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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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