Store Separation

Introduction

Store separation testing is vital before a new store (such as a missile or a fuel tank) can be used in service on an aircraft. Separation tests are used to ensure that a store – when released – clears the parent aircraft safely without endangering the aircraft or pilot.

To meet this requirement, ARA has developed a unique store release simulation capability (see the figure above), which it believes to be the most accurate in the World; positional accuracies can be measured to within 0.05", or 1.25mm, which for a 1/10 scale model is 0.5", or 1.25cm, full scale.  The Two Sting Rig (TSR) system, as it is known, achieves this level of accuracy through monitoring the relative positions of lights mounted in known positions on the store and aircraft, using a calibrated camera system.  The TSR is mounted on the sting of the parent aircraft.  It has three large, rotational units, a translational unit, pitch and yaw heads and a roll unit, which together provide the necessary freedom to enable a store to be positioned arbitrarily in relation to a parent aircraft.  Forces and moments measured on a released store are fed into a 6 degree of freedom model which determines the position that the store will move to over a short period of time.  New forces and moments are then measured and the process continues.  The efficiency of the whole approach allows the movement of the store to be continuous, which in turn leads to very efficient testing; coupled with the accuracy of the system, it is apparent that ARA adds very high value in the area of store release testing, particularly when compared with the very expensive alternative of flight testing.

The Role of Wind Tunnels

Wind tunnel testing is critical in a successful store separation test program. Initial tests are carried out by numerical simulation but these tests are considerably simplified due to the very complicated nature of the air flow physics involved.  Wind tunnel tests provide a large amount of high fidelity data to allow the customer to check the accuracy of their computer predictions of how the store will separate; a complete operational envelope can be covered with a fidelity and speed which is far beyond the capability of today’s computers. Wind tunnel tests frequently uncover unexpected and important information which proves very useful in developing both the store design and the release technique.

The Capabilities of the Two Sting Rig

The TSR rig is used for store release testing to provide both Captive Trajectory Simulation and Grid Survey (these types of testing are explained in detail below) data at subsonic, transonic and supersonic speeds. Stores can be positioned with six degrees of freedom - covering almost any possible combination of position and attitude.

Customers using our facility take advantage of:

• Stores positioned in location and attitude with superb accuracy levels
• Excellent productivity – trajectories can be completed quickly with very high levels of fidelity
• Decades of experience testing a wide range of stores and aircraft
• Efficient test campaigns through our ability to validate all set up data and trial and optimise grid surveys ahead of the test entry 
• Advanced release techniques including extremely high incidence tests of up to -120˚ and releases from inside weapons bays 
• A comprehensive range of simulation parameters including fin deployment, pivot hook release, rail constraint and autopilots
• A comprehensive range of internal balances for measuring store loads
• A secure test environment for classified projects
  

Test Technique 1: Captive Trajectory Simulation (CTS)

By continuously measuring the aerodynamic loads acting on a store using an internal balance, and by solving the equations of motion  subject to these loads, a store separation can be  simulated  to  predict  the trajectory a store would take as it leaves the aircraft.   This type of simulation is known as Captive Trajectory Simulation. ARA has developed and validated a highly accurate and efficient CTS model and enhanced it so that a full range of release conditions can be simulated including ejector forces, rocket forces, control forces and G-forces due to pull-up or push-down manoeuvres.

Test Technique 2: Grid Survey

In the Grid Survey approach, store aerodynamic loads are measured at a predetermined array of store positions and attitudes to create a database of the spatial variation of the loads in the proximity of the aircraft.  This database can then be used as a 'look up table' which, given a store position and attitude, can be interpolated on to provide an appropriate set of loads.  In this way, the database can be coupled to an equation of motion solver and the calculation of numerous store release trajectories, with different starting conditions (ejector force, etc.) computed after the Grid Survey data has been measured.  This type of testing is very efficient, but the overall accuracy of the trajectory predictions will be less accurate than CTS testing in highly non-linear regions of the flow field.

Test Technique 3: Freestream Traversing

The TSR can be used for freestream traversing without the parent aircraft present to measure the aerodynamics of the isolated store. In addition, flow angularity and pressures can be measured around the parent aircraft model using flow angularity meters and pitot rakes fitted to the TSR.

Test Technique 4: Free Drop Tests

In addition to the store release simulation techniques using the TSR rig that are offered by ARA, we have experience in conducting free drop tests using mass and inertia scaled stores. Whilst this technique has fallen somewhat out of favour with the development of CTS capabilities, the technique can nevertheless be useful to predict release trajectories involving extreme store attitudes and tumbling stores. The releases are recorded using high speed digital cameras and the recordings are analysed post-test to determine the trajectory.

Technical Information

TSR Store positioning range:

• X, Y, Z = 700mm (28in)
• Pitch, Yaw =  ±29˚
• Roll = ±160˚
• Parent Incidence from -10˚ to +16˚
• Parent Roll = ±180˚

TSR Store positioning accuracy:

• Y, Z = ±1.3mm (0.05in)
• Pitch, Yaw = ±0.15˚
• Roll = ±0.3˚

TSR Productivity:

• All store movement is continuous
• Grid Survey: up to 30 grids per hour
• Trajectory: up to 15 trajectories per hour
• Freestream: up to 20 traverses per hour

TSR Simulated parameters:

• Specified start conditions/angular rates/ linear velocities
• Ejector Release Units, single or twin ram, store or pylon mounted
• Rocket forces
• Autopilots for guided weapon
• Aircraft pull up
• Pivot hook release
• Fin Deployment
• Rail constraint for rail launch releases
• Induced incidence and damping derivative forces

Free Drop Tests:

• Free drop tests normally use the light body scaling technique

A Wealth of Experience

The TSR has been in use at ARA since 1979 when it was created from a joint research and development program between ARA and the UK Ministry of Defence.

Over the past three decades the TSR has been used to support store integration programs for an extensive range of stores released from a wide variety of production and developmental aircraft. These have included Raptor, Stormshadow and Brimstone integration on Tornado and Harrier in the UK. Overseas customers have included Saab Scania with the Viggen and Gripen, KAI with the T-50 and Lockheed Martin with the F-16.

The TSR rig has recently been refurbished to maintain a world-class level of capability and reliability.