February 2009

Nobody ever said that persuading a car up to a speed of Mach 1.4 is going to be easy.

The words “engineering adventure” are such a mantra on the lips of the BLOODHOUND team that I suspect they all sit cross-legged first thing in the morning and recite “Engineering Adventure” ten times over before hitting the Corn Flakes.

As well they might. For this is uncharted territory.

If a 13th century map-maker had no actual knowledge of what hazards might lie within a certain small area – say, the land-mass of America – he was apt to label it “Here Be Ye Dragons”. Now times are different and the territory between ThrustSSC’s 763 mph and BLOODHOUND’s projected 1,000 mph is the subject of the best research available to 21st century Man. But until overrun by real experience, it is undeniably a territory infested by certain dragons. Some of whom may not prove to be attractive pets.

Aerodynamics is such a region.

The best predictions, based on Computational Fluid Dynamics and maths light-years beyond my ability to count on my fingers, say that all is well. Yet the fact is that if certain assumptions are even slightly askew, then one Andy Green could rather suddenly find himself driving a wheelbarrow. A wheelbarrow travelling at somewhere between 700 and 1,000 mph…..

There is a terrifying precedent for this. We will return to it.

But first, it is necessary to make a nodding acquaintance with the head dragon who rules the territory.

This dragon’s name is Shockwave.

Start up a car engine and the sound waves expand in a concentric circle like ripples on a pond. In air the ripples obviously travel at the speed of sound – call it 760 mph at sea level – but the pond-principle remains the same. However, drive the car at 100 mph and the ripples in front are compressed by that 100 mph. The wavelength in front gets a bit shorter so the sound frequency increases, while the waves behind get longer and the frequency decreases. Hence ‘eeeyoouw’ when a fast car goes past – high frequency in front of it, lower frequency behind. Doppler Effect, just to prove I did get a little way beyond kindergarten.

This constantly-advancing wave-front is – again obviously – constantly creating new waves ahead of it. This establishes a pattern whereby the airflow is pre-shaped to separate and flow around the oncoming object with a minimum of fuss.

Want to upset this happy cooperation? Okay – crank up the speed of the object to the speed of sound. Now the soundwaves cannot run ahead of you carrying the message, and so you crush them all up together into a bad-tempered shockwave. It is this shockwave which is the sound barrier – because it is, quite literally, a barrier. The drag-rise is enormous and the huge instant pressure change produces the supersonic bang. The bang is of course continuously travelling with the craft – you the observer just hear it the once as a bang when the wave-front passes you on the ground.

I find this eerie. You see a supersonic aircraft running in rapidly, but there isn’t a sound. Not a whisper. Well, of course there won’t be, seeing as the device is outrunning its own soundwaves – but being a simple soul, this utter quiet never fails to raise the hairs on the back of my neck. Then just after it passes comes the bang and the huge wall of jet-sound as a brain-numbing testament to the amount of power required to bust through the Barrier.

So there we have it. Reach the sound barrier, one big shockwave, and you either have the power to drag it along or you don’t. Simple. Maybe not that easy to actually do, but simple in principle, right?

Well, no. Not simple. Because it isn’t like that.

Take any object in motion – a bullet, an aircraft, a car – and the speed of the airflow is not the same all over it. Where the shape of the device – an aircraft wing being the prime example – obliges the airstream to bend around it, then the air has to speed up ‘cos it’s got further to go in a given moment. Which is of course how a wing works anyway – speeding up the flow over the top causes a reduction in pressure which sucks the wing up. Easy now we learn it in second form – but it took ten millennia before a scientist called Bernoulli wrote down the apparently simple statement that “If you increase the speed of a gas or liquid you cause a reduction in pressure, and vice versa”. To which the rest of mankind muttered; “Oh yeah, well, I always knew it was something like that….”

Okay. Now crank up the speed of the vehicle to somewhere around 85% of the speed of sound. Mach 0.85, or roughly 650 mph at sea level. The vehicle as a whole is sub-sonic, but is entering the transonic region where some bits of speeded-up airflow around the shape are starting to hit supersonic. 

At which point they produce shockwaves. Shockwaves in different places. Around the nose, around the cockpit canopy, around engine intakes – anywhere the airflow has to bend.

Shockwaves which disrupt the airflow. Big time. As the body continues to accelerate towards Mach 1 these shock-fronts intensify and some may move backwards along the structure. Beyond Mach 1 the position of the shocks stabilises but their intensity keeps right on building.

Which, to those who would go fast, can be more than slightly disconcerting, since the forces involved are huge. Behind the shockwave the airflow very abruptly slows down by as much as 50%. Dutifully following Bernoulli’s Law the air pressure equally abruptly increases – not by a little, but by maybe 1,000 millibars. At Mach 1.4, for example, the airflow directly behind a shockwave is slowed to Mach 0.72, and the pressure instantaneously more than doubles. Or in some cases very much more than doubles. Very, very much moree….

Well, there be ye dragons.

It starts with aircraft

The pioneering work on supersonics was naturally in the field of aircraft. Until scientists found solutions in heavy wing sweepback and other exotica, the transonic region was all too apt to rustle up a shockwave at the fastest point of the airflow over a straight wing – which shockwave, lying roughly along the line of the mainspar, instantaneously dumped 90 per cent of all wing lift. And since much the same was happening on the tailplane, robbing the elevators of authority, the total effect was seriously discouraging. Some fighters of late WWII could achieve this unhappy state of affairs in a dive, rendering a number of terrified pilots unwitting – and sadly very short-lived – pioneers of transonic flight. If they could have accelerated further, up to Mach 1, the shocks would have moved aft the to trailing edges and the wings would have worked again – but they didn’t have enough power to do that. And with the ground rushing up there wasn’t always time to slow down out of the transonic region….

One survivor was Hans Guido Mutke. In 1945 Mutke is one of the Luftwaffe’s elite fighter pilots entrusted with the world’s first operational jet fighter, the Messerschmitt 262. Diving in combat he well exceeds Herr Willi Messerschmitt’s designated VNE – Velocity Never Exceed – and strange things start to happen. Very quickly.

The 262 commences to buffet so violently that rivets pop out of the airframe. Many rivets. Mutke wishes most fervently to pull out of the dive, but the elevators no longer respond. By some divine intervention he tries rolling back the trim-wheel, which in the 262 lowers the leading edge of the tailplane. Still accelerating, the aircraft starts to pull out – at which point the buffeting suddenly ceases and the world’s first jet fighter becomes stable again and answers its controls. Mutke keeps pulling and belatedly thinks to slam the throttles shut. This flames out both engines but he is far beyond caring about minor details. As he recovers the dive and the speed reduces the airframe again abandons all sanity and resumes its imitation of a maniac road-drill. Mutke resigns himself to the hereafter….

But as the speed falls further the shaking stops. And he successfully lands his tattered aeroplane minus a remarkable number of rivets and all of his bravado.

Later it will be conjectured that Mutke may well have been the first homo sapiens to pass through the transonic region and achieve the relative smoothness of supersonic flight. Well, maybe, maybe not – but his hand-quivering experience certainly suggests that he passed through the evil transonic zone and back again. That he was perhaps the very first to travel through dragon country and return to mankind.

But a car is different….

A car essaying the transonic zone has one advantage and one disadvantage.

The advantage is that the car is not trying to create lift – in fact, approximately the last thing in the world a supersonic car wants is lift – so you don’t have to deliberately speed up over-wing airflow to create lift. Which is a Good Thing, because over-wing shocks move aft with speed-rise, which to say the least could be inconvenient for a car. The car does of course create shocks where the airflow speeds up anyway around things like wheels, canopy, jet intake. But these shocks are anchored – they don’t move. They increase their savagery with Mach number, certainly – but by and large they do not move as they do on an aircraft.

Now the disadvantage.

With an aircraft. the massive pressure rise behind the shockwaves dissipates in all directions.

In a car, it does not. Because the ground gets in the way. It is called Ground Effect. It is serious.

The Budweiser Rocket

Cut to the most controversial record car of all time. On this cold day in December 1979 it is called the Budweiser Rocket, but that is merely a recent manifestation of several identities. Created by film director Hal Needham and designed by one Bill Fredricks, the car has for years been the subject of intrigue, court cases, sabotage in the shape of acid poured onto braking parachutes, and even associated with four murders and a suicide. Such is the Wild West of America.

On this day it will run on a desert playa surface at Edwards Air Force Base in the Mojave Desert. Strictly speaking it is not a Land Speed Record contender in the normal sense at all, having only three wheels – a single nosewheel and two rear wheels on outriggers – instead of the four required by FIA rules. Moreover the team have stated openly that they have no intention of turning the car round for the two-way average speed runs needed to qualify for a Land Speed Record. They are after one thing and one thing only – Mach 1.

The car is a sleek torpedo powered by a hybrid rocket main engine with a Sidewinder missile solid-fuel rocket mounted on top. It is the first record car to run on solid wheels. It has carried out 18 previous work-up runs, and today they are going for broke.

The driver is Stan Barrett, a deeply religious Hollywood stunt man. During the countdown to rocket ignition he intones “Lord, into Thy hands I commit myself”. Later he will tell congregations; “That’s a moment that I felt close to the Lord”.
In fact his timing is out by about 20 seconds.

The Budweiser Rocket thunders across the salt. Passing maybe 600 mph – very much maybe, because the instruments are now a blur of vibration – Barrett lights up the Sidewinder for five seconds of extra boost.

This is the point at which he in fact becomes closest to the Lord.

For the back wheels of the Budweiser Rocket lift a foot off the surface and stay that way for a distance of some 700 feet. They touch down again slightly off the centre-track of the front wheel, and Barrett has another deeply devotional moment retaining control before finally bringing the device to rest.

Many things have also gone awry this day. The USAF’s radar speed-tracking doesn’t work properly. The rockets expire just before the car enters the timing trap, which is non-standard anyway. Claims of speed and repudiations of same erupt within 12 hours and will rumble on for years before finally being disproven. Budweiser decides that the whole episode is as close to the Lord as a beer company actually wishes to get – and the car never runs again.

I have nothing to add to the controversy. The certainty is that the project was heroic. The probability is that the Budweiser Rocket ventured into transonic territory.

And thereby prodded a dragon which, if not actually sleeping, had never before revealed its true strength.
Ground effect.

Run a car up into the transonic region and obviously you squeeze the airstream underneath it between car and ground. Which speeds up said airstream no little. On one hand this is a good thing because the reduction in pressure – Bernoulli again – sucks the car towards the ground. This may not be right at the top of the Desirable list but is infinitely preferable to sucking it up into the sky. However this speeded-up airflow tends to go supersonic fairly early in the transonic proceedings. And of course then produces…. a shockwave.

Which creates this enormous high pressure pulse behind it. Much, much, much more enormous than the pulse created by an aircraft, simply because the aircraft’s pulse can dissipate all round, and the car’s cannot. The car’s pulse is sandwiched between the car and the ground. And so the pressure becomes even higher – very, very, very much higher. And as the speed increases further the under-body shock amalgamates with the shock coming off the rear wheels and the pressure behind the combination becomes higher again. Much, much higher….

A high enough pressure for the impact of air to slam inches into the desert surface, pulverising it so that it explodes upwards an instant later as pressure re-equalises.

A high enough pressure, perhaps, to lift the back end of a six-tonne car off the ground…?

Well, the answer is certainly yes. Witness Budweiser. The calculations all say it will indeed inevitably occur – but the question is where it occurs. Ron Ayers, Chief Aerodynamicist, and Ben Evans, guru of Computational Fluid Dynamics, have put in painstaking weeks of calculation to ensure it will be far enough aft for the high pressure burst to occur just a millisecond behind BLOODHOUND’s passage, and so not affect it.

Nonetheless – for this is still definitely dragon territory – the team are going to creep up on 1,000 mph by stealth – or at least as much stealth as a jet-rocket car, which is never going to be exactly furtive, can muster. In Year One the plan is 20 runs with the objective of 800 mph. In Year Two another 20 runs aiming at 900 mph. And in Year Three 25 runs leading to the 1,000 mph….

The dragons will still be there. They are not going to go away. As speed rises, the underbody pressure dragon may or may not become a monstrous opponent – but it will certainly not be the only one.

The most powerful sword the BLOODHOUND team has is their relentless collecting of data and their willingness to adapt thereto. As the process evolves, the car may not be the same in three year’s time as it will be on first-build – or if it is, I for one will be at least mildly astonished.

No – the team’s strength lies in the mantra. Engineering Adventure. Engineering Experiment. Open minds. Can 1,000 miles an hour be done….?