Getting Started with Hybrid Motors

A quick overview of what you'll need

By Doug Pratt

(updated 10/20/06)

Three hybrid systems. There are three distinct types of hybrid rocket motors presently certified and on the market. Each requires different support equipment, although there is some commonality.

The simplest is the Aerotech system, where the oxidizer tank is attached to a conventional reloadable motor casing. The tank is loaded before it is attached and installed in the rocket. A “pyrovalve” releases the oxidizer into the combustion chamber when the igniter is fired.

The second is the floating injector design used by RATT Works, Propulsion Polymers and Contrail Rockets motors. This is sometimes called the U/C Valve, for Urbanski and Colburn, two experimenters who apparently came up with the idea independently at about the same time. In this kind of motor, the casing is a single tube with closures on each end. The inside of the tube is divided into two sections by the injector, which can move up and down in the tube. The space above the injector is the oxidizer tank; below it is the fuel grain. A nylon hose is connected to the injector, and runs out the nozzle to the oxidizer supply tank. On the launch pad, oxidizer flows into the motor through the nylon hose. When the motor is full, a special igniter positioned just under the injector is fired. It burns through the nylon fill hose, which releases oxidizer into the combustion chamber and starts the motor burning. These motors require a launch system that can control the flow of oxidizer into the motor.

The third hybrid motor system in common use is the HyperTEK system invented by Korey Kline. A HyperTEK motor consists of three parts: a tank, a molded plastic fuel grain, and an injector bell that connects the two together. On the launch pad, the motor rests on a special support structure. A metal tube mounted on the structure goes up through the nozzle and the fuel grain, into the injector. Oxidizer flows through this tube into the tank. When the tank is full, a second tube introduces oxygen into the combustion chamber while another mechanism generates a high-voltage arc at the end of an igniter wire in the combustion chamber. This ignites the fuel grain, which burns through straps holding the fill tube in place. This allows the tube to come out of the injector, releasing oxidizer into the combustion chamber. The major advantage of the Hypertek system is that there are no pyrotechnic components anywhere in the system.

Getting Started. Getting started with any of these systems means buying some ground support equipment.

First of all, you need an oxidizer supply tank. A 10 lb tank is adequate for smaller motors, and should cost between $180 and $300 depending on where you buy it. A 20 lb tank is recommended if you plan to fly motors of J level and above. Be aware that tanks sold for use in performance cars tend to be expensive, because they are polished or have special finishes.

Make sure the tank is rated and tested for nitrous oxide (N2O), the oxidizer we use for hybrid motors. While common CO-2 tanks can handle the pressure of liquid N2O, they are often equipped with seals and rings that are not compatible with N2O. Do not use a tank that is intended for CO-2 or for compressed air (SCUBA).

The most common source of N2O is automotive racing (hot rod) stores. While they are usually excellent places to get your tank filled, they are often on the high end of pricing for tanks and fittings. You can expect to pay $4-6 a pound to have your NOX tank filled.

The HyperTEK system also requires a gaseous oxygen (GOX) tank. These can often be rented from welding or medical companies for less money than it would cost to own a tank. Check your local Yellow Pages.

The Aerotech hybrid requires a fill valve and hose that connects to the motor’s oxidizer tank. Since the tanks are filled by weight, you will also need a precise digital scale. The fill hose assembly costs around $70, and you can expect to pay $80-150 for a suitable scale. Once loaded, an Aerotech hybrid is treated like a conventional motor, with no special equipment needed to launch.

As of this writing, there is only one system on the market designed for floating injector motors:  the Pratt Hobbies ULS (Universal Launch System). The ULS is offered in three Modules. This modular approach allows you to buy the parts you need to fill and fire the motors you are interested in, and upgrade later. For floating injector motors, Modules One and Two are used. Module One is the Basic ULS, which can be used to fire conventional solid fuel rocket motors. Module Two is a solenoid valve assembly that attaches to the N2O supply tank and plugs into Module One. It allows you to connect to the nylon fill hose, fill the motor with N2O, fire the igniter, and dump the N2O tank in the event of a misfire.

The HyperTEK motor system requires the most complex ground support. A basic Hypertek system consists of valves, regulators and fittings for the N2O and gaseous oxygen (GOX) tanks, a high voltage system to generate the ignition arc, and associated cables, hoses and controllers.  Also required is a Fill Stem Assembly, the part that contains the metal tubes that introduce N2O and GOX into the motor.  All of this equipment is of very high quality and will be good for years of firing. The only part that can be degraded by continual use is the fill stem, and the vulnerable parts are easily and cheaply replaceable. Furthermore, once you have the basic Hypertek system, you can get inexpensive adapters to use it to fill and dump floating injector hybrids, as well as fitting the Aerotech fill hose. 

Another alternative is to add Module Three to the Pratt Hobbies ULS system described in the paragraph about floating injector motors. Module Three controls ignition of HyperTEK motors. Modules One, Two and Three of the ULS will fill and fire any commercially available sport rocket motor. For motors in the K range and above, a longer control wire is required by the NAR and Tripoli Safety Codes. The ULS comes with a 100 foot control wire; lengths up to 1000 feet can be ordered. With all three modules of the ULS, you can fill and fire any commercially available rocket motor, solid or hybrid.

There are two sources for Fill Stem Assemblies for HyperTEK motors. HyperTEK makes a sturdy steel unit that is reasonably priced and attaches easily to any large rod or rail. X-Rockets makes a Drop Down Launcher system that virtually guarantees successful ignition and release of the fill stem. While the X-Rockets system is more expensive than the basic HyperTEK fill stem assembly, it should be considered for larger rockets and for heavy use.


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