BEST IELTS Academic Reading Test 28
ACADEMIC READING TEST 28 – PASSAGE – 1
ACADEMIC READING TEST 28
READING PASSAGE – 1
You should spend about 20 minutes on Questions 1-13, which are based on Reading Passage 1 below.
THE LIQUID-PROPELLANT ROCKET AND ALTERNATIVES
A liquid-propellant rocket is a rocket whose engine uses propellant in liquid form to power it. The reasonably high density of liquids make them a desirable form of engine power as the volume of propellant tanks used to hold them can be relatively low. Lightweight pumps can also be used to pump the liquid propellant from the tanks into the engine, which means the propellant can be kept under low pressure. Both these factors, smaller tanks and lighter pumps, are advantageous as they effectively lower the rocket’s mass ratio.
One of the most common types of liquid propellant rocket is the bipropellant rocket. Bipropellant rockets generally have two tanks; the top-most one which contains liquid fuel, and a second, typically slightly larger tank, containing a liquid oxidizer such as liquid hydrogen or a hydrocarbon fuel, liquid oxygen combination. The fuel tank and the oxidizer are connected to the combustion chamber by pumps. It is within and chamber that the fuel and oxidizer react and combust. This chamber, in turn, connects to the nozzle through which spent fuel is expelled, generating sufficient thrust to get the rocket airborne.
Liquid-propellant rockets are preferred to other types primarily because they have a higher energy content; thus generating more thrust. Tankage efficiency is another important factor. Liquid propellant will typically have a density similar to water and require only modest pressure to prevent vaporization from occurring. This combination of high density and low pressure permits a very lightweight tank. Gasses, on the other hand, are not nearly as dense and require more pressure to be applied in order to keep them stored within the tank, meaning heavier tankage must be used, which results in a higher rocket mass ratio.
Other advantages of liquid-propellant rockets include the fact that they can be reused for several flights, as happened many timed in the Space Shuttle programme operated by NASA, and the ability to shut down and reignite such rockets multiple times if necessary. That said, the use of liquid propellants has been associated with a number of issues. One such issue is termed slosh (the movement of a liquid inside another object already undergoing motion). Slosh can lead to loss of control of the vehicle and it can also confuse computerized guidance systems, which are not equipped to account for the random path disturbances it can cause. Another major drawback of this kind of rocket is the functional complexity of the liquid-propellant mechanism, which operates high speed moving parts at very high temperatures. This can be a recipe for disaster, as it increases the probability of something going wrong.
The pumps used to pump the liquid propellant, though lightweight, are also very hard to design, and this is another bone of contention with proponents of other forms of rocket propellant. These turbopumps, as they are known, can suffer serious failures, such as overspreading or shedding when operated at high speed.
Essentially, liquid-propellant rockets must be fine-tuned and they operate with a very small margin for error. They are, therefore, very high-maintenance taking into consideration build, design, storage and flight logistics. However, if the logistical complications can be overcome, the reward is a highly effective, precision instrument that is relatively lightweight and can be reused more than once – a huge plus point when the expense of building a new rocket is factorized in.
Solid-fuel rockets have been in existence much longer, and their main advantage over the liquid-propellant rocket is their long storage life. Solid-fuel rockets can be stored indefinitely and can then be readied for redeployment and operation in a relatively short space of time. They are also less volatile and for that reason can be transported more easily from place to place. This explains the military’s preference for solid-fuel rockets when it comes to its missile cache.
On the other hand, while NASA does deploy solid-fuel rockets in the initial stages of a space shuttle launch, primarily for their superior initial thrust, it relies on liquid-propellant rockets in the latter stages as these rockets provide thrust for longer periods.
In an attempt to capture the best aspects of both rocket types – solid and liquid (gas, realistically, being far too volatile) – hybrid rocket models have recently emerged. These are mechanically and functionally simpler machines, as they require one not two liquid tanks. They are also superior from a safety perspective as they can be loaded onsite, so they are effectively transported in a benign state and do not represent a hazard in transit (this type of hazard is a major drawback of conventional rockets). At present, it is mainly space science agencies that are researching the potential of hybrid rockets.
Label the diagram below.
Choose FIVE answers from the box and write the correct letter, A-G, next to questions 1-5.
B. Liquid Oxidiser
C. Combustion Chamber
F. Hydrogen Gas
Complete the notes below. Using NO MORE THAN THREE WORDS from the passage for each answer.
Liquid-propellant Rockets: The Pros and Cons
– Generate more thrust for 6…………………………..
– use of a more lightweight tank helps keep the rocket’s 7…………………………………….low compared to say gas-propellant rockets.
– Can be used more than once and allows the controller to 8………………………………..the rocket again several times after it has been shut down.
– Liquid fuels inside tanks often suffer from slosh, which can adversely affect 9……………………….., and , in some cases, lead to a loss of control of the rocket.
– Functional complexity of the rocket mechanism increase the likelihood of 10……………………………………..in flight.
– Pumps hard to design and prone to failure.
Complete the summary below.
Use NO MORE THAN THREE WORDS from the passage each answer.
Solid-fuel Rockets have been around much longer than liquid-propellant ones. They are also renowned for having a superior 11……………………………….and, not only can they be stored indefinitely, once required for operational purposes, they can be operational again in a very short space of time, hence why they have always been the 12………………………….favoured type of rocket. Solid-fuel rockets also have 13………………………………,which is why NASA uses them in the initial stages of launching a space shuttle.
6. longer periods
7. mass ratio
9. computerizes guidance systems
10. something going wrong / serious failures
11. storage life
13. superior initial thrust