Seems like a lot of extra weight (coil, magnet, relay, battery to keep coil energized for several minutes) compared to a powder charge, igniter and smaller-ish battery.

FWIW, the old school rocketeers would tell you to use what you know works, as better the devil you know than the one you don't. But that's why I don't work in the rocket factories no more, too, it is awful boring for a young engineer when there is a strong bias against innovation.

Sorta odd question.

First... how are You staging between the booster and upper-stage motors?

'Conventional practice' with these small-scale rockets is to have the booster propellant burn-thru the end of the booster motor, transferring burning/pressurized propellant to the upper-stage motor nozzle which usually has exposed propellant grains that readily ignite. A very low strength joint between staged-motors [cellophane tape or a friction joint], then usually keeps the stack together just-until the upper-stage motor ignites... and the propellant ignition-pressure forces stage separation.

Are you designing staging electronics/ignition... so that on booster burn-out there is an electrical signal to the upper-stage motor which ignites a 'squib' which positively ignites that motor? IF so, then battery/triggers/latches/pyro/electromagnetic/electronic mechanisms already make a part of a 'system in-place'.

or...

Are You using some other mechanisms for aligning/joining stages... then transferring energy from the booster to the upper-stage for ignition and unlatching stages??? this could be done with a G-switch or other simple mechanism. This method would be more common for aircraft ejection systems that have pyrotechnic gasses channeled thru tubing to initiator-actuators for rapid/high-energy/irreversible systems sequencing/actuation? Example...

Pilot pulls the ejection handle. Instant pyrotechnic charge then pulses to the canopy/hatch latches and then reaches the jettison actuator and is simultaneously routed to the seat-restraint tightening system and to initialize [wake-up] seat propulsion and stabilization controls and 'other time-delay' pyrotechnics. As soon as delay for canopy/hatch jettison occurs, then these delayed pyro actuators 'fire' to free the seat from the aircraft hold-downs and ignite the rocket-motor to initiate seat movement up the guide-rails... etc...etc...

NOTE. I think the Pershing II Missile had small mating flanges between stages... with locating-pins/holes and a single V-band-strap holding stages together. A pyro charge broke the single joint on the V-band strap, that then came apart instantly with upper-stage motor ignition... separation was clean/fast... and who cared-about the extra debris!

And FYI...just for giggles... because I’m on a roll....

MIL-HDBK-143 Glossary of Guided Missile Terminology
MIL-HDBK-762 Design of Aerodynamically Stabilized Free Rockets
MIL-M-8555 Missiles, Guided, Design and Construction, General Specification For
MIL-STD-1901 Munition Rocket and Missile Motor Ignition System Design, Safety Criteria For
Rocket Propulsion Elements, 9th ED ISBN: 978-1-118-75365-1

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
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I've seen a number of rocket stages held together with the V-band strap. Advantage is that if you want more redundancy in the release system, you can add another explosive bolt. Doesn't matter how many ex-bolts you add, it still lets go.
Too complex for the size of rocket system used by the OP, though.

Slight improvement on the magnet idea: the lower stage has all the gadgetry, the upper only has the magnet(s). The lower stage has the coil, but drive it with a capacitor that was charged before launch, not a battery. The coil is around an iron core, which will be attracted to the upper permanent magnet. When the capacitor fires, the current puts a reverse polarity into the iron, and presto. Charge the capacitor from a high voltage source. Take apart a microwave oven? Discard the capacitor afterward. Inductance will rule the rise-time of the transient current, which itself is also proportional to the rise time (differential equation). Field strength dependent on the saturation limit of the iron (this can be calculated without much difficulty; initial guess value 2T). Better to test this thoroughly on the ground to figure out if it actually works.

No one believes the theory except the one who developed it. Everyone believes the experiment except the one who ran it.
STF