Letter 19 From Screws to Space Stations: All Engineering is Important
Dear Nick and Natasha,
Here I sit at my computer keyboard in a bit of a pensive mood as I begin to write to you. One thing that has always bothered me, and perhaps you too, is that sometimes engineers question the validity of their careers. After all, what real contribution have they made to human civilization? What device, machine, system, software, or process have they invented that has been truly remarkable? I know I’ve had these kinds of discouraging doubts about my own engineering career from time to time.
On the whole, of course, the engineering profession has made major contributions to society throughout the ages. I don’t think I have to remind you of all the technologies that have sprung from the minds and hands of those in our noble line of work. And we have even had geniuses emerge from our midst who are now household names and whose work continues to shape our world. And I take pleasure in that knowledge. But, I am really talking about the individual engineer who sometimes feels their particular work is a bit dull, a tad unimportant, and maybe even altogether pointless. Yet, why should that be so?
On the contrary, it seems to me that all engineering is important. None of it should be taken for granted. If we take a step back and take a look at what engineering has produced, we’ll see several lessons. First, most advances in technology are usually evolutionary, not revolutionary; most occur gradually and build on the work that came before. Second, ideas and inventions that are seemingly trivial and those that are apparently crucial, often have to practically work together hand-in-hand in order to achieve the desired goal. Third, some engineering benefits society in useful everyday ways, while other engineering benefits society by inspiring a big vision; they are both equally needed for human society to keep moving forward.
It’s one thing to say all this and another thing to really know it, feel it, and act on it. So, in this letter, I want to encourage and even persuade you that the engineering that you do—whatever it might be—is really vital. To illustrate this, I want to describe 2 engineering marvels that seem to be on opposite ends of the spectrum of importance and excitement, but which have changed the world in their own unique ways. I beg your indulgence if, at times, I seem to be stating or describing the obvious, yet I do so to make a point. Here are the stories of the screw and the space station (see Figure 19.1).
Figure 19.1 The screw (top) and space station (bottom). V is the escape or orbital velocity.
The Story of the Screw
Historians of technology tell us that the screw was invented around the year 350 BC probably by the ancient Greek engineer Archytas of Taras who, incidentally, was a friend of the philosopher Plato. Screws were not that commonly used by the ancients because the external (or male) threads had to be cut and filed by hand. Yet, by the 1st century AD, the engineer Heron of Alexandria described a screw thread cutting device that could make internal (or female) threads.
But, what exactly, is the screw? The screw is a device made from metal, plastic, or other material, whereby an angled plane is wrapped helically around an axis to produce external teeth-like threads. The screw head, however, has unique cuts, slots, or other features that can receive the tips of tools called screwdrivers. Screws are made using various fabrication techniques, such as 3D printing, lathes, and dies.
How does the screw work? Essentially, when an optimal torque is applied via a screwdriver to the screw head, it advances the screw tip forward into 2 or more materials in order to fasten them together with an optimal holding force. This happens in the following way.
In the initial turning phase, only a small rise in torque is required to turn the screw around its axis and advance the screw farther into the materials. This torque is caused by the friction generated as the screw threads cut into the materials to be joined in order to create matching internal thread-like grooves.
In the seating phase, a rapid increase in torque is required to turn the screw, which ultimately results in a peak torque and peak holding force. These peak levels are caused by the screw head fully being seated or pushed against the top of the materials to be fastened.
However, if the screw is turned beyond this point, then the stripping phase occurs. When this happens, the external threads of the screw and/or the internal thread-like grooves of the materials begin to strip or break. This, in turn, causes a rapid decrease in the torque and holding force.
And we engineers are always glad when there’s a formula that describes a phenomenon. In this case, it is TPEAK = 0.5 FPEAKd (p + fd)/(d – fp), where TPEAK is the peak torque, FPEAK is the peak holding force, d is the screw outer diameter minus 0.65p, f is the friction factor between the screw material and the object material, and p is the screw thread pitch which is the distance between 2 neighboring screw threads.
Now, the bolt is also a type of screw with external threads, except that it passes freely through a smooth hole in the materials to be fastened, and then is tightened into a nut with matching internal threads. A machine screw is also a type of screw, except that its external threads are tightened into pre-existing matching internal threads in the materials to be fastened. The same formula mentioned earlier applies here.
The screw is almost everywhere we look today. It holds together many items that people use every day, like appliances, containers, cupboards, doors, electronics, furniture, plumbing, shelves, vehicles, and so forth. It also fastens together parts used to make special commercial, industrial, and scientific equipment, like airplanes, bridges, computers, machines, medical implants, microscopes, telescopes, trucks, satellites, and so on. A large airplane, for example, can use several million screws and other fasteners.
It’s no exaggeration to say that, in the history of the world, many more screws have existed than people. There are probably trillions upon trillions of screws in use at this very moment as I write this letter. And when screws break, it is often merely annoying. But, it can also result in great damage, financial loss, and even human death. The poet-priest George Herbert wrote a 17th-century proverb that says, “For want of a nail the shoe is lost, for want of a shoe the horse is lost, for want of a horse the rider is lost.” Similarly, I’d say that for want of screws or bolts our modern technologies and, ultimately, our civilization will be lost. The screw is a simple, but fascinating and important, device for the engineer and for the world!
The Story of the Space Station
The aim of the International Space Station (ISS) is to advance knowledge to prepare humanity for its possible future in space. Beginning in the 1990s, the construction of the ISS was finally completed in 2010. At a cost of 150 billion US dollars, it was the most expensive engineering project ever completed in the history of humanity. An endeavor of this magnitude necessarily involved the cooperation, resources, and money of national space agencies from different countries.
Consisting of countless individual components, the ISS required the designing and building expertise of many scientists, engineers, and technicians. This included computer modeling, electronics, manufacturing, materials, mechanics, orbital physics, radiation physics, robotics, and so on. The ISS was not fabricated on Earth and then launched into space wholesale. Rather, individual components were taken into space on at least 42 trips by rockets and shuttles and then assembled there. The Space Shuttle, for instance, used to take 27,000 kg of payload into space in a single trip and then dock with the ISS.
According to NASA, the ISS weighs 420,000 kg, its capsule portion is 109 m long from end to end, its solar panel portion is 73 m long from end to end, and its internal surface area is 3252 m2. It can accommodate 8 spaceships docked to it at once, as well as 20 different research payloads, on the outside. A 16.8-m long robotic arm on the outside is used to move modules, do experiments, and transport spacewalking astronauts and cosmonauts. It has 350,000 sensors to monitor the health and safety of the crew, 50 onboard computers to control everything, 12.9 km of electrical wiring, and 8 solar panel units that provide 75–90 kW of power. It has pressurized sections accessible by the crew, as well as unpressurized sections that are not. It has 6 bedrooms, 2 toilets, 1 gym, 1 viewing bay window, and other areas.
But, how does the ISS stay in orbit? We know from orbital physics that any object launched from the Earth must reach an escape velocity to overcome the planet’s gravity in order to enter orbit at the same velocity. Otherwise, it just crashes back into the planet. Like all artificial near-Earth satellites, the ISS travels in a circular orbit at a velocity that is mathematically expressed as V = , where V is the escape or orbital velocity, G is Newton’s universal gravitational constant, M is the mass of the Earth, and R is the orbital radius. So, the farther out the desired orbital path is from the planet, the slower the escape velocity that’s required, and the slower the orbital velocity that’s maintained. For the ISS, the orbital altitude is 400 km above the Earth at an orbital velocity of 29,000 km/hour.
Astronauts and cosmonauts from a number of countries have performed over 2,700 research experiments to understand the effects of low gravity on the human body and other aspects of biology, chemistry, and physics. Those stationed on the ISS—which can accommodate 6 people at once—have to first undergo months and years of rigorous physical, psychological, and intellectual preparation on Earth. Despite that, they know they are putting themselves in harm’s way. They are exposed to deadly radiation from cosmic rays and solar flares and sometimes need to move to better-protected sections of the ISS. And those who complete a 6-month tour of duty on the ISS lose about 1% of their bone mass each month. They also experience muscle atrophy, immune system depression, cardiovascular degeneration, and optic nerve damage. They often need 1 year of physical rehabilitation once they return to Earth to recover from the effects of low gravity, but some of the damage is apparently permanent. The cost of human understanding, it seems, is human risk.
The ISS is a gateway into deep space. Yet, even gateways need to be designed and built by someone. The scientists, engineers, and technicians who did so remind us that vision, knowledge, skill, and cooperation can be mixed together to produce astounding outcomes. But, equally importantly, the ISS reminds humanity about the challenges and opportunities that still await us out there among the stars!
Which Is More Important?
After having now read about the screw and the space station, which do you think has contributed more to human civilization? It’s really not meant to be a trick question. It seems to me that both have been, and continue to be, highly valuable inventions. I personally don’t see it as a competition. But, I can see arguments for either as more important for those who wish to pick one.
On one hand, the screw is much more practically beneficial to us on a day-to-day basis, since it quite literally keeps our physical belongings from falling apart; but, it doesn’t inspire us to nobility and greatness. On the other hand, the space station is the exact opposite; it doesn’t really benefit us in performing the daily routines of life, yet it holds within itself the promise of a bright future for humanity’s destiny. In my view, they each have an important role. The screw reminds us of the practical and sensible aspects of engineering, while the space station reminds us of the poetry and vision of engineering.
So, What’s the “Take Home” Message of This Letter?
The idea that I want to get across to myself, as much as to you, is that all engineering has its proper place. All engineering, whether it appears trivial or grand, can make a difference to people’s individual lives and to society as a whole. We engineers should never feel apologetic, cynical, or discouraged about what we do on a daily basis in the workplace, whether it’s at the university, in some industry, or with the government. Sometimes it may be exciting, sometimes it may seem dull. But, it always, always, always, makes a difference to somebody somewhere sometime. So, let’s go forth and change the world one screw and one space station at a time.
With best wishes,
R.Z.