The Dawn of the Supersonic Age

It's the age of outsourced memory.  We no longer memorize phone numbers.  We Google the same things repeatedly that we used to memorize.  As that skill atrophies, it is replaced with retrieval; the ability to conjure facts with comparable speed and greater accuracy than mental recall. Things actually remembered are now outliers, and thus exceptional... special. Often because they are important, but also from short-lived bursts of rote repetition that hammered them in there way back when. While I am out of practice when it comes to squirreling away new information, here are some of my indelible mental ever-notes:

• My kids' birthdays.  Sometimes my wife's.
• The conversion factor from feet to meters: 0.3048
• The keyboard shortcut for the ± sign: Alt+0177
• One standard atmosphere in Pascals: 101,325

While I always forget that Home Depot closes early on Sunday until I arrive at its sparse parking lot; every October 14th I somehow commemorate (unaided by a calendar app) the anniversary of the supersonic age. 

On that day, in 1947, Chuck Yeager famously piloted the Bell X-1 through the sound 'barrier', achieving Mach 1.06 at 43,000ft.  That aircraft was purpose built for the attempt, yet it was not until its 50th flight that this milestone, the "most significant since the Wright brothers' flight 44 years earlier", was achieved.

Bell X-1 in flight. Credit: NASA

This was not the first supersonic flight.  By the end of World War II, aircraft had become so streamlined and engines so powerful that such speeds were reached. 

Unfortunately, with the onset of compressibility effects in the transonic regime, the aircraft become uncontrollable, and often lethally unrecoverable.  A phenomenon known as transonic tuck (aka Mach tuck) pushes the center of pressure further and further rearward as speed increases...  

Aircraft are typically designed with their center of pressure behind the center of gravity, producing a nose-down tendency that tends to extract us gracefully from sticky situations like stalls. [This is a similar principle to deliberate understeer in an automobile, where letting off the gas saves our bacon most of the time.] 

When tuck occurs, that nose down moment is amplified, causing an accelerating dive that pushes the c.p. farther back. This positive feedback (divergence) makes the situation monotonically worse until structural failure. 

Transonic flight was thus an opaque no-man's land that responded to interlopers with unmerciful violence.  That was more the 'barrier' than one with any basis in physics, yet this underlines both the risk and the daring of the X-1 undertaking.   

Elevator input, which would normally counteract a dive, was ineffective due to compressibility effects at high speeds. This was confirmed during the X-1's flights, as it was originally equipped with a conventional elevator.  Sure enough, as speeds increased, the aircraft became more and more difficult to control.

The brilliant solution to this was the 'all-flying tail', which actuated the entire horizontal stabilizer rather than a control surface appended to it.  This was a key learning of the X-1 program, and its discovery was of such strategic importance that it was classified for 5 years, while the fledgling* U.S. Air Force took the lead in fielding supersonic aircraft. [*The US Air Force was officially founded that same year.]  All-flying tails are now commonplace and trace their origins to the plucky orange rocket plane.

The aircraft itself seems a combination of practical assumptions and a few dart throws, given what we couldn't have known and have since learnt about supersonic flight.  The fuselage was shaped like a bullet, the best conventional counter-evidence of an absolute sound barrier.  The wings were thin, only 8% of chord length, somewhere around half of typical airfoil thickness, in order to reduce drag. 

And while supersonic aircraft of the last half-century universally exhibit a swept-back leading edge, the X-1 was straight winged.  Sweeping the wing back effectively reduces the local Mach number over the wing.  A 60° [degree sign is Alt+248] sweep (a bit below the max in-flight sweep of the Grumman F-14) reduces local chord-wise Mach number ideally by one half. This reduces drag dramatically. 

With the benefits of wing sweep not well-understood at the time, the straight-winged X-1 was brute-forcing its mission. That notwithstanding, it was a focused mission.  Neither a behemoth post-war radial engine like a Wasp Major, nor a state-of-the-art turbojet powerplant, were chosen to propel it.  Doubtless the designers anticipated myriad issues to solve with these air-breathing engines at Mach 1.  Instead, primarily at the behest of the Army, the X-1 was equipped with four rocket engines, each one with two throttle settings: ON and OFF. It was not a platform aircraft with manifold future ambitions as a fighter or interceptor: it had one job.

Ironically, that one job evolved over the course of the program.  To the National Advisory Committee for Aerodynamics (NACA), the X-1 was to be a transonic research plane.  It had to be controllable close to Mach 1, long enough to collect data on the unique flight regime. If it fulfilled its promise, the X-1 program would fill the gaps between the understood aerodynamic mechanisms well-below and well-above Mach 1, informed by aviation and ballistics, respectively.  The Army, which funded the program, prioritized demonstrating flight beyond Mach 1.

 
The point of the X-1, to NACA, was to fill in that bit in the middle. From: Anderson, Modern Compressible Flow.

One way to think about drag on an aircraft is how much of its energy is transferred into the airflow around it.  If the calories in its fuel are ultimately changing the momentum of the air, that's drag.  An aircraft that slips along with minimal wake is low drag, while one that punches a hole in the sky, accelerating and swirling the air alongside it, is high drag.  Modern supersonic aircraft do this effectively, incorporating the lessons we began to learn with Bell's bullet. 

Schlieren photography, which allows us to visualize shock waves, hints at this.  Many of us have seen such photos showing slanting shock waves extending away from a wind tunnel model.  That slant indicates an 'oblique shock', which in reality is a 'weak shock', meaning that while the flow behind it is slower than in front, the downstream flow is still supersonic.  The faster that downstream flow, the lower the drag, because the aircraft flying through it is not changing the momentum of that air so much.  

Modern aircraft take advantage of this.  Jet engine inlets for supersonic aircraft are shaped to produce a series of oblique shocks that progressively and efficiently decelerate the fluid to subsonic before entering the compressor.  This is why supersonic aircraft inlets appear to have their own movable control surfaces -- hinged intake ramps or telescoping inlet cones -- so that those shocks can be precisely tuned for all airspeeds.

English Electric Lightning inlet cone. Credit: Craig Sunter, License

The X-1 produced a 'bow shock' as it exceeded Mach 1.  Rather than an attached oblique shock, a bow shock is displaced in front of the object and is curved rather than slanted.  It's the shock shape that comes to mind when we think of Apollo capsules during atmospheric reentry.  Directly in front of the object, the 'normal' shock has no slant.  Unlike the weak shock, the flow behind a normal shock is subsonic (with respect to the aircraft itself).  So the aircraft is doing a lot of work to accelerate the fluid around it and pull it along for the ride.  More work done on the fluid means more drag, which means a lower top speed for a given thrust.  This is a very good reason to use powerful rocket engines in the attempt.

In fairness, the reason for the bow shock was simply that there are no oblique shocks at Mach 1, only beyond it.  Mach 1 is a mess all around.  It is a maximum for coefficient of drag, and a body flying at Mach 1 will have transonic flow speeds all around itself, meaning a mix of supersonic and subsonic local flows, unstably toggling, producing momentary shocks and pounding buffeting effects.  [The X-1 had a control yoke rather than a stick so its pilot could hold on during the anticipated wild ride.] But as the aircraft continues to accelerate, and wholesale enters the supersonic regime, the oblique shocks attach, the buffeting subsides and the coefficient of drag actually decays with increasing airspeed.  Metaphorically, the X-1 was doing its job inside thunderclouds, so that its successors could fly above them.

The impact of the X-1 program, and its seminal milestone flight on October 14, are hard to overstate.  With its pilot Yeager, builder Lawrence Bell, and visionary scientist John Stack rightfully recognized for the 1947 Collier Trophy, the achievement stood alongside the first round-the-world flight (1924); development of airways and air navigation facilities (1928); and the invention of the flying boat (1912) that came before it.

Just as those achievements signaled and shaped modern air travel, the sonic boom heard that day propagated through every high speed aircraft design since. The event is a discontinuity in aviation history, like the Wright brothers' 1903 flight before and the landing of the 1969 Eagle landing that followed: in an instant the human race graduated to having done something, from having not.  

References:

https://www.nasa.gov/centers/dryden/history/HistoricAircraft/X-1/techdata.html

https://naa.aero/awards/awards-and-trophies/collier-trophy

https://history.nasa.gov/SP-4219/Chapter3.html

For more history of aerodynamics, read A History of Aerodynamics and Its Impact on Flying Machines, by John D. Anderson.