Guy Norris/LOS ANGELES
Every time a Space Shuttle blasts off, its booming sound waves pass unseen over the forgotten bones of a long abandoned project. Lying at the edge of the Kennedy Space Center, Florida, are the forlorn remains of Boeing's 2707-200 supersonic transport (SST) full-scale mock-up.
Abandoned when the US Congress cancelled the SST in March 1971, the mock-up is the nearest the USA got to flyable hardware since the start of its original supersonic airliner study work in 1952. Yet, even though US interest in a second generation SST has been revived in the 1990s through NASA's High Speed Research (HSR) initiative, the programme once more seems to be threatened with termination.
The HSR project, which is supported by a team of US aerospace companies, is the focus for technology that could be used by industry to develop a high speed civil transport (HSCT).
Part of the reason lies in the payload bay of the Space Shuttle sitting on the launch pad close to the dismembered 2707 mock-up. Pressure is growing on NASA to maintain adequate funding for the high-profile International Space Station, particularly since the Russian element has run into financial problems. NASA has to get the money from somewhere, and some within the US Government believe the HSR effort, along with potentially the agency's entire aeronautical research budget, appears to be one area ripe for poaching.
The HSR has, in its turn, become vulnerable to cuts because of other reasons. In mid-1998, with much of the HSR's Phase I and II targets achieved, Boeing and NASA made a bold, and sudden, decision to aim for far more ambitious noise and cost targets than those outlined in the original charter. Although the decision was thought to be critical in terms of making a future US SST meet the environmental rules of the next century, it also raised the technological stakes. It raised them so far, in fact, that the realistic earliest entry-into-service target for an HSCT has slipped by 10 years to around 2020.
This delay has inevitably called into question the viability of the HSCT effort, particularly from the relatively short term perspective of the US Government's Office of Management and Budget (OMB). This group will pass on recommendations for the President's budget which, in turn, will be used to generate a budget request presented to Congress in late January.
Some members of the team believe the decision to refocus on even tougher noise and cost rules has already been interpreted by the OMB as a sign of failure. "We're looking at the programme as a significant success, but the OMB's success criterion is to see an aircraft programme launched. Therefore, with it moving to the right, they said they don't see it as a success. It makes the advocacy task even tougher," says General Electric's HSCT programme manager, Leigh Koops.
Pratt & Whitney's veteran HSCT programme manager Dick Hines has similar views. "If you are not going to start development before 2015 for entry into service in 2020 you start to wonder what the urgency is. Yet, if you don't keep the momentum going, and keep the team going towards the goals, you may lose some of this and you never know when the competition is going to get into it. If the USA is not in contention for the HSR it will ruin the US aerospace industry," he says.
TOUGHER GOALS
Meanwhile, NASA remains adamant that the decision to go for the tougher goals is still the right one, despite the increased danger of zero funding. "It may be bad timing, but even if it was different, we'd still be doing the same thing," says NASA HSR programme acting director Wally Sawyer. "It would be fruitless to put millions of dollars into hardware and then need something more robust. We were given the job of spending the taxpayers' dollars as wisely as we could and, forgetting about the space station or anything else, we needed to be honest with our stakeholders," he adds.
Boeing HSCT programme manager Malcolm MacKinnon recalls the events leading up to the decision. "Up until about a year ago we were fairly well on track with the HSR programme. The research was going extremely well, but we found the requirements we'd set for the aircraft were changing - particularly the environmental targets. Supersonic boom we'd given up on, but concepts for ozone and low emissions combustors were going well, and exceeded our targets."
Environmental noise remained the big question mark. "We thought Stage 3 would be good enough, but as we got into it we began to think that we probably needed to go quieter. The most important part is cut-back noise, and we saw that subsonic aircraft are certain to get quieter," says MacKinnon referring to recent breakthroughs in NASA's parallel Advanced Subsonic Transport (AST) programme. The AST noise reduction goals include cutting perceived noise levels of future aircraft by half (10dB) from current aircraft, and by 75% (20dB) within 25 years. "By mid-summer, we realised that we were going to have be 8-10dB quieter than Stage 3, and that's pretty much beyond the technology we have developed."
To circumvent the problem, the team began looking at changing the aircraft configuration to increase aspect ratio. This would have increased the lift/drag ratio at take-off, lowering the thrust requirement and therefore the noise produced. Increasing the aspect ratio, however, created more problems than it solved. "We began running into structural problems. The outboard sections of the wing were very slender and flutter became quite a concern. So the weight that we added to fix that, plus the extra weight of the Stage 3 engine and suppressor nozzle, made it all far heavier than we previously thought," says MacKinnon.
The problems were cropping up on the HSR's TCA, or Technology Concept Aircraft. The TCA is not a real aircraft, but was chosen in 1995 to represent a single aircraft concept for all the technologies to be aimed at. The TCA, like an eventual HSCT, is sized around the original goals of a 300-seater with a cruise speed of Mach 2.4, a fatigue life of around 60,000h and a 9,250km (5,000nm) range, or considerably further than the transatlantic design goal of the Anglo-French Concorde. When it was begun in 1990, the projected market was for more than 500 HSCTs by 2015, worth more than $200 billion in sales.
Boeing's target maximum take-off weight (MTOW) was around 340,500kg (750,000lb), roughly the same as the projected limit for the ultra-long range 777-200X/300X. "However, at one stage it got closer to 1,100,000lb. We simply could not put together a 1.1 million pound aircraft - it was too noisy and it came to the point where it became too heavy to make economical sense. So, very reluctantly we had to draw a halt to this and say 'time out'," says MacKinnon.
At no point, says NASA, did it question the validity of the noise concerns. "It is unrealistic to think you could build an aircraft and try to get a dispensation from the international community," adds NASA deputy HSR programme director Alan Wilhite.
Boeing has also raised the cost stakes by urging that the programme should aim for supersonic performance with no appreciable increase in operating costs, and therefore no raising of ticket prices.
At the same time as keeping a watchful eye on the budget process, the NASA team is working on a revised plan for Phase IIA. This is not only aimed at defining the tougher requirements themselves, but also ways of meeting them. "We're looking at how we could be more aggressive in our technologies. We are looking at where the AST programme is pushing technology in areas like noise and emissions, and asking: are our goals as robust as they should be? We need to look at more aggressive goals, and if we do that, how does that change what we are doing," says Sawyer.
"We are looking at what are the real long poles to making this technology viable for one day getting into an aircraft," he adds. Chief among the "long poles" are noise and emissions - both firmly in the area of propulsion research. This is therefore likely to take the lion's share of Phase IIA funding, should it be approved early next year.
BACK TO BASICS
NASA is essentially going back to basics to rescope the entire HSR effort which, by the advent of Phase IIA, was originally intended to be at the fine tuning stage. "It was expected a while ago that the configuration would be firmed up by this stage, and that we'd be doing more detailed work in Phase IIA," says Wilhite.
The changes are so fundamental that even the familiar, four-engined Boeing and former McDonnell Douglas concepts seen in artists' impressions since the days of the 2707 and before, are being questioned.
"We're looking at better high lift and therefore differing fuselage and wing planforms," says Sawyer.
MacKinnon adds that, from Boeing's perspective, "-we are trying to see if there is another layer of technology to solve the problem of noise produced by the engines. Propulsion is the long pole in the tent and we airframe people are stepping back and encouraging the engine people," he adds, saying that "-everything is still in the proposal stage, but we are looking to start IIA sometime in the spring of 1999".
Meanwhile, NASA and its industry team have embarked on a 15-month study period to review as many as 200 technology items, many of which were dropped from earlier phases of the programme.
Airframe related studies include:
Innovative configurations. These range from higher aspect ratio, blended delta planforms to longer fuselage SST designs incorporating hybrid canards and ride control vanes to ensure dynamic stability. Another novel option under study includes a five-engined solution in which a large subsonic turbofan, such as the GE90, would be added to the four SST engines. The turbofan would be used only in boost mode for take-off, allowing the other engines to operate at half power and low noise for the crucial take-off period;increased use of improved lightweight, high performance composite materials such as Glare, which is made of Ti-graphite, a high temperature aluminium composite.
thermal tailoring, a development of hybrid laminar flow control (HLFC) which met 90% of requirements during tests on a NASA Lockheed Martin F-16XL testbed. The theory is that by thermal tailoring of the wing, the onset of turbulent flow can be delayed. This is achieved by altering the temperature locally, therefore affecting the viscosity of the air which, in turn, affects the Reynolds number. Thermal analysis tools to study the differential heating affects on the wing structure are being validated, along with turbulence sensors, in the current series of flight tests on the Tupolev Tu-144 supersonic airlines in Russia.Propulsion related studies include:
Alternative engine configurations to the conventional two-spool turbofan with two-dimensional mixer/ejector sound suppression nozzle arrived at by the end of Phase II. These include variable cycle engines and mid-tandem fan concepts, such as those previously proposed by GE and Rolls-Royce, respectively;noise suppression treatments and concepts;
further work on improved combustor efficiency and reduced emissions; lightweight materials for engines; higher temperature capability, particularly for hot section. 
GE's Koops says "a major decision toll gate" is due around March/April 2000 when "-we downselect to a final technology set" at the end of the study period. Hines adds: "That's when we will decide if the whole thing is feasible or not." The technology studies will continue up to 2006 or 2007, assuming funding is approved for Phase IIA.
Ultimately, NASA believes the wisdom of its more cautious approach will prevail, even if the exact timing of an SST application is made more unpredictable as a result. "We think this technology will one day result in a vehicle," says Sawyer. "We don't know how or when it will happen, but if we don't do it, someone will. We think it is important to continue," he says.
For the sake of the future US aerospace industry, NASA's team is therefore keeping its fingers crossed that next month's OMB recommendation does not repeat the edict which was made 28 years ago.
THE AIR SPIKE : A FAR OUT WAY TO STOP NOISE
Among the more exotic technologies being proposed for study as part of Phase IIA is the use of microwave energy to cut supersonic wave drag and reduce, or eliminate altogether, sonic booms. "It is pretty far out, and high risk, but it could make all the difference if it works," says MacKinnon.
The idea is based on the "air spike" concept developed by scientists at the Russian Rensselaer Polytechnic and Russian Academy of Science's Institute for Problems in Mechanics. Although initially aimed at solving drag and heat transfer problems associated with hypersonic flight, NASA and Boeing believe the concept could have more immediate benefits to the HSR/HSCT effort.
The air spike concept uses concentrated microwave energy projected forward of the aircraft to drive air from its path. The structure of the air in front of the vehicle is briefly altered to plasma, creating an oblique shock wave, reducing the drag and heat transfer. The air spike is, in effect, a blast wave, creating a low density air pocket in front of the vehicle. As the speed of sound rises dramatically in a plasma field compared to air, the HSCT could theoretically fly physically faster than the speed of sound without causing a sonic boom.
Among the most serious unknowns are the weight and size of the microwave generator required to alter the shock wave, and the effect of the concentrated microwaves on the ozone layer.
Source: Flight International
