[Gravityloss]

The problem with these advanced technologies is two fold

1. it requires a large amount of hydrogen - which is not heavy so it looks good on paper but is very cold and voluminous, meaning you need huge insulated tanks. They are expensive to build, have bad mass fraction and are aerodynamically problematic. Hydrogen is also expensive to handle in systems and infrastructure.

2. the dry mass of an air breathing engine sucks because they process gases (rockets process liquids with 1000x density), though SABRE is better than stuff like scramjets. This is especially bad for an SSTO like they are proposing since you carry the inlets and precoolers and all that all the way to orbit.

My bet for cheap spaceflight would be a two stage kerosene-oxygen rocket. A SABRE engine might make a great first stage but I doubt if you would want to carry it to orbit. I haven't performed much calculations though.

[brc]

That was my first thought - Hydrogen has low energy density, even in frozen form. That is the thing that holds Hydrogen cars back more than anything else - tiny range for massive (and costly, and complex) fuel tanks. The same would have to apply to both airplane and rocket designs.