Introduction
Hybrid rocket motors (HRM) are a type of rocket propulsion system that combine the advantages of both solid and liquid rocket motors. They use a solid fuel as the propellant, which is typically a rubber-like material, and a liquid or gaseous oxidizer, such as nitrous oxide or hydrogen peroxide, to sustain combustion.
The concept of a hybrid rocket motor has been around for over a century, but it wasn’t until the 1960s that it began to receive serious attention. However, overall, much less research and development has been completed with hybrids than liquids or solids.
Advantages of HRMs
Hybrid rocket motors offer several advantages over other types of rocket motors, including:
Safety: One of the primary advantages of hybrid rocket motors is their inherent safety. Unlike liquid or solid fuel rockets, hybrids are less prone to explosion or catastrophic failure. This is because they use a solid fuel grain, which cannot explode like a liquid or solid rocket motor.
Controllability: Hybrid rocket motors offer greater controllability than solid rocket motors. The fuel flow rate can be adjusted by varying the oxidizer flow rate, allowing for precise control of thrust.
Restartability: Hybrid rocket motors can be easily restarted, which is not possible with many other rocket motor types.
Cost-effectiveness: Hybrid rocket motors are generally less expensive to manufacture and operate than liquid or solid rocket motors. This is because they require fewer complex components and can be manufactured using less expensive materials.
Environmental Friendliness: Hybrid rocket motors are considered to be environmentally friendly because they produce less harmful emissions than liquid or solid rocket motors.
Disadvantages of HRMs
Hybrid rocket motors have some disadvantages compared to other types of rocket motors. Here are a few:
Complexity: Hybrid rocket motors are more complex than solid rocket motors. They require a fuel grain and an oxidizer, and the interaction between the two can be complex and difficult to predict.
Limited performance: Hybrid rocket motors may not have the same performance capabilities as other types of rocket motors, such as liquid rocket motors. Compared to liquid rocket engines, hybrids typically have lower specific impulse (a measure of how efficiently a rocket engine uses propellant), which means they require more fuel to achieve the same level of performance.
Safety concerns: Hybrid rocket motors can pose safety concerns due to the potential for oxidizer leaks. The use of oxidizers in hybrid rocket motors also requires careful handling and storage, as they can be hazardous materials. Also, because hybrids use a solid fuel, there is a risk of the fuel burning unevenly or in an uncontrolled manner, which can cause oscillations in thrust and lead to structural failure.
Implementation Challenges
Orbital Class Rockets Using HRM
One problem in designing large hybrid orbital rockets using HRM propulsion is that turbopumps become necessary to achieve high flow rates and pressurization of the oxidizer. This turbopump must be powered by something. In a traditional liquid-propellant rocket, the turbopump uses the same fuel and oxidizer as the rocket, since they are both liquid and can be fed to the pre-burner. But in a hybrid, the fuel is solid and cannot be fed to a turbopump’s engine. Some hybrids use an oxidizer that can also be used as a monopropellant, such as nitromethane or hydrogen peroxide, and so a turbopump can run on it alone. But nitromethane and hydrogen peroxide are significantly less efficient than liquid oxygen, which cannot be used alone to run a turbopump. Another fuel would be needed, requiring its own tank and decreasing rocket performance.
Electric turbopumps are also an option. However, this requires the use of electric batteries which increases the weight of the launch vehicle.
Paraffin Fuel
One of the most promising materials for the solid fuel component in HRMs is paraffin, a type of wax that has many desirable properties for rocket propulsion. Paraffin is a low-cost, easily obtainable material that has a high energy density and a predictable combustion behavior. These properties make it an ideal choice for use in HRMs.
Despite its advantages, there are some challenges associated with using paraffin as a fuel in hybrid rocket engines. One of the biggest challenges is ensuring that the paraffin is evenly distributed throughout the combustion chamber, which can be difficult to achieve. In order to overcome this challenge, researchers are developing new techniques for distributing the paraffin, such as using porous materials or developing new fuel grain geometries.
Launch Vehicles Using Hybrid Rocket Engines
Virgin Galactic
Virgin Galactic’s SpaceShipTwo suborbital launch vehicle uses a large hybrid rocket motor to propel passengers to suborbital space. Rocket Motor 2 (RM2) is a hybrid rocket motor utilizing solid hydroxyl-terminated polybutadiene (HTPB) fuel and liquid nitrous oxide oxidizer, providing up to 310 kN of thrust. They do not use a turbo pump to manage oxidizer flow rate.
SpaceShipTwo and its RM2 hybrid rocket motor is operational.
Gilmour Space
Gilmour Space’s ERIS orbital launch vehicle uses hybrid rocket motors to launch payloads of up to 305 kg into LEO. Their Sirius hybrid rocket motors use hydrogen peroxide as an oxidizer and a proprietary solid fuel. They use electric turbo pumps to control oxidizer feed rates. Their launch vehicle uses 4 Sirius for stage one and 1 Sirius for stage two. Each Sirius rocket motor produces up to 110 kN of thrust.
ERIS is forecast to launch in 2024.
HyImpulse
HyImpulse Technologies is a rocket launch company based in southwestern Germany, founded as a spin-off from the University of Stuttgart. HyImpulse Technologies has its origins in hybrid motor development at the German Aerospace Center (DLR).
Hylmpulse is developing SL-1, a three-stage orbital launch vehicle with hybrid rocket engines. The launch vehicle will have the capability to carry a payload of up to 500 kg into LEO. Their hybrid rocket motors use paraffin and liquid oxygen. They use turbo pumps to manage oxidizer flow rates.
The first suborbital launch is forecast for 2024.
Their price point is $20,000 per kilogram to LEO.
Reaction Dynamics
Reaction Dynamics is developing Aurora, a two-stage orbital launch vehicle that uses hybrid rocket motors to carry a 30 kg to 150 kg payload to LEO. Multiple hybrid rocket motors are used in stage one. Each rocket motor produces 21.6 kN thrust. Reaction dynamics has not disclosed their oxidizer or fuel. They do not appear to be using turbo pumps for oxidizer flow management.
Their first suborbital flight is forecast for 2023, and their first orbital flight for 2024.
Reaction Dynamics price point is expected to be between $30,000 and $50,000 per kilogram to LEO.
As of October 17, 2023, the status of the company is uncertain and the website shows as”under construction”.
TiSpace
TiSpace is a Taiwanese aerospace company developing hybrid rocket technology and launch vehicles for the small satellite market. The company was founded in 2016 and is headquartered in Taiwan.
TiSpace developed a family of hybrid rocket engines called Lelien.
In 2019, their HAPITH I suborbital test rocket conducted 3 launch attempts from Australia, but suffered an anomaly and fire on the pad during the final attempt. This provided engineering data to improve their technology.
Their plan was to develop a orbital launch vehicle, HAPITH V, to deliver up to 390 kg to LEO and uses the Lelien engines.
As of October 17, 2023, the status of the company is uncertain.
HyPrSpace
HyPrSpace is a French aerospace startup developing a hybrid propelled micro-launcher called the Orbital Baguette (OB-1).
The OB-1 is designed as a micro-launcher capable of delivering 200-250 kg payloads to low Earth orbit (LEO).
The maiden launch of the OB-1 is scheduled for 2026.
VAYA Space
VAYA Space is an American company headquartered in Florida. They are developing innovative hybrid rocket engines and launch vehicles utilizing 3D printed fuel grains and vortex flow field injection. Their proprietary STAR-3D hybrid engine uses non-explosive thermoplastic fuel grains that only become energetic when combined with liquid oxidizer, making the system safer to handle and transport compared to traditional solid or liquid propellants.
The company’s Dauntless small satellite launcher will utilize the STAR-3D engines in both stages. Dauntless stands 35 meters tall and can deliver over 1000 kg to LEO or 600 kg to SSO. It is designed for rapid launch turnaround, with VAYA targeting under 30 days from build to launch. Dauntless completed a successful suborbital test flight in early 2022, validating the hybrid engine design. VAYA aims to be ready for first commercial orbital launches out of Cape Canaveral as early as 2025, pending final development and testing. The company hopes its hybrid technology will provide a lower-cost and more sustainable launch option.
Other Companies
Other companies that are currently focused on the development of commercial hybrid rocket engines include:
- Firehawk Aerospace: A US-based company that is focused on the design and manufacturing of hybrid rocket engines.
- Pulsar Fusion: A UK-based company that focuses on the development of green and clean hybrid rocket engines.
Summary
Hybrid rocket engines have many attractive properties, including low cost, safety, and environmental friendliness. While there are still some challenges to be overcome, the potential benefits of this technology make it a promising area for continued research and development in the field of space exploration. With further advancements, hybrid rocket engines could play a key role in future launch vehicles. However, the big question that remains unanswered – Is there a sustainable market for small lift launch vehicles?
