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Would it be possible for a aircraft to deliver a payload into space using a turbojet-rocket hybrid? Merits are that they don't have to carry tons of oxidizer like a rocket would have to. It still may need to carry some but only when the atmosphere gets so thin. But even there is a lot of weight saving which means huge savings in fuel cost. Can Projects like the Skylon Space Craft really work or are they just fantasy?

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Turborockets are hybrid systems that aim to use atmospheric oxygen as oxidizer instead of carrying it on board and thereby improve performance. While they do have a real chance of replacing the conventional rockets in use today, there are a number of engineering challenges to be overcome before they become viable.

  • These hybrid systems basically use a gas turbine inside the atmosphere (ramjets/scramjets have also been proposed) to produce thrust. The issue is that the jet engine becomes dead weight outside the atmosphere, so its weight has to be minimized. Skylon uses a slightly different method with a heat exchanger using helium, but the problem is the same.

  • The air intake has to operate over a wide range of speeds, from subsonic to hypersonic; the best option is to have a variable air intake, which adds weight and complexity.

  • The nozzle has the same problem. The area ratio of the nozzle has to vary significantly to cope with widely differing flow conditions.

  • If the vehicle has to take off under its own power, it has to have lifting surfaces, which are dead weight (eating into payload) once out of the atmosphere.

  • The integration of all these extra systems also causes a significant weight penalty.

The addition of all these systems increases weight more than the weight of the oxidizer they replace. This demands extremely high performance materials and very high efficiency systems for the vehicle to be commercially viable.

Jeffrey Bosboom
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aeroalias
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    Snap-off wings that parachute themselves back to the surface for recovery, just like the Space Shuttle's solid rocket boosters! Only half joking – FreeMan Dec 28 '15 at 17:26
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    And this added complexity is only usable for the first minute or two of flight and the last few minutes on re-entry. A lot of dead weight for the vast majority of the flight and weight is the prime commodity on launch vehicles. – casey Dec 28 '15 at 17:54
  • @casey The first couple of minutes are not as trivial as you might think. Keep in mind that the entire burn time for the Space Shuttle's SRBs was only about 2 minutes. The overall shuttle stack burned through half its mass in the first ~90 seconds. Also, once you're on orbit, "dead weight" is kind of irrelevant, as no additional energy is required to stay in orbit. It's just getting up to orbital velocity that is the expensive part (and, for that, mass is indeed critical.) – reirab Dec 30 '15 at 21:13
  • @FreeMan How about just use a snap-off airplane instead? :) No need to worry about the mass of the wings or the turbine engines after separation and the carrier aircraft just returns back to base in preparation for the next launch. Much less expensive recovery process than with the Shuttle's SRBs. Less dangerous, too, as long as you make very sure that the rocket doesn't ignite while still attached to the carrier aircraft. – reirab Dec 30 '15 at 21:16