FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1961
1961 - 0846.PDF
858 FLIGHT, 22 June Missiles and Spaceflight. . . MISSILE RELIABILITY (continued from page 855) and many XM-55 sub-scale engine tests conducted for design infor-mation (see Table II). Both destructive and non-destructive tests had been performedon some of the engine's components, such as the combustion cham- ber. During the initial stages of chamber fabrication, a quantity ofsub-scale and full-scale chambers were subjected to destructive tests to obtain processing and quality information. In addition, allcombustion chambers intended for full-scale engine fabrication were subjected to non-destructive functional tests (such as hydro-testing)prior to use. The number of tests shown in the table of XM-55 component tests probably will double by the time the engine's quali-fication test programme has been completed. Using Gen Schrievers analogy to the iceberg, Dr Ritchey indicated that theactual flight tests of the XM-55 solid propellant for Minuteman will account for only ten per cent of the total engine developmentcost. The other 90 per cent (which, like the iceberg, are not readily apparent), include five per cent for initial design and analysis, 30per cent for component testing, and 55 per cent for engine static tests. Electronic Units Col William H. Congdon of the USAF's new Electronic SystemsDivision at Hanscom Field, Bedford, Massachusetts, told the group that designers must frankly "employ redundancy of com-ponents, circuits and sub-assemblies" in electronic gear to achieve reliability. To get good reliability at low cost, he recommendedduplexing so that two or more parallel but less expensive units were employed instead of one very expensive but very reliable unit."This technique is appropriate to electronic systems which contain common items such as computers, communication channels, dis-play equipment and power generators," Col Congdon stated. The designer considering parallel design techniques, the speakercontinued, must have a complete understanding of the trade-off between the failure rate of the part in question and the effect itsadded weight could have on overall performance of the system. On a missile, of course, weight was a far more important considera-tion than on ground equipment. Expanding on Dr Ritchey's theme about the cost of achievingreliability, Dr Ruben F. Mettler, executive vice-president of Space Technology Laboratories, Inc, declared: "If the reliability objectiveand the reliability actually achieved by the system are too low, excessive costs for maintenance and replacement will be encounteredwhen the system goes into service. Conversely, if the reliability goal is made too high and the necessary development test pro-gramme is laid out to meet that goal, then the development costs and the development time can become excessive." Putting this in practical terms, Dr Mettler posed this situation:"Suppose an enemy plans to knock out a thousand American targets in a surprise blow. Recognizing that he has only, let us say,a two-out-of-three reliability expectancy, he can simply increase the number of missiles in the attack from 1,000 up to 1,500 and stillaccomplish his purpose. What is more, it may be a lot more sensible for that enemy design team to choose two-thirds as the properreliability to work towards, rather than, let us say, to take several additional years and perhaps ten times as much money to try tostrike 1,000 targets using, say, only 1,100 missiles of improved reliability of around 90 per cent." Dr B. P. Blasingame, director of engineering at AC Spark Plug,the Electronics Division of General Motors at Milwaukee, Wis- consin, stated that today's guidance systems for missiles containedfrom 20,000 to 30,000 electronic and electro-mechanical parts, ten times more than early missile guidance systems. As a result,reliability requirements had been set ten times as high, and no ordinary design and test processes were good enough to ensuresuch reliability. He said that two programmes introduced by the US Air Forceas part of its ballistic missile effort and used in industry were helping to assure reliability in the face of growing complexity.One was a Parts Qualification Program under which sample quantities of parts were subjected to an environment much in ex-cess of that which they would experience in the missile itself. Transis- tors, synchro-motors and even tantalum capacitors were putthrough the programme which, over a period of time, had resulted in the qualification of a large number of parts that might be used innew missiles with reasonable safety. Furthermore, he said, indivi- dual systems manufacturers were now interchanging parts testdata through the Interservice Data Exchange Program, or IDEP, set up by the Army, Navy and Air Force for exchangingtest data on ballistic missiles and space systems. TABLE II: XM-55 COMPONENT TESTS Component Tests No of Tests (1) Combustion chamber tests: (A) Subscale (B) Burst (C) Hydro (2) Propellant tests: (A) Subscale ballistic, less than 101 b (B) Subscale ballistic, greater than 101b (C) Physical properties (D) Environmental (3) Nozzle tests (4) Soft insulation tests (5) Hard insulation tests (6) Igniter tests, Pyrogen: (A) Bench tests with initiator (B) Case impact (C) Case hydro (7) Igniter safe and arm tests: (A) Squib (B) Pyrotechnic train ... (C) Housing hydro (D) Environmental (E) Subscale ignition ... (8) Destruct system tests: (A) Destructor (B) Subscale destruct ... (C) Environmental (9) Destruct safe and arm tests: (A) Detonator (B) Pyrotechnic train ... (C) Environmental (D) Subscale destruct 32 2 144" 26,060 696 25,500 5,500 196 700 560 50 3 3 2,000 50 3 6 10 50 21 18 2,000 50 6 85 * Non-destructive test: all others destructive. "Because of IDEP and other records of prior usage of parts,"declared Dr Blasingame, "we have had to test only 20 per cent of the parts on the Titan guidance system. Even so, parts qualificationwork accounts for about five per cent of the development cost of this system." The second programme consists of formalized design reviews.Heretofore, an individual engineer's design was discussed with his supervisor and reviewed informally by an experienced designer.This now had been supplemented by a much more rigid and formal review process. Today each designer was required to compute thestress levels of all parts involved in his design, and then pass this information on to an applications review team which must approvethe design. The result was that many design shortcomings were caught before they appeared in hardware form. Designers at AC Spark Plug were provided with a series ofmanuals to assist them in their work, Dr Blasingame said. These included a parts selection manual kept up to date by the partsqualification programme, a parts de-rating manual, and a manual of design standards. Compiled from prior experience, the manualswere essential to "designing in" required reliability. "Designers begin to think quantitatively about reliability after they have beenworking to the severe environmental specifications called for in missiles," Dr Blasingame concluded. PENDINE TEST TRACK Pendine Sands, on the shore of Carmarthen Bay, is one of thesmoothest and flattest areas in the United Kingdom. In the 1920s they were the venue for successful attempts on the world land speedrecord, and today much higher speeds are reached there by rocket- propelled vehicles on the longest test track in Britain. The WarOffice recently marked the 21st anniversary of the move to Pendine of the Proof and Experimental Establishment by opening thegates to visitors, and although the British scale of operations may appear very modest in comparison with Aberdeen proving ground,Eglin AFB or China Lake, very full use is made of the facilities available. The Long Test Track has a length of 3,000ft, and varyingnumbers of 5in rockets are used to propel vehicles along the welded double rail at from 800 to 3,000ft/sec. Sleds on this track have heenused for the dynamic testing of the warheads of all British Govern- ment-sponsored guided weapons, and static firing of warheadstakes place along the seven miles of sand dunes.
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
