It is one thing starting something. It can be, sometimes embarrassingly, quite another stopping it. In the case of BLOODHOUND a certain Wing Commander Andrew Green OBE, current Land Speed Record holder, sums it up with his best deadpan expression on his face, which is very deadpan indeed.

“Acceleration is optional. Braking isn’t”.

Anyone who has suffered a brake failure in any kind of machine – assuming they survived the subsequent entertainment, of course – will certainly say: “Yeah man, Amen to that!”  I definitely join this congregation, having had several brake failures, two in cars and the rest in aeroplanes. In the car failures I was simply lucky both times. The aircraft failures were more technically interesting because aeroplane brakes operate independently left and right, and in each case I lost just one of them. Mostly – Murphy’s Law being what it is – on short runways in high-performance tail-wheel aeroplanes which were not exactly renowned for their directional stability on the ground in the first place. This produces a certain juggling act between not stopping in time or swerving off to one side, digging a wing into the deck, and rolling the thing up into a ball. By 99 per cent fortune I survived them all without a scratch on either me or the aircraft. After the last occasion I received a sharp note from Lady Luck saying: “So far so good, Buster, but let’s not keep pushing it, huh? I just might be busy with my VAT returns next time you have one of those…”

Well, okay. But my braking problems are as nothing compared to BLOODHOUND’s. As Andy Green says, accelerating is optional. You can always decide you just don’t feel like it today and kill the jet and the rocket and thereby cease accelerating – or you may still feel like it but get a red warning caption or the electronics may do it for you and abort the run whether you like it or not, then sit back with their arms folded while you sort it out. Whichever. The bottom line is that you can always abandon accelerating…

But you can never abandon braking.

By the same token, you cannot leave braking to the robots, be they ever so clever. You can – and indeed have to – leave acceleration control to the electronic monitors to a certain degree. Because they will – it says here – spot a snag quicker than a human being can, and will accordingly – it also says here – instantaneously chop all sources of thrust if they become querulous.

But trigger the braking systems…?

Well… er, no. Or only one of them, under one particular circumstance, anyway.

Look at it like this. If the magic didgeridoos spring an electronic leak and abort a run spuriously under acceleration it is a bloody nuisance. If those same computers or their circuitry which have just gone fizz were also controlling braking – then the braking’s just gone fizz, as well.

Not a great idea. You, the driver – or me the driver, once I’ve carefully removed A Green from the scene – are going to feel a bit of a dork if an electronics failure or wire-cut suddenly also removes all contact with the braking systems…

Answer? Hoss-sense. Good old down-home hoss-sense. Let the black boxes have some – even considerable – control over the acceleration. Which is optional. Then leave the human – the pilot – in control of braking. Which is not optional. Clear-cut. Acceleration – computers involved. Braking – driver.

 

So how do you do it?

So how do you brake from 1,000 mph on the ground?

Well of course the trite answer is ‘Carefully, ho ho ho!’. But that is a complete excess of ‘trite’ because it must also be done quickly. Very quickly. ‘Ho ho – ah, er…hmmm…’

The first reason for this is the topography of the planet Earth. If the BLOODHOUND team could have found a 20 mile long dead flat dried-mud playa desert anywhere in the world then both acceleration and braking could have been a bit more leisurely – but there ain’t no such location, as they’d strongly suspected from the start. Or if there is, certainly none which don’t happen to be bisected by a couple of war zones, an air base the size of Wales, a small city wondering quite how it got there, or any combination of all of the above.

No – the Hakskeen Pan is currently the best place on Earth because it allows a ten-mile track with an extra mile at each end for an emergency over-run.

You need this over-run as a contingency. But if you ever use it on a record run then you are not going the get the record today because (a) you have encountered a snag which will certainly ground the car until said snag is fixed, and (b) you have anyway gone past the turn-around crew who will then have to move, with the result that your chances of turning round and going back through the time-trap within an hour no longer exist.

So you accelerate for four-and-a-half miles, whack through the measured mile in 3.6 seconds, then decelerate for another four-and a half miles. Winding up the speed calls for acceleration which peaks at nearly 3G – so, obviously, slowing calls for equally savage deceleration. That’s yer Laws of Physics, innit.

Okay, you could extend the run-up end – the start-end – by say half a mile to increase the initial acceleration distance. But then you’d be also be extending the braking end by the same amount, if for no other reason than to put yourself at the right start-point for the return trip. Okay, not the end of the world providing you approach it with the mindset that you must be able to stop within four-and-a-half miles, but are prepared to coast for an extra half mile in order to reach the pit crew and the coffee machine. This is indeed a possible run-profile – but it doesn’t change the fact that the mile over-run at the braking end is still sacrosanct. Coast into it for half a mile – well, ye-es, okay. But compromise the mile emergency distance? No f – jolly way.

Which still leaves you with the equal-and-opposite deceleration requirement. So as you pass through the end of the measured mile you cut the jet back to idle, cut the rocket entirely – and receive the equivalent of a whack in the face with the flat of a shovel as you go to 3G deceleration. Not physically impossible – on the general lines of  ‘If you can’t take a joke you shouldn’t have joined’ – but not exactly one of life’s more sybaritic moments.

Then, for a second or two you do… nothing. Having chopped the power sources at Mach 1.4 the supersonic drag whaps you up to 3G slowing or even a bit more all by itself. Period.
(I personally find this almost incredible. Here we have the sharpest, most streamlined shape of ground vehicle ever known to man – and you cut off the power and zap, a 3G deceleration shovel in the face just by drag. But that’s how it is).

But this drag most rapidly decays. Just as the drag-rise went up by more than the square of the speed during transonic acceleration, so does the drag equally quickly wind down whilst slowing. So – how do you sustain the deceleration? Losing 1,000 mph in 40 seconds?

Well, you hit it triple-fold; you chuck out airbrakes, parachutes, and finally use wheel-brakes…

Yee-hah! That’ll do it…!

 

Not that simple…

Well, let’s just stop there for a moment. No pun intended. Because – and this’ll not take you by surprise unless you still believe in the Tooth Fairy – it ain’t that simple. It took 47,500 lbs of thrust to propel BLOODHOUND up to 1,000 mph. Assuming it successfully does. It ain’t gonna take the same power to stop it because you have drag on your side – but it sure as hell is going to take power, or energy, or however you like to call it, to absorb the speed. And plenty of it.

Okay. You stop a car really hard from 150 mph. The mass / energy of the car is converted by the friction of brakes and tyres into heat. Touch the tyres immediately afterwards and it’s like touching a hot kettle. Touch the brake discs and your fingers will probably become welded to them, which might make you say ‘Oh, bother’. Or something like that.

Converting kinetic energy into heat by friction is not, however, an option if you wish to stop quickly from 1,000 mph. If it worked at all it would probably produce enough heat to melt half the car – but in any case it simply won’t work. You might – in fact could, fairly easily – stop the wheels going round. But stopping metal wheels going round on a mud playa is most distinctly not the same as stopping the car, which would go bowling on more or less regardless, achieving an additional speed record for mud-skiing on a desert in the process. Until such time as the bottoms of the wheels melt away of course, whereupon you’ll be in line for the highest-speed somersault in history. For which I believe there is not as yet an established world record protocol.

No – mechanical friction ain’t gonna be a factor at the really high speeds. So that leaves you with what you have got.

Airflow.

Plenty of that about at Mach 1.4. So use airflow.

In fact, more than that. Very much major on airflow. If wheels can’t sustain heavy braking effect at low speeds (BLOODHOUND-speak for under about 200 mph) then you need to do most of the slowing work at high speed while you’ve still got a fast airflow to work against.

First line of offence is of course the airbrakes. Not simple, the airbrakes. Firstly, they will be starting to open – to deploy – at a far, far greater speed than any other airbrake ever on any land-born vehicle. That’s for certain. Some jet fighters can deploy airbrakes at very much higher speeds – which slows them with all the tact and diplomacy of slamming into a brick wall. But ground vehicles…?

A jet fighter might have an airbrake of about 0.7 square metres in area. 

BLOODHOUND will have TWO airbrake doors of 0.7 square metres area apiece, and they will start to open at Mach 1.2 – around 900 mph – a bit like a pair of elephant’s ears. Except that ears belonging to elephants are actuated by an adrenaline rush to the muscles, and although elephants can run faster than most people think – especially a few people who didn’t believe it and are now extinct as a result – there is no record of any elephant ever having had a requirement to open its ears at Mach 1.2.  In fact to stick out the ears at 900 mph requires a very great deal of force indeed. Such as a hydraulic ram producing some 200 bar of pressure. Somewhere around 3,000 psi. Which is a most serious hydraulic ram.

 

A slightly dumb ram

But a hydraulic ram clever enough to be – actually slightly dumb.

Look at it this way. What is the number one nightmare scenario of any ‘elephant’s ears’ airbrakes, particularly ones that are about to produce supersonic bangs – and accompanying drag – all of their own, big-time? Obviously, one side opening and the other not, for whatever reason. However intrinsically stable the car is, it ain’t gonna survive that. So BLOODHOUND’s system doesn’t react each door’s force to the car chassis, but to the opposing brake door on the other side.

Imagine you are standing in a very narrow corridor with your arms outstretched, pushing on doors on each side. The one on your left is tough to open, but the one on your right is slightly tougher. So you lean to your right a tad until the pressure evens up and you’re standing there – chances are feeling something of a berk – in equilibrium. Well, that’s how BLOODHOUND’s airbrakes work – they are not reacting to air speed but air pressure, which means that the slowing forces will be equal each side even if you’ve had to shuffle one way a tad or two. You are in balance. The pressure is the same each side.

Well, that’s the way the airbrakes work.

And they are even – most deliberately – dumber than that.

The brakes open from an angle of zero degrees to a max of 60 degrees. I, the driver – sorry, sorry, of course I mean Andy the driver – hit the brake button passing 900 mph decelerating.

What happens?

Well, this is a fail-safe system. Electronics do not trigger deployment but completely the opposite. They stop it. Solenoids hold the airbrake valves closed against springs. When I press the button I don’t activate the solenoids – I de-activate them, so the dumb hydraulic ram suddenly ain’t being held shut any more and starts shoving the doors open.

Note that I the driver do this. The black boxes do not get their sticky electronic fingers on the actuation process except in the case of a sensor-triggered emergency power shut-down. Lose power – and the airbrakes will to default to Open. (And do this even if the car’s main hydraulic system has gone down, the airbrakes having their own independent circuit pressurised by a hydraulic accumulator).

But even at 200 bar of hydraulic pressure (or whatever it ends up regulated at, which has yet to be finally calculated but won’t be far short of 200) the brakes are not then going to open with an instant SMACK, but start fighting the air pressure and open gradually as the speed drops off and the air pressure reduces. ‘Gradually’ being a kind of relative term, meaning five or six seconds. The idea is that they reach 60 degrees – fully deployed – as the speed unwinds through about 600 mph. (About Mach 0.8). Simply a fixed hydraulic pressure opposing a reducing air pressure. Nothing to go wrong – it says here – and no sudden massive G-spike in the slow-down. More comfortable for me – no, no, done it again, I mean Andy – but more importantly also more comfortable for the EJ200 jet, which is already grumpy in a minor sort of way about being hit with an initial linear 3G deceleration, and could become positively irate if said deceleration had the odd 4 or 5G jerk in it.

So – no downsides to the airbrakes then, correct?

Well… er, no, not quite entirely. There is a downside, and it is called turbulence. Or it’s called turbulence by me. The BLOODHOUND designers for some reason prefer the term ‘unsteady airflow’.

Which of course stands to reason. If you’re ambling along at Mach 1.2 and start opening a pair of elephant’s ears you can hardly be astonished when the airflow breaks up, stalls, swirls, darts every which way and generally becomes bad-tempered. Or ‘unsteady’.

The problem is – just how unsteady? There will certainly be buffeting and vibration as the affronted airflow in the wake of the brake doors hits things like the rear suspension outriggers and wheel fairings, the rear fuselage sides, maybe even the fin. But how much vibration? Enough to ripple Andy’s coffee in the cup-holder? Or enough to shake the back end of BLOODHOUND into an extremely high speed warehouse of spare parts? And if so, at what speed and at what door-opening angle…?

Well, you turn to CFD (Computational Fluid Dynamics) for some answers. But the truth is that conventional CFD – if there’s any such thing as ‘conventional’ CFD – is not at its best with ‘unsteady’ airflows. So you, being worldly engineers on a somewhat unworldly project, do not entirely and absolutely trust the conclusions. Too many variables.

So you design the airbrake doors to be 30% holes. Something like 25 holes of 10cm diameter in each door, which is a lot of holes, with the objective of smoothing out the outgoing airflow – well, a bit.

So when you’re leaning on the balcony late at night – not that any of the BLOODHOUND team are doing much of same at this time, being more inclined to stagger home and crash out instantly – but if you were, what would you think about the airbrakes?

Well, you’re going to think they’re always going to be experimental until they’re – er, not. Until they’re proven. You, the designers, think this in your heart of hearts. Andy the driver thinks this in his heart of hearts. All know it to be true. They’ll be tested on the runway trials, of course, but that’s only up to 200, just maybe 250 mph. No – the airbrakes can only be really tested on the Hakskeen Pan as the run-speeds climb higher and higher. And they’ll only cease to be experimental when BLOODHOUND slows down after zapping through the timing-trap at over 1,000 mph for the second time within an hour. Which will of course be the last time they’re ever used. Then they’ll be proven…

But…

What happens if, for some unimaginable reason, the airbrakes fail? Totally…

 

A warm and fuzzy feeling of reserve…

Well, the standby system is braking parachutes. There is a misconception abroad that the slowing system is airbrakes followed by brake-chutes.

Not so.

No – the airbrakes are the primary stop-mover, if you see what I mean. The paras will be there in case the unimaginable happens and the airbrakes go totally on strike. The paras are a completely independent back-up.

They’ll be tested at very high speed on the desert, of course, because the theory is that either system – airbrakes or chutes – will do the slowing job by themselves, thus creating a warm and fuzzy feeling of having a 100 per cent reserve system. It says here.

These brake-chutes – which are going to be supplied by Survival Equipment Services in Tetbury, Gloucs – are in fact fascinating. The idea is that one, with a 7ft 6in diameter canopy deployed at 600 mph, will do the job. With a second one in reserve in case it doesn’t. They face problems ranging from airflow flutter from the airbrakes to further grumping the EJ200 by snatching a G-spike into the slowing process. (The answer to the latter, apparently, being a brake-chute strop which actually stretches – by 20 per cent, no less – to lessen the impact of deployment).

We’ll look at parachutes in a later article.

As we will at wheel-brakes. Because from 200 mph downwards wheel-brakes progressively become the major factor as the aerodynamics diminish. For the initial runway-runs on rubber tyres BLOODHOUND will have brakes on all four wheels – but for the desert record runs it will only have front brakes, rear brakes being impractical because however you place them you’ll be creating undesirable bumps in the airflow.

These front brakes, which are carbon-carbon, are most definitely not simple. You would wish to vent the very considerable heat for one thing, preferably without sautéing either the driver’s feet or substantial portions of the airframe.

Not an unbeatable problem in an open-wheel racing car with airflow around the brakes – a problem, yes, but not an unbeatable one, even though F1 brakes can peak at 2,000 deg C. A very different proposition for BLOODHOUND, because BLOODHOUND’s front wheels are most definitely not open-wheel, they being very deliberately embedded into the fuselage to prevent any stray airflow getting into the wheel-arches, becoming bad-tempered, and producing drag through turbulence. So if they’re that insulated from airflow, how do you get rid of an awful lot of heat…?

To a point, it gets rid of itself. One thing BLOODHOUND has which F1 cars don’t is 90 cm solid wheels which act as a heat-sink. Andy Green would like to see the theoretical temperature not exceeding 1,000 deg C, because Thrust SSC proved that was cope-able. In an ultimate emergency?  We’ll come back to that as time progresses.

But that’s only one of the problems. Another is that however clever, efficient and lightweight carbon brakes are, they ain’t been, any of them, designed to spin up to over 10,000 rpm. Okay, you’re not going to be using them for braking at 10,000 rpm – 1,000 mph wheel-speed – but you would kinda like to know if they’re going to fly apart under sheer centrifugal force anyway because they were never designed to rotate beyond about a fifth of that. Okay, carbon F1 clutches are tested to 18,000 rpm and more – but they are nothing remotely like the diameter of BLOODHOUND brakes…

Again, we’ll come back to that in future.

Right now, suffice to say there are still what-if’s on deceleration. They are being solved – but there are still what-if’s.

Part of the engineering adventure. Can’t take a joke – shouldn’t have joined…