Douglas Barrie/LONDON
Providing more than twice the punch for the same mass in comparison to solid rockets has inevitably made ramjet propulsion attractive to missile designers. In the air-to-air missile (AAM) arena, however, the benefits of ramjets have previously been literally outweighed by their disadvantages. Ramjet sustainer units have been just too big.
Recent developments in ramjet propulsion have overcome design limitations which, militated against AAM applications: that is, the ability to design a ramjet propulsion unit with a calibre small enough for use on a beyond-visual-range (BVR) AAM.
The UKMinistry of Defence has under way a competition to procure a BVR weapon for the Royal Air Force's Eurofighter EF2000. Whatever the outcome of Staff Requirement (Air) 1239, the winning missile is almost certain to have a ramjet sustainer.
Energy Equates With Survival
For the missile-design fraternity, the key advantage of the ramjet sustainer is that it offers considerably greater energy for similar mass, providing about double the kinematic range of a comparable solid rocket.
While a solid-rocket-motor-powered missile, such as the Hughes AIM-120 Advanced Medium-Range Air-to-Air Missile, will have a greater peak velocity than that of a comparable ramjet design, the ramjet will have a greater sustained velocity. In an air-to-air missile battle, superior energy equates with survival.
Grp Capt Graeme Smith, British Aerospace's military air advisor, says that"-current medium-range weapons suffer from a lack of overall total energy in that they do not have the manoeuvrability required to achieve a kill against a highly agile opponent: that is, they have a relatively small no-escape zone".
It is believed to have been just such a conclusion that prompted the RAFto look beyond a conventional solid-rocket design (for the EF2000, the AIM-120B) to a more capable missile with a greater energy for the "end-game engagement". There is no point in a missile reaching the final stage of the engagement if it cannot deal successfully with a target manoeuvring at 9G-plus. As a rule of thumb for a successful BVR engagement, a missile needs to have a minimum of three times the manoeuvre energy of its target. If a target pulls up to10G in an evasive manoeuvre, then the missile will need to sustain 30G-plus turns at the end of an engagement to record a kill.
Some sources indicate that RAF simulations of the Sukhoi Su-27 Flanker and Flanker Plus derivatives and associated missiles versus the EF2000 with the AIM-120B revealed an unacceptably poor exchange ratio. The focus fell on providing the EF2000 with a missile, which has a far greater no-escape volume at BVR ranges.
As Smith points out, the BVR environment is also expanding, as heralded by the emergence of the Russian Vympel's long-burn R-27RE (AA-10 Alamo). Traditionally, the BVR engagement has gone out to around 40km (22nm). The next generation of BVRAAMs will push the engagement envelope to around 100km.
As well as providing increased absolute range, the rocket-booster/ramjet-sustainer design, more importantly, offers an increased no-escape zone. A ramjet-sustainer AAM potentially triples the volume of space within which the probability of a kill remains high.
Cost Catch
With such an increase in performance, there has to be a catch. In the case of the rocket/ramjet AAM, this equates to cost; this propulsion approach is estimated to be about twice as expensive as that of a solid-rocket design.
This, however, is hardly prohibitive when one considers the potential advantages a rocket/ramjet missile brings to a BVR engagement. Paying more for improved propulsion is infinitely more attractive than losing a $40 million-plus fighter in an air-to-air engagement against a capable opponent.
The UK competition has resulted in the emergence of two teams: BAe (now Matra BAe Dynamics), leading Alenia, Daimler-Benz Aerospace, GEC-Marconi, and Saab in offering the Meteor missile; and Hughes UK, which is proposing an AIM-120 ramjet derivative, teamed with Aerospatiale, Fokker, Shorts and Thomson-Thorn.
In part, the charm of the ramjet is its design simplicity - air intake (or intakes), a combustion chamber, fuel injectors and a fuel tank are its primary constituent elements. Moving parts are unnecessary. In terms of efficiency, the ramjet motor, however, is dependent on being accelerated to around Mach 2 before it achieves anything approaching operational effectiveness. This requires that the ramjet sustainer be designed along with a solid-propellant booster. Integrated nozzleless designs are now the norm as far as the booster is concerned.
While there is a general design consensus in this area, different solutions remain in designing ramjet sustainer motors. These are based around the choice of fuels and how the combustion process is managed. The FMRAAM uses a direct injection ramjet, while the Meteor is understood to be being proposed with a boron-doped solid-ramjet design. Not surprisingly, there is a considerable amount of combustion between the propulsion advocates in both camps.
The basic advantage of the ramjet is that it uses atmospheric oxygen rather than a packaged oxidising agent for combustion. Where the challenge arises is in the management of the combustion process across the altitude regime, given the variance in air pressure.
The simplest solution, as initially used by South Africa's Kentron and Somchem, is effectively to ignore the issue of regulation. While considerably easing design issues, such an approach restricts the missile to an extremely limited-altitude flight regime if the ramjet is to operate at its optimum performance. The solution initially considered by missile-designer Kentron was to fly the AAM down its specific altitude corridor, leaving this only in the final phase of the engagement when ramjet performance was no longer a concern. The limitations of this design approach, however, have led power plant-manufacturer Somchem to begin work on an active mechanical-valve system which would act as a throttle.
Propulsion Solution
Aerospatiale defines four basic ramjet designs: self-regulated solid-propellant ramjets, boron-doped solid-propellant ramjets, direct-injection ramjets and regulated-liquid ramjets. Within the Hughes bid for SR(A) 1239, Aerospatiale, not surprisingly, has responsibility for developing the propulsion system.
The company has drawn on several study projects in developing its propulsion solution for what Hughes calls its Future Medium-Range AAM (FMRAAM). In the late 1980s and early 1990s, it carried out studies into the development of small-calibre ramjets (SPCs) suitable for AAM applications. The company was a subcontractor to Onera and Matra on the Rustique ramjet project for a self-regulating power plant from 1990 to 1995. The SPC 1 programme, which ran from 1988 to 1990, looked at using a smaller derivative of the ASMPramjet for AAM applications, with the design using two intakes 90í apart, rather than 180í apart. Although technically successful, Aerospatiale says that even a smaller ASMP-style engine remained prohibitively costly for the tactical AAM
The SPC 2 programme looked at modifying the fuel-injection architecture, shifting it from being mounted at the elbows of the ASMP's air intakes to the front of the combustion chamber. While again being a technical success, Aerospatiale considered that a solenoid valve remained too heavy, and too expensive, to be used for regulation.
Following on from the SPCprojects, the company is studying further programmes aimed at cost reduction under the composite ramjet-structures programme. The second of these looked to develop a simplified fuel tank, inexpensive pressurisation system, and a compact direct-injection system. In turn, this project fed into the simple regulation ramjet (SRS) project, which began in 1994. This was aimed at drawing together all of the company's recent work on small-calibre-ramjet design.
The end result of Aerospatiale's studies was that it has opted for a liquid-fuel direct injection ramjet, using a regulation technique which resulted from its SRS project. It decided that a regulated solid-fuel ramjet was less suitable because of what it deemed to be an unacceptably high level of technical risk as well as high development and production costs. The self-regulating ramjet, while suitable for the anti-radiation-missile application, was not acceptable for an AAM because of what Aerospatiale viewed as its inherent limitations across the altitude envelope. Matra is more positive about the "self-modulating" approach explored within the MPSR programme. Regulation on the MPSR test missiles was purely by means of the flow rate being sensitive to atmospheric pressure variation.
The regulated-liquid ramjet, as used in the ASMP, while technically acceptable, was financially prohibitive.
Aerospatiale's solution - what it describes as a direct-injection ramjet - uses an elastomer bladder within the fuel tank, which is then linked to a pressure-reducing valve, with the fuel then fed into the combustion chamber via a four-injector assembly.
In choosing a propulsion solution, it also considered that solid propellants, with metallic additives such as boron or aluminium, raised detection concerns in both the visual and radar environments. Unburnt metallic particles, argues Aerospatiale, provide a good radar scatterer, risking exhaust-plume detection. The company also contends that unspent metallic agents in the plume could degrade the guidance datalink performance between the launch aircraft and the missile during the mid-course phase of the engagement. Finally, it raised concerns that a boron-based powerplant courted unacceptably high technical risks.
DASA power plant subsidiary Bayern Chemie, which is involved in providing the ramjet sustainer for the Meteor, contests the Aerospatiale conclusions. It has gone as far as writing to the German defence ministry in attempting to counter the Aerospatiale claims over boron-based solid ramjets.
The company claims that, as far as the technical risks are concerned, it has the experience of several successful technology-demonstrator projects, which it believes validate such a design approach. As far as plume metallic residue is concerned, the company says that tests carried out so far have been encouraging, with the results being nowhere as bad as Aerospatiale would suggest. Further tests are planned.
In prosecuting its case for a boron propellant, Bayern Chemie argues that a solid-ramjet solution (because of the higher fuel density than that of a ramjet design) allows a for a greater amount of propellant energy to be packed into the same volume of space.
It also claims, in supporting the choice of a solid ramjet, that there have been concerns over the combustion stability of small-diameter liquid ramjets at high-altitude flight profiles, suggesting that the BAe Sea Dart missile's Rolls- Royce Odin ramjet suffers from such a problem. Another concern it raises is with the choice of JP10 for the FMRAAM sustainer, suggesting that the corrosive properties of this fuel cause doubts about suitability for long-term storage.
The UK MoD is expected to announce a decision by July of 1997 on which bid has won. Above and beyond a requirement for up to around 1,000 missiles for the RAF, the competition could also determine whether Hughes effectively becomes the de facto standard manufacturer for future BVR weapons.
Were the UK MoD to select the Hughes-led team, it is unlikely that the other European nations would pursue an independent missile solution to meet their own extended-range BVR needs. Developing such a BVR weapon even within a collabrative project is costly. Considering developing a contestant to a UK-supported ramjet AMRAAM derivative could prove prohibitively so.
The irony for Hughes is that, at least for the moment, it cannot publicly bring the US Air Force to the table as a potential collaborative partner. The USAF has no stated requirement for an extended-range BVR missile, although there is little doubt that it will eventually procure a derivative of the AIM-120 with a considerably enhanced engagement envelope.
The USAF's concern is that it does nothing to jeopardise the Lockheed Martin/Boeing F-22 advanced tactical fighter. There are those in Washington who might view a ramjet AMRAAM, coupled with an upgraded F-15C, as just such a threat. Were the USAF not to purchase an enhanced-range AIM-120, it would raise serious issues about the F-15C and the Lockheed Martin F-16 in terms of their BVR capability against emerging threats. The US Navy's position is slightly different in that it has a stated long-term need for a extended range AMRAAM for its McDonnell Douglas F-18E/F now under development.
For Matra BAe Dynamics, winning SR(A)1239 would secure not only the EF2000 nations as customers, but also in all likelihood Sweden for the Saab JAS39 Gripen, and France, for the Dassault Rafale multi-role fighter. It would also guarantee export independence from the USA for a primary AAM for Europe's three fighter projects.
There is concern among the European nations that, in a transatlantic fighter-export battle, the USA could shackle European competitors by dragging its heels over the releasability of the AIM-120. Problems over integrating the AIM-120 on the Gripen for Sweden effectively ended Saab's chances in the competition, won by the F-18. The machinations over this competition resulted in European fighter manufacturers being reluctant to be dependent upon the supply of a US-designed next-generation BVRAAM for their aircraft.
The Meteor team members also contend that key AAM development technology skills need to be maintained within Europe, and that a US solution would jeopardise these, notwithstanding that Hughes UK is leading the bid into the UK money. Those lacking charity might view Hughes UK as being little more than a "wooden horse".
Perhaps the political end game yet to be played is the fact that BAe and Hughes are bidding together for the US Air Force/Navy requirement for a Sidewinder replacement, the AIM-9X, with variants of the BAe Advanced Short-Range Air-to-Air Missile. The US Department of Defense could yet make an overture on linkage between SR(A)1239 and the AIM-9X. The prize of becoming the successor to the Sidewinder - with some 150,000 sold - could prove a tempting bait indeed.
Source: Flight International