All six divots of thermal insulation foam have been ejected from the flight test fixture on NASA's F-15B testbed as it returns from a LIFT experiment flight.

The Shuttle Return to Flight team requested data on the structural survivability of external tank insulating foam debris or "divots" that are shed from the tank during a Shuttle launch. The Lifting Insulating Foam Trajectory (LIFT) flight test series used NASA's F-15B to test these “divots” in a real flight environment at speeds up to about Mach 2.

Small-scale divoting, commonly called popcorning, results from adhesive strength failure of external tank thermal protection system (TPS) foam brought about by decreasing atmospheric pressure combined with increased heating during Shuttle ascent.

According to LIFT project manager Stephen Corda, objectives of the flight tests on the F-15B included determining divot structural survivability in a flight environment; assessing divot stability; quantifying divot trajectories using videography; and providing flight verification of debris tracking systems to be used for Shuttle launches.

"We're using the unique capabilities of the supersonic F-15B aircraft and the aerodynamic flight test fixture to provide a means to eject these debris or divots from the fixture, and then photograph them with a high speed digital video system, where we're able to video these divots in flight at up to 10,000 frames per second," Corda noted.

The debris tracking systems were verified using the F-15B as a surrogate Space Shuttle while the aircraft ejects TPS foam divots. These tracking systems included a Weibel Doppler radar and a high-definition video system aboard a NASA WB-57 aircraft.

NASA' s Space Shuttle Systems Engineering and Integration office at the Johnson Space Center (JSC) in Houston, Texas, funded the LIFT flight tests at NASA Dryden as part of the STS-114 Return-to-Flight effort. JSC aeroscience engineer Ricardo Machin said the current LIFT flight tests will help them validate the models that they use for debris transport analysis.

"In particular, it's going to help us understand whether the divots break up once they come off the external tank, and secondly whether they will trim and begin to fly, or if they'll tumble. The difference between trimming and flying makes a huge difference – the amount of kinetic energy that this piece of debris can impart to the shuttle," Machin said.

The LIFT flight test requires two new capabilities: an in-flight foam divot ejection system, and a high-speed video system to track and record the trajectories of the divots in flight. Both capabilities were developed by Dryden engineers.

Dryden's LIFT team designed, fabricated, and ground-tested four different divot ejection systems, completing 70 ground tests to determine and refine the best approach.

NASA Dryden engineers designed and procured the very high-speed digital video equipment, including development of a system to synchronize the cameras with the divot ejection system. In addition, they developed videography analysis techniques in order to quantify divot trajectories.

The Dryden team completed the design and ground tests of these systems over a compact 2 1/2-month period.