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Does anyone else feels like the engines 45º angle is too much? It's a 30% cosine loss. The SuperDraco is more like 25º, probably even less. Is it possible to change the angle without editing the model? An MM patch?

Right, It's related, but much more loosely. The main source of DV loss in spaceplane design is drag, so there is an advantage in designing a spaceplane to be aerodynamically cleaner. So how does lift help? Well... There are 2 kinds of drag in KSP; induced drag and parasitic drag. Induced drag is a by-product of creating lift. Parasitic drag is the result of pushing a mass through the air. It so happens in KSP that wings only produce induced drag while everything else produces parasitic drag. It also happens that induced drag is *much* lower than parasitic drag in KSP. Using wings to create your lift instead of structural panels, body lift, or raw thrust means that you will create dramatically reduced drag during the launch. This means less DV wasted to drag and cosine losses, which means less DV required to achieve orbit. Best, -Slashy

In the Stock Payload Fraction Challenge, which is a good thread for ascent profiles, it was found that with Rapiers getting them up to speed as early as possible was key. Also for best dV it is a matter of reducing cosine loss from high Angles of Attack. So typically an ascent would start with running off the end of the runway, pitch up to between 5 and 10 degrees, lower for lower TWR, reach 400ms and then keep ascending to 10000m by which time you should be doing 1000ms already and then it's just a matter of tweaking your ascent velocity to keep it from blowing up and stay on air as long as possible to achieve the highest speed. Any big pitch changes cost you dV. Also wing inclination to body helps, a LOT. I only found out how much recently, makes me want to go back and redesign everything.

RCSBuildAid set to engine, red circle is dry mass, important to design plane to be balanced when full and empty. IntakeBuildAid balances intakes for performance and stop asymmetrical flameouts. These days spaceplanes are a matter of maximising your in atmosphere speed. Two whips to a swivel, or a Rapier to 15t. One intake only per engine. Get your speed to max possible by 17km, as close to 1350 m/s as possible. Then switch when acceleration tails off and pitch up a bit for orbit, not too high or you lose effiiency through cosine loss. Rapiers don't hit max thrust till over 400 m/s so you want to hit that as early as possible.

60°, that's crazy! You have 50% cosine loss of trust. I find a 30° pitch is a good compromise, but I frequently just fly 15° above prograde until the atmosphere diminishes. Orbital velocity is a about 2.4 km/s at 20 km. You only need enough pitch to see you to that speed before you hit Ap. Getting the correct pitch earlier just has the added bonus of leaving the last of the significant atmosphere quicker. With that advice, pitch will be related to TWR. Extremely low TWR craft (nuclear planes mostly) may want an entirely different approach. The required pitch for a gravity turn is too great. Instead, they can be better off flying at the edge of the atmosphere (32-35 km) using lift to keep them aloft at suborbital speed. The reduced drag that high grants more efficiency while accumulating orbital velocity. You establish atmospheric orbit and just do a Hoffman transfer out of the atmosphere. This is also likely the most thermally demanding flight profile in KSP today. I last tried it in 1.02 and the guages were displayed the whole flight. Nothing blew up though.

As long as you don't exceed a 55 km Ap before 30km altitude, just cut/reduce throttle until prograde is where you want it. The only losses you suffer is some of the Oberth effect since the coast will have you do more of your burn at a lower velocity. That velocity loss likely has less of an effect on energy efficiency than cosine losses. Efficiency of atmospheric launch of a nuanced thing. You need to balance drag, thrust, and mass against each other. Low (or single) stage launchers complicate this by reducing the flexibility in thrust and mass profiles. At what point does engine mass outweigh TRW gains? Faster acceleration gets you out of the air faster reducing dV losses, but needs more fuel per dV. Taking to the extreme, very rapid exit from the soup could require suboptimal burns to orbital velocity. Is the reduced time in the soup worth it? The mechanism of your gravity turn is gravity applying acceleration towards the surface. This alters the velocity direction via vector addition. The turn rate has an inversely proportional relationship with engine acceleration and a porpotionate relationship with the sine of the angle from vertical of your heading. High TWR designs need an aggressive turn to account for a shorter burn and the reduced work of gravity. OTOH, a shallower accent increases drag losses. Especially since you are flying with greater speed. The important thing is to grain horizontal momentum. That controls your angle from horizon at a given time from Ap. Try fidiling with your initial pitch mauver while trying for that 45° at 10 km. This part of the flight is where you need acceleration the most. If you exceed 110% atmospheric efficiency getting to a 10 km angle of 45° or more, reduce your engine array for mass savings. Don't use throttling if you have too much power down low. Reduced engine weight reduces other requirements. Be warned, due to the sinusoidal nature of the gravity turn, the greater your pitch-over, the more accurate it needs to be. It could be worthwhile to note both the initial pitch-over and the desired angle a minute later so you can correct before you deviate from flight plan too much. Disclaimer: I tend to be bad with high TWR gravity turns in atmo. I much prefer space planes. More active control.

If it's SAS oscillation, then yeah, my answer is "Don't use SAS". IMHO it's awful to fly planes with. When turning an aircraft you should bank and let the plane turn itself, and SAS prevents that. In level flight it's liable to cross-control the plane slightly - you'll be rolled left and yawing right to compensate. Even in stock that means you'll have a small cosine loss on the engine thrust and in FAR it'll increase drag too. I use pitch trim (Mod+W/S) to maintain a steady altitude, and build in roll stability or/and use FAR's wing leveller to keep the plane straight.

Toyotawolf : be aware that in .23.5 the game display days in Kerbin time now (1 day = 6h). There is an option in KSP settings to change that back. Err Yes. Actually it's voluntary. Let me explain why, and how it will be fixed (or not) : in the past I had this wonder full idea that torque would be so much better if we used the current thrust. I changed the code and all was fine at first look. Then I added the "cosine loss" calculation. Then some time later people complained that the translatron was not working anymore, so I reverted some of those code. The problem is that for both code to work together I need the know the "rest" position for a gimbaled engine. The game does not provide that to mods for now (the value is "protected" for those who code). I asked the devs to change that, and wait to see if we get it in .24. If it not changed for .24 then I have an other solution (use reflection, I did it in my Auto Trim mod) but it's a bit messy and would break MJ when the dev do the change (easy to fix, but it would not show up at compile). So for now we wait

Yeah, "cosine loss" IS "considers engine angle".

Actually, I've seen a number of ship designed with off-axis engine mounts where the thrust doesn't act fully along the x-axis of the design. In those cases, a perfectly "normal" ship will correctly have a different result shown for dV if the box is or isn't checked. What's more, "cosine loss" is both mathematically correct and what real aerospace engineers use to refer to the issue.

I eventually figured out this option, but it was still confusing, as the checkbox does nothing on a normal ship. Can you rename it "ÃŽâ€V considers engine angle" or something more understandable than "cosine loss"?

There is also loss of delta-V from the engine not pointing exactly on the line of travel. The term I see used for this on space related forums is "cosine losses".