Bad science in fiction Hall of Shame

[James Kerman]

If we're going by Hollywood examples, most American vehicles seem to be built out of TNT and solid gasoline.

[StrandedonEarth]

Although, later in the movie Arnie was strafing police cars with a minigun, and not a single one exploded until he pulled out the grenade launcher...

40 minutes ago, Gargamel said:

It takes a few seconds of under direct the flame of a blow torch for diesel to ignite. 

I once read an account of a train engineer stuck under the burning locomotive of a derailed train. He decided he was safer in the pool of spilled diesel. It kept him cooler 

Edited February 20, 2019 by StrandedonEarth

[magnemoe]

7 hours ago, James Kerman said:

If we're going by Hollywood examples, most American vehicles seem to be built out of TNT and solid gasoline.

Think this is a bit of an dead meme now as it was so overused. 

[Superfluous J]

7 hours ago, James Kerman said:

If we're going by Hollywood examples, most American vehicles seem to be built out of TNT and solid gasoline.

To be fair, plenty of foreign cars have also exploded in American movies...

[James Kerman]

28 minutes ago, magnemoe said:

Think this is a bit of an dead meme now as it was so overused. 

I have to admit that most modern films depict car crashes more realistically now but you still get the odd director that chooses this option.

[kerbiloid]

30 minutes ago, 5thHorseman said:

To be fair, plenty of foreign cars have also exploded in American movies...

Because they call petrol "gas".
So, whatever they fuel, it can burst.

[p1t1o]

To be pedantic, there are scenarios where a petrol tank can be say, shot, and it would explode in a fireball, but you'd almost never meet those conditions with a car. Its the reason why large fuel tanks, like those you might find in a plane, often have "inerting" systems such as nitrogen pressurisation.

But no, not cars. I'd imagine if you ever met the conditions with a car, the explosion would be small enough that it'd just basically rupture the tank and set fire to the car. Which is about the most you would normally expect to see.

On the other hand, if you set fire to a car, without damaging the tank, you can set up a BLEVE (https://en.wikipedia.org/wiki/Boiling_liquid_expanding_vapor_explosion) scenario, and it can make a decent explosion, but it might take a little while.

Edited February 20, 2019 by p1t1o

[mikegarrison]

Cars may not often explode in real life, but they burn really well. It's still not a good idea to be inside one if it's on fire.

[Gargamel]

8 hours ago, p1t1o said:

On the other hand, if you set fire to a car, without damaging the tank, you can set up a BLEVE (https://en.wikipedia.org/wiki/Boiling_liquid_expanding_vapor_explosion) scenario, and it can make a decent explosion, but it might take a little while.

No, properly designed gas caps have relief valves in them to prevent such a thing.  You'll not get a BLEVE scenario from a standard automotive gas tank. 

[Superfluous J]

3 hours ago, mikegarrison said:

Cars may not often explode in real life, but they burn really well. It's still not a good idea to be inside one if it's on fire.

That's solid advice for most anything.

[StrandedonEarth]

So you basically need to make the standard automotive gas tank rupture onto a tepidly hot surface so it can vaporize before being sparked...

[Gargamel]

1 hour ago, StrandedonEarth said:

So you basically need to make the standard automotive gas tank rupture onto a tepidly hot surface so it can vaporize before being sparked...

In an environment with no wind, next to a massless spherical cow.  

While this might lead to an explosion, most likely it will just flash over, and everybody loses their eyebrows.  You need the stoichiometry of the air/fuel mix to be just right for it combust with force.   This will look like an explosion, but won't really have any concussive force to it.  But it will start a lot of other things on fire, so it will appear to have exploded, but it's more like the fire just spread really quickly. 

You'd be better off putting 1 gallon of gas in a 50 gallon drum, seal it up, and shaking it real hard, and then remotely igniting it. 

Gasoline, in liquid form in a tank, is not explosive.   It's highly flammable, but it won't explode.   The air needs to mix with the gas in the right ratios to make it explosive.  And sitting in a closed tank with not so much air in it, does not tend itself to the right environment for this to happen, and that's by design. 

 

[StrandedonEarth]

14 minutes ago, Gargamel said:

In an environment with no wind, next to a massless spherical cow.  

While this might lead to an explosion, most likely it will just flash over, and everybody loses their eyebrows.  You need the stoichiometry of the air/fuel mix to be just right for it combust with force.   This will look like an explosion, but won't really have any concussive force to it.  But it will start a lot of other things on fire, so it will appear to have exploded, but it's more like the fire just spread really quickly. 

You'd be better off putting 1 gallon of gas in a 50 gallon drum, seal it up, and shaking it real hard, and then remotely igniting it. 

Gasoline, in liquid form in a tank, is not explosive.   It's highly flammable, but it won't explode.   The air needs to mix with the gas in the right ratios to make it explosive.  And sitting in a closed tank with not so much air in it, does not tend itself to the right environment for this to happen, and that's by design. 

 

Well, that all comes down to semantics. What many people call an explosion is just rapid uncontrolled combustion. But if the right stoichiometric mixture is present, then you get a detonation, complete with shock wave. 

A ruptured propane tank doesn’t explode; it rapidly combusts. But if a tank has time to leak just the right amount into an empty room, then the room can explode. 

The ET attached to Challengerdidnt explode; it ruptured and the propellants rapidly combusted. But the general population calls it an explosion. 

I guess I’m walking a thin line here. I tend to consider an actual explosion to be a detonation, but AFAIK a detonation generates a supersonic shockwave and/or combustion front. I don’t know if all fuel-air explosions fit my understanding of a detonation.

The definition of explosion tends to be a little looser. 

I think I just confused myself 

[Gargamel]

1 minute ago, StrandedonEarth said:

But if a tank has time to leak just the right amount into an empty room, then the room can explode. 

Yes, but who crashes a car in an empty room other than Micheal bay?

[magnemoe]

7 hours ago, StrandedonEarth said:

Well, that all comes down to semantics. What many people call an explosion is just rapid uncontrolled combustion. But if the right stoichiometric mixture is present, then you get a detonation, complete with shock wave. 

A ruptured propane tank doesn’t explode; it rapidly combusts. But if a tank has time to leak just the right amount into an empty room, then the room can explode. 

The ET attached to Challengerdidnt explode; it ruptured and the propellants rapidly combusted. But the general population calls it an explosion. 

I guess I’m walking a thin line here. I tend to consider an actual explosion to be a detonation, but AFAIK a detonation generates a supersonic shockwave and/or combustion front. I don’t know if all fuel-air explosions fit my understanding of a detonation.

The definition of explosion tends to be a little looser. 

I think I just confused myself 

Steam and compressed air can explode. It goes boom. Detonation is an explosion there you get an supersonic shockwave, this require an fuel air mixture or high explosives. Gunpowder does not detonate as i understand. 
Now lots of the explosions you see in movies is just rapid expanding burning liquid, this is pretty safe and easy to control, the fireball is also not very energy rich so has low chance of creating fires. 
Real explosions from grenades or bombs are gray from the dirt thrown up. 

[Codraroll]

If I recall correctly, explosions that happen with with a subsonic expansion of the explosion front are called deflagrations. This would include most liquid explosions. If the explosion front is supersonic, the term "detonation" is used instead.

Both of them would be classed as explosions, however. 

[mikegarrison]

9 hours ago, magnemoe said:

Gunpowder does not detonate as i understand.

Black powder does. Modern smokeless powder does not.

[p1t1o]

12 hours ago, mikegarrison said:

Black powder does. Modern smokeless powder does not.

Neither detonate on their own, both are made to detonate in their main function.

Cordite burns about as fast as a candle, black powder burns significantly faster, but very subsonic.

But what happens in a firearm is that they are contained in a pressure vessel (the breech). As soon as the first tiny part starts burning, the gas pressure starts to spike. And reaction rate is proportional to pressure, so reaction rate rises as well - which increases the rate of gas production and pressure rise which  increases reaction rate etc....

So what happens is that it starts to burn, but conditions rapidly change to increase the rate of reaction by many orders of magnitude, causing a detonation.

[DDE]

46 minutes ago, p1t1o said:

So what happens is that it starts to burn, but conditions rapidly change to increase the rate of reaction by many orders of magnitude, causing a detonation.

So a musket wouldn’t fire without a ball?

[Green Baron]

It would not. What should it fire ? The black powder in the primer would fizzle (assuming no flint lock), hopefully a spark would make its way to the cartridge, that in itself would fizzle away. "Psh ... pssssh". Like @p1t1o said. The expansion of the gas in the barrel drives the mass of the bullet. There has to be a pressure for the accelerated reaction to take place. Too heavy a bullet can damage the barrel or the mechanism with possible consequences for the shooter.

Haven't we all played with black powder as boys :-) ?

There was a certain amount of hope(tm) necessary that even a correctly prepared musket would fire on command. Or later. Or not at all. At least not now :-)

Edited February 22, 2019 by Green Baron

[p1t1o]

22 minutes ago, DDE said:

So a musket wouldn’t fire without a ball?

What @Green Baron said. It would fire, but it wouldnt go "bang".

[DDE]

15 minutes ago, p1t1o said:

What @Green Baron said. It would fire, but it wouldnt go "bang".

Good enough for Hollywood.

[magnemoe]

14 minutes ago, p1t1o said:

What @Green Baron said. It would fire, but it wouldnt go "bang".

This, now blank ammunisjon has an plastic or metal cartridge with an rupture zone in front who let pressure build up. For an musket you could probably use cloth or fiber. 

And it makes sense cordite burn slower as its used in cannons who has higher chamber pressure than normal guns. 

[kerbiloid]

1 hour ago, DDE said:

So a musket wouldn’t fire without a ball?

It wouldn't fire without a wad.

[ARS]

That reminds me,  explosions on TV and movies are unusually large, visually impressive fireballs, that appear to be fueled by gasoline, propane or another similar such fuel, even when the object in question has no right to explode at all, much less spectacularly. Sometimes they are (or include) a shower of sparks while in real life, a fireball is often a minor part of an explosion (though not always, as is the case with incendiaries, like phosphorus, uranium, gelled fuels like napalm, etc). The real devastation from an explosion often comes from the force (overpressure) of the blast and the flying debris and shrapnel. Very rarely will explosions look like what they do in real life, an expanding cloud of dust and debris with very little light — and very little left behind (Grenades are very prone to this in movies, since even a "pineapple" frag grenade can produce a napalm-bomb explosion that throws people around, with ironically, no fragments) . Also, most explosions on TV also burn at a ridiculously slow rate compared to what they would on real life, which conveniently enables one to outrun it. In addition, the more powerful the explosion, the more quickly it uses up or blows apart the reactants involved in any combustion, so there's an inverse relationship between how powerful the blast is and how much fire there is. Oh, and fireballs in space. Perhaps some chemical mixtures can make fireballs in a near vacuum, but it's unlikely most space explosions are like that. This occurs in 90% of all onscreen explosions.