Creating a supersonic stealthy vertical take-off fighter is an extremely difficult task, and years of studies —and a healthy handful of initialisms and acronyms — paved the path to today’s F-35B. Jim Smith describes his role on the ASTOVL project and the challenges it faced.
What and when was ASTOVL, and what was your role on the project?
“The Advanced Short Take-Off/Vertical Landing (ASTOVL) programme evolved over a long period, from the early 80s through to the JSF programme as we see it today. While the initial intent was to examine the issues associated with the development of supersonic combat aircraft which could take-off in a short distance, and perform a vertical landing, the objectives evolved significantly over time, culminating in a development programme to provide a 5th-Generation stealthy Strike Fighter for the USAF, USN, USMC and International partners and customers.
During the period of the ASTOVL programme my career path took me in and out of the programme, and the best way of answering these two questions appears to be to reflect my involvement against the changing programme focus and direction.
“I first encountered the programme in the form of the Joint UK-US ASTOVL programme which was an effort to examine the merits of different possible propulsion systems for ASTOVL concepts. Four US and four UK concepts were examined by a team called the Joint Assessment and Ranking Team (JART) in at NASA Goddard in October 1988. My role on the small UK team was to lead on aerodynamics (including ground interaction) and configuration. Other specialists looked at the mission and flight control systems and technical aspects of the propulsion system. US participation was extensive and included USAF, Army and Navy, but with NASA providing US team leadership and specialist input. My responses below will focus on this period, partly because most readers will be less familiar with the activity, but also because these designs were a long way from the operational system of today, and are perhaps less sensitive as a result.”
“Following a period in the British Embassy in Washington, I worked in Future Systems (Air) leading aircraft weights and performance analysis. The ASTOVL programme had evolved substantially, with DARPA and the US Navy both heavily involved in programmes looking at a STOVL Strike Fighter (SSF) programme, which evolved first into a Common Affordable Light Fighter (CALF), and eventually in 1993 into the Joint Advanced Strike Technology (JAST) programme. The UK was still engaged in the programme, leading to my involvement in configuration analysis of various designs, and, later weights analysis of (mainly) Lockheed concepts, in partnership with NAVAIR, leading towards the JSF EMD phase. I was a member of Integrated Product Teams in the US programme covering Aerodynamics, Performance and Integration, and Aircraft Weight.”
“Through this period, the focus had shifted from consideration of ASTOVL technical issues to harmonising a set of designs that could plausibly meet the needs of the USN, USMC, USAF and RN. The RN interest was, of course, replacing the Sea Harrier, and at this time the aircraft was very much constrained by the need to operate off the Invincible Class carriers then operated by the RN.”
“In 1995, the UK signed up as a formal partner to the US JAST programme, which in 1996 morphed into the JSF development effort, and this coincided with my move to the Central Staffs, where I was looking to ensure Defence proposals were evidence-based, and that scientific, technical, risk and programme aspects were all well covered in the process. My direct involvement with the JSF programme diminished, but I was still involved in assessing the suitability of the concepts emerging from JSF for meeting RN needs. At this time, the focus was still only on the Navy, and only on operating from the small Invincible-class carriers.”
“In 2002 I moved to DSTO in Australia to lead the Air Operations Analysis branch. The day before I arrived, the Australian Government announced their intention to procure JSF, and that they intended using the F-35A variant to replace both the F-111 and F-18. While I maintained a strong personal interest in JSF, I had little direct involvement from that point.
The UK subsequently decided to invest in larger carriers, but have maintained their choice of the STOVL variant for both the RN and RAF.”
Which designs did you study?
“I must open here with an admission that all this was thirty years ago, and I don’t feel comfortable that I could describe the various contenders in detail. Instead, I will focus on the propulsion systems, which was in any case the primary topic for study by the JART team.
UK concepts were developed by the four combat aircraft design teams then in existence, from Brough, Kingston, Warton and Weybridge. The US designs were developed by Lockheed, McDonnell Douglas, and I think Northrop, and Grumman.
Although there were four propulsion system designs considered, they were not quite the same. The UK concepts were:
Remote Augmented Lift System (RALS)
Tandem Fan (TF)
Ejector Augmentor (EA) and
Advanced Vectored Thrust (AVT) Kingston
The US systems were:
Mixed-Flow Vectored Thrust (MFVT) (McDonnell)”
Propulsion system Issues:
“Vertical Lift aircraft experience two main phenomena which can reduce available VL weight and cause issues in control in the hover – Hot-Gas Ingestion (HGI), and Suck-down. HGI causes an increase in the temperature of the air at the engine, and a decrease in density, resulting in a loss of thrust. Suck-down is a result of the high-speed exhaust gases flowing under the aircraft fuselage and wing, and increases as the aircraft nears the ground. This is why VL aircraft generally seem to ‘drop’ the last couple of feet as they land – they are being pulled down by these local flows.
HGI is best avoided either by using relatively cool flows, for example by mixing cool fan air with the hot exhaust gases, or by physically separating the hot flows as far as possible from the engine intakes. Suck-down may be reduced by the use of a high wing configuration, and blockers or dams under the aircraft to capture and contain the flows – both of these are readily apparent on the Harrier. Lower exhaust velocity will also help.However, note that use of relatively cool low-velocity jets will impact severely on landing weights unless the mass flow is greatly increased, for example through a supplementary lift fan, or a flow entrainment device such as an ejector-augmentor.
Propulsion systems considered by the JART:
RALS systems essentially used thrust augmentation (afterburning) to increase jet velocity and thrust at the rear nozzles. Disadvantages included difficulty in keeping the hot exhaust gases away from the engine inlets so that hot gas ingestion (HGI) or re-ingestion as the US call it, could be avoided, and the greatly increased pressure and temperature loads on landing surfaces. Depending on configuration, the high-velocity exhaust sheets could induce significant suck-down, although a beneficial fountain effect can also occur with careful management of the exhaust flows beneath the aircraft.”
“Ejector-Augmentor systems duct air from the mixed flow at the rear of the engine forward, and use an ejector system to entrain cool ambient air, increasing flow mass and reducing jet velocity, temperature and noise. These attractive advantages are offset by the complex systems required, by the volume of the fuselage taken up by the ejectors and exhaust ducting, and by the rather unproven nature of the system.
Tandem-fan solutions use the flow through the propulsion system to produce lift at the rear nozzles, and use a clutch and shaft system to drive a forward fan in the fuselage behind the cockpit. In the configuration considered by the JART the Lockheed TF was horizontal, and a complex system of doors were required to provide the flow path for the fan. Significant risk was seen in the clutch and shaft system.”
“The Mixed-Flow Vectored Thrust concept took the relatively cool flow from the fan, mixed this with the jet exhaust and then split the flow between front and rear nozzles to provide balanced lift. This offered the prospect of manageable HGI and suck-down, but with the need to accommodate the necessary ducting to the front nozzles.
The Advanced Vectored Thrust design was essentially a Plenum-chamber-burning (PCB) version of the Harrier concept, offering much greater thrust in the lift and forward flight modes, but with all the challenges of the RALS concept, coupled with doubts about the viability of the PCB concept.”
How would have the best ASTOVL concept compared with the F-35B?
“The real challenge faced by ASTOVL design teams was the management of suck-down, hot gas ingestion, ground erosion and control in the VL phase. But an additional challenge was introduced as the US Stealth programme was revealed to public gaze in April 1990, with the unveiling of the F-117 programme. A clear driver for JSF is the requirement to ‘do all that stuff and be stealthy too’. On top of which commonality of airframe configuration was sought for land-based, carrier-based and STOVL variants.
Without considering stealth, the JART took the view that the RALS and AVT solutions were going to have too much difficulty in managing HGI and ground interaction, and that the bulky packaging and unproven technology of the EA solution was also too risky. The most basic consideration of signature (in hindsight) would be another reason for ruling out the AVT concept, with its short intakes and rear fuselage bathed in hot exhaust gases. Neither the AVT nor EA concepts really provided any scope for internal weapons bays, although this was again not explicitly covered by the JART analysis.”
“The concepts that emerged as most promising were the Tandem Fan and Mixed Flow Vectored Thrust solutions. Compared to JSF, the TF design was longer, because of the horizontal (rather than vertical) shaft-driven fan. It had a simpler drive system, as this was aligned with the propulsion system, but more complex ducting. Also, I believe it had a rectangular nozzle rather than the JSF design, which originates with the Yak-141. The MFVT design by McDonnell Douglas was a very neat configuration, and looked much more attractive than the eventual Boeing X-32.”
Who paid who for these studies?
The JART work was covered by a UK-US MoU. Under MoU conventions, generally each party carries their own costs, so UK studies would be covered by the UK, and US studies by the US. Later, when the UK formally joined the JAST, a financial contribution to the programme was made by the UK.
What was good and bad about the project?
“Technically, the programme was terrific. Good people from both UK and US worked hard to understand each other’s concepts and to come up with an assessment process to which all could agree. The relationships forged in the process were of enduring benefit in my subsequent posting to the British Embassy in Washington, and my later engagement with CALF/SSF/JAST.”
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If I have to identify a weak point, I think we could have done more in advance to formalise the assessment process. I also considered the US approach to technical risk at that time to be relatively primitive. There was an in-built assumption (particularly in Industry) that everything could be solved with time and money, and no real appreciation that system integration was also a source of risk.
With hindsight, the programme would have been better had stealth been explicitly considered, but at the time, US activities were still in the ‘Black’ world, and information would certainly not have been shared with UK Industry.”
Is STOVL a necessary feature for a combat aircraft? Or was there a cultural bias from the USMC and the Royal Navy?
“It depends what your requirements are. The USMC has long sought to deliver rapid reaction strike to support ‘troops on the beach’, and also operates from relatively small carriers – these factors drive it towards STOVL operations. At the time of my involvement, the RN also had small carriers, and at that point could not see a path to the sort of capability it has today.
So, I am not sure that STOVL is actually fundamental for the RN or RAF. It appeared to be so at the time, and certainly still is for the USMC. There is a penalty in range and complexity that comes with STOVL.”
What the hardest thing for the designs to achieve?
“Vertical lift, while also being supersonic. When you add in the tri-service dimension and stealth (which were not part of the JART requirements), you can see that the JSF EMD programme, which required three variants to be demonstrated, to fly supersonically, to meet signature requirements, and to release weapons, was a good stab at reducing what were seen to be the main technology risks.
Unfortunately, US risk processes did not really consider (in the early days at least) the enormous challenges that would be introduced in generating, clearing and integrating the vast amounts of software (and hardware) required to turn the demonstrators into weapons systems.
Which was the worst concept and why?
It’s a tie between the RALS, EA and AVT concepts. There are real doubts that any of them could have been developed into a practical system. AVT and EA would have had additional challenges in meeting a low signature requirement.
If a new STOVL fighter was made today which propulsion concept should it use?
“I’d want to start with studies around the Tandem Fan and Mixed Flow Vectored thrust propulsion systems, and then look hard at the packaging and configuration considerations which will be driven by mission system, point and mission performance and signature requirements.
But I’d also want to see what the pros and cons were of a conventional aircraft designed to similar requirements, just to understand the cost-capability trade-off between STOVL and conventional designs.
Careful consideration of the requirements would be essential, to ensure that the scenarios chosen and the mission, performance and other aspects specified were reasonable, considering future geo-political and technology developments.”
What are the strengths and weakness of the F-35B’s propulsion/lift system?\
“I think the F119 is a first-class engine. The STOVL system appears to be complex, but to work well, although I have not been close enough to the programme to have any real knowledge of this. The control system appears to be notably good in the hover, and to have benefitted from the substantial technical engagement between the UK and US in this area. Creeping vertical landings are clearly a good way of increasing the bring-back mass of the aircraft, just as the use of a ski-jump increases the take-off mass available.
I have not identified any weaknesses, but would observe that the support system for the aircraft looks to be a bit of a challenge, and operators will need to work out carefully how capability and readiness is to be assured at a manageable cost. In the early stages of the programme, weight was a significant challenge, particularly for the STOVL variant. As aircraft endure in service, they tend to grow in weight, and it is likely that a continuing watch will be required on weight, and on structural load and fatigue margins.”
What should I have asked you about ASTOVL?
“I think I have given ASTOVL (as in the UK-US work of the late 80s) a fair go. There are many topics you could have asked me about on the JSF programme, but none that I am able to address.”
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