I'm doing my best to launch an SSTO into orbit. Currently, my primary objective is to replace my LVT-30 with an LV-909 so that I can reap the d/v rewards. Unfortunately that means pushing as high and as fast with the airbreathing engines as possible.

I seem to have hit a brick wall at 1400 horizontal meters per second. No matter what I do, my craft won't beat that speed. Levelling off soon enough to have enough air to push faster means too much drag and the plane explodes.

Is it even possible to break this barrier (~>Mach 4) in Kerbin's atmosphere? If so, how can I re-design my craft to achieve it?enter image description hereenter image description here

I added More Engines™ but only managed to increase to 1456m/s. I've managed to hold that speed for several minutes now at about 10km alt but can't break it- the craft doesn't explode but no longer accelerates.

  • Oh, one more thing I noticed in your screenshots. Get rid of these radial intakes and precoolers. Don't airhog. One large intake (like Shock Cone, which has ridiculously low drag; people use it as nose cone on rockets!) per engine is perfectly sufficient. At altitudes where additional intakes would be useful thrust of jet engines drops so low that the engines won't overcome the extra drag introduced by the intakes.
    – SF.
    Sep 10, 2015 at 8:38
  • The precoolers do seem to really help prevent the engine exploding, but that's not my current problem, so I may well get rid of them.
    – DeadMG
    Sep 12, 2015 at 20:32

1 Answer 1


Not on Turbojets. They rapidly lose TWR above 1000m/s and exceeding 1200m/s with them is quite hard.

It's fairly easy on Rapiers though. Their thrust curve starts below that of Turbojets - they will be weaker on launch and initial ascent, but they provide higher thrust in the range of 800+ m/s and maintains the peak until past 1300m/s. 1400m/s is quite easy to reach and only above that speed they start losing thrust again.

  • Is that still in airbreathing mode though, or closed-cycle?
    – DeadMG
    Sep 8, 2015 at 14:26
  • @DeadMG: Airbreathing. Actually, Rapier's thrust in closed cycle mode is quite meager; 180.0kN closed cycle vs 892.5kN peak air breathing. Rapiers used to have the entire curve worse than Whiplashes before 1.0; currently they start off worse, but can achieve higher peak than Whips.
    – SF.
    Sep 8, 2015 at 14:35
  • The Rapier is very fuel-hungry, even in airbreathing mode. However, I've observed that you can use a weak TWR engine to finish off your ascent if it's at a low angle, like the 909. Now all I need to do is figure out how to jettison the "plane" part.
    – DeadMG
    Sep 12, 2015 at 20:52
  • I wouldn't say "very" - atmospheric ISp 3200 vs 4000 of Whiplash; may feel like more because fuel usage is proportional to thrust and Rapier achieves considerably higher thrust, but that's not a wasted fuel. If you're going for low TWR engines, consider LV-N. A spaceplane can reach orbit on Whiplashes and LV-N alone, although that's a frustratingly slow experience, so adding a third kind of engines, with good thrust plus a little oxidizer for them will shorten the flight by a lot. Poodle, Aerospike, Rhino... I even made one that used SRB.
    – SF.
    Sep 13, 2015 at 22:47
  • ...but where are you going with "jetissoning" any parts? Spaceplanes' advantage is that they never jettison anything so your cost per launch is the fuel cost only. If you're going to discard anything bigger than, say, a pair of drop-tanks, you should definitely reconsider if you need a spaceplane in the first place.
    – SF.
    Sep 13, 2015 at 22:49

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