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)
- 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.
- 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.
- 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.
- From "Burning for Revenge" where the group shoots tankers loaded with Jet
Fuel using assault rifles during the attack on the airbase.
- 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
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
There are three basic ways flammable liquids burn.
- 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.
- 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.
- 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
(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
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:
- 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).
- 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).
- 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)
|Jet Fuel (JP4)
|Jet Fuel (JP5)
|Jet Fuel (JP8)
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
|Mower Petrol Tank
|45 square house, 3m ceilings
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.
||Max possible vapour explosion (measured in equivalent
amount of TNT)
||Mower with Petrol
||21 grams of TNT
||Tanker with Petrol
||150 kilograms of TNT
||1.5 tons of TNT per house
||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
- 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.
- 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
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.
- 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.
- 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.
- 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,
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,
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.