Thank you all. I see some comments here about how to fly with a shield and when you can turn it off, but that wasn't really the question - the question is "what can I give up to take that shield?" It seems that the refinery is the obvious one - though it seems to me that means you ditch the mining laser as well, since you can't pick up fragments without the refinery. I wasn't able to, at least. Maybe I'm missing something. And the cargo rack isn't needed for "materials", so that potentially frees up two slots (r4efinery and cargo rack).
So I would be taking:
1) fuel scoop 2) discovery scanner 3) surface scanner 4) SRV 5) AFMU 6) shield.
There are seven slots on an Asp Explorer, so that's completely doable. However, what do those of you running other ships with 5 or 4 slots - such as a DBX - do it? What do you forgo on your trips?
You don't need the Refinery to pick the up materials that you actually might need while Exploring, nor do you need the Cargo Racks. When the pieces break off, you can see those specific materials in the top right information panel, and you can also see them the Contacts tab of your Navigation Panel. You might want to go mine some rocks before you leave, just so you can see what I'm talking about, though I don't think that would be necessary. Carrying a mining laser is extra insurance in case you get yourself stuck, and need Jumponium materials, but they aren't available on the planets (if you can even land on any of them). You might be able to mine some Germanium from asteroid belts/rings, and get yourself out of a sticky situation. It's not guaranteed (there may not be any belts/rings), but the weight is minimal, and you should take everything you might need. Mining laser, SRV, Shields, and AFMU, namely. You can drop the AFMU if your trip is not going to be a really long voyage, you aren't going to be using Neutron Boosts, and you aren't terribly accident-prone. In short, it's better to have those things and not need them, than to need them, and not have them.
Something else you should do (in my opinion) is get familiar with how to call for a rescue from the Fuel Rats, and also their rescue protocols. Even if you have your map filtered for only scoopable stars, you can still end up getting too many non-scoopables in a row. In the bubble, I have gotten 4 in a row from the route plotter, and in the ship I was flying at the time, it would have been a shameful moment if it had been 5. Everyone thinks that they will be vigilant, but it only takes one too many careless jumps to leave you waiting for the Rats.
Myself, I don't build incomplete ships, or ships that are essentially flying gas cans. I build ships to fulfill whatever their intended purpose is, and leave nothing to avoidable chance. My time is too valuable to have it wasted because I wanted another 2 light-years on my jump range. As such, if I were to take a long voyage, I would have all 4 of the components I mentioned above (2 AFMU's, if the ship had enough slots, because they can repair each other), with a Shield generator that falls somewhere in the middle of the slot sizes available. I also pack enough power for everything, which is not something you will hear often on this forum, but I prefer not flipping switches to turn on my Shields, and causing my Thrusters to go offline at a really bad time, like during a landing. To be honest, I don't even know if this can happen in the first place (haven't tested it), but by having enough power for everything, it isn't even a possibility in my world - removing avoidable chance, just my preference. The concession I might make on this would be having enough power for both AFMU's, because that just isn't necessary. Power enough for one to be on at a time would be sufficient.
I went on longer than I intended - good luck, and enjoy your voyage, Commander!
Riôt