Would it be possible to have a gas-powered drone that has some amount of time it can run on batteries? I'm thinking similar to the old diesel submarines that would go on battery when they wanted to be stealthy.
Yup, lightweight, high-powered gasoline engine, powering a generator, the output hooks into the regular battery and ESC (Electronic Speed Controller) loop. All the rest of the control is the same
The benefit is the energy density of petrol is ~2500Wh/kg, vs about 250Wh/kg for Li-Ion batteries. This is significantly offset by the weight of the ICE engine & generator, but it is still enough to get very good flight times. It is enough to do using selected off-the-shelf components, but to get really outstanding results, everything needs to be optimized for excellent power-weight ratios.
Two big ones in that space, going back-&-forth and beating each others' records are:
And yes, at least the units we built had significant batteries in the loop. The main purpose was to be able to land if the ICE engine/generator failed, but also could be used for a less-noisy mode ("quiet mode" is a bit much for most multirotors, as the rotors themselves are quite noisy).
The least sophisticated way to engineer a drone like this would be to use a consumer grade heavy lift electric drone with an onboard generator and fuel source to charge the batteries
