How do the "floating injector" motors work?

Skyripper, Ratworks, West Coast amongst others manufacture floating injector hybrid motors (sometimes called Urbanski/Colburn or U/C Valve hybrid motors) there design is beautifully simple, there is one long tube with the rear closure/nossel holder, the propellant grain and the bulkhead at the bottom, above the bulkhead is the oxidizer tank and forward closure.

A nylon tube runs through the nossel and up the center of the fuel grain and connects to the injector, which is fixed in the center of the bulkhead.

Anatomy of a Skyripper hybrid rocket motor.

Fuel Grain (Red)    Nozzle (Grey)

Case (Black)       forward/Rear washer/ Retaining Ring (green)

Injector body (Pink) venting bulkhead (Yellow)


N20 is the oxidizer that provides the oxygen to drive the combustion chemical reaction. (Combustion, an oxidation reaction, is the combination of oxygen with some other substance.

Fuel Grain as the name suggests provides the fuel which is the substance that actually "burns" (is oxidized). This reaction between the oxidizer and fuel generates gas and heat, which provides the thrust for the rocket.

The Nozzle directs the gas formed by the reaction out the back of the rocket. The nozzle is formed such that the gasses are accelerated as they pass through the nozzle and provide efficient thrust.

The Injector body forms the top of the combustion chamber and the base of the oxidizer tank, it holds the injector when the fill line burns through it releases the oxidizer into the combustion chamber.

The Venting Bulkhead seals the top of the oxidizer cylinder and provides a visual indication that the cylinder is full by venting excess N20 when full.

The retaining Rings fit in groves at each end of the motor case and holds the componemts inside the motor assembly.

The Case holds all the components of the motor together, and is made to withstand the pressure exerted upon it during the motor burn. 


unlike solid rocket motors hybrids do not have a delay grain or ejection charge, therefore some other method of deploying your recovery system must be used.

Prepping the Sky ripper Hybrid for flight.

To prep the motor,

1. Put the fill tube through the center of the injector and secure it in place with the pressure fitting.

2.Insert the pre heater grain ( a 10mm slice of 29mm white-lightning reload) into the top of The fuel grain.

3. Attach your igniter to the side of the nylon fill tube close to the bulkhead so it will sit next to the pre heater grain, hold it in place with electrical tape.

4. feed the tube through the hole in the pre heater/fuel grain and fit the bulkhead in place.

5. Fit the nozzle to the other end of the fuel grain.

6. Now ensure you have lubricated all the o rings with Krytox and insert the whole assembly into the bottom of the motor casing.

7. Insert the seal disk and the aft cir clip.

8. Finally, lubricate the O rings with Krytox on the forward closure and insert it into the other end of the motor casing, and again fit the cir clip.

9. Connect the fill tube and the igniter leads to the GSE.

10. Retreat a safe distance to your hybrid launch controller.

11. when you get the go from the RSO, hit the Fill switch and the N2O will flow into oxidizer tank.

12. when you see the N20 coming out the vent the tank is full.

13. Do a quick count down from 5, and push ignition button.

14. The igniter starts the pre heater grain burning,which ignites the fuel grain and cuts through the nylon fill tube, releasing the N2O into the combustion chamber and brings the motor up to pressure.

15. We have lift off.