Bipropellant rocket

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Schematic of a pumped bipropellant rocket
Schematic of a pumped bipropellant rocket

A bipropellant rocket engine is a rocket engine that uses two fluid propellants (very often liquid propellants) are stored in separate tanks that are injected into, and undergo a strong exothermic reaction, in a rocket's combustion chamber. In contrast, solid rockets have single solid propellant, and hybrid rockets use a solid propellant lining the combustion chamber that reacts with an injected fluid. Because bipropellant systems permit precise mixture control, they are often more efficient than solid or hybrid rockets, but are normally more complex and expensive, particularly when turbopumps are used to pump the propellants into the chamber to save weight.

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[edit] Properties of bipropellant rockets

Bipropellant rocket engines are extremely powerful rockets- they can provide the highest specific impulse of all current Earth launchable rocket engines whilst at the same time as providing thrust to weight ratios of 70-100+, and permitting extraordinarily lightweight tankage and vehicle structure.

The highest ISP bipropellant rocket engine in existence is the hydrogen/oxygen fuelled SSME which gives very high performance; but in terms of overall performance the dense-fuelled NK-33 is comparable due to better mass ratios; in spite of lower specific impulse.

[edit] Principle of operation

Bipropellant rockets are simple in concept...
Bipropellant rockets are simple in concept...

Bipropellant rockets have to introduce the propellants into the chamber at high pressure, mixing them well to give stable and thorough combustion and stop the chamber from melting.

As propellants need to leave the tanks at sufficiently high rate they are stored under pressure, normally as liquids for maximum density. Gaseous storage can be used but is rarely employed as the tanks are inevitably heavy. Liquid propellants are pressurised by a pressurant gas, either an inert one, often helium, or in some cases the vapourised propellant itself is used. Early experiments by Goddard of directly pressurising the fuel with oxidiser vapour led to frequent in-tank explosions, and this is no longer done; although sometimes a common tank is used with flexible membrane or piston to avoid mixing.

The propellants must be introduced into the combustion chamber at high pressure (typically 2 to 20 MPa (20–200 atm) and reasonably high flow (0.1-1000+ liters per second). This is achieved either via high pressure (heavy) tankage, or from lightweight, low pressure tankage through suitable pumps. The pumps used are typically turbopumps, often powered by tapping off 1-2% of the propellants or using a separate system, such as decomposed hydrogen peroxide and powering the pump via a gas turbine. The exhaust from the gas turbine is either dumped over the side, used to cool the nozzle, or placed into the combustion chamber.

Propellants are introduced to the combustion chamber through injectors. Injectors can be as simple as drilled holes with sharp edges which aim jets of liquid propellants to collide with the optimum mixture ratios. However, liquid fuels are not precisely flammable- the liquids must be first turned to gas before combustion can take place. This readily occurs within the engine, but takes longer, uses up volume in the chamber and can cause combustion instabilities. High performance rocket engines such as the Space Shuttle Main Engines take great pains to gasify the propellants before injection into the chamber. This gives more thorough, quicker and much more stable combustion; and permits the combustion chamber to be smaller and hence lighter.

The injectors' job is also to drop the pressure slightly from the propellant line feeds. This decouples the flow through the injectors from the natural variations in chamber pressure that occur during the combustion process. Failure to drop sufficient pressure in the injectors can cause oscillations in pressure in the chamber that can badly damage the engine and cause 'hard-starts' or even self disassembly of the engine during the ignition process.

The high temperature combustion products accelerate along the chamber from the injectors and then pass through the throat; and then expand out the nozzle, pressing on the inside of the nozzle, accelerating and generating an equal and opposite thrust on the rocket.

[edit] Cooling

The combustion process can generate temperatures as high as 3500 kelvin; above the melting point of almost all materials; two exceptions are graphite and tungsten. Combustion chambers that need to survive for any length of time usually do so by continuously cooling the solid walls.

The coolant methods include:

  1. uncooled (used for short runs mainly during testing)
  2. ablative walls (walls are lined with a material that is continuously vapourised and carried away).
  3. radiative cooling (the chamber becomes almost white hot and radiates the heat away)
  4. dump cooling (a propellant, usually hydrogen, is passed around the chamber and dumped)
  5. regenerative cooling (uses the propellant to cool the chamber via a cooling jacket before being injected)
  6. curtain cooling (propellant injection is arranged so the temperature of the gases is cooler at the walls)
  7. film cooling (surfaces are wetted with liquid propellant, which cools as it evaporates)

In all cases the cooling effect that prevents the wall from being destroyed is caused by a thin layer of insulating fluid (a boundary layer) that is in contact with the walls that is far cooler than the combustion temperature. Provided this boundary layer is intact the wall will not be damaged.

Disruption of the boundary layer may occur during cooling failures or combustion instabilities, and wall failure typically occurs soon after.

With regenerative cooling a second boundary layer is found in the coolant channels around the chamber. This boundary layer thickness needs to be as small as possible, since the boundary layer acts as an insulator between the wall and the coolant. This may be achieved by making the coolant velocity in the channels as high as possible.

[edit] Propellants

Main article: Liquid rocket propellants

Thousands of combinations of fuels and oxidizers have been tried over the years. Some of the more common and practical ones are:

Robert Goddard and his rocket
Robert Goddard and his rocket

One of the most efficient mixture, oxygen and hydrogen, suffers from the extremely low temperatures required for storing hydrogen and oxygen as liquids (around 20 K or −253 °C)) and low fuel density (70 kg/m3), necessitating large and heavy tanks. The use of lightweight foam to insulate the cryogenic tanks caused problems for the Space Shuttle Columbia's STS-107 mission, as a piece broke loose, damaged its wing and caused it to break up and be destroyed on reentry.

For storable ICBMs or interplanetary spacecraft, keeping the fuel cool seems to be an unsolvable problem. Because of this, mixtures of hydrazine and its derivatives in combination with nitrogen oxides are generally used for such rockets. Hydrazine has its own disadvantages, being a very caustic and volatile chemical. Consequently, hybrid rockets have recently been the vehicle of choice for low-budget private and academic developments in aerospace technology.

[edit] Small scale rocket engines

XCOR Aerospace, a California based company, is developing small scale rocket engines to power small airplanes for suborbital flights. They have tested various combination of propellants including nitrous oxide/propane, nitrous oxide/alcohol, LOX/alcohol, LOX/kerosene with success.

[edit] See also

[edit] External links

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