Chapter 4 - Structure

The Skeleton

"Thrust 2" is a 'one off.

The general shape is a long profile divided internally into three longitudinal compartments, consisting of a central engine bay flanked by sidebodies containing wheels, driver and fuel tanks.

The frame structure unites, supports and locates all the various components and transmits the forces applied by or on them to the vehicle as a whole. The major forces are engine thrust, aerodynamic loads, chute pull, brake reaction, vehicle weight and acceleration/deceleration loads.

The skeleton structure should meet the following criteria:

- structural integrity, strength and rigidity for minimum weight, to provide for performance and crashworthiness,
- ease of construction, for economy and speed of completion with minimum facilities,
- minimum jigging and tooling requirement commensurate with the 'one off build, modification possibility, to enable new developments and refinements to be incorporated,
- convenient attachment pickups for components and equipment, use of readily available materials, tools and skills, for fabrication, incorporation of driver's safety cage and external strong points for towing, lashing and lifting.

A compound structure consisting of a skeleton space frame, with internal webs and bulkheads and external frames and skin panels, integrated to form a body/frame structure, would fulfil all the requirements and this was selected for "Thrust 2".

To build an efficient compound structure using a tubular frame and shear web panelling, demands good 'interface jointing' between tube and panel with load bearing fixings. This is conveniently provided by lapping panels directly onto square or rectangular tube and attaching with blind rivets. It also facilitates direct assembly of the `body' on to the skeleton using this as a 'Big' in conjunction with formers and templates.

In the design adopted, all the major components are attached directly to the space frame and the forces produced by them are transmitted through it. The frame is fully triangulated and designed to carry all the mechanical loads, as demonstrated in the 1980 season when the car was run unclad to prove the principles and systems. The integrated webs and skin panels transmit the aerodyamic loads, increase the safety factor and rigidity and improve crashworthiness.

Many competition car space frames are constructed from round tube because when considered in isolation this gives a better strength weight ratio. If the body is attached as an dependant unit this is justified. But the body unit will then require considerable stiffening Of its own, either in form of 'shape' or additional stiffeners.

On "Thrust 2" where the body is integrated with the frame, the convenient and efficient attachment of webs, formers, panels and components dictates the use of square or rectangular tubing. The tube is not working in isolation but becomes part of a compound structure of tube and panel. Exceptions are cross bracing struts and tubes that require bending, such as roll bars and engine hoops. Torsion boxes are formed by using shear webs in compound planes as in the bulkheads, internal webs and skins. These boxes impart considerable rigidity to the entire vehicle.

Fixings

To be effective, structural elements must be connected by load bearing joints. Any joint is only as good as its fixings. For direct fixing of panels to tubework, blind fasteners are required. Internal panels are attached with Avdel Aerospace dome headed rivets. External panels subject to airflow are fitted with similar, but countersunk rivets which are skin milled to a flush finish. Certain internal panels that are removable for access are attached with `taptite' thread rolling screws, these have proved very successful and acceptable for the 'short service life' of "Thrust 2". External panels are attached with countersunk aircraft bolts, or Tridair high shear fasteners.

All major components are attached to the space frame by brackets welded to the cheeks of the tube or by bolted through connections with tube spacers to prevent crushing.

Materials

The choice of material is influenced by:

— suitability for the job; strong and light,
— quality; high for reliability and consistency,
— availability; very relevant on a tight budget,
— workability, with limited facilities available.

For the space frame Reynolds 531 was chosen. This tube has a high strength weight ratio and is famous for its use in racing bicycles and motor cycle frames. The decision was made easy by TI Reynolds Limited offering to fabricate the space frame for us.

The body and webs are aluminium alloy. All flat panels are high strength L73. This material is not suitable for forming without heat treatment facilities, so for folded members and curved panels NS4 half hard was selected as being a material with moderate strength, good workability and corrosion resistance.

Engine mounting

The Rolls Royce Avon and reheat dominates "Thrust 2". It is by far the largest and heaviest component in the car - 25ft long, 40 inches in diameter and weighing 3,700 lb. It exerts considerable forces by virtue of its weight and the thrust it produces. To contain and support this engine and its forces, it is mounted between two full length ladder type longeron chassis frames. The longerons contain all the engine and suspension mountings and support the bending between front and rear wheels over the 250 inch wheelbase. The fins, and lifting and lashing eyes, are also attached directly to the longerons.

Cantilevered out from the longerons are `sidebodies' which contain the driver in his cockpit, the fuel tanks, ancillary bays and drag chute canisters.

Structurally the jet engine is an embarrassment. By its nature it demands an uninterrupted tunnel from nose to tail, eliminating diagonal bracing or transverse webs. The lozenging tendency around hs tunnel between the longerons was catered for by making top and bottom cross beams as deep and stiff in bending as space permitted and corner bracing them to reduce the length of pure bending to a minimum.

As the engine is mounted directly to the longerons, it does not contribute to bending between them and its loads are transmitted longitudinally to the suspension mountings. The front engine mounting does, however, try to splay the front 'U' section of the chassis and this is directly reacted by the front suspension struts.

Scale model

As an aid to both the designer and constructors, a balsa wood model of the space frame was made. The scale chosen has one tenth, identical to the frame drawing and also the already existing wind tunnel model, giving an interesting visual comparison between the clad and unclad models. This frame model also gave useful dynamic guidance, under carefully applied torsion or bending, relative rigidity of elements could be tested.

Safety features

Land Speed Record breaking is historically a dangerous pastime. Although the risk element can never be eliminated, it can be reduced considerably. To this end, designer, driver and team must be made aware of safety as a positive and primary goal. Safety aspects frequently conflict with those of aerodynamic purity, weight, expense and available team effort, but must be given primary consideration in the resulting compromises.

Safety is a wide subject and it is only intended to cover the structural aspects at this stage. Structural failure must not precipitate an accident. The structure must be strong and rigid enough to cater for all known conditions with plenty in reserve to accommodate unforseen loads or situations.

Panels must be securely fastened to resist loosening by vibration or aerodynamic forces. Materials, components and workmanship must be of the highest quality and reliability. Should an accident occur the structure must be designed to protect the driver from direct impact or excessive deceleration. The location of the cockpit is a primary consideration with the middle of the car being the most sheltered position. This area is protected from impact and has considerable structure ahead and behind to allow for progressive collapse and hence reduced deceleration. The 'corners' of "Thrust 2" are comparatively 'soft' to collapse on impact sacrificing structure to absorb energy. The front of the space frame has long crush tubes with 'dog legged' supports designed to crumple under frontal impact.

The driver's cockpit is the nerve centre of the car. From it the driver must gather all his sensory inputs and respond with the controls to perform his objective - the Land Speed Record. The cockpit must, therefore, allow adequate visibility, with enough bonnet for 'sighting' his aim. It should be comfortable with enough room to operate the controls and be isolated from severe vibration, noise and heat. Above all, it must protect that vital and delicate control system - The Driver.

During an accident, driver protection should meet the following criteria:

prevent collapse of structure on to driver,
prevent ingress of engine or components into cockpit,
prevent impact of driver against interior,
prevent loose objects becoming missiles inside the cockpit.

Following an accident, the criteria are:

fire prevention or suppression,
protection from fire and fumes,
quick harness release and driver evacuation,
rescue access to emergency exit.

In "Thrust 2", therefore, the driver's cockpit is contained within a strong 'cage' integrated into the space frame and incorporating substantial 'roll bars' and firewalls. This cage has a collapse allowance margin around the driver, despite the frontal area penalty.

The front, inside and rear bulkheads are built as firewalls. The structure of these walls is based on the 2 inch space frame tube, the external hazard face of which is clad with 28 swg stainless steel sheet, attached with stainless steel rivets and the internal cockpit face is clad with 16 swg L73 light alloy. The 'filling' of the resultant sandwich is 2 inches thick Rockwool mineral 'firebatts'.

The cockpit

It is no good building a massive cage if the driver is free to 'rattle' around inside it, so he must be securely contained. To this end he is supported by a fully-formed seat and headrest and restrained by a 6-point harness restraint with quick release fixings. The seat locates the driver laterally and distributes load over a large body area, it is of sufficient 'depth' to ingest severed struts or sheet metal from the rear bulkhead. All internal cockpit fittings are flush or 'sheltered.' The steering wheel, column and arm rests are collapsible.

Heavy components are located externally when possible, others, such as the breathing bottle, are mounted securely against severe impact decelerations. Fire hazard is suppressed by an automatic extinguisher system and life support air breathing is fully contained within the cockpit. Access to the cockpit is through a single curvature flush fitting 'gullwing' canopy of double skinned light alloy construction and the outer skin incorporates an 'axe out' panel for rescue should the car be inverted.

The transparency is sharply raked and made from single curvature 5/8" thick acrylic, retained in its frame structure by clamping bolts. The edge is rebated to give a flush outside surface and sealed with Thiokol. All double curvature is contained in a 'hoop' structure which blends the screen to the canopy hatch. This hoop structure acts as a driver's 'sunshade' in the bright desert light and as additional rollover protection. It is double skinned, well stiffened and strongly anchored to space frame members.

The outer skin

The outer shape of the car has to envelope all the components, in a low drag, non lifting aerodynamic package, and yet be manufactured by a very small team with limited facilities. Compound curvature was kept to the minimum with slab surfaces blended to single curvature where possible. The 'shaped' outer skin panels are supported by formed frames with webs attached direct to the space frame members. To reduce drag the outside skin is completely flush finished including all fastenings and fittings.

The engine air intake forms the 'nose' of the car and is by far the most curvacious asset. It must provide smooth car penetration into the airstream and airflow into the engine. Structurally it is subjected to considerable suction from the engine and pressure from the airflow. Mechanically it seals the engine intake lip and supports the starter air pipe.

Due to the complex shape, it was decided to break away from the 'standard' construction of the car which we could produce 'in house' and manufacture the intake from aerospace glass reinforced plastic (G.R.P.). The material selected was 10 layers of .011 inch thick glass weave Y244 with Crystic 199 resin. This was reinforced by crossed top hat stiffeners, 'glassed over' foam formers and by metal insert strips along the attachment faces and flanges.

"Sonic I", previously the world's fastest jet car, had considerable trouble with heat buckling of the panels round the reheat pipe. This was relieved by cutting a multitude of louvres to vent and stiffen the area, also generating a lot of drag and stress raisers. Rather than fight the heat problem, it was decided to cool the reheat area with forward facing intakes which could exhaust around the jet pipe, at the same time relieving air from a high pressure area and 'dumping' it into the base area vacuum. The intake was 'generated' by using the bonnet, screen and canopy intersections with little additonal work.

The car has a flush undertray. This reduces drag by fairing in the belly and generates downthrust by the venture effect between the car and the ground.

Twin swept back and tapered fins are fitted for directional stability. Two fins were chosen to reduce the height of the roll movement for a given area and also to utilise the strength of the main longerons as mounting platforms. The fins are removable for transit and bolt directly on to brackets incorporated into the main longeron frames. Their structure is conventional. The main and rear spars being fabricated from folded light alloy sections and webs. The aerofoil is formed by the horizontal ribs and outer skins.

The Land Speed Record rules determine that the 'return run' must be made within the hour. To comply, reliability and accessibility for inspection and servicing are vital. For servicing, all critical components must be accessible through quickly removeable, flush fitting and safely secured hatches. These hatch panels range from small 'hand holes' to large lift off sections covering the engine and front wheel arches.

The wheel covers which must be removed after each run, are particularly critical. They must be large enough to allow wheel removal, yet strong and rigid enough to withstand the lift and transonic shock on the outer surface and ballooning whirlwind on the inside. To meet these conditions they are supported by deep beams at the top and bottom of the panel.

The panel itself is well stiffened and attached with stressed quick release Tridair fasteners, as used on the Tornado aircraft.

Crash test

On 17th June 1982 we put our safety theories to the test - the hard way. "Thrust 2" was undergoing trials at Greenham Common Air Base prior to departure for the USA when the accident happened. Following low speed chute failure due to excessive speed, wheel brakes were applied and the car was steered sharply left off the runway and across 900ft of rough ground, finally coming to rest on hard standing at the far side of the airfield.

The chute failure occurred at approximately 270 mph and wheel brakes were applied and locked for 4000ft until standstill - a world record skid?

The left turn was entered at about 170 mph in a four-wheel slide and the verge impacted at about 125 mph wheels locked. The ramp of the verge and subsequent humps in the rough ground resulted in the car becoming airborne in a series of hops, plunging the nose into the ground. The accelerations experienced by meters in the passenger cockpit were: 7.8g decleration, plus 4g and minus Ig vertical. As these readings were for the centre of the vehicle, much higher values must have been experienced locally at the front.

The car was damaged by its passage across the rough ground and the engine by the ingestion of debris.

Both driver and passenger Mike 'The Chute' Barrett, were completely unharmed and rapidly evacuated themselves. The front port side tyre was flat and all tyres severely shredded. Brake discs and pads showed considerable work had been done before the wheels were locked with tyre to surface skid dissipating further energy. After the accident the brakes were tested on the pedal and were still functioning correctly.

Structurally, the 'soft' front corners and apron collapsed as intended absorbing energy. The suspension suffered bending and displacement of several components but did not break or collapse. Such a failure during the high speed turn would almost certainly have precipitated a disastrous roll.

The driver and passenger remained well supported and restrained throughout the violent manoeuvres and deceleration and did not suffer any discomfort or bruising.

This was a drastic test of the structure safety systems and the following criteria were met:

no rollover, the car remained upright despite the high 'g' turn,
no fire, the three onboard extinguishers were automatically triggered by inertia
switches and still operating after standstill,
no critical structure, or suspension failure,
progressive sacrificial collapse of front end,
no injury, driver and passenger securely restrained,
quick release and exit by driver and passenger.

The accident was followed by a hectic rebuild of engine and car. This involved hacksawing off the front end forward of the front wheels and completely renewing it all, in time to get on the salt by September, but the car had proved its survivability and crash worthiness.