Date of filing Complete Specification: June 22, 1956.
Application Date: March 23, 1955. No. 27552/54.
Complete Specification Published: June 22, 1960.
PROVISIONAL SPECIFICATION
Improvements relating to the Ignition of Gas-Generating Charges in Small Reaction Motors
We, WILMOT MANSOUR & COMPANY LIMITED, a British Company, of Salisbury Road, Totton, Southampton, do hereby declare this invention to be described in the following statement:—
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In jet-propelled model aircraft the gas generated by the combustion of the fuel escapes through a jet orifice that is necessarily small, being in general no greater than 1/16th inch in diameter. The fuel itself is a solid charge contained in a casing which must have no opening other than the jet orifice through which gas can escape. The ignition of this fuel presents a difficult problem, because electrical ignition, which is the obvious method, is not practical in such a thing as a model aircraft, involving as it does the provision of a battery and other complications such as a platinum sparking plug.
Hitherto this problem has been solved by the provision of a fuze entering the motor casing through the jet orifice. To ensure that the fuze, after being ignited externally, will remain alight and ignite the charge, it is necessary to furnish it with a heat-conducting core in the form of a wire, which also serves to facilitate the threading of the fuze through the orifice. There is a risk of particles of the fuze and ash from the charge blocking the orifice, and this risk is materially increased by the presence of the wire. In consequence, it has been necessary as a safety precaution to make provision for the relief of pressure if the jet orifice should become blocked, and this has been done by making an end cap with the jet orifice in it movable against springs, thus complicating the construction and increasing the weight of the motor.
In the design of such small reaction motors, the size of the jet orifice is an important factor. The higher the speed of the gas through the orifice, the more efficient is the motor in acting as a rocket. In a casing of a given size with a given solid-fuel charge, the pressure developed in the casing depends on the size of the orifice, and the smaller the orifice the higher is the speed of the gas through it. Therefore to reduce the risk of blocking of the orifice by increasing the size of the orifice leads to decreased efficiency. In any case, the efficiency is not as high as it might be, because the orifice cannot be reduced below a certain size, even if means are provided for relieving abnormal pressure, the reason being that the orifice must be large enough to accommodate the fuze, Now in practice the minimum diameter of a reliable fuze is about 40 thousandths of an inch, so the orifice diameter must be larger than this.
Similar considerations arise if the motor is used for any other purpose, e.g. propelling a model ship.
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The ignition of this fuel presents a difficult problem, because electrical ignition, which is the obvious method, is not practical in such a thing as a model aircraft
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According to this invention the fuze is wholly contained within the casing and is ignited by heat transmitted locally through the casing wall. This both substantially reduces the risk of the orifice being blocked and enables the size of the orifice to be reduced without changing the dimensions of the casing or the size of the charge. The risk of blocking of the orifice can be practically eliminated if in addition a filter is provided across its inner mouth. Hitheno the provision of such a filter has been impossible because it would have prevented the passage of the fuze through the orifice into contact with the charge. A safety device may nevertheless be provided and consist of a thin metal disc in the wall of the casing, the disc being strong enough to withstand the normal operating pressure but so weak as to break if the orifice becomes blocked and abnormal pressure develops in the casing.
Preferably the heat for the ignition of the fuze is transmitted through a thin disc forming part of the wall of the casing and serving as a safety device as just described.
The heat for the ignition of the fuze is preferably generated by the burning of a short length of fuze in contact with the outer surface of the casing wall. To ensure that both the outer and inner fuzes are in contact with the casing wall tongues of metal may be provided both externally and internally to leave wedge-shaped spaces between themselves and the wall, the ends of the fuzes being forced into these spaces.
If a model aircraft includes an augmenter tube into which the gas jet passes from the jet orifice, it has hitherto been necessary to ignite the fuze by pushing an igniter rod down the augmenter tube. There is no longer any need for this, as the external fuze can be ignited through a short lateral passage made in the fuselage.
In a model aircraft with two jet motors it has previously been difficult to ensure equality of the thrusts produced by two motors. This difficulty can be overcome by using a single charge ignited according to the invention and providing two gas passages through which the gases generated from this charge pass, each passage terminating in a single jet orifice.
For the Applicants:
ARNOLD TICKNER,
Chartered Patent Agent,
239-241, Shaftesbury Avenue,
London, W.C.2.
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According to this invention the fuze is wholly contained within the casing and is ignited by heat transmitted locally through the casing wall.
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