solid propellant rockets -1

         The most important conventional rockets uses solid propellants(fuel in solid fom)  for their propulsion. As the density of solid is greater than that in any other form, we require quiet less volume of fuel tank as compared t o the liquid or gaseous fuelled tanks. There are many other advantages and of course disadvantages too, we will discuss them in this course.

  

Mechanism of burning  in solid propellant rockets.

Doublebase propellants:      

             the propellant surface when heated, degrades exothermally in the solid phase. This zone of decomposition within the solid is known as the foam zone . gases like NO₂, NO and aldehydes are liberated during the decomposition . these gases mix near the surface of the propellant and chemically react to liberate heat. The major reaction is between NO₂ and  aldehydes, where CO and CO₂ are formed. The zone of gas phase reaction is known as the fizz zone.

   

                          Temperature increases in this zone and the zone is highly luminescent. An increase from surface temperature of the propellant T_s to T₁ takes place. The fizz zone is followed by a dark zone in which chemical reactions between ( NO and CO ) and (NO and NH₂) takes place. There is no significant raise in temperature here. This zone is followed by a 2^nd luminous flame zone where in chemical reactions proceed further with heat liberation . Temperature rises in this zone from T₁ to the final value t_f.

                              The extent  of the 3 zones is very small( order of fraction of mm)  and depends very much on pressure. The thickness of the foam zone is much smaller than the fizz zone and dark zone. The intermediate dark zone is at a lower  temperature T₁ than the final temperature T_f. This causes the oveall temperature gradient in the gas phase near the propellant surface to be small and thereby limit the heat transfer from the flame at temperature T_f to the surface of the propellant .When the pressure exceeds about 10MPa, the primary fizz zone merges with the 2^nd luminous zone , giving rise to increased heating of the propellant from the hot gases.

Burn rate :

              The most important parameter for the successful  working and high efficiency of a rocket, is the burn rate of the propellant we used. Not only in the case of solid but  also in liquid and even for hybrid propellant , the controlled burn rate is most necessary. In most cases the acceleration of rocket increases with the burn rate but there also exists few cases where the rocket is decelerated due to heavy loss of the propellant in a short time . It is very important to analyse the effects of the burn rate and also the control measures  the maintain the velocity of the rocket.

               The burn rate of a solid propellant rocket is given by the following equation:

                                          r = a pⁿ

where      r is the burn rate

                  a is a constant  that takes values accordingly.

                P is pressure

this relation is known as Vielle`s law and also as Saint Robert`s law.