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Propulsion systems
Definition:
Different types of propulsion systems:(1) Rockets(2) Jet engines
Chapter 1 The jet propulsion principles
Examples of propulsion:(1) Birds and fish(2) Octopus and jellyfish
(3) Chinese fireworks
(4) V-2 missile (liquid rocket), Saturn V, boostersof space shuttles, etc.
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1.1 Basic concepts: fluid momentum and reactionforce.
Newtons laws of motion :
(1) Generation of thrust (Fig. 1.1, a cold-gas rocket):
(2) Force balance (with the effect of shearing forces
ignored) and thrust calculation:At t = 0:
For t > 0 :
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p is a function of x, y, z, and t for complex geometries-> difficult to determine.
A simpler way to estimate the steady-state thrust of achemical rocket:
wherem(dot) = rate at which mass flows out of the chamberue = averaged exhaust velocity.
(3) Work outputs :
Trust velocity = work/time = power.
Thrust arm length = torque; Torque revolutions/time = power.
An ancient steam-turbine (uses thrust to produce shaftwork) design:
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1.2. Rockets and propellers
1.2.1. Rockets:(1) Chemical rockets (solid-propellant and liquid-propellant). Thrust is produced by the chemicalenergy released from the propellant.(Note that, more energy can be released in ionizationand nuclear reactions)
(2) Nuclear rockets: use nuclear reaction products aspropellant (unthinkable!) or use the heat generated ina nuclear reaction to heat a working fluid.
(3) Electrical thrusters: ionized gases (consisting offree electrons and ions) are accelerated in high-intensity electrostatic fields to very high velocities.
Generation of plasmas: electron bombardment;resistance heating, arc discharging heating, microwaveheating, etc.
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Performance for key rocket technologies:(Rocket performance table)
Remarks:(1) Specific impulse Isp = Thrust/[Propellant mass flowrate). (Sea-level gravitational acceleration)].(2) Cold-gas propulsion system uses a compressed gasto develop thrust.
1.2.2. Propellers
Fig. 1.7
(1) Thrust and efficiency of a propeller or a turbineblade.Thrust developed by the propeller in Fig. 1.7:
Minimum possible fuel-energy consumption rate:
where e = engine thermal efficiency
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= shaft power/input chemical energy rate.
Thus,
Maximum possible value for a propeller:
For chemical rockets:
and the minimum energy consumption rate is :
Combining equations:
For the same (minimum) rate of chemical energy
consumption:
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(2) Aerodynamic considerations.
Fig. 1.8 Velocities at the tip of a propeller blade
To maintain good flow over the blade:
(A) Small angle of attack (turning angle) to preventflow separation.
(B) Relative approach velocity must not be too close tothe speed of sound.
1.3 Air-breathing engines: turbojets, turbofans,ramjets, etc.
1.3.1. Turbojet, turbofan, and turboprop engines (Fig.5.1)
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1.3.2. Ramjets
1.3.3. Fundamental propulsion consideration of air-breathing engines.
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Thrust:
Rate of energy consumption:
where t = thermal efficiency of turbine engines.
At a constant flight speed u,~ air drag ~ u2 -> E(dot) ~ u
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