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I was thinking that placing RCS on the top of the rocket could help manouver it at low speed where the fins are useless. Than i read somewhere that cool gas propellant are not enough.
But we know that SpaceX is developing the SuperDraco engine. Do you believe that it would help?
There was discussion on using the Super Draco design to land the second stage, but SpaceX has mostly given up on second stage recovery for Falcon. (NOT for the BFR booster that will be the vehicle to Mars).
Super Dracos do not gimble, so using them for directional control would not help that much. Except perhaps differential throttling.
The major advantage to a Super Draco is the lower thrust it delivers. The problem with landing a Falcon 9 V1.1 stage is that the Merlin 1D can only throttle down to about 70% and that still generates more thrust (about 100Klbs of thrust) than the empty stage weighs.
This means they cannot hover nor slow down slowly. They have to come in hot, in what they call a hover-slam. This means decelerate down to zero m/s at zero height. Which looks really fast as they come in. This is why they spent so much practice time on it with the Grasshopper test vehicle.
The downside to a SuperDraco engine on the first stage is that it is more dead weight that is otherwise not needed if they could control it without the SuperDraco. On the Dragon capsule it is used on every mission. Either to abort, or to land.
As we saw in the last few launches, cold gas thrusters (the F9 first stage uses nitrogen for recovery maneuvers) seem to be sufficient to maneuver the stage before fins engage.
But yes, Superdracos could be used to help land the first stage but are not needed since the center engine can throttle down far enough to allow landing (and that's already with a much lowered thrust being a single engine versus nine). The second stage, however, has a single engine already, so would need to throttle down nearly an order magnitude more than the first stage center engine. In that case, Superdracos would make sense.
In fact, auxillary landing engines (perhaps SuperDracos) for the second stage are shown in this 2011 video from SpaceX:
Elon Musk mentioned they decided to hold off on second stage reuse for the time since a kerosene/oxygen vehicle doesn't quite have the performance margin to make it worthwhile for high-energy payloads to geosynchronous transfer orbit, but SpaceX recently won a contract from the Air Force to help develop their new high-performance methane/oxygen Raptor engine (eventually for their "BFR") for a Falcon 9 or Falcon Heavy upper stage. Maybe that's just the Air Force talking, but it could mean that they intend a higher performing upper stage, and it would be consistent with previous SpaceX statements if this stage were reusable like shown in the above video, and probably would also require auxiliary landing engines to have low enough thrust to allow reliable landing.
