Problem: the original poster got their units messed up, and the surface of Mars has pressure equivalent to about 35000 meters, not feet. That's about 115 thousand feet. That's nearly 3 times the "altitude without payload" figure listed there.

1.5% in Neptune and 2.3% in Uranus, not enough to matter for buoyant craft.

Didn't forget them, they're just not very good for ballooning. With their hydrogen-helium atmospheres, the only way to get a reasonable amount of buoyant lift is by heating your lift gas, and the low density of those gases means even that gives little lift at a given pressure. Better than Mars, but worse than Earth.

Lift (along with drag) is proportional to the *square* of airspeed, all else being equal. But the rotor blades on a Mars helicopter would have airspeeds in the transonic to supersonic region, with very different airflow, so simply applying a scaling law like that isn't very accurate.

Titan's atmosphere has about 1.5 times the surface pressure of Earth, and the atmosphere is even denser due to the cryogenic temperatures (about 20 K lower, less than the difference between your freezer and room temperature, and it'd start raining nitrogen). The only place in the solar system better for balloons is Venus, they're barely possible on Mars.

I *was* using the density of CO2...which at 273 K and 600 Pa is about 0.012 kg/m^3. It might reach 0.020 kg/m^3 when it's coldest, but it sounds high for a typical surface density.

A helium balloon on Mars would only carry about 10 g of per cubic meter. A hot air balloon would carry far less. (given 273 K ambient temperature and 373 K balloon temperature...rather difficult to maintain with the available power...around 3 g/m^3)

Less drag for the same blade velocity, but less lift in the same proportion, and what matters is lift to drag ratio, which isn't as good at high speeds (and a Martian helicopter would likely require a supersonic rotor).

And fundamentally, a hovering Mars drone is constantly accelerating by 3.7 m/s^2 by accelerating the nearby atmosphere downward. This is energy intensive, entirely apart from the drag losses. The thinner the surrounding atmosphere, the lower the mass flow rate and higher the velocity you have to accelerate it to, and higher the energy requirements...if a 1 kg drone accelerates 100 g of atmosphere (about 10 m^3) per second to 37 m/s to maintain a hover, it's doing about 70 watts of work, without even looking at losses. For reference, Curiosity gets about 125 W of continuous electrical power from its RTGs.

Weather balloons are quite large and delicate. You need something that can be deployed from a rover without any assistance, and which can survive being tethered to that rover while fully inflated...recall that weather balloons are barely inflated at launch because they expand during ascent, when they actually reach those high altitudes they are far larger than they appear on the ground. We're talking tens of meters across, a hundred cubic meters per kg of payload and balloon, made out of a fragile plastic film...and the goal is to make a rover *more* mobile, so it has to be tethered to something trundling along the surface, or self-propelled effectively enough to stay with it.

There would also be a risk of fouling the rover when the thing inevitably ruptures, something there'd be a particular risk of during inflation. A free-flying balloon probe would be possible, though very difficult and limited, a balloon drone to assist a ground rover is much less practical.

A blimp would suffer the same problem, but even worse. A cubic meter of unpressurized helium will only lift about 10 grams at the surface, and there's nothing you can do about that. The helicopter could at least spin its blades faster, though there's the problem of a power source...

Apart from being at the wrong end of a rather long vehicle, for most of the landing process the turbopumps aren't running. The engines do use RP-1 pressurized by the fuel turbopump for things like the gimbaling hydraulics, but the fins have to work even while the engines are shut down, and so have a separate system.

They only use fuel as hydraulic fluid for the engines, the fins use a different system that has to operate when the engines (and the turbopumps pressurizing the fuel) are shut down. If you're out of fuel, you don't need to gimbal the engines. And it was rather clearly not out of fuel, considering the big plume of fire coming out of the bottom of the rocket.

The fluid requirements would depend on how much the fins were actually being moved around during descent. That's something difficult to estimate without ever having actually flown a stage back to the surface under control of the grid fins.

It's not just the pumps and piping, they also save having to carry a power source for those pumps, which all adds up to a mass equivalent to quite a lot of fluid. Are you going to stick a big battery pack and electrically powered pump on the rocket? Or maybe use something driven by toxic hydrazine monopropellant? Or ditch the pump entirely and make the fluid reservoir and one of the existing pressurized helium tanks slightly bigger?

An open system with larger reserves of fluid is also less susceptible to leaks.

Environmental assessment for their landing sites at LC13 at the Cape:
http://www.patrick.af.mil/shar...

Return to launch site has been their goal all along. It's only in the last few months that they started talking about the seagoing landing platform approach, and then only for those situations where there wasn't enough propellant left to return, which were previously expected to require more expensive launches that expended cores instead of recovering them (the Falcon Heavy center core and geosynchronous launches, mainly).