Hydraulic accumulators are the dextrose tablets of the engineering world, providing fast access to stored energy when required.
At supersonic speeds, having instant access to energy reserves is critical; for BLOODHOUND SSC, that could mean making sure the car stops safely before running out of desert. At 1,000 mph every millisecond counts, so Andy Green will rely on powerful air brakes to slow the vehicle from 800mph and below. But there’s also a need for instant energy as a backup – in this case, taking the form of two composite hydraulic accumulators.
The case for composite accumulators on BLOODHOUNDAfter BLOODHOUND's EJ200 jet engine starts, the accumulators are charged. If a loss of hydraulic power occurs at any point, stored energy in the accumulators will be ready to deploy the air brakes at a controlled speed.
Originally, BLOODHOUND SSC was going to house one large steel bladder accumulator, high in the vehicle. However, project engineers were concerned about the potential height of the centre of gravity and symmetry of mass about the centerline axis; at such high speeds, the car must be very stable about the yaw axis.
Product size, weight and placement are also key factors for BLOODHOUND SSC; there’s a huge amount of technology packed into the car. The team needed a lightweight accumulator that could fit into a compact design envelope.
Parker’s project team spotted that using two composite piston accumulators would meet all of these demands. Compared to bladder accumulators, our recommended piston counterparts were much smaller and can deliver more of the available oil capacity. They also weigh considerably less than a standard steel product.
With this in mind, Parker sent some models to BLOODHOUND’s engineers - kick-starting an iterative process to custom-build an accumulator specific to project requirements.
Composite accumulator product development and designHigh-pressure hydraulic products need strong structural components. Normally, industrial applications use high-strength steels, whilst more expensive lower-density alloys are used in lightweight applications. But whilst high-performing metallic alloys are very strong, these benefits can be outweighed by decreased ductility and reduced fatigue strength.
Composites are an ideal reinforcement material for hydraulic products. They offer higher strength and superior fatigue properties compared to most metals. Traditionally, composite accumulators have used metal liners with an outer composite reinforcing structure, due to the high-stresses involved; but Parker’s engineers have been working hard for more than 10 years to develop a high-performance hydraulic product that is almost entirely made of composites.
Rather than simply reinforcing a metallic barrel, Parker developed a specially-formulated plastic liner, integrated into the carbon fibre reinforced epoxy composite product. This plastic liner is lighter and more cost-effective than thin-walled metallic barrels, yet offers superior fatigue life properties over most metals.
The barrel design concept consists of two parts – an inner liner and outer barrel. The fully composite inner liner carries the hoop loads of the internal pressure, while a fully composite outer barrel is responsible for supporting the axial loads. To allow maintenance of seals, the outer barrel has metal inserts integrated into the composite via a patented technology, which provide a threaded interface for the end caps at each end of the barrel. With everything but the threaded end caps constructed of composite, the product is up to 60 per cent lighter than standard steel accumulators.
The fully-composite liner is cheaper to manufacture, lighter, and offers higher burst and fatigue strengths than a composite reinforced metallic liner. The fully-composite outer barrel is also lighter and offers outstanding burst and fatigue strengths.
Table 1 below compares a sample 15-litre Parker composite accumulator to similar performance steel piston accumulators, using data published in several major hydraulic manufacturers’ product catalogues.
Table 1: composite accumulator performance
Typical steel piston accumulator
Parker composite accumulator
Volume [L]
15
15
Pressure [bar]
350-375
380
Weight [kg]
90-131
26
Development testingThe development phase incorporated substantial testing. In particular, numerous burst tests were performed to confirm the structural integrity of the product was well above regulatory requirements. Due to the typically high-cycle lifespans of accumulators, several samples were fatigue tested (up to approaching six million cycles). Drop tests and impact resistance tests were also performed in preparation for future regulatory approvals.
In addition, the product was deliberately designed to have a safe failure mode in the event of over-pressurisation. Before reaching a pressure level where the accumulators could dangerously rupture, they are designed to leak at a rate that prevents further pressurization.
To confirm this for the BLOODHOUND accumulators, Parker made an additional accumulator only for burst testing. As expected, this sample leaked but could not be made to rupture - offering a fail-safe mode to the BLOODHOUND team, should the completely unexpected occur.
Analysing processes was another key step in the chain. This involved exploring options to achieve a good surface finish, make the product round and reduce the likelihood of cracking.
Future applicationSo far, Parker’s composite piston accumulators have primarily been used in controlled conditions - such as for testing in academic environments. However, discussions are progressing about the potential for launching the product to specific niche markets.
Cost-wise, composite accumulators are more expensive than standard steel versions, but around the same price as stainless steel products. Whilst unlikely to be adopted for mainstream applications where traditional steel is more cost-effective, they are ideally suited to corrosive environments and applications where weight is a key priority.
Composites are an excellent option for industries such as oil and gas – offering the equivalent corrosion resistance as stainless steel accumulators, but with the added benefit of being much lighter, meaning lower transportation costs and easier portability. Subject to securing the necessary regulatory approvals, this type of product has significant market potential in the years ahead.
In the meantime, of course, all eyes will be on BLOODHOUND and the test runs scheduled next year. And as pilot Andy Green powers his way across the South African desert with Parker's composite accumulators on board, he’ll have instant access to a big energy boost if and when he needs it.
Feature photo credit: Photographer Stefan Marjoram 
Content by Mark Cattermole, Parker Sales UK (pictured) and Bruce Otte, Hydraulics Division
Related links
Watch the video: Parker’s BLOODHOUND SSC airbrake system
Check out our latest news on the BLOODHOUND website: Hydraulic power packs streamline testing for BLOODHOUND's world record bid
Read the blog: Lightraulics C-Series Actuators: High Pressure Hydraulics for Lightweight Applications
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This post builds on an article covered in The Engineer in January, and provides additional information about Parker’s contribution to the BLOODHOUND supersonic vehicle.