Stratospheric flight: I'm getting slower

[DownLow70]

first thing you did wrong was to not search the forums..there are dozens of posts with help/suggestions like this one:

Requesting help with Darkstar

Hello everyone! Can you explain to me what is the easiest way to reach Mach 9 and maintain it? I’ve been trying to finish the challenge for hours, but sometimes my plane goes to Mach 2.7 and in a second I see my screen zoom in and my speed drops from Mach 2.7 to less than 1… Same thing last night, I was finally able to go to Mach 9, but after maybe 25 minutes where everything was fine, my plane did the same thing and my speed dropped below Mach 5… What could be the reasons for this sudden dec…

or this one:

Cannot get Dark Star above Mach 1.7

Try this: [MSFS | DarkStar Cold & Dark start | Scramjet Tutorial]