(Space)Planes, Altitudes and Efficiency

[Somerled]

So I'm going all Kerward Hughes and ramping up my aircraft industry. Now, I've settled on a working design and I'm aiming for Kerbin circumnavigation. The thing is, I'm obviously getting different fuel efficiencies at different altitudes, if efficiency is measured as fuel used over surface distance traveled. Well, duh! Drag, surface speed, thrust and lift profiles, all of that stuff matters for making my plane fly the farthest.

I want to ask the Lindkerbs around here what they think of my flight SOP. What I'll do is get up to my maximum safe altitude as fast as possible, then use turbojets on low(ish) power and trim to a pitch that maintains airspeed and altitude. My thinking is that there's minimal drag up there, so I can get up to higher speeds using less power. However, there's also minimal lift, so I have to spend more downward thrust to maintain altitude. There's also engine intake and ISP to consider.

I have no feel for the tradeoffs between high- and low-altitude flight in relation to fuel efficiency. Anyone care to straighten me up so I can fly right?

[strongest_2hu]

If you get up high enough you can be making about 2 km/s (depending on how big your plane is, if you're intake spamming, etc) with the throttle as low as you are physically capable of setting it. As opposed to flying along at the surface where you can make about 200 m/s with the throttle all the way up. So yeah, you're doing it right.

Things to consider: B9 engines actually have an atmospheric pressure curve in addition to their speed curve, so if you're using that mod pack you can leave the throttles all the way up and watch as your thrust drops off at higher altitudes, taking your fuel consumption with it. If you're using the stock turbojet the way you get more efficient is by throttling down as your intake air decreases, which it sounds like you've got the hang of. If you're crazy nose-high to maintain level flight, try adding some more wing.