Is Skyhook Viable

SkyHook is a BAe invention, protected by worldwide patents and as a registered trade-mark in the US. The basic concept was originated by Heinz Frick, one of the company test pilots concerned with Sea Harrier flight trials. Frick was impressed by the ease with which the Sea Harrier could be hovered with great precision, regardless of wind variations. If this accuracy in the hover could be combined with a space-stabilized “skyhook”, then there might be the basis for a new means to recover a V / STOL aircraft to a highly agile deck. If the design of the ship precluded any form of ski jump, then the aircraft could presumably take off from the advanced technology crane used to recover it, although disposable load would clearly be restricted by the aircraft’s limited VTO weight.

The notion undoubtedly had both limitations and technical problems, but it offered the prospect of a system that could bring fixed-wing air power within the reach of navies that could not afford a “conventional” carrier for V/STOL aircraft. Another attractive feature of the concept was that it required very little modification to the aircraft itself, the basic need being for an attachment point above the CG, with local strengthening to take the full weight of the aircraft. If the operator wanted the capability to “hot refuel” on the hook, this would mean changes to the fuel system, but the development funds required would still be fairly modest.

Looking at the feasibility of SkyHook on a step-by-step basis, the first problem is to produce an advanced technology crane with a space-stabilized head. In principle, this can be achieved by building something akin to an inertial platform into the base of the crane, feeding data on ship motion to a fast-acting hydraulic system that produces compensating movement of the crane.

The preliminary design of the SkyHook crane has been a combined effort by BAe, Stothert & Pitt, and Dowty Boulton Paul, the last two companies being responsible for the structure and hydraulic system respectively. The aim from the outset has been to enable a V/STOL aircraft to operate from a relatively small ship in any sea state in which conventional fixed-wing operations would be possible from a carrier. This was taken to be sea state 6, which even in the northern North Atlantic would allow uninterrupted operations for 97 per cent of the time.

The computations were basE:ld on the RN’s 4,250-ton Type 42 destroyer (Sheffield class), since it was a suitable size, and data was readily available. Factoring the ship’s response in average sea state 6 conditions gave a design point corresponding to the worst wave in 2,000, for which the system was to provide full compensation. At the design point, the ship is heaving through a total distance of 8. 8 metres, rolling through an angle of 30 degrees and pitching through 12. 25 degrees. To recover under the more severe conditions experienced, transiently the aircraft would have to hover alongside for a minute or two, until the ship’s motion could again be compensated by the crane and the head movement eliminated.

The SkyHook is designed to recover an aircraft weighing up to 13,600 kg, hovering 15 metres above sea level and a similar distance out from the side of the ship. If the SkyHook were to be located at one of the forward corners of the helicopter pad of the Type 42 destroyer, the various ship motion components would combine to move the base of the crane through an elliptic envelope some 10. 7 metres high and 5. 2 metres wide. The structural weight of the crane has been estimated at around 1 0 tons, excluding the reinforcement of the ship. The accuracy of crane head stabilization is estimated at 15cm. Vertical motion compensation requires a peak power of roughly 750 hp, and lateral motion a peak of around 1,300 hp. Fortunately, the two motions are out of phase, hence power demands are not additive. Boulton Paul reckons that, using regenerative techniques, this type of SkyHook would require a continuous input of only 500 hp.

Assuming that there are no insuperable problems in the development of the SkyHook crane, the next challenge is to ensure that the aircraft can really be hovered with the necessary accuracy, bearing in mind the errors in crane-head stabilization and the likely limitations on the engagement system that is used to take out the positional differences between the aircraft and the crane head.

As a starting point BAe reckons that, given good visual cues, the pick-up probe on the hovering aircraft can be positioned within a cube of three-metre sides, which in practice would be centred about 4. 5 metres below the crane head. One of the key elements in the SkyHook system is clearly the development of a display system that will enable the pilot to judge positional errors relative to a space-stabilized point some distance below the crane head. The display proposed is referred to by BAe as a Pilot Hover Slight (PHS). It would be attached to the crane head, and abreast of the cockpit. The PHS consists of a pair of “goalposts” and a three-part arrow, with the two outer parts set forward, so that the three parts are in line with each other and just bridging the gap between the goalposts when the hover position is correct.

To date, BAe appears to have made very little investment in the SkyHook concept, but some preliminary hover tests have been run at Dunsfold Airfield in Surrey, with two types of Harrier and three different pilots. For these trials a hover sight as described above was mounted on a fire brigade turntable, adjusted to Suit a distance of nine metres from the pilot’s eyes, and positioned 15 metres above the ground. In winds gusting up to 25 knots (45 km/hr) and veering up to 60 degrees off aircraft heading, it was found (both with and without auto stabilization) that a hover position could easily be maintained for tens of seconds within 0. 6 metres of datum. Extrapolating to the design conditions, it was felt by the pilots that the planned three-metre hover cube could easily be achieved and that the separation between the pilot’s eyes and the PHS could safely be reduced to around 7. 5 metres.

Once the aircraft is hovering within the “capture envelope”, an automatic system is required to complete the engagement. The system proposed by BAe for SkyHook is one designed by British Robotic Systems, using an IR-absorbing patch bonded to the upper surface of the aircraft around the pick-up probe. An IR system is proposed in order that the aircraft can be illuminated for recovery at night, without adverse effects on pilot vision. Having estimated the aircraft’s positioning errors, the system would lower the lock-on jack in the crane head toward the aircraft’s pick-up probe, any small terminal errors being eliminated by a funnel on the jack guiding the probe to engagement. Initial lock-on is to be indicated to the pilot by a series of lights or by R/ T. He would then reduce power by 5-10 per cent fan RPM, while controlling aircraft attitude by means of the normal reaction control system. Sensing the weight of the aircraft on the SkyHook, the system would raise the jack to lift the aircraft into a docking cradle formed by four pads.

If the object is simply to refuel the aircraft, the engine is to be left at idle while fuel and water transfer takes place. If a normal turnround is planned, the engine is shut down, and the crane head is gradually changed from space-stabilization to deck- stabilization as the aircraft is swung inboard and lowered either on to its undercarriage or to trestles anchored to the deck. The possibility of eliminating the aircraft’s undercarriage was seriously considered since it would have saved around 350 kg of empty weight, but the idea was eventually rejected in view of the loss of the aircraft’s flexibility of operation. On the other hand, the trestle concept was retained since it offered the prospect of take-off and recovery in extremely severe sea states, when deck-handling crews might not be able to secure the aircraft. For the longer term, trestles also hold out the possibility of an automated turnaround in a contaminated environment. They also permit the carriage of a longer centre-line store, such as Sea Eagle.

For the launch phase, the aircraft would be positioned on the flight deck, either on its wheels or on trestles, and the robotic engagement system would provide a lock-on. The SkyHook would then swing the aircraft over the ship’s side, the crane- head gradually changing from deck-stabilization to space-stabilization. The engine would next be started and the pilot would complete his cockpit checks. The lock-on jack would lower the aircraft clear of the four docking pads, the pilot controlling aircraft attitude by means of the reaction controls. He would then select disengagement, and the jack, sensing an upload, would quickly retract, leaving the aircraft in free-air hover.

Operational studies by BAe have produced an estimate of two minutes from an aircraft hovering alongside to the completion of recovery, with the aircraft secured on deck. In refuelling on SkyHook, flow rates of around 500 kg/min are predicted, providing complete replenishment of the Sea Harrier’s internal tanks within five minutes. In launch operations, the studies suggested that a ship with a pair of SkyHooks could have four aircraft air-borne within two minutes.

Operational Use

As indicated earlier, one obvious limitation of the SkyHook technique is that the aircraft is restricted to launching at free-air hover weight, which is far less than STO weight, though slightly better than a Sea Harrier can achieve in VTO from the deck. Free-air conditions reduce hot air recirculation to the intake and induce downwash over the airframe, which may be worth several hundred kilograms. The absence of “ground effects” is, in fact, the first advantage of SkyHook claimed in the patent, the others being the ability to recover to a highly mobile deck, and the reduction in ship cost (since aircraft-related deck space can be limited to that for storage, maintenance and rearming). However, it is also worth bearing in mind that a Harrier hovering 15 metres above the sea is going to produce a considerable amount of spray, and the high-energy jets of the next generation will be far more spectacular.

Weight and performance data for the various members of the Harrier series have never been published in full, but it is possible to make a rough estimate of the disposable load (ie, fuel and armament) that could be taken from a SkyHook launch, and re- late it to the value achieved in STO. If the sales brochures are to be believed, both the Sea Harrier and the AV-8B have a basic weight plus pilot of approximately 6,000 kg. A very good guide to free-air hover weight is provided by the VTO weight of the AV- 8B, ie, 8,700 kg, since this Harrier II has special lift-improvement devices (LIDs) to eliminate ground effects. Either type could therefore leave a SkyHook with a disposable load of around 2,700 kg. For comparison, the maximum STO disposable load for a Sea Harrier is around 6,000 kg, and for an AV-8B this figure would be reduced to 36 per cent, since the STO weight of an AV- 8B is much higher. In short, SkyHook appears to offer less than half the warlord-radius performance of an STO ski-jump launch. It follows that if SkyHook is to be worthwhile, then either the aircraft that uses it must have very attractive maximum warlord- radius characteristics (ie, it has to start out looking good, before its performance is limited by SkyHook), or special operational circumstances must exist, in which very little warlord, radius or endurance is required. For example, it is generally accepted that providing standing patrols for air defence some distance from a carrier requires a vast effort, which might make the idea of placing a Sea Harrier on the radar picket ship quite attractive. It has been said that it takes no less than 16 F-14s to keep one continuously in the air at a radius of 200nm. For a navy consider- ing procurement of Sea Harriers, the size of the force needed to maintain a singleton on standing patrol may be (at a guess) only eight aircraft, but it is nonetheless far more expensive in initial and operating costs than buying two or three and basing one temporarily on the radar picket, using SkyHook.

The Falklands Conflict of 1982, though demonstrating the Sea Harrier’s ability to combat successfully much faster aircraft, also showed up the difficulties in achieving an effective combat air patrol system. With the RN carriers held back 200 nm to the east, so that Argentine bases were around 600 nm away, the Sea Harriers had only five or ten minutes on station at low level in the amphibious landing area. The fact that they shot down 23 Argentine aircraft without loss in air combat says a great deal for the quality of the RN and RAF pilots flying the Sea Harriers, the maintenance crews who achieved an incredible availability level despite appalling conditions, and the kill probability of the AIM- 9L. If the Sea Harrier could reach a firing position, the Argentine aircraft was almost as good as dead, but the Sea Harriers were spending too long in transit to provide anything like continuous air cover. If four Sea Harriers could have been based on the radar pickets throughout daylight hours, the attrition of Argentine aircraft might well have been transformed into a massacre, and the Task Force might not have lost two destroyers, two frigates. a container ship, and an LSL.

In this context, it is relevant to recall that in the last few days of the conflict a significant improvement was effected by the opening of “HMS Sheathbill”, some 250 metres of aluminium plank- ing forming a short runway near the amphibious landing area. This boosted the Sea Harrier’s flight time in the Argentines’ target area to around 30 minutes, since it could remain on patrol down to a very low fuel state. It could then land and refuel, and stay in the air for an extra 20 minutes before returning to the carrier. This concept of operations was undoubtedly far more effective, though the improvement was limited by the lack of rearming facilities at the strip, and by the fact that the strip was available for only nine days out of an air war that lasted 45. It is also arguable that an even bigger improvement would have been achieved if the Sea Harriers could have been brought forward to the picket ships, rather than based in the target area. The Falklands case is a striking example of the potential gains from forward refuelling facilities, in that without “Sheathbill” the Sea Harriers were on station for around seven minutes in a 70-minute sortie. In a very low-intensity conflict, with the Sea Harrier simply held on the ground at the forward site, the improvement in cost-effectiveness relative to a normal standing patrol from the carrier would presumably have been even better than this four-fold gain.

A more general case in which SkyHook may be assessed is that of defending a carrier by means of an aircraft based on a picket ship. For the purpose of a comparative study. BAe took the picket to be 120 nm from the carrier, and positioned on the estimated threat axis. A Backfire bomber was assumed to be heading directly toward the carrier at Mach 1. 3 and 30,000 ft, tasked with launching an air-surface missile at a separation of 180 nm, this figure presumably being limited by the radar horizon from the aircraft’s cruise altitude. The carrier was taken to have no AEW facility, and the radar picket vessel was given a detection range of 230 nm. Sea Harriers were assumed to be launched at two and four minutes from target detection.

Using AMRAAM, which is part of the proposed Sea Harrier mid-life update programme, the first launch would give target intercept outside of ASM range from the carrier, for any intruder approaching within 35 degrees of the threat axis. For the second Sea Harrier (ie, a four-minute launch delay), this angle would be reduced to around 27 degrees. In short, provided that the threat axis can be estimated reasonably accurately, then a pair of Sea Harriers based on a picket ship at 120 nm along this axis can make an economical yet effective contribution to the defence of the carrier. Maintaining a CAP (combat air patrol) above the picket ship would be at least as effective, but would require the use of many more aircraft, and would be far more costly.

Aside from the outer air defence role for carrier protection, BAe argues that SkyHook makes sense in anti-shipping strikes, using a centreline BAe Sea Eagle missile. Detailed performance figures are not available, but the company claims that a target can be hit at a substantially greater radius than with a ship-ship missile in the same size category. Going back to the weight figures derived earlier in this discussion, if the disposable load in a SkyHook launch is around 2,700 kg for a Sea Harrier or AV-88, and if Sea Eagle weighs 600 kg, then the fuel load available must be approximately 2,000 kg. For an aircraft of this size, that amount of fuel should be good for a radius of about 200 nm in a HI-LO mission, with the Sea Eagle range taking the effective strike radius to perhaps 250 nm.

The Way Ahead

On BAe estimates, it would take something in the region of $12m to fund the complete development of the SkyHook system. The company appears to be concerned in feasibility terms mainly with the robotic system that will provide automatic engagement, once the aircraft is hovering within the “capture envelope”. Within two years, BAe would expect to be demonstrating the system’s ability to compute relative aircraft position and successfully engage, though at that stage trials would be limited to an unstabilized land-based system on a “cherry-picker”. Given two more years, sea trials would be mounted with a fully- stabilized SkyHook in containerized form.

The idea of developing a SkyHook suitable for installation on a container ship appeals to BAe, since it would facilitate trials and result in a saleable package. However, discussions with Vosper-Thorneycroft have also led to the preliminary design of a 5,300-ton escort carrier with two SkyHooks, five Sea Harriers, and one helicopter, which would use a conventional landing pad at the rear. in this case the plan is to provide a 75-metre skeletal ski jump on top of the hangar and between the two funnels, thus overcoming the limitations of a SkyHook launch. The ship would have a maximum speed of 26 knots and a range of 4,500 nm at 16 knots.

This writer’s own view is that modern technology can overcome the development problems of the SkyHook concept, but that restricting the aircraft to a SkyHook launch would at present have such an adverse effect of warlord-radius performance that the idea is unlikely to find support. The sort of STO/VL carrier proposed by Vosper-Thorneycroft would largely overcome this objection, although the length of the skijump proposed is less than the desired size, if the maximum take-off weight of the aircraft is to be exploited. One problem with any such proposal is that unless the RN funds SkyHook development, it appears unlikely to go ahead at the present stage. The indications are that BAe is insisting on acting as prime contractor in SkyHook development (rather than licensing an overseas company, such as Mitsubishi), yet is unwilling to use its own funds for this purpose. One might suspect that this is due to reservations on the size of the potential market, rather than on the feasibility of the scheme. At present, it is difficult to judge the probability of RN support being forthcoming, due to the reorganisation in Whitehall and the general shortage of UK defence funds. The basic concept undoubtedly offers the prospect of spreading fixed-wing air cover over large areas of sea at a relatively low cost. For political reasons, it may be especially attractive to Japan. It may also be attractive to countries such as Italy and Spain, in providing an air defence ring for a carrier. However, unless BAe can find a customer willing to invest the necessary front-end money to establish SkyHook’s feasibility, the idea could easily sit on the shelf until the patents expire and VISTOL aircraft have advanced to such a stage that VTO performance is no longer such a limitation. Or perhaps until the Russians have tried it?