Hard nut crackers: the Pentagon's top priority in weapon developments is to find a means to deal effectively yet safely with deeply buried facilities used for the production and storage of chemical and biological materials. This follows decades of penetration weapon development, directed first at bridges and airfields, and later at underground command and control bunkers.

One early challenge was to penetrate runways from a low release. This required a special bomb, first retarded into a steep descent, then rocket-boosted for a high velocity impact. One example is the 52 kg MBDA Kriss submunition, ten of which are carried in the company's Apache cruise missile. Each Kriss is accelerated to 400 metres/sec by a Bayern-Chemie Protac rocket to penetrate the runway and explode beneath it. The same parachute-and-rocket approach is used for the Bazalt Betab submunition.

The US Air Force has long been confident of making medium altitude tactical incursions, allowing hard targets to be attacked with more conventional bombs, using penetration warheads with thick forged steel casings and a Tritonal or PBNX-109 explosive filler. They were initially designated I-500, J-1000, and I-2000 (the numbers indicating approximate weight in pounds), and later BLU-111, -110, and -109 respectively.

With laser spot-homing Raytheon Paveway kits, these penetrators produce outstanding results against targets such as runways and hardened aircraft shelters. One example is the GBU-10G/H/J Paveway II, using the I-2000 or BLU-109/B warhead.

The Lockheed Martin BLU-109/B, the US Air Force's penetration weapon of choice for many years, has a casing made by National Forge. It weighs 874 kg with 240 kg of explosive, and can penetrate up to 2.4 metres of concrete. The BLU-109/B is used in the US Air Force's GBU-24A/B and the US Navy's GBU-24B/B, both with Raytheon Paveway III kits. Some 1181 GBU-24s were delivered by the F-111F during the 1991 Gulf War. The same warhead is used in Britain's P3(UK) Paveway III. Adding a GPS/ IMU unit to provide a semi-precise attack--when cloud obscures the target--produces the EGBU-10/-12/-16/-24 series, which includes the EGBU-24E/B Enhanced Paveway III and Britain's EP2(UK) Paveway II.

The cropped GBU-27/B Paveway III allows the BLU-109/B penetrator to be carried inside the F-117A. It weighs 984 kg and is covered in radar-absorbent material, also known as Ram. The Paveway III combines larger wings to maintain range from a low level release, proportional navigation and a selectable impact angle.

Spain's 917 kg BPG-2000 is a joint development by Raytheon and Explosivos Alaveses (Expal), based on the latter's CPE-800 bomb and ER501 fuze. A lighter development is the Israel Military Industries (IMI) 425 kg PB-500A1, which can be used with the Elbit Systems Lizard laser guidance kit.

An alternative to the penetrator in dealing with one class of hard target is the US Air Force Research Laboratory's Tunnel Defeat project, intended to place up to six Mk 84 bombs from a single aircraft simultaneously within five metres of the aim-point.

For some hard targets the near-precision of GPS navigation is adequate, hence the use of the Boeing Jdam (Joint Direct Attack Munition). The BLU-109 penetrator is used in the US Air Force GBU-31(V)3B and the US Navy's corresponding GBU-31(V)4B, while the J-1000 or BLU-110 penetrator is used in the US Air Force GBU-32(V)3B and the Navy GBU-32(V)4B.

Russia has a smaller series of guided penetration bombs, notably the 525-kg TV-guided Kab-500-Kr and the 1500-kg laser-guided Kab-1500L-Pr. The former has a 100 kg penetration warhead and is intended for bridges, concrete shelters and runways. The latter can penetrate up to 20 metres of soil or two metres of concrete and is intended for command posts and nuclear weapon storage facilities. The TV-guided Kab-1500-Kr appears to use the seeker from the Kab-500-Kr. All are produced by the Region State Research and Production Enterprise.

Longer Ranges

The BLU-109/B is used in the TV-guided Boeing GBU-15(V)3 cruciform-wing glide weapon and its IIR-guided GBU-15(V)32 equivalent (and the GPS-aided EGBU-15), as well as the rocket-powered AGM-130 derivative. The extended-range Boeing AGM-84H Slam-ER has a 227 kg WDU-40/B titanium-cased penetrator with PBXC-129 filling and a Raymond FMU-155/B variable-delay fuze.

The BLU-109 has recently been cleared for use on the Rafael Spice TV-guided glide bomb. Israel produces its own fuzes, the leader being Motorola Israel, whose range includes the solidstate, self-powered ID 260A impact/delay fuze. This can be installed in either the nose or the tail of the bomb and is activated by a lanyard. Time delay is selectable between zero and 200 milliseconds.

The needs of the 1991 Gulf War led to a crash programme that produced the BLU-113A/B, employing casings manufactured by National Forge from surplus howitzer barrels. Two GBU-28s with these penetrators were dropped from F-111Fs during Desert Storm against a bunker complex at the Al Taji Air Base. Some were used from F-15Es over the former Yugoslavia in 1999 and others were released from B-2As over Afghanistan in 2001.

The GBU-28B/B is an upgraded version developed specifically for the B-2A, combining laser and GPS guidance and using the bomber's radar to provide target position updating prior to release. This version weighs 2132 kg and contains 306 kg of Tritonal. Operational testing is due for completion by the end of 2004. Early this year the US Air Force announced plans for an improved GBU-28, introducing Eglin Steel (ES1) and a new explosive, AFX-757. It was recently announced that General Dynamics Ordnance and Tactical Systems is to develop an enhanced BLU-113 under a $ 4.8 million contract from the US Air Force Air Armament Center, with production options worth $ 24 million.

The latest major advance in large US penetration bombs is Lockheed Martin's 874 kg Advanced Unitary Penetrator (AUP) or BLU-116/B, developed under a 1995 contract to defeat extremely hard multi-layer underground facilities. Inside an aluminium shroud is a sub-calibre penetrator, made from a nickel-cobalt steel alloy designated Air Force 1410, and filled with high-energy PBXN-109 explosive. The AUP can penetrate 30 metres of soil, or six metres of concrete. Its effectiveness is enhanced by the void-sensing, layer-counting, penetration-measuring Alliant Techsystems (ATK) Precision Fuze FMU-159A/B Hard Target Smart Fuze, developed in association with Thales Missile Electronics (TME). The US Air Force Research Laboratory Munitions Directorate is working on a system to transmit information back from the bomb fuze to the launch aircraft, to assist in target damage assessment.

TME and ATK are also teamed (with TME as leader) on the Multi-Event Hard Target Fuze (MEHTF) for the Raytheon Systems Precision Guided Bomb (PGB). This is based on the 227 kg Mk 82 IM (Insensitive Munition) and will enter Royal Air Force service in 2007. The MEHTF will provide for air burst, impact or delayed detonation.

The AUP has the same dimensions and aerodynamic characteristics as the BLU-109, and the same attachment points for guidance and control kits. It can thus be delivered as a GBU-10, -15, -24, -27, the Jdam or the AGM-130. The AUP-3(M) version has been selected for the Boeing AGM-86D ALCM (air-launched cruise missile), of which 50 have been produced by upgrading. Up to five AUPs were used operationally in Kosovo in 1999, before the munition was formally approved for service.

A further example of kinetic energy penetrator technology is the 130 kg US Air Force/Boeing GBU-39/B Small Diameter Bomb (SDB), which aims to achieve a penetration (1.83 metres) comparable to that of the 874 kg BLU-109/B, this by combining a slender body of exceptionally hard material with impact normal to the target surface. The SDB combines an advanced anti-jam GPS and INS guidance system. Fitted with the MBDA Diamond Back wing-kit, it will have a maximum range of over 74 kilometres. The 180 cm-long SDB contains approximately 23 kg of explosive with a cockpit-selectable electronic fuze. Boeing was chosen in August to carry out development and production of the bomb and its 145 kg BRU-61/A four-store rack. It is anticipated that 24,000 of these bombs and 2000 racks will be required. First deliveries are scheduled for October 2005. At the opposite end of the spectrum, there have been reports of a 13.6-tonne 'Big BLU' being developed for the B-2A.

The increasing use of penetrators is illustrated by the Raytheon RGM-109H Block IV Tactical Tomahawk Penetrator Version (TTPV), with an ARC-made WDU-34 warhead and HTSF. There have also been tests with a long-rod kinetic penetrator, developed by the US Defense Threat Reduction Agency, capable of defeating three metres of concrete.

Further unitary penetrator development depends on increased impact velocity, better case materials and new explosives. There have been reports of the US Air Force testing rocket-boosted GBU-27/Bs and GBU-28/Bs, increasing impact velocity from around Mach 1.0 to 3.5. In 1994 a photograph was published of the rocket-boosted Betab-500ShP developed by the Bazalt State Research and Production Enterprise.

Some indication of the ultimate limit for an air-launched penetrator may have been provided by an Orbital Sciences test in which an old Lockheed Martin Pershing missile, equipped with a 300 kg steel penetrator, was launched from a B-52. It impacted at Mach 3.5 and penetrated 13.7 metres of granite.

Multi-warhead Systems

While the US has concentrated on heavy, slender forged steel penetrators delivered at high velocity, Western Europe has developed Multi-Warhead Systems (MWS), primarily intended for use against targets such as reinforced concrete aircraft shelters, covered by a thin layer of earth. In essence, the MWS enhances the effectiveness of a small-calibre kinetic energy penetrator by using precursor shaped-charge warhead(s) to weaken the target. A second forward charge may be employed to clear soil or to produce a shallow maximum-diameter hole in the concrete, before the second shaped charge goes deeper.

The MWS allows certain types of hard target to be defeated by a lighter weapon than a kinetic energy penetrator. Judging by the German Air Force requirement that led to the Taurus KEPD350, the Tornado has a practical single-store limit of around 1400 kg, and thus could not use the GBU-28.

Comparative analyses indicate that an MWS is better at defeating concrete, while a kinetic energy penetrator is the weapon for deeply buried targets. Tests prove that an MWS remains effective over a larger range of impact angles.

The most effective current MWS has been claimed to be the TDW Mephisto used in the Taurus KEPD350. Tests have demonstrated that this warhead is capable of defeating over 36 metres of soil or two metres of reinforced concrete. In August 2002, Germany's BWB awarded a 570 million contract to Taurus Systems for production of 600 KEPD350s for delivery between 2004 and 2009. Taurus Systems is two-thirds owned by Eads/LFK and one-third by Saab Bofors Dynamics. The TDW subsidiary of Eads/LFK is also responsible for the Mephisto's Programmable Intelligent Multi-Purpose Fuze (Pimpf).

Britain's Broach MWS has been developed by the RO Defence division of BAE Systems, initially for use in 450 kg form on the MBDA Storm Shadow cruise missile series. The warhead is initiated by a TME Multi-Application Fuze Initiation System. The US Navy has recently signed a $ 4.2 million initial production contract for 42 Broaches of the 227 kg version for the Raytheon AGM-154C Jsow (Joint Stand-Off Weapon), with deliveries to begin in 2004.

Another British MWS is the Ordnance Technologies Lancer, which was proposed in 510 kg form for the Tactical Tomahawk with the aim of defeating six metres of reinforced concrete. In this application a 170 kg shaped charge containing 110 kg of explosive was combined with a 340 kg penetrator holding 25 kg of explosive. In 2002 the British Ministry of Defence agreed to fund two test firings of a Lancer warhead, using an FMU-156 fuze, a shaped-charge precursor and a dense metal penetrator (DMP) developed under an earlier US-UK programme. The tungsten DMP is 200 cm long and 20 cm in diameter. These tests appear to be aimed at developing a warhead technology contribution to America's Supersonic Cruise Missile programme to defeat reinforced concrete up to 15 metres thick.

High Temperatures

America's operations in Afghanistan established a need to attack cave complexes. The best weapon for Afghanistan appeared to be a laser-guided penetration device with a thermobaric warhead, to generate both temperature and pressure. In a 67-day development programme a BLU-109 penetrator was filled with the US Navy's RBXIH-135 explosive, and tested in Nevada on 14 December 2001. Designated BLU-118/B, this thermobaric weapon was first used operationally in a GBU-24 released from an F-15E over Afghanistan on 3 March 2002.

America has an urgent requirement to disable hardened chemical and biological production and storage facilities without releasing lethal agents. The DTRA is working with the US Navy on the concept exploration phase of an Agent Defeat Warhead programme, combining penetration with a warhead that generates high temperatures but low overpressures, and a chlorine disinfectant gas. The Navy work is reportedly based on the GBU-24D/B with an AUP penetrator, and the GBU-31(V)4B Jdam with BLU-109.

The High Temperature Incendiary (HTI) J-1000 is a US Navy/Lockheed Martin proposal, using a 136 kg two-stage fill adapted from the Special Operations Command's Vulcan Fire programme to generate temperatures of around 540[degrees] C. The HTI-J-1000 would be used in the Jassm, the Jsow and the Jdam.

In August ATK announced the receipt of a $ 1.9 million US Air Force research and development contract to develop the Shredder precision-guided penetration bomb, reportedly a BLU-109 modified to neutralize chemical and biological weapons.

Deeper Demands

For deeply buried targets it may be that the only feasible non-nuclear means of attack is a relatively heavy surface-to-surface missile. Later this year the first firing is scheduled for a penetrator version of the Lockheed Martin MGM-140 Atacms; a missile with a range of 220 km. This is a joint US Army/Navy programme in which the Navy is supplying the penetrator. Some 180 Block 1s are due to be modified to Atacms-P standard. Studies are also being conducted into the possibility of using the 32-tonne Boeing LGM-30 Minuteman III with a titanium rod penetrator. The Minuteman has a maximum range of around 10,000 km, and provides an exceptional impact velocity.

The ultimate in hard and deeply buried targets is illustrated by the Pentagon's own secure facilities, such as the old Norad operations centre, buried under 600 (plus) metres of granite. Such targets could be attacked effectively only with nuclear penetrators.

America's fiscal year 2004 Defense Authorization Bill attracted publicity by authorising research by the Energy Department into (previously banned) low-yield nuclear weapons, but it also approved work on 'robust' (one megaton plus) earth-penetrating nuclear devices. This concept is now causing concern since current technology is believed to restrict penetration to around 15 metres of granite, at which depth a megaton weapon would produce massive fallout. The House of Representatives has recently reduced the appropriation for a new nuclear penetrator from $ 15 to five million, arguing that there is no relevant requirement.

The US Air Force's current nuclear penetrator is the 550 kg B61-11, by Sandia National Laboratories, to penetrate soil, not rock. It detonates up to six metres below ground level, creating a shock wave that can destroy targets hundreds of metres deep. The B61-11 has a steel nose cone one centimetre thick. Deliveries began in 1996. The nuclear yield for the B61 series is selectable in nine steps, from 0.3 to 340 kilotons. The B61-11 was reportedly used to threaten Libya regarding its underground chemical weapons facility at Tarhunah in April 1996.