David McMurtry
This article will focus on the life of yet another pillar of transport engineering from Ireland. The extraordinary story of David McMurtry could well be told with only a passing reference to his work in transport, such was his influence on major industrial processes in the last 50 years. But his contribution to transport was both significant for the legacy it left behind, and also in forging the inventive intellect that would go on to having such a profound impact across multiple industrial sectors.
David McMurtry was born in Dublin in 1940, and spent his childhood in Clontarf, on the north coast of Dublin city. He attended Mount Temple Comprehensive (founding members of U2 are also alumni). In 1958, he moved to the UK, starting an appreticeship with Bristol Aeroplane Company (which was later acquired by Rolls Royce). During this time, he worked primarily on engine develompent, and rose to become Deputy head of engine design on the Concorde engine. Perhaps most known for co-founding Renishaw PLC, using a touch-triggered measurement probe to revolutionise metrological processes.
Fig. 1 – McMurtry/Rolls Royce patent (GB1445977A, 1972) Touch sensitive probe patent sketch
While developing precision fan blades for high performance jet engines, McMurtry become frustrated with existing measurement capabilities, so took time to improve the probe head. This meant faster and more accurate 3D measurement of components. This simple and elegant solution would have implications far beyond jet turbine blades. In 1973, McMurtry and a colleague, John Deer, left Rolls Royce and founded Renishaw Electrical. (The Concorde program was relatively mature, with supersonic operation achieved in 1970, and production aircraft build in 1973). They purchased the patent rights to McMurtry’s probe, and never looked back. Renishaw is still a pre-eminent name in manufacturing quality, but has since expanded to additive manufacturing, laser interferometry, neurology and life sciences. McMurtry stepping back from his role as CEO in 2018.
Jet engines.
When considering McMurtry’s career, his time working in jet engine development coincided with some of the most ambitious and successful engineering projects undertaken in post-war Europe. He initially joined Bristol Aeroplane Company as an apprentice engineer, but quickly rose through the ranks to become the firm’s youngest ever Assistant Chief of Engine Design. During this time, he was responsible for over 40 patents on jet engine design, focusing on improving performance and efficiency across different operating points.
The plan to build Concorde probably resulted the most ambitious project brief in aviaition history. The concept of Supersonic Transport was discussed in the early 1950s, and by the second half of the 1950s, both the British and French governments had engaged suppliers to develop concepts. Bristol Aeroplane Company was chosen for the concept development in 1959 (two years after McMurtry had joined as an apprentice). Bristol had particular expertise in engine delivery, and as the Bristish and French governments agreed to go ahead with the development in late 1962, the chosen engine was based on existing Bristol Olympus technology.
To achieve speed in excess of Mach2, a slender delta wing was needed. This wing type allows for good performance at higher speeds, but generates very little lift at low speeds, making both take off and landing a challenge. For the engine design, therefore, afterburners had to be incorporated, to ensure sufficient engine thrust during take off. At the same time, the engine needed to deliver significant efficiency during cruise, so that transatlantic operation would be viable. (Contemporary military aircraft were capable of Mach2, but with range limiting them to intercepting and defence roles). To achieve these divergent targets, the engine design needed to incorporate a number of new solutions, such as variable air inlet geometery to deal with the changes in air pressure at supersonic speeds, air bypass (to feed afterburners, using digital control units).
Fig. 2 – Concorde Air Intake System
Fig. 3 – McMurtry/Rolls Royce patent sketch (GB1306872A, 1972) for Gas Turbine Ducted Fan Engine, showing novel air bypass elements
Active stator blade cooling and temperature resistant titanium- and nickel-based alloy rotor blades. Twin engines incorpated in a single nacelle, but seperated for resilience minimised airflow impact on the wing, improving efficiency. Rotor blades in particular needed solutions capable of dealing with extreme air pressure, temperatures in excess of 1400degC and any foreign objects that might enter. The improtance of engineering precision in design of these components cannot be overstated.
During this extraordinary tenure with Rolls Royce, McMurtry created almost 50 patents.
While the enourmous thrust of the engine and low drag dynamic of the wing were necessary for efficient supersonic flight, these requirements also psuhed the envelope in a number of other areas (some of which are now common-place in ground transport), notably in braking. While normal braking of an aircraft may encompass deceleration during a landing, the ability to Refuse Take Off (RTF) is by far the most stringent requirement. Here, the plane may already be at take-off speed, having consumed significant amount of runway, and have full payload. As with any brake, the system weight should also be minimised.
Concorde was the first example of a Brake-by-Wire system, whereby the pilot’s brake pedal inputs were relayed digitally (green circuit) to hydraulic actuators on the fuselage. These actuators would then create pressure, which in turn was replicated in the wheel brakes. As with today’s automotive equivalent, the system had sensor redundancy for the pilot input, but also a full hydraulic fall-back (yellow circuit), whereby the braking could still be achieved.
Fig. 4 – Concorde BbW system
And while the brake actuation system was cutting edge, the innovative approach to brake modulation was exceptional, resulting in the first fully electric anti-lock braking system. On Concorde, the main wheels were braked, but the nose wheel wasn’t. This meant that the nose wheel could be considered free-rolling during braking, and so could be relied upon as a true representation of the ground speed (vehicle reference speed is a crucial element in any contemporary automotive stability system). The main wheels were, therefore, able to be reliably controlled away from this reference speed. This meant that the main tyre slip could be maximised, greatly increasing the braking efficiency, and allowing for the RTO point to be further down a runway.
Fig 5 – Anti-lock braking system control schematic of Concorde
Fig 6 – Theoretical tyre slip utilisation in braking
The system also incorporate a mechanism to use air-speed as a ground speed reference, meaning that peak braking could be controlled even before the nose wheels had touched down during landing.
Finally, the wheel brakes themselves introduced a new material combination, the carbon brake rotor. This allowed for a 60% weight save on the equivalent steel rotor, as well as up to a 6x increase in service life. While this brake rotor technology is now common in motorsport and performance vehicles, the introduction in Concorde marked another significant braking first.
McMurtry Automotive
Like many of his luminary engineering compartiots, McMurtry maintained a strong interest in automotive engineering. Renishaw has a wide variety of automotive customers, and is even a partner in the Bloodhound Land Speed Record vehicle, which is attempting to achieve Supersonic land speed, and eventually set a new record.
In 2016, McMurtry co-founded an automotive company, with a distinct mission to push the limits of downforce technology and deliver unrivalled driving performance. While his co-founder Thomas Yates took an active role in developing the first vehicles, McMurtry was instrumental in setting the direction for the company and mission descriptions for the vehicles.
The first vehicle, the McMurtry Spéirling, was unveiled at the 2021 Goodwood Festival of Speed, where it set a new record for the Goodwood Hill Climb the following year. Since then, it has set records at numerous race courses, including Hockenheim, Castle Combe, and Laguna Seca, even during the vehicle development phase. The vehicle is powered by twin rear electric motors, developing over 1000hp, weighs less than 1,000kg and has a unique active downforce system which allows for extraordinary downforce to be developed, even while the vehicle is stationary.
Fig. 7 – Scholz /McMurty Automotive (GB2598473B, 2019) patent sketch for downforce system for a vehicle
This downforce system allows for over 2000kg of downforce at all speeds (and in all directions of travel), greatly increasing tyre grip, meaning the vehicle lateral and longitudinal limits can be greatly increased over conventional approaches. Considering Fig 6, the achievable peak of adhesion is increased as tyre normal load increases, meaning in turn that significantly greater tyre lateral or longitudinal forces can be deployed. Or, more succiently, the higher the downforce, the more sticky the tyre.
The Spéirling has certainly delivered, in a very tangible way, on it’s mission to push the limits of downforce technology, and in doing so, is challenging a lot of conventional approaches to vehicle design. It also plays a fitting role in book-ending a lifetime in transport engineering for David McMurtry. From the start of his career, right through to his sad passing, his inventive passion and engineering rigour has redefined what we come to expect of transport solutions.