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As far as I'm currently aware, the Falcon 9 block 5 does not have the capability to retract it's landing legs once they're deployed, however:

If F9 landing legs had the ability to retract themselves (as Elon has mentioned is a goal for full reusability), would the Falcon 9 be able to take off from any reasonably solid surface such as a parking lot or highway?

More specifically:

  • Would the aerodynamics of unfolded legs interrupt the early stages of flight (until the legs are folded in)?
  • Would the exhaust from the engines damage the underside of the first stage or the landing legs in such a way that it would be unable to take off or land later?
  • Would the surface loose integrity and topple the F9 before it could take off?
    • Or can the Merlins start up fast enough to avoid this problem?
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    $\begingroup$ A bigger problem would be that the legs won’t support a fully fueled F9, not even “just” the first stage. They are designed for the dry mass and some small fraction of fuel which remains after landing. $\endgroup$
    – jkavalik
    Commented Apr 26, 2019 at 13:28

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There are a few issues with launching such a rocket without a launch pad.

  1. Probably the biggest is that the rockets do not reach full thrust immediately. This means that there is a regime where the thrust is high enough to move the rocket but not high enough to stabilize it in flight. The launch clamps hold the rocket down until full thrust is reached. Without them, the rocket would topple even if point 2 below wasn't an issue.

  2. The Falcon 9 needs to be placed vertically somehow. They currently use a vertical erector (it comes in horizontal, then is lifted to the vertical and attached to the launch tower). If you wanted to side-step the issue, some rockets do launch on their erector platforms rather than being attached to a different launch structure. If you feel that the platform doesn't count as a "launch pad" per se, and were willing to design one for the Falcon 9 that wouldn't be destroyed by its engines, then "yes" is the answer to your question. But that's a bunch of speculation about a system that does not exist.

  3. The launch pad isn't just a place to hold the rocket until it's ready to go. It supplies power, fueling infrastructure, computer and communications equipment, the list goes on. To give an example of why this is important, one of the big steps in launching a Falcon 9 is transitioning the spacecraft to internal power, which is all batteries during launch. Everything would have to be on internal power to launch without a pad, so you'd need lots of batteries.

So...ultimately the answer is a hard no, because that's not how the Falcon 9 was designed.

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    $\begingroup$ flame trench? deluge system? $\endgroup$
    – user20636
    Commented Apr 26, 2019 at 13:46
  • $\begingroup$ Well, if the only problem was the rocket exhaust I'm sure that can be solved. It's already solved for the launch pad, which is not destroyed every flight $\endgroup$
    – Michael Stachowsky
    Commented Apr 26, 2019 at 13:47
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    $\begingroup$ ad 1, the RCS handles stabilization during landing at nearly-hover over the drone ship quite well. It would require more RCS fuel until thrust is sufficient to have the engines and aerodynamics to stabilize the rocket (and still suffice for landing), but it doesn't seem like a show-stopper. Also, full thrust before launch seems to be an American prerequisite - Russian launchers typically lift off the moment TWR exceeds 1. $\endgroup$
    – SF.
    Commented Apr 26, 2019 at 14:47
  • $\begingroup$ @SF: it would also require RCS thrusters with the thrust to stabilize the rocket fully fueled. I don't know if the Falcon 9 has such thrusters. Ultimately, every point I mentioned above is an engineering problem, not a physical impossibility. But the question was whether a Falcon 9 (as in, currently in use) could launch this way, not whether a rocket could be designed to do so $\endgroup$
    – Michael Stachowsky
    Commented Apr 26, 2019 at 15:02
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    $\begingroup$ The deluge system doesn't protect only the pad - it protects the vehicle itself from the noise of it's ignition, reflected back from the ground. That's an additional issue for the list. You also need to load propellants up to the minutes prior to flight, and detach cleanly when finished - the pad helps support those umbilicals as well. $\endgroup$
    – Saiboogu
    Commented Apr 26, 2019 at 18:17
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Another major problem is the rocket exhaust. Here's what the rocket looks like standing on its legs

Landed Falcon 9

The legs are only providing 3-4 feet of clearance. Here's a Falcon 9 launch. It takes about 2 seconds after ignition for the rocket to start climbing. And during those 2 seconds you have a LOT of thrust energy being produced by a powerful rocket

Falcon 9 Flame

That energy would immediately encounter the ground on a ground launch, so it would rebound back up to the rocket, as well as spread out. That could damage the engines, the engine bells, etc. You could also damage the legs. If you sheared off a leg before the rocket reached neutral buoyancy it would likely topple over. If it didn't explode as a result, now you have a horizontal rocket sliding across the ground...

A flame trench is designed to allow the flames to move downward, away from the rocket. The energy is then dissipated by distance and water.

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