Had a go at the new Experimental Darkstar challenge and it was genuinely so much fun! Reaching Mach 9 was quite cool and flying into the outer atmosphere.
Why does my Darkstar only speed up to 0.98MACH?
You might need to do a nose dive whilst you’re 40,000ft up high I reckon?
And I can’t use this mission to fly this jet. the fps will drop down to lower than 10fps.
but it works well out of this mission.
Wait, can you use the plane on a normal flight?
Yes, you can. Just made a flight over the northpole……with refuelling 
Oh lovely, wasn’t able to find the aircraft on the normal flight aircraft selection
yes…it can
New York to Rome: 1h 30m at mach 10. Amazing.
And in the middle of Atlantic the rise of the sun with the blue of atmosphere and the black of the space. It was the best flight that I ever made. WOW
Thank you Asobo and Microsoft for this wonderful gift.
It would be nice that be possible to flight over 275000 feet (now we are slight under karman line that delimite the outer space, 100 km), maybe at the height of the International Space Station, what a vision that would be.
But already now is a fantastic vision! 
275000 is a limitation of the game not the plane, if you build a rocket and flew it straight up it too would stop climbing at 275000’.
I’ve had that one exact same problem. Did you maintain +1.2 Gs until 20° during the pull after the unloaded roll to upright?