Space Propulsion and Power Systems Flashcards
List the three classifications of thrust generating systems
Thermodynamic
Electrostatic
Electromagnetic
List the four applications of rocket missions
Defence and Research Applications
Space Launches
Time-critical maneuvers such as transfers, ΔV<1000m/s
mass-limited missions, ΔV>2000m/s, Low Thrust
List the four types of conventional thermochemical rocket systems
Liquid Rocket - Fuel + Oxidizer
Soild Rocket - Solid Propellant and Charges
Hybrid Rocket - Liquid Oxidizer and Solid Fuel
Gaseous Rocket - Gaseous Propellant
What is the optimal nozzle expansion ratio?
When Pe=Pa, ε = Ae/At
If Pe<Pa, flow is over expanded
If Pe>Pa, flow is under-expanded
Describe the advantages and disadvantages of a liquid rocket engine
Adv: High performance, variable thrust, can be stopped and restarted
Dis: Complex, less reliable, fuel is cyrogenically stored and hypogolic, and low in density even when stored under pressure
List the four basic components of a SRM
Motor Case/Combustion Chamber
Nozzle
Solid Propellant Grain
Igniter
Describe the advantages and disadvantages of a solid rocket propellant?
Adv: High Propellant Density, Long lasting chemical stability, proven in service, cheaper, simplier and more reliable
Dis: Lower Isp, difficult to vary thrust on demand, smokey exhaust, performance affected by ambient temperature
Describe how a ram-jet rocket operates
Ram-jet rockets are hybrid motors using SRM and airbreathing methods for combustion. Fuel is solid propellant, oxidizer is coming from air. Requires a booster to reach pressure in combustion chamber
List the five assumptions for an ideal nozzle
Gas flowing through nozzle is ideal
Flow is frictionless and no heat or mass is exchanged/lost
No combustion of gases during nozzle flow
Flow is quasi-stationary and quasi-1D
Cross section transitions in nozzle are smooth
How do Pressure, Temperature and Velocity vary through a CD nozzle?
Pressure and Temperature start high, with temperature always being greater. Velocity starts low.
Towards throat, v increases exponentially, T and P decrease exponentially. Plots become vertical in throat, then decay exponentially in diverging section
List losses in the propulsive characteristics of a real nozzle as described in the Landsbaum equation
K-in-d-BL-ic-t
Kinematics
Immersed Nozzle
Divergence
Boundary Layer frictional and heat losses
Inefficient Combustion
Two-Phase Flow
Describe the relationships of pressure and volume in polytropic processes
pV(n) = constant
For isobaric processes, n=0, meaning p=constant
For isothermal processes, n=1, meaning pV=constant
For Isochoric processes, n=inf, meaning V=constant
For isentropic processes, n=k (=gamma for air)
Describe the operation of a liquid monopropellant
Chemical substance in liquid form capable of being decomposed into hot gases when in contact with catalyst
low power but simple and reliable
Used in satellite thrusters and APU’s gas generators
Common example is passing Anhydrous hyrdrazine over iridium
Describe how liquid bipropellants are stored under normal conditions and in space
Normal conditions:
Liquid under normal conditions and pressures, stored in tanks and reserviors on board rocket. Liquids are often hypogolic
Space Storable conditions
Liquified gases are stored cyogenically. They have very high performance and used in demanding civil applications
Describe what is meant by ‘green’ liquid bipropellants
Minimal environmental impact from reduced production of greenhouse gases, ozone depleting chemicals and acid rain
Low or non-toxicity, which improves safety and reduces costs
An example is hydrogen peroxide + ethanol, which produces mainly CO2 and water
Describe what gelatinous liquid propellants are
Continuous solid skeleton enclosing a continuous liquid phase.
Gelling improves safety, handling and storage properties. Combine the advantages of solid and liquid propellants. Common examples include silicon dioxide or cellulose
Describe how hybrid rocket motors operate
An liquid oxidizer is connected to the main solid fuel tank with a valve. The oxidizer is injected into the tank and ignited. Varying injection rates enables thrust control. More complex than SRM but more manoeuvrable
How are different propulsion method’s performances measured?
Using:
Characteristic velocity, c * = (Pc * At)/mdot
Specific impulse, Isp = c * x Cf
Thrust, T = Isp x mdot = c * x CF x Sb x r x rho
List the key properties of solid rocket propellants (3,3,4)
Characterstic velocity, c*
Specific impulse Isp
Burning rate r
Density
Mechanical properties
Storage properties
Combustion signiture
Vulnerability and safety in handling
Environmental Impact
Life Cycle costs
List the three main types of solid rocket propellants and their basic composition
Homogeneous (Double Base):
Molecule that contains oxidiser and fuel, such as nitroglyerine
Heterogeneous (Composite):
A crystralline oxidiser in a hydrocarbon based polymetric fuel/binder
Composite double base:
Double base composition with added oxider and metallic fuel
Describe and compare two methods used for creating double base propellants
Extruded - NC/NG mixtures extruded horizontally through die then inserted into motor. Higher c* and burn rate, grains have smaller web thickness
Cast - NG-based casting liquid added to an NC-based casting powder, set at elevated temp inside motor for a few days. Better mechanical properties, single grain can be made with boost and sustain sectors
Describe and compare the two types of composite rocket propellant
Plastic:
Mixture of Ammonium perchlorate, aluminium and preformed thermoplastics. Tends to slump
Rubbery:
AP and Al in cross-linked elastomeric binder/fuel, formed by reacting liquid prepolymer with a curing agent. Usually cast for large motors but can be extruded for small.
Describe additives in double based propellants
Stabilisers: Increase propellant storage life by delaying autocatalytic decomposition of NC and NG
Burning rate catalyst: Increase propellant burn rate and plateauing buring
Non-energetic plasticisers: improve mechanical properties of grain at low temperatures
Describe an additive in rubbery composite propellants
Antioxidants - Extend grain life by suppressing reaction between oxygen and carbon double bonds. Degrades mechanical properties
Describe the advantages and disadvantages of double base (homogenous) propellants
Adv:
Minimum smoke combustion
Super-rate and plateau burning
Dis:
Relatively weak mechanical properites
low energies
Describe the advantages and disadvantages of rubbery composite (hetreogeneous) propellants
Adv
Good mechanical properties at low temperatures
High energy
High densities
low temperature coefficients
Wide range of burning rates
Dis:
Smoky combustion signature
List the 5 zones of burning homogeneous propellant
Virgin Material
Foam Zone
Fizz Zone
Dark zone
Flame Zone
What is erosive burning and why is it detrimental?
Erosive burning is when high-speed combustion gases increase the burning rate of the popellant. It is caused by grain geometry and high initial operation conditions.
It is detrimental as it increases initial thrust and affects stability. Can also damage exposed motor case
Describe the thrust profiles for various military and space rockets
Military rocket motors prefer nearly constant thrust as this minimises the thickness of chamber
Space rockets have regressive thrust to avoid over-accleration for fragile payloads
Air-to-air rockets have large initial thrust followed by continued decreased thrust
Missile Launchers have huge acceleration to burn out before missle clears launch tube
List common propellant geometries and their burn characteristics
Front cigarette burning: No oriffices, long low thrust burn
Central Oriffice: Progressive short burn
Central Oriffice + Slot: Regressive short burn
Star: Progressive with neutral region, long
Describe the upper and lower extremes of recombination reactions for actual expansion
In nozzle, recombinations are too slow due to low temperature from expansion slowing down reaction rates, This makes flow ‘chemically frozen’ and reduces performance
In the case recombination reactioons do occur along flow, it is called equilibrium expansion. Increased performance
Actual performance falls between these two
What is stoichiometric combustion and why is do engines not use stoichiometric ratios?
proportions of fuel and oxidizer are those for which, if the reaction were complete, no excess of either would be left
Ensures maximum heat release per unit mass, however this can clash with limits of materials melting temperature
Jet engines and gas generators in rocket turbopump operate far from stoichiometric ratio
Rocket engines use reactant proportions close to stoichiometric to achieve maximum high specific impulse
Name and Describe the two basic combinations of SRM case and propellant grain
Case Bonded Charge - Propellant grain molded directly in motor case. Thermally insults SRM case during burning
Cartridge Loaded Propellant - Propellant grain molded separately to motor case. Case requires internal thermal insultation
List flight conditions considered when deciding SRM case strength
Must be able to withstand:
Internal combustion pressure
Lateral inflight g forces
Longitudinal g forces during launch
Typical safety factor of 1.2 for UTS
Design pressure: Pd = (1.1 Pc +10 ) bar
List and Describe options for material selection of SRM case
Ratio of max stress to density used as merit
Steel - high performance, low cost, heavy
Aluminium or Titanium alloys - Lighter than steel, high performance, lower thermal resistance and higher costs
Composites - light, good thermal resistance and low conductance, higher cost
What is “Inhibition” in the context of SRM rockets
Inhibition prevents the exposed propellant grain surface from burning. This is done by stopped the propellant from separating under load, and by having similar thermal expansion between propellant and inhibitor
List and Describe various methods for cooling nozzles
Radiation cooling: nozzle is made of lightweight, heat-resistant material. Nozzle walls reach thermal equilibrium.
Heat Sink: Heavy, uncooled nozzle with large thermal capacity used as heat sink. Only works for short burn times
Ablative Cooling: nozzle is lined with fiber reinforced organic material which undergoes progressive endothermic decomposition
Insulation: Low thermal conductivity coatings such as graphite or ceramics used as insulation
Film cooling: A regenerative method using a cool burning propellant at nozzle end to form boundary larger within nozzle
Why are bell shaped nozzles only used for space applications?
On a graph displaying ratio of L/Dt on y axis and Ae/At on x axis, the ideal bell shape gives best performance and lowest losses but also has highest L/Dt ratio
