Micro8 Hybrid Rocket Motor with two paper reload kits

MicroHybrid Quick Start Guide

Before any assembly the motor should be inspected for any damage, wear or dirt, special attention should be paid to the internal circlip groves, these should have a crisp edge and not be obstructed with dirt or residue build-up, this is the most common cause of motor mishap, the circlips not holding when under load of Nitrous release or firing due to worn or dirty grooves.

1) If using a paper fuel grain ensure that it fits into the motor casing, some paper fuel grains require either a wrap removing or sanding, If using a plastic fuel grain place paper disk at either end of the grain to stop it from sticking to the nozzle and injector housing.

2) Wrap the Nitrous Oxide bulb with masking tape to make it a snug fit inside the motor casing it must still slide without having to use force, ensuring a central fit within the body tube Nitrous bulb neck must be centered to align with the piercer.

3) Grease the two large O-rings and small O-ring with Nitrous safe grease such as Krytox or simply Dawn dishwashing liquid.

4) Prepare the Piercing unit / injector and injector housing as follows:

a) Place a pre-heater grain in the pre-heater well (the shallow but larger diameter well) My preference is a coil of PIC coiled with a tail coming from the centre that extends out of the nozzle to an external igniter, or this can be an Aerotech D21 fuel grain with the outer paper peeled off and inserted into the pre-heater well and then trimmed flush with the top of the well. Ensure the slot in the fuel grain extends over the hole through the centre of the housing, if it does not then drill a hole through the centre of the pre-heater; this is best done from the front of the housing.

b) Into the piercing unit well of the injector housing insert a burst disk followed by small O-ring and the injector, the injector should be positioned with the flat side against the O-ring and the point facing out.

5) Assembly is started at the forward end of the motor by inserting the forward inner circlip.

Due to the Circlip manufacturing process stamping them from sheet metal, they may have a rounded edge on one side and a flat side the opposite side, ensure the circlips are installed with the flat sides to the outside ends of the body tube so that when under load they don’t try to slip out the rounded side is prone to slippage.

6) Put in the T-nut with the threaded section forwards and the flat to the rear (inside).

7) Insert the nitrous bulb rounded end forward, do not push all the way in, and leave the neck of the bulb protruding for the moment.

8) Holding the casing vertically (forward end uppermost) at all times until instructed otherwise (or you may move the injector / piercing unit. Place the prepared injector housing/injector and preheater assembly under the nitrous bulb and gently insert it in to the rear end of the motor housing. Be very careful not to dislodge or move the injector on its O-ring. The point of the injector should be centered in the indent of the nitrous burst disk.

9) Insert one of the pre greased large O-rings into the rear of the motor housing so it is against the injector housing.

10) Insert the fuel grain (if using a plastic grain it is advisable to place a paper disk on each end to stop the plastic sticking when hot). Slide all of the way in, until the T-nut is securely against the forward circlip keeping it pressed in place you can now turn the whole assembly over.

11) If using a plastic fuel grain ensure the paper disk is on the nozzle end, if not insert one now. Insert the second large O-ring into the motor housing and place it against the end of the fuel grain.

12) Insert the nozzle so that the O-ring will seat into the grove in the forward face of the nozzle. If using a PIC preheater ensure the PIC tail exits the nozzle whilst fitting the nozzle.

13) Insert the rear circlip ensuring it is the correct way round (rounded side inwards flat side outwards) and properly seated in the groove.

14) Lastly insert the outer circlip into the grove at the rear of the motor housing.

15) Insert an igniter through the nozzle so that it touches the pre-heater grain and secure in place. Or if using a PIC preheater attach the igniter to the pic tail outside of the nozzle. Just before you are ready to launch your rocket arm the Nitrous by tightening the arming bolt, there may be a slight his as the bulb seals over the injector, this should stop as you finalize tightening the bolt, do not over tighten the bolt or you may damage the O-rings and injector.

If using the PIC with tail pre-heater ensure the RSO is aware that there will be second or two's delay from ignition to launch. With other preheaters there may be a second delay but the motor will come to life with some noticeable flame and noise. If you have a misfire then holding the motor at arm’s length (wear suitable gloves for handling Nitrous) slowly loosen the arming screw slightly until the Nitrous start to escape from the front of the motor casing, keep clear of the front end the Nitrous is extremely cold and can cause frost bite.

After firing the motor be careful as the motor casing and internal components may be hot. Disassemble the motor in reverse order inspecting all components for wear, replace any worn components, carefully inspect the nozzle for wear, cracks or chips and replace if there are any suspect areas. Clean all parts of residue whilst fresh to ease clean-up, again take care cleaning the soft graphite nozzle it is easily damaged.

By arming or firing this motor you agree to do so at your own risk and release us of any liability in case anything should happen to you or your property. It is your responsibility to provide for the safety of yourself and others during arming or firing the microhybrid.