Space Propulsion and Power Systems Notes PDF

Preview

Summary

Space Propulsion and Power Systems NotesThe ‘Thrust’ PrincipleDepending on thrust generating mechanism we can classify propulsion systems into:  Thermodynamics systems Gas is “generated” in the system by “thermo” process, in flow of gaseous particles produces a pressure forces against “chamber” wal...

Description

Space Propulsion and Power Systems Notes The ‘Thrust’ Principle Depending on thrust generating mechanism we can classify propulsion systems into:  Thermodynamics systems Gas is “generated” in the system by “thermo” process, in flow of gaseous particles produces a pressure forces against “chamber” walls, nozzle accelerates gas particles and the momentum is obtained (principle of operation of most large rockets, nuclear, and some electric…)  Electrostatic systems Ions are accelerated by electric field - Coulomb’s Law a) Ion engines b) Hall thrusters c) etc.  Electromagnetic systems Gas accelerated by electromagnetic forces (Lorentz Force) a) Magneto-plasma-dynamic MPD Thrusters b) Pulsed Plasma Thrusters – PPT c) etc.

The ‘Energy Source’ Principle Depending on energy source:  Chemical (Thermal)  Liquid Propellant a) Monopropellant b) Bipropellant  Solid Propellant  Hybrid  Gaseous  Nuclear Thermo  Electrostatic or Electromagnetic  Solar Thermo  Electrostatic or Electromagnetic

Mission Application 1. Defence and Research Applications: a) Sounding Rockets Solid Propellant, 1-4 stage rockets b) Military Tactical Missiles Solid Propellant , 1-2 stage rockets c) Long Range Missiles - ICBM Solid or Liquid Propellant , 2-3 stages 2. Space Launch: Solid, liquid or both, 2-4 stages 3. Impulse ΔV for space applications (time-critical manoeuvres, energy change from elliptic orbits, plane change from elliptic orbits, non-fuel-limited situations, ΔV ≤1000 m/s): a) Small Solid Rocket Motors. (Apogee kick, etc) b) Bi-propellant liquids (storable) c) Monopropellant liquids (storable), d) Nuclear thermal 4. Low-Thrust ΔV in space (Mass-limited missions ΔV ≥ 2000 m/s, non-time-critical missions, small continuous orbit corrections, near-circular orbits a) Solar-electric systems b) Arcjets (a bit faster, less Isp)

c) d) e) f)

Hall, Ion (slower, higher Isp) PPT (precision maneuvers) Nuclear-electric systems Direct solar-thermal

Thermochemical Rocket Propulsion    

A rocket is an internal combustion engine! A rocket carries all its own fuel and oxidizer for combustion process inside the rocket engine A rocket is self-contained and works anywhere (in air, water or in space (vacuum)). A rocket is usually utilized when high thrust (propelling force) and short duration is required

Types of Rocket System    

Liquid Rocket Engines → Liquid Rocket Propellant (Fuel + Oxidizer) Solid Rocket Motors → Solid Rocket Propellant and Charges Hybrid Rocket Engines → Liquid (gaseous) Oxidizer and Solid Fuel Gaseous Rocket Engines → Gaseous Rocket Propellant

Performance Measures      

Thrust Coefficient (Cf) Specific Impulse (SI) Characteristic Velocity (C*) Delta V calculation Equilibrium Pressure Overall Design Constraints and Limitations for comparison and mission applications

Thermochemical Rocket Two principal components: 1. Combustion chamber (or chamber for gas generation – conversion of chemical energy into the potential energy of pressure) 2. Nozzle (conversion of potential energy into the kinetic energy of accelerated gas particles) Pressure ratio is always sufficient to choke flow so convergent-divergent nozzle is always used to generate supersonic exit gas conditions.

Rocket Science Principles 



  

The chemical energy released from a high-pressure combustion reaction of propellant component, usually a fuel and oxidizer, permits the heating of reaction product gases to very high temperature (2500–4100ºC). Since these gas temperatures are about twice the melting point of any metallic material used for production of combustion chambers, it is necessary to cool or insulate all the surfaces that are exposed to the hot gases. These gases, subsequently, are expanded in a nozzle and accelerated to high exit velocities (1800– 4300m/s). A rocket contains both the fuel and oxidizer on board of the vehicle whereas an air-breathing engine (e.g. turbojet, turbofan or ramjet) takes in its oxygen supply from the atmosphere. Consequently, airbreathers are generally superior to rockets in terms of efficiency.

Fundamental Laws of Rocket Science     

Newton’s first law Newton’s second law Newton’s third law Boyle-Marriote’s Law and Ideal gas equation

Newtons Laws 1. A body in motion will travel in a straight line unless acted upon by an external force 2. The acceleration of the body will be proportional to the external force acting on the body a = F/m 3. For every action there is an equal and opposite reaction

Application of Newtons First Law   

In the absence of contrary forces, the speed and direction of an object’s movement will remain constant (inertia) A rocket’s thrust must be great enough to lift its total mass from the launch site or it will not fly Gravity and air resistance (drag) must be taken into account in determining both the required thrust and computing its trajectory to the ultimate target

Application of Newtons Second Law   

Acceleration is the ratio of the applied force to the inertial (rest) mass of the object (F = ma) The greater the amount of thrust developed relative to the mass of the total vehicle, the faster the rocket will move through the air If enough thrust is developed, the speed can be built up until the vehicle can escape the pull of earth’s gravity and move into outer space (11.2 km/s)

Application of Newtons Third Law  

For every force exerted by one mass on another, there is an equal and opposite reaction exerted by the second mass on the first Expulsion of combustion products (gases) through the nozzle produces a reactive force on the rocket in the opposite direction which causes it to be propelled

Application of Boyle-Marriots Law and the Ideal Gas Equations     

Reducing the volume of a container within which a gas is held causes its pressure to increase in direct proportion P1V1=P2V2 As the temperature is raised on a fixed mass and volume of gas the pressure increases PV = nRT (Ideal Gas Law) Propellant combustion increases n and T

Rocket Thrust     

Rocket expels mass at a given momentum rate from the nozzle and receives a thrust in the opposite direction. There may also be a thrust component due to pressure field in nozzle. Thrust may be increased by either increasing propellant flow rate or exhaust velocity. Propellant burning in chamber raises chamber pressure Pressure difference causes gases to accelerate from chamber and through nozzle at high speed

Thrust Equation      

Momentum = mass x exit velocity Thrust (Force)=mass flow rate x exit velocity Thermal to kinetic energy conversion, gas is expanded in nozzle, expanding gas cools and accelerates, thermal energy converts to kinetic, energy is conserved. As the rocket ascends, the ambient pressure decreases so the pressure thrust contribution increases F = mV + (Pe – Pa)Ae This effect is important in nozzle design

Thermochemical Engines and Motors There are several classes of chemical rocket propulsion devices. These are: 1. Liquid propellant 2. Solid propellant 3. Gaseous propellant 4. Hybrid propellant

Liquid Propellant       

Use liquid propellant components are fed under pressure from tanks into the combustion chamber The propellant usually consists of a separate liquid oxidizer and a liquid fuel. Cryogenic or hypergolic propellants, as sub type of liquid propellants Low density propellants Usually very difficult to handle and toxic/dangerous materials In the thrust chamber, the propellants react/combust to form hot gases, which are expelled at high velocity through the convergent-divergent nozzle Liquid Rocket Engines provide high performance, variable thrust profiles but increased complexity and reduced reliability (compared to the solid rocket propellant motors). Liquid rocket engines can be started and stopped, but they are very expensive.

Solid Propellant      

  

Solid propellant–propellant contained within the combustion chamber or case The solid propellant charge (or grain) contains all the elements required for combustion Once ignited, it usually burns smoothly at a predetermined rate on all the exposed surfaces Initial burning takes place at the internal surfaces of the cylinder perforation or slot the internal cavity grows as the propellant is consumed The resulting hot gases flow through a supersonic convergent-divergent nozzle to produce thrust Basic Design Features - four basic components: 1. motor case/combustion chamber 2. nozzle 3. solid propellant grain 4. igniter. High propellant density (volume-limited designs). Long-lasting chemical stability Readily-available, tried and trusted, proven in service

     

No field servicing equipment & straightforward handling Cheap, reliable, easy firing and simple electrical circuits Lower specific impulses (compared with liquid rockets) Difficult to vary thrust on demand Smokey exhausts (especially with composite propellants) Performance affected by ambient temperature.

Gas Propellant   

Use high-pressure gas such as air, nitrogen or helium as their working fluid or propellant. Hybrid propellant rocket engines Use both a liquid and a solid propellant.

Applications of Solid Rocket Motors in (Space) Propulsion    

Space launch vehicle propulsion- Booster Engines ICBM – ballistics missiles Free flight (unguided) rockets, rocket propelled grenades Guided weapons – air to air missiles, anti-ship or anti-tank missiles

Ramjet Rockets    

Air Breathing Solid Propellant Motors Hybrid motor employing both SRM and air-breathing methods for combustion. Fuel is solid propellant and the oxidizer O2 is coming from air Need booster to reach the pressure in combustion chamber

Basic Nozzle Definitions   

Under expanded (Pext – Pa) > 0 Fully expanded (ambient) (Pext – Pa) = 0 Over expanded (Pext – Pa) < 0

Energy Transformation in Nozzle  

The conversion of (PE → KE) Assumptions for Ideal Nozzle : a) Ideal gas is flowing through nozzle. b) Flow is frictionless with no heat or mass exchage/losses. c) No combustion of gases during the flow in the nozzle d) Flow is Quasi stationery and flow is quasi-one-dimensional. e) Cross-sectional transitions in the nozzle are smooth.

MATHS A LOT IN LECTURE 3

Propulsive Characteristics of the Real Nozzle  

Efficiency is always less than 1. Losses can be classified as losses due to: a) Two-phase flow

b) c) d) e) f)

Divergence Kinetics Boundary layer (friction and heat transfer) Immersed nozzle Inefficient combustion

Thermodynamic Processes        

Reversible – Process slow enough that may be reversed. Irreversible – Cannot be reversed Isobaric – At Constant Pressure Isochoric – At Constant Volume Isothermal – At Constant Temperature Adiabatic – No Heat Transfer Occurs Isentropic – No Change in Entropy Cyclic – Initial State = Final State

Polytropic Processes This is a common functional relationship between p and V, whereby:  p Vn = constant  Isobaric, n = 0 (p = constant).  Isothermal, n = 1 (p V = constant).  Isochoric, n = ∞ (V =constant).  Isentropic, n = k (= γ for air)

1st Law of Thermodynamics         

States that for any thermodynamic cycle, cyclic integral of heat equals cyclic integral of work, or where +W is work done by system +Q is heat transferred to system Essentially a Conservation of Energy principle, energy can be neither created nor destroyed, i.e. energy of universe is constant. If concerned with processes rather than cycles, whereby system undergoes change of state, then system energy (E) may also change. Fixed control volume so no mass enters or leaves. Property E represents all available energy, internal (U) + kinetic (KE) + potential (PE). Kinetic (KE) – energy of motion= ½ mV2 m = mass, V = velocity Potential (PE) –e.g energy of position= m g Z g = gravitational acceleration, z = elevation Internal (U) – total microscopic or molecular energy.

p-V Work & Enthalpy    

If process occurs at constant pressure, energy may be transferred to system through internal energy and flow work or mass entering/leaving. Such processes make use of definition of enthalpy (H) = internal energy + flow work, SI unit is J for enthalpy and J/kg for specific enthalpy. Enthalpy may be used to calculate the internal energy of a system during a constant pressure process, using u = h – pv.

2nd Law of Thermodynamics      

Nothing in 1st Law to suggest impossibility of heat engine which could transform continuous amount of heat into equivalent amount of work (i.e. 100% thermal efficiency). 2nd Law says that this is impossible in reality. 2nd Law also says that heat passes naturally from high to lower temperatures A heat engine cannot produce an equivalent amount of work from a heat input. Thermal efficiency (ηther) = ratio of work output (W) to heat input (Qs) must be < 1 and situation shown in figure is impossible Heat always flows naturally from hot to cold bodies

Reversibility & Irreversibility  

Complete reversibility means that a system and its surroundings may be completely restored to its original conditions after a process. Causes of irreversibility include: a) Fluid friction (viscosity); b) Solid friction; c) Electrical resistance; d) Unrestricted expansion; e) Heat transfer; f) Mixing; g) Nonequilibrium chemical reactions;

Entropy               

There is a property called entropy (S) A small change in its value is given by: dS = δQ / T Measures dispersal of energy – how much is spread out or how widely spread out it becomes as a function of temperature. SI units are J / K or J / kg.K (for s) Says that the entropy of the universe increases spontaneously, i.e. energy changes from being concentrated to more spread out if left unhindered. Relates to irreversible processes and the direction they take May also be considered as a measure of the unavailability of a system’s thermal energy to do work (lost work). Entropy is a way of quantifying the 2nd law – the 2nd law says that it can never decrease in a closed system. It is not a measure of a system’s disorder, as commonly believed. Not a property/parameter of the process as the pressure is. No absolute zero - relative to arbitrary reference Entropy change ΔS calculated along any reversible path between 2 states applies to any path between these two states, whether reversible or not. Entropy is thus a property and is a function of other properties, e.g. p & T. For reversible adiabatic (isentropic) process dS = 0. Cycles may be plotted on T - S axes – area under curve = Q for reversible process.

The Energy Equation   

Using the 1st Law of Thermodynamics to study steady, 1-D duct flow between stations 1 and 2 with no work or heat transfer (adiabatic): Expresses local velocity non-dimensionally and is a measure of compressibility effects. Varies locally with changes in u and also with T due to

Subsonic & Supersonic Flows 

Pressure waves cannot propagate upstream to influence flow if supersonic, unlike for a subsonic case.

Isentropic Flow Area Relationship  

Equation m=ρAu applies to isentropic flow in any duct. So: a) if M < 1, negative du/u for positive dA/A b) if M > 1, positive du/u for positive dA/A

Normal Shock Waves   

Only occur when Mach number before shock > 1; Mach number after the shock is then < 1. A normal shock is a nonisentropic process, and acts as a sharp discontinuity to the flow. Static pressure and temperature rises across shock, total pressure drops.

Oblique Shocks          

Mach number behind it is still > 1. Total pressure drop is less than for normal shock. Static pressure, temperature and density rise is less than for normal shock. Shock is “attached” to leading edge of surface. An oblique shock wave may be formed by forcing a supersonic flow to change direction via a wedge. Flow upstream and downstream of shock may be decomposed into components normal (un) and parallel (ut) to shock. Normal components then analysed as if passing through normal shock Parallel components unaffected For any given initial Mach number M1 and flow deviation δ, there are two possible values of the exit Mach number and static and total pressure ratio. The one with the smaller inclination and smaller static pressure ratio is usually referred to as the weak shock (most common) and the one with the higher inclination and static pressure ratio as the strong shock.

Chemicals for Liquid Rocket Propulsion    

Liquid engines are state of the art of internal combustion engines complex design and heavier structure than solid motors Subsystems like pumps, valves, gaskets, pipelines, required Liquid engines can be throttled – variable thrust increases missile manoeuvrability

    

Propellants more difficult to handle and operate than solid prop. (toxic, corrosive) Gelatinized propellants as alternatives Liquid Propellants are less expensive than solids Oxidiser and fuel, stored in separate tanks. Mixed and ignited in the combustion chamber. Storable under “normal conditions”

Monopropellants     

Chemical substance in liquid form capable of being decomposed into hot gases (exothermic reaction) in contact with catalyst – usually metal compound Low-power rocket engines but simple and reliable used as satellite thrusters, as well as APU’s- gas generators Anhydrous hydrazine (N2H4) is most widely used: 2N2H4® N2 + H2 + 2NH3 (over a metal catalyst – iridium) Ammonium Dinitramide (AND) also used (NH4+N(NO2)2¯ in CH3OH/H2O) Dimethylaminoethylaziden(DMAZ)

Bipropellants       

Derivatives of hydrazine with lower freezing points Positive heats of formation (ΔHf) More energy released on reaction Hypergolic (ignite on contact) with many common liquid oxidisers Toxic (0.1 ppm – dangerous concentration). Carcinogens. Monomethylhydrazine (MMH), CH3NH-NH2 Unsymmetrical dimethylhydrazine (UDMH), (CH3)2N-NH2)

Liquid Hydrogen    

  

To exist as a liquid, H2 must be cooled below hydrogen's critical point of 33 K. However, for hydrogen to be in a full liquid state without evaporating at atmospheric pressure, it needs to be cooled to 20.28 K (−252.87°C) Density - 70.85 kg/m3! In most rocket engines fuelled by liquid hydrogen, it first cools the nozzle and other parts before being mixed with the oxidizer (liquid oxygen (LOX)) and burned to produce water with traces of ozone and hydrogen peroxide. Practical H2/O2 rocket engines run fuel-rich so that the exhaust contains some unburned hydrogen. This reduces combustion chamber and nozzle erosion. It also reduces the molecular weight of the exhaust which can actually increase specific impulse despite the incomplete combustion.

GREEN’ LIQUID BIPROPELLANTS • Minimal environmental impact from production of: – ‘greenhouse’ gases – ozone depleting chemicals – acid rain • Non-toxic or very low toxicity – improves safety and reduces costs • Example: hydrogen peroxide (H2O2, oxidiser) + ethanol (C2H5OH, fuel)

– Combustion products mainly CO2 and water

GELATINOUS LIQUID PROPELLANTS • Continuous solid ‘skeleton’ enclosing a continuous liquid phase • Gelling improves safety, handling and storage properties – combines advantages of solids and liquids • Common gelling agents include silicon dioxide and cellulose • Oxidisers are more difficult to make gelatinous than fuels (more corrosive)

HYBRID PROPELLANTS • Solid/liquid bipropellant system – oxidiser is usually ...