So, what makes a Hybrid Rocket Motor a Hybrid?
Every rocket motor, by definition, is a reaction engine that carries all of its propellant with it. Typically, the energy that is used to throw mass in one direction (to push the rocket in the other direction) comes from a “redox” (reduction/oxidation) chemical reaction between a fuel and an oxidizer — in layman’s terms, something burns.
We generally classify rocket motors by the physical state that their propellants are in prior to ignition:
- In solid rocket motors, both the fuel and oxidizer are solids.
- In liquid rocket motors, both the fuel and oxidizer are liquids.
- In (hot) gas rocket motors, both the fuel and oxidizer are gaseous.
- “Cold gas” rocket motors rely on a high-pressure gas expanding to a lower pressure, rather than a chemical reaction, to supply their energy.
- In a Hybrid rocket motor, the fuel and oxidizer are in different physical states. For example, a solid fuel and a liquid oxidizer.
In hobby rocketry, Hybrid Rocket Motors almost always use a solid fuel of some sort, and liquid nitrous oxide as the oxidizer.
Why Nitrous Oxide?
Nitrous oxide has a few important characteristics that make it the oxidizer of choice for hobby hybrid rocketry:
- It is a liquid over a wide temperature range when stored under reasonably-high pressure in tanks. We can store and transport it easily.
- That wide temperature range is “human-friendly” — we’re talking about comfortable “shirtsleeve weather”, and not “if you were to get this cold and fall down, you’d shatter” cryogenic temperatures. (Liquid oxygen, on the other hand, is only liquid several hundred degrees below zero, and requires special vacuum-insulated dewars to keep it from all boiling away before you use it.)
- It’s a gas at those same temperatures when under less pressure (a few hundred psi, for instance). This means that it will vaporize on its own when it sprays, under pressure, into the combustion chamber.
- It’s widely available for not too much money, due to its widespread use by the “hot rod” community.
- It requires high temperatures (hundreds of degrees) to provide enough heat energy to cause the N2O molecules to dissociate. This means that the oxygen atoms are likely to remain “tied up” (and unable to oxidize anything we don’t want it to) until we very deliberately initiate ignition.
How does it all work?
Typically, a hybrid motor will have some sort of solid hydrocarbon fuel grain — often a roughly tubular chunk plastic, urethane rubber, or paper — in the combustion chamber. In fact, the fuel grain may even be the combustion chamber, if there will be enough left of it at the end of the burn to maintain structural integrity. At the aft end of the combustion chamber there will be a nozzle. Above the combustion chamber there will be a tank of some sort, which will hold liquid nitrous oxide under pressure.
At the moment of ignition, something energetic will be started burning near the top of the combustion chamber, and liquid nitrous oxide will be allowed to spray through some sort of injector into the flames. This will start the decomposition of nitrous oxide into nitrogen and oxygen gasses. In the presence of heat and lots of oxygen, the fuel grain will start burning, as well. The burning fuel grain (and the excess nitrogen from the decomposing nitrous oxide) will cause pressure to build up inside the combustion chamber, and hot gasses will be pushed out through the nozzle throat, producing thrust. As the burn continues, the heat of the burning fuel keeps dissociating more of the incoming nitrous oxide, maintaining the flow of oxygen into the chamber.
There is a built-in negative feedback system at work, preventing the chamber pressure from rising too far or too fast. As the chamber pressure rises, the pressure drop across the injector drops, and the flow of nitrous oxide slows down. This slows the rate of combustion, allowing the chamber pressure to drop back down as hot gasses continue to exit through the nozzle.
As chamber pressure drops again, the pressure difference across the injectors increases again, and the nitrous oxide starts to flow faster. The combustion rate increases, again. (This feedback cycle is fast, but not instantaneous, and may be the reason for the typical buzz, squeal, or “flatulent” noise that is commonly heard from hybrid motors.)
Can you be a bit more specific about ignition?
Yes and no. There are a few common methods of igniting a hybrid motor, but the details vary quite a bit. Ignition is also closely related to the “fill method” (how the nitrous oxide gets into the flight tank aboard the rocket), and different manufacturers handle things in different ways.
The most common ignition and fill method uses the so-called “U/C valve”, which gets its name from the initials of the two folks who first came up with the idea, independently — Bob Urbanski and Bill Colburn.
In this method, nitrous oxide fills the flight tank through a piece of nylon tubing that runs up through the nozzle and combustion chamber to a compression fitting on a bulkhead separating the combustion chamber from the flight tank. A chunk of solid propellant (typically, a short ammonium perchlorate composite propellant solid fuel grain is used, but Pyrodex pellets and other similar items can be used instead) is attached to the tubing near the bulkhead. When this “ignition charge” is lit, it burns through the nylon fill line, starting the flow of nitrous back into the chamber, and simultaneously providing the initial flame and high temperature needed to start the dissociation of the nitrous oxide. The burn-through of the fill tube acts as the “valve” that starts the flow of oxidizer.
U/C valves are used by most of the commercial hybrid motor manufacturers, and most amateur experimenters, because of their simplicity. RATTworks, Propulsion Polymers, Skyripper, and Contrail hybrid rocket motors all use U/C valves, with some minor variations.
HyperTEK GOX Ignition
HyperTEK (owned by Cesaroni) uses a completely “non-pyrotechnic” ignition system, adding a bit more complexity in ground support equipment in order to ensure that their motor reloads are essentially inert, and shippable by any carrier with no special packaging or permits needed.
HyperTEK hybrids are filled through a special “coaxial” fill tube assembly, which has a smaller-diameter stainless steel tube running down the center of a larger one. The fill tube assembly goes up through the nozzle, and passes through the combustion chamber, just like the nylon fill tube in a U/C valve. But, instead of being attached with a compression fitting, the center tube seats into an O-ring in the injector, with the outer tube ending just below the injector, and not sealing against anything. The whole fill tube assembly is held in place with a nylon wire tie or two, to keep it from falling out, or being pushed out of place by the pressure of the nitrous oxide. A piece of two-conductor speaker wire is taped to the outside of the fill tube assembly, with the end of the wire stopping short of the injector, as well.
Upon ignition, a high-voltage neon sign transformer is switched on, generating sparks across the cut end of the speaker wire. Simultaneously, a solenoid valve is opened, starting the flow of gaseous oxygen (aka “GOX”) through the outside tube of the coaxial injector. In the presence of pure oxygen, the spark is sufficient to start the wire’s insulation burning, and then the interior of the fuel grain starts burning, as well. The flames come out the nozzle, and burn through the nylon wire ties. With nothing holding it in place now, the fill tube assembly falls out, and the nitrous oxide starts flowing into the combustion chamber. The flames start the decomposition of the nitrous, and you’re off to the races.
AeroTech used to manufacture a series of “pre-filled” hybrid motors. These motors had a DOT-rated flight tank that the user would pre-fill with nitrous oxide using a special fill tube, and a scale to measure the fill quantity. The tanks had a spring-loaded “pin valve”, with the pin held in place by a pellet of compressed black powder. The filled tank would then be attached in place of the forward closure on a standard AeroTech motor casing, with either a rolled paper fuel grain, or a special “turbo hybrid” plastic fuel grain (which was essentially an extremely fuel-rich traditional APCP solid fuel grain).
Upon ignition with a standard solid motor igniter or electric match, the black powder pellet would start burning, and the spring-loaded pin valve would be released. The valve would open, and nitrous oxide would start flowing into the combustion chamber. The still-burning black powder pellet would start the dissociation of the nitrous oxide, and ignite the fuel, sending your rocket skyward.
AeroTech’s hybrid motors, while unique in their ability to be launched from any launch pad, without any specialized ground support equipment at the pad, have been out of production for a number of years, although there are still occasional rumors that they might restart production at some point in the future.