Propellants

[surya kiran]

Hello all,

@amardeep mishra @The Deterrent @Skull and Bones @Nilgiri

Any idea what purpose this serves in solid propulsion motors? We just placed a tender for a $hit load of this stuff.

1. Dioctyl Adipate

Also we placed an order for HTPB. This is better known as the fuel in the Shourya and Agni. Production increasing?

 

[Skull and Bones]

HTPB can be compressed, so large amount of propellant can be confined in a pressurized chamber.

 

[surya kiran]

HTPB can be compressed, so large amount of propellant can be confined in a pressurized chamber.

OK. What about the first one? Any idea where it can be used?

 

[Skull and Bones]

OK. What about the first one? Any idea where it can be used?

Maybe that's the oxidizing agent. I'm not a chemist.

 

[surya kiran]

Maybe that's the oxidizing agent. I'm not a chemist.

It is supposedly a plasticizer. On googling I found this. Maybe, somebody else can put better light to why one needs HTPB and the first one. Since, it says here that HTPB is used as a plasticizer/secondary fuel.

Energetic material pyrotechnic compositions, especially solid rocket propellants and smokeless powders for guns, often employ plasticizers to improve physical properties of the propellant binder or of the overall propellant, to provide a secondary fuel, and ideally, to improve specific energy yield (e.g. specific impulse, energy yield per gram of propellant, or similar indices) of the propellant. An energetic plasticizer improves the physical properties of an energetic material while also increasing its specific energy yield. Energetic plasticizers are usually preferred to non-energetic plasticizers, especially for solid rocket propellants. Energetic plasticizers reduce the required mass of propellant, enabling a rocket vehicle to carry more payload or reach higher velocities than would otherwise be the case. However, safety or cost considerations may demand that non-energetic plasticizers be used, even in rocket propellants. The solid rocket propellant used to fuel the Space Shuttle solid rocket booster employs HTPB, a synthetic rubber, as a non-energetic plasticizer/binder/secondary fuel.

 

[amardeep mishra]

1. Dioctyl Adipate

Also we placed an order for HTPB.

Hi @surya kiran and @The Deterrent
I am not into propulsion,although i have taken rudimentary course in propulsion and i think DOA is a plasticizer.Now plasticizer is basically any substance that improves the physical properties of a binder/propellant combination. you see,basically AP(ammonium perchlorate or NH4ClO4) burns very rapidly as it has 4 oxygen- it is however amorphous. The problem is to cast it into any meaningful shape from missile perspective.Now the challenge is how to do that?-Thats where your binder comes into picture-HTPB(basically a long chained rubber wherein double bond between carbon are cleaved and it forms a huge chain like structure- just like any other rubber).Binder essentially provides a shape - that can be firmly bonded to the missile casing! Now to improve the structural properties of your binder,various substances are added.Now you'd naturally ask,why do we need to improve the structural properties of binder?Well the answer to the question lies in the fact that we want our missile to work flawlessly(when fired) during the estimated shelf life(shelf life is directly contingent upon the structural properties of your binder).
Missiles typically undergo various kinds of loading during the estimated life inside their canisters or otherwise. Lets say the binder just gave in to these bendings(some due to gravity) and developed a "minor" crack somewhere along the surface.Now what?
Just look at this equation- m'(generated)=r'*S*rho---------------------------------- 1
Where m'= rate of generation of gases(dm/dt)
r'= regression rate(rate at which surface is burning)- usually another substance known as "stabilizers" are added to the binder to have some sort of control over r'- ethyl citrolite
S= surface area exposed to burning
rho= density of propellant.
From this above equation we can clearly see that the generation of gasses is directly proportional to the surface area exposed to burning.An open crack(developed due to various stresses acting on the binder) provides "extra" area for burning!What it implies is,a slight increase in surface area leads to a corresponding increase in m'.
Similarly the expression for m'(out)= (P/sqrt(gamma*R*T))*sqrt((2/gamma+1)^((gamma-1)/(gamma+1)))--------2
where gamma,R are Cp/Cv and gas constant respectively and T is the temperature,P your chamber pressure.
or m'(out)=P*Cd*At--------------------------3
where P=chamber pressure,At=area of throat
Engineers normally equate eqn 1 and 3 to find the operating point.But this lil crack just messes things up and and "extra" gas generated increases the chamber pressure exponentially. If this exceeds the mehanical tolerances of your missile casing then ,missile will simply explode mid air!
I hope now you understand the importance of "plasticizers" and "stabilizers" in solid fueled missiles?

 

[amardeep mishra]

I forgot to mention the dependence of regression rate on chamber pressure-
r'=A*(P^n)
Where A is pre exponential factor and P your chamber pressure.
For stability of operation we'd ideally want our n<1 . Because it wont blow out of proportion and explode.
n>1 results in exponential increase in r' with slight increase in P
@surya kiran