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In HSC Physics one of the dot points is about a contributor to rocket science. Here is a completed research task (as you may be asked to complete one) of Robert Goddard.
Robert Goddard, being born in 1882 in America, is informally known as the father of modern rocketry due to his work on liquid fuels for his rockets. Several years after his death, the US government for $1million, showing how the opinion of society had changed due to scientific discoveries and achievements, bought Goddard’s patents.
Rockets in a Vacuum
In a vacuum, there is no air but rather an absence of any matter. The most special thing about a vacuum is the way the dispersion forces work. Dispersion forces that hold a lot of substances together now work in reverse as the substance attempts to fill the space in which it is occupied (which is boundless). As such there have been serious concerns in the past about whether or not a rocket could travel in a vacuum.
To understand this, an understanding of how a rocket works must be achieved. A rocket works through the Law of Conservation of Momentum (momentum initial=momentum final), and Newton’s 2nd and 3rd laws of motion ( F=ma and every action has an equal and opposite reaction). By this it is meant that a rocket is a reaction engine, what it does, has an affect that in turn powers the rocket. Through combining the equations we get F=(mv-mu)/t (force = change in momentum / time)
The matter is caused by a combustion reaction that releases chemical potential energy and converts this to kinetic energy, moving it out of the engine of the rocket at a high velocity . Despite this matter weighing very little, the velocity that it comes out at means that it has a relatively high momentum, compared to being stationary. This in turn through the law of Conservation of Momentum means that the rocket is moved in the opposite direction to the direction that the matter is thrown out of . Then although the rocket has a reaction force acting upon it, the effect of this force is divided by the time over which it is felt. In some modern rockets the thrust can be greater than 3.3×107 N; however Newton’s 2nd law of motion means that the acceleration of the rocket is the force divided by the mass, significantly lowering the acceleration .
So the contribution from Goddard towards this aspect of rocketry was not the theory, but rather the testing of the theory. He did this through the use of a ballistic pendulum with a rocket and a rocket at first, measuring the height the rocket with rock reached on the pendulum. Years later he tested the theory again through the use of a calibrated spring and firing the rocket into it, calculating the thrust and proving that there needed to be no air to push against for a rocket to provide thrust. This allowed for the commencement into the development of space exploration technologies without the risk of failure and a massive waste of resources.
There are a number of advantages to the use of liquid fuel rockets over solid fuel rockets. These include:
- Liquid rockets able to be stopped once started
- Able to be pumped through pipes
- Variable thrust
- Boosters are more re-useable than solid rocket boosters
However there are disadvantages to liquid fuelled rockets:
- Fragile (they contain many complex parts)
- The liquid oxygen must be kept liquid (-183’C)
- Less thrust per size than solid fuelled rockets
Liquid rockets, however, still use combustion reactions. Combustion reactions release heat and kinetic energy when the chemical potential energy is released from the compounds. The rocket immediately uses the kinetic energy, however the sound and heat energy needs to be transferred to another device to convert them into kinetic energy.
Attachments to Rockets
While proving that rockets could work without the presence of air to push against, he also measured the efficiency of which rockets use the chemical energy released by the combustion of various materials. By firing a rocket whilst submersed in water, he found that only about 2% of the amount of energy available in the chemicals being used was actually being used in the thrust of the rocket. This was calculated through the rise in the water temperature, and calculated using the specific heat capacity of water. To solve problem with the waste of energy, Goddard looked at ways of converting this ‘wasted’ energy into the thrust of the rocket. As most of this energy was lost as heat, Gustav De Laval who had designed a steam turbine that could reach speeds of faster than the speed of sound relative to the surface of the earth, had already solved the solution to the loss of heat for Goddard [H1]. This device transferred the heat energy into the turbine, and then converted the heat energy to kinetic energy through using steam [H7]. The use of this steam turbine increased the theoretical efficiency of Goddard’s rockets to roughly 60%, greater than other steam powered turbines due to the temperature at which a rocket operates. This increased the amount of matter being propelled backwards thus increasing the thrust of the rocket, as discussed earlier.
He developed pumps suitable for the rocket fuels, cooling rocket motors and other such contraptions designed to carry man to outer space. Goddard’s prototype rockets reached an altitude of 1.6x103m (1.6km). These inventions have allowed for the commencing research into space exploration and as such have had a significant impact on the space community.
Another area that Goddard looked deeply into was that of guided rockets. In this area he used gyroscopes to control the motion of the rocket while it was in flight, steering the rocket with small vanes in the sides of the rocket. This allowed the motion of the rocket to be predicted and a suitable landing point constructed. Gyroscopes work using the law of conservation of momentum, and stay unchanged relative to a set point. This means that if a gyroscope was used and spun around in circles, the centre of the gyroscope would still be unchanged (not spinning) relative to the starting point, even if the rest of the object was.