What are our 3 conservation equations?
Mass (continuity), momentum and energy.
Can mass be created or destroyed in a system?
No! Mass flow in is equal to mass flow out.
How is energy stored in a system ($ E_{stored} $) quantified?
$$ E_{stored} = E_{in} - E_{out} $$
What is the format of a substantial derivative?
$$ \frac{D}{Dt}(x) = \frac{\partial (x)}{\partial t} + \vec{v}•\nabla (x) $$
Do turbofans have only 1 exhaust stream?
No, they have 2 or more.
What happens to the thrust generated by a propeller plane as $ v_0 $ increases (also applies if $ \frac{v_0}{v_e} $ decreases)?
It decreases.
What happens to the propulsive efficiency and thrust as $ v_e $ is reduced to be closer to $ v_0 $ for a propeller plane?
The propulsive efficiency increases, but the thrust reduces.
What are some flow machines that have a heat input, but no net power?
Turbojets, ramjets, scramjets, and pulse jets.
When discussing the effect of flight velocity, if a system used work addition only, what would the thrust depend on?
The velocity ratio $ \frac{v_e}{v_0} $ and the actual value of $ v_0 $.
When discussing the effect of flight velocity, if a system used heat addition only, what would the thrust depend on?
Only on the velocity ratio $ \frac{v_e}{v_0} $ and not the actual value of $ v_0 $.
What is the purpose of a compressor?
To increase the pressure in the system - helps with the combustion reactions to make them more stable and easier to ignite, also allows higher power density to be obtained and can be started from zero velocity.
What two efficiencies are multiplied to get the overall efficiency?
Propulsive $ \eta_p $ and thermal $ \eta_{th} $ efficiencies.
What is the thrust specific fuel consumption (TSFC) for?
Measuring the overall efficiency.
How can specific impulse be formed with respect to the thrust specific fuel consumption (TSFC)?
By taking the inverse of TSFC.
On the plot of propulsive efficiency $ \eta_p $ against the velocity ratio $ \frac{v_0}{v_e} $, does the line corresponding to rockets lie above, below, or overlap the line for turbojet or propellers?
It lies above.
What do hydrogen fuels open up in terms of missions compared to hydrocarbon fuels?
Higher Mach operations.
True or false, the cold bypass stream adds heat, but not work.
False - it adds work, but not heat.
Is it true the core, or hot stream, adds heat but not power, and work is extracted to power the cold stream?
Yes.
As the bypass ratio $ \beta $ increases, what happens to the contribution to specific thrust from the hot and cold streams?
The hot stream decreases non-linearly from 100% and the cold stream increases non-linearly from 0%. The contributions equal one another around a bypass ratio of 1.
What happens to the propulsive efficiency as the bypass ratio increases?
$ \eta_p $ goes up.
What is entropy?
A measure of disorder in a system (ie, energy that cannot be used).
What are the pros of an unmixed turbofan?
Less weight if mixer is removed, can vary fan pressure ratio independently of compressor and turbine pressure ratio.
What are the pros of a mixed turbofan?
Controlled mixing can reduce losses, easier to integrate with the airframe, and reduces exhaust temperature signature.
What are the benefits of a fully mixed internal (to the aircraft) turbofan?
Smaller size which reduced ram drag and overall weight.
What industry consumes the largest amount of aviation fuel in the U.S.?
Commercial aviation at 85%, general aviation at 8%, and then military and government at 7%.
What percent of commercial fuel is used in single and twin aisle planes?
93% - 57% for twin aisle and 36% for single aisle.
For the Brayton Cycle Efficiency, what are two ways to increase $ T_{t4} $?
Burn more fuel and increase the compressor pressure ratio $ \frac{P_{t3}}{P_{t2}} $.
Is it true increasing $ T_{t4} $ decreases the specific thrust?
No! It increases the specific thrust.
What are the assumptions of an ideal engine used for a turbojet with an afterburner?
Perfect gas, no power loss in the shaft, negligible fuel mass flow, and processes are either isentropic and adiabatic or isobaric (constant total pressure).
What is one thing that could modify the total pressure upstream of the inlet?
Shockwaves!
What is the purpose of the diffuser?
To slow fluid and increase static pressure.
What happens to all of the power extracted by the turbine in an ideal engine?
It goes to drive the compressor.
Does a ramjet have a compressor and turbine?
No, it has an inlet, a combustion chamber, and a nozzle.
Why are engine lips smoothed?
To accept flow form all angles
For Fanno Flow, what decreases, increases, or remains constant for a subsonic inlet? (Supersonic inlets have these characteristics flipped).
Static pressure, static temperature, and total pressure decrease. Velocity and Mach number increase. Total temperature remains constant.
For Rayleigh Flow, what decreases, increases, or remains constant for a supersonic inlet? (Subsonic inlets have these characteristics flipped).
Mach number, total pressure, and velocity decrease. Static pressure, static temperature, and total temperature increase. Nothing remains constant.
What are the challenges of supersonic inlets?
They introduce shockwaves and variations in stream tube capture area. The inlet compression becomes a more significant fraction of the overall compression.
What’s the primary goal of a nozzle? What’re secondary goals?
Expand flow to extract maximum possible thrust from exhaust stream. Reduce noise generated, provide thrust vectoring, and reduce signature.
Is it true a variable geometry is not worthwhile for subsonic air transport?
Yes.
What is needed of the fuel in order to ignite it?
It must vaporize.
What are the three T’s of combustion?
Temperature of reactants, turbulence for mixing, and time for combustion to complete.
How is the fuel to air ratio calculated?
The molecular weight of fuel divided by the molecular weight of air.
When is the fuel-to-air equivalence ratio lean, stoichiometric, or rich?
Lean when $ \phi $ is less than 1, stoichiometric when $ \phi $ is equal to 1, and rich when $ \phi $ is greater than 1.
When does the theoretical maximum flame temperature occur?
At stoichiometric conditions.
Given heat of formation, what does a negative value indicate about the energy?
The energy is released in forming the compound.
In terms of chemistry, what states have a values of zero?
Reference states (diatomic gasses).
What is detonation?
The flame front accelerates until it merges with a shockwave.
What is a subsonic flame called?
Deflagration.
For rockets, when does peak temperature occur?
At stoichiometric conditions.
When a rocket runs fuel rich, what does it leave in the exhaust?
H2 - diatomic hydrogen.
In turbo machinery, true or false, stators and vanes are stationary, and rotors and blades are rotating?
True.
What are the 3 components to a velocity triangle and what does each component stand for?
$ \vec{c} = \vec{w} + \vec{u} $; $ \vec{c} $ is the absolute velocity, $ \vec{w} $ is the relative velocity, and $ \vec{u} $ is the velocity of the reference frame.
What are the loss sources for compressors?
Boundary layers, wake mixing, tip vortices, shockwaves, rotor-stator interactions.
What is the compressor dominated by?
Aerodynamics.
What are compressor challenges?
Adverse pressure gradient make blades more likely to stall, very thin, more stages leads to more interaction among stages.
What is the turbine dominated by?
Heat transfer.
What are turbine challenges?
Extremely high temperatures, highly-loaded blades, more stress at the blade root, exposed to combustion products, hot spots from tip leakage.
What direction do compressor airfoils turn the flow towards?
Axial.
What direction do turbine airfoils turn the flow towards?
Tangential.
What are the propeller model assumptions?
Streamtubes entering and leaving defines the flow distinctly, pressure far ahead and far behind match ambient values, and propeller is replaced by an actuator disk that is infinitely thin - no rotation is imparted by the disc and thrust is uniformly distributed over the disc.
How does induced velocity change for a propeller as the coefficient of thrust is increased?
The induced velocity increases.
True or false, higher velocity stream reduces propulsive efficiency.
True.
In terms of propellers, what is pitch?
The distance a propeller would advance if there is no slippage.
What is the difference between fine and coarse pitches for propellers?
For fine pitches, it is appropriate for smaller forward velocities and the thrust will drop off as $ v_0 $ increases and angle of attack decreases or goes negative. As for coarse pitches, they start with a much higher angle of attack and generate significant thrust even at high $ v_0 $.
What is the definition of advance ratio?
How far a propeller advances in one revolution normalized by the propeller diameter.
What are maintenance requirements of rockets?
Frequently overhauled by a team of experts, operated by experts, SSME reliability of 99.95% for an abort on the pad.
What is important about the Mach number in the combustion chamber that allows us to treat static and stagnations quantities as equal?
Mach number is low.
Is it true that conical nozzles are simple and cheap?
Yes.
What is a drawback of conical nozzles?
Flow does not perfectly align with the flight direction which leads to losses.
What is the trade off between small and large conical nozzle angles?
Smaller cone angles will keep the losses small, but larger nozzles will keep the nozzle shorter - cut down on weight. Larger cone angles also risk separation and additional losses.
What are some characteristics of bell nozzle geometries?
Produce a longer, smooth nozzle, but modern nozzles adopt more turning closer to the choke to shorten the nozzle. Still likely has a degree of thrust loss less than 1 ($ \lambda < 1 $) due to truncation or exit pressure variation.
At sea level, will the plume likely be overexpanded or underexpanded?
Overexpanded.
For varying coefficient of thrust $ c_F $ with altitude, what is a benefit of the ideal nozzle?
There are no losses.
For a given chamber pressure to exit pressure ratio, as gamma decreases, what happens to the thrust coefficient?
It increases.
What is the difference between cryogenic and conventionally-stored fuels?
Congenitally stored fuel is liquid at standard atmospheric conditions, whereas cryogenic propellants are stored at extremely low temperatures to liquify.
What is a problem with cryogenic fuel as it slowly vaporizes?
It can cause an increase in tank pressure and will cause a safety issue if not relieved - hence the tanks cannot sit for long periods of time.
What is the difference between hypergolic and ignitable fuels?
Hypergolic fuels can ignite without an initial spark where as ignitable fuel do need an initial spark.
What is the different between bipropellant and monopropellant systems?
In bipropellant systems, the oxidizer and fuel start out separately, are mixed together, then combusted. As for monopropellant systems, they are ready to ignite in their current state, but are inherently unstable.
What is a cold gas thruster?
A single propellant containment in a pressurized reservoir.
Is the Isp peak more pronounced for vacuum or air rocket engines?
Vacuum rocket engines.
What is residence time in terms of rocket combustion?
How long a kernel of flame remains in the combustion chamber.
What does turbulence in a rocket engine depend on?
Injection methods and processes.
Is there coupling between acoustic waves and heat release from combustion reactions?
Yes! This leads to instabilities.
In terms of combustion, what are the 3 instability modes?
Helmholtz resonance (low-frequency rumble typically less than 50 Hz), longitudinal resonance (mid-frequency oscillations ~100 Hz), and transverse modes (high-frequency screech modes greater than 1000 Hz).
What does resonance depend on in combustion chambers?
The combustion geometry and size, propellant flow rate, and rate of heat release (O/R ratio).
Where does the combustion rate increase the most?
At peak pressure of traveling acoustic waves and decreases in low pressure troughs.
What are some techniques for addressing combustion instabilities?
Adding Helmholtz resonators to change resonant frequencies, decouple fuel line acoustics by choking or detuning, break the symmetry in the combustion chamber, or sending pulses to fuel to counteract instabilities.
What are two common options for propellant suppling for rockets? What is the simplest option of the two?
Turbopumps to pressurize propellant and inject into the combustion chamber and pressurized feed systems. The pressurized feed system is the simplest design.
How do pressurized feed systems save weight compared to turbopump designs?
They eliminate the weight from the turbopumps.
Where is the peak heat load in a rocket engine?
Near the throat.
How is cooling often done on rocket engines when convective cooling is not an option/ideal option?
Regenerative cooling.
What is the purpose of a gimbal for a rocket engine?
It connects the engine to the rocket and allows for thrust vectoring. It also carries the full thrust in operation.
What is special about solid rocket propellants?
Solid propellant contains fuel and oxidizer.
Do solid rockets need thrust vectoring?
Typically no, but they can use vectoring.
Between what temperature range do solid rocket propellants typically ignite?
Between 250-350 °C.
What are the type of solid fuels and what are their differences?
Homogeneous Propellant and Composite (Heterogeneous) Propellant. Homogeneous - fuel and oxidizer compounds are mixed and are indistinguishable. Heterogeneous - mixed together as particles of oxidizer and fuel to make grains and flake fuel.
What happens to the burning rate for solid propellants as pressure builds?
The rate increases, then stabilizes.
What is desired for compounds over a pressure range of interest for solid fuels?
The burning rate remains near constant.
What option is correct - with solid fuels, you can get either (a) increasing, (b) decreasing, (c) constant, (d) all of the above, burning areas.
(d) - the grain shape can be adapted to give all of the following burning area options.
What is the relationship between burning area and thrust?
Increasing burning area increases thrust.
How is burning rate commonly found?
Through empirical methods.
For solid rockets, what is known when the burn area and throat area ratio ($ \frac{A_b}{A_t} $) have been chosen?
The chamber pressure.
How can cracks in the grain affects the burn area and, hence, chamber pressure?
They increase $ A_b $ which can lead to sudden increases in $ P_c $.
What can we assume about the throat mass flow rate and produced mass flow rate for solid rockets when operating in steady-state?
They are equal.
In terms of chamber stability for solid rockets, when does the mass flow and chamber pressure relation become linear?
When there is choked throat mass flow.
For solid rockets to be stable, does n have to be greater than or less than 1?
Less than 1 - negative feedback.
In terms of chamber stability for solid rockets, what is the worry when n is greater than 1?
When a small perturbation is introduced in the chamber pressure, a significant increase in the mass flow occurs which is too great to pass through the throat. This increases the chamber pressure, which further increases the mass flow rate. This cycle leads to a runaway affect increasing the chamber pressure.
True or false, oxidizers and fuels are mixed together in a solid structure for hybrid rockets.
False, the oxidizer is often in liquid form and the fuel is in solid form for hybrid rockets. They mix in the boundary layer near the solid fuel surface.
What do hybrid fuels tend to be comprised of?
Polymer-based.
What would be the benefit of adding metal powders to hybrid fuels?
Extra specific impulse.
What are the benefits to using hybrid rockets?
They can stop or throttle the engine, reduced pressure vessel size, increased Isp (still lower than liquid, but higher than solid), able to control thrust/Isp, safer storage than solid, less concern about cracking.
What are some tradeoffs with using hybrid rockets?
It is difficult to keep the O/F ratio constant as the fuel burns and they tend to need multiport systems.
What is a use of the RCS (reaction control system) on spacecraft?
For small delta-v adjustments.
What is the difference between the source efficiency $ \eta $ and the conversion efficiency $ \eta’ $ for spacecraft systems?
The source efficiency describes losses in power generation (RTGs, solar panels, losses in electronics, etc.) and the conversion efficiency is based on the ability for the engine to accelerate propellent.
For spacecraft engines with a constant power level, how will a higher exit velocity impact the thrust?
It will result in a lower thrust.
What propellant is often used for nuclear thermal propulsion? Why?
H2 - diatomic hydrogen is used because it help increase the specific impulse.
What are some concerns with nuclear thermal propulsion?
The engine mass, safety with radioactive materials, throttling is a challenge because flow rate of propellant is tied to reactor cooling.
What is the principle mechanic of electrothermal propulsion?
They apply heat to pressurized propellants to produce thrust.
What is the performance and lifespan limited by for electrothermal propulsion?
The temperature/pressure the chamber material can survive.
For electrothermal propulsion, where are the losses tied to mainly?
Heat transfer losses either through the heating element to the casing, the hot gas to the thruster walls, or the hot propellant radiation to the surroundings.
How do resistojets work?
Use an electric resistance heater running AC or DC power to heat up propellant passing by to the nozzle.
How do arcjets work?
Heat provided by an electrical arc is used to heat propellant.
How do microwave electrothermal thrusters (MET) work?
Microwaves heat gas to form a hot plasma stream.
What are internal modes for molecules?
Electronic energy, vibrational energy, and rotational energy.
Is it true that larger molecules have more degrees of freedom?
Yes.
In terms of a nozzle, how can more residence time be included for the flow to reach equilibrium?
Increasing the length.
How do gridded ion thrusters work?
They collide electrons with neutral propellant atoms to produce ions which are accelerated by a grid.
Why are electrons injected in the exhaust stream for electric propulsion?
The positive ions need to be neutralized, otherwise they will attract back to the spacecraft and momentum will be lost. Spacecraft would also accumulate negative charge.
How does a Hall effect thruster work?
It uses magnetic fields to contain and spin an electron cloud - this acts as the grid in gridded ion thrusters. When propellant passed through this electron cloud, its orbiting electrons are shredded and a positive ion is created.
Why is Xenon used as a propellant for electric propulsion?
It is inert, has lower ionization energy than other noble gasses, and has a higher mass which gives more momentum gain as it leaves the spacecraft.
What is a pulsed plasma thruster?
Uses solid propellant that is vaporized by a spark. The gasses are accelerated by the force of the spark and a magnetic field is then created by the electrical arc; the propellant is fed via a spring feed.
What are some pros and cons to pulsed plasma thrusters?
They can accurately throttle thrust produced by varying the frequency of sparks. They are also simple and have a high Isp, but have high mass flow, low efficiency, and are difficult to scale up to larger sizes.
What is the difference between a self-field and applied-field magnetoplasmadynamic thruster?
The self-field uses the magnetic field naturally produced by the electron flow through the plasma to create a strong magnetic field; this is used to accelerate the plasma. For the applied-field, an additional electric field is used to further accelerate the plasma.
What are the trade-offs of magnetoplasmadynamic thrusters?
They offer potential to scale up to higher power levels and have a high specific impulse when operating at high power levels. However, both options (self- and field-induced) are still in the development phase because they’re difficult to test on the ground and high power units are not readily available.