Building momentum

The construction of the Bloodhound SSC supersonic vehicle is nearing completion.

Airfix
Andy Green sits in position amid the various components of Bloodhound SSC, now being assembled

Amid the clatter and buzz of power tools and the occasional clang of metal on metal, Bloodhound SSC’s engineering lead for mechanical design, Mark Elvin, rubbed a hand across his scalp and considered the half-built car whose components were piled up around him. “We’ve been very good at being a virtual car for the past six or seven years,” he said. “It’s been necessary, because it’s difficult to attract sponsors to something that doesn’t actually exist yet, so you have to talk about it like it’s real. It’s a little bit strange to think that by the end of July we’ll be rolling out the finished car for the first runway tests at Newquay Airport.”

Indeed, each subsequent visit to the Bloodhound Technical Centre, an anonymous industrial unit on the western edge of Bristol, has revealed a more and more complete-looking car. From the boxy shape of the lower chassis to the arrival of the insectile curved form of the carbon-fibre composite monocoque that will protect driver Andy Green during the car’s runs on the stony desert surface of Hakskeen Pan in South Africa’s Western Cape, to the long, arched stretch of the upper chassis, the solid reality of Bloodhound is steadily making the full-scale model of the show car, familiar from its visits to industrial exhibitions and schools around the country over the past few years, look more and more like the working sketch that it is.

Bloodhound1
Mark Elvin (at the back) oversees chassis construction

At The Engineer’s most recent visit at the end of February, Elvin was overseeing the completion of the upper chassis, the mostly metallic section running from the back of the monocoque to the rear of the car. Quite deliberately, it resembles the fuselage of a slightly old-fashioned passenger or cargo aircraft. “It’s a DC3, basically,” Elvin said. “The skin is titanium rather than aluminium, but that’s placed over aluminium ribs. It’s an aircraft structure because that’s what we on the team know about.” There was consideration during the design phase to use a more motorsport-derived composite structure — “but the cost and lead time were unpalatable for us”, Elvin said. “You might end up with a lighter structure and it would certainly be stiffer, but the process to get there was just ridiculous.”

The upper chassis sits on the steel lower chassis, which was the first component to be completed, about two years ago. “We know it’s heavy, but it’s going to get beaten up by the desert surface. It could be titanium, but the company we were working with was working in steel, and they did it for free, which is our favourite price.”

CLICK HERE TO ACCESS ALL OF THE ENGINEER’S BLOODHOUND SSC COVERAGE

All through the project, the building of the car has been a compromise between the engineering demands, cost and the capabilities of sponsors and partners. “The entire rear of the car was a product sponsorship from the Nuclear AMRC at Sheffield,” Elvin said. “They wanted to show off what their new machining equipment could do, so Smiths Group provided the material billets, the AMRC provided programming and machine time and we got three quarters of a million quid’s worth of brilliantly produced component for nothing.”

Things have changed now. “We’ve got grown-up commercial contracts because our lead times are so much shorter now — eight weeks, not eight months. So we have to pay for things properly.”

Everyone on the project is now aware that the clock is ticking. A big notice board by the entrance to the construction bay displays the project deadline: Bloodhound is to be finished by 25 July. “People have been here seven years now: that’s a long time to be working on this,” Elvin said.

Bloodhound5
The EJ200 engine installed in the upper chassis for the first time

Assembling the upper chassis was a multi-stage process. After the supporting ribs were produced, they were lined up on the lower chassis; then the car’s jet engine — a Rolls-Royce EJ200, the world’s most energy-dense turbojet and the power source for the Typhoon jet fighter — was drawn up into the space under the ribs to check clearances. “We know how much we expect the EJ will move, so we had to check we had that space before the skin went on. The titanium skin was then placed over the ribs, drilled and fixed, then bonded and autoclaved. The final step of emplacing dozens of pressure sensors under the skin was being completed during our visit, with spaghetti strands of PTFE tubing snaking from the sensors around the inner surface of the fuselage; they will carry the wiring from the sensors to the onboard interface block, where data will be collected during the car’s runs and from where the engineering team will download to plan the speed progression.

These sensors are all over the car, placed where computational fluid dynamics (CFD) modelling has indicated that airflow is particularly crucial to Bloodhound’s safe operation, especially where the flow will change direction. More often used in aircraft and motorsport, they connect to holes about a millimetre wide drilled in the monocoque, chassis and other parts of the car, such as the yet-to-be-attached winglets on the tailfin and ahead of the cockpit. These holes are somewhat smaller than those that are usually used, to avoid
them being blocked by desert dust.

When the car is running, data from these sensors will be compared with CFD studies of the car at the same speed. If there is a good match, Green will be cleared for a faster subsequent run. “Discrepancies aren’t necessarily bad; there could be an error in our favour,” Elvin said. “But if we see greater lift or downforce than we’re expecting, we’ll deploy the winglets to trim the car’s performance.” This will not be active control: the winglets will shift to a defined angle for a defined speed. Green will not be in control of this, with the code to deploy the winglets written by the engineering team. “Andy’s job is to get into the car and drive the run profile the engineering team tell him to drive,” Elvin said. “He will do that with a split-second accuracy, because he’s a highly trained RAF fighter pilot with a first in mathematics and he’s the only person ever to have driven a supersonic car.”

With the completion of the upper chassis, the focus of the engineering effort has now shifted somewhat from the main body of the car to some of its key components and subsystems. In another corner of the workshop, RAF engineers are piecing together Bloodhound’s tailfin, one of its most distinctive features and a magnet for members of the projects fan club, hundreds of whom have paid £10 each to have their names inscribed on it. Somewhat larger than the fin of the show car and cruciform, rather than T-shaped, with its aerodynamic winglets set half way up its height rather than at the top, the fin is very similar to that of BAE Systems’ Hawk jet trainer aircraft, familiar to most from its role in the Red Arrows display team.

bloodhound4
RAF engineers start work on Bloodhounds tail fin

But it will undergo very different stresses. “It’s the hardest-working tailfin in history,” Elvin said. “Relative to a fin on a supersonic aircraft that flies at 45,000ft, it’s being pushed through treacle; the air at sea level is that much more dense.” Attaching the fin to the car has also caused some concern. “It’s quite important to keep the root of the fin cool. It’s aluminium, so we don’t want it getting above 150–160°C. But it’s near the hottest part of the engine,” said Elvin. “We’ll have heat shielding and thermocouples and sensors in there so we can monitor the conditions. When we’re running, it’s less of an issue, because there’s air flowing through the car, but when we stop, the case of the engine will be radiating heat. We’ll plug in cool air supplies through ducts at the sides
and the main intake; we should just get a momentary temperature spike between stopping and getting the cooling supplies in.”

Off site, production has started on Bloodhound’s desert wheels, which are forged from solid aluminium. This project is being managed by Glasgow-based Castle Precision, with the wheel blanks being forged by Otto Fuchs in Germany from billets supplied by nearby metals specialist Trimet. The wheels are made from a special grade of aluminium developed specially for Bloodhound. “At one point in the project’s early stages, we had four of the foremost aluminium specialists in the world sitting around one table,” said Yan Tiefenbrun, Castle’s director of operations. “That’s never happened before and it will probably never happen again.”

Bloodhound3
Assembly work on the new oxidiser pump

The team is also adjusting to a new auxiliary power unit (APU), the engine that drives the pump that supplies high-test peroxide to oxidise the solid fuel in Bloodhound’s rocket engines. During tests of the hybrid rocket system, it used a Cosworth 12-cylinder Formula One (F1) engine, but Cosworth withdrew from the project when it exited F1, so the new APU is a five-litre supercharged V8 engine supplied by Jaguar.

This has an advantage over the Cosworth, Elvin said: “It has so much torque that we can go from 1,000rpm at start to 6,000rpm redline with no holes in the torque curve and no loss of performance.

The Cosworth would stall below 30,000rpm; it’s a racing engine and is optimised for that kind of performance. But this is a big, lazy road car engine, and it lets us modulate the speed of the oxidiser pump to an extent with the throttle.” Again, this will be in the control of the engineering team, which will preset the engine’s run profile for each run, determining how fast the oxidiser will be delivered to the rockets and therefore what thrust profile they will produce. “Andy just fires
the rockets at the right moment,” Elvin said.

Bloodhound2
The Jaguar engine that will power Bloodhound’s rocket oxidiser pump

The pump that the APU will run is also new; the previous version was not efficient enough – only 30–35 per cent, Elvin said, whereas the team needed 50–55 per cent. This involved redesigning the pump impeller. “At the intake, we have things that look like blades, Elvin explained. “They don’t suck, in the sense of creating a vacuum; they slice pieces of HTP off, then they get flung out radially and that creates the pressure.”

Rather than running through a conventional gearbox, to save space and weight the engine is connected to the pump via a chain-drive, derived from those used in Land Rovers and Range Rovers in the transfer box for four-wheel-drive systems. This is a simple twin-sprocket system, with the chain running between two sprockets sized to give the appropriate step-down from engine to pump.

The clutch system is also a special design, which uses hydraulic pressure to keep it closed; if Bloodhound’s hydraulic systems fail, for example from a stone flying up and severing the fluid line, the clutch will disengage the engine from the pump and the rocket will stop.

Despite all the simulations and studies, the way Bloodhound will actually perform in South Africa is still a big unknown. “Every run will be a real-time experiment,” Elvin said. “A lot of the unpredictability is interaction with the shockwave and the ground. We can’t test it and we can’t predict what the desert is going to do; we just have to run the car over it and see what it does.”

Bloodhound is a very different shape from its predecessor, Thrust SSC, and Hakskeen Pan is different from the Black Rock Desert where Green last broke the Land Speed Record in 1987; from simulations, the team believes that Bloodhound’s shockwave will be less destructive than Thrust’s, which broke up the surface ahead of the car and forced the wheels to run through relatively soft material.

The team is also not under the impression that its mission this year, to break Green’s existing record, and next year, to break 1,000mph, will be a holiday. “Ambient air is about 35°C, which is going to be hard for us,” Elvin said. “It’ll take us a while to acclimatise. Fitness will be an issue for working in those conditions. Changing the rocket motors over will be challenging, in those temperatures, with no shade and full PPE gear; a chemical suit if you’re on HTP duty, breathing equipment, fireproof gear — it’s all tricky stuff. Nobody’s under any illusion that it’ll be pleasant. It’ll be exciting and exhilarating but it won’t be fun.”