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"Super-green" airliners powered by ... natural gas?

Five years ago, aircraft industry insiders laughed at the idea of an electric-power airliner, says Boeing's Marty Bradley. Today, no one is laughing.

Bradley heads up Boeing's SUGAR effort, which stands for Subsonic Ultra-Green Aircraft Research. SUGAR is a NASA contract that aims to examine technologies that might be viable for subsonic commercial aircraft to meet environmental requirements in the years 2030 to 2050.

The SUGAR team has been working for the past four years and has come up with a laundry list of technologies that look promising for powering aircraft 20 years from now. This year, the team, which includes Boeing Research & Technology, Boeing Commercial Airplanes, General Electric and Georgia Institute of Technology’s Aerospace Systems Design Laboratory, has told NASA the list of possibilities is longer than they first anticipated.

The team reported to NASA in late February 2012. Its findings, summarized in a report titled “N+4 Advanced Vehicle Concept Study,” described looking at the performance of a methane-fueled aircraft concept and the technology development of several advanced fuel and energy technology options for the 2040 to 2050 timeframe. These included hybrid battery-gas turbine propulsion, fuel cells, fuel cell-gas turbine hybrid propulsion systems, cryogenic fuels (liquefied natural gas/methane and hydrogen), cryogenically cooled engines and associated technologies, advanced batteries and open rotor/turboprop technologies.

Interestingly, the team found that “liquefied natural gas (LNG), though not an obvious choice for a future aviation fuel, does offer lower fuel burn and emissions as well as potential cost and availability benefits.” The report went on to say that, “cryogenic LNG also would enable fuel-cell hybrid electric propulsion and would be a step toward clean liquid hydrogen fuel. However, there are environmental issues with methane emissions from LNG production, as well as safety and infrastructure issues....The cost to add LNG infrastructure is probably the biggest challenge. We hope this will inspire others to do more detailed studies that look at cost and infrastructure impacts.”

The cryogenic fuels would apparently do double duty as coolant for propulsion system components. Also, the need for cryogenic tanks would increase the amount of energy the plane would consume, though the LNG itself weighs less than the equivalent amount of jet fuel, Boeing says.

Hydrogen could also be a potential fuel for commercial aircraft in 2040 to 2050 but there are challenges there as well. They include potential leakage, material compatibility, density, and the fact that it takes a lot of energy to make hydrogen fuel.

Phase 1 of the SUGAR study looked at technologies for the years 2030 to 2035 and found that hybrid electric engine technology was a “game-changing technology” because it could potentially reduce fuel burn, greenhouse gases, nitrous oxide emissions, noise and field length. That concept is still getting a close look during Phase 2 of the study, which will continue for another two years, Bradley said.

Another Boeing spokesperson remarked that, "The long pole in the tent is battery technology. The batteries necessary to generate enough power for a 737 would be so big you couldn’t get the thing off the ground. But we have opened the eyes of companies in the battery industry to the idea of a battery powered plane. The objective would be to come up with a module you could pop in and fly. Who knows what people might come up with in 15 years."

In that regard, Bradley emphasized that the purpose of the SUGAR study is to “start the industry thinking about and planning technologies that future vehicles will need in 2030 to 2050. While Boeing is interested in developing environmentally progressive vehicles, it would be premature to conclude that any of the concepts we study under this contract will replace any of Boeing’s commercial products.”

Boeing recently produced a video about the SUGAR project: http://www.youtube.com/user/Boeing

More on the SUGAR project: http://www.boeing.com/Features/2010/06/corp_envision_06_14_10.html

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