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The first stage of SpaceX's Nov 18 Starship launch was the Super Heavy. According to the Wikipedia page, it packs 3,400 tones of fuel. In the launch video, it shows the rocket reaches 27 km of altitude at 2600 km/hr at t+01:44. The engines start turning off at 02:40. Assuming a constant rate of propellant consumption, the first stage spends about two thirds of its 3,400 ton propellant to get out of 1960s' SR-71's flight envelop. With similar analysis, it got out of A380's flight envelop at 6 km altitude and 950 km/hr at t+00:50, after spending about 31% of it's propellant.

Turbofan engines have a specific impulse 6-20 times more than rocket engines. The exponential relation in the rocket equation means an engine with higher specific impulse would greatly benefit in cost, weight and payload. In addition to having better specific impulse, airplanes need much less thrust than rockets for the same payload because the engine doesn't have to directly lift all the weight. Jet engines are just as complex as rocket engines, if not less, but arguably more mature as a technology.

In this scenario, a space launch system that uses a specially designed airplane as the first stage and rocket engines for later stages may save a lot in fuel and weight while not adding much complexity. The first stage will be reusable by design. For private space companies like SpaceX, that have drastically cut costs for launch and operations, and designed all of the rocket system on their own, this approach looks worth considering. This is different from projects like SABRE. What I mean is a totally airplane design, horizontally thrusting jet engine and wings providing the lift, much like the Pegasus project but with a specially designed first stage instead of a commercial plane.

What is the largest barrier to using launch system that has an airplane as the first stage and rockets for later stages?

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    $\begingroup$ there's a whole air-launch tag on the site to research. This question is going to be a duplicate of one of those $\endgroup$
    – Erin Anne
    Dec 3, 2023 at 23:24
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    $\begingroup$ The first stage airplane - could possibly be practical for small satellites. But to design an airplane that could lift 3-5 thousand tons, you would spend hundreds of billions of dollars practically for no use. $\endgroup$ Dec 3, 2023 at 23:28
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    $\begingroup$ @pakieng The accepted answer on that question mentions general points that aren't specific to the SABRE concept, as does my answer. A jet booster is feasible, as Dani Eder's proposal makes clear; it's just that it's only worthwhile if you're going to go with a reusable booster, and there have been relatively few serious attempts at large reusable first stages. $\endgroup$ Dec 4, 2023 at 0:30
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    $\begingroup$ "much like the Pegasus project but with a specially designed first stage instead of a commercial plane." Stratolaunch uses an airplane specifically designed to launch rockets. Does this meet your criteria? $\endgroup$ Dec 4, 2023 at 0:30
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    $\begingroup$ @pakieng I do not know how you can so badly misread that answer to conclude it "probably misses the point of using the airplane as an alternative just for the first stage." The various answers to that question thoroughly cover what would be the answer to your question, which is "yes (and it's been done), but [the answers to that question]." $\endgroup$
    – Erin Anne
    Dec 4, 2023 at 2:26

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