MustI go to the true spped of 17,500 m.p.h to maintain space flight otherwise I always fall back to earth no matter what the altitude???
The answer to your question is actually quite simple. FSX does allow spaceflight however to actually reach orbital altitudes and maintain them requires a different approach than regular flight. I have personally flown to over 50 million feet in FSX, however this took 11 hours to achieve.
to fly in space you must use slew mode to avoid stalling out at your aircrafts peak operating altitude. to engage slew mode press "y" on your keyboard, then tap f4 to head on up (throttle is no longer important).
to return to earth tap f1. I like to make it as realistic as possible so I use f1 on descent to about 100,000 feet then disengage slew mode by pushing "y" again and making an 80,000 foot freefall nosedive (to keep from losing control) and pull out of the dive at about 20,000 feet. throttle as necessary from there to reach your landing destination.
Happy Space Flight - can't reveal all the secrets of spaceflight but that'll get you started!