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Small Supersonic Airliner - Early Work

Between 1963 and 1995, there had been a total of at least 22 such studies or projects on developing small supersonic civilian airplanes. Academic insti-tutions performed six of them, sometimes as student projects or theses, and all during either the 1960s or the early 1990s. The aircraft industry initiated eight more, starting with one by Spains Construcciones Aeronauticas Sociedad Anonima (CASA), which at the time was building supersonic Northrop F-5s under license. Although Boeing internally examined a supersonic 10-passenger plane concept in 1971, which was delta winged like its canceled 2707-300 SST, Fairchild Swearingen conducted the first serious design project published by an American company. It started with a feasibility study involving several major airframe and engine manufacturers. The company then drew up preliminary designs for four two-engine configurations with a range of 4,000 miles and an ability to cruise subsonically as well as supersonically using a modified version of the Concordes proven Rolls-Royce Snecma Olympus 493 engine. By 1985, concerns about weight, the FAAs Stage 3 noise restrictions, and the sonic boom brought the project to an end.

In early 1988, while Douglas and Boeing were engaged with NASA in stud-ies for the HSCT, Gulfstream Aerospace began studying market and technical criteria for an SSBJ. (Grumman had started Gulfstream in 1958 as part of a diversification strategy into civilian aircraft but divested itself of the brand in 1972.) The company, which catered to the high end of the executive jet market, drew up plans for a 125-foot, 100,000-pound, Mach 1.5 airplane with ogive-delta wings (i.e., with their trailing edges angled forward, much as the leading edges were swept back). In a preliminary attempt at sonic boom minimization, the designers were able to lower its predicted overpressure from 1.0 psf to 0.6 psf but only at the expense of some increased wave drag.

Meanwhile, the sudden end of the Cold War (and the unraveling of state funding for the Russian aircraft industry) led the Sukhoi Design Bureau, which had been studying a 114-foot, cranked-arrow wing SSBJ (the Su-51), to seek an international partner. At the 1989 Paris Air Show, Sukhois chief designer and Gulfstreams chairman agreed to explore joint development, taking advantage of the formers expertise with supersonic fighters and the latters expertise with successful business jets. The companies aimed at a speed of Mach 2 and range of 4,000 miles as they considered design options, but the problems of weight versus performance requirements proved to be beyond current technologies. Although variable-cycle and ejector-mixer engine designs might partly mitigate the level of jet noise, Gulfstream concluded that a concerted effort by the FAA, NASA, industry, and academia would be needed to solve the problem of sonic boom acceptability.28 Even after the two companies parted ways in 1992, Sukhoi continued pre-prototype design work in the hopes of forming another partner-ship in the future.

In addition to the university and company projects, NASA conducted or sponsored eight SSBJ-related studies between 1977 (4 years after it dropped sonic boom minimization from the SCAR program) and 1986 (just as it initiated studies on the HSCT). The first, by Vincent Mascitti of Langley, explored five possible configurations for an eight-passenger, Mach 2.2 supersonic execu-tive aircraft based on the latest SCAR research findings and technological advances. Although reduced engine noise was an objective, none of the options were designed with the expressed goal of sonic boom minimization, so a trans-atlantic range of 3,200 nautical miles was one of the criteria.

Also in 1977, Boeing completed a feasibility study for NASA on a subscale SCAR demonstrator followed in 1979 by North American Rockwells proposed supersonic business jet presented at the last Supersonic Cruise Research Conference. The next NASA study, left unpublished in 1981 as a possible casualty of the Reagan administrations abrupt cancellation of the SCR program, was the first phase of what had been planned as a three-phase market survey for super-sonic business jets. The same year, however, also marked the completion of the first of four SSBJ studies performed for Langley by the local technology division of Kentron International (later PRC Kentron). Each of the studies applied the latest technical advances to various SSBJ concepts during the period between the SCR and HSCT programs. Kentrons 1981 report presented concepts for an advanced droop-nose, two-engine Mach 2.7 business jet carrying eight passengers a distance of 3,200 nautical miles.

Reflecting advances since Mascittis study in 1977, the researchers assumed the use of the latest titanium- and superelastic-formed diffusion bonded materials to reduce its weight from 74,000 pounds to 64,000 pounds and a scaled down version of the GE 21/J11 variable-cycle turbofan engine for propulsion. As regards its sonic boom, the predicted overpressure of 1.0 psf at the start of cruise and 0.7 psf at the finish (due to reduced fuel weight) would still prohibit overland operations.

The next study, completed in 1983, examined the use of a more fuel-efficient turbofan engine, the smallest possible eight-passenger compartment, and only one pilot to reduce takeoff weight to only 51,000 pounds. The result was a 103-foot-long, arrow-winged Mach 2.3 executive jet with a range of 3,350 nautical miles at Mach 2.3. Using Carlsons simplified overpressure prediction method with additional area-rule calculations, former NASA supersonic aerodynamicist A. Warner Robins hoped the combination of low wing loading, high cruise altitude, and modified flight profiles for climb and acceleration would alleviate the sonic boom problem on cross-country flights. The plane was also designed to fly 2,700 nautical miles at Mach 0.9 if necessary when cruising over land.

In 1984, the same Kentron researchers completed the concept for a 114-foot-long executive jet with variable-sweep wings for better low-speed performance, which would eliminate the need for a droop nose as on the previ-ous configuration. Although such adjustable wings had been found infeasible for the SST in the 1960s, the researchers hoped lower weight materials and advances in stability and control technology would make them more practical (which subsequent analysis proved overly optimistic). This latest design would have a ramp weight of 64,500 pounds with eight passengers and a two-person crew. Its performance included a range of almost 3,500 nautical miles at Mach 2.0 and over 5,000 miles at Mach 0.9 with takeoff and landing distances of less than 5,000 feet. Using the same prediction method as before, the overpressures at Mach 1.2 and Mach 2.0 varied from 0.9 psf to 2.0 psf depending on weight and altitude, making speeds no higher than Mach 0.9 mandatory for overland cruise.

The last of the NASA studies was completed in 1986. For possible expan-sion of the customer base, the Kentron design team assessed the feasibility of an eight-passenger, long-range SSBJ with a planform similar to the 1981 and 1983 studies that could be converted into a missile-carrying interceptor (presumably for foreign sales). With a takeoff weight of 61,600 pounds for the civilian version and 63,246 pounds for the military version, its low-bypass-ratio turbofan engines would give it a range of more than 3,600 nautical miles or a combat radius of more than 1,600 nautical miles, both at Mach 2.0. Takeoffs would require a 6,600-foot runway. By flying an optimum profile for climb and acceleration, sonic boom overpressure was calculated at 1.0 psf, but the plane could also cruise transonically for 3,780 nautical miles at Mach 0.96.

Although NASA and the major aircraft manufacturers focused on the HSCT for the next decade, the idea of a small supersonic plane continued to intrigue many in the small airplane manufacturing and general aviation com-munities. The rapidly growing corporate jet market appeared to have room for higher speeds, perhaps using more fractional ownership arrangements. The main roadblocks were the complex technology and considerable resources that would be required to develop, test, and produce such an advanced aircraft.



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Page last modified: 29-01-2017 17:00:02 ZULU