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Proper way to launch in 1.8.1


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I haven't played since 1.13, I updated to 1.8.1 and it seems that aerodynamic model has changed drastically. 

I understand that drag is now factored in or something. For the life of me I can't seem to figure what's proper way to launch a rocket, before I could make it in stable LKO on 3,000 dV or less, now sometimes 3,500 seems to not be enough. 

What's best way to launch a rocket and what's best way to launch a Space plane SSTO ?

I think I figured it out, thanks to a tip on Youtube reply. Tip was be at 500 m/s at 45 degrees - do it slowly and gradually. I figured 9 degrees of inclination per 100 m/s. It worked, I launched 2 different rockets, and got them in orbit with 500 m/s dV remaining, so I did the launch in 2,900-3,000 dV. One of them was a 100 ton rockets. Still need confirmation, and question still remains for SSTO

Edited by Zamolxes77
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Yes, many things have changed a bit.

But what it all ends up meaning in the case of your question is that there is no best or proper way. A rocket needs to be launched with an efficient gravity turn -- but that depends heavily on the drag. For spaceplanes, it depends even more on your exact drag, design, engine performance, and wing area. There is no single best way.

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I haven't noticed any big changes in several versions; It's purely anecdotal, but the first thing I do in every update is to launch a stock KerbalX to a Munar landing and return to Kerbin.

One thing I have consistently done for many versions is be able to complete the mission with about 4-500 m/s Δv remaining.

It is just my personal way of checking new versions.

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Assuming you're launching a rocket, vertically, you need to do a gravity turn.

This amounts to tipping toward the east (D key, if you built the rocket with the default pod position in the VAB) by a few degrees when your rocket gets between 50 and 100 m/s, then following the prograde marker until apoapsis is around 80 km.  Throttle down to zero and make a circularizing maneuver at your AP marker, and when that's done, shazam! you're in orbit.  The amount of tilt needed at the start of the gravity turn is dependent mainly on your TWR -- if it's 2 you'll need a lot more tilt than if it's 1.2, because you'll gain speed along your heading faster, and therefore have less time to build up horizontal velocity that tips the prograde marker toward the horizon.

This method hasn't changed significantly since at least 1.2 (I first played in 1.2.2).  Earlier (before 1.0?) there was an alternate method required by the "soup" of the atmosphere, to fly to 10 km before turning over, then immediately yaw to around 45 degrees and follow prograde (from there on, it's pretty much the same) -- but the atmosphere was changed in 1.0 or 1.1 to have less drag at sea level and to gradually taper off, more like a real atmosphere, so the gravity turn works better now.  The one exception is if you're trying to launch something like a complete space station all at once -- something big, fragile, and draggy may still get to orbit best by launching vertically to clear most of the atmosphere, then turning over to the east to build horizontal velocity after you have enough vertical speed to coast completely above the air.

For spaceplanes, the technique is completely different from what I've given here, but since I've never flown a horizontal launch spaceplane to orbit, I'll let those who have advise on how to do that.

Edited by Zeiss Ikon
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I experimented a bit recently and my standard launch profile now is to start with an atmospheric TWR of about 1.8 on the pad, tilt a few degrees east pretty much as soon as it's off the ground, and then follow prograde.  Once the Ap gets 60 seconds ahead of me (not sure if this is easy to find on the base game but KER's HUD will display it) I throttle back to maintain 60s.  This means that the angle steepens and less of the burn is done vertically, thus reducing gravity losses.  Once above about 50km I throttle up again, this costs a bit more dV than sticking with 60 seconds but makes quite a bit difference to the total time required.  This means I now build my second stages withy around 1.5 TWR, which is a lot less than I used to.

Less than 60s is more efficient but 60 give me some leeway if my upper stage turns out to be a bit underpowered.

 

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@RizzoTheRat It's hard to say if your method is any more efficient than the one I gave (stop thrust when AP is high enough), but mine is certainly easier to fly.

I'll admit, I've been influenced by playing Realism Overhaul.  With that mod set (RSS, Real Fuels, FAR, a bunch of historical engines parts packs, most of which don't throttle, have limited burn time, and have only a single ignition) you burn all the way from ground to circularization with minimal staging delays (because you have no control without thrust), and tailor your thrust profile by controlling the mass, hence TWR, of each stage (plus its payload, which includes all later stages).  Historically, a coast phase like the one SpaceX uses with the Falcon 9 (second stage shuts down when apogee reaches desired height, then restarts near apogee for final insertion) just wasn't practical until engines with significant throttling and restart capability existed.  After manually piloting a rocket to orbit in RO once, I'm generally happy to let MechJeb do the heavy lifting.

I'm pretty sure MechJeb is doing something similar to "chasing AP" -- based on the way i see, for instance, an early Atlas-alike stage-and-a-half rocket pitch up and down at various places in its launch -- but Positive Guidance is much more sophisticated than just that -- it actually pitches below horizon when it gets close to desired altitude and is short of velocity, which avoids raising AP as it raises PE to produce a circular orbit without burning at the existing AP or PE.

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For me it is: at least 1.2 twr on the pad, not more than 1.5

And gravity turn goes gradually, for me there are two main points: 45° at 10km and nearly entirely sideways at 40km, so it goes like: 27° at 5km, then the 45/10, 60 or so at 20km, 75° at 30km. That is quite efficient for simple rocket. Watch your drag (F12 for overlay I think?) If the red line is too long, throttle down.

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@Zeiss Ikonthe blue line on that graph in the linked thread is full throttle until the required Ap is reached (I think the method you're using), and then coast to Ap before circularising.  I don't think that throttling back is more efficient than can be achieved with a full throttle burn, but it's able to achieve pretty much the same efficiency from a smaller initial pitch manoeuvre, which gives a better margin for error.  So in that graph a full throttle launch with a 14 degree initial pitch gave a pretty efficient profile, but a 15 degree pitch was too much and it didn't make it to orbit.  A 12 degree pitch and then throttling back was able to achieve the same efficiency as a full throttle 14 degree pitch, so by throttling I could start at anything between 12 and 14 degrees and still achieve about the same efficiency as a full throttle launch at an optimised angle without the risk that I've overdone the initial pitch and don't make it to orbit.  Definitely agree it's more complicated to fly but I'm using kOS so that's not an issue.

However this rocket was clearly over powered, I tend to start most rockets at around 5 degrees these days.  The experimentation on that thread has lead me to change my designs quite a bit, in particular I now use much lower powered upper stages.

Presumably in RSS/RO your higher DV requirement means you don't tend to make rockets as overpowered as quite a few end up at in the standard game.

Re. circularisation, my script currently burns a bit nose down before Ap and nose up after Ap as it's aiming for the delta between the current velocity and the required velocity at it's current altitude.  I don't think it's as efficient as it could be though.  I believe the Apollo missions used to circularise after Ap, so presumably this allows for a decent amount of margin, you can burn to a higher initial Ap, and then correct it to the desired Ap with the circularisation burn.  Iv'e not experimented with this approach yet, as usually with KSP there's about a dozen projects I want to do and haven't had time to start yet :D  I also want to look at the impact of flying a few degrees off prograde, this again would give margin for error but at the cost of increased drag, however at higher altitudes that drag is pretty minimal.

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