It’s practically tradition. Brave knights - from Beowulf to Prince Philip - have been casually blocking gouts of flame since before fantasy was a genre. But you’ve been wondering (as I have):
What really happens to a metal shield when subjected to dragon fire?
Prince Philip confronts Maleficent in Disney's 'Sleeping Beauty', disney.fandom.com
Making some sweeping assumptions and ignoring things like enchanted shields or flame that melts through stone bridges (see Maleficent, about ten seconds after the scene above), I’ll attempt to answer this question with the powers of engineering.
This will require several posts. Today we’ll set up the problem (usually the most difficult part) and learn about conduction, convection, and radiation. Next week we’ll flesh out the model, learn about flame-throwers and medieval blacksmithing, and make some sweet charts.
But first, another knight facing another dragon:
But first, another knight facing another dragon:
Beowulf fighting the dragon, as depicted by J. R. Skelton in 1908, Public domain, via Wikimedia Commons
For any engineering problem, the first step is always the same: take something beautiful and real and turn it into a c̶o̶l̶d̶,̶ ̶d̶e̶a̶d̶,̶ ̶b̶o̶r̶i̶n̶g̶,̶ ̶l̶i̶f̶e̶l̶e̶s̶s̶,̶ ̶o̶v̶e̶r̶l̶y̶-̶s̶i̶m̶p̶l̶i̶f̶i̶e̶d̶ analyzable diagram. Here’s where I started:
Not a great start. This is not only more boring than the original, it’s also not very useful. To get something we can work with, we need to know what’s going on with the shield itself:
OK, now we’re getting somewhere. This is the heat transfer equivalent of the Free Body Diagram (a term you may or may not have had shouted at you repeatedly by a frustrated engineering professor in your sophomore year). It shows the heat entering and leaving the shield.
A note about heat. It moves around in three basic ways. Engineers call these:
The flame on the right is dumping heat into our shield via convection (from the super-heated air doing to the shield what an air fryer does to a French Fry) and radiation (from said super-heated air doing to the shield what a campfire does to your hands while leaving your butt freezing cold).
On the knight’s side of the shield, heat is leaving the shield via convection (the gentle kiss of the cool summer breeze) and conduction (heat traveling through the knight’s homespun woolen sleeve and into the tender flesh of his burly forearm). Technically radiation too, but we tend to ignore that when things aren’t super hot or super cold.
Finally, heat is moving via conduction within the shield itself. One side is technically hotter than the other (just like the bottom of a cast iron pan is technically hotter than the top).
Now for some s̶w̶e̶e̶p̶i̶n̶g̶ ̶a̶s̶s̶u̶m̶p̶t̶i̶o̶n̶s̶ totally reasonable simplifications. Let’s say that the cooling on the knight’s side is negligible compared to the heating on the dragon’s side. Let’s also say that the whole shield is at basically the same temperature. We’ll check those assumptions later, but for now it will allow us an easy first stab at solving this problem:
A note about heat. It moves around in three basic ways. Engineers call these:
- Conduction (heat moving in a solid.) This is how the handle of your cast iron pan gets hot when you leave it on the stove.
- Convection (heat moving between a fluid and a solid). This is how your cast iron pan cools off (and cracks) when you run it under cold water.
- Radiation (heat moving in the form of light). This is how the sun warms things across the emptiness of space.
The flame on the right is dumping heat into our shield via convection (from the super-heated air doing to the shield what an air fryer does to a French Fry) and radiation (from said super-heated air doing to the shield what a campfire does to your hands while leaving your butt freezing cold).
On the knight’s side of the shield, heat is leaving the shield via convection (the gentle kiss of the cool summer breeze) and conduction (heat traveling through the knight’s homespun woolen sleeve and into the tender flesh of his burly forearm). Technically radiation too, but we tend to ignore that when things aren’t super hot or super cold.
Finally, heat is moving via conduction within the shield itself. One side is technically hotter than the other (just like the bottom of a cast iron pan is technically hotter than the top).
Now for some s̶w̶e̶e̶p̶i̶n̶g̶ ̶a̶s̶s̶u̶m̶p̶t̶i̶o̶n̶s̶ totally reasonable simplifications. Let’s say that the cooling on the knight’s side is negligible compared to the heating on the dragon’s side. Let’s also say that the whole shield is at basically the same temperature. We’ll check those assumptions later, but for now it will allow us an easy first stab at solving this problem:
Totally manageable. Next time we’ll start throwing some numbers and equations at this thing so we can get to the answer we seek.
Something I missed? Questions about heat transfer? Favorite dragon-vs-knight stories? Something else you'd like me to analyze? Leave a comment!
Something I missed? Questions about heat transfer? Favorite dragon-vs-knight stories? Something else you'd like me to analyze? Leave a comment!