Gunston ‘Stealth VTOL’ Concept Assessment
Warplanes of the Future was my favourite childhood book. The cover featured an exciting vision of futuristic jet fighters. The lead aircraft was a creation of the author, the inimitable Bill Gunston. We asked Jim Smith, who was involved in the assessment of real-life advanced VTOL combat aircraft if Gunston’s delta jump-jet could have worked.
Following some recent social media exchanges, Hush-Kit’s Joe Coles has asked me to write an assessment of the Stealth ASTOVL project mooted by Bill Gunston, published in a 1980s book ‘Advanced Technology Warfare’ (among others).
Illustrations of the ‘Stealth VTOL’ concept are attached, and are of considerable interest to me since the use of the term Stealth and the form of the concept presented, suggest that it lies broadly in the period in which I was assessing ‘real’, i.e. US and UK Industry-developed ASTOVL concepts as part of a Joint UK MoD and US DoD technical assessment project.
My role in the project was to assess the concepts’ aerodynamics, and in particular to advise on issues associated with ground interaction with the airflow about concepts with differing propulsion arrangements during take-off and vertical landing (VL). The criticality of this aspect is important to note. Different approaches to providing vertical thrust can have very different outcomes in terms of stability, control, and achievable landing weight.
In our programme, we did not consider VTO in detail, mainly because there is little operational benefit. At sufficiently light weights, most aircraft that can achieve a VL could achieve a VTO, but there is generally little to be gained by operating with reduced fuel and payload when you can avoid this with a Short Take-off, especially if assisted by use of a ski-jump ramp.
The key factors involved are discussed briefly in the section that follows. This can be skipped if you want to avoid some detail, but, if you stick with it you will have a better understanding of why I think the Gunston concept has some problems.
Propulsion interaction factors affecting aircraft during a Vertical Landing
There are two principal ground-caused mechanisms which can cause difficulties in performing a VL. These are:
- Hot Gas Ingestion (HGI) also known by the US as Hot Gas Re-ingestion
- Aerodynamic Suckdown
There are also some propulsion factors that may be important:
- Ground environment impacts, typically noise, erosion and temperature
- Intake momentum drag, which can adversely affect stability and control
- Impact of propulsion system packaging on airframe layout
- Stealth considerations
- Propulsion system failure cases
Hot Gas Ingestion
As an aircraft approaches to make a VL, the propulsion system is generating high thrust, generally in the form of columns of high-velocity hot exhaust gas. On striking the ground, this gas travels outwards, and then rises upwards towards the aircraft. If it is then sucked into the aircraft’s engine intakes, the lower density of the exhaust gases due to temperature, and the potentially unsteady nature of the gas flow, can cause a serious loss of thrust, or cause the engine to surge. The severity of this problem depends on the temperature of the exhaust flows, the location of the intake, and any measures to prevent re-circulation of the exhaust gases.
As the jet-borne aircraft nears the ground, the exhaust gases rush outward across the ground surface in a high-speed outward flow under the aircraft. This results in reduced pressure below the aircraft, sucking the aircraft down toward the ground. Factors increasing the severity of this effect include high jet velocities, a low wing, and a large, flat under-surface. Measures to control this effect include the use of barriers and dams to capture the upward flows occurring between pairs (or double pairs) of nozzles, the use of a high wing, and using higher mass flow, but lower velocity jets.
Ground environment impacts, typically noise, erosion and temperature
The jet thrust which supports the aircraft when it is performing a VL is a result of the exhaust flows from the lift system. The nature of these is highly dependent on the details of the propulsion system. At one extreme would be the hot, high-velocity exhaust from an afterburning turbojet engine, while at the other extreme would be the large-scale but low-velocity flow under a helicopter rotor. Lying in between are a range of possibilities, including, listed in order of decreasing flow velocity and temperature:
- Unaugmented core engine exhaust – Harrier rear nozzle, Yak-38 lift engine
- Unaugmented exhaust from the fan of a turbofan engine – Harrier front nozzle
- Mixed Flow from a turbofan engine – McDonnel MFVT project, Boeing X-32
- Ejector-Augmentor systems – jet flows that entrain large volumes of ambient air – Northrop-Grumman concept in MoD/UK programme
- Cold fan flows – F-35B front fan, current eVTOL multi-ducted fan projects
Jet noise scales with exhaust velocity to between the 5th and the 8th power. Hot gas exhausts can damage aircraft structures, and high-velocity ground flows cause erosion and can damage equipment. All of these are dangerous to groundcrew, so current thinking is to reduce jet temperatures and velocity as far as possible, while still providing enough thrust for a VL.
Intake Momentum Drag
Intakes are designed to slow down the air flowing into the engine to allow the compressor to function efficiently. In wing-borne flight, we think of the air flowing past the aircraft, due to the aircraft’s motion, being slowed and having its pressure raised to allow the engine to work efficiently. From the aircraft’s perspective, the air is slowed from the free stream value, and the work done in achieving this manifests itself as intake momentum drag. If the intakes are ahead of the centre of gravity (which they will be), and if the aircraft is in a crossflow of any sort – perhaps due to transitioning laterally to land on an aircraft carrier, intake momentum drag can be quite destabilising, especially in yaw. This is certainly the case with the Harrier, and for a slow-moving aircraft nearing a VL, the effect is compounded by the relatively low control power available to the pilot.
Impact of propulsion system packaging on airframe layout
The measures required to manage suckdown and ground environment impacts tend to make low velocity jet flows desirable. However, the various means that have been used to achieve this generally suffer from significant penalties in terms of propulsion system volume. This is coupled with the constraint that in the hover and VL, the engine thrust has to be vertically downwards, and balanced around the aircraft centre of gravity. These propulsion factors can have a dominating effect on the packaging of the airframe, a problem which only becomes more pronounced if the airframe is also required to be stealthy, and to operate at supersonic speeds.
Without drawing on any protected information, one can observe a number of features that are common to aircraft intended to have low radar signature. These are:
- Alignment of edges and surfaces as far as possible
- Avoidance of anything resembling a corner reflector
- Screening and shaping of engine intake and inlet duct so as to avoid direct visibility of the engine front face
- Carriage of weapons and stores in internal bays
- Avoidance of opening doors and panels as far as possible, and treatment of those that sre unavoidable
- Use of conductive coatings for the canopy, and specialist coatings elsewhere
Measures will also be required to reduce Infra-red signatures. These might include, in addition to the above:
- Mixing of ambient air with exhaust flows to reduce temperature
- Screening of hot components, and managing cooling flows ao as to avoid hot exhaust flows and airframe hot spots.
Propulsion System Failure Cases
Failure of lift system components is likely to be a critical event for any aircraft performing a VL. However, the degree of criticality does vary. For example, engine failure for a multi-engine helicopter may be an inconvenience rather than a critical flight emergency. On the other hand, failure of one of the lift engines of a Yak-38 results in rapid loss of control, the impact was sufficiently severe that the aircraft was fitted with an auto-eject system.
The Gunston Concept
So, what does all this mean for the Gunston concept?
In the language of ASTOVL assessment, the Gunston design would have been characterised as (Advanced) Vectored Thrust. I have enclosed ‘Advanced’ in brackets here, because it is not clear whether the design is intended to be supersonic or not. If it is designed to be supersonic, then the engine would need some form of plenum chamber burning (PCB) – an afterburner for the front nozzles of a four-poster vectored thrust lift-engine. If this is not used, then the design is essentially a re-packaged Harrier.
Looking at the general issues I have identified, I’ll offer some views on each, as applied in the Gunston design.
Hot-gas Ingestion might not be too much of an issue, although a forward dam might be needed to limit flows from the front nozzles reaching the intake. The likelihood is that the front nozzles will be discharching fan air flow, which would be relatively cool, unless PCB is to be used, in which case HGI would be a significant issue due to the intake being close to the front exhaust flows and the high temperature of the exhaust.
Suckdown would be a major problem for this concept, and possibly more difficult if PCB is employed. The low wing, the short undercarriage, and the location of the rear nozzles under a large flat plate (the wing) will all increase the degree of difficulty. A box-shaped barrier ahead of the rear nozzles, with sidewalls extending forward to the HGI dam would be required to capture the fountain effect likely to be formed between the front and rear nozzles, presumably deploying when the undercarriage is lowered. The problems will be compounded if PCB is used, because of the high temperature of the front jets. One of the drawings appears to show doors in the relevant area of the fuselage, but these may be to allow engine removal for maintenance.
Ground environment impacts, noise, erosion and temperature impacts will be highly dependent on whether PCB is used. If it is not, the aircraft may be regarded as a repackaged Harrier, albeit with additional suckdown problems due to the low wing and short undercarriage, and HGI problems due to the proximity of the front nozzles to the intake. If PCB were to be used, my first concern would be that the system was never really proven to work. Setting that on one side, and assuming a working system, the augmented jetflow under the aircraft would probably present insuperable problems in all of the three elements noted.
Intake Momentum Drag is specifically mentioned as a factor to be managed using the rotating reaction control valve in the tail. Intake momentum drag was an issue to be managed carefully in the Harrier, as it significantly destabilised that aircraft in yaw, and I assume this is why it was raised for this concept. I am not convinced it would be as great a problem for the Gunston design, but the combination of the lack of a vertical fin, and the possibility of strong vortices being shed from the slender fuselage nose, do suggest that the RCV may well need to deal with both pitch and yaw control separately, rather than by simple rotation.
Impact of propulsion system packaging on airframe layout This is a particular problem for AVT designs, because the engine has to be located so that the exhaust thrust is balanced about the centre of gravity to enable a vertical landing. As a consequence, the engine ends up in the centre of the configuration, rather that, for example, towards the rear of the aircraft, which would be typical for a Conventional Take-Off and Landing (CTOL) aircraft. Knock-on consequences of this are the inevitability of a relatively short intake, and difficulty in packaging other elements of the aircraft, including weapons, fuel and other systems. These constraints also make aircraft development in service difficult – when an improved radar was fitted to produce the Sea Harrier FRS 2, the fuselage had to be lengthened so that additional avionics elements could be located to the rear of the engine installation to better balance the aircraft.
Stealth Considerations are perhaps the most challenging aspect for the Gunston concept. I am assuming that measures such as gold-flashing the cockpit, and the use of LO treatments in the intake system will prove adequate. The short intake duct, and its positioning on the upper surface, may cause engine integration issues, but let’s assume these are able to be resolved, particularly since the design is clearly intended as a strike aircraft, rather than an air superiority fighter. These are not the main problems.
There are three key issues, the first of which is that the only practical location for the weapons bay is well behind the centre of gravity of the aircraft. We know this has to be essentially midway between the vectored jet nozzles. Carrying four stores in the weapons bay will move the centre of gravity well aft, and will pose significant problems for the flight control system. Let’s assume that this can be resolved for wingborne flight through the use of a full-authority computer-managed flight control system like that used on Eurofighter. Even with this assumption, there remains the problem of landing with the centre of gravity well aft of the centre of thrust. This, of course, will be managed with the swivelling reaction control valve (RCV), I hear the reader say.
Well, maybe, but the geometry suggests the thrust required to be approximately half the weight of the weapons remaining in the weapons bay. If the weapons weigh 4000lb (1814 kg), that RCV has to deliver 2000 lbf (8.9 kN) which is a big ask, and does not seem likely to be available as a bleed flow from the engine. This is the second significant issue, because committing to release all weapons on every sortie is implausible. If a high and continuous thrust is required from the RCV simply to trim the aircraft in the hover, it is also not evident how the proposed RCV can also be actively controlled to manage the aircraft in yaw, particularly since the design may well be directionally unstable.
The third issue is that the underside of the aircraft is continuously bathed in the hot exhaust flows from the propulsion nozzles. In consequence, there can be no way to visualise this concept as having low observables in the infra-red. Were PCB to be used for the forward nozzles, of course, IR signature would be a yet greater challenge.
Propulsion System Failure Cases are likely to be no worse than for other single-engine STOL and STOVL designs. Control system failures would be more critical than aircraft like the Harrier as this aircraft appears likely to be both longitudinally and directionally unstable.
Examination of the Gunston concept is interesting, not least because it provides insights into the difficulties faced in packaging a stealthy strike aircraft around a four-poster propulsion system, like that of the Harrier. Regrettably, the problems identified in delivering a VL capability from this concept do appear insuperable.
However, the concept, if re-imagined as a CTOL aircraft, and unmanned, appears remarkably prescient and foreshadows designs such as the Boeing X-45, Northrop-Grumman X-47, BAE Systems Taranis and Dassault nEUROn unmanned aircraft.
In a CTOL configuration, with the engine moved to the rear, and stores, sensors and fuel packaged around the centre of gravity all of the issues identified with the concept as a stealthy VTOL platform would be resolved.