Structural Design

Mark Chapman  

When it comes to the structure of BLOODHOUND SSC it’s easy to fall into the trap of forgetting its scale, all too easy when it’s on your screen and doesn’t look so big.  Then you start to come to terms with the fact it’s getting on for 13m long, weighs close to 7 tonnes and has to deal variously with 220 kN of thrust, going through the sound barrier, 2.5+g acceleration followed pretty swiftly by 3+g deceleration, and going over the odd bump, it’s less racing car and more supersonic truck.

The reason I say this is to make the point that whereas with a Formula 1 or Le Mans type car the route to structural success is, if not simple, at least a fairly well trodden path.  You’ll make a monocoque carbon fibre tub, attach a structural engine and gearbox, fix on some suspension and ensure all the aero loads feed into strong bits of the tub or gearbox and off you go racing.  The sheer scale of BLOODHOUND SSC means that there isn’t a similar single solution that can be applied throughout the car
 
So, breaking it down piece by piece;

The front of the car, this contains the front wheels and suspension, the cockpit with Andy, the ice tanks for cooling the auxiliary power unit (APU), the front winglets, various ancillaries and for better or for worse the HTP tank.  Of all the car this is the most “normal”, from a motorsport perspective – aside perhaps from the tonne of HTP.

Making it as a large carbon fibre tub makes sense as it makes best use of the properties of a carbon structure.  It’s a small enough section that can be realistically put into an autoclave, the body surface is quite complex so would probably have to be made in composite anyway, and there are few apertures - composite structures like to be as continuous as possible.  Most importantly, it’s the best material to provide a rigid safety cell to protect Andy in the event of an accident. To see the resilience of well designed carbon structures, you just have to look at the accidents in F1 where drivers have been able to walk away from car-destroying crashes where the safety cell around them has remained intact.

The next section of car rearwards is split in two at approximately the centreline of the EJ200, the upper section being carbon, the lower a steel space frame.
 

The upper section is challenging as there's the gaping hole for the intake duct - just about where you'd like a nice deep section beam to take the considerable bending moment.  This was one of the main advantages of the early bifurcated intake design which meant there could be a continuous spine running the length of the car, unfortunately what looked good structurally didn't work aerodynamically, so we'll have to deal with the gaping hole.
 

Although we looked at a number of space frame options, it was difficult to get enough structure where it was needed without greatly increasing the frontal area - Ron's bête noire.  As the complex external profile around the intake would need to be made from composite anyway,  the decision was made to go with a composite upper section, and use a composite web to take the load from the spine under the rocket to the front bulkhead.

The lower section is a more traditional space frame structure and contains the APU and the EJ200.  One thing we've had to bear in mind is access, and where there are systems in the car we need to get to, the tendency has been to go for a metallic frame with non-structural access panels.  In the case of the APU, some jobs will be possible through the framework, but for anything more complex, the engine complete with the gearbox and pump assembly can be removed from the top of the car on its own structural frame. 

The rear of this section runs under the EJ200, again, accessibility is an issue.  The EJ200 is normally mounted in the Eurofighter Typhoon with enviable access through the opening bay doors - all the bits you could possibly need to get to are on show.  In our case those few items that do need checking or changing on a regular basis, such as oil and hydraulic filters, magnetic chip detectors and potentially some limited borescope access, will have to be managed through the confines of the lower frame.  For anything more major, an engine removal is required.

This brings us along to the next section which encases the top of the EJ200.  Now, on the Eurofighter, engine removal is by lowering the engine out of the bay, on BLOODHOUND SSC we'd need some pretty big axle stands to achieve this.  So, and this is the clever bit, as all the EJ200 mounts attach to the top frame, to remove the engine the rear frame is wheeled away, and the top frame is lifted complete with EJ200 attached.  This top section also includes the front half of the twin beams that make up the spine of the car under the rocket.  These help distribute the combined thrust loads from the jet and the rocket, and conversely the braking loads from the twin chutes and the airbrakes.

The final section is the most highly loaded on the car, this is based around two steel bulkheads which pick up the inboard mounts for the rear suspension and a base plate that carries the rockers for the dampers, the front of the dampers are reacted against a third plate that makes up the front bulkhead.  As well as the suspension loads, it also has to cope with thrust from the rocket, which is reacted at a mount on the nozzle, the parachute loads - the parachute strops are mounted to the spine between the rocket and the jet, and the aero load from the winglets being used to control the car's stability.

This all adds up to a pretty massive structure, which from an aero perspective needs to be boat-tailed to reduce the base drag at trans-sonic and supersonic speeds.  This has had a big impact upon the current design.
 
One aspect of the structural design which is currently up for debate is that at present, where we have a space frame, the intent is for this to do all the work and not to use the skin to carry significant load. 

The reason for not initially designing a structural skin is twofold;

Firstly, for the skin to take load we need a reliable way of getting the load into it.  For a composite skin that would mean bonding it to the frame, for a metal skin that would mean bonding and/or a lot of rivets, the problem with both of these is loss of accessibility to the underlying components - putting lots of access panels into a stressed skin rather defeats the object.

Secondly, and this one really applies to a composite skin, is the stiffness benefit of a stressed skin panel, say 40mm thick, greater than increasing the hoop radius of the underlying structure by an equivalent amount?

We're hoping that a CAE technique known as Topological Optimisation will help us to answer this question, and also with refining the structure of the space frame and the structural composite panels. 

The best way of thinking of an optimiser is that it's full of little termites that eat away all the low stressed parts of a component (blue bits tasty, red bits nasty), and what remains is a framework showing where the highly stressed areas are, ie. exactly where you do need the structure.  The example below shows a cube of material that is supported at its four corners and has a load placed on the centre of the top face.  The optimiser progressively nibbles away at all the blue and green lowly stressed material, till all that remains is the red, stressed, framework that is required to support the load.