Rocket efficiency increases with higher pressures,
this requires stronger casings. This means that it would be better to
small casing and load more fuel into the rocket as it is used.
The concept is to make one small engine and load individual grains into it while it fly's. I know this has been tried before, however I plan on combining several different concepts. Each grain would burn for 1-3 seconds and then a small motor will breach load a new grain. The thrust of the engine would then cause the fuel grains to move down the tube and small flaps would prevent them from moving back up the tube as the shuttle moves back.
Next I want to explore high pressure engines since the chamber is so small a high pressure can be handled without making it very heavy.
The only problem is "detonation". I plan on making an automatically adjusting aerospace engine to control the pressure as the fuel increases it's burn rate.
Click for a larger image.
The fuel pellet would have a hole in the center
and would burn on both the outside and the inside.
A small amount of heat sensitive starter material would be added to the end of the pellet.
Ablative coatings would prevent the shell from overheating.
Pros: higher thrust.
Cons: More complex, changes balance point.
The mechanical version (to the right) works by allowing some of the high pressure gasses to push a piston, which reduces the required spring pressure. Then when a pressure change occurs the small hole prevents the piston from changing pressure and the valve opens or closes faster.
March 18 2006 update