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Looking at the launches and animations for the Falcon 9, you can clearly see some rockets firing at the top/side of the stage to maneuver it clear of the second stage and orient it correctly for return & landing. None of the documentation I've looked at ever mention those engines. I would think, given SpaceX's general preference to remain consistent, that they are Draco engines.

My questions are: What kind of engines are these, how many are there, how much fuel are required for them and where are they located?

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  • $\begingroup$ I just found this page indicating they are Cold Gas Nitrogen RCS $\endgroup$
    – neelsg
    Mar 3, 2015 at 9:15
  • $\begingroup$ Neelsg, you can answer your own question. $\endgroup$
    – Deer Hunter
    Mar 3, 2015 at 11:54
  • $\begingroup$ @DeerHunter I know, but I don't think that is the whole answer $\endgroup$
    – neelsg
    Mar 3, 2015 at 11:57

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SpaceX controls the reentry of a Falcon 9 first stage with at least three different approaches.

  1. Main engine gimballing
  2. Cold Gas Nitrogen thrusters
  3. Grid Fins on the interstage

After the main engine of a first stage is complete, the cold gas thrusters rotate the stage into the direction of flight which is quite a feat considering its size. But it mostly out of the atmosphere so it is possible.

Three of the main engines re-ignite and decelerate the forward motion vector of the stages flight path. Depending on the mission profile, either enough to return to the launch site, or just enough to land on the ASDS barge, per-positioned downrange underneath it.

The cold gas thrusters then reorient the stage as needed so it is pointy end down (Engines pointing down) so that when it hits the thicker atmosphere it is most aerodynamic to survive the heating.

They fire a single engine (center engine) again to slow down the interface with the heavier atmosphere so that the stage can survive it. The cold gas thrusters are used for attitude control throughout this process as needed.

Then they start using the grid fins to fine tune the flight path to the designated landing location.

Finally in the last few seconds of flight the center engine ignites again and tries to hover slam the landing. They have too much thrust for the remaining weight, so they cannot hover, rather they need to time the landing so that they hit 0 altitude with 0 velocity. Thus the term hover slam, which they practiced with the Grasshopper vehicles at the McGregor, TX test site. (Also tested the grid fins on that vehicle.)

Whenever the main engines (Merlin 1D) are firing they can gimble and offer some level of control. But it is limited as was seen in the first ASDS landing attempt. They lost grid fins hydraulic fluid and came in off course and the main engine was desperately trying to correct on its own and failed.

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    $\begingroup$ On the first stage, also the nine engines in octagrid layout are all variable thrust (throttleable) and can be cut off independently. Not sure how many can be restarted tho, they only ever restart up to three of them. But all that ought to add to thrust vector control too. $\endgroup$
    – TildalWave
    Mar 3, 2015 at 13:12
  • $\begingroup$ @TildalWave, all engines are restartable multiple times (well, enough to land, anyway). $\endgroup$
    – marked-down
    Mar 3, 2015 at 19:48
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    $\begingroup$ @Antilogical While it may be true they are all restartable, it is only demonstrated that the middle, and 2 other engines are restartable. Not all 9 are used in landing. It is possible that differing amounts of TEA/TEB igniters are present on otherwise similar engines. Just as an example. It is possible that the center engine, which is lit at least 4 times in a landing mission has some other differences from the 8 surrounding it. $\endgroup$
    – geoffc
    Mar 3, 2015 at 19:59
  • $\begingroup$ @geoffc, sort of. Three engines are used for boostback+reentry, but in the event of an engine out event during these phases, Hans Koenigsmann has mentioned it would be possible to "switch" which three engines they use. The only requirements being they form a line and the central engine is not the one which flames out. F9 is totally redundant until the landing burn (which requires the central engine and central engine only). Whether the central engine carries more TEA-TEB I am not sure. $\endgroup$
    – marked-down
    Mar 3, 2015 at 20:02

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