Thrust SSC - Engineering

The Parachute Braking System

by Bruce White, ThrustSSC Parachutes

The Parachute Braking System is employed to slow the car to a speed at which the wheel brakes can safely be used. Although the method of parachute deployment is the same, two distinct standards are provided, one for low speed use on, say a runway at deployment speeds up to some 200 mph, and the high speed system for use up to a speed of 600 mph.

Two parachute containers are fitted. At low speeds, both contain identical parachute assemblies, the second assembly intended only as a reserve, for use in the unlikely event of a malfunction occurring on the primary system. Either system can be used as primary or secondary.

Robbie Kraike and Brian Palmer fitting parachutes
(Robbie Kraike and Brian Palmer fitting parachutes. Photo: Jeremy Davey. Digital camera courtesy Canon (UK) Ltd)

Deployment is initiated by a button for each system on the steering wheel; each of these serves to fire a pyrotechnic cartridge which propels a metal 'bullet' from one of two guns at the rear of the car, adjacent to the containers. The containers are closed by fabric flaps secured by a small pin, which is connected to the bullet. The function of the bullet is firstly to withdraw a small auxiliary parachute stowed at the rear of the main parachute pack, and connected to it by a strop. This auxiliary then inflates in the air stream and withdraws the main pack from the container. As the pack is pulled away, firstly the main parachute strop, followed by the rigging (sometimes called shroud) lines, are sequentially deployed. When these are fully extended the canopy, which is retained by flaps within the pack, is then released; but does not separate from the pack again until it is fully extended. In this manner the deployment is totally controlled, obviating any possibility of canopy inversion, line irregularities or other malfunctions. The main canopy is then free to inflate and provide the braking force. The bullet, auxiliary and pack are jettisoned, together with sundry items such as rubber bands, which are used to stow the lines neatly within the pack.

The low-speed system utilises a ring-slot type parachute of nearly 15 feet diameter (for most common parachute types the stated diameter relates to that of the canopy when it is laid flat; in flight the 'mouth' diameter is usually about two-thirds of the flat diameter). This parachute is a strengthened version of a type used to extract loads from the tailgate of an aircraft such as the C130 Hercules in the supply-dropping role. At 200mph it will produce a braking force of some 6 tons. It flies approximately 33ft behind the car. The auxiliary parachute is of the ribbon type, of approx. 5 ft diameter, originally designed as a drogue parachute for an unmanned surveillance drone. (An auxiliary parachute serves only to pull out a second or main parachute, whereas a drogue slows down its load before being released to pull out the main.)

The high-speed system uses a very strong ribbon parachute of 7ft 6in diameter, originally designed some 30 years ago against a Ministry of Defence requirement. It will produce a force of 10 tons at 600 mph. (Some idea of its strength may be gained from the fact that each of its 24 rigging lines has a breaking strength of 1 tons. They also make good tow ropes.)

A single parachute unit, which is designated to be deployed at 600 mph is attached to a strop of 50,000 lb. breaking strength, and of length such that the parachute will fly at some 100 ft behind the car. This system is exactly the same as that used for Thrust2, being the strongest available without the need for a very costly development programme to provide a higher speed deployment capability.

For Thrust2, a second stage comprising a cluster of three of the same type of parachutes, flying at some 40 ft behind the car, was provided for optional use at 400mph, once the car had slowed to that speed on the single parachute first stage. On deployment, this cluster will produce a retarding force of about 12 tons in addition to the single, at that speed providing about 4 tons, giving a total of some 16 tons. On Thrust2 this produced about 4g retardation. In the event Richard did not find it necessary to use the cluster very often, due to the space available to slow down, the single proving sufficient.

ThrustSSC with single parachute deployed during Farnborough tests
(ThrustSSC with single parachute deployed during Farnborough tests. Photo: Jeremy Davey. Digital camera courtesy Canon (UK) Ltd)

For ThrustSSC, although the second stage cluster remains an option, and has been provided, Ron Ayers has estimated that two singles deployed sequentially in cascade should provide adequate retardation. In this case also, the second parachute will fly nearer to the car than the first, at about 75ft, but it may well be that some loss of effectiveness will result due to the proximity of the two canopies, in which case we may well have to adjust the separation distance.

The high-speed systems utilise a strong, fairly open weave fabric auxiliary of about 3ft diameter, derived from the same MoD project.

The parachutes are packed into fabric bags designed to control the deployment sequence. The pack for the single high-speed parachute has internal divisions to separate the strop and lines from the canopy, each section being closed by flaps secured by a breakable tie cord, which are positively broken in the sequence. At the rear of the pack is a small compartment for the auxiliary parachute. This pack, for reasons of economy, also serves to house the low-speed parachute system.

The pack for the cluster system is similar in principle, but due to the extra bulk to be contained within the same confines imposed by the containers, the pack incorporates a lacing system which enables the diameter to be reduced after the parachutes are stowed in the pack. After placing the pack in the container the lacing cord is withdrawn so that as the pack is pulled out by the auxiliary, it may assume a larger diameter, allowing the three parachutes to exit without damage. In each case the parachutes are also tied in to the pack by break cords, ensuring full system extension before release of the pack.

On Thrust2 the attachment point for the parachutes was at the top of the body, and it was found that the single parachute tended to fly high. Some were therefore modified by removing one or two of the circumferential ribbons. This had the desired lowering effect with no noticeable loss of retardation. As yet we do not know if the same measures will be necessary for SSC due to, in this case, a very low attachment position. Hopefully all will be revealed at Al Jafr, and based on our experience with Thrust2, we are confident that all will be well, providing deployment is not initiated above 600mph. However, in the design certain reserve factors are incorporated, so that should an emergency arise the system may well survive a higher speed deployment.

Bruce White

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