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Old 04-17-2016, 02:25 PM
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Gus Gus is offline
7/21/61
 
Join Date: Sep 2005
Location: North of Detroit
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International competition events use electrical ignition for all rockets, but fuse still has a number of uses in those events.

Fuse ignition of upper stages in Scale and Scale Altitude if often used. The electrical ignition of the booster simultaneously ignites a fuse, precisely timed to ignite the upper stage. The U.S. team used this very successfully on their Bumper Wac models at the last several championships. This is easy to do, but extremely difficult to do well. Lots of important design considerations and tons of practice are required.

Fuses are also used, with varying degrees of success, to delay ignition of upper stages in altitude models which use direct motor staging (similar to using Estes booster motors). Euro motors have VERY tiny nozzles. A piece of fuse stuck in the nozzle of a sustainer engine can provide delayed ignition by completely occluding the nozzle. Instead, the booster motor ejects flame which gap stages to the short piece of fuse in the sustainer, short delay while the fuse burns and then ignites the sustainer motor. Lots of debate as to whether this really helps, also very difficult to get precise delays.

A third, and more common use of fuses in international competition is for dethermalizers (similar to free-flight glider events here in the U.S.). The FAI timed rocketry events all have maximum times. FAI events also require 3 rounds and you must fly 3 rounds with two rockets. This means you must get at least one back to have all 3 flights. The max in parachute duration for example, on A motors, is 5 minutes. In order to get 5 minutes you have to catch a thermal but it is very common to just have your model thermal away after the 5 minutes. So a dethermalizer of some sort that deflates your parachute at 5 minutes is very helpful. A number of teams use a piece of fuse connected to a parachute riser line which holds half the shroud lines. The fuse extends out through the side of the model (often through the lower edge of the nosecone). As soon as they get the launch command from the RSO they ignite the fuse, then push their electrical launch button. This is a simple and effective method but requires you have a parachute folding technique that is guaranteed to open even if the parachute has been packed a day or week before (you can't really do the precise packing with dethermalizer out on a windy launch field). The U.S. does not use this techinique because of the difficulty of practicing here in the U.S. (we're just all way too nervous about attaching long burning things to our parachutes).

So although I've never seen a booster ignited directly by fuse in an international event, I have seen the very effective use of fuses in a number of circumstances.
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