Stresses and strains of a world beating supercar

When Thrust SSC set a new supersonic world land speed record of 763.035mph or Mach 1.0175 staff at Assystem offices in Preston and Bristol were jubilant at their own vital role in this very British success story.

Assystem was selected by Richard Noble to design the nose cone and engine intakes for the Thrust SSC project.

Having developed and verified the aerodynamic shape of the SSC using Computational Fluid Dynamics (CFD) and run a rocket-powered scale model at the Pendine test track, the Thrust Team was confident that it had very good correlation between the analytical data and its test results. At this point, Assystem was asked to handle the detail design work to create the front end profile which would keep the 7 tonne vehicle on the ground and ensure it stayed in one piece.

In addition to experience with composite materials Assystem was able to apply aerospace design techniques for this 'plane on the ground' vehicle. The chassis design, in the form of a 2-D data model, was provided for conversion into 3-D solid models from which the surface design was developed for the sub-structure which transmitted the loads from the panels into the main chassis. To speed the design process and to meet the tight schedule stress engineers, designers and tooling manufacturers worked together on each element, concurrently, without producing detail drawings at each stage.

Because of the complex nature of the body shape, Assystem used CADDS 5 NURBS for defining the complex surfaces. Point loading data was converted into a 3-D model facilitating the definition of smooth surfaces to speed up the design process.

Parametric Modelling

The engine intake barrels were surfaced and modelled using parametric solid modelling derived from the point data. This was used to carry out some small but vital changes. To assist further in defining the body profile, Assystem also used Finite Element Analysis to help identify which panels would be aluminium, which would need to be carbon fibre because of the high pressure loading and which needed to be made from titanium to withstand the heat from the exhaust. It was calculated that when the car was travelling at Mach 1.1, pressures in excess of 15 tonnes would be experienced on the body panels.

Uncharted Territory

Assystem designers were familiar with air flows and forces relating to trans-sonic and super-sonic aircraft. However, there was no empirical data for supersonic shock waves in ground effect. maintaining the integrity and freedom from aerodynamic noise of the skin structure was of paramount importance. The basic aerodynamic shape of the vehicle was contrived to ensure that the pressure forces on top of the car would keep it on the ground, as well as stable and on course at high speed.

The tail section or empenage was more complicated and also subject to heavy stresses.

Following a review of the stiffness of the structure under load, Assystem designed all the panels in the tail section as well as the rear structural steel framework of the vehicle.

Assystem ran some unit cases to analyse deflections under loading to provide steady stress analysis on the fin and the rear fuselage structure. The likely deflection characteristics of the tail were assessed by applying loads horizontally, vertically and with twisting moments. The data obtained from these computer models would enable the Thrust Team to factor up the Assystem results when they had a better idea of the actual displacements of the structure, in order to ensure the necessary stability in action.