Rocket Science by Mark Denny & Alan McFadzean

Author:Mark Denny & Alan McFadzean
Language: eng
Format: epub
ISBN: 9783030280802
Publisher: Springer International Publishing

You see from Fig. 5.1 why tailoring the shape of a mass of solid propellant changes the rocket motor thrust and thrust duration. This geometrical characteristic tells us why the propellant must be very homogeneous, and why any cracks that develop in the solid mass might be catastrophic. A crack will increase the burning surface area and so will increase the burn rate in an unpredictable and unintended manner. Such a process can become a runaway event (heat from burning causes cracks that increase burn rate, which increases the heat further, which causes more cracks...) and result in an explosion—this has happened many times.

A neutral burn means a constant rate of gas being generated, constant thrust and constant stress upon the structure of the rocket. A progressive or regressive burn will have increased stress at the beginning or the end, so that the rocket structure will have to be engineered to cope with this increased stress. This fact means that the rocket is over-engineered for most of its burn, which is inefficient.

Here is another consequence of the fact that solid propellant burn rate depends on its shape, its geometry: once it starts, it goes on until there is no more propellant to burn. It is hard to stop a solid propellant from deflagrating once it has been ignited. Thus, solid propellants are hard to control. They are used in cases where we know in advance that we want the motor to burn all its fuel in 15 seconds, or 250 seconds, or whatever the solid propellant shape and size dictate.

Are there circumstances where we might want a progressive or regressive burn, despite the inefficiency? Yes there are. Consider, for example, a rocket that has blasted off from the Earth’s surface and is heading vertically up and out of our atmosphere. As it burns propellant, its weight drops sharply and its speed increases rapidly. The rocket has been engineered to cope with a certain maximum heat load brought about by aerodynamic drag; if it accelerates too fast then the rocket might burn up in the atmosphere. In this case a regressive propellant burn would reduce thrust in the latter stages, so capping the rocket speed and thus the thermal stress. From this simple illustration you can perhaps envisage the many interacting variables and multifaceted nature of space rocketry, balancing A against B, and taking A and B (and don’t forget Q) into account when considering solid propellant geometry .

Solid rocket propellant chemistry, and the explosion (an appropriate term) in the variety and power of available propellants, peaked during the Cold War. We encapsulate this progress in Fig. 5.2, which shows how the specific impulse of the best available solid rocket propellant increased over the decades. Specific impulse is a measure of propellant effectiveness; we have encountered it before, but let’s recap. The impulse of a rocket is the average thrust generated by its motor multiplied by the time that the motor can operate before it runs out of propellant. Thus impulse measures how fast a given rocket can get from a standing start.

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