ. An ADR is generally used
when the total fuel impulse does not adversely impact the powered
range. The performance of the LFIRR or GFRJ is typically superior
to the other four variants shown.
Although the ramjets shown conceptually in Figure 3 are viable
vehicle propulsion systems, none can produce static thrust. Figure 4
shows three types of hybrid or combined cycle ramjet engine cycles
that can. The first embeds a turbojet engine within the main ramjet
engine and is usually liquid fueled and called an air-turboramjet
(ATRJ) (Fig. 4a). Here, the turbojet produces the required static and
low-speed thrust for takeoff (and landing if required) that may or
may not be isolated from the main ramjet flow at supersonic speeds.
An alternative to the ATRJ is the air-turborocket (ATR) in which
alow-to-moderate pressure rocket motor is used to drive a turbine
and to provide a gaseous fuel for the ramjet (Fig. 4b). The turbine, in
turn, drives a compressor, the combination of which produces static
thrust. At supersonic speeds, the compressor, again, may be isolated
from the main ramjet flow and the turbine idled so that the vehicle
can operate as an ADR. The final hybrid ramjet cycle capable of
producing thrust is ejector ramjet (ERJ) shown in Fig. 4c. Here, a
rocket motor or gas generator produces a high-pressure, generally
fuel-rich, supersonic primary or ejector flow that induces secondary
air to flow through the engine even at static conditions. The ejector
effluent and air then mix and burn (at globally subsonic speeds) and
finally expand in the convergent–divergent exit nozzle.
There are three basic types of integral/rocket ramjet engines,
namely, the LFRJ, the solid-fueled ramjet (SFRJ), and the ducted
rocket (DR) as shown in Fig. 3. In the LFRJ, hydrocarbon fuel is
injected in the inlet duct ahead of the flameholder or just before
entering the dump combustor. In the SFRJ, solid ramjet fuel is cast
along the outer wall of the combustor with solid rocket propellant
cast on a barrier that separates it from the solid ramjet fuel. In this
case, fuel injection by ablation is coupled to the combustion process.
An aft mixer or other aid is generally used to obtain good
combustion efficiency. The DR is really a solid fuel gas generator
ramjet in which high-temperature, fuel-rich gasses are supplied to
the combustor section. This provides for flame piloting and utilizes
the momentum of the primary fuel for mixing and increasing to-
tal pressure recovery. Performance of the DR may be improved by
adding a throttle valve to the primary fuel jet, thereby allowing larger
turndown ratios. This is known as a variable flow DR (VFDR). The
reader is directed to Zucrow, 6 Dugger, 7 and others 8 for additional
ramjet fundamentals.
Supersonic Combustion (Scramjet) Cycle Behavior
Turning now to supersonic combustion engines, Fig. 5 shows a
generic scramjet engine and two hybrid variants. Figure 5a shows a
traditional scramjet engine wherein air at supersonic or hypersonic
speeds is diffused to a lower, albeit still supersonic, speed at station 4.
Fuel (either liquid or gas) is then injected from the walls (holes, slots,
pylons, etc.) and/or in-stream protuberances (struts, tubes, pylons,
etc.), where it mixes and burns with air in a generally diverging
area combustor. Unlike the subsonic combustion ramjet’s terminal
normal shock system, the combined effects of heat addition and
diverging area in the scramjet’s combustor, plus the absence of a
geometric exit nozzle throat, generate a shock train located at and
upstream of the combustor entrance, which may vary in strength
between the equivalent of a normal and no shock. The strength of this
shock system depends on the flight conditions, inlet compression or
inlet exit Mach number M4, overall engine fuel/air ratio ER0, and
supersonic combustor area ratio (A5/A4).
The unique combination of heat addition in a supersonic airstream
with a variable strength shock system plus the absence of a geometric
throat permits the scramjet to operate efficiently over a wide range
of flight conditions, that is, as a nozzleless subsonic combustion
ramjet at low flight Mach numbers, M0 = 3–6, and as a supersonic
combustion ramjet at higher flight Mach numbers, M0 > 5. At low
M0 and high ER, the combustion process generates the equivalent of
a normal shock system and is initially subsonic, similar to that of a
conventional subsonic combustion ramjet, but accelerates to a sonic
or supersonic speed before exiting the diverging area combustor,
which eliminates the requirement for a geometric throat. As ER
decreases at this same M0, the strength of the precombustion shock
system will also decrease to the equivalent of a weak oblique shock,
and the combustion process is entirely supersonic. At high M0, the
strength of the shock system is always equivalent to either a weak
oblique shock or no shock, regardless of ER. This is referred to as
dual-mode combustion and permits efficient operation of the engine
from M0 = 3toM0 = 8–10 for liquid hydrocarbon fuels and up to
a) Supersonic combustion ramjet
b) DCR
c) ESJ
Fig. 5 Schematics of generic supersonic combustion engines.