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Most fuel on board a launcher is used during the first couple of minutes. Most of the propellant is needed to carry other propellant. For example, Saturn V (kerosene) and Delta IV (hydrogen) use about a tenth of their launch mass during the first half of a minute. That's more than their payload delivered to orbit.

Why not hose (I mean pumping through a flexible tube) liquid oxygen or kerosene to the rocket during the first tens of seconds or so, in order to reduce its mass? While it still moves slowly at low altitude. Not having the hose with explosive fuel destroyed by the rocket engine exhaust might be an engineering challenge, but not as crazy as the often advocated sci fi space elevator tether idea. I could invoke the carbon nanotube dream as solution.

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  • $\begingroup$ What's the altitude at which you're envisioning the hose decoupling? $\endgroup$
    – Russell Borogove
    Feb 18, 2016 at 0:05
  • $\begingroup$ @RussellBorogove At the optimal altitude, of course ;-). A few kilometers at least, I suppose. A launcher should be able to unwind and carry an empty hose that high. But one with high pressure fuel inside of it, well, I do admit that this fantasy has a TRL comparable to that of a space elevator. This is brainstorming. $\endgroup$
    – LocalFluff
    Feb 18, 2016 at 0:55
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    $\begingroup$ Your pumps would have to accelerate the propellant to match the speed of the booster. And the booster would have to carry the weight of the hose. It seems unlikely to provide any benefit. $\endgroup$
    – Organic Marble
    Feb 18, 2016 at 2:03
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    $\begingroup$ Compute the pressure required to lift fuel to an altitude of several kilometers, and the strength of the hose required to contain that pressure. Then figure insulation for cyrogenic fuels... If I did the math right, kerosene at about 0.8 gm/cm^3 works out to 80 kg/cm^2, or about 1100 psi, which is getting close to scuba tank pressures. $\endgroup$
    – jamesqf
    Feb 18, 2016 at 6:38
  • $\begingroup$ There are many problems involved with filling tanks while the engines are running. During launches of the Space Shuttle and Ariane 5 there is a time (7 seconds for the Ariane) where the main engine is already running, but the boosters are not ignited, so the rockets stay put on the pad. Even during this time, they don't top off the fuel tanks. $\endgroup$
    – Rikki-Tikki-Tavi
    Feb 19, 2016 at 15:10

5 Answers 5

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There exist one way for this to work

As pointed out by others before, a hose is heavy to carry along. However, if the propellant station had the same altitude and velocity as the rocket, it may be pretty simple engineering. And the obvious way of propelling the tank is by the means of a rocket engine. This is known as "propellant cross feed" or "asparagus staging". Although not strictly a hose, the concept is substantially different from traditional strap on rocket boosters. The first actual use of this scheme is planned to for the Falcon Heavy.

Another concept based on cross-feed is a bimese, in which two close-to-identical rockets travel together, sharing propellant, until the empty one returns, and the other one continues with full tanks.

Why do ground support have to be on ground?

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  • $\begingroup$ This is totally changing the premise - the question asked about pumping it from the ground. $\endgroup$
    – Organic Marble
    Feb 19, 2016 at 3:34
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    $\begingroup$ @OrganicMarble Make the ground fly! $\endgroup$
    – SE - stop firing the good guys
    Feb 19, 2016 at 5:36
  • $\begingroup$ A related concept is the Black Horse (en.wikipedia.org/wiki/Black_Horse_(rocket)), a spaceplane that uses aerial refueling. $\endgroup$
    – Pitto
    Jul 27, 2020 at 22:56
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You'll lose more energy hauling that hose up than you could possibly save by pumping fuel in. A hose full of fuel is going to be heavy, not to mention you'll have quite a time keeping it out of the rocket exhaust.

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    $\begingroup$ The fuel could be carried by balloons at aircraft altitude, with hoses hanging down to the launcher. @Fred too. $\endgroup$
    – LocalFluff
    Feb 18, 2016 at 8:19
  • $\begingroup$ You're a person of vision @LocalFluff ;) $\endgroup$
    – GdD
    Feb 18, 2016 at 9:05
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    $\begingroup$ @GdD Ignorance is helpful for fantasy. $\endgroup$
    – LocalFluff
    Feb 18, 2016 at 10:11
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    $\begingroup$ Balloons would not help much because of the relative velocity problem. If the booster flies by the balloon at Mach 2, your pump will have to accelerate the fuel to Mach 2. $\endgroup$
    – Organic Marble
    Feb 18, 2016 at 14:22
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    $\begingroup$ @LocalFluff: I've thought some more about your alternate idea of having hoses dangle from above. You say in an earlier comment that the hose could be "a few kms at least". That situation would be similar to what happens in some very deep South African mines - the deepest being 3.9 km. The mines have to be supplied with water from the surface. To reduce the energy of the fluid in the pipes, due to gravity, the system has to be interrupted at regular intervals so the fluid pressure can return back to atmospheric pressure. Relieving the fuel pressure for a rocket would be a major problem $\endgroup$
    – Fred
    Feb 20, 2016 at 2:56
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The simple solution is to launch from much higher up. Either an air launched rocket or launch from a raised platform. (Both have been described, and one has been used.)

Or just begin the launch with a supergun.

Really, there are so many engineering problems associated with hoses... just put the fuel in detachable boosters. (If you think about it, putting the fuel in a strap on booster and making it support its own weight is equivalent to using your hoses.)

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  • $\begingroup$ What about an electric cable instead? The launcher's first stage would have access to all the power from huge power plants to heat its hydrogen propellant. No need to carry oxygen. A nuclear rocket with the nuclear power plant on the ground. Opening the floodgates of huge dams and let the water masses that fall through the turbines raise the rocket through the sky. $\endgroup$
    – LocalFluff
    Feb 18, 2016 at 10:15
  • $\begingroup$ Well in that case don't use cables - use electricity on two rails, i.e. a railgun a few hundred kilometres long to accelerate then loft the second stage. There are all manner of speculative technologies we could include... $\endgroup$
    – Andy
    Feb 18, 2016 at 10:39
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    $\begingroup$ Launching from higher won't give you extra speed, which is much more important than height $\endgroup$
    – Antzi
    Feb 18, 2016 at 11:06
  • $\begingroup$ Launching from higher won't give you higher speed - but it's still more practical than hoses! :) To be fair we're talking extremely far out technologies anyway. Reusable boosters are almost here... $\endgroup$
    – Andy
    Feb 18, 2016 at 11:33
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    $\begingroup$ @Andy: Yes, I was agreeing with you that almost anything is more practical than kilometers-high hoses. $\endgroup$
    – jamesqf
    Feb 19, 2016 at 18:50
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Even better than refilling a rocket through hoses as it rises is to put the rocket on a flying platform. It is quite possible to lift a rocket up to 50 miles or higher with the right platform.

This platform is of course, known as "the first stage".

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Additional problems I see are having a pump that can move the required amount of fuel at the rate required to the height required while overcoming shock losses as the hose unrolls and travels upwards.

Also, a trailing hose would be problematic with rocket stability and streamlining the rocket as it moves through the air.

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    $\begingroup$ The pump is by far my favorite answer here. The pumps on a rocket are ridiculously high performance, and so we propose to maintain that kind of flow rate plus a kilometer or more of head! $\endgroup$
    – Erin Anne
    Apr 19, 2018 at 5:38

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