## 1 Introduction  
The renewed interest in civil supersonic transport (SST) leads to growing concerns about high levels of community noise, i.e., noise around airports produced by such aircraft. Community noise is airport-dependent but can be related to the certification noise metric defined by the International Civil Aviation Organization (ICAO) [1]. Typically, the lower certification noise of an aircraft leads to lesser noise exposure around the airport. Subsonic aircraft has achieved impressive reductions in their certification noise levels during the last several decades, primarily due to using higher-bypass-ratio turbofan engines. However, such engines have larger diameters that limit their application for SST because of the increased drag at cruise with supersonic speeds. This contradiction in requirements for SST engines constitutes one of the major problems for next-generation SST design: low noise levels at landing and takeoff regimes imply high bypass ratios, while aerodynamic efficiency and low fuel burn at supersonic cruise require low-bypass-ratio engines. There are a number of available technologies for noise reduction, which include technologies for turbofan noise reduction at the source [2] and technologies relying on installation effects such as shielding [3]. However, they are not sufficient to ensure that SST complies with the current noise certification requirements for subsonic aircraft, which is a necessary condition for public acceptance of SST. As a result, a different approach has to be proposed for SST to achieve substantial noise reduction. One approach to reducing the certification noise of next-generation SST consists of transferring a portion of the turbofan engines' thrust at takeoff to buster engines [4]. The idea is to reduce the speed of the turbulent jet, which is a dominant noise source for low-bypass-ratio turbofan engines. This allows for achieving significant noise reduction because the jet noise level is proportional to the eighth power of jet speed in accordance with [PERSON]'s theory [5]. The thrust deficit due to this jet speed reduction at takeoff is compensated for by turboprop buster engines, which have lower noise levels than those of high-speed jets. When the aircraft reaches a certain altitude and flight Mach number, the thrust of the turbofan engines is increased, while the blades of the turboprop buster engines are folded to avoid deterioration of aerodynamic characteristics of the aircraft at high flight speeds. It is envisioned that transferring a large portion of thrust to the buster engines (35% or more) would allow such an SST to comply with the existing noise regulations for subsonic aircraft [4]. Another approach to certification noise reduction consists of using advanced takeoff procedures [6]. Whereas the reference certification procedure for subsonic aircraft prescribes that a significant part of departure flight has to be performed at maximum thrust, with the constant configuration of high-lift devices (HLD), etc., this approach proposes to modify the requirements for next-generation SST. It capitalizes on the excess thrust capability of SST engines at takeoff: in contrast to subsonic aircraft, supersonic aircraft have more thrust available than is necessary for a safe takeoff. As a result, it is possible to change the engine operation or airframe configuration in order to reduce noise during the departure procedure. To make the idea viable it has to be ensured that the control of engine thrust, or airframe configuration is fully automatic, so that it is performed not only during the aircraft certification but is reproduced in day-to-day operations of the aircraft as well. The aircraft subsystems that realize this automatic control are called variable noise reduction systems (VNRS) [7]. The major objective of the present study is to assess the VNRS efficiency in terms of noise reduction for SST. There are several studies that considered the implementation of different VNRS for supersonic aircraft noise reduction [8; 9; 10; 11; 12; 13; 14; 15; 16; 17].