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Aviation History
1961
1961 - 0141.PDF
PLIGHT? 3 February 1961 Missiles and Spaceflight . . . the liquid oxygen is as cold as possible are usually worthwhile,and it has even been found profitable to cool some to the tem- perature of liquid nitrogen immediately before starting, yieldinga crain in suction head of 121b/sq in. From the engine side, the problem may be approached by reducing the pump acceleration;but there is a limit to this, since extended operation at a low thrust level may lead to combustion instabilities. Severity ofcavitation is influenced by the specific and latent heats and the liquid/vapour density ratio of the liquid being pumped. A highlatent heat is favourable, in that the production of a small mass of vapour results in a considerable cooling of the remaining liquid.Likewise a low ratio of the liquid/vapour densities tends to be beneficial, and on this count liquid oxygen provides a bettercavitation performance than many other propellants. Cut-off problems are in many respects similar to thoseexperienced on starting. Just as it was found necessary to bleed dead ends and enclose spaces in order to achieve proper priming,it is equally important to allow the vapour generated from trapped liquid oxygen to escape, and it frequently happens that the samebleed lines or points can serve as vents. The main source of trouble at cut-off comes from the liquid oxygen trapped betweenthe propellant valves and injectors, for as the chamber pressure falls below the s.v.p. it immediately evaporates. In the case of thegas-generator the resultant high mixture ratio can damage the turbine blades or guide vanes, and in the main chamber it resultsin an unpredictable increase in total impulse. Arrangements may be made for the oxygen to be shut off before the fuel, and forthe injector cavities to be purged with an inert gas; but both measures result in a further increase in complexity. Having successfully completed a test or flight the next step isto examine the instrumentation records. Steady-state evaluation is theoretically relatively uncomplicated, involving measurementof the propellant flows, and the magnitude and direction of thrust. 141 Transient behaviour is much less straightforward. The importanceof such behaviour cannot be over-emphasized, since the rate at which thrust builds up and dies away affects the magnitude ofthe dynamic stresses in the vehicle. Furthermore, incorrect mixture-ratio conditions, though persisting for only a short time,can have a dramatic effect on turbine or chamber life. By far the most revealing measurement is that of pressure,either absolute or relative to some other part of the system, and in examining such records in liquid-oxygen lines one receives a rudeshock. A glance may indicate that the pump inlet experienced a pressure surge several times the steady running value, and cer-tainly in excess of that which the inlet lines were stressed to with- stand. Similarly, a calculation of mixture ratio from the differencebetween oxygen injection pressure and chamber pressure suggests impossibly high starting values. In contrast, pressures appearstrikingly steady once steady-state operation has been achieved, and at cut-off the transducers record a consistent decrease inpressure with no alarming symptoms. The reason for this anomalous behaviour lies in the way the transducers are installed.Since many are unsuitable at liquid-oxygen temperatures, it is common practice to interpose a length of connecting line to insu-late them from the effects of low temperature by a column of gas. During the engine start, the increase in pressure forces theliquid along the transducer line, compressing the gas ahead of it, until the pressure in the transducer is equal to that in the mainsystem. But the momentum of the column of liquid further raises the pressure within the transducer until this momentum isdestroyed. The process then reverses itself, and the record takes the form of a damped oscillation. The liquid oxygen within theline gradually evaporates, and at cut-off a more faithful record is obtained as the low-density gas escapes from the transducer line.This inherent ambiguity can be overcome only by the rather costly flush transducer with a heated sensing element. Accordingly,accurate diagnosis of starting transients is difficult, and evidence of abnormal oxygen flow often has to be obtained either from itseffect on other parameters, or, in the worst cases, components. EUROPE'S BIG DECISION By the time this issue appears the decision whether or not Europewill have its own space-launcher programme—and, if so, whether Blue Streak will be used—will probably have been taken. Toreach such a decision is the only main purpose of the twelve- nation conference in Strasbourg, convened jointly by the Britishand French Governments, which began last Monday (January 30) and was expected to last four or five days. The Minister of Aviation, Mr Thorneycroft, rounded off hisEuropean travels in preparation for the conference with a visit to Stockholm on January 26-27. In Strasbourg on the eve of theconference he denied French press reports that the motive of the British plan was to recoup losses on the Blue Streak missileprogramme. "In fact," he said, "we are making Blue Streak a gift to our European friends. As far as Britain is concerned we arewriting it off. It is a gift . . . free." These are the first photographs to be released showing the only Blue Streak launch emplacement completed at Woomera. Almost identical to the C-3 installation at Spadeadam ("Flight," September 16, 1960), the launcher is mounted at the end of a causeway on which runs a huge servicing tower, seen enclosing the launcher in the right-hand photograph COMMUNICATIONS SATELLITES IN EUROPE The Federal German Post Office is to send a group of tele-communications specialists to Britain to discuss British plans for a communications satellite. This announcement on January 27followed talks in Bonn between Mr Reginald Bevins, Postmaster General, and the Federal Minister of Posts, Herr RichardStridden. It was stated that these talks had been arranged before the visit to Germany of the Minister of Aviation earlier last month. •*•*.; *-
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