Backyard Ballistics (tm)

Hybrid Motor Rocketry Page
Content Copright 2001, 2002, 2003, 2004 by Private Data, LLC

Hybrid Rockets and Motors

I'm interested in hybrid motors for rocketry. Why? I don't know. But I'm into the techie side. And I love the loong burns. And the hybrids are also attractive for the higher power rockets (L2 & L3) as the reloads are much cheaper - and certified motors readily available - something difficult to say on both accounts for composites (though particularly on cost).

My club that flies at Whitakers has HyperTek equipment, though I'm told that it's essentially not used So HyperTeck is an option. And recently Doug Pratt donated the adapter set to convert the club's HyperTek equipment to launch U/C valve hybrids.

The Ratt Works are nice, and have H class motors (perhaps nice for getting started), and can be fired from HyperTek equipment with an adapter. Reloads readily available. The disadvantage is the motors are LONG - essentially due to the 29mm tubes they're based on (one motor, the I (?) is 36" long (!)) They've now got 38mm tubes. and have also a 64mm K-240 and L/M-900 motor set.

The Propulsion Polymers seem like an improvement on the Ratt Works - for me mainly due to the 38mm tube yields a shorter motor. Pratt Hobbies has them, though they don't list the "smaller motors" that span into the Ratt Works range. RATT now has 38mm motors. They've also added an L/M combo.

I am also thinking that a Rocket designed for a long 38mm tube could also take the long Cesaroni & Aerotech motors, which would add a lot of flexibility in launching.

Hybrids require electronics and there is no motor ejection.

Hybrids also require special builds as the motor tubes are much longer. This often conflicts with placement of as-designed dual deployment systems as well as, for custom, shifting the balance point for rocket design. There's also the issue of lower average impulse so they need somewhat lighter rockets. The long burns make it all worthwhile though!


I've scratch built my own GSE as I've been frustrated at my apparent inability to sync up with the right people with the right equipment at a particular launch. So for the last 4 launches or so, I've had my motors, but was unable to launch them for various reasons - that boiled down to not having my own GSE. This situation will certainly improve overall - but I want to fly! The GSE availability at Whitakers is resolved, and the equipment typically in the trailer.

I've built a master / slave relay control panel along the lines of the "12 volt relay launcher". I've modified it though to use CAT-5 cable (through DB-9 connectors) and by doing so I get power from the slave and can also add NOX dump/fill and O2 lines. I won't be using the GOX, but am adding support for it so that I can launch HyperTek's later (either mine or others).  I will post plans, schematics, and progress.

I've completed my GSE for all extents and purposes. It's complete with respect to handling all fill/dump/gox relays and one pad. The launch pad box has installed relays for pads 2, 3, but those aren't wired to the second DB9 or the breakout strip. The LCO box will fully control the launch pad box with respect to pad 1 and the hybrid relays. It does not yet have ANY support for pads 2 and 3 nor does it yet support single box launching (using just the LCO box  for a "close, non hybrid pad").

You can look at more details on my Hybrid GSE Page.

I occasionally find myself desiring or needing (sometimes out of field necessity) to adapt equipment to my own or from my own to others. In that light I'm starting to work out the Hypertek GSE Pinouts. Click on that link for what information I've got.

Fill Tubes, Relief Cuts, and Sheething

On U/C valve type motors it is important to get the fill tubing to burn through and separate as close to it's exit point from the injector. The lower the burn through the lower the average thrust, the higher the O/F ration, the higher probability of a burn through, esp if the tubing is slightly bent and pointing even slightly towards a side of a grain. (It's quite common for the tubing to have a slight bend and very difficult to eliminate it).

A common technique, that is really the minimun you should do, is to overwrap the fill tube, for the length of the motor grain and a bit more, tightly with an overlap to two thicknesses of  electrical tape. Leave just enough of a gap to match the thickness of the pre-heater grain.

Dave Griffith of Monterey Machine (RATTWorks) now supplies cardboard tube sheaths for his 29mm motor reloads. These work great

Dave also came up with the idea of using some high temp fiberglass sheathing for, in particular, his 64mm K-240, Tribrid, and L/M motors.

Here's a picture of a K-240 TriBrid fill tube with fiberglass sheathing installed.

RATT K-240 Fill Tube with Fiberglass Sheath

Dave: "For the Ratt K- 240 5/16 dia fill lines use 1/4 i.d. sleeving (part#  8819k44) mount about 1" below injector and long enough to reach past nozzle. use black tape to hold in place. On the Ratt TriBrid and L/M 600-900 motors with 3/8 fill lines use 5/16 i.d. sleeving (part# 8819k45) mount right at below  groove and long enough to go past nozzle and tape in place. Price is only .31 per foot.

For the larger nylon fill tubings, 5/16" for his K-240 and 3/8" for his TriBrid and L/M combos, Dave cuts a relief groove in the fill tubing to further encourage U/C valve separation at the desired opint.

Here's a close-up of that same fill tube illustrating the relief groove.

RATT Fill Tube Relief Groove and Fiberglass Sheath

This tubing is high pressure 3/8" nylon tubing. The ID is 0.25" providing a measured wall thickness of 0.065". The OD of the relief groove is 0.325"  so 0.050"  has been removed. providing a new calculated wall thickness of 0.065 - (1/2)(0.050) or 0.040" so we've removed about 1/3rd of the wall thickness.

Pyro-Free GSE (ignition)

The HyperTek system seemed cumbersome requiring all the GSE. Now that I'm building out  my own GSE, adding O2 seems only more of a slight additional expense.

Basically it's a flare fitting T. The leg would carry the "ignition gas" to the outer sleeve. One end of the T would have a compression with the larger ignition gas tubing; the other end of the T would have the N2O tubing running through it. The specific orientation is only to allow the N2O tubing, which goes through the fitting, to go straight through. The ignition gas tubes terminate at the fitting so can be on the leg and end of the fitting.

West Coast and Propulsion Polymers have been up to this. Here's the West Coast Hybrids instruction document and they're Pyro-Free GSE plans; and the Propulsion Polymer's instructions (well, one variant anyway) has pyro free info.

Here's a NOW Hybrids page on Pyro Free for RATTWorks Hybrid Rocket Motors.

Propulsion Polymers

I've purchased a Propulsion Polymers I160 from Pratt Hobbies. I've since purchased the other tubes and floating injector in the set. I  hoped to then use the larger J140 tube to attain my TRA L2 certification. But for various reasons that didn't pan out and I got my L2 cert on a solid. I bashed Periwinkle's Pride, my PML Black Brant VB, and re-christened her "Periwinkle's Pride Gets N2Oxious". The J-140 motor took up most of this kits's body tube  I doubled it's length adding a recovery module and main chute bay.

I've flown Periwinkle half dozen times or so on the I-160 and J-140 tubes. both are sweet rides. Nice kick off the pad and nice long burns. They don't have the raspy "farty" note of the RATT Works motors, so some folks don't recognize it for a hybrid flight.

Propulsion Polymers now has a nice 54mm "Penta Max" motor with two tubes and configurable injectors.

Do see Designing Rockets for Hybrids down below and, in particular, my efforts at bashing Periwinkle.

Micro Hybrid

I'm  interested in one of  Rene Caldera's Micro Hybrid. It offers something unique but also inexpensive flights as well. Plus fancy hardware for my tool fetish page. Roy Trzeciak-Hicks and Caveman Rocketry are producing finished motors. Since I don't have a lathe to finish a kit, I bought an assembled motor from Caveman Rocketry on a Rocketry Online Auction.

I've received this motor and plan on building a rocket for it. Since the motor has no recovery provisions I'm planning on using a Magnetic Apogee Detector to fire the main chute. These are inexpensive (~$30) and quite compact. I've got a "Kinda Jayhawkish" LOC kit for it.

Update: I've picked up some N2O micro-cylinders from Ebay (cheap!), but haven't done anything with it yet. I think I'll try it first in a Art Applewhite UFO so I can test it in a rocket not requiring recovery (I haven't anything small that has electronic recovery - though I DO now have a MAD detector on my bench). And the LOC kit on the bench too.

Recovery Electronics

This page discusses rocket recovery electronics. The focus is on altimeters, timers, tracking transmitters, beepers, etc. Look elsewhere on this index page for other electronics.

Designing Rockets for Hybrids

Designing a rocket for Hybrids is a little more challenging. Partly due to the longer motor tubes (very long in the RATT, particularly the M-900!) Also the need for electronic deployment. And, if one wants dual deployment, the weight of those components is shifted farther forward. For such a more neutral balance (vs short motors in the ends),  either a longer rocket is required OR smaller fins. This to keep the rocket "stable" but not over stable. Small fins requires a longer launch rod/rail depending on the initial motor thrust. The other issue is the relatively low average impulse of hybrids. Long motor burns, yes, but lower average impulse. Fortunately, most thrust curves are regressive providing a nice kick off the pad.

Note that Apogee Rocketry's Hybrid Motors In RockSim page they erroneously state that the worst case scenario is when the motor is empty. This is not correct. The worst case scenario is when the nitrous level matches the CP in the rocket. It is at this point that the nitrous is "doing it's worst" making the CG/CP relationship unstable as it's at this point that the nitrous weight is pulling the CG back the most. The situation actually improves again as the nitrous is consumed.

So CG/CP is at it's best when motor is full
is at it's worst when nitrous matches CP
is, well, somewhere inbetween when nitrous consumed.

Here's a specific link to a cool rocket. It's built for the HyperTek, but the ideas apply. This link is from Star Rocketry. They also have some good build info.
 Todd Lumpkin's Scratch Built Purple Reign
There doesn't otherwise seem to be much info, concentrated at least, for designing for hybrids.

I've been  bashing Periwinkle and have her ready to go. I plan on launching her at August Whitakers. I've had her ready for a while but this time I'll have my own GSE so that I'm self-sufficient and can launch no matter who attends otherwise. I missed August Whitakers, and misplaced my large floating injector for September Whitakers, so am not targeting October Whitakers, with a bashed periwinkle, with my new GSE.

RATT Works

RATT Works and HyperTek were the early certified sport hybrid rocket motor vendors. To many, the RATT K-240 is the quintessential  hybrid rocket motor.

As of this writing they have 29mm ,38mm, and 64mm motors. That they didn't have 38mm motors was one reason I bought my Propulsion Polymers set though I must say I'm very happy with that set. The 64mm motors, now spanning a K-240, through K-350, through an L/M-900 combo, are an odd size, but fortunately PML 2.5" BT is a perfect MMT and CRs are available so there isn't any real reason, from a build perspective to not consider these motors.


I have the H-70. It's a nice easy flyer. 29mm and 18" long. I bought it to fit my PML BullPuppy 2.1 after a slow Kosdon G-40 skywriting session shortened the rocket so it wouldn't balance for my favorite Kosdon 29-150 loads. It was long enough for this motor to use the rocket as a booster section and I built a payload bay with dual deploy and have been getting nice 1,000 foot flights off her.

K-240 / K-350

To many this is the quintessential hybrid rocket motor. A nice, loud, farty resonance, 8 second burn, it is recognized by many. As folks say "A serious ride for a 10lb rocket." Now available is an (uncertified at this writing) new injector that ups the impulse and ISP and provides a full K flight and higher average impulse as K-350.

I have one from a ROL Auction (motor and 4 reloads for $230). I am looking to build a new booster section for "Ignorant Of My Destiny"  to fly this motor. I am working with Dr Zox on a "ratfink" fin theme just for fun. I will probably call it Ratfink too. Right now, this is looking to be a 14lb rocket. I need to do the sims to see if it needs to be flown on the K-350 injector or whether the K-240 stock motor will be safe on available rails.


Recently introduced is this L/M motor combo. 900 Ns average thrust with a nice pad peak on a regressive thrust curve. It's still 64mm and this the M variant is long - about 78" whereas the smallest M HyperTek is about 40" it comes as a 42" long L-900 along with an extension tube that makes it an M-900. Aside from it's length it looks like a sweet combination.

I'm considering using one for my L3 Certification Rocket. It's flexible and nicely priced at $375 for the L/M combo and reloads are about $80 for the L and $95 for the M vs $125-$150 for the HyperTek M reloads..

One minor advantage, perhaps, is I maybe be able to use it for larger EX flights, moreso at least than if I bought a HyperTek. and in the larger sizes, I'm not sure if there is an easy L/M HyperTek combo.

Now hybrids has a RATT resource page with McMaster part numbers. It's here.

Aerocon / Bill Colburn

Bill Colburn produced some 38mm H/I hybrid rocket motors with the intention of certifying them. They were never certified, but I managed to pick one up on a ROL Auction. Since it's now an EX motor at this point, I'm documenting it and my activities on my Experimental Hybrid Rocket Motor Page.

SkyRippers / West Coast Hybrids / HyperTek

I mean no disrespect glomming these vendors together. It's mostly that I have less to say about motors that I don't own. ... which is no longer true about HyperTEK ,...


Aside from RATT Works, HyperTek is the quintessential sport hybrid rocket motor. They're available for 54mm J motors through farily large 98mm M motors. They require more complex GSE (get your club to buy it; Whitakers has it) but motor prep is much easier than for a U/C valve motor. Overall, it's your preference, but you'll find a wider selection of motors in the larger sizes (J and up) with HyperTek. Many of their motors are configurable using different injector combinations for different flight characteristics and one of the J loads can be flown twice.

The only reason I don't have one (at this time of writing) is that now having my own U/C GSE I can fly anywhere ther's a rail. Whitakers is my main flying home now, and they've got a HyperTek GSE, so there's less reason for me to not consider one.

Well, I've got a HyperTEK Hammerhead J now and will be flying it sport and EX
Here's the HyperTEK Manual
Here's the current HyperTEK dimensional drawings.

Here's a favored way of preparing the hv speaker wire. Well, several. The "split" and also the "Breathe Right Strip" Technique:

West Coast Hybrids

WCH has a nice 38mm motor and I think is working on others. It seems to be popular on the Western US. I believe they're working on a 54mm motor.

Sky Rippers

Sky Rippers is a newer entry in the market. They have a nice 38mm tube set and are gaining in popularity. One nice feature is they use snap ring cases (made by Woody at so fit standard retainers. I've also considered one as a candidate for EX Hybrids as I may need to do nothing more than slip in a Loki 38mm o-ring'd nozzle and fly some pipe. I believe they're working on a 54mm motor.

Experimental Hybrid Rocket Motors

I am intending to fly experimental hybrid rocket motors and will document my activities on my Eperimental Hybrid Rocket Motor Page.

General / links

Bill Shamblin - Bill was an early hybrid flyer at Whitakers and has an experiment hybrid rocket motor page.
Robert Gaejis (sp) - the inventor of the MAD Apogee sensor has been flying experimental hybrid rocket motors.

Pratt Hobbies
Now Hybrids
Star Rocketry

Propulsion Polymers
Ratt Works
West Coast Hybrids
Sky Rippers

Cold Fusion N2O systems - ask for Mike and for the eBay pricing; should be lots off their list.

Forums - The home of the "Thigh Altitude Hybrid Project"; this tends to deal less with sport and more with science and serous large rocket design. - An active forum for sport and some experimental hybrid rocket motor flying.
http://fredddy - somebody else's hybrid rocket forum.
http://xxxxx - The Micro Hybrid forum