Re: Re. motors
There are a number of things going on here, many of which have been discussed previously, but for the sake of completeness:
A processor on its own supply which is
- talking to the GPS unit
- talking to the comms transmitter
- talking to a logging MD card
- and of course triggering the pyro stuff
But what we really really want to avoid is the chance of the rocket going off accidentally, and most of all when someone is close to it... so we don't want to start the thing, for example, and then connect the wires with volts on them. To avoid this, the multi-stage arming requirement where the operator is looking to ensure there are no warning lights at any stage: no power to the relay *before* the relay is connected; no power *from* the relay before the wires are attached. For obvious reasons we don't want to attach and then just turn stuff on...
Equally, we don't want the risk of the thing coming down for whatever reason with a live rocket motor *and* live power. In such a condition - with the risk of mechanical shock or damage when it hits the ground - we can't rely on either the processor or the relay to be working properly; there's a risk of just touching the thing and it going off. So the idea of the fuse is a second-line defence to to ensure that there is no power in the igniter circuit as soon as practicable after we expect the launch to occur.
Managing that by using a normally-open relay held shut is complicated by the expected life of the batteries at very low temperatures.