A long time ago the story of powered flight was started by bicycle makers on beaches on either side of the Atlantic – the Wright brothers in America and John Gaunt here in our very own Stockport. The earliest planes were, quite literally, fragile air kites made of wooden struts and canvas, with wheels and propellers powered by cogs and chains.
At the threshold of the 21st century, flight has developed to a point where huge passenger aircrafts furnished with gyms and beds, bars and entertainment, have launched into our skies. These airbuses, as reported in the previous issue of WTF, have been made possible by the incredible technological development of engineering, chemistry, and electronics, where the actual way things are made and the materials from which they are created have been applied to avionics in order to reduce the weight and bulk of aircraft to an all time low, thus allowing larger aircraft to fly safely.
Amazing stuff, yet where does it leave us, and what is in store for avionics as it goes forward through the next century? Have they really taken it as far as it can go with the Airbus, or will we see commercially viable supersonic, even hypersonic, flights in the future? Because despite all the hype surrounding the Skycar and Virgin Galactic (as reported in WTF issue 1 – see www.wtfmagazine.co.uk for articles), the commercial application of the physics of aviation hasn’t really pushed forward very much since the huge strides made in the mid 20th century when airflight went from the subsonic speeds of the Wright brothers and the early aircraft of the 20th century through to the transonic speeds of commercial carriers of the 1930s-1940s. And viewing Concorde as the blip that it undoubtedly was in 20th century avionics, modern commercial flights still cruise at subsonic speeds owing to the problems associated with breaking through the ‘sound barrier’ or to be more precise, travelling faster than the speed of sound.
Ah, the sound barrier, which was one of the great urban myths of the early 20th century, because scientists were for a long time baffled by its very existence and whether it could be ‘broken’ and supersonic or hypersonic flight achieved. Despite misgivings (and remember that they had no computers, simulations or animated design packages then with which to test their theories), physicists worked with engineers through the middle decades of the 20th century until the quest to create an airplane that could fly supersonically culminated in the iconic Concorde. Beautiful and technologically advanced as Concorde was, however, it was never commercially viable and the last one was withdrawn from service in 2003. And thus the achievement of commercially viable speeds beyond that of sound seemed to be dealt a deathblow that was hard to believe given the phenomenal advances in technology that was happening all around it. Only avionics seemed to be going backwards rather than forwards, and as Jeremy Clarkson, another technology and speed buff like ourselves, and who was one of the passengers on the last BA Concorde flight on October 24, 2003, said, paraphrasing Neil Armstrong to describe the retiring of Concorde: “This is one small step for a man, but one huge leap backwards for mankind”.
So why is the application of technology to airplanes so complex and why is it so difficult to make aircraft that fly faster, even much faster, than the speed of sound, commercially viable? And what is the speed of sound, anyway, and why does it matter to aircraft?
The speed of sound
If you think of air as being like water (it’s easier to visualise than air for all us who aren’t pilots, physicists or uber-geeks), then when an aircraft moves through it at speed, the molecules become disturbed and agitated and leap about and over the aircraft, like water will around the prow and body of a ship or submarine. This then affects the speed of the aircraft by ‘dragging’ at it, but exactly how and to what extent is dependent on the ratio of the speed of the aircraft to the speed of sound.
The speed of sound is typically measured at 760 mph, and is given the special Mach parameter (it’s known as Mach in honour of Ernst Mach, a physicist who studied gas dynamics in the late 19th century) of Mach 1. When an aircraft flies at very much less than the speed of sound, it’s said to be subsonic. Typical speeds for subsonic aircraft are less than 250 mph, and the Mach number M is therefore much less than one (and is written as M << 1). For subsonic aircraft, compressibility effects are practically nil and the air density remains nearly constant, which basically means it’s pretty calm and unchanged by the aircraft that is flying through it.
The first powered aircraft to fly subsonically was the Wright Brothers’ 1903 airborne bicycle, and modern general aviation and commuter airliners continue to fly at this speed because propellers provide a very fuel-efficient propulsion system, thus making the aircraft very economically viable. The wings of subsonic aircraft are typically rectangular in form and made of lightweight aluminium, although we have already noted, the earliest planes used wood and cloth in their wing construction.
As aircraft go faster, however, some of the aircraft’s energy (speed, heat) compresses the air and changes its density where it is close up against the plane. This compressibility effect changes how much pressure and drag there is on the aircraft, and this obviously becomes more of a factor as the speed increases and the aircraft approaches what is known as transonic flight. Typical speeds for transonic aircraft are greater than 250 mph but less than 760 mph, which is the speed of sound. In transonic flight, the Mach number M is nearly equal to one (M ~= 1) and the small disturbances in the airflow are transmitted to other locations isentropically or with constant entropy. Lost you yet, have we?
Ok, think of water flowing through a nozzle. When the flow is constant and ideal (not dribbling or gushing) it is scientifically described as ‘isentropic’, a combination of the Greek word “iso” (same) and entropy. Entropy is the second law of thermodynamics (energy is the first law), and without going in to the formulae, their relationship is the basis of pretty much everything, with the first law expressing how things remain the same, while the second law expresses all that which changes and what motivates the change ie the fundamental time-asymmetry in all real-world processes. So to get back to our nozzle, what comes through (ie the air in this instance) is constantly in motion and change (entropy), but in a constant manner (the same = iso). So in transonic flight, the air disturbances stay fairly constant in so far as they are ruffled, with localised changes. It’s simply a complicated way of saying it’s a bit choppy but constant.
However, a sharp disturbance, such as a power surge into supersonic speed or a boost into the realms of hypersonic speed, generates a shock wave that affects both the lift and drag of an aircraft as it is pushed beyond the speed of sound. And when it does that all sorts of interesting things start to happen.
Now, the first powered aircraft to explore transonic flight were the high performance fighters of World War II, and these aircraft seemed to encounter a so called ‘sound barrier’ where it was found that drag was increasing faster than the thrust. This led to speculation in the mid-1940s that manned flight was not possible at speeds above the speed of sound, even though the muzzle velocity of rifle bullets is supersonic.
So what is supersonic exactly? Its when the aircraft is going faster than 750mph, the speed of sound, but slower than 1500mph, so the Mach ratio is 1 < M < 3, and initially this was thought to be an impossible thing for human beings to endure. However, the flight of the Bell X-1A in 1947 proved that people could fly faster than sound, and when Concorde (Mach 2.03) was developed it became the supersonic pin up, a luxurious and incredibly fast design icon that meant any person with enough money could fly supersonic. In style
The problems inherent with supersonic flight are, however, myriad, include increased drag, air compression and air density, which we have already discussed and which, in supersonic flight, are negatively affected by shock waves, mass flow choking, and sonic boom.
Like an ordinary wave, shock waves carry energy and are characterized by an abrupt, nearly discontinuous change in the characteristics of the medium. Across a shock wave there is always an extremely rapid rise in pressure, temperature and density of the flow, thus affecting the speed.
Mass flow choking
To understand mass flow choking we must return to our nozzle, and understand that when the air gets too hot or pressured as it tries to move through the nozzle it can change its density and become static, or choked. This then causes loss of thrust, and therefore loss of speed.
The noise of Concorde, and of all supersonic jets, was distinctive and unavoidable. The result of sonic bomb, it is where the shocks caused by the aircraft passing through the air creating a series of pressure waves in front of it and behind it, similar to the bow and stern waves created by a boat. These waves travel at the speed of sound, and as the speed of the aircraft increases the waves are forced together, or compressed, because they cannot avoid each other, and eventually merge into a single shock wave travelling at the speed of sound (761 mph), 167 megawatts per square meter, and exceeding 200 decibels. Basically it generates enormous amounts of sound energy, much like an explosion. While this isn’t unknown in the natural world, because thunder is a type of natural sonic boom created by the rapid heating and expansion of air in a thunderstorm, it’s pretty unpleasant if you happen to live in the flight path. The upshot is that you used to be able to get to New York before the time you left Paris or London, as Concorde’s cruising speed exceeded the top speed of the solar terminator ie it was able to overtake or outrun the spin of the earth. Neat, and much publicised by BA with their “Arrive before you leave” campaign.
So what happens when you make aircraft go even faster? Well then you get in to rockets, Virgin Galactic and Moller Skycar territory, and the realms of hypersonic flight.
When aircraft speeds are much greater than the speed of sound, the aircraft is said to be hypersonic. Typical speeds for hypersonic aircraft are greater than 3000 mph and Mach number M greater than five (M > 5) although NASA’s experimental space scramjet, the X-43A, set a new speed record for aircraft on November 16, 2004. In the unmanned test flight, the plane reached Mach 10, 10 times the speed of sound, or about 6,600 miles per hour.
The chief characteristic of hypersonic aerodynamics is that the temperature of the flow is so great that the chemistry of the diatomic molecules of the air must be considered. At low hypersonic speeds, the molecular bonds vibrate, which changes the magnitude of the forces generated by the air on the aircraft. At high hypersonic speeds, the molecules break apart, producing an electrically charged plasma around the aircraft, and large variations in air density and pressure then occur as a result of shock waves and expansions. For Mach numbers greater than 5, the frictional heating of the airframe by the air becomes so high that very special nickel alloys are required for the structure and so in some hypersonic aircraft it is proposed that the skin will be actively cooled by circulating fuel through the skin to absorb the heat.
So can we have manned hypersonic flight? Well, we already have, with the X-15, the SpaceShipOne, and the Space Shuttle during re-entry. Others that are in the development phases for commercial application are the Virgin Galactic and the Moller Skycar, both of which we have reported on previously.
SpaceShipOne was an experimental air-launched (it was slung below the belly of the ‘White Knight’, a turbofan-powered airplane that carries the SpaceShipOne up to 45 to 50,000 feet) suborbital (ie it didn’t go right round the Earth) space plane that used a hybrid rocket motor to propel it and the SpaceShipOne and its pilot, Michael Melvill, to an altitude of 62.5 miles (100 km) above the Earth’s surface. Which officially makes Michael an astronaut.
SpaceShipOne was developed by aviation company, Scaled Composites, wholly without government funding. On June 21, 2004, it made the first privately-funded human spaceflight, and on October 4, it won the $10 million Ansari X PRIZE. The competition challenged independent designers to safely put three people into space twice in two weeks with a reusable spacecraft. It did so, and in addition, during its testing regimen SpaceShipOne set a number of important ‘firsts’, including first privately funded aircraft to exceed Mach 2 and Mach 3, first privately funded spacecraft to exceed 100km altitude, and first privately funded reusable spacecraft.
Much like Branson and Moller, SpaceShipOne’s creators at Scaled Composites, the company behind the project, envision a world where space travel is a thriving commercial business catering to anyone who has the desire to venture to the stars. However, with development costs estimated to be $25-million plus, this isn’t likely to be any time soon, although with ever accelerating technical achievements in the field of avionics, it may yet be sooner than we think.
So where do you stand?
Is the future supersonic as spearheaded by the ‘Save Concorde’ group’s campaign, who are committed to returning a Concorde to service, or hypersonic with SpaceShipOne? Or perhaps you, like Richard Branson, who keeps his foot firmly in both camps, think there’s room for both? After all, the future is flying closer all the time.
© Claire Burdett. Please only reproduce this article with permission, in its entirety and with a hyperlink to www.claireburdett.com. Thank you.