British inventors have been responsible for many of the innovations that have made carrier aviation possible. The ‘ski jump’ was first developed in the 1970s to enable the Sea Harrier jet to launch more safely and efficiently and is a feature of the new QEC aircraft carriers, helping launch the latest generation of jets. Here we look at the history, design and purpose of the ramp.
The first known use of an angled ramp to launch aircraft at sea was in 1944 when a temporary wooden ramp was installed on HMS Furious. This crude construction was employed specifically to help underpowered, Fairy Barracuda aircraft stagger into the air armed with 1,600 lb armour-piercing bombs for operation Tungsten, an attack on the German battleship Tirpitz in Norway.
The Harrier GR1 first entered with the RAF in 1969 a land-based close-support aircraft but various trials were made with the aircraft operating at sea. The RN’s conventional aircraft carrier project (CVA-01) had been axed in 1966 and many in the navy began to realise the Harrier had the potential to provide a basic fixed-wing fighter than could operate from the much smaller vessels of the future. British Aerospace was keen to produce a navalised Harrier and studies were well underway before the Navy Board formally approved the development of the Sea Harrier in April 1975. Although the Harrier could take off vertically when given any kind of useful weapon and fuel load, this was impossible and a short rolling take-off was required. It was determined that wind over the deck and a 299-metre runway would be needed to launch a Harrier with a full war load. The RN needed a solution to the Short Take-Off (STO) problem as the Invincible class ‘through-deck cruisers’, then under construction, had only a 200-meter flight deck.
Lt Cdr Doug Taylor, studying at Southampton University in 1973 was tasked to write a thesis looking at options for resolving this problem. His apparently simplistic idea of a curved ski ramp was initially met with some scepticism but further investigation quickly revealed that the concept showed real promise. Hawker Siddeley Aviation in Kingston, responsible for designing and manufacturing the Harrier (before it was consumed by British Aerospace in 1977), and the MoD explored and verified the concept further using simulation and computer modelling techniques.
On 5th August the 1977 first ski-ramp trial was conducted at RAE Bedford using a test aircraft (The first Sea Harrier did not fly until 20th August 1978). Within a year, the ramp had been tried at angles between 6.5 – 20º and the tests showed that the aircraft was indeed able to get airborne with much heavier loads. The ramp also added a considerable margin of safety, even if the ship pitched down at the moment of launch, the aircraft would still have enough height above the sea. If the engine failed during launch, the pilot had about three times the amount of time to eject than if launched off a flat deck.
In simple terms, the aircraft does not fly off the ski jump, instead, forward momentum is partially converted into vertical thrust by the ramp. Together with upward thrust generated by the vectored jet nozzles, the aircraft follows a semi-ballistic trajectory for a few hundred yards until it has enough forward speed for the wings to provide all the lift. As the aircraft moves along the ramp, there is additional force applied to the landing gear but both the Sea Harrier and F-35B were designed with heavy vertical landings in mind which often put far greater stress on the undercarriage.
The first ski jump was added to HMS Invincible while she was fitting out in Barrow and was set at a conservative 7º angle, so as not to interfere with the firing arcs of the adjacent Sea Dart missile launcher. Invincible commissioned in July 1980 and on 30 October, test pilot, Lt Cdr David Poole made the first Harrier launch from a ski jump at sea. HMS Illustrious, hurriedly commissioned in 1982 was also fitted with a 7º ramp while HMS Ark Royal, commissioned in 1985, was built with a 12º ramp from the outset. The first two ships were subsequently retrofitted with 12º ramps which was found to be the optimum angle. Improving the performance of the Harrier was deemed of much greater importance than a small limitation on Sea Dart firing arcs and the entire missile system was subsequently removed entirely to provide more space for aircraft and stores.
The ski jump is a relatively cheap and simple addition to the carriers, being a straightforward steel construction with no moving parts. However it was discovered, once in service that apparently small differences in the build quality of the ramps of the three ships affected the life of the Sea Harrier undercarriage. The original design work assumed an absolutely smooth ramp but small ruts or imperfections in the surface were enough to cause cracking on some aircraft landing gear. This issue was expensively resolved and the lesson led to higher design tolerances being specified for the QEC ramps. Additionally, the F-35 has a wide tricycle gear which is more affected by small bumps, demanding more careful ramp design than for the Harrier’s tandem main gear. The centre section of the QEC deck is slightly cambered to help water runoff, further complicating the interface with the ramp.
By 2003 the decision had been made that the QEC carriers would be configured for STOVL operations and would have a ski ramp like the preceding Invincible class CVS. The QEC design is an ‘adaptable carrier’, configured for STOVL but capable of being fitted with catapults and arrestor gear, if required at some point in the future. (The brief flirtation with CATOBAR configuration between 2010-12 is a controversial rabbit hole, well beyond the scope of this article). This meant the ski jump would be a removable structure that was added to the forward flight deck, rather than a fully integrated into the bow design of the ship (as seen in the 2002 QEC ‘Alpha’ concept).
Detailed work on the ramp design was started but in 2007, once Lockheed Martin had done enough simulator work and had developed mature flight control models. The QEC ramp was designed by BAE Systems with input from LM, rather than the shipbuilders. It is not immediately obvious but the ramp has two very subtle curves. The entry section is a long ‘cubic’ curve that leads to a second let-down or ‘ellipse’ section where the aircraft is launched.Evolution-of-Ski-Jump-Design-Royal-Navy
The next generation
Work on ski jump trials with the F-35B began in 2014 in the United States at NAS Patuxent River, initially using offline and manual simulation. Most of the work involved exploring what would happen if problems occur during take-off, such as a sudden drop in wind velocity, loss of engine power, blown tyres or nose wheel failure. A UK company, Williams Fairey Engineering Limited (WFEL) was awarded a £2M contract to construct a test ramp in the Centre Field at Pax River. The design was based on the CVS ramp profile and completed in 2009, although the first F-35B ski-jump STO was not made until June 2015.
By June 2016, 31 test launches had been made testing a variety of approach speeds and internal loads with speeds off the end of the ramp ranging from 65-95 knots. Some issues were discovered during testing but nothing serious and the results informed the minor design changes to flight control software. A second phase of trials numbering around 150 launches was begun in 2017 to understand the characteristics of the aircraft during overspeed or underspeed take-offs and carrying external weapons, including asymmetric loads. When the first jet was successfully launched from HMS Queen Elizabeth’s ski jump on 25th September 2018, years of simulations and preparation ensured it was considered a very low-risk aspect of the programme.
Despite its vertical landing and ski-ramp launch, the 21st Century F-35B Lightning II has very little in common with the Harrier designed in the 1960s. The Harrier had four side-mounted swivelling nozzles used to direct thrust down or aft. The supersonic F-35B has a more conventional jet tailpipe but is fitted with a 3-Bearing Swivel Module (3BSM) which rotates down from the horizontal to generate 18,000 pounds of downward thrust from the engine exhaust. To provide longitudinal balance, a cold air shaft-driven lift fan directly behind the cockpit also provides up to 20,000lb of additional vertical thrust during take-off and landing. The lift fan incorporates a vane box which can direct thrust upto 50º aft to provide extra forward thrust during the rolling take off. There are also roll posts under each wing which can deliver up to 2,000 pounds thrust to give lateral balance. To maximise forward thrust, the roll posts are turned off for a few seconds during the STO run and switched back on just as the aircraft leaves the ramp.
Automation has made both landing and take-off, much simpler procedures for the F-35 Pilot. A Harrier pilot had a tricky job to manually operate the stick, the throttle, and the nozzle lever with his hands during launch. Advanced control law software in the F-35 means the pilot must only push the throttle to full power, release brakes, and steer towards the centre of the ski-jump. Sensors on the aircraft detect the change in pitch rate and attitude, taking the controls into ‘ski-jump mode’ it automatically sets the horizontal tail position, engine nozzle angles, and changes the balance of thrust between the lift fan and the tail nozzle. When on the ramp, more of the forward thrust comes from the lift fan, but as the aircraft leaves the ramp the 3BSM is very gradually moved so that the engine thrust moves from about 45º to the horizontal. Once flying, the lift fan is disengaged and the doors closed to aerodynamically ‘clean up’ the aircraft.
It is interesting to observe that while the F-35B is exceptionally capable and has low pilot workload, its mechanical complexity is staggering with the greatest number of moving parts ever put into a fast jet.
The British ski ramp was quickly copied by many other navies and remains in use today. The Russian, Chinese, Indian, Italian and Thai navies all posses aircraft carriers with ramps. The Australian and Spanish Juan Carlos-class LHDs also have ramps with an eye to potential F-35B operation. In 2018 it was announced the Japanese Izumo class ‘helicopter destroyers’ would embark F-35Bs and the addition of a ski ramp would clearly be desirable.
The US Navy has never adopted ski ramps for its Harrier and F35-B-equipped assault ships, although the benefits of the ramp are fully appreciated and well understood by the Marine Corps aviation community. Unlike other nations, whose STOVL aircraft are the primary armament of the ship, the Gator Navy’s main purpose is to deliver Marines ashore and the helicopters have priority. With limited flight deck space, a ramp would take up at least two helicopter spots. The USMC concept of operations also sees the fixed-wing aircraft being sent ashore at the earliest opportunity to work in close support of the troops while flying from small airstrips. There is an unspoken political concern within the USN that the addition of a ramp might see the assault ships become perceived as small aircraft carriers, undermining the case for the giant conventional carriers (CVN) that are the centrepiece of the surface fleet. For the Royal Navy, this relatively simple invention will continue to play a key part in enabling fixed-wing operations from its two aircraft carriers for many years to come.