Nitrous oxide has to be heated to 500OC to start it breaking down to release the oxygen this is the function of the pre-heater to start the initial breakdown until the fuel is starting to burn.

The pre-heater in the micro hybrid is also used to burn through the burst disk to initiate the flow of nitrous oxide.

Nitrous Oxide

In hybrid engines the fuel and oxidiser is kept separate until ignition and then the oxidiser is injected to the fuel chamber. The nitrous oxide for the micro hybrid is contained in ISI whipped cream charger bulbs each bulb contains 8g of N2O

Burst Disks

Burst disks are precision cut from P.E.T.E Plastic the thickness of this is important as if it it to thin it will not hold back the NOX and may rupture prematurely, if it is to thick it may not rupture at all.

Hybrid Combustion Process Principal

A generic hybrid rocket works by burning an inert solid fuel grain in the presence of a gaseous or liquid oxidiser, for model rocketry this tends to be Nitrous Oxide as it is easy to handle and relatively easily obtained.

The process of burning an inert fuel with the oxidiser is very similar to the way a candle burns, as shown below:

hybrid candle

The heat melts the wax, creating a rich vapour of wax fuel particles that are swept up with the rising heat to be mixed with oxygen being drawn in by the heat rising, this fuel is burnt at the flame front, creating the flame and the convection crurrent that draws in more oxygen and heating more wax to create move fuel vapour.

In a hybrid rocket the oxidizer flows down a port or ports in the solid fuel grain and reacts with the solid fuel. This produces the hot exhaust gases required to produce thrust. This process can be seen in the following image:

hybrid combustion

The oxidizer and the solid fuel are able to react, because the hot gases from the combustion process cause a small layer of the fuel to vaporise. This fuel then reacts with the unused oxidizer to produce more combustion gases. This process is shown in the following image:

Micro Hybrid Combustion

The initial reaction and fuel vaporisation is caused by the pre-heater element.
It is important to get the correct flow of oxidiser, if the flow is too low the flame will not be able to develop and may even go out. If the flow is too high it can blow out the flame. This can be shown by the example of putting candles in a tube and blowing down the tube. as depicted below:

Flow Rates

The Micro Hybrid is the smallest available hybrid motor, it is classified as an experimental motor as it has not been put forward for certification by NAR or CAR. The Microhybrid uses 8g cream whipper nitrous oxide chargers as the source of oxidiser and a solid tubular safe fuel grain of varying materials from paper through different plastics and composites.

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