The concept helps in other ways, allowing Gulfstream to design an axisymmetric inlet that minimizes both drag and boom. Inlet spillage, caused when intake supply exceeds engine demand and air overflows the cowl lip, increases supersonic drag and sonic boom. To avoid spillage, the inlet must capture the centerbody tip shock, but with a conventional external-compression inlet, zero spillage is almost impossible to achieve. With the high-flow nacelle, the engine cowl is inside the aeroshell, allowing the outer intake to capture and bypass spillage from the inner inlet.
“Classic inlet designs have good pressure recovery and low flow distortion, but produce a larger sonic-boom contribution due to high nacelle profile blockage and off-design spillage,” says Tim Conners, principal engineer for supersonic propulsion. “So we've come up with a hybrid-compression design which has a large secondary flowpath for bypassing flow around the engine. This produces a cowl-lip region within the outer mold line, but doesn't need variable geometry.”
Bypass drag is significant, but nacelle drag is reduced, while engine performance increases and interference drag is reduced. Gulfstream has combined the concept with relaxed isentropic external compression, which moves the initial shock to defocus the compression field at the inlet lip and tailors the terminal shock to reduce the cowling slope and increase core-stream pressure recovery, lowering drag and sonic boom.
“We get high inlet performance consistent with a mixed-compression design, but with the shock stability of external compression,” Conners says. Relaxed compression also weakens the cowl shock, for an 80% reduction in near-field overpressure, according to CFD analysis. Performance is improved, with 50% less cowl drag and 9.9% lower installed specific fuel consumption.
The relaxed-compression inlet was tested at NASA Glenn Research Center, at up to Mach 2, with good results for flow quality and shock stability. “This allowed us to move to the next step, but we knew the shaping we'd tested was not sufficient to solve supersonic boom,” says Conners. Gulfstream and Rolls then produced a notional flight demonstrator configuration aimed at showing nacelle bypass can enable additional low-boom shaping.
Flight-scale ground testing of the low-boom supersonic nacelle, including inlet and nozzle, was conducted on a Tay engine installed on a Gulfstream IV testbed in late 2009, “demonstrating stable inlet flow and excellent performance at static conditions,” he says.
This was followed by supersonic tunnel tests at Glenn in 2010. A single-flowpath model investigated relaxed compression and use of micro-ramps on the centerbody to manage the boundary layer and avoid inlet bleed. The second, dual-stream model evaluated the performance and stability of high-flow nacelle bypass at Mach 1.7. Scaled from a flight nacelle sized for a Tay, this bypassed about 40% of the captured flow.
The tests showed “we should have a low-boom inlet with excellent pressure recovery and very stable angle of attack from minus three to plus five degrees,” says Conners, adding that the concept “can outperform a single stream.” Further study is underway with the universities of Illinois and Virginia as well as Purdue University, plus Rolls and NASA. One discovery by Illinois was that a vortex trapped between the inner and outer lips entrained flow and improved inlet performance at low speed. Ground tests were conducted on a G450 “to see if the trapped vortex was real—and it was.”
While Gulfstream is working on what NASA calls the “N+1”—or first-generation quiet supersonic transport—Boeing and Lockheed Martin are studying N+2 and N+3 concepts for notional 2025 and 2035 timeframes, respectively. Each generation is larger, with more-stringent targets for sonic boom and airport noise. The concepts tunnel-tested in 2011 were aimed at NASA's N+2 goals for a 35-70-seat, Mach 1.6-1.8 jet with an 85-PLdb boom and noise 12 EPNdb below Stage 4.
While the focus was sonic boom and cruise performance, Lockheed Martin's Phase 1 study included acoustic tests of two different low-noise nozzle concepts—a mixer-ejector design from Rolls-Royce Liberty Works and an inverted velocity-profile nozzle from GE Aviation. Both are intended for use with commercial variable-cycle engines developed using technology from the U.S. Air Force Research Laboratory's Adaptive Versatile Engine Technology (Advent) program.