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From what I understand (not much), you need good specific impulse to travel long distances with, say, flyboard.

I know that RP1 has good specific impulse. I wonder if he used it, and what type of oxidizer/propellant would be most efficient for this task, and feasible (very small engine, not that high temp) .

To be specific: The question is if the efficiency (in terms of travel distance per unit mass of fuel) could be improved by choosing another type of engine/fuel?

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    $\begingroup$ Can you be more specific on what your question is? I'm not sure I understand what you are asking. Also, you may be confused between jets and rockets. Flyboard is powered by a jet, which uses ordinary air from the atmosphere as the oxidizer. Rockets in space have to carry separate oxidizers, but jets do not. $\endgroup$ – Daniel K Aug 6 '19 at 1:03
  • $\begingroup$ Yes, it is reasonable . My main interest is rockets, so I didn't even think about it. $\endgroup$ – user2679290 Aug 6 '19 at 9:45
  • $\begingroup$ Using a rocket engine with another oxidizer is out of question? $\endgroup$ – user2679290 Aug 6 '19 at 9:46
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I'm not directly familiar with the engines Zapata uses on the Flyboard Air, but I'd hope they're using a kerosene derivative like JP-1 or commercial Jet-A. Alternatives (gasoline, diesel, various alcohols) have no better energy content per volume, and some (alcohols) considerably less.

There is no case (for low altitude atmospheric flight) in which a rocket can beat an air-breathing jet for specific impulse/specific fuel consumption -- because of the need to carry oxidizer, instead of just using the air for that purpose, the amount of fuel/oxidizer available is limited by tankage. This is why airliners, warplanes, and private business aircraft use jets or turbine driven propeller engines instead of rockets (not to mention the far higher durability of a turbine compared to a rocket engine).

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  • $\begingroup$ Your answer is very good. I guess that we can dive into various options involving battery and I guess that compared to these cases too, jet is considerably better. $\endgroup$ – user2679290 Aug 6 '19 at 13:04
  • $\begingroup$ Fossil fuels beat the energy density of the best batteries by a factor of, as I recall, five or six. The small efficiency increase of a fan or ducted fan vs. a jet is nothing by comparison. $\endgroup$ – Zeiss Ikon Aug 6 '19 at 13:35
  • $\begingroup$ @ZeissIkon By a factor of 20. This is how you get 20,000 km jets, but only 1,000 km is potentially achievable for an electric aircraft. $\endgroup$ – Therac Aug 6 '19 at 17:24
  • $\begingroup$ According to an old copy of their web site, it runs on Jet B. $\endgroup$ – Michael Hampton Aug 6 '19 at 19:26
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For rockets, specific impulse is constant regardless of speed. For jet engines, specific impulse is very high at zero speed and drops off as velocity increases. At relatively low speeds (say below Mach 1), a jet engine will beat the pants off a rocket in terms of specific impulse (i.e. better by a factor of 20 or more). See for example some comparisons here: https://en.wikibooks.org/wiki/Jet_Propulsion/Performance Since the flyboard is definitely a low speed craft, you would not improve it by using a rocket instead of a jet.

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Feasible, but then you have to carry rocket fuel and oxidizer (such as liquid Hydrogen and liquid Oxygen, both refridgerated and under pressure), vs just readily available fuel (Jet-A), and getting oxidizer (O2) from the air all around us.

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  • $\begingroup$ You seem to be conflating a turbojet with a rocket. Because the jet uses atmospheric air as its oxidizer, its apparent specific impulse is many times even hydrolox rockets. $\endgroup$ – Zeiss Ikon Aug 6 '19 at 12:37

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