After a couple of interesting posts to the Arocket list on hybrid motors I began chatting with Bill Colburn about the history of this motor type. . If you want to email him, address comments to space1space@sbcglobal.net

Amateur Hybrids and N2O use- Primarily the incubation period to 1995
Outside the scope of the Arocket Archives

• N2O and Coal were used in a hybrid motor in the 1920's in Germany.
• Max Valier may have used liquid N2O in a rocket powered racing car, the one in which a motor explosion killed him. This was about 1930.
• GIRD in Russia launches the first flight hybrid in 1933 using LOX and a Gelled Hydrocarbon (my thought is that "Gelled Hydrocarbon" is a mistranslation of asphalt) in the 1930's.
• Rocket Motor Research Society launches first Hypergolic Hybrid using Mixed acids and asphalt/sugar/doped with Potassium Chlorate on the surface, June 1951. (also uses GASEOUS N2O for an oxidizer in
  Bi-Props)
• PRS launches many hybrids in the early 1950's. LOX and Presto Logs, LOX and Wax/Cotton mixture, LOX and Thiokol Rubber (used then for sealing toilets!) Altitudes to 20,000 feet are claimed.
• Not much hybrid work at an amateur level until Bill Wood starts pushing N2O as the "perfect Amateur Hybrid Oxidizer". My first
  acquaintance with Bill's Idea was in 1990. One of the ideas he mentioned right off was a reminder to me of N2O as a self
  pressurizing oxidizer for hybrids. Since I had only used it as a gaseous oxidizer previously, it hadn't occurred to me to go any farther with it.
• I have Mark Grubelich's Sandia paper on an N20/HTPB sounding rocket. When asked directly, he did not remember if he had gotten the idea for using N2O from Bill Wood or not, although they were in constant contact and Bill was a very voluble person.
• Dave Oback, a friend of Bill Wood, asks me to help him with a tiny
  hybrid using the whippet N2O cylinders, 8 grams of N2O. This was
  about mid 1992 IIRC. He wanted to use expended Estes motors for the grain/motor case. Very frugal, Dave. He needed help with a release system. I built a device which punctured and valved the N2O through a 1/16" tube. Dave ran hours of tests (all on endless video) using everything imaginable and unimaginable to ignite these little motors. So Dave is the first to use cellulose and N2O. But I am sure he won't challenge Gary's patent. In about 2 years we had one operating with a self-puncturing device, 1.5 lbf thrust for 1.5 seconds. It had an Isp of 100 seconds. It did not function every time and we dropped it in
  favor of more ambitious sized motors. Later, Rene Caldera came up
  with the same idea independently and successfully.
• At an RRS launch in 1994, Kory fired a 350 lbf thrust hybrid. It
  burned an incredibly long time. It was made up of pipe, copper tubing
  and a group of 20 lbm NOX cylinders to make up the high flow rate
  required.
• I machined a motor for Keith Batt. Keith fired it also in 1994. It
  was a prototype of a 54 mm system which could fly in an HPR Rocket,
  using a lecture bottle and a 2 inch diameter motor case.
• At the next RRS launch I fired one of the same motors. It was N2O
  with an acrylic grain. It was directly connected to a 20 lbm N2O tank
  instead of its flight tank. Embarrassingly, it bent the clamp holding
  it and dragged the tank across the desert. Kory seemed very excited
  by it and exclaimed "It must have had 100 pounds of thrust!". This
  was also the first time I turned a valve into a hybrid motor! I could
  not shut the motor down because the valve seat (neoprene) had been
  burned away in the valve without destroying the valve body.
• I began a long series (three years) of testing at Rocket Ranch,
  firing hybrid motors up to 140 seconds in duration, from 1.5 to 1600
  lbf thrust. I used metallized grains, cardboard, HTPB, Polyurethane,
  polyethylene, partially oxidized grains (tribrid), polyester, epoxy,
  EPDM, Silicone, wood, all in hybrid fuel grains. There was not a
  thought of going into the consumer market with these motors, although
  some were sold to specific qualified individuals.
• I started AEROCON and published the Hybrid manual. Here we introduced the use of electrical conduit for inexpensive static test motors and the use of various types of polymer hose and tubing as fuel grains. Jimmy Arakaki, Dave Oback, and Terry McElheran also contribute ideas to the manual for valving, ignition and general design. Hundreds go out across the nation. Guys were building hybrid motors in nearly every state.
• Suddenly Hybrid Motors were in the news (amateur rocketry news) in 1993-1994.
• I talked with Kory, Kory talked with Gary, I talked with Gary.
  Everyone seemed to be keeping their info close to their chest, as it
  were. Kory did reveal to me, however, the secret of the Kline Valve
  far before it was used at Hypertek. In 1993 I made a connection with
  Lake Valve Company and they sent me some of their axial solenoid
  valves. Later that year I mentioned the valve to Gary. The valve
  showed up in the first ad he ran. He did not go with the valve in the
  final product, however. Gary also checked my intentions by asking if
  I were going to offer the motors for sale to consumers. I said no.
• The ads hit the magazines, Hypertek first and Gary secondly. I ran
  ads for my Hybrid Manual.
• I proposed some ideas for hybrid designs to JPL in 1994, to Leon
  Strand. He offered some encouragement and then JPL closed down their propulsion group and transferred him to planetary missions. Amongst those ideas was the tank-in-motor concept and the motor-in-tank concept. These both offered a greatly reduced package size for
  hybrids. (the SORAC booster is so designed). I also proposed the use
  of a rod-in-tube grain with the rod support also supporting a plug
  nozzle. Another concept was the radial flow grain where stacked discs
  with alternating central perforations and radially disposed
  perforations with spacers to permit cross-flow.
• Terry McElheran did much original work with hybrids including the use
  of a thermitic material to open a diaphragm for valving. He managed
  to open the whippet cartridges in the same way. He used everthing
  from cast polyethylene to toilet paper rolls!
• Mark Geislinger did some interesting work with hybrids adapting
  off-the-shelf items for various functions.Mark also flew Aerocon
  Hybrids.
• HALO Project in Huntsville, Alabama did some original work with
  N2O/Asphalt motors. Russ Bruner developed a cupped-splash plate
  injector about the same time as the SORAC Project and the HALO Group. This had the effect of taking the "roughness" out of the combustion of a hybrid motor.
• John Urbanski and I both came up with the idea for using polymer
  tubing as a combination filler, burst element, and valve, also in
  1994.
• Dave Grifith took up Hybrid Motoring and developed very slick designs
  using the U/C Valve and a floating Piston. 1995 is my guess for
  Dave's entry at that level.
• The SORAC Project starts in 1995 and is continuing to innovate hybrid
  propulsion systems. We launched the largest amateur hybrid motor,
  12.5 inches in diameter producing 3400 lbf of thrust. Out of 8 trials
  we have two successful flights with no recovery and 6 completely
  successful propulsion systems.
• From 1995 on, just check the arocket archives! Here the activity is
  profuse and profound. I certainly may have missed a point or two and
  heartily apologize to anyone I have missed offering credit. This is
  only my personal recollection from 1951 to 1995, and is certainly not
  to be taken as the "official word". Lots of details of the evolution
  of the amateur hybrid motor have been omitted, clearly. I encourage
  others to contribute to this history also. I would be open to
  answering specific questions. Bill Colburn space1space@sbcglobal.net