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I would like to find out why there is a straight hole down the middle in all solid rocket engine motors.
I thought it only makes sense in hybrid engines where pure oxygen needs to be blown down the tube.
And why isn't there a hole down the middle in model rocket engines?
A bore in the solid propellant grain increases exposed surface area and allows for a higher burn rate to increase thrust. There might be several grain geometries used, to meet launch vehicle's ascent profile needs through grain regression and with it control flow rate as the solid propellant core burns. From Wikipedia on Solid-fuel rocket - Grain geometry:
Solid rocket fuel deflagrates from the surface of exposed propellant in the combustion chamber. In this fashion, the geometry of the propellant inside the rocket motor plays an important role in the overall motor performance. As the surface of the propellant burns, the shape evolves (a subject of study in internal ballistics), most often changing the propellant surface area exposed to the combustion gases. The mass flow rate (kg/s) [and, therefore, pressure] of combustion gases generated is a function of the instantaneous surface area $A_s$, (m2), and linear burn rate $b_r$ (m/s):
$$\dot{m} = \rho \cdot A_s \cdot b_r$$
Some of the grain geometries and their corresponding thrust curves might look like below examples:
Common solid propellant core cross-sections of grain geometries, including circular, finocyl and non-circular bores.
Other profiles in use are a C-slot (wedge cut out at the side of the core), Moon burner (off-center circular bore), and so on. Grain geometry can also use three-dimensional cross-sections, usually achieved by stacking two-dimensional cross-section bored core segments one atop the other. E.g. Ariane 5 SRBs (EAP P238 and P241) use such stacked segments to achieve a three-dimensional grain profile, but there are other launch vehicles that use that, too.
It depends on the particular engine.
Thrust from a solid rocket is approximately proportional to the burning surface area of the fuel (also called the grain). A long solid rocket motor with a channel along its length is burning more surface area than an "end-burning" motor, so produces more thrust. Typically solid rocket boosters are used to provide very high levels of thrust at liftoff, so the long channel in the grain is needed.
If the channel is a simple circular bore, the area increases as fuel burns away, making thrust increase over time. This generally isn't what you want, because mass is decreasing as the fuel is spent, so the rocket accelerates too fast. For this reason, other grain profiles are used as described in TildalWave's answer to control the thrust-versus-time curve.
Model rockets usually fly with a much higher thrust-to-weight ratio than full-sized orbital rockets - higher than 5:1 TWRs are common for model rockets, while large rockets are often below 1.5:1 at ignition. End-burning grain, therefore, is more than sufficient for them.
And why isn't there a hole down the middle in model rocket engines?
Model rocket engines are available in many "sizes"--varying thrust and duration. One configuration has the hole down the middle. They do make a lot of thrust for a short time. These were designed to be used for booster stages.
Fwoosh instead of boom.
Others noted that the model engines may or may not have a bore, and that bores may or may not be round. To that I'll point out that some of the bores aren't actually cylindrical, either. Various tapers are used, too.
It's to maximize thrust, certainly, but that's not just burn rate.
You have to toss mass away from (as in separate and remove, creating a reactive force) or smack mass into the mass you want to move. Gases wandering about don't help, excepting as they increase the efficiency of the parts that are. (or happen to hit the mass appropriately)
Thus, the other biggie is shaping the flume. With a naked burning cylinder end, you do get thrust, but the lateral expansion is almost utterly wasted. With burning from the center out (or plugged with appropriate nozzle) that lateral expansion increases the pressure of the main body of the flume and, thus, the thrust. (Any nozzle used needs to be fitted to this purpose correctly or it will get popped right off.)
Check out "shaped charge" for further edification. See also why guns have a chamber. These are all linked.
(Ed.: My friend pointed out that insulation against the burn is quite important, as well, to prevent the destruction of the shaft and also reduce the benefit of the nozzle.)
