In considering rocket engines I think the research and development has a serious gap. With the invention of the liquid fueled rocket by Robert Goddard we have had around 90 years to research rocketry. There was some small engine development the the years of the world wars. Then the next major development in rocker engine was during the space race of the 60's and 70's. There was a large leap in the size of rockets being built and the fuel for our mordern space machines was switched to hydrogen. Hydrogen has a higher specific impulse than other fuels but come across more logistical prolbems such as keeping it cold to keep in liquid form. when dealing with rockets the weight of the engine and vehicle is the largest factor. That has to cautiously weighed against the specific impulse of the fuels.
The space race caused a gap in the gradual development in the size of rocket engines. The demand of the space race calling to put a man on the moon demanded larger rockets to be developed. With that call there was a lack of development of small to mid size rockets. I am of the opinion that small to medium sized rockets will be the ones to transfer the crews to orbit and the lager rockets will be for heavy lifting and cargo transportation. In the future the people will be moving in and out of space quite a bit where the cargo will be shot up in large loads at regular intervals. I think that the various new space companies are going fill specific roles. I will comment later on the positions of the various positions of the new companies and their roles in the future of space.
Comments are encouraged on my opinions, but in leaving comments please use references for information as to encourage more learning and understanding of this subject.