Aircraft Emergency Evacuation slide inflation system: Why is a mixture of CO2 & N2 used instead of pure N2?

Question

I was intrigued knowing from this SE-Aviation post that modern, aircraft Emergency Evacuation-Slide inflation-cylinders are often filled with a CO2+N2 mixture.

See here:

Which gas is used to fill the evacuation slides of modern aircraft?

Intuitively the reason behind the CO2 addition instead of using simply pure N2 is not very obvious to me. Digging deeper into the technology, Wikipedia reveals that "The CO2 is used to slow down the rate at which the valve expends the gases."

Any idea about the science behind this? Why would CO2 slow down the inflation rate? And why do we want to slow the inflation in the first place? Fabric damage?

Also, what is the CO2 percent in the mix, anyone know?

Answer 1

Let me first give some background information on the art of inflating an evacuation slide. There are two ways to fill them:

1. Expand gas from a high-pressure bottle: This will cool the gas as it expands, so ice may form at the aspirator outlets where sucked-in air and the expanding gas mix, blocking the flow of outside air into the slide. Also, storing compressed gas needs bulky and heavy steel vessels.
2. Pyrotechnic gas generators which use an exothermic reaction to create a large amount of hot gas. They are more compact and lighter than a pressure vessel for the same amount of gas produced, but their hot gas could scorch the slide material and will contract when it cools down after the filling process, so a slide filled by a pyrotechnic gas generator will sag after a while.

Because of the disadvantages of both pure stored gas and pure pyrotechnic inflation devices, current state of the art emergency evacuation slide systems use a hybrid inflator, which comprises a stored compressed gas together with a pyrotechnic gas generator. The pyrotechnic gas generator augments the stored compressed gas by providing additional gas as well as heat to counteract the effects of the expansion-induced cooling of the compressed gas as it expands out of the pressure vessel.

Operation of a hybrid inflator

The typical gas generator uses solid ammonium nitrate which is electrically ignited and produces nitrous oxide (N$_2$O) and water (H$_2$O) when heated. Nitrous oxide will exothermically decompose into nitrogen and oxygen when heated above 650°C or 1200°F, so even more energy and gas is produced. Note that the decomposition of two moles of nitrous oxide will produce two moles of nitrogen (N$_2$) plus one mole of oxygen (O$_2$), thus adding 50% more gas volume. This hot mixture of nitrogen, oxygen and steam is then piped into the pressure vessel and mixes with the gas flowing out from it, thereby heating the emitted gas.

The pressure vessel contains gasses like nitrogen or carbon dioxide. Other candidates are argon or helium, but their higher price has held them back. To increase the yield from the pressurized gas, the trick with the nitrous oxide can be repeated here: By storing nitrous oxide in the pressure vessel, again 50% more gas volume can be produced from what is stored, while the exothermic decomposition produces additional heat that offsets, to some extent, the heating of the stored gas.

Nitrous oxide is also attractive because it offers the best manufacturability, storage and handling of the possible decomposable gas source materials. Although classified as an oxidizer, in practice it is generally nontoxic and non-corrosive and is relatively inert up to temperatures of about 200°C. Also, nitrous oxide, as compared to gases such as oxygen, nitrogen and argon, liquefies relatively easily at ambient temperatures - helpful when the gas in the pressure vessel is stored in liquefied form to reduce its bulk.

Heated to its decomposition temperature, pure nitrous oxide will undergo a rapid disassociation chain reaction unless a coolant gas is added to absorb some of the heat generated by the exothermic reaction. Therefore, the mixture in the pressure vessel contains 50% or more of carbon dioxide which has a latent heat of vaporization of approximately 66 Calories per gram. As a three-atomic gas it is better suited for this task than two-atomic alternatives. Thus, it does not serve as a diluent (as aeroalias wrongly states), it absorbs heat and stabilizes the disassociation process.

Why a mixture of nitrogen and carbon dioxide

Now we can answer your question: Nitrogen is contributed both by the pyrotechnic gas generator and by the compressed gas and is produced from the dissociation of nitrous oxide (N$_2$O). Carbon dioxide is added to dampen the exothermic dissociation and absorb some of the heat produced. Without it, the gas generator would experience a runaway chain reaction! Its concentration in the compressed gas is between 50% and 75%, so the concentration in the gas generator mix is around 35% to 50%.

The high speed flow of gas from the pressure vessel is now piped through an aspirator, which sucks in ambient air by using the Venturi effect, and from there into the slide. This contributes up to three quarters of the total gas in the slide, so most of the nitrogen in the slide is from ambient air. In the slide, the concentration of carbon dioxide is diluted to around 10% to 15%.

Aspirator (from BF Goodrich's US Patent 4368009). High-speed gas from the gas generator (red arrows) enters from the angled pipe on the left and pulls ambient air (blue arrows) with it when shooting from the nozzles on the right. Contrary to aeroalia's claims, an aspirator is not just another name for a gas generator, it is an entirely different thing altogether.

Answer 2

The $\text{CO}_{2}$ is used as a diluent.

• The pressure vessels (for inflating the slides) usually have nitrous oxide (or similar) compounds, that undergo exothermal thermal decomposition during inflation. For example, $\text{NO}$ reaches temperatures of $\sim 650\,^{\circ}\text{C}$ during thermal decomposition. As $\text{CO}_{2}$ absorbs heat during its expansion, this prevents any damage to the fabric.

• For the same reason, the presence of $\text{CO}_{2}$ controls the reaction rate of the decomposition of the nitrous oxide as it is usually impossible to prevent decomposition from generating unpredictable amounts of gas.

The $\text{CO}_{2}$ forms around 50% to 75% of the mixture.

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The amount of gas required for inflation of the slides in the aircraft is quite high. If all these are carried in pressure vessels, either of the following has to be done

• increase the pressure (already $\sim200$ bar) of the gas in the vessel (its usually in liquid form), which increases the pressure vessel thickness and consequently the weight, or

• increase the number or size of pressure vessels, which again increases the weight.

Also, a gas generator (usually pyrotechnic) can be used, which generates gas by exothermic reaction. This too, has its disadvantages:

• The gas generated is at high temperature (via thermionic reaction), resulting in damage to the slide fabric.

• Also, as the gas cools, the evacuation slide may sag (as gas contracts). This is not good as the slides are expected to act as life rafts.

The size of the gas bottle can also be reduced by using an aspirator, which draws in ambient air to mix in a ratio of about two-thirds air to one-third gas from the bottle.

To overcome these, the aircraft slide inflation system uses is hybrid system that uses two techniques:

• Instead of carrying the gas itself, carry a compound capable of undergoing thermal decomposition such that the number moles of gas produced by the inflator is greater than the number of moles of gas stored. Nitrous oxide is used because of manufacturability, storage and handling concerns.

  Two moles of Nitrous Oxide $\text{N}_{2}\text{O}$, when undergoing thermal decomposition, forms two moles of diatomic nitrogen ($\text{N}_{2}$) and one mole of diatomic oxygen ($\text{O}_{2}$)

$2\text{N}_{2}\text{O} \ = \ 2\text{N}_{2} \ + \ \text{O}_{2}$

Because the universal gas constant is the same for all gases, the three moles of nitrogen and oxygen produced by the decomposition of two moles of nitrous oxide occupy 50% more volume than the two moles of nitrous oxide would have occupied at the same temperature and pressure. Thus, the size of the container is reduced by a third (compared to situation where we use undecomposed gas).

• A pyrotechnic gas generator (or an aspirator) that augments the stored compressed gas by providing additional gas.

For more details, see US patent application US 6877698 B2, Aircraft evacuation slide inflation system using a stored liquified gas capable of thermal decomposition

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In case the amount of gas required is low (for example, in some helicopters), only the pressure bottles are carried.