I wonder why nobody ever proposed a space launch system like Starship. Methane as a fuel and all the theory behind are well known, so why?
Is it solely because of IT progress that made returning the stages possible?
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I wonder why nobody ever proposed a space launch system like Starship.
What exactly do you mean by "like Starship"? Systems like Starship have been proposed before, although differing in the details, going back to Von Braun's mid-1950s Mars expedition concepts. Starship is ambitious in several ways, but it's more evolutionary than revolutionary.
Starship is fundamentally a large, two-stage-to-orbit launcher with full reusability of both stages.
It's about 60-70% larger by liftoff mass than the largest successfully flown orbital launcher (the Saturn V), but much, much larger rockets have been seriously proposed (e.g. Sea Dragon).
Two-stage-to-orbit with full reusability of both stages was seriously considered for the US space shuttle program (as described in The Space Shuttle Decision), but the budget didn't allow for development of a fully reusable booster. The fully reusable shuttle concepts relied on winged, horizontal landing rather than vertical propulsive landing, but the pros and cons of the two approaches are endlessly debatable. Propulsive vertical landing has been demonstrated and proven at smaller scales (Surveyor, Apollo LM, several Mars landers, DC-X, Falcon 9).
Large methane rocket engines and full-flow staged-combustion engines haven't been flown before, but both concepts have had development work done previously.
Fundamentally, it's because of economics. There simply wasn't any demand for a large rocket between today and the space race.
Let's analyze what (I think) makes the Starship concept special:
Now, let's go over why nobody's seriously suggested something like Starship before (although there are exceptions, which I'll get to later).
Size: A bigger rocket is more efficient, primarily due to the square-cube law reducing non-propellant mass in the rocket the bigger it is and certain elements, such as avionics, which do not scale significantly in mass when the rocket gets larger. Large rockets have been proposed and built before (Saturn V, N1, SLS) however the rockets have always been built to suit demand. The Saturn V and the N1 were built for the space race and the SLS is being built because NASA wants to send heavier payloads to the Moon. In the past, large rocket proposals have mostly faltered against the question "But why do we need a big rocket when a smaller one will do just fine?" Only recently, with burgeoning interest in space exploration and Elon Musk's vision, has manned, beyond-orbit, spaceflight become attractive again to a wide audience, which enables the building of bigger rockets.
Reusability: Arguably, Starship is not new in this regard because it's aiming to achieved the dream that the Shuttle Program was designed for but never achieved: The spacecraft lands, the astronauts get out and high-five the next crew, which boards, and then they take off again after quickly refueling. Originally, the Space Shuttle was supposed to be just that, a shuttle but for reasons that are out of the scope of this answer, it never happened. Starship is attempting something similar, with Elon's goal of multiple reflights in a day being very ambitious. Again though, incorporating reusability in a rocket system really only makes sense when there's enough demand that the financial investment it requires makes sense compared to disposable rockets.
Budget: One of the iconic hallmarks of the Starship program is that it's being essentially welded together in a field while most other spacecraft manufacturers work in clean rooms while wearing in bunny suits. This is, in my opinion, an advantage that commercial enterprise has over government development: since SpaceX doesn't need to justify themselves to politicians and ultimately taxpayers, they can get away with a lot more. If a NASA rocket explodes, people will complain about their tax dollars being "wasted" but if SpaceX's prototype blows up... who cares? SpaceX only needs to appease their limited and hand-picked investors, which is much easier than justifying tax dollars. This institutional pressure to get it right on the first try leads to a cautious approach so that if something goes wrong, it's nobody's fault. Why haven't we seen this before? It's because successful commercial launch companies (that aren't defense contractors) are very new.
Mars compatibility: This is again, in my opinion, due to the traditionally conservative nature of space exploration. Typically, things are done in steps. For example, first, we send an orbiter, then we send a lander, and finally a rover to make sure everything will work. Building Starship with ISRU in mind is jumping over several steps and, again, not something that a government-funded program would be able to justify to a taxpayer easily.
Now, to the exceptions and what's been proposed/done before:
As other answers imply, the reason is likely not technical but economical. May be this question could attract some interesting answers at this sister site. I think it is a good example of an entrepreneur venturing into a field that has been always controlled by governments and exercising opportunities to "fail fast" and the availability of capital, both unencumbered by politics. In a sense, this is agile development at a cosmic scale.
I have been thinking about this and sought this site to try to find an answer. I don't think it was really technical because both SpaceX and Blue Origin have achieved similar results in the same rough time period. I don't think it was economic because the resources for SpaceX and Blue Origin are smaller than what NASA could conceivably put in such an effort. I don't think it was a market issue either, because the first time SpaceX successfully landed their Falcon 9 rocket, it was obvious this would usher in a new era of cost reduction for space flight in the future.
I think it was due to billionaires.
By being billionaires, Musk and Bezos could focus on accomplishing the task at hand. They were also new at rocketry, with no organizational and cultural baggage to hold them back. NASA is hamstrung to accomplish something like this, because no one person is given the authority to execute a singular vision like this, despite the obvious benefits. Any vision would be watered down by committees and sniping power blocs that would sap resources. Also, every new POTUS thinks its fine and dandy to scramble NASA's missions every 4 or 8 years.
I think the big lesson to consider here is why couldn't an organization like NASA do this? They really should have. And they probably could have done 10+ years earlier, at least from a technology standpoint. I have nothing against billionaires, but if gov't organizations simply can't do some things that wealthy individuals can do, its a sad state of affairs. FWIW, I don't think a wealthy person in 1960 could have flown people to the moon and brought them back, so NASA used to be able to do these things, but not so much anymore.
The most important aspect of Starship is that it is (supposed to be) reusable. For most of rocket history it was almost impossible*.
Because of lack of good enough computers. You would need to put a pilot in it and the first few landings would end in RUD. The Space Shuttle got around this by landing as a plane.
Only after someone demonstrated it's possible to land an unmanned rocket (some time before SpaceX) did the idea of the reusable rocket move from a dream to something that can be actually done.
* There are other ways to achieve at least partial reusability that could be done sooner (parachute for engine and aerial capture), but there was no market for it (if the government is paying for a few rockets a year regardless of price, you as the supplier don't want cheaper rockets).