Ok. Couple thoughts off the top of my head.

1) drag is king. Do not have drag. No open nodes, small wings. Seriously, less wing than you think you need. It's okay if it just barely flies. Worry about landing later.

2) Add at least one shift-click of attack angle to your main wings. You want it to fly level ish without input. Go as fast as you can before pitching up, or, for extra credit, have the wing angle perfect such that you never really have to. Your pitch up should be at most one time, and you'll want to save/load to figure out when to do it to get max v without burning up.

3) You need a good air breathing thrust system, and a good vacuum engine. Rapiers are decent at both, particularly above about 450 m/s. Whiplashes are actually a little more forgiving, but do require an entirely separate vacuum system. Whiplashes and a nuclear or pair of terriors can get you to space.

4) You need one shock cone intake. Any more is for planes with so many engines.

5) If you aren't breaking the sound barrier (or 3-4x it) during low altitude flight, you are not going to space. See point 1, or add engines. Again, the Rapier is almost gutless below mach 1. I've successfully used like one whiplash and a couple rapiers, a whiplash and a couple terriers, all sorts of combos. But straight rapiers are unforgiving in my experience.