Seminar Presentation on the topic Rocket Based Combined Cycle Engines

1. Overview of Rocket Based
Combined Cycle Engines
(RBCC Engines)
AKSHAY REJI
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2. Introduction
Rockets
Carry their on oxidizer.
• Characterized by high engine
thrust to weight ratio ( 70 -
100) and relatively low
trajectory average specific
impulse ( 350 – 450 sec for
LH2/LOX systems).
Air breathing engines
• Don’t carry a substantial
amount of onboard oxidizer.
• Higher specific impulse ( 1500 –
500 sec for LH2).
• Low thrust to weight ratio
around 5 – 15.
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Jet engines can be classified into air breathing and non air breathing
engines ( rocket engines ).

3. Terms and Definitions
• Specific Impulse : Total impulse delivered per unit
of propellant consumed and is dimensionally equivalent to the
generated thrust divided by the propellant mass flow rate or weight
flow rate.
• Thrust to weight ratio : Dimensionless ratio of thrust to weight of
a jet engine, or a vehicle propelled by such an engine that indicates
the performance of the engine or vehicle.
• Mach Number (M) : ratio of the velocity of a fluid to the velocity of
sound in that fluid.
Subsonic : M < 1 Transonic : M = 1
Supersonic : 1 < M < 5 Hypersonic : M > 5
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4. Rocket Based Combined Cycle (RBCC) Engines - 1
Defined as a combined cycle engine whose primary propulsion element
is the chemical rocket.
• Also known as air – augmented rocket or air breathing rocket engines.
• The rocket and ramjet share the same flow path, thus leaving out
complex structures and reducing redundant weight.
• Offers thrust to weight ratios of 25 – 40 and specific impulse that are
higher than rockets (420 – 800 sec)
• Completely integrated flow path used throughout the entire ballistic
trajectory so
that it can take full advantage of various thermodynamic cycles.
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5. Rocket Based Combined Cycle (RBCC) Engines - 2
• The high-temperature and fuel-
rich gas ejected from the rocket
nozzle is called the primary flow
and the entrained air is called
the secondary flow.
• The ability to utilize the rocket as
an ejector increases the engine
mass flow, therefore, thrust.
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Fig 1. Layout of a RBCC Engine
Zeyu Dong, Survey on key techniques of rocket-based combined-
cycle engine in ejector mode.

6. Need of an RBCC Engine
• Engines with a single mode cannot work effectively in the full
speed range.
• Rocket engines are currently the only engines with the
ability to launch an aircraft into orbit.
• Additional propulsion system is needed to implement orbit
change and the re-entry mission.
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7. Working process of a typical RBCC engine – Part 1
Ejector Mode
• Before the period of Mach 2.
• The eject rocket is ignited and
the fuel-rich plume mixes and
reacts with the secondary flow
in the main combustor.
• The rocket acts as an ejector to
pump
the airflow into the engine.
Ramjet Mode
• Between Mach 2 and Mach 5.
• The free flow is compressed and
slowed down after a normal shock
wave in the inlet, before mixing
and reacting with the fuel in the
combustor.
• The eject rocket is shut down, but
can also supply fuel to the
combustor.
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8. Working process of a typical RBCC engine – Part 1
Scramjet Mode
• Speed exceeds Mach 5.
• The free flow’s total enthalpy
is too high to maintain an
effective reaction in subsonic
conditions.
• inlet and isolator may still slow
down the free flow by
generating a series of shock
waves.
Rocket Mode
• When the inlet fails to capture
enough airflow to maintain
combustion, the eject rocket is
re-ignited and the inlet is closed.
• The rocket provides all the
thrust in this mode and no
secondary flow passes into the
flow path.
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9. Operational Modes
• Fig. 2 Operational modes of an RBCC engine
Zeyu Dong, Survey on key techniques of rocket-based combined-
cycle engine in ejector mode.
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10. Components of RBCC engines - 1
• Inlet / Fore body
Capture freestream air and prepare it for the combustor section while
producing minimum drag losses.
The goals of inlet design are to produce uniform flow at the combustor
entrance, no boundary layer separation, low drag, low heat transfer
and a minimal total pressure losses
Act as the primary compressor of the ambient air.
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11. Components of RBCC engines -2
Combuster
• Heart of the combined cycle design.
• Integrates the various propulsion modes into a single propulsion unit.
• The rocket ejector is the primary feature of the combustor section
operating in three of the four propulsion modes: as the ejector in air
augmented rocket mode; as a fuel injector in scramjet mode; and
alone in all-rocket mode for orbital insertion.
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12. Components of RBCC engines -3
• Nozzle
• Optimally expand the products of combustion in order to maximize
potential thrust.
• The nozzle must be integrated with the preceding engine section and
the following section – afterbody.
• Excessive drag at transonic and supersonic speed due to large
expansion ration nozzle can be reduced through the use of external
hydrogen burning.
• The nozzle is the thrust producer of the engine and its exhaust system
has potentially strong influence on vehicle stability and control.
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13. Basic operating principle of the RBCC ejector mode
• Used from takeoff through to the low supersonic flight regime.
• Rocket acts as an ejector to pump the airflow into the engine.
• Under the effects of viscous interaction, the low-velocity secondary
flow is ingested into the high-velocity primary flow in the mixing duct.
• Exhaust is discharged into the atmosphere through the RBCC nozzle.
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14. Ideal thermodynamic cycle of ejector mode
• Black line and the red line represent the
thermodynamic process of primary flow
and secondary flow.
• Propellant is pressurized by the fuel
supply system
• Combustion process in the primary rocket
chamber.
• Burned gas at high temperature and
pressure carries out an isentropic
expansion in the rocket nozzle.
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Fig 3. Ideal cycle of a RBCC engine in ejector mode
Zeyu Dong, Survey on key techniques of rocket-based combined-cycle
engine in ejector mode.

15. Features of RBCC compared to other combined cycle engines - 1
• Afterburning in rocket-ejector mode, using the ramjet/scramjet fuel
injectors, increases the thrust and specific impulse.
• Ratio of the bypass air to the rocket exhaust mass the specific
impulse continues to increase as the cycle more
closely resembles ramjet operation.
• In ramjet and scramjet modes, the rocket could be advantageously
used as a fuel injector and mixing enhancer.
• In the rocket-only mode, the use of the engine duct as a highly
expanded nozzle at high altitudes increases the specific impulse of
that mode of operation.
• Reduction in the amount of onboard oxidizer required.
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16. Features of RBCC compared to other combined cycle engines - 2
• Vehicle propellant mass fractions for RBCC-powered vehicles are projected
to be around 70%, as compared to 90% for all-rocket vehicles ( propellant
mass fraction - portion of a vehicle's mass which does not reach the
destination).
• In the rocket-ejector mode, RBCC systems can provide vehicle thrust-to-
weight ratios greater
than one and are therefore capable of vertical takeoff and landing.
• The cryogenic fuel can be used in airbreathing modes as a heat sink to
increase the density of the inlet airflow, thus increasing the work output.
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17. Challenges
• Braod range of chemical constituents produced as a result of
combustion.
• Difference in the operation condition of various modes.
• Cooling techniques and active cooling must be defined.
• Realizing smooth transition among working modes.
• Propulsion/ airframe integration design, and thermal protection
design.
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18. Summary
• RBCC engine
• Working modes
• Different components
• Ejector mode of a RBCC Engine
• Features
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19. Conclusion
• Combined cycle engines are the future of reusable launch vehicles
and RBCC engines development is a necessity to create SSTO vehicles.
• Due to technical limitations it not easy to generate a practical RBCC
engine powered aircraft. However, research on RBCC engine
techniques will greatly benefit the
related disciplines and the payoff for future space round trips.
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20. References
1. ZHANG, Zhen-guo WANG, Wei HUANG, Jian CHEN, Ming-bo SUN - The
overall layout of rocket-based combined-cycle engines: a review, 2019
2. Zeyu Donga, Mingbo Suna, Zhenguo Wanga, Jian Chenb, Zun Caia -
Survey on key techniques of rocket-based combined-cycle engine in
ejector mode , 2017
3. Kent T. Chojnacki and Clark W. Hawk - An Assessment of LlleRocket-Basec
Combined Cycle Propulsion System for Earth-to-Orbit Transportation
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