There are several things that might (or might not work)

1. Drop re-usable liquid fuel engines and concentrate on reliability and thrust and forget about refurbishment. While other parts might be re-usable, rocket engines take such an incredible beating that it would be cheaper to start with new ones than to try to get used ones working again.

2. Air breathing boosters for at least part of the launch. The specific impulse of hydro-ox rockets is somewhere around 4,500 m/s. Air breathing engines have much higher specific impulses. There are several strategies for reducing rocket size by using air breathing boosters, including:

2.1 Manned carrier aircraft capable of at least 1,000 m/s;

2.2 Air breathing ramjet boosters.

3. More energetic fuel-oxidizer combinations, at least for storable fuels for long space missions. I do not know if anything will beat liquid hydrogen-oxygen as a propellant pair in practice. There are a few really exotic oxidizers such as OF2 but they are not very environmentally friendly. There is a small possibility of adding some fraction of liquid ozone to liquid oxygen, but there are big problems with that too, especially separation of the mixture into its components. While a mixture of 25% ozone and 75% oxygen is not explosive, if the stuff separates, big hazards await. Anything that increases the specific impulse even slightly will have a big payoff, if implementation problems do not make it impractical.

For deep space missions, I do not see anything on the horizon better than a fission reactor powering a plasma rocket engine. The problem is reliability for several years without any maintenance whatsoever. I think it may be possible to produce a hermetically sealed reactor, generator, & heat sink unit that will stand up to space conditions for a decade. But, so far, it has not been done.

The plasma rocket has been demonstrated, but it needs a large power source. I do not see radioisotope generators supplying enough power for deep space missions, but I could be wrong on this. For missions to Mars, photovoltaic power just might work.

I think that the next generation of space hardware will reduce costs slightly, if at all. I doubt if any evolutionary improvement will result in significant cost savings. Therefore, I expect to see some revolutionary improvement in the next decade if space travel is to become routine.

Also, I really do not see any need for manned exploration of space. Robot spacecraft bring back far more information per dollar spent. I know that this statement is heretical, but that is my honest opinion.

David F Mayer