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Aviation History
1963
1963 - 0461.PDF
riAHS 1913-1963 FLIGHT International, 28 March 1963 439 GUIDING BLUE STREAK . . . mined amount (dependent on range) short of that which would have given the correct range. • Choice of axes system was governed by the following require ments: minimum overall complexity; reduction of accuracy speci fied for precision components; ease of generation of demand signals for pitch and yaw steering; simple ground monitoring and adjust ment; the need for a simple flight control system. The systems considered were: rotating axes, with their origin at the centre oi the Earth; non-rotating space axes, with a fixed origin at the ientre of the Earth; falling axes (i.e., non-rotating space axes starting with their origin at the centre of the Earth but falling in the Earth's gravitational field); non-rotating space axes moving through space with the Earth's rate velocity component along each axis at the instant of launch; combinations of the last three. I After taking into account all the requirements, the fourth system was chosen, namely: a right-handed system of non-rotating space axes which, at the instant of launch, had its origin at the launch point and was moving through space with a velocity equal to the Earth's rate velocity at the launch point at the instant of launch, and was subsequently accelerating under lateral gravity. . The axes system had to be set up in the missile an appreciable time before lanuch. This was achieved by setting up the Ox axis along the required line of thrust and defining an angle of 36° to the horizontal at the launcher, with the Oy axis horizontal, and then constraining the complete axes system to rotate at Earth's velocity until the time of launch. By this means the initial conditions for the non-rotating space axes system can be defined. Navigation requirements were met as follows:— 0) (2) P) (4) The non-rotating space axes were established by gyroscopes, thus denning an orthogonal three-axis system. A stable platform, on which the gyroscopes were mounted, was maintained in coincidence with the gyroscopes. Accelerometers, mounted on the stabilized platform, measured apparent accelerations in each of three directions (x, y and z). A computer subtracted computed gravity terms gx and gz from the apparent acceleration terms to give true x and z acceleration values. The y accelerometer output was unmodified. (5) One stage of electro-mechanical integration was then performed in each of the three channels to obtain real velocity in x and z directions, and apparent velocity in the y direction (Earth's velocity was not added to the velocities obtained). i.6) A further stage of electro-mechanical integration was carried out to produce a position signal for each channel. As already noted, accurate control to a particular acceleration, velocity and displacement programme would have imposed severe requirements on the control system, so in practice Blue Streak would lave taken off and turned over about the pitch axis on its control gyros alone. During this stage dispersion from the ideal trajectory was expected to occur. The detailed ideal trajectory was pre-com- puted for a given launch site and target, allowance being made for Such factors as the rotation of the Earth, gravitational anomalies and re-entry effects. The differences between the ideal cut-off con ditions and the conditions obtaining at any time in the actual missile were computed by the inertial system, and guidance had to be such that at actual cut-off, these differences were balanced in the sense piat they resulted in no range or line errors at impact. 3lue Streak powered trajectory ivent times Blue Streak static firing at Spadeadam Considerable simplification was possible if range and line errors could be taken as linear functions of the differences between ideal cut-off and actual conditions, and this was permissible if the differ ences were sufficiently small. Since the control gyros were not ex pected to be able to hold the missile axis sufficiently accurately to a given body angle to ensure that cut-off would occur where linear computing was sufficiently accurate, some form of pitch guidance was essential. The range error would have been zero at cut-off since it was this error which instigated cut-off; but the guidance system had to ensure that, at cut-off, the line error was zero. An actual missile trajectory would have differed to some extent from an ideal trajectory, and total time of flight would have dif fered from the ideal. Due to the rotation of the Earth, the target would have had a velocity in space relative to the launch point, and this would have been allowed for by an angular offset of the launch direction from the great-circle bearing of the target from the launcher. This effect would obviously have depended upon the total time of flight, so that any deviation from this time would have resulted in impact errors. These errors would, in general, have had components both in range and in line so that, if a correc tion had to be made, the philosophy of both motor cut-off and yaw steering would have been dependent upon the consideration of time-of-flight errors. The philosophy of pitch guidance was treated as a self-contained aspect of the complete guidance philosophy, but, as discussed above, motor cut-off and yaw steering considerations were both dependent upon time-of-fiight considerations, and it could be shown that yaw steering was dependent upon range error (and thus motor cut-off) considerations. The pitch system finally evolved was designed to ensure that dispersions from the ideal in velocity (z) and position (z) due to gyro errors, variations in the turn-over rate, burning rate, mass and specific impulse, etc, would have been reduced to acceptable limits by cut-off time. Yaw steering would have been achieved to give zero apparent acceleration, velocity and displacements until approximately 15sec before cut-off, at which time corrections for time of-flight errors could have been introduced. Sperry overcame great difficulties in the design and development of an inertial system for Blue Streak; but even more complex prob lems are met in launching a Polaris from a moving vessel. Britain's decision to undertake a programme of four Polaris submarines will have implications for British industry of a magnitude only now being fully considered. It is thus natural that Sperry London should be interesting itself in these problems, particularly as Sperry New York holds the basic navigation-system contract for the US Navy Polaris fleet. The British Government has made it clear that it may well buy the navigation system complete from American sources, which include a host of subcontractors to whom American Sperry have delegated part of the work. This still leaves Britain with considerable problems related to specialized components for the Polaris warhead, and our own national management burden which is the responsibility of the senior officers already appointed at the Admiralty, the Ministry of Aviation and the Admiralty Com pass Observatory. A particularly urgent requirement might be the setting up of training facilities, complete with simulator. , 0 2 uc RANGE VI ERROR I.':, PRIOR TO
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