What can we learn from Skunk Works’ Darkstar in Top Gun?

The Darkstar from Top Gun Maverick – How close could this be to reality?

Hush-Kit has asked me to have a look at the cinematic hypersonic fighter, Darkstar, shown in various teaser clips and publicity material for the Top Gun Maverick movie. The movie version gains an added cachet because Lockheed-Martin are stated to have assisted with the design concept, and with the construction of a full-sized mock-up for filming.

The movie Darkstar is not to be confused with the real Lockheed Martin Darkstar, an unsuccessful high-flying UAV reconnaissance platform of the late 1990s, but does bear a striking resemblance to renderings prepared by Lockheed-Martin of their ‘SR-72’ concept. So much so, that this article will look at the SR-72 renderings as indicative of the Darkstar movie concept, and consider how close to reality the Darkstar, and by implication, the SR-72 could be.

Clearly, any publicly available rendering of the ‘SR-72’, and the movie Darkstar, is not going to be a faithful representation of ‘the real thing’, and indeed, the programme is unlikely to be delivering ‘the real thing’ just yet. Consequently, this article is speculative. It looks at the material that is out there, on the net and open-source, and at the technologies that might be required to realise the stated objectives, noting that these may themselves be plausible rather than accurate.

Configuration

So, what can we observe about the SR-72 concept? The rendering shows what we can infer to be a large twin-engine slender delta aircraft, with a single fin. The engines are mounted below the wings, on a basically flat undersurface. This surface consists of a slender triangular forebody, with highly-tapered wings, located about halfway along the fuselage. The wings have approximately 60 deg leading edge sweep, and a 20 degree forward sweep on the trailing edge.

Statements about the Darkstar refer to it as being ‘hypersonic’, and the SR-72 is also stated to be intended to achieve a maximum speed of about Mach 6 at high altitude. As might be expected for a high-speed vehicle, the propulsion system consists of two large nacelles, but, unlike the slower SR-71 Blackbird, these nacelles are rectangular in section rather than circular, and lack the inlet cones which are distinctive features of the Blackbird.

High-speed intakes are a specialist subject in themselves, but the shape of the Darkstar/SR-72 inlets suggest that the intake shock structure is managed by variable ramps in the inlet, as opposed to the translating shock cones of the Blackbird. For more on supersonic intakes see my article here.

The exhaust system extends well behind the trailing edge of the wing, and, perhaps unsurprisingly, is not shown in any detail in the rendering, but appears to be contained within a rectangular shroud.

Statements about the SR-72 suggest that its role is to deliver rapid, reactive strike, on a global scale, at short notice, a very different role to that of the Blackbird. A possible reason for this is the apparent preference of the USAF to consider persistent, stealthy, high-flying unmanned platforms as future ISR and communications nodes, as indicated here. That said, the renderings do not reveal and stores carriage, weapons bay or targeting sensors, and the ‘SR-72’ is clearly depicted as unmanned.

In contrast, the movie Darkstar, no doubt driven by the imperatives of the script, is a manned aircraft. The teaser clip appears to include a laser weapon, suggesting, as might be expected from the nature of the film, an air combat role.

An interesting feature of both concepts is that the slender triangular forebody is followed by an essentially parallel-sided centre and rear fuselage, this starting ahead of the wing leading edge. In planform, then, there are two discontinuities: at the rear of the slender blended forebody; and at the junction of the wing leading edge. Moreover, while the forebody appears to lie within the Mach cone which would be generated from the fuselage nose at Mach 6, the wings will extend beyond this, which has implications for aerodynamic heating and wave drag.

Other mild surprises include an abandonment of stealth as a feature of the aircraft. I make this comment because of the evident corner reflectors in the airframe between the fin, the wings and the nacelles, and because of the essentially straight engine ducts.

There are various reasons for this, but a simple explanation is that the aircraft is supposed to be invulnerable because its high speed and its operating altitude would make it extremely difficult to intercept. It will also, of course, be subject to such high surface temperatures that its infra-red signature will enable detection and tracking by infra-red sensors with some ease.

Technology Aspects

The premise of the movie is that Darkstar is a manned aircraft capable of flight at ‘hypersonic’ speeds, with an air combat capability. The supposition for the SR-72 is that this is a Mach 6-capable reconnaissance and weapons delivery platform that is unmanned but reusable. Both aircraft are to operate from, and be recovered to, conventional runways.

So why, given the strides that have been made in aircraft and space technologies, would this be difficult? The difficulties arise from the ways in which the stated aspirations create conflicting requirements. For example, if we want to be able to deliver kinetic effects anywhere on earth within a short period (say an hour or so), we can already do this using either tactical or strategic ballistic missiles. To achieve a greater degree of unpredictability, we might use a boost-glide vehicle as the delivery system, a capability I have written about here.

But neither of these established solutions are re-usable, neither operate from conventional runways, and neither are manned (or optionally manned). Why does this make a difference? After all, the Space Shuttle was manned, re-usable, and was at least landed on a conventional runway.

The critical factor is the desire to operate from a runway in the same way as an aircraft, with some additional elements to be considered for a manned platform. Why would operation from a conventional runway be desired? Principally so that the system can operate at very short notice, without using special launch facilities, so that a responsive system is available, and so that, if necessary, flights can be made without signalling this intent. A long-range manned vertical launch system would be restricted to operation from pre-prepared locations, with large rockets, fuel tanks and launch facilities, and would be incapable of delivering strategic surprise.

A second factor is that, to operate from a conventional runway, wings will be required that are large enough in area to provide acceptable take-off and landing speeds from normally available runway lengths. The wings allow sustained cruise flight in the upper atmosphere to achieve the range required, but will themselves face the challenges of prolonged exposure to kinetic heating throughout the flight.

The kinetic heating issue affects many aspects of the design of such a vehicle, particularly in the areas of structural materials, thermal protection systems, aerodynamics and propulsion. In the case of a manned aircraft, additional complexities arise because of the need to protect the pilot, and to provide external vision and, for a combat aircraft, targeting sensors. These latter aspects apply to Darkstar, but possibly not to the SR-72 concept, which is consistently portrayed as unmanned.

Possible Propulsion Systems

There are many challenges in designing a system with the capabilities that appear to be required, but propulsion is perhaps the critical one to consider. The SR-71 used variable geometry within its J58 engines, which, in concert with a complex intake system allowed the engine to operate both as an afterburning turbofan, and at high speeds essentially as a ramjet. This approach was clearly successful in enabling speeds of Mach 3.2 to be achieved; however, a different approach is required to achieve the anticipated Mach 6.0 of the SR-72.

The available material on the SR-72 suggests that the powerplant will be a turbine-based combined cycle (TBCC) engine, operating as a conventional turbine engine up to Mach numbers of perhaps Mach 2.5, but changing over to a supersonic combustion ramjet (scramjet) at higher Mach numbers. A major issue has been that scramjets have required a speed of about Mach 4.0 to operate, and in consequence, a dual mode ramjet might be required, capable of operating from Mach 2.5, and becoming a scramjet at speeds greater than Mach 4.

All of this technology must be regarded as very flaky, but relatively recent statements from Lockheed suggest that technology breakthroughs are emerging to allow speeds of Mach 6 to be achieved. Advances in manufacturing, modelling and 3-D printing are hinted at as having enabled this breakthrough.

In considering potential propulsion solutions, I came across references from the UK, China, Japan and Lockheed, all indicating that an engine incorporating an internal airflow cooling system could be critical in enabling a hypersonic powerplant.

With this in mind, let’s have a look at the SABRE powerplant, which had been under development in the UK, with Single-Stage To Orbit (SSTO) capability as the intended application. SABRE is an acronym which stands for Synergetic Air Breathing Rocket Engine, referring to a combined cycle rocket engine which features an air-breathing mode for use within the atmosphere. Critical to the concept is a precooler located within the intake which is capable of very rapidly cooling the intake flow, increasing the density and leading to a very high pressure-ratio within the engine when operating in its air-breathing mode. This system is used up to a Mach number of 5.4, after which the intake is closed, and SABRE run as a rocket engine, using liquid oxygen and hydrogen as fuel.

Engaging, beautifully illustrated and fascinating, The Hush-Kit Book of Warplanes.

The precooler concept dates from NASA work as far back as 1955, and was picked up in the UK, initially for use in the HOTOL project, and latterly in the SABRE engine, which is stated to be capable of producing high thrust efficiently, from zero speed up to Mach 5.4. The precooler is critical in achieving this, and represents the highest technical risk aspect of the engine. Initial work by Reaction Engines at Westcott has quietly transitioned to a DARPA project based in Colorado, where demonstration activities of the precooler heat exchanger have been successful, apparently leading, in late 2019, to successful cooling of 1000 deg C intake flows to a target of -150 deg C. In achieving this, a liquid helium cooling loop is used, which avoids hydrogen embrittlement issues which had affected earlier attempts to develop a precooler, but introduces additional complexity.

The demonstrated cooler used 16,800 thin-walled tubes, and has been shown to achieve the dramatic cooling required in 0.01 seconds, which seems remarkable, to say the least. 3D-printing would seem to be an ideal technology to construct such a device, and this, coupled with the move of the facility to Colorado, and the involvement of DARPA, may indicate that this precooler represents the breakthrough suggested by Lockheed-Martin. The application would not be to achieve a SSTO capability, but to enable a propulsion system capable of sustained hypersonic flight in a vehicle such as the SR-72.

All this is speculation, but given a claimed air-breathing performance delivering high efficiency thrust from zero airspeed to Mach 5.5, it would not be surprising if Lockheed-Martin were to be highly interested in the potential of such a system. And maybe, a little development of the intake system, the nozzle, and perhaps other elements of the engine installation, might make the aspiration of Mach 6 capability achievable.

If you are interested in this technology, here’s a technical paper from China, which may add some credibility to my speculation on the relevance of precooling systems.

Other technical challenges

As indicated earlier, kinetic heating of the vehicle is likely to pose significant problems. However, at least for limited exposure times, suitable materials have been developed for many other space-based applications, particularly those featuring wing-based re-entry and landing. A combination of Carbon-Carbon composite materials, ceramic tiles, insulating and thermal protection systems have been developed for systems such as the Space Shuttle, X-37, and other hypersonic projects.

A host of additional technology issues may be anticipated, but, again, the variety of space-based activities and other high-speed projects may be maturing solutions to issues such as:

Thermal protection systems;

Crew environment management;

Vision systems;

Sensor, targeting and communications systems;

Deployable approach path guidance.

Noting that missions may involve much greater time at elevated temperatures than a re-entry manoeuvre, any of the above issues may prove more challenging for an SR-72 than, for example, the X-37. The greater heat soak may prove to be a challenge in areas which might otherwise be regarded as mature, such as hydraulic systems, or even wheels and tyres.

Weapons, weapons bays, and carriage and release systems are specialist areas where new technologies may be required to deliver kinetic effects from a high-speed platform. Aerodynamic and heating loads look to be a significantly challenging issue, quite apart from target assurance – the need to be sure that the correct target has been identified and located.

Another issue needing consideration is whether, and what, defensive aids might be required. Such systems might be required because it seems likely that a hypersonic strike platform would be highly detectable, and would also be likely to have limited manoeuvre capability when flying at high speed and altitude. Of course, proponents will argue that such systems are essentially invulnerable because of the difficulty of a successful weapon engagement against such a fast-moving high-altitude target. Maybe so, but it is surprising what can be achieved, given sufficient incentive to find a solution.

Are these concepts feasible?

My answer is – closer than you think, if my speculation about propulsion is correct.

The big issue for the SR-72 is whether a subsonic, stealthy, high-flying unmanned system can do the ISR job at a lower cost, albeit without the rapid response capability of hypersonic speed. If that should be the case, then the SR-72 looks like a very niche, very expensive, capability as a reactive precision strike aircraft. So, the biggest threat may be cost and relevance, rather than feasibility. When has that ever stopped the US?

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The issue for Darkstar, if conceived as a hypersonic fighter, is the sheer implausibility of manoeuvring air combat in that class of platform. With the added complexities and penalties of carrying a pilot, and the difficulty of integrating information and providing sensor and weapons capabilities. In my view, this is a much more difficult ask than the SR-72, and would be likely to be an even more expensive solution. My advice – don’t go there.

Unless the Chinese are going there already …

  • Jim Smith

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