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The function of the propulsion system is to produce thrust, which is the force that moves a rocket through air and space. Different propulsion systems generate thrust in different ways, but always through some application of Newton’s third law of motion. In any propulsion system, a working fluid is accelerated and the reaction to this acceleration produces a force on the system. A general derivation of the thrust equation shows that the amount of thrust generated depends on the mass flow through the engine and the exit velocity of the gas. The propulsion system of a rocket includes all the parts that make up the rocket engine: tanks, pumps, propellants, power head, and rocket nozzles.
Technological advances in propulsion are steadily reducing costs and improving efficiency, reliability and safety, in order to keep up with satellite demand and maintain the competitiveness in space. Constant environmental concerns result to an increasing need for non-toxic propellants that do not harm life on the ground or in the atmosphere.
In a solid rocket fuel grain, all the components required for vigorous combustion are mixed together and packed into a solid cylinder, into one substance. Once the combustion starts, it proceeds until all the propellant is exhausted. There will be an oxidizer (usually a salt such as ammonium perchlorate or potassium nitrate), a fuel (HTPB – Hydroxyl Terminated Polybutadiene) or some other solid hydrocarbon and an accelerant (sulphur, powdered aluminium, or other easily oxidized metal). When lit, the fuel grain will burn energetically, releasing a large volume of hot gases that are used to provide thrust.
In Liquid fuel propulsion, propellant is comprised of two composites: fuel and oxidizer. They are stored separately in tanks in liquid phase and are pumped into the nozzle combustion chamber where burning occurs. Engine can stop the combustion and the thrust by turning off propellant flow. Liquid rockets tend to be heavier and more complex because of the pumps and storage tanks.
Solid rockets inherently have fewer working parts, which typically relates to lower risks and costs. Storage life of solid propulsion systems is also greater than that of liquid systems, in that they do not require regular maintenance. The disadvantage of standard solid propulsion has been the inability to be “throttled”, thus the initiative to enhance controllable solid rocket propulsion technologies. Another advantage of variable thrust is the ability it provides for altitude compensation for the exit cone and optimum expansion at all boost and sustain thrust modes.
Hybrid propulsion systems, are hybrids or a cross between other types of rocket motor, in particular, liquid fueled rockets and solid fuel rockets. They were conceived to overcome the complexities of liquid bi-propellant engines and the lack of controllability of solid rocket motors.
Controllable solid rocket motors combine the simplicity of solid rockets and the thrust variation ability of the liquid and hybrid engines. Variable solid rocket thrust propulsion technology has been explored by government and industry since the end of the 20th century to find a solution to the propulsion requirements for the next generation of missiles and launch vehicles.
Controllable solid propulsion systems have several advantages over liquid propulsion systems for lander systems. Liquid propulsion systems use multiple fuel tanks, a combustion chamber, several valves, and a control system to manage valve functions. In comparison a typical solid propulsion system has only one igniter and nozzle, and a configured solid propellant grain in the combustion chamber.
Hybrid rocket technology has encountered increased interest in the last two decades. This is due to several parallel advancements proving that high performance, high regression rate fuels can be combined with safe oxidizers in relatively simple, cost-effective solutions. In a hybrid rocket engine, the fuel regression rate is the rate that the fuel surface recedes over the course of a burn and this quantity has a first order impact on the configuration (i.e. combustion chamber length and diameter) and therefore the performance of a motor. Opportunities arising from additive manufacturing of fuel grains including ease of implementing complex geometries, swirling channels etc.
Increased market needs for small launch vehicles and Space 4.0 trends lead to focus on reliability, decreasing launch costs and hybrid rocket propulsion has potential for meeting New Space requirements, thus it is of interest of numerous entities focusing on future commercial operations.
Hybrid Motors Fuels
While historical developments used mostly LOX and storable oxidizers based on nitric acid and dinitrogen tetroxide, current developments utilise HTP and N2O along LOX. Focus is given to environmentally friendly solutions which increase safety and provide potential for simple and low-cost operations. Increased interest in use of higher concentrations of HTP can be seen, unlike in the beginnings of the space age. Moreover, mixtures of N2O and GOX (NYTROX) are considered nowadays. Several flights of new systems worldwide prove that various non-toxic propellant combinations and motor technologies may be used in cost-effective operational space transportation systems in the next years.
ESP Propellants
ESP Propellants are safe and volumetrically efficient solid motors that can be throttled, extinguished, and re-ignited like liquid propulsion systems. ESPs are inert unless ignited with an electric potential of sufficient magnitude. Once electrically ignited, burning rates can be increased by up to 20x (throttled) using electrical power and then switched off (extinguished) by removing the electrical power. Once extinguished, they can be re-ignited by applying electrical power.
ESP formulations are based on high molecular weight (poly)vinyl alcohol, which swells and intermingles to create a tough plastisol solid when added to the ionic liquid oxidizer, hydroxylammonium nitrate stabilized with 5% ammonium nitrate (S-HAN5). The S-HAN-5 is conductive and exhibits pyroelectric behavior (generating heat when subjected to an electric current) in the propellant. This property contributes to the safety of the plastisol, which cannot be ignited by spark or flame (in marked contrast to most solid propellants).
ESP formulations are well-suited for long-term space applications and can be transported safely on the long duration trip to the Mars surface from Earth. They contain no volatile compounds and are manufactured using a water-based, solventless process, which allows them to be stored and operated in vacuum. ESPs are stable and safe to handle, and are classified by the Department of Transportation as Hazard Division 1.4 materials (minor fire or projection hazard). They can be classified as “green” propellants and are relatively non-toxic.
ESP formulations produce only carbon dioxide, nitrogen, and water as major combustion products. As a result of these properties, ESPs are well-suited for use in a cartridge-loaded propellant system, in which propellant is cast into multiple cartridges and the cartridges are loaded into the motor as needed. This allows thrusters to be re-used multiple times simply by replacing spent propellant cartridges.
Hybrid Thruster and motor demonstrations
Aerospace engineering company Valley Tech Systems has developed a thruster technology that was evaluated in a hot-fire test at the company test facility in Lovelock, Nevada, US. During testing, the controllable solid-propulsion thruster design prototype successfully achieved the longest-duration-to-date operation. The technology has been developed under an Air Force Research Laboratory (AFRL) programme to support wider use of solid propellant thrusters for missile and space launch systems.
Raytheon Missile Systems has developed and tested on/off/restart solid propellant thrusters which are controlled only by electrical current. This new patented class of energetic rocket propellant is safe, controllable and simple. The range of applications for this game changing technology includes attitude control systems and a safe alternative to higher impulse space satellite thrusters.
Aerojet Rocketdyne’s controllable solid rocket motor
Aerojet Rocketdyne, Inc., has recently achieved a milestone by completing a successful hot-fire test of a controllable solid rocket motor under the U.S. Air Force Demonstration and Validation Post Boost Study A program.
Post boost propulsion systems are a critical element of strategic and missile defense systems for final payload positioning and deployment. The Post Boost Study program was aimed at identifying emerging propulsion concepts using a trade study approach and selecting high potential candidates for demonstration of technical maturity.
Vice President of Defense Advanced Programs Tyler Evans said this program allowed us to evaluate various advanced technology propulsion systems and select one of the most promising – a controllable solid rocket motor system for hardware demonstration testing. He continued that maturing this critical technology now will undoubtedly pay future dividends as the nation looks to modernize its strategic deterrent capabilities in the coming years.
In 2007, Aerojet tested the largest successful firing of the company’s controllable solids product line using a high-power electromechanical valve control system. The electromechanical valve control system was developed by Aerojet to provide faster responding, smaller and more reliable controllable solids that can be used on larger scale applications, like ORION, and will reduce development risks for controllable solids. “In addition to proving the electromechanical valve control system, this test successfully concludes a one year effort to demonstrate a lower cost method of fabricating the high temperature device used to control thrust and flow direction in our controllable solid products,” said Aerojet vice president of Space Programs, Julie Van Kleeck. “Aerojet is making steady progress toward improving the performance, cost and reliability of our controllable solids products.” Aerojet’s controllable solid product line is critical to stabilizing the flight of space vehicles used in our nation’s missile defense and by NASA.
Valley Tech Systems Demonstrates New Controllable Solid-propulsion Thruster Tech
Valley Tech Systems, Inc. has developed a new thruster technology under an Air Force Research Laboratory (AFRL) program. Valley Tech Systems has validated the controllable solid-propulsion thruster design in a hot-fire test at the company’s test facility in Lovelock, Nev. A prototype successfully performed continuously for hundreds of seconds – the longest firing ever of a system of this type.
“This breakthrough in long-duration throttling opens the door to the advantages of solid propellant thrusters across a broader spectrum of national security programs, as well as government and commercial space applications,” said Russell Carlson, vice president, Aerospace Division, Valley Tech Systems. “Along with long-duration performance, our design focuses on low lifecycle costs and fielded system safety. Key elements of our approach include advanced high-temperature refractory metals and reusable high-temperature insulation material, combined with a fully DoD-qualified solid propellant manufactured by Nammo, our strategic partner for advanced energetic materials.”
The sustained performance achieved with the company’s long-duration throttling makes solid-propulsion thrusters a safer, more affordable alternative for applications in which liquid-propulsion thrusters currently are used. Storable liquid bi-propellant-fueled systems typically have high lifecycle costs due to their toxicity, whereas the solid fuel used by Valley Tech Systems is less hazardous. The Valley Tech Systems technology regulates the ballistic thrust performance of solid rocket fuel, dramatically boosting its energy efficiency.
AFRL funded development of the long-duration controllable solid-propulsion thruster technology through its Small Business Innovation Research program for the development of advanced technologies for post boost propulsion systems, which steer and orient payloads after they are launched into space. The technology also can be applied to booster reaction control systems for the launch of small satellites.
Applying Controllable Solid Propulsion Tech to Army Rocket Modernization
Valley Tech Systems, Inc., has designed an Active Nozzle Propulsion System as a screw-in upgrade for U.S. Army rockets that can significantly increase range and maneuverability. Specifically, it can increase lethal range by more than 67 percent and decrease turning radius by approximately 70 percent over static, fixed-thrust-profile rocket motors. Our design features an all-in-one axial propulsion system with integral thrust vector control, deep throttling and drag reduction. The Active Nozzle Propulsion System applies our controllable solid propulsion technology to the Army’s long-range precision fires modernization priority, enhancing weapon system target acquisition and lethality, in addition to range and maneuverability.
In the Army’s second Expeditionary Technology Search, or xTechSearch 2.0, competition in 2019, we were selected to conduct a proof-of-concept demonstration as one of 12 finalists out of an initial field of 170 entrants. For the demonstration, we developed a prototype and conducted a hot-fire test, which showed the relevance of our Active Nozzle Propulsion System design to Army needs and the efficacy of our technical approach. The results of the test are shown under “Successful Hot Fire Test with Flight Modeling.” We currently are pursuing opportunities for further development, testing and maturation of this application of our controllable solid propulsion technology.
Application to Army Modernization Priority for Long-range Precision Fires
Our active thruster nozzle design directly responds to the Army’s modernization priority for long-range precision fires that provide long-range and deep-strike capability critical to winning in a fight against a peer adversary. This capability is essential to enabling “multi-domain forces to penetrate and neutralize enemy A2/AD (anti-access/area denial) capabilities while ensuring military overmatch at every echelon,” as stated in the 2019 Army Modernization Strategy.
Because we have conceived of our active thruster nozzle design as a screw-in upgrade, it would be applicable to modernizing current weapon systems, as well as to future weapon systems. Our current system design could provide an upgrade capability for the Army’s 2.75-inch Advanced Precision Kill Weapon System (APKWS). Other potential upgrade applications for our active thruster nozzle technology include tactical air-to-air, air-to-surface, surface-to-air and surface-to-surface missiles and the strategic long-range cannon, as well as lower-tier missile defense. In addition, there are potential commercial applications for the technology in the mining and oil and gas industries.
“Smart” Nozzle Increases Range and Precision
Our active thruster nozzle is essentially a “smart” nozzle. Replacing an existing missile or artillery nozzle with our active thruster nozzle would result in a “smart” rocket motor from end to end. This design increases lethal range by increasing operation time through either continuous thrust throttling with options for extinguishable and reignitable operation. With flexible on-demand thrust amplitude capability, the design enables options for air-to-air, air-to-surface, surface-to-air and surface-to-surface engagements.
The design improves precision maneuverability by decreasing turning radius through an innovative multi-nozzle operation that simultaneously throttles the axial thrust while steering the missile through attitude control thrust. Our technical approach employs a portfolio of proprietary hardware, system architectures, innovative low-cost materials and state-of-the-art manufacturing methods. Our technology enables size, weight, power and cost (SWaP-C) solutions to improve overall performance, capability and safety.
The Active Nozzle Propulsion System design includes the following.
- Advanced control algorithms
- Highly accurate proportional thrusters
- Hot-running materials
- Advanced structural insulators
- Thermal coatings
- Wooden round solid propellant with safe handling and storage for warfighters
- Safe, insensitive munitions-compliant ignition systems
Air-to-Surface Engagement Scenario Example
As an example, in an air-to-surface engagement where the ordnance needs to shoot over a hill to hit a distant target, the active thruster nozzle propulsion system would go through this sequence, which also is shown in the following illustrations. This sequence of high-level boost, low-level sustain and precision targeting manages the kinetic energy to extend the engagement range and improve targeting accuracy and lethality.
A: Non-Operation to Ordnance Initiation – to arm and launch
B: High-g Turn – to ensure clearance of obstacle
C: Boost – to achieve peak velocity for ascent
D: Sustain – to optimize flight and drag profile to increase range
E: Precision Thrust Vector Control – to precisely steer to the target
F: End-game Boost – to increase lethality
Successful Hot Fire Test with Flight Modeling
In August 2019, we successfully completed a single-axis hot-fire test of a 2.75” representative thruster nozzle prototype, which mitigated major risks and provided data to anchor performance benefits. This test brought the system to technology readiness level 4 (TRL-4) (component and/or breadboard validation in laboratory environment).
Here is a summary of the test results.
- Successful ignition
- Performance within desired prediction bounds
- No jamming, clogging or excessive ablation
- Thermal margin and thermal shock survival demonstrated
- No combustion instability or chugging observed at low pressures
- Ballistics verified
- Thrust calculated for post-test modeling
- Data collected with no anomalous data
Using hot-fire test data to anchor performance benefits, the flight modeling was updated as follows. Results indicate >70% increase in maneuverability and >67% increase in range. Performance is enabled by thrust vector control, deep throttling, and drag reduction.
ISRO successfully tests hybrid motor
The Indian Space Research Organisation (ISRO) reported in Sep 2022 to have completed a successful test of what it called a hybrid motor. According to ISRO, the test of flight equivalent 30 kN hybrid motor demonstrated ignition and sustained combustion for the intended duration of 15 seconds. The motor performance was satisfactory, the Indian space agency added.
Conventional motors developed by ISRO burn a chemical combination in similar states (either solid or liquid), i.e., they are powered by solid fuel and solid oxidizers or liquid fuel and liquid oxidizers. However, the latest Hybrid motor uses a Solid fuel Hydroxyl-terminated polybutadiene (HTPB) as fuel and liquid oxygen (LOX) as an oxidizer.
In the hybrid motor, the use of Liquid oxygen as oxidizer enables throttling of the motor and the control over the flow rate of the Liquid oxygen enables the re-start capability.
Dr S. Unnikrishnan Nair, Director, of Vikram Sarabhai Space Centre said that rocket motors powered by solid fuels and solid oxidizers are a single-use category. This essentially means that they are discarded (fall back into the ocean) after they burn out the fuel contained within. These motors also cannot be re-started and fired again. However, in the case of motors that use liquid fuel and a liquid oxidizer, the motors can be restarted and the amount of thrust can be throttled (controlled).
It was explained that using liquids makes throttling easier and that managing the flow rate of LOX makes it possible to restart. Both HTPB and LOX are environmentally friendly, but LOX is safer to handle, according to ISRO.
References and Resources also include:
https://www.sciencedirect.com/science/article/pii/S2667134421000614
