Why are the spolette on top.

[Dalle]

Why do they put the spolette on top of the rocket instead of in the bottom whith the charge who send it up in the air?

 

[Zumber]

Where is rocket?

Are you taking about round or cylindrical shells?

Both method works fine.

If it is placed at top of shell it is ignited by quick match flame and if it is placed at bottom it is ignited from lift charge.

[cmjlab]

The image you show depicts normal cylinder shells with a lift charge built to be fired from a mortar, not rocket headers.

- Left Image: depicts black match extending from the mouth of the mortar to the spolette, then a Passfire from spolette to lift charge (in left image), which is how most cylinder shells are built.

- Right Image: Depicts black match extending from the mouth of the mortar, down side of the shell, and into the lift charge and spolette, accomplishing ignition of both lift and timing in one step.  Some small shells are built this way, I wouldn't if using a spolette.

Rocket Headers:  typically have a Passfire tube filled with Black Match(not a spolette) facing down into the top of the rocket motor, to take fire from the rocket once the delay finishes its burn and spits fire through the top of the spent rocket motor (either clay bulkhead with Passfire hole, or delay used as bulkhead passing fire to the header). 

There are people on here into rockets way more than I am, and they can likely explain why there would be a Passfire tube or spolette in the top of the rocket header, if there is such a thing (though I've not seen them that way).  

I have seen a few who use smaller tubes to create a bottom shot, and they are placed on the side facing up, and could look similar to a really thick or large spolette.