A lot of discussion here on what is science fiction and what is fantasy. To my mind, science fiction requires that science be essential to the plot. The original The Outer Limits was (mostly) solid sci-fi. Star Trek was a mix. Taking any old story and changing the setting by putting it on a space ship or replacing swords with blasters or light sabers does not make it science fiction. Star Wars is fantasy. You could stick it alongside Conan the Barbarian and replace starships and with boats and tauntauns with camels and hardly touch the story. The Force is just magic. It's a standard fantasy quest in a different setting, perhaps what the original poster meant by science fantasy.
The problem with that is that if you actually look at the history of the genre, you will find a rather large number of fundamental classics of the field which contradict that assessment, from Wells'
The Time Machine (or even, really,
The War of the Worlds, when you get down to it), to Kuttner & Moore's "The Children's Hour", "Mimsy Were the Borogoves", and "Vintage Season", to Sturgeon's
More Than Human, Heinlein's
Stranger in a Strange Land (or even
Beyond This Horizon), nearly anything by Ballard, Fred Brown's "Arena", Kornbluth's "The Little Black Bag", Clarke's "The Nine Billion Names of God", Asimov's "Eyes Do More than See"... the list goes on and on and on. In many, many stories where "science is essential to the plot", you still end up with something which is much more, rationalistically speaking, in the fantasy camp than in that of science fiction:Miller's
A Canticle for Leibowitz, for instance, or Asimov's "The Last Question" or "The Ugly Little Boy", Heinlein's "Waldo", and so on.
The fact is that sf is an outgrowth of the much older and much broader literary field of fantasy*; it is speculation usually (though not always) based in a scientific or at least pseudo-scientific premise and with a more-or-less rationalistic frame of reference behind it... but beyond that, the two fields are really much closer together than the adherents of one or the other tend to want to believe... which is why, through the bulk of its history, sf and fantasy have walked not only side-by-side together, but often half-and-half in the same tale.
*Once again, I feel I need to remind people that the "standard fantasy quest" is only one very, very tiny facet of the immensely broad category of fantasy itself, despite its current vogue as
the idea of what constitutes the genre.