Missile Models

STK Premium (Air) or STK Enterprise
You can obtain the necessary licenses for this tutorial by contacting AGI Support at support@agi.com or 1-800-924-7244.

Capabilities Covered

This lesson covers the following STK Capabilities:

• Aviator Pro

Editing Missile Properties

STK's Aviator Pro capability enables you to model the performance characteristics of missiles in addition to aircraft. You can create a missile model in the User Missile Models catalog of the Aviator catalog interface or catalog manager.

The Missile properties window is used to define the global settings and performance models of a missile model in your User Missile Models catalog. Any changes that you make in this window are applied to the missile's definition in the catalog.

The Missile properties window is comprised of three tabs - Performance Models, Aerodynamics, and Propulsion. The Performance Models tab defines the basic turning, climb and descent transition, cruising, and attitude transition characteristics of the missile, while the Aerodynamics and Propulsion tabs allow you to select and define strategies to model attitude and propulsion characteristics, respectively.

When you are done editing the missile's properties, click Save to save any changes you have made and click Close to close the window; you will be prompted to save any changes before closing the window.

Performance Models

The Performance Models tab is comprised of five sections - Level Turns, Attitude Transitions, Climb, Cruise, and Descent - as described below. As they do with aircraft models, performance models define the behavior of the missile in flight.

Level Turns

The Max Load Factor that the missile can withstand while maneuvering. The value specified for this parameter is the level turn value for the missile. Aviator adheres to this value when possible, but in procedures where the turn is non-level the value may be adjusted to maintain the correct relationship between this and other interrelated parameters.

Accel Maneuver Mode Field

You can toggle if the aircraft turn or push/pull calculations use atmosphere density scaling. Alternatively, you can define specific parameters used in the turn calculation for more realistic results.

Table -Accel Maneuver Mode Options

Attitude Transitions

Missile attitude is determined using a 123 Euler angle sequence of Bank, Angle of Attack The angle between the body X axis and the projection of the velocity vector onto the body XZ plane. The velocity vector is the velocity of the object as observed in the object's central body fixed coordinate system., and Sideslip, originating from a velocity aligned, nadir constrained set of axes. Attitude rates may be violated in the case of very short - or zero distance - procedures.

Table - Attitude Transitions Parameters

Parameter	Description
Roll Rate	The standard roll rate - the rate at which the missile bank angle changes - of the missile in a turn. When Aviator violates the specified Turn Roll Rate, the probable cause is an unrealistic climb or descent model, or the use of climb, descent, level turn and speed change parameters that aren't well matched to the roll rate parameters of the missile.
AOA/Pitch Rate	The pitch rate when transitioning between attitude modes, between procedures, and between uncoordinated maneuvers when necessary.
Sideslip/Yaw Rate	The yaw rate when transitioning between attitude modes, either triggered by changes in the acceleration performance model or between takeoff/landing, normal flight, weight-on-wheels, or hover mode.

Always Ignore Flight Path Angle for Climb and Descent Transactions

When you are working at the design limits of an aircraft model, you may commonly encounter problems with:

• pushing over at high path angles
• pulling up at low flight path angles

For example, an aircraft flying at high altitude and high speed may not have enough control authority to push over as a procedure requires. Or, the aircraft may need to violate another constraint such as the procedure ceiling.

In these situations, you can select the Always Ignore Flight Path Angle check box to ignore load factor limits. This option enables you to suspend these limits without needing to change to the aircraft model that you are using.

Climb

The Climb performance model is comprised of a simple set of parameters that define the flight characteristics of the missile while climbing.

Table - Attitude Transitions Parameters

Parameter	Description
Airspeed	The standard airspeed of the missile while climbing. You can select the airspeed reference of the performance model - true airspeed (TAS True Airspeed: the speed that the aircraft is moving relative to the airmass that it is flying in.), calibrated airspeed (CAS Calibrated Airspeed: the speed reported by the airspeed indicator, corrected for position and instrument error.), equivalent airspeed (EAS Equivalent Airspeed: the airspeed at sea level (International Standard Atmosphere) at which the dynamic pressure is equal to the dynamic pressure at the aircraft's current true airspeed and altitude.), or Mach The ratio of the aircraft's speed and the speed of sound at the aircraft's altitude, with local atmospheric conditions. number - using the drop-down menu.
Min Flight Path Angle	The minimum pitch angle of the missile's flight path while climbing.
Max Flight Path Angle	The maximum pitch angle of the missile's flight path while climbing.
Fail Climb when performance is insufficient	Select this check box to have a mission not propagate if the missile does not have sufficient Specific Excess Power (Ps) for the climb performance model.

Cruise

The Cruise performance model is comprised of a simple set of parameters that define the flight characteristics of the missile while cruising.

Table - Attitude Transitions Parameters

Parameter	Description
Max Airspeed	The maximum airspeed of the missile while cruising. You can select the airspeed reference of the performance model - true airspeed (TAS), calibrated airspeed (CAS), equivalent airspeed (EAS), or Mach number - using the drop-down menu.
Default Cruise Altitude	The missile's default cruising altitude. You cannot specify an altitude below sea level (0). If a procedure is specified that requires the missile to fly below sea level, the performance models in effect at sea level will be applied to that procedure.
Enable low speed at high altitude	Checking this box prompts Aviator to ignore aerodynamic lift considerations when computing minimum airspeed for a given flight condition. This enables you to build trajectories even if the vehicle cannot fly the specified trajectory. Features that depend on having enough lift to perform the feature may fail, but features that don't explicitly require the ability to generate sufficient lift should succeed.

Descent

The Descent performance model is comprised of a simple set of parameters that define the flight characteristics of the missile while descending.

Table - Attitude Transitions Parameters

Parameter	Description
Airspeed	The standard airspeed of the missile while descending. You can select the airspeed reference of the performance model - true airspeed (TAS), calibrated airspeed (CAS), equivalent airspeed (EAS), or Mach number - using the drop-down menu.
Min Flight Path Angle	The minimum pitch angle of the missile's flight path while descending.
Max Flight Path Angle	The maximum pitch angle of the missile's flight path while descending.
Fail Descent when performance is insufficient	Select this check box to have a mission not propagate if the missile does not have sufficient Specific Excess Power (Ps) for the descent performance model.

Limit Fields

Optionally, you can define limits to the performance model system to augment the values used for climb, cruise, and descent performance model speeds. Select the check box next to the applicable limit(s) to use.

Aviator does not check if these limits are compatible with the specified Climb, Cruise, and Descent speeds. If they are mismatched, the system can allow the speed during a climb or descent to momentarily violate one of the limits, even though the ending speed satisfies the limits.

Table - Limit Parameters

Parameter	Description
Limit Total Temperature	The maximum allowable total temperature.
Limit Mach Number	The maximum allowable mach number. This will override any speed values specified for Climb, Cruise, or Descent when enabled.
Limit Equivalent Airspeed	The maximum allowable Equivalent Airspeed. This will override any speed values specified for Climb, Cruise, or Descent when enabled.

Dynamics/Moments

The Dynamics/Moments tab allows you to specify control parameters for the aircraft. These allow calculation of aircraft performance characteristics, but are not used to calculate trajectory. Required Control Authority allows you to specify what angular rates are expected to safely control the vehicle. Control Surface Buildup allows Aviator to calculate control forces available in each axis and is used to compute flight profile Control Force Required and FullControl.

Table - Dynamics/Moments

Parameter	Description
Surface Area	Surface area of relevant control surface; aileron, rudder, or elevator or X, Y, Z axis respectively
Moment Arm Length	Distance of center of force applied by control surface from aircraft center of gravity specified in configuration inertia tab.
CLMax	Maximum lift coefficient used to calculate maximum force per unit area control force can apply.

Aviator calculates control forces as torques throughout the flight. The max force available for the torque is determined from Surface Area, CLMax, and flight conditions based on:

Item	Description
F	Force
p	Air density determined by aircraft altitude
V	Aircraft velocity
S	Surface area determined above

The max available control torque along an axis is the product of this force and the proscribed Moment Arm Length.