I think most people usually don't consider Tom Clancy to be a science fiction writer, but these two books (his best two by far, FWIW) both feature speculative technology central to the story. Red October features a magneto-hydrodynamic drive for a submarine; MHD drives work for real in the small scale but without great superconductors it doesn't scale well to work for a full scale military submarine. In Red Storm Rising there are F-19 stealth jets based off rumors of the F-117. They get a few things wrong (smooth continuous curves instead of flat facets, poor stealth from below, armed with anti-aircraft missiles, etc) but on the whole they were reasonably grounded speculation.

If you're not convinced, here is a longer argument for Red October (the film) being hard science fiction: https://qntm.org/october

For something different, think "navel warfare in space", the Lost Fleet series from Jack Campbell dealt with relativistic effects in space, with regard to combat, and the difficulties of predicting where/when people were.

It's basically 21st Century Naval Warfare in space. I quite enjoyed it. A lot of thought was put into the world.

The Empire of Man novels by Weber and Ringo.

Belisarius series by Eric Flint and David Drake.

The General series by S.M. Stirling and David Drake.