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Improving Performance of Solid Rocket Fuel through Advancements in Materials Science


RT&L FOCUS AREA(S): General Warfighting Requirements

TECHNOLOGY AREA(S): Air Platforms; Materials / Processes; Weapons

The technology within this topic is restricted under the International Traffic in Arms Regulation (ITAR), 22 CFR Parts 120-130, which controls the export and import of defense-related material and services, including export of sensitive technical data, or the Export Administration Regulation (EAR), 15 CFR Parts 730-774, which controls dual use items. Offerors must disclose any proposed use of foreign nationals (FNs), their country(ies) of origin, the type of visa or work permit possessed, and the statement of work (SOW) tasks intended for accomplishment by the FN(s) in accordance with section 3.5 of the Announcement. Offerors are advised foreign nationals proposed to perform on this topic may be restricted due to the technical data under US Export Control Laws.

OBJECTIVE: Design and develop efficient near-throat controlled cooling technologies that would improve solid fuel performance by enhancing condensation of gaseous metal suboxides.

DESCRIPTION: Typically, solid rocket fuel is packed in a tubular motor case and consists of hydroxyl-terminated polybutadiene (HTPB) mixed with fuel additives in the form of metal powders, such as aluminum or boron. Upon deployment of a missile, the available fuel is ignited; and the combustion pressure that develops is funneled through the nozzle assembly. The throat, a critical component of the nozzle, is located near the exhaust end of the motor case. The nozzle diameter is intentionally reduced here in order to alter the flow and maximize performance.

The science of combustion of a solid fuel system is highly complex and it is believed that complete combustion of metal additives could dramatically increase performance [Ref 1]. A substantial part of combustion energy is released during condensation of the gaseous metal suboxides. A significant portion of this energy can be emitted as a flux of light. It could constitute more than 50% of the overall energy produced during the combustion of metals [Ref 4]. This energy should be dissipated from the condensation zone in order to enhance condensation. The thermal conductivity [Ref 5] and jetness of the nozzle are major parameters that control the rate of energy dissipation.

The condensation of metal suboxides occurs concurrently with the exhaust gas expansion in the nozzle. As such, the latter should be thoughtfully designed in order to:

(a) enable the most favorable conditions for condensation of gaseous metal suboxides into its condensed oxide form,

(b) accommodate the majority of condensation energy, and

(c) withstand a radiative heat flux of high intensity formed as a result of a localized light emission subsequent to condensation of gaseous metal suboxides.

This SBIR topic seeks to design and develop efficient near-throat controlled cooling technologies that enhances heat removal, and therefore enhances condensation of gaseous metal suboxides. The manufacturing materials of the nozzle assembly must be able to withstand temperatures of combustion gasses on the order of 3000 K, high-radiative heat fluxes (greater than 10 MW/m2 [Ref 8]) caused by intense emissions of light, erosion, stress, thermal shock [Refs 2, 3], and other factors involved in the operation of a solid fuel rocket engine. The legacy materials are mostly based on specialty carbons, such as a carbon-carbon composite, or isostatically molded graphite [Refs 6, 7]. The proposed solution must meet all of the properties of the standard carbon materials to include, but not be limited to, to withstand erosion, stress, and thermal shock. Additionally, the throat insert must have controlled material properties such as thermal conductivity and jetness, so that it can remove heat from the condensing gas and from associated light emission at an ultrahigh rate, at least 10% more efficiently than the traditional materials.

PHASE I: Design and develop a numeric model for the purpose of tuning the throat nozzle assembly’s material properties to induce a more efficient condensation near the throat. Initial prefeasibility studies with newly fabricated materials should be undertaken at the bench scale level. Deliver a prefeasibility report that outlines the results from the model and the delineation of nozzle assembly properties, which should at the very least, meet the performance characteristics of existing standard throat nozzle assembly materials at the end of Phase I. Outline a plan for improvement of material properties. Include prototype plans to be developed under Phase II.

PHASE II: Demonstrate that the new materials will be at least 10% more efficient at withdrawing heat from condensing gases and light emission sources. Qualitative modeling will be used to estimate exactly how the prototype parts would benefit the thrust. All other physical and chemical properties of throat assembly will be at the same performance level as standard throat assembly materials. Perform testing to validate the technology can withstand stress, shock, and erosion.

PHASE III DUAL USE APPLICATIONS: Finalize and mature the technology for transition and integration into surface-to-air and air-to-surface munitions, mobile targets, and space vehicle programs. Solid rocket fuel engines are heavily employed by various branches of the U.S. Navy, other branches of the DoD, and NASA to include commercial space exploration missions. Other applications include, but are not limited to, industrial uses for high-density electrically conductive graphite used in refractories, reactor components, and specialty liners for chemical vessels. Another application would be in industrial burners and in the design of exhaust of elevated temperature combustion engines, to include automotive applications.


  1. Balas, S. and Natan, B. “Boron oxide condensation in a hydrocarbon-boron gel fuel ramjet.” American Institute of Aeronautics and Astronautics, Inc., Journal of Propulsion and Power, 32(4), February 24, 2016, pp. 967-974.  
  2. Essel, J.T.; Acharya, R.; Sabourin, J. L.; Zhang, B.; Kuo, K. K. and Yetter, R. A. “High-temperature behavior of graphite under laser irradiation.” International Journal of Energetic Materials and Chemical Propulsion, 9(3), January 2010, pp. 205-218.  
  3. Thakre, P.; Rawat, R.; Clayton, R. and Yang, V. “Mechanical erosion of graphite nozzle in solid-propellant rocket motor.” American Institute of Aeronautics and Astronautics, Inc., Journal of Propulsion and Power, 29(3), May 7, 2013, pp. 593-601.  
  4. Altman, I.S.; Pikhitsa, P.V. and Choi, M. “Key effects in nanoparticle formation by combustion techniques.” Springer, Dordrecht, Gas Phase Nanoparticle Synthesis, January 2004, pp. 43-67.  
  5. Altman, Igor. “On energy accommodation coefficient of gas molecules on metal surface at high temperatures.” Elsevier B.V., Surface Science, August 2020, p. 698.   
  6. “Technical Data Sheet CGW™ Graphite.” GrafTech International Holdings, Inc., 2011.
  7. Albers, T.; Miller, D.J.; Lewis, I.C.; and Ball, D.R. “Low CTE Highly Isotropic Graphite (U. S. Patent 7,658,902 B2).”  
  8. Cross, P.G. “Radiative heat transfer in solid rocket nozzles.” Journal of Spacecraft and Rockets 57(2), October 2018, pp. 1-14.
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