3 Types of Chemical Rocket Engines
Chemical Rocket Propulsion
Chemical rockets use a chemical fuel and oxidizer (collectively ‘propellants’) to react inside a combustion chamber. The chemical reaction produces hot gases. The ejection of rapidly expanding hot gases at high speed from the rocket nozzle creates thrust.
Chemical rockets are mainly used when high thrust to weight performance is required such as powering space launch vehicles, guided missiles, and spacecraft. A Hybrid Rocket Engine (HRE) is one of three categories of chemical rockets – the other two being Solid Rocket Motor (SRM) and Liquid Rocket Engine (LRE). To better understand the advantages of Firehawk’s 3D-Ultra™ HRE technology, it is important to have a basic understanding of each of the three Chemical rocket forms.
In an SRM, solid propellants – oxidizer and fuel are mixed with binding agents, additives, and curative materials to cast-mold a cylindrical shaped propellant grain. Preferred oxidizers for missile and space launch applications are Ammonium Perchlorate and Ammonium Dinitramide. Aluminum (in micron particle size powder-form) is the most common fuel blended with Hydroxyl Terminated Polybutadiene (HTPB), a synthetic rubber binder. Because the oxidizer and fuel are in intimate contact, SRMs are subject to potential explosive detonation, which can be triggered by grain flaws, seal failures, and air pockets or by outside stimulus such as high-energy impacts and exposure to fire or electrical discharges. The propellant grain can be designed to burn from the center outward to the motor case wall or as an ‘end burner’, in which case the propellant is combusted from the aft end forward. In the former, an internal void is formed to create a combustion chamber which vary in their geometry as a means to tailor the thrust profile to the specific application and mission. SRMs cannot be conventionally throttled nor stopped once ignited until all propellant has been exhausted. A few experimental examples have been designed with moveable vent plugs to divert or terminate thrust. Some have even demonstrated pulsing as a means of throttling. Specific impulse performance as high as 295 sec. vac. has been demonstrated by Ammonium Perchlorate Composite Propellant (APCP) SRMs.
Because they can be long-term stored ready for immediate use, they are favored for military applications and because their propellant density and thrust per volume performance is superior to Liquid Rocket Engines, they are often used to power launch vehicle first stages and boosters.
Liquid propellant rockets are subdivided into two sub-categories: Monopropellant and Bi-Propellant. Monopropellant Rocket Engines generate thrust through a process of chemical decomposition, usually with a catalyst; whereas, Bi-Propellant Rocket Engines generate combustion by mixing a liquid oxidizer and a liquid fuel under high pressure within a combustion chamber.
Liquid Monopropellant rockets, though significantly lower in specific impulse compared to Liquid Bi-Propellant rockets, are mechanically much less complex and thus considered more reliable. They can be throttled, stopped, and restarted on-command. Those commonly used for satellite, upper-stage, and spacecraft propulsion have used hypergolic propellants like Unsymmetrical Dimethyl Hydrazine (UDMH) that are toxic, environmentally unfriendly, and carcinogenic. However, in recent years satellite thrusters using ‘green’ propellants such as AF-M315E, a hypergolic hydroxyl ammonium nitrate fuel/oxidizer blend, Nitrous Oxide, and Hydrogen Peroxide have been developed and tested. Liquid Monopropellant rockets range in Specific Impulse from 76 sec. vac. for a typical cold gas generator to a high of 220 sec. vac. for a hot gas thruster using Hydrazine propellant. Some hypergolic engines have demonstrated specific impulse as high as 322 sec. vac.
Liquid Bi-Propellant Rocket Engines offer much higher specific impulse than other classes and types of chemical rockets. Pump-fed engines make use of lighter weight tanks than pressure-fed versions, albeit with significantly higher complexity and cost. They are favored for space launch applications, particularly for upper-stages and spacecraft. Bi-propellant pump-fed rocket engines range in specific impulse depending upon the propellant combination from a low of 303 sec. vac. for rockets using Liquid Oxygen and RP-1 to a high of 455 sec. vac. for those using Liquid Oxygen and Liquid Hydrogen. Cryogenic propellants like Liquid Oxygen and Liquid Hydrogen, though inexpensive, add even more complexity and potential sources of failure, many of which can cause the vehicle to go out of control, catch fire, or explode. Because their propellants cannot be stored long term on-vehicle and require considerable preparations for use, they are rarely used today for military applications.
And then of course there are Hybrid rocket engines. Despite the use of the term ‘hybrid’, a hybrid rocket engine (HRE) is a distinct form of rocket engine that uses both solid and liquid propellants. ‘Liquid-Solid’ Rocket Engine or ‘L-SRE’ would be a more accurate term. Classically designed HRE’s using a liquid oxidizer and a solid fuel operate more closely to a LRE than a solid rocker motor (SRM). In a SRM, oxidizer and fuel are intimately blended and formed into a propellant grain; whereas, a HRE fuel grain contains no oxidizer and the port(s) dually serves as the engine’s combustion chamber; and through a phase-change ablation process, the source of fuel. Classically designed HRE’s using a liquid oxidizer and a solid fuel operate more closely to an LRE than an SRM. In an SRM, oxidizer and fuel are intimately blended and formed into a propellant grain; whereas, an HRE fuel grain contains no oxidizer and the port(s) dually serves as the engine’s combustion chamber; and through a phase-change ablation process, the source of fuel. While traditional hybrids suffer from a low regression rate, inconsistent run-to-run thrust profile, excessive vibration and many other flaws that prevent it from becoming a widely used chemical engine, Firehawk's hybrid engine does mimic those characteristics. By 3D printing our fuel grain we have solved the hybrid problem.
The many benefits of a Firehawk Hybrid Engine can be read in our previous post.