Hybrid Propellants

Hybrid propellants use a combination of liquid and solid reactants. Most frequent is to have the oxidizer in liquid state and the fuel in solid state. The following types of hybrid combinations exists:

a) Classical hybrid: liquid oxidizer – solid fuel

b) Inverse hybrid: solid oxidizer – liquid fuel

c) Quasi hybrid: Liquid oxidizer – fuel rich solid propellant

d) Inverse Quasi hybrid: Oxygen rich solid propellant – liquid fuel

Combination b) and d) are hard to impractical to do and will not be further discussed herein. As mentioned above, combination a) is most common whereas c) has only seen experimental use. However, there are some claims that SpaceShipTwo is using oxygen doped fuel grain in order to achieve sufficient regression rate (burn rate).

A classical hybrid rocket motor uses an oxidizer in liquid state. The solid fuel part is different from the solid propellant that it does not contain any oxidizer (oxygen rich salt). The fuel is thereby totally inert, which makes the classical hybrid rocket engines very safe. The following table lists some propellant combinations at chamber pressure of 3.45 MPa.

Propellant O/Foptimum [-] Flame Temp  [ºC] Vacuum Specific Impulse  [s]
Oxygen + HTPB 2.05 3500 318
90% H2O2 + HTPB 6.50 2800 299

The fuel part is HTPB, which is an abbreviation for Hydroxyl-Terminated PolyButadiene. HTPB is an artificial industrial polymer often used as binder in solid propellants.

Other inert fuel types often used are high-density Polyethylene (HDPE), Paraffin Wax, ABS and Polyurethane. HTPB has seen most use since it is well known from the solid propellant world. In its pure form HTPB needs to be strengthen with additives in order to get sufficient mechanical strength. The molecule is complex and has a complex cracking process which take some energy away (specific impulse). The more readily available, less complex molecule and less costly polyethylene (plastic) gives higher performance than HTPB for the same type of oxidizer. However, its regression rate is often less, which means relatively speaking that a larger evaporation surface is needed. Paraffin wax has seen much attention in recent years, mainly due to its high regression rate and high specific impulse. Low regression rate is often a design challenge when using or considering hybrid rocket propulsion. Rates as low as 0.3 mm/s to 1.5 mm/s is not uncommon. Paraffin wax has amplified these values up to five times, making hybrids more attractive. An important drawback with paraffin wax if not treated with additives, is its low melting point. The best performing wax melts around 65 ºC which may limit its used as propellant.

Hybrid combustion occurs somewhat differently than in solid propellant combustion. Solid propellants deflagrate through chemical combustion on the evolving burning surface. In hybrids, oxidizer is injected at the head-end of the engine over the inert fuel. The injected oxidizer should be in fine droplet state (mist) or even better at a heated gaseous state. The solid inert fuel needs to be converted from solid state to gaseous state. That phase change requires energy depending on the chemical compound or composition used. The gasified fuel leaves the fuel surface and mixes in with oxidizer at some distance from the fuel surface. In the mixing zone (boundary layer) exothermal reaction occurs producing gas products at elevated temperature. The thermal energy generated is in transferred to the inert fuel by part convection and by part thermal radiation. An important limiting factor in hybrids are the fuel rich zone that emerges from the evolving evaporation surface. As thermal heat has to be radiated through this zone in order to gasify new fuel, some of that thermal energy may be absorbed and transported away, by thus limiting the heat transfer to the fuel and the regression rate. Turbulent flow may enhance the heat transfer momentum to the fuel surface through the boundary layer.

Regression rates may be increase by adding metallic particles like aluminum (Al) or boron (B). Chemical compounds like magnesium hydride (MgH2), Alan (AlH3) or Lithium Aluminum Hydride (LiAlH4) may both increase regression rate and the specific impulse.  Aluminum has shown to increase regression rate with 2 to 3 times by increasing the combustion temperature. The down side is two-phase flow, slag and increase erosion.

Energetic compounds have shown interesting results in lab-scale motors. However, they are costly, unstable and can create toxic fumes.

Regression rate can also be enhanced by turning to swirl combustion. To create vortex combustion, the oxidizer should be is injected into the motor and fuel through a dedicated swirl injector. Swirl combustion has proven to increase the regression rate up to 5 times compare to traditional axial injection for the same oxidizer mass flux condition. Swirl flow and combustion is complex but seems to solve many of the classical limitations in hybrid propulsion by improving regression rate and improving combustion efficiency.

Regression rate is normally not dependent upon pressure as for solid propellants. It is dependent on mass flux. Mass flux is the ratio between mass flow over port surface area. Often regression rate is related to the oxidizer mass flux. The propellant mass flux is rarely not higher than 700 kg/m2-s. Too high mass flux can lead to flooding, meaning to much oxidizer limiting the ability to burn. Regression rate level will stagnate. Too low oxidizer mass flux will often mean that regression rate is unpractical small, dominated by melting process.

A well-designed hybrid rocket motor will be very robust and safe. Since the combustion mechanism is not pressure dependent as in solid propellant rocket motors, any crack or debond (larger burn surface) will have little to no effect on the motor performance. A crack or debond in a solid motor is often enough to cause catastrophic failure by over-pressure.

Hybrids are more complex than solid propellant motors since the need a separate volume for the oxidizer, pressuring system, tubes and valves. However, they are less complex than a liquid rocket engine. Hybrid has shown to be excellent for throttling, on demand termination and restart. Specific impulse may be higher than most solid propellants.

Some researchers have studied and tested hybrid rocket motors with oxygen doped fuels in order to overcome some of the important and difficult limitations in the classical hybrids. An oxygen doped fuel is a step towards a solid propellant. There is an important difference here. If the fuel is doped to certain degree, the fuel may not be able to deflagrate at ambient pressure level. When able to at higher pressure, it will behave as solid fuel gas generator. The temperature is low as the specific impulse. But when the gas is mixed with e.g. oxygen, the combustion temperature and specific impulse may increase to high levels. There are different types of quasi hybrids. Some has the oxidizer injected at the front end as in the classical hybrids. Others has the oxidizer injected into a chamber downstream the gas generator charge. The latter type seems to be the most promising way. The engines may become very compact, have good combustion efficiency and still be able to throttle, terminate and restart. The down side is increased complexity.

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This article is a part of a pre-course program used by NAROM in different courses, for example Fly a Rocket!