Only a handful of surface-to-air missile systems powered with ramjet sustainer engines remain in service today, but, as Douglas Barrie and Joseph Dempsey suggest, renewed interest and investment in high-speed guided-weaponry could prompt a comeback for ramjet engine technology.
Surface-to-air missile (SAM) systems with ramjet sustainer engines could be about to make a comeback. Such engines were central to European, Soviet and United States medium-to-long-range SAM system research and development in the 1950s and 1960s. Of the systems to emerge from this work, today only a handful of the Soviet-era 2K11 Krug (SA-4 Ganef) and 2K12 Kub (SA-6 Gainful) remain in service. Norwegian energetics and propulsion specialist Nammo would like to change that.
Nammo is working on the development of a ramjet engine that it believes could provide the basis for an extended-range SAM, offering an engagement capability far in excess of most of today’s Western systems. The company claims a range envelope beyond 400 kilometres (250 miles) for high-altitude fly out, although a Nammo official recognised that this poses targeting issues.
Ramjet engines differ from solid-propellant motors in that they use atmospheric oxygen rather than including an oxidizer as part of the solid motor. A ramjet-sustainer missile of the same size as a solid-propellant missile will provide greater range and a higher average speed. The mass of the oxidiser, which constitutes most of a solid motor, is replaced with fuel.
What a SAM using ramjet propulsion also offers is the ability be used against comparatively fast high-altitude targets while still under power, at ranges at which solid-propellant missile motors would have burned out, with the missile rapidly losing energy.
Form and fit
Nammo showed a concept model of a ramjet-powered SAM design during the recent Defence and Security Equipment International exhibition held in London on 10–13 September. The design showed a chin-mounted intake with narrow-chord wings, and was of a size that would fit into a National Advanced Surface-to-Air Missile Systems (NASAMS) launch canister.
NASAMS uses a variant of the AIM-120 Advanced Medium-Range Air-to-Air Missile developed by Raytheon. The company is partnering with Norway’s Kongsberg Defence and Aerospace on both NASAMS and the upgraded NASAMS II. Nammo manufactures the solid-propellant motor for the AIM-120 family of missiles. According to the IISS Military Balance+ database, alongside Norway, NASAMS II operators include Finland and the Netherlands.
Nammo is now looking for possible partners who might be interested in using its ramjet motor as the basis for a SAM, or indeed for other missile applications. In the meantime, the company is working toward an initial ground test-launch of the ramjet sustainer by 2022–23.
Solid requirements
Ramjets require a solid booster of some form to reach a transition speed at which ramjet combustion is able to be sustained efficiently. In the case of the Nammo design, which uses an integrated solid booster, this is around Mach 3. The current design is optimised to cruise at Mach 3.5 at 15.2–18.3 km (9.5–11.4 miles).
The chin-mounted intake is positioned below a reduced-diameter forward section, maintaining a constant overall diameter. This design was dictated by the aim of fitting the missile within the existing NASAMS canister. Two of the missile’s four strakes that extend along the length of the body act as bypass ducts, and by varying the air flow can be used to throttle the engine.
As well as surface-to-air applications, Nammo said the ramjet engine could be used as the sustain motor for an extended-range air-to-air missile (AAM). European missile manufacturer MBDA’s rocket/ramjet Meteor AAM is already in service with Sweden and the United Kingdom.
However, a nearer-term development for Nammo is to carry out firings of its ramjet-boosted artillery round, dubbed HE-ExR (High-Explosive Extreme Range). Projected ranges for this round are up to 150 km (93 miles), and live-fire tests are due to begin in 2020. In June 2019, Nammo signed a development agreement with Boeing Defense, Space and Security to work on the HE-ExR.
Whether or not Nammo’s SAM ramjet goals are met remains to be seen. What is clear, however, is that renewed interest and investment in high-speed guided-weaponry is going to see the wider adoption of ramjet and scramjet (supersonic-combustion ramjet) engine technology in both offensive and defensive roles.