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Aviation History
1942
1942 - 1657.PDF
v>. AUGUST 6TH, 1942 F QfQ HT »55 PREVENTION of :KET MOTOR BURN-OUT The Advantages of Concentric Feed in Liquid-fuel Rocket Propulsion / By CEDRIC Technical Chairman of the American Rocket Society I OXYGEN INLET W3M H THE main obstacle to th^fuccess of the-'Iiquid-iuel rocket motor (a type oj^et propulsion where all the necessary fuel and^ft^gftiHS^arricd along) has been the tendency of the motor to corrode and burn out. With the burning combustibles at times reaching to a tempera ture of 3,000 deg. C, none of the usual methods of counteracting motor burnout has 1 een entirely satisfac tory, and even with most of the difficulties involving them eliminated, motors still burn out at frequent intervals. In most types of internal-combustion engines intermittent •firing takes place, and high temperatures are reaped only for a very short time. Because of this, the problem of sustaining these high temperatures is less difficult, while in a rocket motor with continuous combustion the rase is somewhat different. The best materials for motors would have physical properties of a high ""Inelting point, good thermal con ductivity, high tensile strength and good corrosion resistance. Where in the past brass and aluminium have been used almost exclusively, it is reasonable to believe that the substitution of monel, duralumin or a similar metal for parts of the motor tending to burn out would be to better advantage. It is also pos sible that a future alloy or plastic may have enough of these properties to become an ideal combustion-chamber material. Various types of cooling systems, although written and spoken of highly, usually bring into the picture so many construction complications that they partially defeat the purpose for which designed. Many methods are used to protect and cool the chamber walls—from the addition of water to the fuel to dampen the combustion temperature to external water cooling or regenerative systems where one or both of the propellants are circulated through a motor jacket to Assist in cooling the motor and preheating the propejjant. Cause of Burning The use of refractories for lining the inner wall of the chamber has often been considered favourably in view of the fact that they generally have higher melting points than most metals. But, owing to their poor conductivity of •^heat, low tensile strength at high temperatures, and diffi culty of application, many of these heat-resisting materials lose their effectiveness in usage. Although each of these methods may be employed in dividually, or together, they still do not cover all essen tials to the problem adequately, and two additional less well-known methods for reducing motor burnout will be discussed at length. The usual cause of the burning of the chamber walls within the motor is due to the action of the liquid oxygen. Once the walls of the combustion chamber become heated from the high temperature, the nearly pure oxygen strik ing the walls consume them as a fuel. If the greater part of the oxygen can be kept away from the walls, the walls may be allowed to reach a higher temperature than other wise. As the burnout points are usually at the feed inlets and the choke of the nozzle, both of these pourfs would need to be better protected than the axial chamber walls. Probably the simplest as well as the most practical way of preventing motor burnout is by surrounding the liquid oxygen with a thin layer of fuel. With " concentric feed " the oxygen is injected into the motor encircled by a dynamic ring of fuel which protects the walls of the motor from the scoring action of the oxidiser, and introduces a '*' Fua MNLET Diagram of a concentric feed reaction motor. The object is to surround the oxygen with fuel. gradient of lower temperature. The insulating area of fuel protects the walls until the greater part of the oxygen is well mixed with the fuel and is on its way to complete combustion. By having the inlets in the head of the motor, the liquids are set in motion effusing in a straight line towards tho motor mouth while being homogeneously mixed. Another important factor is the reduction or elimination of the need for regenerative systems. The absence of such systems results in a lowering of the motor weight, and also leads to simpler motor construction. Motor units having full concentric feed should be able to use alcohol and liquid oxygen or propellants of lower combustion tem- ^^_ peratures without resorting to cooling systems. When using petrol and - oxygen or higher temperature com bustibles, probably some form of motor cooling will be necessary. Owing to the time lag in the complete mixing of the components, the motor will be of greater length than usual. As diameters of full concentric leed motor are increased lengths will vary accordingly. One of the main reasons for doubting the feasibility of concentric feed when first introduced was the belief that the exploding gases in the combustion chamber needed to push directly against the solid back inner wall of the chamber. Reaction motors concentrically fed, as well as jet propulsors using external combustion, have shown that propagated explosion waves will deliver a reaction thrust even when travelling a short distance back against the emanating stream of efflux gases. Apparently the first successful liquid fuel rocket motors employing concentric feed were the ones designed and built by Mr. Nathan Carver' in 1936 for the Greenwood Lake (N.Y.) stamp planes. Although the planes, for one reason or another, nose-dived and broke wings, the motor fired perfectly, driving the planes along the ice over the state line. Since then the majority of successful reactors have used some type of concentric feed in,their construction. A modified form of concentric feed is being used in the present Wyld motor which was tested by the American Rocket Society during the past summer. In this motor the oxidising agent is injected from the centre of the motor head through a ring of small pinholes, while the fuel is induced through radial inlet openings, where it intersects and encompasses the oxidiser. Outside Burning A second method of defeating motor burnout may be accomplished by making use of the principle of "outside burning." in an article published in the American Rocket Society publication Astronautics, for April, 1940, the writer described the theory of external motor combustion. In general, outside burning is a method of protecting the rocket motor walls from the oxidising action of the hot gases, by placing the peak flame temperature area outside of the motor mouth. The ultimate efficiency is attained when the flame place ment is such that the greatest possible reaction is obtained with the least possible oxidisation of the motor metal. If the metal melting point of the combustion is inside or too near the motor mouth a burning of the metal will take place. While if too great a distance from the motor the result will be a lowering of the reaction thrust available for propulsion. The area of highest temperature may be changed at will by varying the ratio of propellants or adjusting the pro-
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