What kind of fuel is in my model rocket engine?

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What chemicals power your rocket motor?



That is a good question, and the answer is in the realm of rocket science.  First let's clear up the word "rocket fuel."  To be accurate, in rocket science it's best to use "propellant" if you're referring the chemicals that make it go.  Almost all rockets use two propellants, that when mixed correctly will burn at high pressure and produce a lot of hot exhaust gases.  Those propellants might be two liquids, or a combination of solid chemicals.  So rockets may be powered by liquid-propellant or solid-propellant engines.  (The Space Shuttle uses two big solid-propellant boosters to help get it off the ground and up to speed, plus three big liquid-propellant engines to push it on to orbit.  Your model rocket engine almost certainly uses a solid-propellant.

Most model rocket engines (the Estes-type motors are an example) are propelled by black powder, and made quite similar to fireworks skyrocket powerplants.  They pack a stiff cardboard tube with black powder, seal one end, and ram a clay nozzle into the tail.  Actually it's not nearly that simple, but that is the general idea of how they're made. 

So what kind of propellant powers your Estes-type model rocket motor?  It's a well-milled, tightly-rammed and highly compressed grain of black powder.  Black powder is a specially prepared and extensively treated mixture of potassium nitrate (saltpeter), charcoal, and sulfur (sulphur).  You cannot make an equivalent mixture, by the way, by simply mixing the correct ingredients in the correct proportions.  Black powder must be extensively milled and processed to reach the high energy and power of a small commercial model rocket motor. 

The characteristics of black powder propellant can be adjusted and modified with various additives.  Excess charcoal (especially coarse charcoal) will slow the burn rate and produce long streamers of sparking exhaust).  Certain chemicals could be added to produce color smoke trails, or other special effects.

But there are also some bigger, more powerful model rocket motors out there, too.  Most of these are RELOADABLE (reusable) units with metal cases and plastic fittings.  The company "AeroTech" pioneered these high-power engines, and now they come in many sizes and configurations. 

AeroTech-type model rocket motors don't use black powder propellant.  These advanced engines are designed for specific formulations of a "composite propellant."  Composite propellants, the kind that power the Space Shuttle solid boosters, use a high-energy oxidizer (often ammonium perchlorate) thoroughly mixed into a high-energy polymer (synthetic rubber) fuel-binder.  Most AeroTech motors are designed to use HTPB composite solid propellant.

HTPB?  Yes, HTPB, not PBAN.  That is to say, there are several "flavors" of composite solid propellant.  AeroTech and the Space Shuttle use the fuel-polymer hydroxyl-terminated polybutadiene (or HTPB, for short).  Other composite formulations use a different fuel, polybutadyene acrylic acid acrylonitrile (PBAN), for instance.  But that's taking us beyond "Rocket Fuels 101."

For more in-depth propellant chemistry, scan the bookshelves for the kinds of books in our eBay Store.


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