SpaceDev Hybrid Rocket Propulsion Technology

Hybrid rocket propulsion is a very safe and low-cost technology that has tremendous benefits for current and future space missions. Our hybrid rocket propulsion technology features an elegantly simple design, the ability to be restarted and is throttleable. Additional benefits include the ease of transportation and handling and storage.

Our background and expertise in hybrid propulsion technology was derived from the initial knowledge base produced by American Rocket Company (AMROC). We obtained the technical rights, proprietary data and patents produced by millions of dollars worth of hybrid rocket motor research by AMROC. AMROC designed and hot test fired a wide variety of hybrid rocket motors of all sizes, utilizing non-toxic, storable propellants, which are the critical enabling technology for sub orbital manned space flight. AMROC completed approximately 300 hybrid motor tests from 100 to 250,000 pounds of thrust.

We have performed studies and analyses to determine the preliminary requirements, concepts and propulsion design parameters for expendable launch vehicles and a modular, scalable family of orbital Maneuvering and Transfer Vehicles (MoTVs) that use hybrid rocket technology. Additionally, we have received several motor contracts and grants by the California Technology, Trade and Commerce Agency and other government agencies, related to our hybrid-powered MoTVs. Under these contracts, we have built and successfully test fired multiple variations of hybrid motors, with the final unit utilizing replaceable fuel cores.

We were  awarded Phase I and Phase II of a contract to develop a Shuttle-compatible propulsion module for the Air Force Research Lab (AFRL). Our previous work for other government agencies on our Secondary Payload Orbital Transfer Vehicle (SPOTV) and its MoTV, combined with this project for the AFRL, gave us important and unique capabilities in the area of placing, inspecting and protecting space-based assets. The MoTV was designed to provide on-orbit maneuvering and orbit transfers of customer microsatellites and payloads launched from expendable launch vehicles. Successful completion of the AFRL project gave us additional capabilities to deliver customer payloads to their desired orbits, and include NASA Shuttle Hitchhiker Experimental Launch System (SHELS) compatibility.

For fuels, most organic substances will work, however, for manufacturing purposes, HTPB (synthetic rubber) or PMMA (Plexiglas) are preferred. Other oxidizers were tested, including liquid oxygen, nitrous oxide, and hydrogen peroxide (LOX, N₂O, H₂O₂). All have benefits, and none are considered toxic, but they do have different handling requirements. Nitrous oxide is our oxidizer of choice because it is benign, storable, and self-pressurizing to 700psi at room temperature. The combination of HTPB or PMMA and N₂O is totally benign and non-toxic. We have worked with different combinations of fuel and oxidizer and we choose the most appropriate (tried and tested) fuel and oxidizer for each intended use. The oxidizer to fuel ratio varies for each, but the technology does not and hundreds of successful motor firings verify this.

While the somewhat lower Isp and reduced mass fraction associated with hybrid propulsion systems tend to make them less desirable candidates for launch vehicle boosters and high-performance upper stages, hybrid propulsion offers several benefits over conventional liquid and solid systems. For sub-orbital manned vehicles, hybrid is ideal because the performance is adequate to achieve altitude, but the safety features of using hybrid propulsion in conjunction with these vehicles far outweigh other performance factors. The simplicity of their operation makes hybrid rockets ideal for high-reliability applications.

Hybrid Rocket Propulsion Systems

Ground Handling Equipment

Minimal ground handling equipment is required: a motor lifting device; a gas cart; and an electrical test set. After manufacture, the hybrid stage can be processed horizontally. If desired, it can be mounted to a pallet and handled via forklift. A basic "gas cart" for N₂O loading is needed, which can be palletized or mobile and consists of simple valves and gauges. Depending on loading procedure, a scale for weighing ullage may be needed and the operator may need a small avionics test set to test the ignition system and the main oxidizer feed valve.

Rocket Test

Very little development testing is necessary. At a minimum, fuel regression rate verification tests, and some oxidizer injection flow tests are required. Under a contract awarded by the California Space Authority (CSA), we built its mobile hybrid motor test facility. All development tests can be done at full scale and/or some can be done with lab scale (~300 lbf) motors. Motor qualification test with flight hardware can then be performed. Additionally very little ground equipment is required, essentially a thrust stand and a PC-based Data Acquisition System (DAS), all of which is provided by us, fully integrated and tested.

Mass Fraction and Specific Impulse

Hybrid rocket motors can be tailored to meet a variety of different mission requirements. For launch vehicle boosters that help deliver payloads to orbit, the motor design is optimized to maximize both Isp and propellant mass fraction, which increases size and cost. Conversely, a small orbital transfer vehicle is optimized for delta-V capability to minimize size and cost. For a manned sub-orbital space plane, Isp and mass fraction are traded off for low-cost and high-reliability within the boundaries of acceptable performance and size constraints.

The Isp of a LOX/HTPB hybrid motor is comparable to solid propellant motors and LOX/kerosene rocket boosters. The propellant mass fraction of a hybrid motor is typically 20% to 30% less than these competing systems. However, new developments in hybrid motor fuels and oxidizer injection techniques show great promise for closing the gap on mass fraction.

Safety

It is difficult to find a rocket motor safer than one using rubber/plastic and laughing gas. The rubber / LOX combination has even been rated by the Vandenburg range as a 0 lb of TNT equivalent. N₂O is even safer (it is used as a pressurant for whipped cream). The bottom line is that the fuel has to be vaporized in the presence of an atomized oxidizer with a high temperature igniter in order for it to burn. Even in failure mode, it’s safe.