Speed is the New Stealth: The SR-72 Challenges the Future at Mach 6

The popular image of a stealthy, fifth-generation fighter and subsonic drone-dominated future in military aviation appears to have a serious challenge at hand. On 1 November 2013, Lockheed Martin’s ‘Skunk Works’ development department unveiled the ‘SR-72’ project designed to replace the retired Mach 3 capable SR-71 Blackbird spy plane. According to the exclusive report by Aviation Week, the SR-72 will be a remotely piloted aircraft with strike as well as intelligence, surveillance, and reconnaissance (ISR) capabilities. It is to cruise at Mach 6 above 100,000ft using revolutionary engines which combine a high-speed turbojet and scramjet. Although previous attempts to develop practical hypersonic aircraft have failed due to excessive costs and technical difficulties, Skunk Works claim to have created an ‘affordable’ solution to the problem of producing an aircraft capable of flying faster than Mach 4 that can also take off from a runway.

Any project to replace the legendary Blackbird is guaranteed to attract a great deal of interest in aviation and defence circles. Yet the hypersonic SR-72 deserves particular attention as it challenges many widely held ideas about what the future of military aerospace will look like.

Speed is the New Stealth

The original SR-71 used its extraordinary speed to accelerate away from hostile surface-to-air missiles (SAMs). With a projected top speed of more than 2km/s, the SR-72 would be twice as fast as its predecessor. The lack of warning time and difficulty in intercepting an intruder at such speeds has led Al Romig, Skunk Works’ engineering and advanced systems vice president, to claim that ‘speed is the new stealth’.

This is a substantial break with mainstream thinking on the next generation of combat aircraft. Thus far, the most widely discussed penetration capability of current and next generation combat aircraft has been stealth, rather than top speed. Hence the F-35 Lightning II, F-22 Raptor, B2 Spirit, RQ-170 Sentinel, Taranis, Sukhoi PAK FA and many other cutting edge aerospace projects all currently rely on stealth rather than top speed as a means to penetrate defended airspace.

The quest for immunity to interception through sheer speed is traditionally associated with a previous generation of aircraft designs developed during the 1960s such as the SR-71, B-70 Valkyrie and Mig-25 Foxbat. However, it is interesting to note that despite great advances in Surface-to-Air-Missile (SAM) and radar technology during the Blackbird’s service life, no SR-71 was ever shot down despite multiple launches against its overflights. The principle of evasion through acceleration would therefore seem to be sound.

Certainly a Mach 6 cruise at operating altitudes of around 100,000ft would make SR-72 extremely challenging to intercept with traditional SAM systems. However, it is possible that progress in the anti-ballistic missile (ABM) field might challenge this theory. The Raytheon Standard Missile 3 (SM-3) series recently demonstrated its capability to shoot down both low-orbit satellites and ballistic missiles. Although the SM-3 is not designed for anti-aircraft work, its ability to intercept targets at extremely high altitudes and at closing speeds of over 22,000 mph could suggest that by the time SR-72 might enter service around 2030, SAMs could possess similar performance. 

In terms of propulsion, the SR-72 certainly promises a quantum leap in military aircraft potential. Lockheed Martin is unsurprisingly silent on the details of the proprietary turbine-based combined cycle (TBCC) engines that are intended to power the SR-72. However, if Skunk Works can deliver on their stated plan to produce a Mach 6 capable, optionally-piloted scaled demonstrator by 2023, they will radically alter the possibilities for high-end military aerospace designs. If the F-22 sized demonstrator aircraft can maintain level flight at Mach 6 using the new technology, a new international race could be triggered to develop hypersonic combat aircraft comparable to the fifth generation stealth fighter rivalry between the West, Russia, India and China.

Advantage Over Stealth Drones

If the turbine/scramjet propulsion system proves practical then the future of air strike missions may look very different to the stealth drone dominated picture currently envisioned. Whilst drones such as the MQ-9 Reaper and RQ -4 Global Hawk have proven extremely potent against targets in Afghanistan and Pakistan, they are not capable of penetrating well defended airspace due to slow speeds and lack of evasive manoeuvrability. Stealth drones do not necessarily remedy this lack of capability as the loss and capture of a stealth RQ-170 Sentinel over Iran in December 2011 showed.

Furthermore, small stealth platforms may be compromised in future by technologies being developed in Russia and elsewhere such as meter-band radar and passive arrays. The hypersonic penetration capabilities promised by SR-72 could be utilised in daylight and in highly protected airspace where even stealth drones would be at serious risk. However, persuading the Pentagon and Congress of the requirement for such capability may be more of a challenge than proving the technology behind SR-72 is practical in itself.

Will it Fly?

Skunk Works is famously tight lipped when it comes to ‘black projects’, so the fact that they have publicly revealed SR-72 suggests the management of Lockheed Martin have confidence in the future of the programme. The project fits into the US Air Force’s hypersonic roadmap which looks for hypersonic ISR and strike capability by 2030. This is further complemented by the fact that the company has a great deal of experience with US military procurement.

However, following the huge cost increases and delays suffered by the F-35 programme and the substantial budgetary constraints facing the US military, many analysts are not optimistic about the prospects for further technologically ambitious, large scale procurement projects for the USAF in the near future. Lockheed’s competitor, Boeing, appear to have reached a similar conclusion and have focussed much of their recent efforts in the military aerospace sector on upgrade packages for existing airframes such as the F15SE Silent Eagle. 

On the other hand, there is growing recognition of the capability gap left by the retirement of the SR-71 in terms of real-time surveillance for the military inside protected airspace. The theoretical capability of the SR-72 to carry missiles for pinpoint strikes without warning is also a significant draw for the Pentagon in face of Anti-Access/Area Denial (A2/AD) strategies deployed by Iran, North Korea and others which could increasingly call into question the US military’s ability to conduct strikes at will without large scale defence suppression. There is also the fact that SR-72 would give the US capabilities that are currently far beyond those of its rivals, which might prove especially attractive in view of the alleged Chinese theft of F-35 technology in a cyber attack, as well as emerging challenges to US dominance in drone capabilities.

Lockheed Martin’s willingness to invest considerable sums in research and development (R&D) of the SR-72 suggests a belief that in the next 10-15 years, there will be considerable US government funding for high speed ISR and surgical strike capabilities that can operate within airspace protected by modern air defence networks. However, according to the announcement, the project is nearing the stage where further progress with full scale flying demonstrators will be impossible without governmental funding. If this is not forthcoming, Lockheed Martin will face a difficult choice; to continue with the project and risk potentially enormous sums on R&D without any guarantee of return, or to shelve the concept and forfeit most of what that they have spent and achieved with the project in the past seven years.

In either case, this is definitely a project to watch!