Advancements in Solid Ramjet Fuel Development
Navy SBIR 2014.1 - Topic N141-011
NAVAIR - Ms. Donna Moore -
Opens: Dec 20, 2013 - Closes: Jan 22, 2014

N141-011 TITLE: Advancements in Solid Ramjet Fuel Development



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OBJECTIVE: Develop and demonstrate innovative Solid Fuel Ramjet (SFRJ) technologies based on novel high performance fuels.

DESCRIPTION: Air-breathing propulsion, in the form of Liquid Fuel Ramjet (LFRJ) or Solid Fuel Ramjet (SFRJ) systems, is a highly competitive solution to tactical systems requiring long range and/or high speeds. While significant development has occurred, including development of several operational systems using traditional LFRJ technology and ducted rocket systems, few resources have been devoted to SFRJ’s despite the significant potential demonstrated in the programs that have been completed [5]. SFRJ systems have a clear advantage over rocket-based systems due to their inherent high specific impulse values which greatly improve the range and kinematic performance of the system.

The use of high-speed air-breathing propulsion for tactical applications has a long history in the U.S. dating back to the Navaho, Bomarc, and Talos systems of the late 1950’s and early ‘60’s [1]. All variants of the Ramjet, e.g. SFRJ, Ducted Rocket, LFRJ, etc., allow significantly higher effective specific impulse (Isp > 1200 seconds in the case of LFRJ and SFRJ cycles) compared with rocket propelled systems and can also possess design simplicity and safety advantages. For this reason, a large number of strategic and target development programs utilizing air-breathing propulsion have been conducted over the last 40 years including the Advanced Low Volume Ramjet (ALVRJ), Advanced Strategic Air Launched Missile (ASALM), Advanced Common Intercept Missile Demonstrator (ACIMD), Variable Flow Ducted Rocket (VFDR) [1], and most recently, the Navy’s GQM-163A "Coyote" Supersonic Sea-Skimming Target (SSST) ducted rocket target drone [2]. The Navy’s Ram Air Rocket Engine (RARE) program conducted boron and magnesium loaded SFRJ flight tests at Mach 2.3 in 1955 with good success [3]. A SFRJ development program in the 1970’s, culminated in a free-jet test at NASA-Lewis Research Center in 1980 [4]. Other Air Force programs in the 1980’s demonstrated high combustion efficiencies and the efficacy of metal fuel additives such as magnesium and boron. Additionally, modern flyout analyses comparing a min-smoke solid propellant rocket system to a similarly sized HC/boron-fuelled SFRJ predicted a 5-fold increase in range using the SFRJ technology.

The SFRJ cycle is the same as the LFRJ cycle except that the fuel exists in solid form within the chamber and the stoichiometry of combustion is controlled by the regression rate of the fuel. The fuel is not a propellant in the solid rocket motor sense, but a pure fuel without the addition of oxidizer particles. A wide range of fuels can be used from polymers such as polymethyl methacrylate (PMMA) to long-chain alkanes or cross-linked rubbers such as Hydroxyl-terminated polybutadiene (HTPB). Because the fuel exists in the solid form, inclusion of solid additives (e.g., metal powders, strengthening additives, etc.) is relatively easy and can increase potential performance gains without sacrificing safety and handling characteristics.

SFRJ’s offer some significant advantages including:
• Simple in design compared to a liquid-fueled rocket or LFRJ. There is no need for pumps, external tankage, injectors, or plumbing for fuel delivery.
• Higher fuel density in the solid phase for many pure hydrocarbons and even higher if performance additives (such as metals) are used.
• Easy inclusion of solid performance additives (such as boron, magnesium, or beryllium) which raise the heat of combustion, and/or the density and therefore the density impulse capability compared with liquid ramjets.
• Solid fuel can act as an ablative insulator, allowing higher sustained combustion chamber temperature levels (and hence specific thrust) with less complexity.
• Fuel is stored within the combustion chamber allowing for more efficient packaging and higher mass fractions than liquid ramjets.
• Can be stored as "wooden rounds" in the same fashion as traditional solid rocket motors with minimal logistical concerns associated with liquid fuels.
• Have potential for favorable Insensitive Munition (IM) properties due to the minimization of oxidizers within the solid fuel grain.

To further advance the state-of-the art in SFRJ technology, advanced fuel formulations with high regression rate, high specific impulse and density impulse capability need to be developed. The fuel formulations need to generate stable and efficient combustion with air. Methods of increasing fuel-regression rates should not be at the expense of fuel inertness. Fuel formulations should contain a volumetric heating value of >825 BTU/in3 and demonstrate a combustion efficiency >90%. Additionally, system level concepts must consider precepts outlined in referenced Military Standards [6] & [7].

PHASE I: Develop innovative fuel formulations and verify performance gains over existing solid rocket motor systems through system level calculations and testing. Demonstrate operational advantages of fuel formulations and develop a plan to transition fuels into flight-qualified system.

PHASE II: Design and develop a prototype system capable of generating positive thrust force under realistic operating conditions. Perform subscale tests to assess theoretical predictions, mechanical properties, and burn rates associated with selected propellant formulations. Conduct direct connect or free-jet tests using a prototype system in order to demonstrate the performance gains of the newly developed fuel formulations over a varying range of operating conditions.

PHASE III: Integrate the selected fuel formulations and motor geometry into a munitions system capable of being integrated into the Navy warfighter capability.

PRIVATE SECTOR COMMERCIAL POTENTIAL/DUAL-USE APPLICATIONS: Advanced solid-fuel formulations developed for ducted rocket-ramjet applications can have a dual-use application in hybrid rocket motors. Commercial space flight is an emerging area that could benefit from such formulations.

1. R. Wilson, C. L. (1996). The evolution of ramjet missile propulsion in the U.S. and where we are headed. 32nd Joint Propulsion Conference and Exhibit. Lake Buena Vista, FL: AIAA.

2. Hewitt, P. W. (2008). Status of Ramjet Programs in the US. 44th Joint Propulsion Conference and Exhibit. Hartford, CT: AIAA.

3. Fry, R. S. (2004). A Century of Ramjet Propulsion Technology Evolution. Journal of Propulsion and Power , 20 (1), 27-57.

4. Limage, C. R. (1996). Solid fuel ducted rockets for ramjet/scramjet missile applications. 32nd Joint Propulsion Conference and Exhibit. Lake Buena Vista, FL: AIAA.

5. Fink, L.E. (1981). Chronological History of SFRJ Flight Tests. Engineering Technology Boeing Aerospace Company. Seattle, WA.



KEYWORDS: Solid Fuel Ramjet; Sfrj; Fuels; Airbreathing Propulsion; Ramjet; Ducted Rocket

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