swilliam@dtoa1.dt.navy.mil (Williams) (10/18/90)
From: swilliam@dtoa1.dt.navy.mil (Williams) More information on SR-71: Apparently Air Force officials feel that it is no longer cost effective to operate and maintain the SR-71 fleet compared to alternative systems. It cost $208 million per year to fly the Air Force's six SR-71s, accoring to General Larry Welch, Air Force chief of staff and a former SR-71 pilot. Estimates are that each mission costs in excess of $8 million. Just starting the engines racks up a $50,000 tab. In contrast, $8 million can keep between three and four F-16s flying for a year. Then Defense Secretary Carlucci once said that the aerial refuelling makes SR-71s too expensive to operate. Since the planes use a unique fuel (JP-7) some speculate that a fleet of tanker aircraft must be dedicated to serving their needs. In addition, the SR-71's Pratt & Whitney J58 engines have been out of production since the late sixties and replacement parts are increasingly dear. Extensive maintenance procedures also make this aircraft expensive to keep in the inventory. For example, after each flight, seven postflight checklists with more than 650 items must be completed. Five structural specialists take an average of six hours to complete their checklists which includes inspecting every titanium and plastic spot weld on the wings' upper surface. The people needed to conduct these stringent SR-71 maintenance programs are said to require 18 to 24 months of training before being allowed to work on the plane unsupervised. That fact, plus low retention among Air Force personnel, makes using higher paid civilian technicians a necessity. In all, the aging aircraft seems to have become too expensive to maintain. Skunk Works Design With little more computing power than a slide rule and only the most rudimentary in composite technologies, Clarence "Kelly" Johnson led the design of the Blackbirds. Being a "black," or special access weapons program, Johnson had the luxury of dealing with less outside interference and a more streamlined bureaucracy than today's defense contractors. And he had almost unlimited funding. Cold War fears of the Soviet Union put the SR-71 on the front burner. Design criteria for the Blackbird grew out of vulnerability studies and estimates of Soviet military technology. Studies showed that the next generation spy plan would need a cruising speed in excess of Mach 3, a cruising altitude of over 80,000 feet, and a low radar cross-section. It would also have to carry state-of-the-art electronic countermeasures and communications equipment. For safety considerations, it was also decided that the aircraft should have at least two engines. Engineers at Lockheed and Pratt & Whitney had to overcome many previously unknown design problems to make the Blackbird fly. One of the most difficult and pervasive involved was the high temperature environment associated with flying at Mach 3. At that speed, aircraft skin temperatures range from 450 degrees to over 1,200 degrees F. This ruled out all but alloys of titanium and stainless steel as structural materials. With its superior strength-to-weight ratio, titanium was the final choice. It makes up over 90% of the Blackbird's basic structure. Lockheed engineers first had to get an understanding of how to work with titanium before they could complete the project. One manufacturing breakthrough dropped the cost per foot for machining wing extrusions, thousands of feet of which went into SR-71s, from $19 to $11. Initially, drill bits had to be tossed out after drilling only seventeen holes. They were soon replaced by bits that could drill one hundred holes and then be resharpened. Lockheed also instituted a complicated quality control program. For all but the first few titanium parts, the company can determine the mill pour from which their 13 million titanium parts come. For the most recently made 10 million parts, the firm even has records of the grain direction in the sheet from which the part has been taken. Once work began on the skin of the aircraft, Lockheed quickly found that heat caused warping in the large titanium wing panels. Lengthwise corrugations proved to be the solution. The corrugations deepen a few thousandths of an inch at cruising temperatures and then return to their original shape upon cooling. These grooves also add structural strength to the wing, provide more heat dissipating surface area, and do so with little added drag. Designers also discovered a few quirky ways in which titanium reacts with elements like chlorine and cadmium. Wing panels spot welded in the summer failed quickly, while those constructed in the winter would last indefinitely. Engineers traced the problem to the chlorine added to the Burbank water supply in summer to control algae. A distilled water wash proved to solve that problem. Another difficulty was that the heads of engine bolts fell off when engine temperatures climbed into the 600 degree F range. This problem was tracked to the cadmium-plated tools used to install the bolts. Tools left just enough cadmium on the bolts to cause failure. The extreme heat also proved a challenge for Pratt & Whitney engine designers. A new fuel, JP-7, had to be developed that could withstand high temperatures. The fuel also became a major component of the cooling system. The relatively low temperature fuel is used as a heat sink to cool the crew, aviaonics, and even the landing gear. Since this leaves little cooling capacity for engine electronics, a chemical ignition system was developed. Tetraethyl borane (TEB) is used for starting both the main engines and afterburners. Engineers managed to wring one more use out of the fuel. It also serves as the engine hydraulic fluid, acutuating bleeds, afterburner nozzles and so on. The fluid passes through the engine hydraulic system once and then is sent through the engine for burning. The distinctive shape of the plane was dictated by speed, altitude, and range requirements. It was built as a modified, tailless delta-wing with a blended forward wing. The 130-foot long fuselagee stores the estimated 80,000 pounds of fuel along with the landing gear and payloads. The forward wing, called a chine, was added to reduce wing drag; it turns the forward fuselage into a fixed canard and develops lift in flight. To take advantage of the vorteces this chine creates, the dual vertical tails are canted inward. This causes directional stability to increase as the angle of attack increases. Engine inlets are pointed down and inward to also take advantage of the chine air flow. Special care also had to be taken to protect the crew. Engineers had to design an ejection system capable of working at Mach 3 and at altitudes above 80,000 feet. An integral part of this ejection system is the custom-made pressure suits the flight crews wear. Just recently, when an SR-71 crashed into the China Sea, this ejection system was able to save both crewmen. Additional information: KC-135 tankers used to aerially refuel SR-71s are outfitted with a communications line within the refuelling boon, allowing the planes to operate using a secure radio link and maintain radio silence. SR-71s would have to descend to 28,000 feet to rendezvous with KC-135 tankers, and then climb back to the 80,000 altitudes to continue their missions.
clements@cs.utexas.edu (Paul C. Clements) (10/20/90)
From: clements@cs.utexas.edu (Paul C. Clements) In article <1990Oct18.021420.7298@cbnews.att.com> swilliam@dtoa1.dt.navy.mil (Williams) writes: >Engineers had to design an ejection system capable of working at Mach 3 and >at altitudes above 80,000 feet... Just recently, when an SR-71 crashed into >the China Sea, this ejection system was able to save both crewmen. Was the bailout at speed and altitude? Any other details available about the system -- should I envision your basic Martin-Baker seats plus the suits? And do we know what caused the crash? >KC-135 tankers used to aerially refuel SR-71s are outfitted with a >communications line within the refuelling boon, allowing the planes >to operate using a secure radio link and maintain radio silence. I assume it isn't really a *radio* link at all, but more of an intercom? Real question: any reason why this isn't a really good idea no matter WHAT kind of tactical a/c you're refueling? Thanks for a fascinating article. P. C. Clements
tighe@uunet.UU.NET (Mike Tighe) (10/24/90)
From: convex!tighe@uunet.UU.NET (Mike Tighe) >From: clements@cs.utexas.edu (Paul C. Clements): >>From: swilliam@dtoa1.dt.navy.mil (Williams): >> Engineers had to design an ejection system capable of working at Mach >> 3 and at altitudes above 80,000 feet... Just recently, when an SR-71 >> crashed into the China Sea, this ejection system was able to save both >> crewmen. >Was the bailout at speed and altitude? The aircraft that was lost was #64-17974, aka.," Habu/ichi ban". It was lost on 29-Jan-89 over China Sea shortly after takeoff from Kadena AFB. Pilot (Lt. Col. Dan House) and RSO survived. Aircraft was recovered 10 days later (AW&ST: 29-Jan-89). -- Michael Tighe, tighe@convex.com