N.R. Jenzen-Jones & Trevor Ball
Editor’s Note: This blog post is linked to entries in the Open-source Munitions Portal (OSMP), a joint project between ARES and Airwars, available at https://osmp.ngo/.
The use of the Small Diameter Bomb (known in U.S. service and elsewhere as the GBU-39) is increasingly being reported upon in the context of the ongoing conflict in Gaza and Lebanon, including in air strikes which have caused notable collateral harm to civilians and civilian objects. The GBU-39 has several components that may survive as ‘diagnostic remnants’ after the munition functions. This short article outlines those components and provides general information on locating and identifying them. Please adhere to all necessary safety precautions in the field. Arms and munitions should never be handled without specialist training.
The GBU-39 Small Diameter Bomb (SDB) is a guided aerial bomb in the 250-pound weight class, developed by Boeing in the United States. General weight classes are used to broadly categorise air-delivered bombs by their mass, but do not necessarily indicate the total weight of a specific munition. The GBU-39/B, for example, weighs 268 lbs (122 kg). It is 70.8 inches (1.8 m) long and 7.5 inches (19 cm) wide. It contains 37 lbs (16.8 kg) of AFX-757 explosive composition—a plastic-bonded explosive which has ‘insensitive munitions’ (IM) characteristics. This is a small explosive payload for an air-delivered bomb. The general arrangement of the GBU-39/B is shown in Figure 1.1. Although the GBU-39 has been in service for nearly twenty years (reaching initial operational capability with the USAF in October 2006), the munition been used more frequently in recent years, particularly in Iraq, Syria, Gaza, and Lebanon. The unit cost for a GBU-39/B is given by the U.S. Air Force (USAF) as $40,000 USD.
The GBU-39 is a precision guided munition (PGM), a term used to refer broadly to a range of munitions, both powered and unpowered, which can alter their flight paths to strike a target with a high degree of precision. The GBU-39 is also considered a ‘glide bomb’, being fitted with a Diamond Back wing assembly made by MBDA Missile Systems that permits longer, flatter ‘gliding’ attack vectors—giving it a range of more than 110 km from the release point. It has four tail-fin control surfaces to adjust the munition’s trajectory in flight in response to GPS and inertial guidance (an ‘inertial navigation system’, or INS), thus improving precision. The U.S. Air Force describes the munition’s targeting and guidance as follows:
The system can be targeted and released against single or multiple targets. SDB target coordinates are loaded into the weapon before release either on the ground or in the air by the aircrew. Once the weapon is released, it relies on GPS/INS to self-navigate to the desired impact point.
Several variants of the SDB are in service with the USAF or under development. The most common variant of the GBU-39 is the GBU-39/B, which features a hardened nosecone to provide a penetrating capability. Other variants include the GBU-39A/B Focused Lethality Munition (FLM), with characteristics to further reduce collateral harm, and the GBU-39B/B, featuring semi-active laser (SAL) guidance. The later Small Diameter Bomb Increment II (SDB II), in U.S. service as the GBU-53/B, is a distinct munition produced by Raytheon. Raytheon call their weapon the StormBreaker; the earlier SDB is sometimes referred to as the SDB I to distinguish it from the SDB II. The GBU-53/B offers infrared, millimetre wave radar, and SAL guidance, in addition to GPS and INS guidance. Whilst this provides significantly more targeting flexibility—including the capability to strike moving targets—this advantage comes at a higher unit cost. (A ground-launched modification of the weapon is not addressed herein.)
Where the body of the GBU-39 survives a strike intact (i.e., in cases where the munition fails to function) it may feature a variety of diagnostic markings, although these can be abraded or obscured as a munition passes through, for example, masonry. Markings will sometimes include a data label with a clear designation (GBU-39/B) and unique serial number, as well as other information such as the National Stock Number (NSN; 1325-01-526-8728) and Department of Defense Identification Code (DODIC; EC53). Even if the GBU-39 functions as intended, diagnostic remnants will often survive. The components which are recovered as primary remnants from a GBU-39 strike (see Figure 1.2) often remain relatively intact due to factors such as their construction and exposure to explosive forces during detonation. The solid nose cone of the bomb body (yellow, 3), the fuzewell (red, 4–7), the tail actuation section (pink), and parts of the wing assembly (orange) are the key diagnostic remnants. They are outlined below with reference to real-world entries from the Open-source Munitions Portal (OSMP):
Nose Cone
The nose cone (Figure 1.3) is made from hardened steel to enable the munition to penetrate more than three feet (one metre) of reinforced concrete. The heavy metal construction of this component means it typically survives functioning, despite its direct proximity to the explosives in the munition.
Fuzewell
The fuzewell (Figure 1.4) that is inserted in the base of the bomb body often survives functioning intact. The initial detonation of the fuze, along with the fuzewell’s IM features, likely contribute to the fuzewell being ejected from the bomb body, as the primary explosive charge in the body of the bomb detonates shortly after fuze functioning.
Tail Actuation Section
The tail actuation section (Figure 1.5) is located at the farthest point of the munition from the bomb body and explosive content. The tail actuation section is often found partly or largely intact following the munition’s functioning. This section contains the thermal battery and four fins, some or all of which may be damaged or absent.
Wing Assembly
The deployable wings (Figure 1.6) from the wing assembly mounted to the top of the GBU-39 may also be recovered after the blast. The wings themselves are often sheared off as the bomb penetrates structures before detonating, which can result in their being found outside the building or floor where the munition functions. Other parts of the wing assembly may also be found, but these are generally less distinctive.
Comparatively small air-delivered munitions like the GBU-39 are often selected specifically to minimise the chance of harm to civilians or civilian objects. Indeed, the GBU-39 was one of several weapons originating with a U.S. Air Force requirement to provide aircraft with a strike option that can minimise collateral damage, whilst being capable of carriage in the smaller, internal weapon bays of the F-22 and F-35 fighters. Smaller munitions also allow aircraft to make more strikes without returning to an airfield for re-arming, however, and are not necessarily used to reduce collateral harm.
The BRU-61/A bomb rack enables the carriage of four GBU-39/B munitions for each MIL-STD-1760 store location—essentially permitting aircraft to carry four SDBs for each 2,000-pound-class MK 84 aerial bomb that might otherwise be carried. For most strike fighter aircraft, this will equate to a total load of eight SDBs (some specialised aircraft can carry more). The munition is in service with several air forces around the world, including Australia, Israel, the Netherlands, South Korea, Sweden, and the United States. The weapon has also been supplied to Ukraine, where the BRU-61/A bomb rack has been seen fitted to Soviet-designed Mikoyan MiG-29 fighter aircraft (see Figure 1.7).
Sources
Armament Research Services (ARES) & Airwars. Open-source Munitions Portal (OSMP). Various entries. <https://www.osmp.ngo>.
Australian Defence Magazine. ‘RAAF F-35As receive small diameter bombs’. 23 August 2019. <https://www.australiandefence.com.au/news/raaf-f-35as-receive-small-diameter-bombs>.
Boeing. ‘Boeing Delivers Final Focused Lethality Munition to US Air Force’. 19 December 2013. <https://boeing.mediaroom.com/Boeing-Delivers-Final-Focused-Lethality-Munition-to-US-Air-Force>.
Boeing. Small Diameter Bomb (SDB) GBU-39/B Weapon. 2015. <https://www.boeing.com/content/dam/boeing/boeingdotcom/defense/weapons-weapons/images/small_diameter_bomb_product_card.pdf>.
Brown, Cate, Meg Kelly, Kareem Fahim & John Hudson. ‘Experts say Israel used U.S.-made bomb in deadly Rafah strike’. The Washington Post. 29 May 2024. <https://www.washingtonpost.com/world/2024/05/29/rafah-strike-us-munition-israel/>.
Gould, Joe. ‘Guided-Bomb Makers Anticipate GPS Jammers’. DefenseNews. 31 May 2015. <https://www.defensenews.com/air/2015/05/31/guided-bomb-makers-anticipate-gps-jammers/>.
Jakes, Lara. ‘A Small American Bomb Killing Palestinians by the Dozen in Gaza’. The New York Times. 8 June 2024. <https://www.nytimes.com/2024/06/08/world/middleeast/us-israel-bomb-gbu39-gaza.html>.
Jenzen-Jones, N.R. & Jack Shanley. ‘Precision Strike: A Brief Development History of PGMs’. RUSI Journal, Vol. 166 No. 5 (2021). <https://rusi.org/explore-our-research/publications/rusi-journal/precision-strike-brief-development-history-pgms>.
MBDA Missile Systems. ‘Diamond Back Wing Assembly’. n.d. <https://mbdainc.com/products/diamond-back/>.
Parken, Oliver. ‘A-10 Warthog To Soon Carry 16 Small Diameter Bombs In Combat’. The War Zone. 25 April 2023. <https://www.twz.com/a-10-warthog-to-soon-carry-16-small-diameter-bombs-in-combat>.
Raytheon. ‘StormBreaker Smart Weapon’. n.d. <https://www.rtx.com/raytheon/what-we-do/air/stormbreaker-smart-weapon>.
Tyler Rogoway. ‘Check Out These F-35s Dropping Full Loads Of Small Diameter Bombs During A Test Mission’. The Warzone. 22 August 2020. <https://www.twz.com/35931/check-out-this-f-35-dropping-a-full-load-of-small-diameter-bombs-during-test>.
Saab. ‘Ground-Launched Small Diameter Bomb’ n.d. <https://www.saab.com/products/ground-launched-small-diameter-bomb-glsdb>.
United States Air Force. ‘GBU-39B Small Diameter Bomb Weapon System’. n.d. <https://www.af.mil/About-Us/Fact-Sheets/Display/Article/104573/>.
United States Air Force – Eglin Air Force Base. ‘Small Diameter Bomb II approved for operational use’. 13 October 2020. <https://www.eglin.af.mil/News/Article-Display/Article/2380292/>.
United States Department of Defense. ‘Contracts for April 15, 2019’. 15 April 2019. <https://www.defense.gov/News/Contracts/Contract/Article/1814812/>.
United States Navy – Naval Air Systems Command (NAVAIR). ‘Small Diameter Bomb Increment II (SDB II)’ n.d. <https://www.navair.navy.mil/product/Small-Diameter-Bomb-Increment-II-SDB-II>.
Cover image source: U.S. Air Force.
Remember, all arms and munitions are dangerous. Treat all firearms as if they are loaded, and all munitions as if they are live, until you have personally confirmed otherwise. If you do not have specialist knowledge, never assume that arms or munitions are safe to handle until they have been inspected by a subject matter specialist. You should not approach, handle, move, operate, or modify arms and munitions unless explicitly trained to do so. If you encounter any unexploded ordnance (UXO) or explosive remnants of war (ERW), always remember the ‘ARMS’ acronym:
AVOID the area
RECORD all relevant information
MARK the area from a safe distance to warn others
SEEK assistance from the relevant authorities