FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1956
1956 - 1726.PDF
890 FLIGHT MISSILE TESTING Weapons on Trial: a Series of Controlled Experiments IN the development of aircraft the first machines of eachparticular type (or, in earlier days, the prototype) arethoroughly tested over a period of months or years. A con- siderable proportion of the desired information is already knownbefore the wheels of the first aircraft leave the runway, as a result of rig tests and ground programmes on more or less completeaircraft. When flight trials do begin, any amount of instrumenta- tion can be carried and the records of every item which needsto be measured can be analysed relatively at leisure, and design modifications fed in with assurance.Should something completely unforeseen occur, it is usually possible for the pilot to make corrections on his own initiativeand also to obtain detailed first-hand observations of the malfunc- tion. Should the aircraft have to be abandoned, the pilot's state-ment may in itself enable the design team to pin-point the source of the trouble; and it is also frequently possible to recover virtu-ally all the records from the airborne instrumentation. By contrast, guided weapons present serious difficulties. Themere fact that there is no pilot is a tremendous handicap, for it places the design team in absolute dependence upon the readingsof the airborne test equipment and the results obtained from ground stations. In most cases a missile cannot be fired morethan once, and inability to fit a satisfactory recovery system— coupled with the fact that missile ranges may lie over the sea—frequently make it impossible to retrieve anything. Typical test vehicles, with their instrumentation, may cost tensof thousands of pounds and the number that can be fired is accordingly strictly limited. The basic planning of the firing pro-gramme is therefore a direct compromise between the limitations of the permitted expenditure and the statistical approach to theproblem. A statistical approach is essential, because no two missiles can be made exactly alike. Wings, for example, willvary from round to round as a result of minute differences peculiar to individual machine-tool operators, jigs or manufacturers; andrivets or bolts may be tightened by different amounts, giving vary- ing degrees of structural joint elasticity. Even if two rounds weretheoretically identical, varying atmospheric conditions would affect the aerodynamic constants and the performance of themotor. The best that can be done is to use standard statistical methods to calculate the minimum number of firings from whichsufficiently accurate final results can be obtained. At the outset very little may be known of the conditions underwhich a particular missile series may have to operate, and the planning of a trials programme inevitably involves a number ofcalculated risks. It should not be inferred that these are risks to life and limb;this is evident from the fact that nobody in the guided-weapon industry is paid danger money, and residents of the areas sur-rounding firing ranges require no special insurance and do not receive regular danger warnings. The element of chance is intro-duced by reason of the fact that the total number of firings from which the final results must be obtained do not permit the pro-gramme to be advanced in as many logical steps as might be considered desirable. Components may frequently have to be used in flight beforea significant number have been flown in survival trials and proved satisfactory. Ideally it is desirable to evaluate the performanceof, for example, a gyroscope in a test vehicle which is not dependent on that gyroscope for control. After a number of suchfirings it would be possible to assess the performance of the new gyro and any shortcomings could be made good before the unithad to take its place in a complete missile system. It is, of course, quite impossible to monitor behaviour of eachelement in the long chain of guidance and control in an actual missile. Recordings are therefore made of those quantities whichare expected to be the most rewarding, and the performance of the remainder is estimated purely by inference. The instrumenta-tion will vary from round to round. For recording the behaviour of the control and guidance equipment it is likely to involveeither scratch recorders or the telemetering of data to receivers and recorders on the ground. The first method is satisfactoryonly when the recordings are likely to be recovered and is there- fore basically restricted to use over land ranges, such as those atWoomera and Larkhill (Salisbury Plain). It is also limited to the recording of movements, such as the displacement of an accelero-meter-weight or the angular position of a control surface. Telemetering, however, is independent of the eventual fate ofthe missile and is also capable of converting into a suitable radio signal any type of input in the form of a d.c. voltage or a changinginductance. Transducers, of a variety of types, are employed to obtain such inputs from the various factors which have to bemeasured, such as liquid pressure, strain value, temperature and acceleration. Each telemetry sender can accept several inputs(usually 24 or more), which are sampled at high speed and trans- mitted to the ground receiver, where diey are placed on a per-manent film record for future analysis. Telemetry was discussed at some length in ourissue of October 23, 1953. The original trans- miner in British missile work was the reliableS.R.D.E. sender Mk 2, or the Metrovick Type 232. A modern equipment is the E.M.I, orMurphy-made Type 620, far smaller and more efficient. Aerials may be suppressed into the bodyor wings or they may be small spikes projecting several inches from the body. A sampling switchis necessary to feed the transmitter with infor- mation from the various picking-up points, whichmay comprise 20 air-pressure tappings (dynamic and stagnation), a dozen mechanical strain gaugesand several other transducers or pick-ups for temperature or control positions. It sometimeshappens that several readings are required to be taken at precisely the same time, and this neces-sitates complete duplicated systems. One of the most fundamental pre-requisites tothe accurate evaluation of a missile flight is that the position of the missile should be known atany given time. This can be determined by any of, or a combination of, several methods. In one arrangement the range is obtained byintegrating Doppler frequencies. The Doppler principle is now generally well understood, and itdepends on the fact that the frequency of a radio signal picked up by a moving body will beincreased if the body is moving towards the
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
