Do tankers go "Bang"?

A comment before we begin:
None of the following really matters. The power of these stories do not rest on the physics but on the people and the situations. John Marsden is simply not a "physical detail" guy. Instead his genius is in character and plot. Please keep this in mind when you read the following discussion of the physics of some of the attacks, which I put together simply because I was interested.

Several times throughout this series, John Marsden has the characters use petrol as an explosive (and, on one occasion, Natural Gas)

1. In "Tomorrow, When the war began" Ellie causes a lawn mower to explode, killing two soldiers and injuring a third. She uses a trail of petrol to detonate the lawn mower's (open) fuel tank.

 
2. The last attack in "Tomorrow, when the war began" where the group uses a petrol tanker to blow up a bridge, using rope dipped in petrol as a fuse.

 
3. The last attack in "The Dead of the Night" when they turn on gas appliances in Turner Street and set a delayed action spark generator.

 
4. From "Burning for Revenge" where the group shoots tankers loaded with Jet Fuel using assault rifles during the attack on the airbase.

 
5. The last scene is the attack on the roadhouse in "The Other Side of Dawn" where Ellie puts plastic explosive in the underground fuel storage tanks.

We are surrounded by lawn mowers and petrol tankers, which regularly crash, and by gas appliances that go out, but it is quite rare to have them disappear in an instant, catastrophic, explosion. So how reasonable is it to use them as bombs ?

Let's take a look

First, a caveat. Explosions are a complex subject and subject to a very large amount of variability. For some idea of the limitations of simulation and discussion, take a look at the summary of the TWA FLIGHT 800 loss investigation that the Californian Institute of Technology has put together. First a summary (gets quite complex) then some facts (in laymans' terms) and then some common misconceptions (again in laymens' terms).

Now, some background.

Explosions result when combustion occurs so rapidly that there is a large build up of pressure from the combustions by-products, the shock wave from these by-product gases expanding is the explosion. If combustion is not this rapid, you just get a fire.

Having the combustion inside a container helps create an explosion as the walls of the container concentrate the shock wave (by restraining it until the internal pressure rises sufficiently high to rupture the container).

To get combustion you need a fuel but you also need oxygen. Purpose built explosives (including the Ammonium Nitrate bomb they use in "The Third Day, the Frost") have the oxygen locked up in the material of the explosives already, but petrol, jet fuel, natural gas, etc do not.

Another thing to be aware of is that liquids do not burn. It's the vapour from them that burns and to get the vapour to burn you need it in the right concentration relative to the oxygen in the air.

One more thing is that fuels, as they don't have their own oxygen mixed into them, have a much greater energy density then explosives. For all the fuels used in the "Tomorrow" series (Natural Gas - essentially Methane, Petrol/Gasoline, and Jet Fuel (JP4, 5 or 8) the energy density is about 12 times that of TNT or Semtex (plastic explosives) or about 50 Megajoules per kilo (vs 4.1 per kilo for TNT)).

There are three basic ways flammable liquids burn.

1. Where fuel is sprayed into the air then ignited (Such as in a car engine, a plane crash, a military Fuel-Air-Explosive bomb or in a Boiling Liquid Expanding Vapour Explosion) If the conditions are just right (such as in a car engine or when a Fuel-Air-Explosive bomb works correctly) you can get an explosion, otherwise you get a fireball.

 
2. In a 'pool fire' where a puddle of liquid burns. This can still result in an explosion when confined in a container and a) the burning is sufficiently rapid that the failure pressure of the container is exceeded before the gases produced can vent and b) there is sufficient air in the container to produce enough combustion by-products to pressurise the container past its failure point.

   A pool fire is what happened when spilt fuel is ignited.

   Note: You can't get a 'pool fire' in a container unless the ratio of fuel vapour in the container to air is less than the fuel's "Upper Explosion Limit" and you can't get a pool fire at all unless the temperature of the fuel at the top of the pool is over its "Flash Point" temperature.


 
3. The ignition of a vapour cloud, which is the gas given off by a fuel when it is heated past its "Flash Point". Vapour cloud explosions can be extremely dangerous, but only occur in quite limited circumstances. For petrol they normally occur in 'empty' fuel tanks when someone does some work on them and create a spark.

   Several aircraft carriers were lost to such explosions in World War Two, when leaking petrol stores gave off enough gas to create an explosion several hours later. Most notably USS Lexington and IJNS Taiho

   Flight TWA-800 (a 747) was also lost to a vapour explosion in one of its fuel tanks

A vapour cloud explosion is most likely what was happening to the tin cans Ellie and Homer used to shoot at, followed by a fireball as the remaining fuel was ignited and sprayed through the air.

One of the interesting things about the explosion of fuel containers is that the less fuel in the container, the bigger the potential explosion (as the more air there is available to react with the fuel). The limit to how big an explosion you can get with a fuel container is the amount of air available to burn. The amount of fuel actually consumed in burning all the available air is normally quite trivial.

Now for an explosion in a container the normal way to get an explosion is via a the 3rd path, a vapour explosion. For vapour explosions there are a number of factors to be taken into account. These are listed below:

1. The "Lower Explosive Limit" (LEL)
   The minimum concentration of inflammable vapour in the air that can lead to sustained ignition. (Below this the mixture is "too lean" to burn).

 
2. The "Upper Explosive Limit" (UEL)
   The maximum concentration of inflammable vapour in the air that can lead to sustained ignition. (Above this the mixture is "too rich" to burn).

 
3. The "Flash Point".
   The flashpoint of a liquid is the lowest temperature at which the liquid gives off enough vapour to be ignited (start burning) at the surface of the liquid.

   The importance of the "Flash Point" is that you can't get a liquid to burn at less than its "Flash Point". Diesel has a Flash Point of 49C, Petrol -40C. If you throw a lit match into a bucket of diesel at room temperature you get a wet match but if you throw a lit match into a bucket of petrol at room temperature you would get a major fire.

OK, so its vapour (gas) that burns, the vapour has to be in a certain mix with air (varying by material) and it all has to burn at once to get an explosion. The last means is only the vapour present at the time of ignition will participate in the explosion. This means there is an upper limit on the size of the explosion you can create inside a given container with a given fuel.

That is the volume times the Upper Explosion Limit times the energy density of the material times the vapour density of the vapour relative to air times the density of air (assumed to be 1.25g/l).

We can now do some calculations on just how big a bang you can get in various circumstances (best case)

Material	LEL	UEL	Vapour Density	TNT Equiv	Flash Point
Natural Gas	5%	15%	0.55	12	<0C
Petrol	1.4%	7%	4	12	<0C
Diesel	0.7%	5%	4.5	12	49C
Jet Fuel (JP4)	1.3%	8%	4.5	12	<0C
Jet Fuel (JP5)	0.7%	5%	4.5	12	60C
Jet Fuel (JP8)	0.5%	5%	4.5	12	38C

LEL = Lower Explosion Limit
UEL = Upper Explosion Limit
Vapour Density = Density of the vapour compared to air (air is about 1.25g / litre)
TNT Equiv = number of kilos of TNT one kilo of this fuel is the equivalent of
Flash Point = Lowest Temperature where the fuel will produce vapour

 

Container	Volume
Mower Petrol Tank	5 litres
Tanker	35,000 litres
45 square house, 3m ceilings	1,250,000 litres

If we assume the container is just about empty of liquid fuel, we get the following upper limits on the size of the explosions in each attack.

Note: There is a factor missing here, which is the amount of oxygen available to burn. For some combinations of fuel and vapour pressures, there will not be enough oxygen to fully combust the available fuel, so these are very definitelyupper limit on the size of the explosion.

 

Attack	Mechanism	Max possible vapour explosion (measured in equivalent amount of TNT)
Mower	Mower with Petrol	21 grams of TNT
Bridge	Tanker with Petrol	150 kilograms of TNT
Turner Street	Natural Gas	1.5 tons of TNT per house
Airfield	Tanker with Jet fuel	190 kg of TNT per tanker

The truck stop is a special case as a significant explosive charge is used to burst the tank and a type 1 explosion is possible.

OK, so we can get some reasonably significant bangs, so lets take a look at each scenario in more detail

1. The lawn mower.

   If the tank was about empty, and if the explosion was constrained (i.e. did not just vent through the fill cap) and if the vapour in the tank was not above the UEL, then we could get an explosion with a force of about 1/5th of the charge in a hand grenade.

   This could do some damage, especially if it splashed lots of unburnt fuel around, but there are rather a lot of ifs here and petrol tanks (that have fuel in them) don't normally explode. The article on misconceptions from Caltech about TWA-800 (which was lost due to a vapour explosion in a almost empty fuel tank) explains why. The relevant sections are reproduced below:

   "Cars contain fuel pumps and wiring inside the fuel tanks - why don't they blow up more often?

   Gasoline tank vapour spaces are almost never flammable while Jet A tanks in airplanes will always pass through a flammable regime during normal flight operations.

   The ullage of gasoline fuel tanks in automobiles is almost always too rich to be flammable except at very low temperatures. This is due to the much lower flash point (about -40 C) of gasoline in comparison to Jet A. The vapor space in a partially-filled gasoline tank does not become flammable until the temperature has dropped below about 10F and a serious hazard will exist below 0 F down to about -40 F for a typical gasoline (Reid Vapor Pressure (RVP) of 9.5 psi, flammability limits between 1.4 and 7 % by volume). [See W.F. Marshall and G. A. Schoonveld, SAE Transactions, Vol. 99, No. 4, 594-617, 1990]

   For this reason, the probability that a fuel tank containing liquid gasoline has a flammable vapour space is extremely small in most climates except in the artic regions. The exception to this is when the tank is removed for servicing and the fuel is drained from the tank. Since gasoline has such a high vapour pressure, the tank can have a flammable vapour space even if there is no liquid fuel visible inside that tank. For example, the complete vaporization of about 1-2 tablespoons of gasoline will result in a flammable mixture inside a 15-gallon capacity automobile tank! This is the reason why welding on or near "empty" gas tanks is extremely hazardous and thorough purging of the tank with steam, carbon dioxide, nitrogen, or other inert gas is required before repair work is started.

   ...

   Fire and explosions occasionally (about 3 accidents out of 1000) do happen when gasoline-powered vehicles crash. Post-crash fires are a serious safety issue for automobiles and are the subject of ongoing study, legislation, and litigation. Fires occur after the gas tank is ruptured and the accident results in an ignition source, often arcing electrical wiring or exposed hot lamp filaments. It is actually much harder to ignite gasoline by spilling it on a moderately hot surface that it is to ignite Jet A so that tailpipes and exhaust manifolds are not good ignition sources for gasoline. On the other hand, gasoline makes a large vapour cloud very quickly and if a high-temperature ignition source is introduced, a very rapid or "flash fire" will be the result. This can serve as an ignition source for the puddle or pool of fuel under the leaking tank, causing a pool fire that may destroy the vehicle. "

   So, overall, I would have to say the lawn mower incident is not credible. They would get a nice fire, but an explosion is very hard to justify.


 
2. The Bridge

   The bridge has essentially the same problem the Lawn Mower, though on a larger scale. The tanker is also just about full so there is very little air inside to burn and cause the tanker to explode. Now tankers do explode. Here is an incident report of one such explosion that occurred just north of Sydney, Australia in the year 2000. Note the very limited blast effects in the photos, and the localised damage over all. There was a massive fire, and there were explosions but it was the fire that was the problem, not the blast.

   Now the bridge attack does not rely on blast, all they need is for the tanker to break open and burn, and it would certainly do this. The heat from the burning compartments may well also lead to structural failures in the tanker, some minor explosions and a huge and spreading pool of fire. Therefore the success of the attack is credible, but the description is not.


 
3. Turner Street

   Looking at the numbers you can certainly get quite an impressive explosion using gas. The problem is the amount of gas required. For the full sized bang you need about 125 kg of gas per house, or a bit over 6 gigajoules. Now a big gas heater will deliver about 25 Megajoules per hour. (about ½ a kilo). To fill the house for the biggest bang you would need such a heater to vent gas for about 250 hours. Modern appliances in Australia are also required to have 'auto off' flame failure detectors so they will cut off the gas supply when there is a problem.

   The best thing to have done would likely have been to cut the gas line to the appliance (after unplugging the gas line to prevent sparks from being a problem - those gas lines are wire reinforced) and then plug it back in. If you did that you might well get 250 MJ per hour out. After half an hour that is the equivalent of 30 kg of TNT and would demolish most houses. It would NOT produce a huge explosion similar to Krakatoa, but may well do the job - except for one more problem. The Lower Explosion Limit. You need at least 5% methane by volume for an explosion and the house has a lot of volume. It is also lighter than air, so turning on any ceiling fans would likely be a good idea to help circulate the gas in the room.

   So, if you cut the gas line, turned a ceiling fan on and set the timer the most likely result would be nothing at 30 minutes as the LEL had not been reached, but as the gas accumulated and the toaster continued to spark you may well reach the LEL at some later stage in the night and get an explosion that is the equivalent of a few hundred kilos of TNT. Still not Krakatoa but nice enough.

   So this attack is also credible, so long as you change the means of attack a bit. The problem is you are using normal appliances and the amount of energy available for a bang is limited by the amount of gas they can put out. You need to get round that limit.


 
4. The airfield

   This attack has the most problems. To understand why, pretend the refuelling tankers were half full of diesel rather than jet fuel. Would you expect them to explode ? No, of course not, you have to go to a lot of trouble to get diesel to ignite. Shooting a big tanker of diesel with a rifle isn't going to do it.

   Why is that a problem ? Well, modern Jet Fuel is much more like diesel than petrol. Have a look at the numbers for "Flash Point" for JP8 vs diesel vs petrol. (38C vs 49C vs -40C). Neither diesel nor modern jet fuel is considered "inflammable" (JP8 is the military equivalent of Jet A, the standard commercial jet fuel, see here for a discussion of jet fuels). Just like Diesel, modern jet fuel, at normal temperatures, will not ignite from a spark.

   There are only two ways for this attack to work. 1) They are using JP-4 (and obsolete fuel with a flash point similar to petrol) or 2) the tanker's internal temperature is over 100 degrees F.

   OK, so the most likely result of shooting at the refueling tankers is a very short lived disappointment followed by a lot of dead main characters, but for the sake of the story lets assume they are using JP-4 or it is a VERY hot November morning, the tankers have been sitting in the sun and are painted dark colours.

   Now, if that is the case, then knocking a hole in the tanker in the region where there is vapour could create an explosion with a force between 10 and 100 or so KG of explosive, if the vapour in the tanker was below the UEL. This would be enough to rupture the tanker and spray the fuel around nicely, accomplishing the effect desired, but it is hardly going to toss a 5 ton truck that is a decent distance away another 50 meters.

   If you hit in the fuel filled portion, you get a hole and a leak, which might ignite but can't flash back into the tanker as there is not path for the flame.

   If you hit in the air filled portion and the contents vapour is above the UEL (which is probably is if using JP4 - see the discussion of automotive gasoline tanks under the mower attack section), then you get a hole, a leak and likely a flame as the vapour vents out. Neither of these are going to destroy the tanker quickly. Now JP4 was dropped due to loss of aircraft due to fires, but aircraft (especially in flight) are much more vulnerable to even the smallest fire than a tanker on the ground is.

   It is very hard to construct a scenario where you can create mayhem by shooting the refueling tankers, and even if you did, you aren't going to get a huge blast, rather a very impressive fire. To me, all the above adds up to the airfield attack being rather far fetched. The idea for it is apparently based on a newspaper article about a partisan attack on an airfield in WWII, but those tanks may well have just had vapour in them (making them potentially quite explosive), or it was very cold (below 0 degrees F for a petrol tank void to become explosive when it contains liquid petrol) - and would have been petrol rather than jet fuel in the tanks anyway.


 
5. The truck stop

   Well, here they go about it the right way. Ellie drops some plastic explosive in the underground storage tank and that should rupture it nicely. All concerns about LEL and UEL disappear.

   The issue with this attack is they the explosion appears to be mainly underground, which is not what you would expect. Fuel needs air to burn after all. What you are much more likely to get is a very nice fireball as the fuel is heated and thrown into the air and the characteristics of a fireball is large amounts of radiant heat rather than a big shock wave. You might just luck out and get a proper 'fuel air explosive" but they are very hard to get going, even with specially built bombs and even then the shock wave will above ground rather than through it. To get a big bang you have to bring the fuel and air together. You can't get the air to the fuel, so you have to take the fuel to the air, and that means an above ground explosion or fireball.

   Nice description though.

So there you have it. Some of the attacks are credible, some are not, but none are likely to happen in the way or with the effects described.

Note: JM mentions in Marsden on Marsden that his explosives consultant was his niece, Elizabeth Farran who has her "shot firer's" ticket (she has the licence required to use explosives). If she has OKed what he wrote, then maybe it works, but I am stuffed if I can see how. If Elizabeth ever stumbles across this page, please take pity and send me a note explaining.

There are lots of links on this page, some of which lead to very interesting information on how explosions happen and are well worth a visit. The ones on aircraft crashes (NASA), TWA-800 explosives investigation summary, general facts and common misconceptions (Caltech) and fuel air explosives and vapour / dust explosions in general (Federation of American Scientists) are particularly worth a visit. So are the material safety data sheets for methane (natural gas), diesel, JP4, JP5, JP8/Jet A and gasoline. The specs for JP5 and JP8/Jet A, particularly Flash Point, were quite a surprise - I had always thought jetfuel was highly explosive - but the opposite it true.