Principles of Propulsion and its Application in Space Launchers Prof. Dr.-Ing. Uwe Apel Hochschule Bremen 13.07.2012REVA Seminar1

REVA Seminar 1

Principles of Propulsion and its Application in Space Launchers

Prof. Dr.-Ing. Uwe ApelHochschule Bremen

13.07.2012

REVA Seminar 2

Overview

• How Rockets are Propelled• Thrust Generation in a Rocket Engine• Rocket Engine Performance Parameters• Classification of Space Vehicles• Application of Rocket Engines• Classification of Rocket Propulsion Systems• Physical Limits of Chemical Space Propulsion• The Rocket Equation• Staging of a Rocket13.07.2012

How Rockets are Propelled

• The Change of the state of motion of a rocket follows the principle of repulsion

• Newton‘s law applies:

ACTIO = REACTIO

Any force acting on a mass creates an force of the same size in the opposite direction!

• By ejection of a mass at a high velocity (usually a hot gas flow ) from the rocket engine a force is produced changing the momentum of the rocket. Important: According to Newton‘ law of momentum conservation

the sum of the momentum changes of working fluid and vehicle equals 0 !

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Functional Principle of a RocketThrust is generated

exits nozzle with velocity


Thrust Generation in a Rocket Engine


Rocket Engine Performance Parameters


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The Rocket Equation

• Describes Movement of a rocket in force-free space

• Calculates velocity change achievable with a rocket geaturing a certain mass ratio and average specific Impulse

• Differential form:

• Integral form:

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Classification of Space Vehicles


Classification of Rocket Propulsion Systems

• Origin of propulsion energy– Chemical– Nuclear– Solar

• Propellants and their aggregate state– Solid propellants– Liquid propellants– Hybrid engines– Cold gases

• Thrust level– High thrust (> engine weight)– Low thrust (< engine weight)


Application of Rocket Engines


Typical Performances of Rocket Engines


Rocket Engine Performance Map

thrust to mass [N/kg]

acceleration [m/s]

spec

ific

impu

lse

[m/s

]


∆V Requirement

• The ∆V requirement of a space mission is dependent on:– Size and orbit of launch planet– Size and orbit of destination planet– Propulsion concept (thrust level, propulsion

time)– Chosen trajectory and resulting flight time– Accuracy of orbit and attitude control system– Vehicle aerodynamics


∆V Calculation

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Typical ∆V Requirements

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Elements of a Space Transportation System


Elements of a Rocket

The take-off mass of a rocket consists of three major mass elements:• Structure and Engine(s)

– Body and tankage– Engines and related equipment– Non-usable propellant residuals– Usable propellant reserve – Recovery equipment (parachutes, wings, landing gear, etc.)– Instrumentation and avionics

• Propellants– Expected propellant consumption during flight– Propellants expended prior to lift-off

• Payload


Design Parameters• According to the rocket equation a maximisation of the ratio between the initial

mass m0 and the cut-off mass mc is required for a high velocity capability• Thus 80% ÷ 90% of the initial mass of a rocket is propellant mass• This requires an ultra-light structural design and small, efficient engines with a

very high power density!• Key design parameters of a rocket are:

– The propellant mass fraction

– The propellant ratio

– The payload ratio


Technological limits for a rocket

• The performance of a single-stage rocket is limited by the technologically achievable values for the mass ratio R and the exhaust velocity C and the ∆V requirements of the mission:

• Limits: – useful minimum payload mass fraction of l >= 1 %– achievable propellant mass fraction of µ = 0.90 – today’s engines performance of C0 = 4300 m/s

Cvac = 4600 m/s

–minimum velocity increment to reach orbit ∆V = 9100 m/s

• Thus, it is very difficult to design a one-stage launch vehicle!


High Development Risk!

Technological Limits: Single-stage to Orbit (SSTO)

mpayload

mstructure

mpropellant


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Staging of a rocket• The problem can be overcome by "staging" the rocket which means

distributing the total propellant mass over more than one tank for each propellant component and not further accelerating empty tankage by cutting it off

• In theory a rocket with an infinite number of stages would provide a maximum payload ratio

• Practically the number of stages is limited by the propellant mass fraction of each stage which increases with decreasing stage size because tanks and engines cannot be downsized linear

• For transportation in orbits around Earth, 2-3 stages provide an optimum performance depending on the selected propellant combination and other design aspects

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Influence of staging on payload mass (example)

Assuming a launch vehicle based on following design data:

Mission velocity requirement(Earth to orbit): ∆V=9200 m/s

Average specific Impulse of engines: C=4400 m/s

Launch mass: m0=100 Mg

Propellant mass fraction: µ=0.9

One-stage design

Two-stage design

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Influence of staging on vehicle mass and payload

One-stage design Two-stage design

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Optimum staging of a launch vehicle

• Optimum distribution of total ∆V between the stages of a rocket depends of specific impulses of stage engines and stage propellant mass fractions

• For a two-stage vehicle, the payload mass fraction l of the rocket with respect to a given mission ∆V can be obtained from the following equation

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Optimum staging of a launch vehicle

• For a rocket with the same average specific impulse and propellant mass fraction in each stage, the l -Function has its maximum at U1=U2=∆V/2

• This means, that the first stage of a two-stage rocket should have a mass which is 3.6 times the mass of the second stage if the same technology is used in both stages

• For a launch vehicle going from Earth‘s surface to an orbit the described theoretical optimum is additionally influenced by the ascend trajectory due to:– gravity and drag losses (changes theoretical ∆V distribution)– engine performance (C depends on ambient pressure)

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REVA Seminar 26

Optimum staging of a launch vehicle (Example)

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Documents

Principles of Propulsion and its Application in Space Launchers Prof. Dr.-Ing. Uwe Apel Hochschule Bremen 13.07.2012REVA Seminar1