Understanding Solid Fuel Rockets: Tail or Head, What's Best?

[Alkhimey]

Can someone explain me how solid fuel rockets works?
Where should I start the reaction, at the tail or at the head? What fuel is the best?

 

[Astronuc]

The burn would start at the tail of course. If the burn started at the head, the pressure would blow the remaining solid propellant out, or it would simply blow the rocket apart.

Solid rocket fuel implies the fuel is solid form. The propellant contains an intimate mixture of fuel and oxidizer, and perhaps some amount of inert material.

There are two designs depending on size, hollow core and solid core. Solid core would be small rockets, and burns from bottom to top. This designs results in high heat load on the shell containing the propellant, hence it is limited in application.

The hollow core solid rocket burns from the inside to outside with the gases blasting down the core. This is the design used in solid rocket booster of the shuttle.

http://www.centennialofflight.gov/essay/SPACEFLIGHT/solids/SP13.htm

The propellant mixture in each SRB motor consists of an ammonium perchlorate (oxidizer, 69.6 percent by weight), aluminum (fuel, 16 percent), iron oxide (a catalyst, 0.4 percent), a polymer (a binder that holds the mixture together, 12.04 percent), and an epoxy curing agent (1.96 percent).

http://www.space-shuttle.com/srb.htm

WARNING! Do not make this at home or without supervision. Working with perchlorates is extremely dangerous if one is inexperienced, and likely such activity requires a license.

 

[Alkhimey]

Thank you for your reply.

The burn would start at the tail of course.

But how is enough pressure generated to make the rocket fly? In my opinion the gasses leave the rocket tank the moment they are created, so there is no pressure concentration.

 

[Danger]

The principle of rocket propulsion doesn't rely upon pressure as you seem to think of it. It's the Newtonian reaction force to the escaping gas that moves the motor forward.

 

[cefarix]

Yes, the gases that are ejected have mass. Since every force has an equal and opposite reaction, the rocket is pushed forward.

 

[cefarix]

In terms of pressure, you can think of it like this: Pressure is applied by the gases to all the sides of the rocket. However since pressure on one side is balanced by pressure on another side, this results in no net force. Pressure is also applied to the forward of the rocket, but this is not balanced by any pressure being applied to the back of the rocket (there's nothing to apply it to...its just a hole). The imbalance in pressure causes the rocket to go forward.

 

[Alkhimey]

Yes I understand.
But there is olso the gravity force you need beat. The gasses must get out of the rocket with force greater then mg.

I made a drawing to illustrate it.

(btw what is the letter for pressure force)

pressure=(-pressure) (action reaction)
-pressure is the reaction of the force of the pressure.
As you can see in order to make the rocket fly the pressure force must be grater than the the gravity force.

 

[Shawnzyoo]

here is a good link for low to high powered rocketry
http://www.nakka-rocketry.net/

 

[Astronuc]

Yes the propelleant turns from solid to gas, and the gas travels at high velocity out of the nozzle. The gas atoms are initially at rest in the solid (more or less), and then the molecular bonds are disrupted and the chemical energy goes into heating up the gas (several thousand K). The gas then leaves the rocket chamber at high velocities on the order of several thousand feet per second (i.e. supersonic flow).

Force = Pressure x Area, and also F = dp/dt, where p is momentum = mv.

See also - http://en.wikipedia.org/wiki/Rocket_fuel

http://exploration.grc.nasa.gov/education/rocket/rktth1.html

To accelerate, the force generated by the rocket motor must exceed Mg, where M is the mass of the rocket, including propellant (which is decreasing with burn time), and g is acceleration of gravity. Also, as the rocket speed increases in the atmosphere, the air resistance also increases until the rocket travels upward to where the air density is greatly decreased.