Three formulations are currently being presented. These are the "basic" formulations that are considered to be the simplest and safest to make. These deliver a fair performance and are well suited to motors up to and including "J" class. Other formulations are being developed that are projected to have a higher performance, and will be well suited to "K" class motors and larger.
The three basic RNX formulations are: Note: all are mass percentages
|Formulation||Epoxy brand||Resin : hardener ||Potassium Nitrate (%)||Epoxy (%)||Ferric Oxide (%)|
|RNX-57||East Systems||6:1||70.0||22.0  ||8.0|
|RNX-71V||West System||6:1||68.0||24.0  ||8.0|
 by mass
 18.9 % resin, 3.1% hardener
 20.6 % resin, 3.4 % hardener
 17.2 % resin, 5.8 % hardener
The three propellant formulations are very similar in most respects, including physical & mechanical properties, performance and burning characteristics. The RNX-71V formulation has the highest burn rate of the three, RNX-73 has the lowest. There is one key difference, however, in regard to preparation of these propellants. The RNX-57 and RNX-73 formulations, made with East Systems and Mr.Fiberglass epoxy, respectively, are the simpler of the two to prepare. The ingredients are simply weighed out, blended, mixed, then packed into the grain mould. The RNX-71V formulation, made with West System epoxy, requires an additional step in preparation, that is, vacuum degassing.
It had been discovered early in my experimental work with the RNX formulations that those prepared using West System epoxy reacted in such a manner that a small amount of gas was generated when combined with the potassium nitrate. This is likely a result of a reaction between a component of the hardener and the small amount of residual moisture that remains in the potassium nitrate, even after oven drying. The result is a propellant with a profusion of microscopic voids or bubbles, as many as 4000 per cubic centimetre, constituting 10-15% of the grain's volume. The consequence of such voids is a propellant burn rate that increases dramatically with chamber pressure. The pressure exponent (n) was typically found to be in the range of 0.75 to 0.90, significantly too high for a safe, reliable & predictable rocket propellant.
It was later learned that degassing the mixed & uncured propellant in a vessel largely evacuated of air (near vacuum) results in the unwanted gas being effectively expelled from the mixture. The resulting density of the cured propellant dramatically increases to typically 94% or 95% TMD, up from the 88% to 89% TMD that is obtained without degassing. The pressure exponent consequently decreases to a more practical and safe level in the order of 0.4.
The required apparatus and procedure for degassing is, fortunately, quite simple and is fully described in the Vacuum Degassing section.
This will result in a particle size distribution largely in the range of 60 to 125 microns. As the burn rate of potassium nitrate based propellants is not greatly affected by particle size (such as it is for AP based propellants), there is no need for great accuracy to be achieved in this pulverizing process.
- Potassium Nitrate
- The grade (purity) of the potassium nitrate required for the RNX propellants needn't be any better than "technical" grade of 98-99% purity or thereabouts. As such, fertilizer grade or certain brands of Stump Remover are perfectly fine. The potassium nitrate, however, should be dessicated prior to usage (to minimize gas formation on contact with epoxy resin). This is done by spreading the material on a cooking sheet, lined with parchment paper, and placing in a preheated oven at 150oC. for about 2 hours. The potassium nitrate, which typically is obtained in the form of prills or granules, must be milled into the form of a fine powder. This is accomplished by pulverizing the prills in an electric coffee grinder, typically 30 seconds per tablespoon.
The cosmetic grade Ferric Oxide comes in the form of a very fine powder and as such, no further processing is required. The Granastar pigment comes in the form of prills (to make it pourable), however, these prills break up into a fine powder when blended together with the potassium nitrate in the electric mixing drum (the inclusion of small oblong pebbles or glass marbles during the mixing process ensures this).
- Ferric Oxide
- I've used two different sources of Ferric Oxide, chemical (cosmetic) grade, and pigment grade. The latter is Granastar Red Brick pigment used for colouring concrete, bought at the local Home Depot hardware store. Note that chemically, these are both red ferric oxide, and are anhydrous (Fe2O3). Other forms of iron oxides are available, such as brown iron oxide (Fe2O3.H2O), and black iron oxide (Fe3O4 ). It is currently not known how effective these would be as substitutes. Note that brown iron oxide can be converted to red (anhydrous) iron oxide by heating in a crucible at high temperature.
Generally epoxies become too thick to be mixed at
temperatures below 10o C (50o F.). A mixing temperature of about 15-20o C (60-70o F.) is the best compromise.
- RNX-57: East Systems
1032 Resin & 834 slow hardener 6:1 mass ratio
RNX-71V: West System
105 Resin & 206 slow hardener 6:1 mass ratio
636 Thin resin & 3:1 Medium hardener 3:1 mass ratio
Although other brands of epoxy may be candidates for propellant usage, it has been found that the burn rate characteristics may vary significantly from one brand to the next. As such, there is currently no direct substitute for the epoxies used in RNX-57, RNX-71V and RNX-73. Other candidate epoxies are discussed below.
Note that the resin/hardener ratio for East Systems and for West System is different than the 5:1 ratio suggested by the manufacturers. The 6:1 mass ratio is used to prolong the pot life, reduce viscosity and to minimize self-heating during curing.
Temperature greatly affects the curing rate of epoxies. In theory, a temperature change of 10 degrees C (18 degrees F.) will double or half the potlife and cure time of an epoxy. Higher temperatures will lower the viscosity (thin) the epoxy, but also reduce
the working time.
As epoxy is self-heating when mixed, spreading out the mixed epoxy instead of keeping it concentrated will extend the potlife.
Other epoxy systems
Epoxy systems are also available at certain craft stores or retail stores such as Wal-mart. Two examples are NU-LUSTRE-55 (at the local Wal-mart) and ENVIROTEX LITE, sold at a local crafts superstore. Both of these come in the form of a kit contain two 8 oz. (235 ml.) bottles (BPA resin & polyamine hardener). These kits are typically used for creating a thick, durable surface on tables or other furniture. The cost for the NU-LUSTRE-55 is $12.84 CAD (=$9.63 USD), the unit price works out to $7.86 USD/lb., which is less expensive than the West System or East Systems epoxy. The quality appears to be similar to the other epoxies currently being used for the RNX formulations. To date, these alternative epoxy systems have not be tested for usage in an RNX propellant formulation.
Although not nearly as economical to make as the sugar propellants, RNX propellants are relatively inexpensive, especially when compared to the high performance AP based propellants. The most costly constituent is the epoxy resin & hardener. West System epoxy currently costs about $96 USD for 1.20 gal. (4.55 litres). This quantity has a mass of 11.5 lbs. (5.23 kg), enough for producing over 48 lbs. (21 kg) of RNX-71V propellant. The unit cost works out to $8.35 USD/lb. ($18.40/kg). East Systems epoxy has a similar cost. Mr. Fiberglass epoxy is much more economical, costing $49 USD for 1.33 gal. (5.0 litres), resulting in a unit cost of $3.89 USD/lb. ($8.57/kg).
The cost for red ferric oxide is very much dependant upon the source and grade. I use Granastar brand "cement colourant", which is essentially pure ferric oxide, and has performed very well. The cost for this material, at Home Depot, is $14 CAD (=$10.5 USD) for a container of 700 grams (1.54 lbs.), which works out to a unit cost of $6.82 USD/lb. ($15/kg).
Potassium nitrate varies significantly in cost depending on the source and grade. Fertilizer grade, which is perfectly suitable, can cost as little as $0.50 USD/lb. However, a more representative cost is about $1 USD/lb. ($2.20/kg).
Based on these constitutent costs, the unit cost for RNX-71V and RNX-73 may be calculated as:
Unit cost = 0.68 (1.00) + 0.24 (8.35) + 0.08 (6.82) = 0.68 + 2.00 + 0.55 = $3.23 USD/lb. ($7.10/kg).
Unit cost = 0.69 (1.00) + 0.23 (3.89) + 0.08 (6.82) = 0.69 + 0.89 + 0.55 = $2.13 USD/lb. ($4.70/kg).
As can be seen, the dominant constituent with regard to cost is the epoxy.