**1. Introduction**

The use of chemical propulsion systems for rocket engines is quite common for over half a century. Hydrazines are the major chemical space propellants of choice due to their good performance and reliable track record. A majority of low earth orbit (LEO) satellite propulsion systems are based on monopropellant hydrazine thrusters. The Israeli Offek LEO satellites employ such a hydrazine system [1–3]. **Figure 1** depicts the Offek satellite top plate with monopropellant hydrazine thrusters, being the space facing part of the propulsion module. **Figure 2** depicts the propulsion system module and its schematic, which identify the construction and major parts and components of a typical monopropellant space propulsion system.

#### *Aerospace Engineering*

Hazards have been identified as an elemental part of such work with materials, which for the sake of performance are required to react as energetically as possible. The hazards are part of the technology throughout the life cycle, from manufacturing, handling, transport, and storage, through actual firing in rocket motors and eventually disposal.

The term "green propellants" has been generally used to describe propellants that have the benefit of reducing any of the abovementioned hazards. Based on the European Space Agency (ESA) definition, a "green propellant" is one that has the potential to have reduced adverse impact, either to the environment or to personnel with whom it may come into contact, while still having the performance to meet mission requirements. The term "reduced hazard propellant (RHP)" has been appropriately coined to describe the propellants for which any of the hazards are reduced.

As part of the ongoing deepening and widening of safety concerns throughout the world, there is an ongoing regulatory process in Europe, which has brought about the subject of RHP to be of high priority. This includes decisions at the European Parliament level of the establishment of the European Chemical Agency (ECHA) and to effect the regulation REACH for Registration, Evaluation, Authorization and Restriction of Chemicals in Europe in mid-2007 [4, 5].

At the onset of the "green propulsion" age, RHP alternatives to propulsion have been emerging. The introduction rate of these into space systems is very slow due to the conservatism of the space propulsion industry [6]. The only "green" satellite propulsion technology that has to date gained actual space heritage as monopropellant replacement is the ADN-based monopropellant by ECAPS that made its debut in 2010 aboard the Swedish Prisma satellite and was recently launched aboard the American SkySat constellation [7–18]. While the ADN-based monopropellant technology has thus gained the highest technological readiness level (TRL) among the emerging "green" monopropellants, it is still being evaluated in R&D programs, such as the European Horizon 2020 [19–23], US "green" propulsion evaluation programs [15], and before that the European FP7 Green Advanced Space Propulsion (GRASP) program in which the corresponding author too had actively contributed to the information generated by the program [18].

Whereas for monopropellant systems there already is some space heritage with the ECAPS "green" propulsion system, or RHP, which is comparable to existing

**3**

**Figure 2.**

*The hydrazine propulsion module [3].*

ment model thruster systems.

outlined proof-of-concept firing program [23].

*Green Comparable Alternatives of Hydrazines-Based Monopropellant and Bipropellant Rocket…*

hydrazine-based systems, then for bipropellants, having higher specific impulse *Isp* and density impulse *ρIsp*, the situation is less advanced. Recently, an innovative hypergolic system, based on kerosene and hydrogen peroxide, has been developed by NewRocket© [24], which is similar in performance to MMH/N2O4. The NewRocket Green Propellant (NRGP) hypergolic bipropellant is based on concentrated hydrogen peroxide as oxidizer and on a kerosene-based fuel. NRGP has been made robustly hypergolic by addition of a minute amount of a solid energetic activator to the fuel, which is maintained homogeneously distributed in the fuel by its suitable gelation to a shear-thinning yield-stress fluid. This, while neat HTP and kerosene are not hypergolic. The shear-thinning feature of the fuel enables its full functionality in propulsion systems, including pressurized or pumped feed flow

and injection to the reaction chamber, just like any liquid propellant.

**Figure 3** depicts a bipropellant module schematic that is identical to the comparable MMH/N2O4 systems, with the regular components. The thruster assembly consists of a thrust chamber assembly (TCA) with injector, combustion chamber, and converging-diverging nozzle, as well as flow control valve (FCV) that controls fuel and oxidizer feeds. These feeds are provided by regulated pressure from their storage via dedicated manifolds with necessary valves, such as check valves, or inline valves that can be of various types: shape memory alloy actuators (SMA), pyrotechnical valves, or bistable latching valves (LV). The system feed and pressurization are serviced via fill and drain valves (FDV) and fill and vent valve (FVV). **Figure 3** also depicts the bipropellant firing test setup which has been realized at the Technion for various NRGP proof-of-concept and develop-

Section 2 describes the proposed concept of gradual migration from monopropellant hydrazine propulsion systems to equivalent RHP systems. The concept is based on dual capability of an entire monopropellant chemical space propulsion system. Details are presented of the actual risk reduction program that has been employed for satellite hydrazine propulsion systems that may function also as "green" satellite propulsion systems employing an ADN-based RHP system. Concluding remarks are presented including a conceived way forward with an

*DOI: http://dx.doi.org/10.5772/intechopen.82676*

**Figure 1.** *Offek satellite top plate with monopropellant hydrazine thrusters [2].*

*Green Comparable Alternatives of Hydrazines-Based Monopropellant and Bipropellant Rocket… DOI: http://dx.doi.org/10.5772/intechopen.82676*

**Figure 2.** *The hydrazine propulsion module [3].*

*Aerospace Engineering*

eventually disposal.

reduced.

Hazards have been identified as an elemental part of such work with materials, which for the sake of performance are required to react as energetically as possible. The hazards are part of the technology throughout the life cycle, from manufacturing, handling, transport, and storage, through actual firing in rocket motors and

The term "green propellants" has been generally used to describe propellants that have the benefit of reducing any of the abovementioned hazards. Based on the European Space Agency (ESA) definition, a "green propellant" is one that has the potential to have reduced adverse impact, either to the environment or to personnel with whom it may come into contact, while still having the performance to meet mission requirements. The term "reduced hazard propellant (RHP)" has been appropriately coined to describe the propellants for which any of the hazards are

As part of the ongoing deepening and widening of safety concerns throughout the world, there is an ongoing regulatory process in Europe, which has brought about the subject of RHP to be of high priority. This includes decisions at the European Parliament level of the establishment of the European Chemical

Agency (ECHA) and to effect the regulation REACH for Registration, Evaluation,

At the onset of the "green propulsion" age, RHP alternatives to propulsion have been emerging. The introduction rate of these into space systems is very slow due to the conservatism of the space propulsion industry [6]. The only "green" satellite propulsion technology that has to date gained actual space heritage as monopropellant replacement is the ADN-based monopropellant by ECAPS that made its debut in 2010 aboard the Swedish Prisma satellite and was recently launched aboard the American SkySat constellation [7–18]. While the ADN-based monopropellant technology has thus gained the highest technological readiness level (TRL) among the emerging "green" monopropellants, it is still being evaluated in R&D programs, such as the European Horizon 2020 [19–23], US "green" propulsion evaluation programs [15], and before that the European FP7 Green Advanced Space Propulsion (GRASP) program in which the corresponding author too had actively contributed

Whereas for monopropellant systems there already is some space heritage with the ECAPS "green" propulsion system, or RHP, which is comparable to existing

Authorization and Restriction of Chemicals in Europe in mid-2007 [4, 5].

to the information generated by the program [18].

*Offek satellite top plate with monopropellant hydrazine thrusters [2].*

**2**

**Figure 1.**

hydrazine-based systems, then for bipropellants, having higher specific impulse *Isp* and density impulse *ρIsp*, the situation is less advanced. Recently, an innovative hypergolic system, based on kerosene and hydrogen peroxide, has been developed by NewRocket© [24], which is similar in performance to MMH/N2O4. The NewRocket Green Propellant (NRGP) hypergolic bipropellant is based on concentrated hydrogen peroxide as oxidizer and on a kerosene-based fuel. NRGP has been made robustly hypergolic by addition of a minute amount of a solid energetic activator to the fuel, which is maintained homogeneously distributed in the fuel by its suitable gelation to a shear-thinning yield-stress fluid. This, while neat HTP and kerosene are not hypergolic. The shear-thinning feature of the fuel enables its full functionality in propulsion systems, including pressurized or pumped feed flow and injection to the reaction chamber, just like any liquid propellant.

**Figure 3** depicts a bipropellant module schematic that is identical to the comparable MMH/N2O4 systems, with the regular components. The thruster assembly consists of a thrust chamber assembly (TCA) with injector, combustion chamber, and converging-diverging nozzle, as well as flow control valve (FCV) that controls fuel and oxidizer feeds. These feeds are provided by regulated pressure from their storage via dedicated manifolds with necessary valves, such as check valves, or inline valves that can be of various types: shape memory alloy actuators (SMA), pyrotechnical valves, or bistable latching valves (LV). The system feed and pressurization are serviced via fill and drain valves (FDV) and fill and vent valve (FVV). **Figure 3** also depicts the bipropellant firing test setup which has been realized at the Technion for various NRGP proof-of-concept and development model thruster systems.

Section 2 describes the proposed concept of gradual migration from monopropellant hydrazine propulsion systems to equivalent RHP systems. The concept is based on dual capability of an entire monopropellant chemical space propulsion system. Details are presented of the actual risk reduction program that has been employed for satellite hydrazine propulsion systems that may function also as "green" satellite propulsion systems employing an ADN-based RHP system. Concluding remarks are presented including a conceived way forward with an outlined proof-of-concept firing program [23].

**Figure 3.** *The bipropellant module schematic (left) and test firing setup (right).*

In Section 3, a similar comparable attitude is proposed for the hypergolic system based on kerosene and hydrogen peroxide, similar in performance to MMH/N2O4. Results are presented of the firing tests of the proof-of-concept and development model systems and of the NRGP fuel rheological characterization. The results of various engine types demonstrate the capability to operate this technology in both pulse and steady modes and in various thrust levels. This bipropellant technology offers a promising alternative to the presently employed hydrazine-based systems, through the fact that the fuel and oxidizer show very robust hypergolicity and short ignition delays, as well as characteristic velocity efficiency (*ηC*\*) exceeding 98%.
