It’s a ‘Go’ for the Unmanned Space Vehicle at CIRA
The Renaissance Italians gave us Machiavelli, Michelangelo, and Leonardo Da Vinci. So it shouldn’t be surprising to discover that the 21st century Italians are also putting their name on Europe’s aerospace renaissance. CIRA, the Italian Aerospace Research Centre, is contributing valuable research and development know-how to numerous European projects.
One project where researchers are harvesting know-how at CIRA is the development of the Unmanned Space Vehicle (USV for short). Aimed to assist the international research community, the in-development USV will be a multi-purpose, multi-mission flying laboratory capable of atmospheric re-entry from low-earth orbit. Designed to act like a civil aircraft without human pilots, the USV is actually a flying test bed where scientists can test materials, verify structural and aerodynamic behavior, develop advanced guidance and navigation and control functions as well as other typical technology associated with aerospace research. Eventually, the USV team at CIRA hopes to be able to offer the USV’s flight and lab space as a service to validate space equipment to ESA or NASA standards.
Castore: The first flight
The first prototype USV-1 (named Castore) took its maiden voyage above the sunny skies of Sardinia, Italy on March 24th, 2007. This first campaign addressed such space ‘basics’ like aerodynamic performance and flight behavior during transonic flight. In other words, the goal was to make sure that the winged USV-1 could withstand the scorching heat and Mach speeds of the planned re-entry trajectory.
On the big day, the 1250 kg USV with its 3.54m wingspan was airlifted using a stratospheric balloon and released approximately 20 km above the Earth. From this perilous point, the USV cruised at a transonic speed of Mach 1.1 – powered solely by gravity. During the deceleration phase, a three-stage parachute system acts like a set of Ferrari F1 space brakes, allowing the USV to plunge into the sea for recovery and re-use.
During the first flight, data was collected using a system of 500 sensors and relied off ESA’s Artemis satellite to the Italian ground team. The data collected from this first flight is currently being studied to improve the second planned USV prototype: Polluce, a vehicle that will be able to reach 82,000 feet (25km) altitude and travel at Mach 1.2 (1500km/hr).
Preparing for the big day
Prior to the first flight, years of back office work went into the USV project. One aspect was the ground vibration test or GVT to confirm the vessel’s structural soundness under extreme aerodynamic loads. This critical aerospace test was in the hands of Vincenzo Quaranta, PhD., Senior Scientist, Experimental Vibro-acoustics Dean at CIRA. It was his team’s task to make sure that the “bird” could withstand the extreme aeroelastic dynamic loads.
Designing a test strategy
Starting from the FE model, Dr. Quaranta and his team designed the test strategy and determined the optimal location for sensors and shakers, using numerical modal data. LMS Virtual.Lab was used to find the optimal location for sensors and an internal solution OFP (Optimal Force Pattern) was used to determine the number and location of the shakers. This is known as the virtual GVT.
To replicate the ideal world of numerical simulation as close as possible, a dedicated suspension system was manufactured specifically for the test. During the actual GVT, the USV prototype itself was suspended by bungee cords in this dedicated suspension system.
To prepare for the actual tests to be run on a 100-channel LMS SCADAS III system in combination with LMS
Test.Lab, the USV-1 was instrumented with 74 accelerometers and 4 shakers. MAC (Modal Assurance Criterion) was used to verify the quality of the modal results. This same MAC criterion was used to correlate the test modes with the ones predicted in the models.
The streamlined LMS solution
Using the LMS solution, Dr. Quaranta and his four-person team performed the GVT over several days. Both phase separation and normal mode techniques were used during the GVT to identify the structural resonances in the respective frequencies and acquire FRFs (Frequency Response Functions) used to extract the modal parameters with the LMS Test.Lab PolyMAX tool. From this information, Dr. Quaranta and his team could provide the right data to predict the flutter parameters of the USV and approve the structural design for its first flight.
“While performing the GVT on our CIRA Unmanned Space Vehicle, I was really amazed by the impressive performance of the latest version of LMS Test.Lab. Not only were we confident that the LMS GVT solution could provide us with the right type of results – an accurate and reliable data set – it also runs on a standard Windows XP laptop. This is much more streamlined compared to the older version we used to run on a powerful yet bulky UNIX workstation. This new set-up is ideal for our lab at CIRA. With a campus this size, we need a solution that
can easily be packed up and moved from test building to test building,” stated Dr. Quaranta.
He concluded, ''LMS’ specialized team - especially the on-site team in Novara, Italy really have helped us get the most out of the LMS Ground Vibration Testing solution which combines LMS Test.Lab software and LMS SCADAS hardware. They not only helped to customize our solution exactly to our needs – integrating our own code where we needed to - they also were there to help find a solution to any issue that we had.”
