As a baseline im on an est 12 month trip in my Anaconda. I've had several mishaps with 5B afmu's and I've used 2400 munitions with my B grade ones. My powerplant are now down to ~96% with everything else still on 100%. I've travelled over 3000 systems in that time (1 month).
Since powerplant damage is the only exploration stopper at the moment lets use that as a guideline to the time to turn back home.
At that rate lets say I'm using 500 munitions per 1% damage meaning I'd need around 50,000 to keep everything repaired until my powerplant went boom. Considering my AFMU's stock 7,300 each I've got enough to drop my powerplant down to ~70% before my AFMU's go empty. That's also keeping everything else repaired, even the things I could let break. If you be economical only repairing what you need you can probably half that number.
That's a crazy amount of accidents though, probably over 10 already... I've not been paying too much attention sometimes (too many crashes during the christmas 24 hour livestream).
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I hope that made any kind of sense.
For the Asp it's all a bit less cost because modules should be smaller and have less health (probably), being economical and letting things like power distributor take damage then you can probably get away with 200 munitions per % powerplant damage, maybe less with a 3A as discussed above. As above from my pre-horizons experience the 2x class 3 AFMU's will be more than enough for a 2-3 month trip or less exploring. For a single AFMU 5A should be good.
To be fair if you are clued in and never play tired or drunk then you can avoid crashing altogether and not need an AFMU ever except for the occasional binary star system that cooks you a bit, a heatsink used correctly will avoid even that! There's some people that have been out for years without proper AFMU support because they simply don't crash.
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