Ever wonder how a hybrid rocket motor gets lit up?

You may recall from my previous post about hybrid motors, that they consist of 2 separate parts - a hollow tube-like component filled with solid (in itself inert) fuel, and a tank of oxidizer fluid with some plumbing that forces the fluid to get flushed through the aforementioned hollow part of the tube-like solid fuel.

That's cool and all, but without the presence of a spark that can light this whole thing up, and start the chemical reaction, all you are left with is what we in the biz call a 'cold flow'.

A 'cold flow' means just that, no spark, no fire, no reaction, just some cold fluid flowing through some solid rocket fuel. To the observer, this is typically rather uneventful. Though it is kinda cool to watch.

Cold Flow test. The mass flow rate change was the testing of different positions of valve opening settings.

Now, cold flow tests are great to have. They prepare you for the real deal. You get to test all your instrumentation, validate your sensors, make sure your timing sequences are working properly. You get to see how your mechanical components are likely to behave during the real test, you see how quickly valves open and close in response to commands you send, so these tests are very valuable.

As valuable as they are, these cold flows are still just one step on the way towards the actual Pièce de Résistance - The Hot Fire.

As the name suggests, the 'Hot Fire' consists of a very hot ongoing reaction, and lots of fire. It takes place  when everything is prepared, and 100% certified for safety, and the team lead says it's time for the powerful fury of the hybrid chemical reaction to be unleashed upon the world.

In order to make that happen, one would have to apply a source of ignition. Which is typically a small piece of solid fuel (NOT inert) similar to the fuel used in model rocketry. The fuel and oxidizer are built into one solid chunk, and when exposed to heat.. BAM!!!! You've got yourself a rocket, or bomb, you know, either way, depending on how you use it. *Note- Do take extreme caution when messing with this stuff, and better yet, consult a professional.

Left to right: Wires to solid chunk lighting chunk, combustion, and more combustion!

You can consider that solid fuel chunk as lighter fluid in a bonfire. The wood will burn if you light it without lighter fluid too, but it'll happen instantaneously if you pour some on there.

There is one more step, just like the lighter fluid on its own does nothing, and needs fire, the same thing holds true for our little NOT inert solid rocket fuel chunk.

For that one little spark that we need to make all the magic happen, we turn to the underappreciated Electric Match!

According to wikipedia, an Electric Match is: "A device that uses an externally applied electric current to ignite a combustible compound". Our combustible compound of course being our lighter fluid chunk, and our external power source being a wonderfully useful 12V car battery.

Generally an Electric Match has 2 parts -  Pyrogen, and Bridge wire.

The pyrogen is a mixture of energetic materials that when combusting, produces large amounts of hot particles. Thus ensuring the ignition of whatever it is we are trying to ignite. The pyrogen itself combusts in a non detonative way, and is a self sustaining exothermic reaction that does not rely on external sources of oxygen to keep going. It usually coats the bridge wire.

The bridge wire is a relatively thin resistance wire, that heats up to a high temperature when a sufficient electric current is passed through it. The bridge wire is usually coated with the pyrogen, and when it heats up to the temperature required to initiate the self sustaining exothermic reaction of said pyrogen, the combustion process begins. The bridge wire itself then melts away in the flames and disappears into oblivion, rendering the original electric circuit formed to start the process, an open circuit.

So there ya have it!

To make rocket fire you need a 12V car battery, attached to a bridge wire, coated with a pyrogen. Bridge wire will heat up a lot when the right current is applied to it. That heat will then start the self sustaining exothermic reaction of the pyrogen which is wrapped around the bridge wire. The pyrogen reaction will kick off our lighter fluid i.e. non inert solid fuel chunk, which will then initiate the actual hybrid rocket combustion reaction between the inert solid fuel and the oxidizer fluid flowing through the motor.

The result: Hot Fire.

Hot Fire

And in case you wanna see a couple more cold flow videos.. here you go.

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