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> But even with helium, airships are still very fragile and dangerous. True. But with high speed landing and takeoff aeroplanes are extremely dangerous too.

by worik 4y ago

> But even with helium, airships are still very fragile and dangerous. True. But with high speed landing and takeoff aeroplanes are extremely dangerous too. Thousands of gallons of high octane fuell in the tanks on board does not help

criddell 4y ago
> high octane fuel Doesn’t jet fuel have a relatively low octane rating compared to most liquid fuels?
- LarryMullins 4y ago
  Yes, but I think he's using it in the colloquial sense that means "highly energetic".
- bruce511 4y ago
  It depends on the plane, but both your point and the parent point are correct. Piston engines run on high octane gasoline (Avgas) . This is the stuff that powered planes up to, and just past, ww2. Today it's still used in planes from that era, and some smaller general aviation planes. Jet fuel (jet a1) is basically paraffin. All turbine engines (think "jets", but also turbofan etc) run on this. It's a lot less flammable than Avgas, but, well, still makes a big bang if you fly it into a mountain. In short both are dangerous because they are high-density liquid energy. Hydrogen is also dangerous, and there does appear to be a double standard here.
  - avereveard 4y ago
    You can dump plane fuel before an emergency landing, making the whole process safer. it's not just double standards.
    - bruce511 4y ago
      Yes and no, but mostly no. Planes dump fuel to get below maximum landing weights. This typically happens when planes encounter an issue right after takeoff, and need to return to the airport. There are two kinds of emergency landing - those that happen in a hurry, and those that happen slowly. If it happens in a hurry there's no time to vent fuel. If it happens slowly fuel can be vented, but typically just enough to get below max landing weight. It's not like they "empty the tanks".
  - throwaway894345 4y ago
    > Hydrogen is also dangerous, and there does appear to be a double standard here. I don't have beef with hydrogen, but I suspect it's a lot easier to secure fuel in a liquid state versus a gaseous state. Putting a lot of hydrogen in a relatively small steel container for use in an engine seems quite a lot safer than putting it in a big bubble and then dangling people from it. But I am not an aerospace engineer, could be wrong, etc.
    - bruce511 4y ago
      I too am not an aerospace engineer, but venting hydrogen after touch down seems easy enough. After all if you open the cells the gas just goes away. By contrast a broken fuel tank leads to liquids spilling on the ground, which seems like fun. That all said, liquids don't explode, gasses do. So if you were unlucky and the hydrogen exploded then life would get very exciting very quickly.
      - worik 4y ago
        > That all said, liquids don't explode, gasses do There may be a technical definition of explosion that says an exploding tank of fuel is not "exploding". Do you care? A ball of fire is an horrific thing
        bruce511 4y ago
        It's probably helpful to ignore Hollywood here. We see exploding cars all the time, but in real life cars don't explode. They can burn of course, and a fireball is a fireball, but an explosion creates shock waves, and all kinds of ancillary damage. Recall the Beirut explosion recently - the fire was relatively contained, but the damage from the explosion is vast. So yeah, talking about gasoline here - the liquid doesn't explode, it burns. However the gas (as in gas, not liquid) can explode, rupturing the tank and spraying burning liquid everywhere. The worst case is a tank mostly empty - the fumes explode, spraying the rest of the liquid. The best case is a full tank. Liquids can absorb a lot of heat, without expanding or creating pressure. Once they boil though (which requires that the liquid doesn't already "fill the tank", pressure builds leading ultimately to the rapid disassembly of the container. Incidentally this is why throwing an _empty_ aerasol can on a fire is very dangerous, possibly more dangerous than a completely full one (depending on the contents). But to your point, and explosion and a fire are very different animals, with very different outcomes.
- impossiblefork 4y ago
  Yes, but this octane-fire mixing is a bunch of confusion to begin with. Gasoline is less ready to ignite than diesel~=jet fuel, but has fumes. Diesel~=jet fuel has little fumes, but is easier to ignite by heat, i.e. in an engine, but it will almost never be ignited outside of an engine. Meanwhile, gasoline is hard to ignite with heat and pressure in an engine, but easier to ignite in air than diesel. Octane also has a higher boiling point than for example heptane, so higher octane fuel is probably not related to easy of ignition due to the fumes either.
  - dredmorbius 4y ago
    The situation's more complicated than that. Gasoline is easier to ignite than diesel fuel. Both fuels burn as vapours rather than as liquids. The vapour pressure of gasoline at any given temperature is higher than diesel, that is, gasoline evaporates far more readily, making liquid gasoline far more hazardous to store, as the vapours can ignite and explode. Diesel fuel resists autoignition under compression to a greater degree than gasoline, and so can be used in high-compression engines without spark ignition. The flash point is the temperature at which vapours of a fuel can ignite given a spark. These are given for various fuels here: <https://www.engineeringtoolbox.com/flash-point-fuels-d_937.html https://www.engineeringtoolbox.com/flash-point-fuels-d_937.h...> For gasoline: -42.5 C / -45 F Diesel: 37.8 -- 54.4 C / 100 -- 130 F (depending on grade) Kerosene / Jet Fuel: 37.8 -- 72.2 C / 100 - 162 F Yes, the autoignition temperature of diesel is modestly lower than that of gasoline, but in practice the principle concern is vapours igniting from a spark rather than net ambient temperature reaching the autoignition point. (I'd expected gasoline to have a lower autoignition temperature than diesel, this was a surprise for me, though multiple sources seem to indicate a lower ignition temperature for diesel. I learned something researching your comment.) The autoignition point of various fuels is given here: <https://www.engineeringtoolbox.com/fuels-ignition-temperatures-d_171.html https://www.engineeringtoolbox.com/fuels-ignition-temperatur...> For gasoline: 246 - 280C / 475 - 536F Diesel: or Jet A: 210 / 410 Kerosene: 210C / 410F
    - impossiblefork 4y ago
      Ah, I was actually trying to express precisely what you're saying here, and don't immediately see the difference between the views we're laying out.

by worik 4y ago

criddell 4y ago

> high octane fuel Doesn’t jet fuel have a relatively low octane rating compared to most liquid fuels?

LarryMullins 4y ago
Yes, but I think he's using it in the colloquial sense that means "highly energetic".
bruce511 4y ago
It depends on the plane, but both your point and the parent point are correct. Piston engines run on high octane gasoline (Avgas) . This is the stuff that powered planes up to, and just past, ww2. Today it's still used in planes from that era, and some smaller general aviation planes. Jet fuel (jet a1) is basically paraffin. All turbine engines (think "jets", but also turbofan etc) run on this. It's a lot less flammable than Avgas, but, well, still makes a big bang if you fly it into a mountain. In short both are dangerous because they are high-density liquid energy. Hydrogen is also dangerous, and there does appear to be a double standard here.
- avereveard 4y ago
  You can dump plane fuel before an emergency landing, making the whole process safer. it's not just double standards.
  - bruce511 4y ago
    Yes and no, but mostly no. Planes dump fuel to get below maximum landing weights. This typically happens when planes encounter an issue right after takeoff, and need to return to the airport. There are two kinds of emergency landing - those that happen in a hurry, and those that happen slowly. If it happens in a hurry there's no time to vent fuel. If it happens slowly fuel can be vented, but typically just enough to get below max landing weight. It's not like they "empty the tanks".
- throwaway894345 4y ago
  > Hydrogen is also dangerous, and there does appear to be a double standard here. I don't have beef with hydrogen, but I suspect it's a lot easier to secure fuel in a liquid state versus a gaseous state. Putting a lot of hydrogen in a relatively small steel container for use in an engine seems quite a lot safer than putting it in a big bubble and then dangling people from it. But I am not an aerospace engineer, could be wrong, etc.
  - bruce511 4y ago
    I too am not an aerospace engineer, but venting hydrogen after touch down seems easy enough. After all if you open the cells the gas just goes away. By contrast a broken fuel tank leads to liquids spilling on the ground, which seems like fun. That all said, liquids don't explode, gasses do. So if you were unlucky and the hydrogen exploded then life would get very exciting very quickly.
    - worik 4y ago
      > That all said, liquids don't explode, gasses do There may be a technical definition of explosion that says an exploding tank of fuel is not "exploding". Do you care? A ball of fire is an horrific thing
      - bruce511 4y ago
        It's probably helpful to ignore Hollywood here. We see exploding cars all the time, but in real life cars don't explode. They can burn of course, and a fireball is a fireball, but an explosion creates shock waves, and all kinds of ancillary damage. Recall the Beirut explosion recently - the fire was relatively contained, but the damage from the explosion is vast. So yeah, talking about gasoline here - the liquid doesn't explode, it burns. However the gas (as in gas, not liquid) can explode, rupturing the tank and spraying burning liquid everywhere. The worst case is a tank mostly empty - the fumes explode, spraying the rest of the liquid. The best case is a full tank. Liquids can absorb a lot of heat, without expanding or creating pressure. Once they boil though (which requires that the liquid doesn't already "fill the tank", pressure builds leading ultimately to the rapid disassembly of the container. Incidentally this is why throwing an _empty_ aerasol can on a fire is very dangerous, possibly more dangerous than a completely full one (depending on the contents). But to your point, and explosion and a fire are very different animals, with very different outcomes.
impossiblefork 4y ago
Yes, but this octane-fire mixing is a bunch of confusion to begin with. Gasoline is less ready to ignite than diesel~=jet fuel, but has fumes. Diesel~=jet fuel has little fumes, but is easier to ignite by heat, i.e. in an engine, but it will almost never be ignited outside of an engine. Meanwhile, gasoline is hard to ignite with heat and pressure in an engine, but easier to ignite in air than diesel. Octane also has a higher boiling point than for example heptane, so higher octane fuel is probably not related to easy of ignition due to the fumes either.
- dredmorbius 4y ago
  The situation's more complicated than that. Gasoline is easier to ignite than diesel fuel. Both fuels burn as vapours rather than as liquids. The vapour pressure of gasoline at any given temperature is higher than diesel, that is, gasoline evaporates far more readily, making liquid gasoline far more hazardous to store, as the vapours can ignite and explode. Diesel fuel resists autoignition under compression to a greater degree than gasoline, and so can be used in high-compression engines without spark ignition. The flash point is the temperature at which vapours of a fuel can ignite given a spark. These are given for various fuels here: <https://www.engineeringtoolbox.com/flash-point-fuels-d_937.html https://www.engineeringtoolbox.com/flash-point-fuels-d_937.h...> For gasoline: -42.5 C / -45 F Diesel: 37.8 -- 54.4 C / 100 -- 130 F (depending on grade) Kerosene / Jet Fuel: 37.8 -- 72.2 C / 100 - 162 F Yes, the autoignition temperature of diesel is modestly lower than that of gasoline, but in practice the principle concern is vapours igniting from a spark rather than net ambient temperature reaching the autoignition point. (I'd expected gasoline to have a lower autoignition temperature than diesel, this was a surprise for me, though multiple sources seem to indicate a lower ignition temperature for diesel. I learned something researching your comment.) The autoignition point of various fuels is given here: <https://www.engineeringtoolbox.com/fuels-ignition-temperatures-d_171.html https://www.engineeringtoolbox.com/fuels-ignition-temperatur...> For gasoline: 246 - 280C / 475 - 536F Diesel: or Jet A: 210 / 410 Kerosene: 210C / 410F
  - impossiblefork 4y ago
    Ah, I was actually trying to express precisely what you're saying here, and don't immediately see the difference between the views we're laying out.