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Project Overview:
Project Overview:
Project Role: Team Founder/Lead
Propellant Type: Metallized APCP
Solids/Metals: 82-83% / 8.6-12%
Delivered Specific Impulse: 215-230s
Chamber Pressures Tested: 300psi - 1580psi
Successful Firings: 12
Successful Flights: 1
Motor Classes Manufactured: J, K, L, M
Total Weight Mixed: 48lbm
Geometries Tested: BATES, Star/Cylinder/Cone
Motor Cases Used: COTS Threaded/Snap-Ring and Experimental CFRP
Special Thanks: ASU SDR Solids (Michael S, Krushna V, Lena B, Nate Y), Alexander Patrus of Castelion Corporation
In 2024, I hit a limit in how fast I could reasonably send a 2" diameter rocket using commercial casings and propellants. Coincident with this, I started to get deeper and deeper into the propulsion side of rocketry, and naturally, these paths crossed. I set out to restart ASU's capability to design, manufacture, and fire experimental solid rocket motors, with performance matching or exceeding commercial offerings, flexible grain geometries, and the capability to make high L:D motors for especially fast flights. In just 3 months, the scrappy team I gathered developed a formula, conducted test mixes, and executed 9 successful hotfires, qualifying the propellant for flight motors in under 1 semester of work. Fast forward to January of 2025, and our first M-class test (pictured left) was completed, and qualified for flight.
Developing a Formula:
Developing a Formula:
Throughout all parts of the process of bringing up a propellant, it was important to keep in mind the end goals and priorities for the propellant. I set forth the requirements for delivered motors and the propellant to be in the range of 225+s Isp, be capable of being flown in ~15:1 L:D ratio motors with relatively high (85%+) volume loading, not contain exotic precursors or materials not readily available to hobbyists, and to have a pourable process to reduce processing variations and inconsistencies. This all led to this propellant formula (only the high-performance variant is shown, although a cooler-burning 82% solids variant is also qualified).
Solids - 83%
AP350 - 43.00
AP90 - 27.00
Aluminum (30u) - 8.00
Magnalium (3u) - 4.00
Ammonium Chloride - 1.00
Liquids - 18%
To achieve a high solids loading mixture with pourable characteristics, a bimodal 350-90 micron mixture was used to reduce the amount of particles of one size in the propellant. A healthy dosage of magnalium was added, in order to ease aluminum combustion in small motors (~2" diameter). Ammonium chloride was used as a low-cost burn rate suppressant, bringing the burn rate down to ~0.3 in/s @ 1ksi.
While not heavily refined and tweaked (3 variations have been hotfired), this formula has proven to meet our requirements, provide excellent performance, and have extremely reliable processing characteristics, helping achieve near-perfect propellant densities on every mix.
Propellant Parameters:
a - 0.0172
n - 0.415
Density - 0.0605
Gamma - 1.20
Molecular Weight - 24.47
Combustion Temperature - 2,955K
Developing a Process:
Developing a Process:
The process for this propellant was tweaked over a number of mix sessions, changing mix steps, vacuum steps, pouring methods, and timings for it all.
The first series of steps in the process is to mix in all liquid ingredients and processing aids, except for the curative (added last). After this, a vacuum is pulled, to remove moisture from ingredients and reduce bubbling later on. The next step is to mix in all of the metal fuels (aluminum and magnalium), before vacuuming this mixture. Once a satisfactory vacuum is pulled, the larger particle size of ammonium perchlorate is added, mixed for 1 hour, and vacuumed for 45 minutes. This long wait time helps AP react with the HX-878 bonding agent, as well as sucking out trapped air. Next is the smaller particle size AP, which is mixed for 1 hour. At the end of this mix, the ammonium chloride suppressant is added, and mixed for 20 minutes. Next is another 45-minute vacuum step. At the end of this, the curative is added, mixed for 7 minutes, vacuumed for 7 minutes, and the propellant poured into casting tubes or thermal liners.
This process has (and still is being) been developed, to account for changing mixing conditions and temperatures, bigger and smaller batches, higher and lower solids mixtures, and more. Throughout each mix, a thorough log of visual telltales, executed timings, and more, is kept, to both log what works and what doesn't, as well as to ensure consistency between batches.
Inspection of our test-ready solid propellants is done through three primary means. The first is visual, spotting any visible inconsistencies or voids. The next is through checking densities versus theoretical - a 100% of theoretical density propellant grain would be guaranteed to have few or no voids. The last is through hardness testing using a durometer. On the Shore A scale, our propellant regularly tests on a 60-70A rating, approximately 1 week post-casting, and 99.5%+ of theoretical density is standard for our propellant's density, with grains at 100% being measured.
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