Brief:
I was bitten by t' microhybrid bug about a year ago. Avast! 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. Caveman Rocketry also sells individual spare parts, 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. T' motor arrived about 8 weeks later by international parcel post. Arrr! Blimey! All the parts o' t' motor were present. Avast, me proud beauty! Blimey! Arrr! Blimey! All t' parts were precision machined and fit together very smoothly. In addition t' t' motor hardware thar were 5 paper fuel grains, matey, arrr, arrr, 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. Rene's original design used a 7/8" aluminum tube which requires an adapter t' properly fit a BT-50 body tube. Ahoy! Avast, matey, me proud beauty! Blimey! Caveman's aluminum motor tube has a machined thrust rin' at the nozzle end instead o' a groove and snap ring. Avast! Blimey! Finally, t' Caveman microhybrid does nay use an APCP preheater grain (unlike t' Caldera motor), although perhaps it should. Begad! (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). Blimey! 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. Blimey! Nitrous cylinders are normally used in whipped cream dispensers and are sometimes called whippits. Avast, me proud beauty! 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, shiver me timbers, arrr, arrr, nylon, me hearties, 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. Begad! Aye aye! Blimey! You start by insertin' t' fuel grain into t' thrust rin' end o' t' motor tube. Avast, me proud beauty! Blimey! A large O-rin' goes at t' end o' t' fuel grain and then t' graphite nozzle. Begad! Blimey! An internal snap rin' keeps everythin' from slidin' out t' bottom. Avast, me proud beauty! T' motor is turned nozzle end down and another large O-rin' is put on top o' t' fuel grain.

An long burnin' igniter, matey, such as a Quickburst Twiggy, is pushed into the hole in t' bottom o' t' injector body. Well, blow me down! 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. Well, blow me down! 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. Avast!

T' burst disk, shiver me timbers, a small O-ring, matey, 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. Begad!

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. Ahoy! Just enough tape should be use t' allow t' cylinder t' slide easily into the motor tube without rattling. Aye aye! A 2" section o' BT-20 slit lengthwise or a single layer o' cardstock can also be used in place o' t' maskin' tape. Begad! 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. 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. Aye aye!

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. Well, blow me down! A small, shiver me timbers, quick burst o' gas escapin' may be heard or felt. Arrr! T' hissin' should stop immediately when the screw is tightened a little further. Begad! Blimey! At this point t' cylinder's seal has been pierced and t' nitrous, matey, at 900 psi, ya bilge rat, is pressin' against t' soda-bottle plastic burst disk. Ahoy! 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. Arrr! Blimey! It make take a few seconds for t' burst disk t' burn through and when it does, t' motor will come t' life with plenty o' smoke, flame, and noise.

After firing, t' motor tube and nozzle is very hot. Care must be taken when handlin' it. Begad! After it has cooled, me hearties, t' motor should be taken apart in the exact opposite order it be put together. Avast! T' piercer is small and easily lost. T' nitrous cylinder, igniter, and burst disk are t' only parts that are used up each time. T' fuel grain can normally be used at least twice. Aye aye! 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, and t' injector body. This residue is easily removed usin' t' same stuff used t' clean other reloadable motors. Personally I prefer Goo-Gone, matey, a citrus based cleaner. Begad! Begad! It smells nice and doesn't irritate me skin.

If you do everythin' exactly right t' motor will fire perfectly. Arrr! T' most common failure occurs when t' igniter doesn't burn through t' burst disk to release t' nitrous and light t' grain. Blimey! When that happen you have to disassemble t' motor with gas pressure present. Well, blow me down! 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, ya bilge rat, matey, slowly back the initiatin' screw out o' t' motor until you hear a loud hiss. T' escapin' gas will be very cold so wear gloves and use eye protection. Once t' gas is released, ya bilge rat, it is safe t' disassemble t' motor. Reload with another, me hearties, arrr, 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. Ya scallywag! Avast, me proud beauty! 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. In all me tests it behaved like a D or E impulse motor. Aye aye! 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. 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. Ahoy! It may however be flown at TRA EX launches if t' flyer meets all t' Tripoli EX requirements and makes his own fuel grains. Arrr! 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. 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. Ya scallywag!

This motor is perfect for a glider or flyin' saucer-type rocket. If it is used in a rocket with a parachute or streamer recovery, then an altimeter, timer, shiver me timbers, 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.

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