Characterizing engine ice, solving a GPS mystery

By Ashlie Flegel, Dale Ferguson, and Justin Likar|December 2016

The Atmospheric and Space Environments Technical Committee encourages the exchange of information about the interactions between aerospace systems and their surroundings.

NASA’s Glenn Research Center in Ohio this year completed two tests in its Propulsion Systems Laboratory (PSL) 3 facility focused on increasing the understanding of engine ice-crystal icing. This phenomena can lead to power-loss events and can occur when jet engines ingest a large amount of ice crystals during flight in clouds.

A full-scale engine ice-crystal icing test was completed in November 2015. This test utilized a heavily instrumented engine model, specifically an unmodified ALF502 engine that had previously experienced an uncommented loss of thrust event called a rollback. Engines of this model have been modified to eliminate engine icing issues. Advanced instrumentation and internal cameras characterized the cloud entering the engine, detected and characterized ice accretion, and visualized the ice accretion in the region of interest. During this test, PSL replicated key in-flight icing event points including a rollback and nonrollback event. Icing videos inside a full-scale engine during ice-crystal icing were acquired for the first time. This study has enabled advanced instrumentation development, expanded the capabilities of PSL, and provided a dataset for developing and validating in-house icing prediction and risk mitigation codes.

In addition, NASA Glenn conducted a test investigating the fundamentals of ice-crystal icing utilizing a static airfoil. A mixed-phase cloud, which simulates the conditions inside the compressor region of the engine, impinged on the airfoil placed at the exit of the PSL contraction operating as a free jet. Ice accretions were generated on the airfoil and recorded on video. This data will help NASA develop ice-crystal accretion modeling tools that ultimately seek to reduce icing issues for future engine designs.

Turning to the space environment, the U.S. Air Force Research Laboratory (AFRL) in New Mexico in April reported novel results in the study of flashover events and electrostatic-discharge contamination damage to solar arrays. The results were presented at the 14th Spacecraft Charging Technology Conference in the Netherlands. Evidence is mounting that excess power loss observed on GPS satellites is due to contamination from arcing on the solar arrays. Flight contamination monitors confirm that solar cell coverglass contamination is responsible for the degradation. Radio-frequency detectors developed by Los Alamos National Laboratory for GPS satellites experience short-timescale events that have not been dispersed by the ionosphere and so must come from the satellites themselves. Rates of these signals are highly correlated with the three-hour fluences of surface charging electrons at greater than 10 kiloelectron volts as determined with the AE9/AP9/SPM empirical climatology model developed by AFRL, the Aerospace Corporation and the National Reconnaissance Office. A ground- based surveillance campaign in 2015 reported preliminary detection of signals that may be radio emissions from the GPS arcs.

The detected emissions are consistently seen in on-source observations but do not show up in off-source observations even with the same receivers 10 minutes apart. Ground tests in AFRL space plasma chambers show that GPS-like arrays arc at thresholds consistent with the voltages that may develop on GPS satellite arrays in their orbital environment and, after thousands of arcs, current-voltage, or I-V, curves show degradation consistent with the GPS on-orbit power losses. Rates of the events sensed by the Los Alamos detectors, although rapid, are also consistent with the amounts of contamination necessary to produce the observed power degradation. The unexpectedly high rates of arcing may be evidence for the effectiveness of power-supply filtering employed on modern satellites. If the detection is confirmed, arc-mitigation techniques may be used on succeeding GPS satellites. Elimination of the excess power degradation would permit future GPS satellites to be launched with 20 percent smaller solar arrays or with a 25 percent increase in end-of-life power. ★