There are certainly countless solid propellant formulations that have been tried by amateur experimentalists over the years. Some have been very successful and have gained worldwide popularity, although undoubtedly most formulations have been simply downright failures, or at best, marginally successful. Surely anyone who has ever been involved in AER can cite formulations that they have tried without success, including myself. The very first propellants that I experimented with were zinc/sulfur as well as black powder. For whatever the reasons, I had no luck with either. Other experimentalists, however, have had good (or even great) success with these. The rocket propellant that provided great results for me in my early work was the potassium nitrate-sucrose formulation (KNSU). Interestingly, certain other experimenters at that time had only limited success with it. Clearly, it can be said that many propellant formulations simply won't work (e.g. bad chemistry), some work, but are of limited value (due to cost, difficult to produce, safety concerns, lack reproducibility, etc.), and yet there are other formulations that will function very well -- conditionally. What exactly does this mean?
In hindsight, the reason that I did not have success with the zinc/sulfur or blackpowder propellants was certainly due to the methodology I employed in usage, not a fault with the propellants, per se. The key to success with any viable propellant is knowledge of the propellant with regard to its particular properties and its usage. For example, the exact ratio of constituents strongly affects how well a propellant will perform. "Perform" is meant in a broad sense, encompassing such parameters as specific impulse, burn rate, mechanical properties, castability (or formability), reproducibility, etc. Getting it right such that all these performance parameters are met, to a reasonable degree, is a daunting task. What adds to the challenge is that this is only part of the picture. There are other complicating factors that have a strong impact on whether a propellant will perform satisfactorily. For example, grain (particle) size of the oxidizer, or its purity, effective blending of the constituents, cure time, moisture content, porosity, burn rate behaviour (in particular its "pressure exponent") and even the particular shape of the propellant grain may or may not be suited to a particular propellant. It can therefore be said that the key to consistent success with a formulation (that is known to work well historically) is to follow the correct methodology in preparation and appropriateness of usage. Meeting these conditions will greatly enhance the likelihood of success.
With amateur rocketry, unlike professional rocketry, the availability of materials, the facilities and processes by which rocket propellants and motors may be produced, as well as available financing, vary greatly. And clearly these pale in comparison with professional resources. As such, a clear distinction must be made between the needs of the professional, and the needs of the amateur rocket engineer with regard to requirements defining an ideal rocket propellant. More importantly, what works well for one person, may not work at all for another. Expanding on this thought, what is suitable for one person, may not be at all suitable for another. Therefore, the list that follows is not presented in any particular order, as the importance of each would vary by individual. The exception are the first two items, which must always be of primary importance.
Compendium of Solid PropellantsThe following is a partial list of solid rocket propellants that have been used successfully by amateur rocket engineers (or have potential). Note that there are are certainly other formulations that I am not presently familiar with:
1-7 These are known as the "sugar" propellants. KNSU is the "classic" formulation, which I first learned about while reading Brinley's Rocket Manual for Amateurs way back in 1971. I used this propellant with great success for all my early rocket experiments. Original formulation presented in Brinley's book:
Click for excerpt from Brinley's book (continued)
Potassium nitrate-sugar was originally a development of Bill Colburn in the 1940's (see The KN-Sucrose Propellant -- A Historical Look Back). KNSB was developed in the late 1970's by the Belgian rocketry group VRO. KNDX was developed in the mid 1990's by the author. KNSU, KNSB and KNDX have been researched extensively by the author, including full characterization of their burn rate - pressure behaviour.
KNFR was developed some years later, but has so far found limited usage due to its great affinity to moisture. The burn rate- pressure relationship for KNFR was found to be identical to that of KNDX, based on strand burner tests conducted by the author. KNER is a fairly recent development. As far as I am aware, Scott Fintel was the first person to develop and document the properties of this particular sugar propellant. KNER has some attractive attributes such as great resistance to moisture and slower burning rate than the other sugar propellants which can provide for greater volumetric loading. It is difficult to ignite, however (which makes it even more safe to make and use). Mannitol-based KNMN has been used successfully by a number of rocketry enthusiasts. Burn rate-pressure characteristics of both KNER and KNMN (as well as KNSB) were investigated by amateur rocketry researcher Magnus Gudnason for his Bachelor Thesis in Chemical Engineering Characterization of Potassium Nitrate - Sugar Alcohol Based Solid Rocket Propellants. Magnus's results indicated that the burn rate- pressure behaviour of KNMN is identical to that of KNSB. I am also aware of some research that has been conducted regarding the use of mixtures of sugars, such as sucrose and sorbitol.
A sugar substitute, xylitol, has been used by the author to a limited extent, with good success. The main drawback is cost of xylitol, however, the KNXY propellant has benefits such as very low hygroscopicity and slower burning rate than most other sugar propellants. Xylitol also has the advantage of an exceptionally low melting point (92 degrees Celsius).
KNSU, KNSB, KNDX, KNER, KNMN, KNFR and KNXY all use the standard 65/35 oxidizer/fuel ratio.
There are two other variation of sucrose-based sugar propellants that have been developed in recent years and have been used with good success. James Yawn has developed a version deemed "RCandy" that is made by solution and recrystallization. A specific recipe for "cooking" the ingredients, which involves the addition of water, needs to be followed meticulously to drive out the residual water to achieve a "crystal mush" stage. Further heating leads to a putty-like consistency that is then packed into the rocket motor casing. The approximate formula is:
Rocketry enthusiast Dan Pollino developed a variation that he deemed "Flexifuel". Owing to residual moisture in the propellant, it remains flexible after casting and as such, has the advantage that the propellant can be case bonded. Dan used Flexifuel successfully in many rocket flights.
8 RNX was developed by the author with research beginning in 2001. I was intrigued by the potential for a cold-cast propellant using potassium nitrate as the oxidizer after being given a sample slug of epoxy mixed with potassium nitrate by fellow rocketry experimenter Marcus Leech. When ignited, the slug burned in a slow but stable manner, and showed good promise.
RNX consists of potassium nitrate, epoxy and red iron oxide and is a moderate-impulse propellant that has many positive attributes. Besides the obvious advantage of being cold-cast, RNX possesses good machineability, is relatively slow-burning and has great resistance to accidental ignition. Two variants were developed: RNX-57 and RNX-71V, differing primarily in the brand of epoxy used.
9-12 Serge Pipko has done some innovative development work on enhanced performance sugar propellants. These four formulations are ones I find particularly interesting:
13 Zinc/sulfur was a popular amateur propellant during the 50's and 60's. Due to it's low impulse, safety concerns regarding working with fine powders and rapid and uncontrollable burning rate, it has limited contemporary appeal. The roar, bright yellow flame, and copious smoke does admittedly make for a spectacular launch. A typical ratio is
14 Pyrotechnic "skyrockets" as well as Estes type of model rocket engines use blackpowder as a propellant. The latter has propellant in the form of a highly compressed pellet comprised of:
If sulfur is added to increase the burn rate, the recommended sulfur content is 3% (apparently, the higher the sulfur content, the more "temperamental" the motor):
15 A successful ammonium nitrate composite propellant utilizing aluminum powder as fuel was developed by the author following extensive experimentation that began in 2004. The A24 formulation proved to be particularly successful, being utilized in both static test motors and flight motors. These formulations utilize neoprene (chloroprene) as a binder, extracted from contact cement. The grains are formed by compression using a hydraulic press. Specific impulse in the range of 200-215 seconds is typical.
16 CP Technologies has developed a composite propellant that is comprised of:
17 Of all the polymers used for composite propellants, polyurethane has one of the highest heating values. As such, polyurthane may be used as a fuel without a thermic agent. According to one source I came across, a composition in the range of 85-90% ammonium nitrate and 10-15% polyurethane works well as a propellant.
18 An interesting formulation I came across in Jared B. Ledgard's Preparatory Manual of Blackpowder and Pyrotechnics, this formulation uses stearic acid (a.k.a. aluminum stearate), which is a white waxy powder, as a binder. The result is a "plastic-like" propellant that can be heat-cast. Performance is reportly good with a burn rate of 5-6 mm/second at 1000 psi.
19 The use of GE Silicone II (GE280) as
a fuel/binder with AP as an oxidizer is discussed in the paper
Silicone II -- a New Fuel and Binder for Fireworks by Ken
Burdick (see Journal of Pyrotechnics #8, 1998). Intrigued by the potential use of a simple and commonly available binder for an AP-based propellant, a few years ago I started experimenting along these lines. Indeed, GE Silicone II does make a very nice binder and the resulting propellant cures fully into a hard rubbery non-porous grain. Potential drawbacks were found to be very high burn rate and high pressure exponent. As such, I experimented with the addition of ammonium chloride as a burn rate suppressant. This resulted in a good experimental propellant.
20-21 Detailed information on making PBAN-based AP composite propellant may be found in Terry McCreary's book Experimental Composite Propellant. The oxidizer is ammonium perchlorate (AP), the resin is PBAN (polybutadiene), and the curative is epoxy. The addition of aluminum results in an increased specific impulse, as the reaction of aluminum (with steam in the exhaust) is very exothermic. A drawback with the use of PBAN is the requirement that curing occur at an elevated temperature (140oF) for several days. A typical starter propellant is:
22 HTPB has the advantage over PBAN of curing at room temperature. The uncured mixture is typically puttylike and packs nicely into a mould. Trapped air can be a problem, creating voids in the grain. Likewise, voids and bubbles can result from gases given off during curing (as a result of moisture absorption). Drawbacks also include the limited pot life once the curative has been added to the mixture. Quite a few ingredients may be required (binder, plasticizer, Tepanol, cross-linking agent, surfactant, burn rate modifiers, etc.) although this is not necessarily the case.
Gordon N. Campbell's booklet How to Formulate and Process Composite Propellants, published by Propulsion Systems, provides detailed information on AP/HTPB based propellants intended for the amateur rocket builder, and which provides formulas for a number of AP/HTPB propellants. An example is:
23 Recently I developed an experimental AP-based composite propellant that utilizes epoxy as a binder. As a thermic agent, iron powder is used to good effect, based on a suggestion from rocketry enthusiast John Ashcroft (aluminum powder is another candidate). Finding a suitable epoxy to safely use with AP was a challenge, as many epoxies (such as West System) result in a hazardous compound rather than a safe propellant. NuLustre epoxy, which is a two-equal part (resin-hardener) system, produces a safe propellant, but must be cured under a pressure of approximately 400 psi to eliminate porosity, This is of essential importance to avoid rapid and violent disassembly of the motor.
24 I have been made aware of a successful composite propellant which utilizes powdered PVC (polyvinychloride) as the fuel/binder, and AP as the oxidizer. The stoichiometric ratio is AP 79.3% and PVC 20.7%. Unfortunately, I do not presently have any additional information on AP/PVC compositions.
25 Apparently an early "sugar" formulation used by rocketeers in the 1960's, I read about this in The Encyclopedia of Space (an English translation of La Grande Aventure de l'Espace), one of my favourite books as a teenager. Click for excerpt from the book.
I have just recently developed a performance-enhanced version of KNSB, which utilizes potassium perchlorate as a supplemental oxidizer. This propellant is prepared and cast in an identical manner to standard KNSB, and features a fast burn rate and moderately high pressure exponent.
27 "Formula Two" from the booklet The Homemade Solid Rocket Engine published by Spartan Scientific consists of a propellant with the formulation:
28 This formulation was under development by the Aurora Project Group for their sounding rocket project. The experimental propellant consisted of Potassium Perchlorate oxidizer, epoxy binder and red iron oxide. Following successful small scale motor firings, a large motor utilizing a cast grain with a star-shaped core was test fired. The motor CATO'd and work on this formulation was discontinued. Potassium perchlorate formulations are known to suffer from a high pressure exponent. This was likely a factor that led to the unexpected CATO.
29 Brinley's book describes an advanced amateur rocket (being built at the time) that utilized 100 lbs. of propellant consisting of 75% potassium perchlorate and 25% asphalt. GALCIT 61-C used in commercial JATO units consists of the following formulation:
30 A successful research propellant based on potassium perchlorate with a polyester binder is described in the technical report FTD-HT-66-730 Solid Rocket Propellants by Krowicki & Syczewski. The polyester binder is Polimal 110 (55% Polyester/45% Styrene). The formulation is:
A successful dual-oxidizer propellant that utilizes two-part silicone as a binder was developed by Dave from Australia. Burn rate reported to be 8 mm/sec at 500-600 psi. Recommended Kn is 300-330. This propellant was used in an 80mm motor to successfully launch an experimental rocket to an estimated 14 kft (4.3km).
32-33 Boris du Reau and Philippe Huguenin Bergenat (
) have successfully developed a propellant that utilizes two-part casting silicone as a binder/fuel with either Ammonium Perchlorate or Potassium Perchlorate as oxidizer.
Based on static test data, the formulation with AP will produce a chamber pressure of around 900 psi with a kn=200. The formulation with PP will produce a chamber pressure of around 550 psi with a kn=100.
35 I recently developed this propellant which is quickly becoming a favourite of mine. It is essentially the same as  with the sole exception that aluminum powder is used as a thermic agent. As a binder, I have used both NuLustre epoxy and New Classic epoxy with essentially the same result. Both are two-equal part (resin-hardener) systems. Curing under a clamping pressure of approximately 400 psi eliminates porosity and results in a propellant with near-ideal density. As-obtained specific impulse has consistently topped 200 seconds in motor static testing. Chamber pressure of around 800 psi is obtained at kn=430.
Some Professional Propellants of InterestProfessional solid propellants are those used in commercial and military rocket motors. Needless to say there are countless variations and formulas, each tailored to the specific needs of the application. Nowadays, nearly all are AP-based composite propellants utilizng HTPB or PBAN as binder. This is true due to the proven reliability, performance and plenitude of engineering data available based upon abundant research that was conducted in years past to fully understand and characterize ammonium perchlorate as a propellant oxidizer. However, it is worth noting that early rocket propellants utilizing other oxidizers and other binders were successful in their own right. This is apparent when one studies the history of sounding rockets. Sounding rockets, used for meteorological and upper atmospheric research (for example) are generally "small" rockets not too dissimilar to those designed and built by amateur rocketeers. As such, I feel there is value in presenting a few of these propellants that may serve as inspiration for us amateur rocket engineers.
The Loki Dart is one of those sounding rockets of interest. Originally developed by JPL for the military, but never put into service, the Loki-Dart found its niche as a highly successful sounding rocket, of which several thousand were flown. A number of different propellants were used in the earlier Loki-Dart rockets (ref. Richard B. Morrow's Small Sounding Rockets - A Historical Review of Meteorological Systems 1955-1973):
 70% as received; 30% ground 12 micron
 57% as received; 43% ground 5 micron
 Grain size distribution for AP Blend
And of course any list of professional rocket propellants of interest to the amateur rocket engineer would be amiss if it did not include the Space Shuttle Solid Rocket Booster formulation: