Tuesday, March 1, 2011
What is Aerospace?
To kick off “Aerospace Month” on the Engineer’s Pulse, let us first examine what the term aerospace refers to. As the word indicates, aerospace studies the motion of objects through both air (in the Earth’s atmosphere) and space (outside the Earth’s atmosphere).
The field of study can therefore be broken down into two principal areas: aeronautical (within the atmosphere) and astronautical (within space). Examples of aeronautical navigation include aircrafts (aviation), helicopters, and even hot-air balloons. Astronautical navigation is limited to spacecrafts and satellites.
There is no real line in the sand, where the atmosphere becomes space; there is just a very gradual transition between the two extreme states. Near the surface of the Earth, the medium known as air is a reasonably dense gas, with roughly 1 atm of pressure (101.3 kPa); this figure depicts how tight the molecules of the matter are packed together. In space, at about 300 km altitude, we begin to see the other extreme, where molecules like Oxygen may travel a kilometre before colliding with another molecule.
Although the altitude up until 10,000 km is considered to be the atmospheric domain, 300 km is a convenient line in the sand. It is where the pressure may be approximated as zero, and the flight dynamics are astronautical in essence. 300 km also happens to be the altitude at which the International Space Station (ISS) orbits the Earth.
Some perspective with regards to the scale of altitude is helpful to appreciate aerospace. A commercial pilot will often announce a cruising altitude of about 30,000 ft, which translates to about 10 km. This is a typical distance from the Earth’s surface where aeronautical navigation takes place. The 300 km altitude mentioned previously is in the region of LEO (low Earth orbit). Even this altitude is a stone throw away from the Earth in a relative sense, given that the radius of the Earth is about 6,378 km. Cities on Earth that we consider nearby are usually more than 300 km apart.
The difference between travelling in air and space is immense. A bird flying and a fish swimming have more in common than do a rocket and an airplane. Airplanes, birds, and fish, make use of the fluid around them to propel themselves, whether that fluid be a gas or liquid. Although spacecrafts must first travel through the atmosphere to reach space, they are designed for navigation once they reach space – a place with no surrounding matter to make use of.
From a Newtonian physics point of view, aeronautical ships have four forces acting on them. The first three involve the air: the thrust force from the engines, the drag force opposing the thrust force (due to wind resistance), and the lift force on the wings due to air pressure gradients. The final force acting on an airplane is the gravitational one. When a rocket or satellite travels through space (outside the atmosphere), the gravitational force is the only force acting on it for the majority of the time. It is the high velocity and gravitational force that cause satellites to orbit the Earth – to gravitate around it.
On occasion, when a rocket is in orbit, it may want to alter its course. If its engines worked like those of an airplane, it would be unable to move. Whereas an airplane propeller intakes and outputs the air around it for propulsion, a rocket must carry its fluid (gasoline) with it. A spacecraft engine makes use of Newton’s third law: it exhausts the fuel that the rocket carries at extremely high velocities (opposite to the direction in which it wants to move) to impart an impulse on the entire rocket. If a rocket orbiting out in space were to run out of fuel, it would be stranded. With virtually no surrounding fluid to impose a drag force on it, it would continue to orbit indefinitely.
The field of aerospace is one of man’s most exciting realms of research and development. Our species, always fascinated by the world around us, saw the birds that navigate the air around them with ease as a direct challenge. However, we had to wait until the early twentieth century to participate in the fun. On December 17, 1903, the Wright brothers performed the first powered, controlled flight.
Aviation has seen a multitude of advances since that historic flight over a Century ago. The sheer quantity of people transported via commercial aircraft today is astonishing. It is estimated that in the year 2014, the number of man flights will reach 1.26 billion (3.5 million man-flights per day).
From a technological standpoint, the principal goal continues to be to make lighter airplanes for increased energy efficiency. Material advances ensure continuous improvement in this area.
While aeronautical travel is interesting, astronautical travel is where the excitement of aerospace resides. Nothing captivates the human spirit more than the exploration of faraway lands. Indeed, the 1969 lunar landing remains a benchmark in human history – possibly man’s crowning achievement to date.
Despite the allure of interplanetary missions and other aerospace research endeavours, many believe that we should focus our resources on more immediate issues that exist on the surface of the Earth, such as poverty, hunger and disease. I believe there is a careful balance to be found here.
Economies that do not advance technologically stagnate, and no area (short of military applications) creates as many spinoff technologies as does aerospace. Materials that are developed for aerospace are later employed by other industries. For example, composite materials, which continue to be developed for aircraft use, are later used for hockey sticks. Velcro was originally developed for astronautical applications, but today, it allows little Jimmy to do up his shoes.
International poverty must be addressed, but only economically healthy countries can afford to send aid. Economic health may only be found on the borders of cutting edge technology, and this is where aerospace lives.
Still, the most compelling reason to take to higher altitudes remains the human spirit. We are still in awe by the birds. Today, we see the endless cosmos as a challenge, and wish to explore it. A major advancement in astronautical travel will be necessary if we are to truly address this challenge. This advancement will be the topic of my next article.