Controlling Explosions — By Timers, Barometric Devices, Switches and More

How does the ter­ror­ist of 2010 ensure that their device det­o­nates at the intend­ed time or loca­tion? Andy Oppen­heimer stud­ies the var­i­ous con­trol mech­a­nisms that can be used and what they might look like on phys­i­cal inspec­tion or under X‑ray exam­i­na­tion.

On Christ­mas Day 2009, an attempt by 23-year-old Niger­ian Umar Farouk Abdul­mu­tal­lab to blow up North­west Air­lines Flight 253 bound for Detroit failed — most like­ly due to a faulty det­o­na­tion sequence. The would-be bomber, whose device incor­po­rat­ed a cock­tail of explo­sives hid­den in his under­pants, is said to have tried to ini­ti­ate it by inject­ing an acid into the main charge of Pen­taery­thri­tol (PETN), a proven, sta­ble high-explo­sive com­bined with the per­ox­ide-based home­made explo­sive Tri­ace­tone Triper­ox­ide (TATP). It is believed the inject­ed acid was intend­ed to ignite the TATP in order to, in turn, det­o­nate the PETN — but the nor­mal­ly high­ly volatile TATP did not ignite as planned, as the acid inject­ed into the explo­sives may not have pro­duced enough heat.

This botched attempt at mass mur­der in the skies (and the failed Times Square, New York car bomb attempt on 1 May 2010) illus­trat­ed a gap in the ter­ror­ists’ learn­ing curve with regard to det­o­na­tion sequences and more gen­er­al­ly, the body of knowl­edge and exper­tise on mak­ing bombs explode at exact­ly the right time, place, and to the max­i­mum effect desired. Unless an air­borne impro­vised explo­sive device (IED) has all the com­po­nents func­tion­ing at once, fail­ure is pos­si­ble – and the more links in the chain of ini­ti­a­tion, the more like­ly it is to mal­func­tion. Such devices, although appear­ing crude, are com­plex enough to require test­ing in real time and in the actu­al set­ting for the attack – a vir­tu­al impos­si­bil­i­ty for most ter­ror­ist groups.

Learn­ing Curve

While sui­cide ter­ror­ists tend not to need advanced delay timers and remote-con­trolled ini­ti­a­tion, the Tal­iban and al-Qae­da are mov­ing towards greater con­trol of IED explo­sions for max­i­mum effect, more pre­cise tar­get­ing, and avoid­ance of pre-emp­tion. Ad-hoc self-starters, dis­parate groups and fanat­i­cal indi­vid­u­als who may try to hijack and blow up an air­craft may not have access to exper­tise, com­mand, con­trol, and sup­ply of mate­ri­als. The cur­rent less defin­able per­pe­tra­tors have less for­mal and recog­nis­able struc­tures and sup­ply chains. These ter­ror­ists must there­fore resort increas­ing­ly to home­made explo­sives (HME) and mechanisms.

They also face mod­ern deter­rence and pre-emp­tion. Had the Christ­mas Day bomber had the lux­u­ry of choos­ing the best type of det­o­na­tor for his mis­sion, he would have used a blast­ing cap or an explod­ing wire det­o­na­tor to set off the PETN in his device, which would have sent a rapid shock wave into it. How­ev­er, com­mer­cial det­o­na­tors may well show up on a stan­dard air­port scan­ner There­fore, although it is hard to detect PETN and some oth­er reli­able con­ven­tion­al, sol­id explo­sives at the secu­ri­ty check, the stan­dard scan­ning sys­tem for checked bag­gage — enhanced world­wide fol­low­ing the Locker­bie air­craft bomb­ing – may have pro­vid­ed a vital deter­rent to effec­tive detonation.

Of grow­ing con­cern, how­ev­er, is the like­li­hood that the bombers will adapt to the cat and mouse game of coun­ter­mea­sures. If they can’t smug­gle com­mer­cial det­o­na­tors on board, they could attack the ter­mi­nal, inside or out­side, with a com­mer­cial-det­o­na­tor IED or impro­vised chem­i­cal device (ICD) before search­es take place. This threat reared its head when ter­ror­ists, hav­ing failed to set off car bombs in Lon­don in August 2007, drove a vehi­cle loaded with gas cylin­ders into the ter­mi­nal at Glas­gow Airport.


the ‘key’ to all bombs Com­mer­cial elec­tri­cal det­o­na­tors work by send­ing a rapid shock into the main explo­sive charge. They are nec­es­sary to set off sta­ble high explo­sives; less so in insta­ble HME such as the per­ox­ide mixes.

Det­o­na­tors often mal­func­tion. When they become detached there is no explo­sion, and many explo­sives ord­nance dis­pos­al (EOD) oper­a­tions depend on dis­rupt­ing or detach­ing the det­o­na­tor (and the tim­ing and fir­ing mech­a­nism) from the explo­sive charge.

Faulty ini­ti­a­tion seems to be the main cause of sev­er­al recent failed attempts. The Lon­don 2007 inci­dents, although not air­borne or at an air­port, demon­strat­ed this. As in many oth­er attacks around the world, the ter­ror­ists attempt­ed mobile phone ini­ti­a­tion to blow up two cars parked near a night­club. The ‘det­o­na­tors’ were high­ly impro­vised — con­sist­ing of two mobile phones wired to a light bulb sur­round­ed by match heads. A phone call was sup­posed to trig­ger the home­made device ignit­ing the vapours swirling inside the cars. Only one det­o­na­tor sparked, but even that was quick­ly snuffed out because the mix­ture of petrol and gas was too thick and there was insuf­fi­cient oxy­gen to trig­ger the devices.

The Times Square bomb sus­pect, Faisal Shahzad, told police that he lit a fuse which he expect­ed would det­o­nate the car bomb with­in five min­utes as he walked away. The tim­ing device was a sim­ple alarm clock. How­ev­er, the bomb mal­func­tioned, emit­ting smoke that attract­ed the atten­tion of the street ven­dor who report­ed the sus­pect vehi­cle. Inves­ti­ga­tors believe Shahzad pur­pose­ly made the device out of less potent fer­tilis­er and M88 fire­crack­ers to avoid detection.

Chem­i­cal det­o­na­tion: the grow­ing threat

Today’s air­borne ter­ror­ists are more like­ly to use chem­i­cal det­o­na­tion, as attempt­ed by the Christ­mas Day bomber. These are less rel iable meth­ods than stan­dard com­mer­cial det­o­na­tors. Where a com­mer­cial det­o­na­tor can­not be used, the per­ox­ide-based HMEs, TATP or Hexa­m­eth­yl­ene triper­ox­ide diamine (HMTD) – used in the Lon­don July 2005 bomb­ings – may be the detonator.

“…a phone call was sup­posed to trig­ger the home­made device ignit­ing the vapours swirling inside the cars…”

TATP was select­ed as a det­o­na­tor by the so-called ‘shoe bomber’, Richard Reid, when he, on 22 Decem­ber 2001, tried to blow up Amer­i­can Air­lines Flight 63 only months fol­low­ing the 9/11 attacks, by try­ing to ignite with match­es two TATP-PETN devices hid­den in his shoes. The device had been made with a small thread of TATP run­ning through 100g of PETN, attached to a pow­der fuse run­ning through the shoelace. As with the Christ­mas Day bomber near­ly a decade lat­er, TATP was seen as an unusu­al trig­ger for PETN, but it was acknowl­edged as a sophis­ti­cat­ed com­bi­na­tion to avoid detec­tion. The com­plex ini­ti­a­tion sequence for the devices intend­ed to down sev­en transat­lantic air­lin­ers in the 2006 liq­uid explo­sives plot revealed a sophis­ti­cat­ed attempt at chem­i­cal det­o­na­tion. The ter­ror­ists planned to use soft drinks con­tain­ers and replace their orig­i­nal con­tents via a syringe with a home­made liq­uid explo­sive — con­cen­trat­ed hydro­gen per­ox­ide mixed with a pow­dered fruit drink which would help detonation. 

“…the 2006 liq­uid explo­sives plot revealed a sophis­ti­cat­ed attempt at chem­i­cal det­o­na­tion…”

A small amount of HMTD – was intend­ed to chem­i­cal­ly det­o­nate the per­ox­ide soft drinks mix and was dis­guised in hol­lowed-out bat­ter­ies. An adapt­ed minia­ture light bulb or sim­i­lar heat source was to pro­vide the elec­tric ele­ment, with its tail leads and with the ele­ment exposed, and would have been con­nect­ed to a dis­pos­able flash cam­era to ini­ti­ate the det­o­na­tion of the liq­uid explo­sive. The IEDs were intend­ed for easy assem­bly and det­o­na­tion dur­ing the flights and the sep­a­rate com­po­nents – the bat­ter­ies, drinks bot­tles, and cam­eras at least – could arguably have passed through secu­ri­ty as inno­cent items. 

Tim­ing is of the essence

Tim­ing mech­a­nisms evolved through neces­si­ty – the need to fire off a bomb at a des­ig­nat­ed time and place and the need to avoid being too close to the det­o­na­tion being upper­most. The IRA, which also usu­al­ly tried to min­imise civil­ian casu­al­ties, devel­oped most of the tim­ing mech­a­nisms used by the Tal­iban and oth­ers today, from watch­es and ana­logue and dig­i­tal clocks to cen­tral-heat­ing, VCR and elec­tric fence timers. They also tar­get­ed air­craft with timed IEDs.

How­ev­er, one of the IRA’s minia­turised tim­ing mech­a­nisms failed in its most seri­ous attempt to down an air­craft, on 23 July 1974, when a flight bring­ing sev­er­al MPs and the Roy­al Ulster Constabulary’s Chief Con­sta­ble from Belfast to Lon­don was divert­ed to Man­ches­ter fol­low­ing an IRA phoned warn­ing that four bombs were on board. After evac­u­at­ing the air­craft, a 1kg device was found under a seat. Its ana­logue wrist­watch timer had failed because a draw­ing pin fixed to the watch’s face was cov­ered by a thin lay­er of paint. This had act­ed as an insu­la­tor and pre­vent­ed elec­tri­cal con­tact with the hand of the watch.

Once lug­gage scan­ning became more wide­spread, ter­ror­ists tried to use less detectable plas­tic or liq­uid explo­sives det­o­nat­ed by minia­turised and benign-look­ing timers. By the mid-1990s this strat­e­gy for small­er devices was epit­o­mised by the so-called Bojin­ka plot, when three men were accused of plan­ning to put 12 bombs on US air­lin­ers car­ry­ing 4,000 peo­ple across the Pacif­ic Ocean to US west coast gate­way air­ports in Jan­u­ary 1995. Each device would have includ­ed a timer craft­ed from rewiring a Casio dig­i­tal watch, which was to be con­nect­ed to a sta­bilised form of liq­uid nitro­glyc­erin, stored in a bot­tle that appeared to con­tain con­tact lens solu­tion. The timer was crude – a met­al rod taped to the foot of one of the conspirators.

Under pres­sure: baro­met­ric timers

Baro­met­ric timers work in response to the lev­el of air pres­sure inside the air­craft – which is low­er than sea-lev­el pres­sure. Once the plane ascends and atmos­pher­ic pres­sure falls below 950mB a baro­met­ric or altime­ter trig­ger­ing device, which is sen­si­tive to high-alti­tude atmos­pher­ic pres­sures, will move a diaphragm which push­es a switch con­nect­ed to a bat­tery, and which by elec­tric charge blasts the cap and det­o­nates the bomb.

The Sem­tex bomb brought aboard Pan Am flight 103 which blew up over Locker­bie on 21 Decem­ber 1988 was said to have been set off by a baro­met­ric switch­ing process cou­pled with a long-delay timer, a frag­ment of which was found in the wreck­age. As the plane cruised at about 30,000 feet, the baro­met­ric sen­sor is said to have com­plet­ed a cir­cuit which start­ed the timer.

“…its ana­logue wrist­watch timer had failed because a draw­ing pin fixed to the watch’s face was cov­ered by a thin lay­er of paint…”

Blow­ing up an air­craft at cruis­ing alti­tude (29,000–45,000 ft) is like burst­ing an inflat­ed bal­loon with a pin-prick. Only a small amount of explo­sive would be need­ed to blow a small hole suf­fi­cient to destroy it or at least cause it to crash.

Com­pact enough to be con­cealed in a radio cas­sette play­er, the Locker­bie IED was recon­struct­ed and said to have incor­po­rat­ed a basic com­mer­cial det­o­na­tor and a cir­cuit board for a small timer. Recon­struc­tion of the device showed that inside the cas­sette play­er case a large bat­tery was wired to a baro­met­ric sen­sor — the bel­lows unit from an aneroid barom­e­ter — con­cealed under the cassette-player’s motor. This in turn was set to start the MST-13 count­down timer, which in turn was wired to a det­o­na­tor to set off the 350g of foil-wrapped Sem­tex slot­ted into space for the loud­speak­er. There is dis­pute as to whether the Locker­bie bomb was a dual device con­tain­ing a baro­met­ric switch and a timer, or a sin­gle trig­ger device, which was acti­vat­ed by only a timer.

The advan­tage of baro­met­ric timers for ter­ror­ists is that they are only acti­vat­ed once the plane is air­borne; the bomb will not go off on the ground if the plane is delayed. The air­craft has to take off and gain height so that some sev­en or eight min­utes into flight, the air pres­sure drops enough to acti­vate the baro­met­ric timer, which would be set to go off 30 min­utes lat­er, that is, 37 or 38 min­utes after the flight took off. The time delay on the Locker­bie bomb timer (claimed as a MST-13, but this remains con­tro­ver­sial), had been set at about four hours to allow the air­lin­er to be far above the Atlantic. This was meant to guar­an­tee that every­thing on board would be lost with­out trace. But as Flight 103 depart­ed late, the explo­sion took place over Scot­land and remains Britain’s worst ter­ror­ist event.

Spot­ting the components

Nowa­days most of the com­po­nents in the Locker­bie device would show up on scan­ners and be spot­ted by oper­a­tives trained to find odd wires and switches.

Some com­mer­cial det­o­na­tors such as blast­ing caps, and con­ven­tion­al det­o­na­tors based on ful­mi­nate mer­cury or lead may show up on X‑ray scan­ning. How­ev­er, small­er, thin-walled det­o­na­tors (as described by Bruce Kof­fler in ASI, Feb­ru­ary 2009 issue) may be more eas­i­ly con­cealed or made to resem­ble oth­er items in the lug­gage. Paper-shelled and oth­er impro­vised det­o­na­tors and are even hard­er to detect.

The main fear is that ter­ror­ists adapt their meth­ods and tac­tics to evade scan­ners and oth­er deter­rents by not using con­ven­tion­al timers and det­o­na­tors. The Christ­mas Day attempt did not include timers or stan­dard det­o­na­tors but was still not detect­ed as it was con­cealed in the bomber’s under­wear. This has prompt­ed the need for selec­tive body scan­ning and spec­tro­scop­ic scan­ning for liq­uid chem­i­cals, in addi­tion to the cur­rent restric­tions on liq­uids. The grow­ing use by ter­ror­ists of liq­uid explo­sive mix­es in small devices, as evi­denced in the liq­uid explo­sives plot, and attempts at chem­i­cal det­o­na­tion is still a chal­lenge to scan­ning sys­tems world­wide. There­fore, secu­ri­ty staff in any air­port must be well trained and extra vig­i­lant in spot­ting oth­er fac­tors alert­ing them to a pos­si­ble sus­pect device.

© Avi­a­tion­se­cu­ri­ty­in­ter­na­tion­al
Issue Octo­ber 2010 

About The Author:

Andy Oppenheimer

Andy Oppen­heimer is an inde­pen­dent UK-based spe­cial­ist in CBRNE (chem­i­cal, bio­log­i­cal, radi­o­log­i­cal, nuclear weapons and explo­sives) and coun­tert­er­ror­ism. He is Edi­tor of Chem­i­cal & Bio­log­i­cal War­fare Review and G2 Defence Intel­li­gence & Secu­ri­ty, for­mer Edi­tor of Jane’s Nuclear, Bio­log­i­cal and Chem­i­cal Defence, NBC Inter­na­tion­al, and Jane’s World Armies, and is a Mem­ber of the Inter­na­tion­al Asso­ci­a­tion of Bomb Tech­ni­cians and Inves­ti­ga­tors. His book IRA: The Bombs and the Bul­lets — A His­to­ry of Dead­ly Inge­nu­ity (Irish Aca­d­e­m­ic Press, 2008) is regard­ed as the sem­i­nal work on the mil­i­tary cam­paign of the Irish Repub­li­can movement. 

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