Mikrotron Cameras Provide Needed Speed for Testing Objects Ejected from Military Jets

Summary

Aug. 5, 2022 - In military aircraft, ejecting ammunition, capsules, missiles and auxiliary fuel tanks during flight is fraught with danger. From the moment "the store" is ejected, there is an interaction between the aerodynamic environment and the dynamic characteristics of the store that can result in it colliding with the aircraft, potentially causing severe damage or a total loss.
 
Before the 1960s, there were practically no widely used or generally accepted methods for pre-flight prediction of store separation trajectories other than wind tunnel testing. With the advent of modern attack aircraft, the need arose to carry more stores and release them at ever-increasing speeds. Today, the most common store separation test technique is a pitch drop followed by in-flight testing.
 
Performed on the ground, pitch drops are necessary to establish that the aircraft / store configurations meet a minimum degree of safety before the far more dangerous flight test. As the name suggests, the store is dropped from the aircraft and its pitch is measured. The established method of acquiring the store's trajectory data during a pitch drop is "photogrammetry," that is, the interpreting of images of recorded radiant electromagnetic energy and other phenomena. Photogrammetry relies on high-speed cameras and 3D software to determine the position and attitude history of the store while it is still under nonuniform aerodynamic interference near the aircraft. The largest drawback to this method is that reconstruction and analysis of the 3D separation path is done post-processing, which is time-consuming, labor-intensive, and creates delays for the in-flight testing.
 
In Brazil, the Flight Research and Testing Institute (IPEV) has carried out store separation tests for years. In order to eliminate the use of the traditional 3D commercial software tools required for photogrammetry, and to have access to near real-time analysis in pitch drop testing, IPEV and its partners developed a new imaging solution that reduces the number of stores needed for in-flight tests by maximizing the engineering data collected during pitch drops. Benefits of the solution include improved use of resources, minimization of workload, and reduced costs and time. 
 
To capture images of the store as it leaves the vicinity of the aircraft, cameras were mounted directly on the aircraft. IPEV selected two Mikrotron MotionBlitz Cube color cameras configured at 400 frames per second and synchronized with each other using an internal camera mechanism to capture multiple images simultaneously.

In these types of tests, a very high frame rate is needed since the typical store will travel from its initial captive position to the bottom of the camera view in 0.2 to 0.4 seconds depending on the distance of the camera and the lens chosen. At 400 frames per second, this typically will produce 80 to 160 usable frames of data in these types of tests.
 
To perform the testing, both Mikrotron cameras were mounted externally on the aircraft in a photographic POD to record movements during the release of the store, and were oriented to maximize overlapping field-of-view. The rear camera was configured with region of interest (ROI) of 1184 x 1040 pixels with the front camera set at 1248 x 968 pixels. The aircraft used for testing was an EMBRAER Xavante military jet carrying an inert store weighing approximately 130 kg. Testing was performed at the IPEV facilities in São José dos Campos, SP, Brazil.
 
Markers were identified on the aircraft's wing and pylon to help verify the position of the store relative to the aircraft. Both are rigid structures that will experience very little variation of position during an actual flight or separation. In addition, before the tests were carried out, a calibration of the cameras was done using a geometric calibration field built by IPEV. Calibration is required to determine the distortion characteristics of lens curvature on the measurements. Markers and calibration are crucial to the system's ability to accurately determine the translational and rotational kinematics of the stores and to support decisions about the potential safety risk of the test when done in-flight.
 
After each pitch drop test, the Mikrotron camera images were downloaded and a time slice of 0.5 to 1 second was defined. Applying the calibration of the lenses, plus a survey of the aircraft's target and the image data tracked to the markings on the wing and pylon, IPEV was able to estimate the location of the store using an iterative solver in near real-time.
 
The photogrammetric solution proposed by IPEV is promising, as it obtained errors of the order of millimeters that are considered acceptable for this type of test. Relying upon the performance of Mikrotron high-speed cameras is assisting IPEV in its continued development of a real-time analysis solution for store separation testing.