Hybrid Rockets

RATIONALE FOR THE STUDY

A hybrid rocket is a rocket with a rocket motor which uses propellants in two different states of matter - one solid and the other either gas or liquid.

Hybrid rockets exhibit advantages over both liquid rockets and solid rockets especially in terms of simplicity, safety, and cost. Because it is nearly impossible for the fuel and oxidizer to be mixed intimately (being different states of matter), hybrid rockets tend to fail more benignly than liquids or solids. Like liquid rockets and unlike solid rockets they can be shut down easily and are simply throttle-able. The theoretical specific impulse performance of hybrids is generally higher than solids and roughly equivalent to hydrocarbon-based liquids. as high as 400s has been measured in a hybrid rocket using metalized fuels. Hybrid systems are slightly more complex than solids, but the significant hazards of manufacturing, shipping and handling solids offset the system simplicity advantages. 

OBJECTIVES

·         To study hybrid rocket

·         To study the components of hybrid rocket.

·         To study the advantages and disadvantages hybrid rocket.

·         To study the problems associated of hybrid rocket.

                     METHODOLOGY OF STUDY

ROCKET PRINCIPLES:

A rocket in its simplest form is a chamber enclosing a gas under pressure. A small opening at one end of the chamber allows the gas to escape, and in doing so provide a thrust that propel the rocket in the opposite direction. A good example of this is a balloon is compressed by the balloons rubber walls. The air pushes back so that the forces on each side are balanced. When the nozzle, with space rockets, the gas is produced by burning propellants that can be solid or liquid in form or a combination of the two

Newton’s first law’s for rockets

Newton’s first law can be stated as:

“An object at rest tends to stay at rest and an object in motion tends to stay in motion unless acted upon by an unbalanced forces.”

Newton’s second law can be stated as:

This law is essentially a statement of mathematical equation. The three parts of equation are mass(m), acceleration(a), and force(F). Using letters to symbolize each part, the equation can be written as follow:

F=ma

Newton’s third law can be stated as:

Every action has equal and opposite reaction.

HYBRID ROCKET:

The sketch below depicts a hybrid rocket. The hybrid normally uses a liquid oxidizer that burns with a solid fuel although reverse hybrids liquid hydrogen burning with solid oxygen.

           

FUELS

The fuel for a small hybrid rocket motor is generally a tube of combustible material (most frequently Polyethylene or Acrylic, although even cardboard is used in some cases!). The tube is known as the fuel grain. The hole down the centre of the tube is called the fuel port. For a larger hybrid rocket motor, multi-port grain geometries are common, where there will be several separate ports in the fuel grain, with oxidiser injected down each port.

REGRESSION RATE

·         In a hybrid rocket motor, liquid oxidiser is fed into the combustion chamber from the oxidiser tank, where it is ignited by an ignition source such as a pyrotechnic igniter. The fuel is then ignited and burned in the presence of the oxidiser, where it vapourises, and burns along the length of the fuel grain. The rate at which the fuel burns, is called the regression rate, and is measured in metres per second (m/s).

·         The combination of fuel burn rate and oxidiser flow rate is called the mass flux, and is measured in kilograms per metre squared seconds (kg / m² s). The oxidiser flow rate affects the rate of regression of the solid fuel, and enables the following equation for solid fuel regression rate to be derived:

·         r = aGⁿx^m

·         where;

r = fuel regression rate (m/s)
G = propellant mass flux (kg / m² s)
x = length along the fuel grain port (m)
a,n,m = regression rate constants

·         The regression rate then, is dependent on the mass flux and the length along the fuel grain port.

OXIDISER TO FUEL (O/F) RATIO

·         A hybrid motor differs fundamentally in terms of combustion behaviour compared with solid and liquid rockets, in that the Oxidiser to Fuel ratio (O/F), varies along the length of the hybrid fuel grain, i.e., it has an axial dependency.

·         In a liquid rocket, the injectors generally inject both the fuel and the oxidiser at one end of the combustion chamber thus there is no axial dependency.

·         In a solid rocket motor, there is no injector head, and every particle is bound of fuel and oxidiser, thus ensuring the O/F remains pretty much constant.

COMBUSTION CHAMBER

The combustion chamber in a hybrid rocket motor not only provides the location for propellant combustion, but also contains the whole fuel grain. The length of the combustion chamber is determined by the fuel grain configuration (e.g. a single port or multi-port fuel grain configuration). Also, the longer the combustion chamber, the more stable the combustion, since the propellant has more opportunity for even mixing.

FUEL GRAIN CONFIGURATION

From checking sources such as Spacecraft Propulsion Analysis and Design (Humble and Larson) and Rocket Propulsion Elements (Sutton), as well as following through with the calculations, a single cylindrical port geometry for a hybrid fuel grain, provides more volumetric efficiency for any high power or amateur rocket, than does a multi-port geometry.  The disadvantage of the single port configuration however, is that it generally requires long length to diameter ratios compared to a multi-port configuration. The multi-port configuration can be made quite short and compact, with length to diameter ratios of between 3-7.

In general however, for high power or amateur rocketry, the single cylindrical port geometry configuration is probably the best choice for most hybrid propulsion applications.

INJECTION SYSTEM

There are two methods of injection that can be used for injecting oxidiser into the combustion chamber of a hybrid rocket motor:

• Direct injection into the fuel grain port.
• Injection into a pre-combustion chamber.

For hybrid rocket motors on the high power and amateur rocketry level, where a single circular port geometry is most frequently used, direct injection of the oxidiser is the best approach, since there is no need to inject multiple oxidiser streams down multiple ports, and hence less requirement for a homogenised oxidiser stream from multiple injector nozzles.

Ingestion system:

PRE COMBUSTION CHAMBER

Injection into a pre-combustion chamber in a hybrid rocket motor is more useful for larger hybrid motors, or hybrid motors where a multi-port geometry is used for the fuel grain, since multiple injectors are more common, and even mixing of the oxidiser stream needs to be achieved before it is passed over the fuel grain.

POST COMBUSTION CHAMBER

Post combustion chambers are used in hybrid rocket motors to enable more complete burning of the combustion products before the combustion products are expelled through the nozzle throat on into the nozzle and the exhaust. Post combustion chambers are less widely used than pre combustion chambers since they can cause quite severe erosion of the combustion chamber in the region of the post combustion chamber,and the benefit of using the post combustion chamber can often be outweighed by the increased mass of the insulation required to prevent burn through of the post combustion chamber.

IGNITION

There are a variety of methods of igniting Hybrid motors. On larger hybrid rocket motors, a solid rocket motor pyro grain, or a complete small solid rocket motor attached at the injector end of the hybrid rocket motor combustion chamber are used to ignite the fuel and oxidiser.

On smaller hybrid rocket motors where a plastic fill line is used to fill the oxidiser tank, a variety of methods of ignition are used, pyrovalve/pyrograin with the fill line running through a hole in the pyrograin, PIC wrapped around the plastic fill line, and Gaseous Oxygen filling of the combustion chamber combined with High Current electrical ignition. Where a metal fill line is run into the oxidiser tank through the combustion chamber, Gaseous Oxygen high voltage spark ignition can also be effected.

ADDITIONAL CHALLENGES NOTED

Non-Technical Challenges

•         Lack of technological maturity

•         Hard to compete against established solid and liquid technologies

•         Established propulsion industry is fine with the status quo

•         Smaller group of rocket professionals relative to solid and liquid rockets

Technical Challenges

•         Low regression rates for classical hybrid fuels

–        Results in complicated fuel grain design

•         Low frequency instabilities

–        Instabilities are common to all chemical rockets

–        They need to be eliminated

–        Expensive and long process

•         Lack of benign, high performance, cost effective oxidizers (common to all chemical rockets)

EXPECTED CONTRIBUTION

·         To help in study of hybrid rocket.

·         To implement the hybrid rockets for future space mission purpose.

LIMITATIONS

·         Take off problems from normal roads.

·         The width of wings when the vehicle is on road mode.

·         Types of fuel used for thrust generation