What advances in materials science/energy density?
A materials science advance would not help much, particularly since advanced materials (at least at first) can be quite expensive.
In terms of energy density, it doesn't really make sense- the only propulsion technology that I can think of that compares favourably to chemical is NTR, and while NTR might revolutionise the physics constraints spacecraft have, it will act pretty badly on the economic constraints. That's no longer a rocket engine, it's now a nuclear reactor and a rocket engine!
Let me put this straight; I hate all the NIMBYers who scream "EARTH-HATE" when the first syllable of the world "nuclear" is spoken. I'm a vehement supporter of nuclear propulsion in the face of NIMBYism, but the problem with nuclear propulsion is that all the stuff that you need to make its vehement support justifiable will tend to make it uneconomic.
We can go on about "cool materials", but it kinda gets like the whole Evil Population Growth thing (not nearly as bad, but still bad). The point is, we can't rely on something that doesn't exist, that we don't know would exist or how it would exist. It becomes magic too easily. We can say, "if X, Y, and Z were developed", then they could maybe revolutionise stuff in manners A, B, and C.
The major problems I see with spaceflight, and these are really the two omnipresent problems:
1. The severe cost-intensity of spaceflight as a whole.
2. The inadequacy of surface-LEO lift.
Now the latter is a problem, because you've got to have a dV of ~9 km/s to get to LEO. The funny thing is, if you're using, I dunno, some sort of ion drive, maybe even a nuclear engine, a nuclear-electric engine... you could probably do 9 km/s quite easily out in space. The problem is, when you're launching from Earth, you've got to fight gravity, you've got to fight air resistance, you've got to accelerate relatively fast, or else you end up fighting gravity for too long or whatever and your whole launch strategy becomes horribly inefficient.
In short: your options are to either use chemical propulsion, or nuclear propulsion. Chemical propulsion isn't that effective rocket-equation wise, relatively speaking. Nuclear propulsion is better physics wise, but comes with a phylum of problems. Chemical is cheaper and far less offensive, but still very expensive to work with.
If you want to reuse your launch infrastructure, you have to return it to Earth, and that's difficult. If you use chemical propulsion, you either have to make a very lightweight, fragile structure, or break up the launch vehicle into several parts which have to be recovered seperately. If you want to use nuclear propulsion you have to contend with the radioactive rocket engines whether you throw them away or not. With both you face the complexity issues that can actually make it less economically sensible to recover and refurbish infrastructure than it is to build it and then throw it away.
You face the economic challenge of infrastructure built to pour money into a corporate operation, to help senators keep their job. No space-flight here, no. The real business is politics.
Spaceflight is incredibly intensive, and incredibly dangerous. Just look at the infrastructure it takes to launch a bullet out of a rifle at less than 1000 m/s. Spacecraft have to be travelling at 7.5 km/s to reach Earth orbit. That's a lot of kinetic energy going into that ship. That's a lot of ship. There's a reason the shuttle stack is so gigantic; that orbiter has to be sped up to roughly ten times the velocity that the rifle bullet does. And it's not just a little bullet weighing a few grams; it's a spacecraft weighing over 100 tons.
Those engines put out gigawatts of power; many times the output of a municipal power station, out of a vehicle a fraction of the size and a fraction of the weight (it may only be for a few minutes, but those are a damn scary few minutes).
On reentry the surface of the craft can get up to 1000 C or more in places. If things change on the order of seconds, the crew can die instantly. On-orbit the vehicle is contending with, for example, temperature changes- the orbital environment is not nearly as strenuous as that of launch or reentry, but is still hostile. With a mind-boggling amount of components needed for the vehicle to keep the crew alive and keep the vehicle operational.
Space travel is so intensive, that the failure rates that we have start to look amazingly good. And we have them because a huge amount of effort and patience and intelligent- and money- are put into making sure the vehicles are safe, making sure the components are within fine tolerances, making sure that things are tested and re-tested to ensure that they do not fail.
Now, you can reduce the costs, if you slash all that effort. But the problem is that vehicles will start to fail at a prodigious rate; LOC/V events will occur everywhere you look. And that is naturally unacceptable, especially if we are trying to popularise spaceflight to any degree.
The only way out of that huge cost intensity is to slowly practice and build up confidence and build up ability. And that will be difficult. It would be, often literally, an uphill battle.
So we've got a universe outside our little microcosm that we couldn't be bothered about since it's so useless, and on top of that spaceflight is exceedingly difficult. So naturally you can see why we're not making any progress.
Manned spaceflight is for all practical purposes useless, yes. But if the environment for it was just a little bit better, likely far more people would be interested. If one part of the Antarctica analogy is in favour of spaceflight, it is that there is far more interest in space, than there is in Antarctica. You don't see many people saying they want to colonise Antarctica or study it or whatever, even though it's far easier to get to.
That spaceflight "meme", the spaceflight idea, that's still alive, even if it is largely ignored, largely forgotten, and often ridiculed. There's still interest somewhere. We are proof. Other people on the internet are proof. Space programs- the mostly useless space programs of the world- they're proof too, kinda.
If it was just a little easier, then you'd see people going into space, paying for a ride (and not at ridiculous prices ranging from tens to hundreds of millions of dollars). If it were just a little easier, it'd be far easier to justify politics-wise, and indeed would need less justification due to its lower 'drain' on things.
If it was just a little easier. But it isn't.
And partially at least, you can blame it on the transistor; the transistor enabled capable communications satellites to be launched on relatively small boosters. They'd be relatively small satellites, too. And all the control they would need would be from the ground.
Now imagine that spaceflight 'took off' a bit earlier, or that the development of the transistor was delayed or its impact somehow came later or whatever, or even a combination of all of those things. And imagine that there was a telecommunications demand.
Now you have a demand for fairly large satellites, because they have to contain heavy, primitive stuff like vacuum tubes. And not only do these satellites require larger launchers, they require a crew of some sort as well- since they are so primitive and large and complex that they need people to watch over them and supervise their operation. So you get an incentive to develop cheaper manned spaceflight, because there's actually a need for manned spaceflight. When the transistor comes around, it's a boon, not a hindrance, as while it may crash the communication-station market, it finalises the delivery of the "easy(ier) spaceflight" that we ask for in our era. Now all the other stuff, like exploratory missions, are far easier to justify. There's an interest in space, far more people have travelled in space per amount of time, and things are generally better off for spaceflight.
Of course, one could argue that the transistor and spaceflight were pretty much destined to be realised in roughly the same time period, or that the latter was not possible with the former, or whatever. But that's not the point. The point is that if there was, and potentially if there is a need for manned spaceflight, it becomes far more justifiable, and it becomes easier, which in turn makes it more justifiable still.
But the problem is, we can't come up for a reason for manned spaceflight to be a necessity. Short of uninventing the transistor. :facepalm: