Basic Question: There doesn't seem to be a lot of propellant margin for payload.

Edit: I think I'm no longer confused, d. seems like the reasoning is not wildly wrong, and /OlympusMons provides a technical explanation that arrives at the same conclusion.

I noted that during IFT-4, both booster and ship had only a sliver of propellant left after completing their primary burns.

I'm not sure how much extra propellant is burned to loft a 100t payload but naively assume its a decent chunk.

I also didn't notice the commentators mention anything about propellant venting.

So Im confused/curious and a casual Google didn't turn up anything. Would anyone in the know be able to enlighten me?

Possibilities as I see them:

a. Less propellant is loaded than the maximum to allow for no payload. This seems unideal from a testing POV and I imagine would be actively communicated, so I am mostly discounting this option.

b. Venting is taken place, just not mentioned / I didn't notice.

c. Changes since the original starship design have eaten up all the margin, and the current vehicle is actually not capable of getting anything close to 100t to orbit. So the stretched versions of starship aren't just an upgrade but at this point are necessary for the system to be a viable launcher. This would also explain why they're listing the current payload capacity as N/A and aren't using a dummy payload.

d. (Edit, and apologies for thinking out loud) Most of the penalty of extra payload is the extra fuel required. Since they are already paying the penalty by loading to the hilt, maybe it does indeed only require a tiny extra silver of propellant to actually take a payload.

But while the difference between (only ship to orbit fuel + empty ship mass) and (ship to orbit fuel + payload to orbit fuel + ship empty mass + payload mass) is much larger than just the payload mass, the latter is still ~8% of the ships wet weight. So naively expect 8% of fuel left over.

But in that scenario, the ship is burning less fuel as it ascends, offsetting the fact it's lighter due to no payload. Assuming the offset is essentially total, the amount of extra fuel could naively be expected to be around the expected payload, or ~50-100t (as pointed out below, starship expected payload is currently stated at 50t).

50t is ~4% of total fuel. This would not be inconsistent with the amount shown by the livestream telemetry given its low resolution.

Does that reasoning hold? I might need to go learn basic rocket maths...