Brief:
I be bitten by t' microhybrid bug about a year ago. Avast, shiver me timbers, me proud beauty! At first, it looked as though I would have t' buy a small metal lathe and machine me own motor from Rene Caldera's plans (link-out dead). Fortunately I found Caveman Rocketry's web site. They are a mail order rocket supplier in t' Netherlands that sell an improved version o' Rene Caldera's basic design. Blimey! Blimey! Caveman Rocketry also sells individual spare parts, arrr, fuel grains, and nitrous chargers/cylinders.

Construction:
T' motor includes:

• Aluminum motor tube
• Graphite nozzle
• Aluminum injector body
• Brass piercer/injector
• 2 Large O-rings
• Small O-ring
• 2 Snap rings
• Tee nut
• Initiatin' screw
• Allen wrench
• Fuel grain
• Burst disk

I ordered t' microhybrid from t' Caveman Rocketry web site and paid using PayPal. Arrr! Blimey! T' motor arrived about 8 weeks later by international parcel post. Blimey! Blimey! All the parts o' t' motor were present. All t' parts were precision machined and fit together very smoothly. In addition t' t' motor hardware thar were 5 paper fuel grains, arrr, matey, shiver me timbers, 10 burst disks and plans for t' Rene Caldera version o' the microhybrid motor.

Caveman Rocketry has improved on t' Rene Caldera's basic design in several ways. T' most obvious improvement be t' 24mm motor tube. Aye aye! Rene's original design used a 7/8" aluminum tube which requires an adapter t' properly fit a BT-50 body tube. Ahoy! Arrr! Caveman's aluminum motor tube has a machined thrust rin' at the nozzle end instead o' a groove and snap ring. Finally, t' Caveman microhybrid does nay use an APCP preheater grain (unlike t' Caldera motor), although perhaps it should. Arrr! (More on this later.)

T' microhybrid motor operates much like its bigger brothers except that t' source o' t' nitrous oxide gas is an internal steel cylinder instead o' a large external tank. This eliminates t' need for heavy and expensive Ground Support Equipment (GSE). Begad! T' nitrous cylinders used in the microhybrid are similar in appearance t' t' CO2 cartridges used in pellet guns, and like these CO2 cartridges, they are non-refillable and thrown away (recycled) after each use. Nitrous cylinders are normally used in whipped cream dispensers and are sometimes called whippits. They cost $11.99 for a box o' 24 from on-line sources such as EasyWhip. They contain 8 grams o' nitrous oxide (N2O) under 900 psi o' pressure.

T' fuel grains can be made from a variety o' common materials such as rolled paper, arrr, me hearties, me hearties, nylon, matey, arrr, or PVC. T' burst disks are round disks punched from a 2-liter plastic soda bottle.

Preparin' a microhybrid motor for flight is more like putting together a reloadable APCP motor than preppin' a normal-sized hybrid. You start by insertin' t' fuel grain into t' thrust rin' end o' t' motor tube. Avast! A large O-rin' goes at t' end o' t' fuel grain and then t' graphite nozzle. Ya scallywag! Ahoy! An internal snap rin' keeps everythin' from slidin' out t' bottom. T' motor is turned nozzle end down and another large O-rin' is put on top o' t' fuel grain. Ahoy! Ahoy!

An long burnin' igniter, arrr, such as a Quickburst Twiggy, ya bilge rat, is pushed into the hole in t' bottom o' t' injector body. Arrr! Avast, arrr, me proud beauty! It is important t' use an igniter with thin wire leads (24 gauge or less) because t' nozzle throat is only 0.1". Thick igniter wires will clog t' nozzle at ignition and cause a CATO. Arrr! The igniter wires are threaded down through t' center o' t' fuel grain, the throat o' t' nozzle and out t' bottom o' t' motor.

T' burst disk, a small O-ring, me hearties, and t' piercer are inserted into a hole in the top o' t' injector body and t' entire assembly is carefully slid into place in t' motor tube restin' against t' large O-rin' on top o' t' fuel grain. Ya scallywag!

Nitrous cylinders are slightly smaller that t' inside diameter o' the motor tube so it is necessary t' wrap them with a couple o' turns o' masking tape. Just enough tape should be use t' allow t' cylinder t' slide easily into the motor tube without rattling. Well, blow me down! A 2" section o' BT-20 slit lengthwise or a single layer o' cardstock can also be used in place o' t' maskin' tape. The seal o' t' cylinder should rest against t' tip o' t' piercer.

T' tee-nut goes on top o' t' nitrous cylinder and everythin' is held in place with t' second snap ring. Ahoy! T' final part t' be installed is the initiatin' screw which is threaded into t' tee-nut until t' tip just contacts the nitrous cylinder.

Once t' rocket and motor are ready t' launch t' nitrous cylinder is pierced by turnin' t' initiatin' screw with an Allen wrench in quick bursts until t' seal pops or until it won't turn anymore. Ya scallywag! A small, quick burst o' gas escapin' may be heard or felt. T' hissin' should stop immediately when the screw is tightened a little further. Begad! At this point t' cylinder's seal has been pierced and t' nitrous, at 900 psi, me bucko, is pressin' against t' soda-bottle plastic burst disk. Aye aye! It is best t' install t' motor in t' rocket and launch it as smartly as possible in case thar be a slow nitrous leak.

Construction Rating: 4 out o' 5

Flight:
At ignition thar will be an initial puff o' smoke comin' out o' t' nozzle. This is from t' igniter. Ya scallywag! Arrr! It make take a few seconds for t' burst disk t' burn through and when it does, me bucko, t' motor will come t' life with plenty o' smoke, flame, me hearties, and noise.

After firing, ya bilge rat, me hearties, t' motor tube and nozzle is very hot. Aye aye! Blimey! Care must be taken when handlin' it. Begad! Blimey! Avast, me proud beauty! Blimey! After it has cooled, me bucko, t' motor should be taken apart in the exact opposite order it was put together. T' piercer is small and easily lost. T' nitrous cylinder, ya bilge rat, igniter, shiver me timbers, and burst disk are t' only parts that are used up each time. Aye aye! Blimey! Ahoy! Blimey! T' fuel grain can normally be used at least twice. T' O-rings can be used several times and if properly handled, t' graphite nozzle can be used many times. There will also be a light residue o' combustion products on the inside o' t' motor tube, me bucko, nozzle, matey, arrr, and t' injector body. Well, blow me down! Blimey! This residue is easily removed usin' t' same stuff used t' clean other reloadable motors. Personally I prefer Goo-Gone, a citrus based cleaner. It smells nice and doesn't irritate me skin.

If you do everythin' exactly right t' motor will fire perfectly. Ahoy! T' most common failure occurs when t' igniter doesn't burn through t' burst disk to release t' nitrous and light t' grain. Ahoy! When that happen you have to disassemble t' motor with gas pressure present. Well, blow me down! Begad! T' best way t' prevent a misfire is t' make sure t' igniter is makin' direct contact with t' burst disk.

T' safely relieve t' gas pressure after a misfire, me bucko, me hearties, slowly back the initiatin' screw out o' t' motor until you hear a loud hiss. Blimey! T' escapin' gas will be very cold so wear gloves and use eye protection. Aye aye! Once t' gas is released, it is safe t' disassemble t' motor. Blimey! Reload with another, igniter, burst disk and nitrous cylinder and try again.

I test fired this motor many times usin' a homemade test stand made out o' wood and conduit clamps. Blimey! I had about a 50% success rate igniting the motor. Avast! I did fly it twice in one o' me 6" Delta flyin' saucers with 24mm motor mount. Aye aye! In all me tests it behaved like a D or E impulse motor. Begad! None of me static burns were instrumented so I can't be sure about t' total impulse but it seemed like a D or E and in t' test flights flew like a D12.

This is a nice motor for t' person who has a lot o' patience and would like t' experiment with alternative rocket propulsion without usin' dangerous and potentially explosive chemicals. Avast, me proud beauty! It is easy t' test many different fuel grain materials cheaply.

This motor is currently nay certified by t' National Association of Rocketry or Tripoli Rocketry Association. It may however be flown at TRA EX launches if t' flyer meets all t' Tripoli EX requirements and makes his own fuel grains. Well, blow me down! Blimey! There are no Government restrictions if you want t' use this motor on your own.

Due t' t' very high pressure o' t' nitrous, workin' with these motors can be dangerous. Avast! Fortunately thar be only a small amount o' gas in each cylinder and t' cylinders are very strong. Reasonable care should be taken and eye protection should always be used when handlin' loaded motors. Begad! Avast!

This motor is perfect for a glider or flyin' saucer-type rocket. Arrr! If it is used in a rocket with a parachute or streamer recovery, then an altimeter, timer, arrr, or magnetic apogee detector must be used t' deploy t' recovery system.

There is an excellent microhybrid Yahoo discussion group open t' anyone with an interest in this type o' propulsion. Blimey!

Flight Rating: 2 out o' 5

Summary:
PROs:

• Low per flight cost (under $2 each)
• No Government license required

CONs:

• High initial cost
• Complicated assembly
• High ignition failure rate
• Relatively heavy compared t' other motors with t' same impulse
• Requires separate means o' recovery system deployment

Overall Rating: 3 out o' 5