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Rockets are heavy because they need to transport the fuel as well as the oxidizer agent. For example, the hydrogen is the fuel and the oxygen is the oxidizer. The problem is it makes the rocket heavy.

Why not use an aerodynamic rocket-plane to go very fast through the atmosphere where there is oxygen and the steer up through space?

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  • $\begingroup$ Because you need to add a piece of equipment to do what, at a lesser cost, a rocket do automatically and swiftly enough $\endgroup$ Commented Jun 30, 2018 at 17:35
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    $\begingroup$ Hey Mmeyer. It is a cool topic but not really world building. Check this out: en.wikipedia.org/wiki/North_American_X-15. - they did exactly what you said for these little rocket planes, carrying them up with a B52 then dropping them off at altitude. There are schemes to lift rockets to the edge of the atmosphere with balloons - "rockoons". en.wikipedia.org/wiki/Rockoon. If you want to make this about world building then visit the help page, scheme up how you can make this question fit the criteria for the site, then edit it up. $\endgroup$
    – Willk
    Commented Jun 30, 2018 at 17:45
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    $\begingroup$ Humm... Off the top of my head, there's Spaceship One en.wikipedia.org/wiki/SpaceShipOne Spaceship Two en.wikipedia.org/wiki/SpaceShipTwo Orbital Science's Pegasus en.wikipedia.org/wiki/Pegasus_(rocket) and probably others. $\endgroup$
    – jamesqf
    Commented Jun 30, 2018 at 18:34
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    $\begingroup$ Play KSP for half an hour and you will see for yourself. $\endgroup$ Commented Jun 30, 2018 at 18:47
  • $\begingroup$ What you are asking about is called a scramjet. This is the abbreviation for Supersonic Combustion ramjet. One proposal for this technology is discussed in @b.Lorenz's answer (see below). More information can be found here: en.wikipedia.org/wiki/Scramjet $\endgroup$
    – a4android
    Commented Jul 1, 2018 at 1:30

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This was considered many times, eg.: https://en.wikipedia.org/wiki/SABRE_(rocket_engine)

The main problem was always that the craft needs a very specialized design to attain hypersonic speeds at an atmospheric density still considerable enough to help as reaction mass. So there are wings and control surfaces, reinforced fuselage... much more weight and complexity when compared to a tubular rocket.

And even the maximal speeds practical in atmosphere (say 5000 km/h) are small compared to the velocity needed to orbit Earth (about 28000 km/h)

So unless extremely carefully designed and using advanced technology, the spaceplane could easily loose more than it gains... But it is an amazing possibility, that may become reality on a day.

I have voted to close your question as I feel it would belong more to Space Exploration Stack Exchange

EDIT: The complete hard science elaboration of the idea is beyond me as the design space is enormous, but I can present some real word data to illustrate the hardships I mentioned:

First compare Space Shuttle Orbiter (or Dream Chaser or any other vehicle designed for vertical rocket launch and plane-like reentry and landing) to SR-71 (or X-15 or any vehicle designed for sustained atmospheric hyper or supersonic flight)

You will notice that the slower planes are lot more "pointy" and "aerodynamic" than the much faster space vehicles. The reason for this is that the definition of "aerodynamically favorable" changes with flight regime. Basically at hypersonic speeds you want pointy shapes and carefully placed and shaped intakes so that shock waves are reflected the right way. While at reentry speeds Blunt Body Theory tells us that blunt shapes perform better because they can deflect most of the hot air/plasma generated away from the body of the craft.

Since you would probably want to reuse your nifty spaceplane, it has to reenter after it made orbit, but it is extremely hard to design a fuselage adequate for both environments.

The other problem is the mass-ratio advantage of conventional rockets. The max takeoff weight of the X-15 is 15,420 kg while is empty weight is 6,620 kg. Even if the difference would be wholly fuel and oxidizer, its mass ratio is only 2.46. A contemporary tubular rocket, the Atlas missle has an empty mass of 3,050 kg + 2,347 kg = 5,397 kg and a gross mass of 119,000 kg- giving a mass ratio of 22.

Since the X-15-s insides are mainly taken up by fuel, it is a credible assumption that the venerable rocketplane approaches the maximum mass ratio such a vehicle can have. So even if a spaceplane would reach space fully fuelled and at the top speed of X-15: 2020 m/s, and we would very generously assume an Isp of 450 s, its remaining deltaV after leaving the atmosphere would be no more than 4100 m/s. Not enough to make orbit even when taking Earth's rotation into account. And this was calculated without any payload and with the assumption that it can reach X-15-s max speed using no fuel and only atmospheric gas as propellant. (for example with beamed energy propulsion)

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    $\begingroup$ While your answer is useful and a good one, you should be made aware that it is regarded as bad form to both answer a question and vote to close it. $\endgroup$
    – a4android
    Commented Jul 1, 2018 at 1:33

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