Propulsion Final conceptual questions and answers 2023
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Does the thrust of a turbojet engine increase or decrease with altitude? Why?
The thrust of a turbojet engine decreases with altitude, principally because of the lower
air density and hence the lower mass flow rate of air ingested into the engine.
Does the thrust of a rocket engine increase or decrease with altitude? Why?
The thrust of a rocket engine increases with altitude, principally because of the
decrease in ambient pressure and hence the increase in pressure thrust.
State two reasons why constant pressure combustors are better than constant area
ones
1. For the same inflow Mach number M3 and stagnation temperature ratio, stagnation
pressure loss is less. 2. Constant pressure combustors do not choke.
Name the three types of combustors used in gas turbine engines. Which is the one that
is in common use today and why?
1. Can type, 2. Can-annular type, and 3. Annular type. Annular type is the one
commonly used today, because the stagnation pressure losses are smaller.
Why is hydrogen easy to ignite and burn compared to kerosene?
Smaller molecular chain; fewer molecular bonds to break.
Why is a normal shock inlet not used at Mach numbers above 1.6 and what is preferred
instead?
The stagnation pressure loss becomes excessive in normal shock inlets when the Mach
number increases above 1.6. External and mixed compression inlets, which make use
of one or more oblique shock waves are preferred instead.
Why is a bell nozzle preferable to a conical nozzle?
For the same expansion ratio, a bell nozzle is shorter and hence lighter.
When is a nozzle a) under-expanded b) over-expanded?
The nozzle is under-expanded when pe > pa and over-expanded when pe < pa.
Why is a perfectly-expanded nozzle preferable in both air-breathing and rocket
engines?
A perfectly-expanded nozzle maximizes the thrust coefficient and hence the thrust.
What are 4 loss mechanisms for nozzles?
1. Under expansion - not expanding flow to pa 2.Divergence - non-uniform exit flow, 3.
Frictional losses, 4. Non-equilibrium chemistry losses
What is the principal advantage of an aerospike nozzle?
It is perfectly-expanded under all ambient pressure conditions.
What velocities do U, C and W indicate in a rotor of a turbomachine?
U is the tangential velocity of the rotor, C is the absolute flow velocity (as seen by a
stationary observer) and W is the relative flow velocity (as seen by an observer moving
with the rotor).
Does the angular momentum of the fluid increase or decrease across the rotor of a
turbomachine?
The angular momentum increases across the rotor in a compressor because the rotor
works on the fluid. It decreases in a turbine because the rotor extracts power from the
fluid.
, If the reaction is 0% (impulse type stage), which blade (rotor or stator) is symmetrical
about its midpoint?
For a 0% reaction stage, the rotor blade is symmetrical about the midpoint. For a 100%
reaction stage, the stator blade is symmetrical about the midpoint.
For what reaction value are the velocity triangles of the stage symmetric?
For a 50% reaction stage, the velocity triangles are symmetric.
Why is a 50% reaction preferred for a compressor but the preferred turbine is an
impulse turbine (0%)?
Because of the adverse pressure gradient faced by the boundary layer in a compressor,
it is desirable to load both the stator and the rotor roughly equally in a compressor and
therefore a 50% reaction is preferred. In a turbine, the favorable pressure gradient
enables impulse turbine to be preferable because of the high power that can be
extracted per stage.
Why is a centrifugal compressor not used in modern high thrust engines and an axial
compressor preferred?
For the same thrust (and hence the mass flow rate), a centrifugal compressor would
have a larger frontal area and hence a higher drag than an axial compressor.
Does the stagnation enthalpy change across a stator? Why?
While the torque on a stator is no-zero, since it is not moving, no work interaction can
take place. Therefore, the stagnation enthalpy (absolute) does no change across a
stator.
What are the two principal advantages of a centrifugal compressor?
It can deliver a high pressure ratio per stage (as much as 10) and a robust and stable
off-design performance, albeit at a lower efficiency.
What type of turbine does the MIT microturbine use and why?
The MIT microturbine uses a radial-inflow turbine, because it has to be fabricated on a
microchip, which rules out axial flow devices.
What propellant combination does Atlas V first stage engine use? Falcon-9? Name
these engines.
Both use RP-1/LOX propellant combination. Atlas V uses Russian RD-180 engine and
Falcon-9 uses Merlin-1D engine.
What is the most commonly used monopropellant and what is the oxidizer commonly
used with in hypergolic bipropellant engines?
Hydrazine (N2H4) is the most commonly used monopropellant. Nitrogen tetroxide
(N2O4) is the most common oxidizer used in hypergolic bipropellants.
Why is a RP-1/LOX hydrocarbon engine desirable for use in the first stage of a launch
vehicle?
The fuel used in a RP-1/LOX engine has a high density (800 kg/m3) and therefore leads
to less bulky and less massive fuel tanks, compared to a cryogenic engine (LH2/LOX),
with fuel density of only 71 kg/m3.
Why does the liquid hydrogen pump require far more power than the liquid oxygen
pump in the SSME turbopump, even though both liquids are injected into the thrust
chamber at roughly the same pressure?
The power needed by a pump is proportional to the head of the liquid being pumped,
which is the exit pressure divided by the liquid density. Liquid hydrogen has a very low
rated A+
Does the thrust of a turbojet engine increase or decrease with altitude? Why?
The thrust of a turbojet engine decreases with altitude, principally because of the lower
air density and hence the lower mass flow rate of air ingested into the engine.
Does the thrust of a rocket engine increase or decrease with altitude? Why?
The thrust of a rocket engine increases with altitude, principally because of the
decrease in ambient pressure and hence the increase in pressure thrust.
State two reasons why constant pressure combustors are better than constant area
ones
1. For the same inflow Mach number M3 and stagnation temperature ratio, stagnation
pressure loss is less. 2. Constant pressure combustors do not choke.
Name the three types of combustors used in gas turbine engines. Which is the one that
is in common use today and why?
1. Can type, 2. Can-annular type, and 3. Annular type. Annular type is the one
commonly used today, because the stagnation pressure losses are smaller.
Why is hydrogen easy to ignite and burn compared to kerosene?
Smaller molecular chain; fewer molecular bonds to break.
Why is a normal shock inlet not used at Mach numbers above 1.6 and what is preferred
instead?
The stagnation pressure loss becomes excessive in normal shock inlets when the Mach
number increases above 1.6. External and mixed compression inlets, which make use
of one or more oblique shock waves are preferred instead.
Why is a bell nozzle preferable to a conical nozzle?
For the same expansion ratio, a bell nozzle is shorter and hence lighter.
When is a nozzle a) under-expanded b) over-expanded?
The nozzle is under-expanded when pe > pa and over-expanded when pe < pa.
Why is a perfectly-expanded nozzle preferable in both air-breathing and rocket
engines?
A perfectly-expanded nozzle maximizes the thrust coefficient and hence the thrust.
What are 4 loss mechanisms for nozzles?
1. Under expansion - not expanding flow to pa 2.Divergence - non-uniform exit flow, 3.
Frictional losses, 4. Non-equilibrium chemistry losses
What is the principal advantage of an aerospike nozzle?
It is perfectly-expanded under all ambient pressure conditions.
What velocities do U, C and W indicate in a rotor of a turbomachine?
U is the tangential velocity of the rotor, C is the absolute flow velocity (as seen by a
stationary observer) and W is the relative flow velocity (as seen by an observer moving
with the rotor).
Does the angular momentum of the fluid increase or decrease across the rotor of a
turbomachine?
The angular momentum increases across the rotor in a compressor because the rotor
works on the fluid. It decreases in a turbine because the rotor extracts power from the
fluid.
, If the reaction is 0% (impulse type stage), which blade (rotor or stator) is symmetrical
about its midpoint?
For a 0% reaction stage, the rotor blade is symmetrical about the midpoint. For a 100%
reaction stage, the stator blade is symmetrical about the midpoint.
For what reaction value are the velocity triangles of the stage symmetric?
For a 50% reaction stage, the velocity triangles are symmetric.
Why is a 50% reaction preferred for a compressor but the preferred turbine is an
impulse turbine (0%)?
Because of the adverse pressure gradient faced by the boundary layer in a compressor,
it is desirable to load both the stator and the rotor roughly equally in a compressor and
therefore a 50% reaction is preferred. In a turbine, the favorable pressure gradient
enables impulse turbine to be preferable because of the high power that can be
extracted per stage.
Why is a centrifugal compressor not used in modern high thrust engines and an axial
compressor preferred?
For the same thrust (and hence the mass flow rate), a centrifugal compressor would
have a larger frontal area and hence a higher drag than an axial compressor.
Does the stagnation enthalpy change across a stator? Why?
While the torque on a stator is no-zero, since it is not moving, no work interaction can
take place. Therefore, the stagnation enthalpy (absolute) does no change across a
stator.
What are the two principal advantages of a centrifugal compressor?
It can deliver a high pressure ratio per stage (as much as 10) and a robust and stable
off-design performance, albeit at a lower efficiency.
What type of turbine does the MIT microturbine use and why?
The MIT microturbine uses a radial-inflow turbine, because it has to be fabricated on a
microchip, which rules out axial flow devices.
What propellant combination does Atlas V first stage engine use? Falcon-9? Name
these engines.
Both use RP-1/LOX propellant combination. Atlas V uses Russian RD-180 engine and
Falcon-9 uses Merlin-1D engine.
What is the most commonly used monopropellant and what is the oxidizer commonly
used with in hypergolic bipropellant engines?
Hydrazine (N2H4) is the most commonly used monopropellant. Nitrogen tetroxide
(N2O4) is the most common oxidizer used in hypergolic bipropellants.
Why is a RP-1/LOX hydrocarbon engine desirable for use in the first stage of a launch
vehicle?
The fuel used in a RP-1/LOX engine has a high density (800 kg/m3) and therefore leads
to less bulky and less massive fuel tanks, compared to a cryogenic engine (LH2/LOX),
with fuel density of only 71 kg/m3.
Why does the liquid hydrogen pump require far more power than the liquid oxygen
pump in the SSME turbopump, even though both liquids are injected into the thrust
chamber at roughly the same pressure?
The power needed by a pump is proportional to the head of the liquid being pumped,
which is the exit pressure divided by the liquid density. Liquid hydrogen has a very low
