Aerodynamics and Propulsion Analysis
The Aerodynamics and Propulsion tabs of a missile model function together as an analysis system that performs a trim calculation - as the missile flies - to compute lift, drag, thrust, throttle, and fuel consumption parameters at any given flight condition for the specified trajectory. The input to this system is the mission flight path while the outputs are the aerodynamics and propulsion data. The fuel flow is integrated into the weight of the missile, but these values do not directly influence the flight path.
The system generates warnings when AOA limits are exceeded or when thrust deficits exist indicating the missile design is not capable of flying the path specified. The path is still flown as designed. This feature allows you to explore a missile design (perhaps based on high accuracy wind tunnel and engine test data) and its suitability to perform a desired mission.
Aerodynamics
The Aerodynamics tab is used to define the methods used to compute lift, drag, , sideslip and intermediate / derived values. There are four aerodynamics strategies to choose from, which are covered below.
Advanced Missile
The Advanced Missile aerodynamics strategy allows you to enter the dimensions of the missile's body, wings, and tail, and calculates the aerodynamic performance of the missile in terms of the specific size and shape that has been defined.
The Plot Aero... option enables you to evaluate how parameter changes affect the missile's performance.
High Fast
The High Fast aerodynamics strategy uses thrust to generate a lift vector, which provides the ability to track fuel burn during lift. Additionally, it generates the forces perpendicular to the velocity vector to provide maneuvering.
The High Fast aerodynamics strategy must be paired with its High Fast propulsion model counterpart.
Parameters
| Parameter |
Description |
| Drag Area |
A vehicle's drag is a function of dynamic pressure. Drag area is calculated by multiplying drag coefficient by the reference area, which is then multiplied by dynamic pressure to understand the vehicle's drag force. |
Missile External Aero
The Missile External Aero strategy uses aerodynamic data supplied by an .aero file.
Parameters
The Aero File field displays the currently selected file; click 
to browse to the file that you want to use. The Reference Area field defines the area of the lift surface of the aircraft.
The Plot Aero... option enables you to evaluate how parameter changes affect the missile's performance.
Click Save... to save a copy of the file to your catalog file area, which is the default location that STK's Aviator capability opens when you select an .aero file to use. Click Reload if you want to update the scenario copy of the currently selected file with changes that have been made to the source file.
Simple Missile
The Simple aerodynamics strategy uses several parameters to define the aerodynamic performance of the missile in terms of a single lifting surface.
Parameters
| Parameter |
Description |
| Reference Surface Area |
The area of the lifting surface of the missile. |
| Cl Max |
The maximum coefficient of lift. |
| Cd |
The coefficient of drag of the lifting surface, representing Cd0 in the equation, Cd = Cd0 + K * Cl^2. |
| Angle of Attack |
By default, the missile's Angle of Attack is 0. Select Calculate AOA to allow the value to be determined by STK's Aviator capability and, if so, define a Max AOA value. Aviator will compute the lift curve slope as: Cl Max / Max AOA
|
Propulsion
The Propulsion tab is used to define the rate at which the missile speeds up or slows down and provides a method for computing the fuel flow. This involves computing the thrust requirements and the throttle setting for any given flight condition which requires a full aerodynamics calculation.
The propulsion models provided with STK separate the acceleration and deceleration speed changes from the thrust available as computed by the models. This is done for ease of use and to allow for quick construction of flight paths without constraints imposed by the propulsion system. AGI recommends that you fine tune these separate parameters so that they result in a faithful representation of actual performance.
There are seven propulsion strategies to choose from, which are covered below. For any strategy, select No Thrust When No Fuel to prevent the missile from thrusting once it has expended all of its fuel.
Missile - Ramjet
The Ramjet strategy models a ramjet engine, which provides a simple propulsion system for high-speed flight.
The Plot Prop... option enables you to evaluate how parameter changes affect the vehicle's propulsion performance.
Parameters
| Field |
Description |
|
|
The Altitude, Number, and Thrust that comprise the design point of the engine. |
| Engine Temperature |
The maximum temperature that the engine material can support. |
Click Advanced... to define additional properties such as pressure ratios.
Boost / Sustain Mode
For Ramjet, Scramjet, and Turbojet strategies, you can select Use Boost / Sustain Mode to model an engine that uses a boost phase to achieve a target velocity and then transitions to a sustenance phase to maintain that speed. The Boost Fuel Fraction parameter defines the amount of fuel that is consumed during the boost phase, while the Boost Fuel Flow parameter defines the amount of fuel flow during that phase.
Missile - Rocket
The Rocket strategy models an aircraft that uses a rocket engine for propulsion.
The Plot Prop... option enables you to evaluate how parameter changes affect the vehicle's propulsion performance.
Parameters
| Field |
Description |
| Nozzle Expansion Ratio |
The exit area divided by the throat area. |
| Nozzle Exit Diameter |
The diameter of the nozzle exit. |
| Propellant Specific Heat Ratio |
The ratio of the propellant's constant-pressure specific heat to the constant volume specific heat. |
| Propellant Characteristic Velocity |
A method to evaluate combustion performance independent of nozzle performance:
Chamber Pressure * Throat Area / Mass Flow Rate of the engine |
| Combustion Chamber Pressure |
The pressure in the combustion chamber. If you are modeling a rocket engine using the Boost / Sustain mode, this property will only define the pressure during the sustenance phase.
|
Boost / Sustain Mode
Select Use Boost / Sustain Mode to model a rocket engine that uses a boost phase to achieve a target velocity and then transitions to a sustenance phase to maintain that speed. The Boost Fuel Fraction parameter defines the amount of fuel that is consumed during the boost phase, while the Boost Combustion Chamber Pressure parameter defines the combustion chamber pressure during that phase.
Missile - Scramjet
The Scramjet strategy models a scramjet engine, which is a low-weight engine that provides high thrust and is suitable for hypersonic flights.
The Plot Prop... option enables you to evaluate how parameter changes affect the vehicle's propulsion performance.
Parameters
| Field |
Description |
| Design Point |
The Altitude, Mach Number, and Thrust that comprise the design point of the engine. |
| Limits |
The maximum compression temperature and maximum burner temperature that the engine can support. |
| Fuel Type |
The type of fuel used by the engine. |
Boost / Sustain Mode
For Ramjet, Scramjet, and Turbojet strategies, you can select Use Boost / Sustain Mode to model an engine that uses a boost phase to achieve a target velocity and then transitions to a sustenance phase to maintain that speed. The Boost Fuel Fraction parameter defines the amount of fuel that is consumed during the boost phase, while the Boost Fuel Flow parameter defines the amount of fuel flow during that phase.
Missile - Turbojet
The Turbojet strategy models a turbojet engine.
The Plot Prop... option enables you to evaluate how parameter changes affect the vehicle's propulsion performance.
Parameters
| Field |
Description |
| Design Point |
The Altitude, Mach Number, and Thrust that comprise the design point of the engine. |
| Turbine Temperature |
The maximum temperature that the turbine material can support. |
| Compressor Pressure Ratio |
The maximum compressor pressure ratio. |
Click Advanced... to define additional properties such as pressure ratios and efficiencies.
Boost / Sustain Mode
For Ramjet, Scramjet, and Turbojet strategies, you can select Use Boost / Sustain Mode to model an engine that uses a boost phase to achieve a target velocity and then transitions to a sustenance phase to maintain that speed. The Boost Fuel Fraction parameter defines the amount of fuel that is consumed during the boost phase, while the Boost Fuel Flow parameter defines the amount of fuel flow during that phase.
Missile External Prop
The Missile External Prop strategy uses propulsion data supplied by a .prop file.
The Plot Prop... option enables you to evaluate how parameter changes affect the vehicle's propulsion performance.
Parameters
The Prop File field displays the currently selected file; click to browse to the file that you want to use. The Reference Area field defines the area of the lift surface of the aircraft.
Click Save... to save a copy of the file to your catalog file area, which is the default location that Aviator opens when you select a .prop file to use. Click Reload if you want to update the scenario copy of the currently selected file with changes that have been made to the source file.
Simple Missile
The Simple Missile propulsion strategy uses basic Max Thrust and Fuel Flow parameters to define the thrust performance of the missile.
High Fast
The High Fast propulsion strategy provides propulsive force, which overcomes drag and provides the force to accelerate.
The High Fast propulsion strategy must be paired with its High Fast aerodynamics model counterpart.
Parameters
| Parameter |
Description |
| Lift Specific Impulse |
The measure of how much thrust force the aircraft gets per unit of fuel flow. |
| Max Thrust Lift |
The maximum lift force that your system can generate.
|
| Propulsive Specific Impulse |
The potential energy of a propulsion system, measured in seconds. Specific impulse measures how long an engine exerts a continuous pound of force until fully burning through a pound of propellant. |
| Max Propulsive Thrust |
The maximum thrust when the system is at maximum throttle. |