a compendium of tech stuff

Mar 19, 2011

On 11:29 AM by Lalith Varun   488 comments

Once a system has been started by some external power source, the working fluids sustain the process on a self sustaining basis, such a system is known as a bootstrap system. A gas turbine engine once started, sustains a bootstrap operation, because the turbine continues to drive the compressor that feeds air to the engine. Another bootstrap system used by aircrafts is the bootstrap air conditioning system which provides adequate cooling to the cabin and electronic equipment.
The bootstrap air conditioning system mainly consists of two units, the cold air unit and a heat exchanger, both of which are mounted on a common shaft. This assembly was supported on ball bearings until they were replaced by air bearings which provide a lighter solution, requiring less maintenance. The compressor increases the air pressure with a corresponding increase in temperature. The increased temperature is now reduced in the heat exchanger which is cooled by ram air. This reduction in temperature might lead to formation of water, especially when the aircraft is flying in humid conditions. The water extractor placed at the turbine inlet removes most of the water thus preventing damage to the turbine blades and spraying water into the cabin and electronic equipment. As the air expands across the turbine, the temperature can drop below 0o and hence a cold air bypass line is used to vary turbine outlet temperature to get the required temperature for cabin and equipment cooling. 


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Mar 15, 2011

On 9:29 PM by Lalith Varun   11 comments


A fin is a surface used to give directional stability to any object moving through a fluid such as water or air. The size of the fins, their shape, the number to use and their placement on rocket are all questions that can be answered only by experimentation. Even in the professional field, the character of the aerodynamic surfaces is determined only by extensive tests in wind tunnels and later flight tests on missiles and rockets. There is no quick and easy mathematical formula that can guide you in the selection of fin designs

 One amateur rocket society which has operated for many years on the Pacific Coast has worked out a formula for determining the fin area required for the average amateur rocket which it claims it has used with good success in building most of its rockets. This formula gives the area of one side of one fin, regardless of the shape of the fin and applies only to a three fin arrangement.
Here is the formula, ((d+0.5) X L)/6
where d is outside diameter of the rocket tube
and L is length of the rocket without nose cone
the value of 0.5 is always added to the diameter of the rocket.
The design formula further provides that width of each fin should be at least 1.25 times the diameter of the rocket body.

The cut and try method of fin design is probably the most widely used and the most successful. Even if you hit upon some formula for fin design you would still have to test the result. Most groups build small scale models of the rocket they are designing and test them with a variety of fin arrangements until they hit upon one that seems to give the rocket the best stability.

Steel and aluminum are the most widely used materials for fin construction as they are both strong and easy to work with. Wood is used to a certain extent, but wood cannot be bent without breaking and is not strong enough to withstand stresses involved. Lightness, is an important consideration for any component of the rocket, but do not sacrifice the strength in the fins of your rocket in order to cut down weight. The most common problem is that fins fall off or bend by the force of blast off. To avoid this fins are made of steel or aluminum 1/16th or 1/8th of an inch thick.

For greatest stability in flight it is important that the fins be inexact longitudinal alignment with the rocket body. Your fins will never stand the stress of takeoff if they cant stand being dropped on the floor a few times. Whatever method of attachment you use, be sure that it is strong and secure. There should be no wobble or flutter of the fins. Do not drill holes into your rocket combustion chamber in order to bolt a flange. The only place that bolts or screws can be used are the points where the nozzle and forward bulkhead are attached.

There you have it, try out all the fin designs shown, test them on your rocket and see your rocket sour to greater heights.