Dragons are part of folk tales all over the world, and their appearance varies according to time and region. For the sake of simplicity, this article will only consider the Western European dragon of the High Middle Ages. It was often described as winged, horned and able to breathe fire9. Hamilton et al. proposed in their work, that climate change could cause a resurgence of the dragon era. The research demonstrated a correlation between the increasing frequency of dragons referenced in literature and rising temperatures10. It is therefore imperative to gain a deeper understanding of the biological characteristics of dragons in order to ensure an appropriate response to the advent of a new dragon era. Consequently, this article seeks to propose a hypothesis regarding the means by which
a dragon may generate a flame.
It has been hypothesised that the flame is produced by an air/methane mixture, analogous to a Bunsen burner, which can be adjusted in terms of heat and range by varying the amount of air inhaled and the quantity of methane produced. Similarly to cows, the dragon may possess ethane-producing bacteria, such as Methanobrevibacter ruminantium, within its stomach. The methane generated from feed fermentation could be collected in a structure known as a “methane bladder” and subsequently expelled through the mouth in a regulated manner by valves, along with the inhaled air. However, it is challenging to ascertain the quantity of fermentable food a dragon must ingest to produce a flame, given the variability in methane production among individuals. Nevertheless, an attempt is made. The following calculations provide an estimation of the quantities of air and methane required to generate a flame with a length of five metres. The chemical reaction for the complete combustion
of methane is:
CH4 + 2O2 → CO2 + 2H2O
Furthermore, the combustion of methane releases a specific amount of energy. The heat of combustion of methane is approximately 50.1 MJ/kg. The length of the flame is influenced by the flow
rate of the gas, the mixing of the fuel and oxidizer. The author assumed ideal combustion and perfect mixing and used the energy release to estimate the volume of methane required. The length of the flame is proportional to the energy released per unit time.
The power requirement for a 5-meter flame is 𝑃 watts. A rough estimate could be around 10 kW (this is a very rough estimate and should be refined based on specific characteristics of the dragon). Given the heat of combustion of methane
(Δ𝐻combustion) is:
Δ𝐻combustion = 50,100 kJ/kg
the mass flow rate of methane (𝑚 CH4) required can be found using the power requirement:
𝑚CH4 =𝑃/Δ𝐻combustion
After substituing the values, 𝑚CH4 is ≈ 0.2 g/s. To find the volume flow rate of methane (𝑉¤CH4
), the density of methane (𝜌 = 656 g/m3) is needed.
𝑉CH4 =𝑚¤ CH4𝜌CH4 ≈ 0.305 L/s
For complete combustion, two moles of O2 are needed for every mole of CH4. Considering air is 21% O2, the volume ratio of air to methane is:
2 mol O2 0.21 × 1 mol CH4 ≈ 9.52
Thus, the volume flow rate of air is:
𝑉air = 9.52 × 0.305 L/s ≈ 2.9 L/s
This means, that the required methane flow rate is 0.305 liters per second (L/s) and air flow rate is 2.9 liters per second (L/s). Assuming that the dragon, breathes fire for 5 seconds, it needs to release ≈ 1.5 L methane. Danielsson et al. found out, that in average, cattle produces 12.4 g/kg (= 17.3 L/kg) methane per dry matter intake. If a dragon eats 1 kg of dry matter, it can produce a 5 m flame for 5 seconds eleven times before the methane storage is depleted. However, this would mean that the dragon must
be predominantly vegetarian, as meat produces far less methane than plants.
A final question that remains to be answered is how to ignite this gas mixture. One possibility would be a catalyst, which would facilitate the reaction of methane to carbon dioxide. Dragons could obtain these catalysts by eating knights and their metallic armour and weapons, or by producing a spark through electrical discharge, as in electric eels. Nevertheless, this hypothesis is unlikely to be substantiated, given that the armour did not contain the necessary metals, and that electric eels require water for electric discharge. The most plausible explanation is that a dragon produces a snap, like the snapping shrimp, which produces a short flash of light equal to a temperature of 4,700°C, igniting the gas mixture11.
9Steer, D. (2004). Expedition in die geheime Welt der Drachen.
10Hamilton, A. J et al. (2015). Here be dragons. Nature, 520(7545), 42–43. https://doi.org/10.1038/520042a
11Spencer, E. (2022, 12. Dezember). The Real Power of the Pistol Shrimp. Ocean Conservancy. https://oceanconservancy.org/blog/2020/09/10/pistol-shrimp/