Falcon 9 rocket consumes approximately 30 tons of fuel for the first 15 seconds of flight while it's still less than 350 meters in altitude.

Is it possible to build a giant tower near the launchpad, so the rocket can be refueled during the first seconds of flight? If so, what would be the main technical challenge of that idea?

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    $\begingroup$ Whether or not it is possible, it would also be possible to build the launch pad at a location that is already at 350 meters altitude.. $\endgroup$ – Andrew Thompson Apr 22 '16 at 1:56
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    $\begingroup$ The point of that idea is to have fully fuelled rocket while it already has some kinetic energy, not to launching it from the top of the 350 meters tower. $\endgroup$ – user3715778 Apr 22 '16 at 2:05
  • $\begingroup$ If the flow is "guaranteed", then it could be just one of the propellants. It would be a heck of a lot easier to pump the RP-1 (kerosene) than it would be to keep 2 tons of LOX per second from boiling while passing really really fast through hundreds of meters of hose. $\endgroup$ – uhoh Apr 22 '16 at 7:16
  • $\begingroup$ May be it will be easier to lift additional tank directly attached to the side of the rocket on some kind of crane or elevator and separate it as soon as it drains out. $\endgroup$ – user3715778 Apr 22 '16 at 7:40

The main technical challenges with this idea:

  1. You've got a giant hose (2 tons/s capacity) hanging off one side of the rocket, making the weight distribution very unsymmetrical. You could relieve the weight by suspending the hose from a winch, but then you'd have to synchronize that exactly with the movement of the rocket.

  2. You'll need to keep that hose out of the exhaust, so you need to reel it in/out during liftoff. That's not easy to do while the hose is full of pressurized liquids, one of which is cryogenic.

  3. Clean separation gets difficult. You have no time to properly drain the hose, so you have to eject the (heavy) hose from the rocket while making sure it doesn't bang into the rocket after separation.

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  • $\begingroup$ Roughy speaking, how big/heavy would the hose actually be? Compared to a mass of 1.2 million pounds with a vectorable thrust of 1.5 million pounds, is it even a 1% effect, quantitatively speaking? $\endgroup$ – uhoh Apr 22 '16 at 7:11
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    $\begingroup$ @uhoh the issue is more puncturing the thin rocket body than disturbing its orientation. $\endgroup$ – Antzi Apr 22 '16 at 8:02
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    $\begingroup$ 1) One way to do this easily is to use a smaller rocket to lift the hose as it clears the pad. Then you could feed the hose from a small tank on the smaller rocket. Oh wait...did we just invent cross feed staging? $\endgroup$ – Aron Apr 22 '16 at 8:08
  • $\begingroup$ @uhoh The mass of the hose is the least of your worries. Its the impulse/momentum that you should be worried about. As Antzi says, it should easily punch straight through your rocket. $\endgroup$ – Aron Apr 22 '16 at 8:11
  • $\begingroup$ @Aron Thanks! I like your answer. I'm mostly worried by the use of words like like "large" and "small" without accompanying numbers or at least a shared understanding (e.g. "Law of Large Numbers"). I worked out the transverse force for a 12 inch ID hose in this comment. The image of implementing such a hose is worrying, the transverse momentum transfer not so much. $\endgroup$ – uhoh Apr 22 '16 at 9:25

Your main challenge is Newton's 3rd Law.

You are delivering 2ton/s of fuel to the side of the rocket.

Depending on the velocity of flow (which is inversely proportional to the size of the hose, and proportional to the ratio of density of the exhaust and propellant). You will be thrusting the rocket side ways.

Assuming that your hose is the same diameter as your rocket nozzle. Then the velocity of flow will be a percentage of the exhaust velocity.

This the thrust exerted sideways will be that same percentage of the exhaust velocity...

The result is the rocket will go sideways. Not good.

Assuming a smaller hose will result in worse outcomes (fyi the rocket nozzle is often a large proportion of the diameter of the rocket)...

This is the same reason that KSP style asparagus launch systems aren't used IRL.

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    $\begingroup$ 2T/S should be offset easily by gimbaling the engine $\endgroup$ – Antzi Apr 22 '16 at 8:03
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    $\begingroup$ It can be solved by using not just one, but two or more smaller hoses from the opposite sides of the rocket. $\endgroup$ – user3715778 Apr 22 '16 at 8:07
  • $\begingroup$ @Aron here are some numbers. From here you can see that the density of the RP-1 (kerosene fuel) and LOX (liquid oxygen oxidizer) are roughly 1.0 and 1.2 g/cm^3, varying a bit with temperature. Let's just call it 1.0. That's 1000 kg/m^3. At 2 tons/second that's 2 cubic meters/sec, and with a (large) 30cm ID tube that's 28 meters/sec, or a 56 kN force. The thrust is about 6800 kN, so the thrust would have to be vectored such that 1% was sideways - if the hose were sideways. Luckily it does not mean a 1% drop in vertical thrust! $\endgroup$ – uhoh Apr 22 '16 at 9:12

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