[BLEEP] Gravity Turns, How Do They Work?

[Wayfare]

So I've been wondering about gravity turns lately. Ever since I watched

and

(YouTube videos) struggle with the launch profile of my Munshine V Apollo-style Mun rocket, and reading comments by others who have had difficulty getting Munshine launchers to perform as advertised, it's struck me that there seems to exist a strong tendency to do inefficient gravity turns during launch.

When I started playing in 0.16 common wisdom held that you launched straight up, turned to 45 degrees around 20km, then kept this heading until your apoapsis hit the desired orbital altitude and circularized there. Today the usual method seems the same, except you turn over at 11km. But when I launch, I begin my turn gently at 5000 meters or so, just 5 degrees to the east. I then take it in steps by the speed my rocket builds up, going halfway to the 45-degree marker at 250-300m/s, then to 45 degees at 500-600m/ms, halfway from there to horizontal at 800-900 m/s and flat out around 1200m/s. This saves an amount of launcher fuel you wouldn't believe! Plus it makes launch a slightly more hands-on approach, which is good because let's face it, after a few hundred launches they all start to look the same.

Now I'm thinking that this may have to do with the way I design launchers. Serial staging means you have a fairly wide range to thrust-to-weight ratio as you drain fuel from each stage. The Munshine V goes from something like 1.2 to 3+ on the first stage. Putting a lot of that high-end TWR into horizontal velocity makes sense, atmosphere be damned. Asparagus launchers, of course, maintain a far more even TWR throughout the flight and this may affect the ideal launch profile.

What's your basic launch profile and what are your thoughts?

[Awaras]

I start turning once my altitude hits 10Km, and get to around 45 degrees before hitting 20km.

I turn all the way towards the horizon as soon as my apoapsis goes over 45-50km and keep it there until I increase my apoapsis to the desired height.

This leaves me with having to make only a relatively small burn (a few hundred m/s dV) to circularize once my craft reaches apoapsis.

[SofusRud]

I start my turn at around 10km, then i gradually lead my prograde marker towards the horizon. This means that i usually only need about 5-400m/s of delta-v to circularize

[lajoswinkler]

Mechjeb starts turning at 45° at 5 km, at least that's what I remember it was doing. I turn to 80° above 10 km and then gradually towards 5°.

[Johnno]

Depends on what I'm flying but my usual launchers have plenty of TWR so I try to get to 10km at around 200m/s, trying to keep my TWR below 2 until then, throttle up and aim for 45°, turn off engines as my AP hits 75km (or whatever I want to get to), coast to AP and burn there.

Wayfare I downloaded your Munshine V and flew it as I normally would my own, got it to 76/77km orbit with 235m/s delta-v left in the second stage. Don't really see any issues with it.

[Van Disaster]

If you use FAR you can actually do a proper gravity turn with a well built rocket - ie just tip it over a bit just off the pad & follow the prograde marker. If you can't, you can at least get quite close to it, I start at 300m & usually I'm at 40-50 degrees by 10km with about 500m/s surface velocity. By 45km I'm almost horizontal, usual circularization burn at AP is in the order of 150m/s at most.

FAR or not, if your pad TWR is too low there'll be a large dV loss to gravity at launch, so I like at least getting off the pad smartly even if it's with short duration SRBs and I slog up to 25km without accelerating that much.

[Wayfare]

Depends on what I'm flying but my usual launchers have plenty of TWR so I try to get to 10km at around 200m/s, trying to keep my TWR below 2 until then, throttle up and aim for 45°, turn off engines as my AP hits 75km (or whatever I want to get to), coast to AP and burn there.

Wayfare I downloaded your Munshine V and flew it as I normally would my own, got it to 76/77km orbit with 235m/s delta-v left in the second stage. Don't really see any issues with it.

Thanks, that's encouraging Did you fly the .20 or the .21 version? The latter is a lot more forgiving during launch because the new small fuel tanks allowed me to build a much lighter payload.

So apart from Van Disaster I'm not seeing anyone initiating their gravity turns below 10km. I really recommend it folks - that little five degree nudge at 5000m makes a lot of difference!

[Seret]

Once, you've pitched over and are at a reasonable height so that the atmosphere isn't too much of an issue the most efficient thing to do AIUI is burn along your prograde vector. It's going to depend on the actual ship (and payload) and where you started your turn and at what speed though. There's no point piling on tons of horizontal velocity if you fall back to Kerbin before you circularise. I think aiming for 45° soon after 10km is a good start, and you can play around with things at the top end of your burn to see what gets you the best result.

Pushing over a few degrees somewhat before your gravity turn isn't a bad idea if you're going to be jettisoning stages, you don't want to rain boosters on the pad. Destructible KSC buildings is on the devs' roadmap, and you'll be wanting to have things come down in the drink for recovery. Best to start developing good practice now.

[regex]

Before I started letting MechJeb do all my ascents (before the SAS sucked, basically) I'd start my turn around 6km for high TWR rockets and 8km for lower TWR rockets. Turns were gradual affairs and I'd end at about 60 degrees at 28km, burning until I had the desired apoapsis. That seemed to work pretty well for me. I notice MechJeb starting all the turns at 6.9km.

[Richy teh space man]

Typically, I do a 45° turn at around 10km up, then turn mostly horizontal at around 30km

[Cipher29]

I like to set my gravity turn markers via speed rather than altitude so I can have the same profile with different T/W profile. I start my turn at 100-200 m/s which typically with a rocket from 1-2 T/W will be before 10 km. Following the prograde marker down you should hit 45 degrees at around 450 m/s, 30 degrees at 600 m/s, 20 at 800, 10 at 1000. By then your appoapsis should be at about 45-50 km several km down range and you can walk it in to your desired orbit from there.

In short:

Start turn at 100-200 m/s

45 degrees at 450 m/s

30 degrees at 600 m/s

20 degrees at 800 m/s

10 degrees at 1000 m/s

[SpaceSphereOfDeath]

You should start your turn at no lower then 8km and no higher then 13km if you want to be efficant-ish

[Whackjob]

I go straight up until all the fuel burns out except for the last stage. On the way back down, I kick the engines on, accelerate down, and barely miss Kerbin. Twelve days later, I circularize around Duna. Which stinks, because I was after Eve.

Oh well. Hit and miss. Rocketry by random dart throw is fun, too.

[The Destroyer]

Usually go between 8-10 km gravity turn, FULL THROTTLE!

[Randox]

My gravity turns reflect my early rocket flying style. I started by doing strait shots to the Mun, no turn, no orbit. There were flaws with the way I balanced and steered my ships in the air which prevented them from safely performing a gravity turn before at least 20km up. When I practiced orbital rendezvous in 0.16, I had to make a special smaller launch rocket beause nothing in my current fleet could actually establish an orbit in the 90 to 110km range without falling back into it, and I needed that range because I had the numbers and guides for it.

I still usually establish orbits in the 100 to 110 range, and never lower than 90, out of habbit, and I still gravity turn late. My lifters are currently turning no earlier than 15km, and they have to wait until Ap to do the bulk of the orbital velocity component. Might try bringing myself to start turning around 12km, and to roll over a bit further, past 45.

[Designer225]

I frequently start turning to 60~70 degrees when I hit 8 km, then 55~60 degrees when I hit 20 km, or when the apoapsis hits 30km, whichever comes first, later 45~50 degrees when I hit 30km, or when the Ap hits 50km, whichever comes first; next up 35 degrees when I hit 40km (or when Ap hits 60km, whichever comes first), 20 degrees when I hit 50km (or when Ap hits 80km, whichever comes first), 10 degrees when Ap hits 85km, 0 degrees when Ap hits 90km, -20km when Ap hits 95km or when the Ap hits 90km ahead of plans, whichever comes first. I generally stops at 100km, but sometimes at higher orbit. In very few cases the Ap was on the other side of the planet. In the case with Periapsis below the ground, I have some 400m/s of Delta-V to burn, but in the case with Pe above the ground, I have less than 366m/s of Delta-V to burn.

[arq]

If you want to get a good idea of 'efficient' launch profiles, try this challenge from awhile back

http://forum.kerbalspaceprogram.com/showthread.php/39196-Launch-Efficiency-Exercise-Updated-for-0-21-1

If you can get it to orbit, you're using an efficient profile that is within maybe 3% of optimal.

I tried ten or twenty different profiles (manually, so a bit of personal error there) and found that even seemingly different profiles ultimately perform almost the same. If you start making a turn at 10km and don't turn too slow or too fast (and that's a surprisingly large swath), the profiles differed by less than 100dV from each other (usually even less).

The burn->coast to apo->burn cost virtually the same as trying to just burn all the way to apo. Also, following the prograde marker really isn't as important as you'd expect. That will probably matter more when there is a proper drag model.

[mellojoe]

On smaller crafts, the standard: vertical until 10k, rotate to 45* for a while, then as AP hits 60k+ go horizontal

On larger cafts, it is a bit more involved: usually I do my gravity turn much slower, staying as vertical as possible longer and taking a smoother turn to 45*. Then once the AP hits 60k or so, start a gradual turn toward horizontal. Usually leading the prograde marker by a bit and not steering too quickly.

[SkyRender]

I've pretty consistently done it the same way for ages, since it seems to work fairly well.

1. Slight turn east by around 2 to 5 degrees near launch; mostly to keep debris from hitting the space center.

2. Controlled speed upwards keeping below the speed of sound (~100m/s below 2500m, working gradually up to ~150m/s around 6400m, and ~240m/s around 10000m; it's fairly free-for-all past that height).

3. 5 to 10-second turn towards 45 degrees at 10,000m.

4. Gradual decline towards 90 degrees as I rise; the main focus at this point is twofold: keep time to apoapsis between 50 and 70 seconds, and keep the prograde marker no more than 10 degrees from my current heading.

I admit it's not the absolute best method, but it does let me get craft into orbit with a reasonable combination of "fast" and "efficient". I also design my staging so that my ship's TWR via staging during ascent never jumps wildly. The Skipper engines made that much easier to do, since their TWR is absolutely perfect for controlled ascent via 3 sets of 2 asparagus-staged orange tanks.

[Thiel]

I usually start my burn right off the pad. How much depends on the rocket.

[UmbralRaptor]

Every time someone suggests a sharp 45° turn at 10 km, Jebediah launches a kitten into oblivion with a pile of boosters.

The best starting altitude varies. With stock aerodynamics, between ~5-10 km (later for low TWR rockets/earlier for high TWR ones) works with an, initial pitch of 80-85°. Then chase prograde (lagging with a low TWR rocket, leading with a high TWR one) until horizontal or you have an apoapsis outside of the atmosphere. FAR should work similarly, but you can begin turning even earlier (perhaps right at launch)

[SkyRender]

Every time someone suggests a sharp 45° turn at 10 km, Jebediah launches a kitten into oblivion with a pile of boosters.

The best starting altitude varies. With stock aerodynamics, between ~5-10 km (later for low TWR rockets/earlier for high TWR ones) works with an, initial pitch of 80-85°. Then chase prograde (lagging with a low TWR rocket, leading with a high TWR one) until horizontal or you have an apoapsis outside of the atmosphere. FAR should work similarly, but you can begin turning even earlier (perhaps right at launch)

Funnily enough, I design all of my rockets to have the same lift regardless of payload. Nothing ever exceeds 2 TWR, mostly because that's the point at which Clamp-o-Tron Sr. ports break. So while my ascent profile of choice only works for one particular type of lift setup, that lift setup is literally the only one I ever use.

[Wayfare]

Yeah I do think TWR figures into this significantly. My heavier launchers require a more gentle gravity turn than the lighter ones. Even so I feel a little tip somewhere well below 10km can do wonders for most launchers. Vertical speed only gets you out of atmo - it's horizontal speed that gets you into orbit.

[Randox]

I could add that I usually have the terminal velocity values handy during launch, and I'll frequently refer to them to check my own velocity, up until 15-20 km at which point it's really about going as fast as I can without breaking the ship (on Kerbin). I usually check the numbers for each km, especially at first when you can waste a lot of fuel.

[FITorion]

I don't really pay attention to the altitude exactly... I pay attention to the atmosphere thickness meter. I stay straight up until I get to the end of the lower atmosphere. Then I slowly nose over throughout the middle level atmosphere until I get to 45 near the end of it.

At this point I check the orbital map and see where my apo is. I point myself at the direction of motion indicator or at 45 until it gets there... until apo is where I want it and shut down. Then plan my maneuver to circularize into a orbit and wait until it's time to burn. then burn.

Works out well mostly for me... except when I have rather low TWR launches... low for me is anything under 3... and really low is anything under 2...