Need a rule of thumb to eyeball my gravity turn

[Oddible]

I'm not sure if such a thing exists, or if it is wildly different for different craft with different TWR/ISP/etc. but I'm wondering if there is some sort of rule of thumb that I can follow. For instance, I know that I get more bang for my dV pushing my peri at my apo and pushing my apo at my peri. During liftoff, can I use my 'time to apoapsis' as a guide?

Just wondering if there is something that I can do to be more efficient about my gravity turns without breaking out the graph paper and setting a bunch of explicit targets.

[Scarecrow88]

I seem to recall somewhere that someone once advocated keeping the time to apo at just under a minute.

[Tex_NL]

In the past I used a pretty simple rule of thumb.

Under 25km keep acceleration to no higher than 2G. Anything above that is wasting fuel fighting air resistance.

Keep a constant eye on the atmosphere gauge. At the altitude where the needle passes from light blue to blue your prograde marker should be tilted at roughly 30 degrees. When it passes from blue to dark blue you should be at 60 and when it passes from dark blue to black you're all the way to 90 degrees.

This will give you a nice steady gravity turn pretty close to ideal. It even works on Duna, Eve and Laythe.

[Deutherius]

What I recall from learning the game some time ago, the Standard Non-FAR Kerbal Rocket Gravity Turn^TM is just:

"Go straight up to 10-12 km, turn into a 45° climb and burn until you get whatever Ap you want."

This works pretty much every time you have a decent TWR (~2) after the turn. If you have less than that, turn initially toward 60°-70° and continue slowly turning toward 45° - as your craft burns fuel, it's TWR will increase, making it easier to not start falling down again. If you have much more TWR, throttle down a bit (or get a smaller engine) - if you get your desired Ap (and shut off engines) but you're still at 25 km altitude, the atmosphere is going to drag you back down to Kerbin.

You can do two things for extra points:

- don't go too much above terminal velocity in atmo (generally when you start getting reentry flames/mach effects, you are going too fast... Or you can get a mod that tells you the terminal velocity for your current atmo pressure, like KER). This one is for efficiency.

- during the 45° climb, try to keep Ap about a minute in front of you - if it gets under one minute, nose up a little. If it's rising too fast, throttle down a bit (or nose down... but only if your nose is up by a lot, DO NOT put your nose below the prograde marker). This will ensure that you can easily inject into a nice orbit.

... should be tilted at roughly 30 degrees. When it passes from blue to dark blue you should be at 60 and when it passes from dark blue to black you're all the way to 90 degrees...

I know what you meant, but it might be better to say 60°, 30° and 0° to avoid confusion.

Edited March 7, 2015 by Deutherius

[GoSlash27]

I run mine by pitch benchmarks. At each waypoint, I want to be at a certain altitude. I also continually reduce my thrust to maintain an acceleration of 2sin(pitch).

Vertical boost: Pitch=90* and acceleration =< 1.7G Maintain until 6km altitude.

Gravity kick: Pitch=80* and acceleration =<2.0G Maintain until prograde marker falls into line with heading bug, then let the nose fall in line with prograde marker.

68* pitch: Altitude=12km and acceleration=<1.9G

45* pitch: Altitude=20km and acceleration=1.4G

30* pitch: Altitude=35km and acceleration=1.0G

Switch to map view.

Injection: Burn prograde at 1.0G acceleration until apoapsis achieved. Pitch to 0* and throttle as necessary to maintain apoapsis.

Circularization: At 30 seconds to apoapsis, throttle to maintain apoapsis distance. Repeat for 15 seconds distance and 5 second distance. Hold apoapsis 5 seconds ahead until circularized.

Best,

-Slashy

[Oddible]

@TEX_NL that's pretty much what I've been doing but I've noticed some things that have negative impacts when I do that. For instance, it doesn't take into account vertical speed and I find that without attentiveness to keeping a constant vertical speed it impacts my efficiency getting to orbit.

@GoSlash27 interesting, will have to look at adding acceleration as a factor.

@Deutherius I'm looking at improving the efficiency of gravity turns.

Currently I've been using something similar to @TEX_NLs 3 benchmark method but am adding both Time to Apo and Vertical Speed to my eyeballin'. I keep my vertical speed constant while increasing my horizontal speed (not sure of a speet spot for vertical speed yet, may vary per craft). Like was mentioned above I try to keep my Time to Apo under a minute at least until the end.

[capi3101]

My old rules of thumb were dependent on time to Apoapsis - after a turn to 45 degrees at 10k, I'd hold that attitude until I was 35 seconds to Ap before lowering to 20 degrees. At 45 seconds, I'd lower to 10 degrees, and after 55 degrees I'd lower to the horizon. That generally gets me an Ap at or around 100k with a simultaneous Pe around 10k, and an orbital insertion burn of less than 100 m/s every time. Not exactly ideal, but it works for me...

[Brainlord Mesomorph]

I started a poll on this and got lots of interesting opinions.

to answer to OP: Start at about 5 km altitude, or 100 m/s speed or whenever the SRBs burn out (they are tough to steer), then a niiice sllooow turn that's 45 degrees by 15 or 20 Km, and 90 degrees by 60 km, or whenever coast phase begins.

HTH

edit: a nice balanced rocket will turn for you after you start it. you just have to keep the nose of the rocket in front of the course ( the V attitude reticle within the O prograde reticle) or lock to prograde

Edited March 9, 2015 by Brainlord Mesomorph

[kfunk]

at launch i set my nav-ball to orbit.

i burn straight up until my orbital prograde vector is 45° above the horizon, then maintain that by yawing toward the horizon until my apopasis hits about 40km

then i burn along my prograde vector until app his about 60km

then i head for the horizon.

i have good luck with it and it's basically independent of the ship you're flying, although you have to mind the staging. if I go from a high TWR to a low TWR sometimes that oribital prograde marker will fall faster than I want it to.

[michaelhester07]

All of my recent burns are all based on the current apopasis.

Burn straight up till APO > 20km, then 70 degrees until 40km, then 45 degrees till 60km, then 0 until APO is where I want it. These are vessel altitude reading.

After that for orbit I adjust up or down during the insertion burn to keep my APO at my ship for a circular orbit. Most orbits I target for 75-80km. If you burn above the prograde marker (in the blue) during this burn the APO location will move forward. If you burn below it (in the brown) the apo will shift backward.

I use the Kerbal Engineer mod to read out the APO but the same reading is on the map if you're pure stock.

[Kelderek]

Burn straight up till APO > 20km, then 70 degrees until 40km, then 45 degrees till 60km, then 0 until APO is where I want it. These are vessel altitude reading.

This is a very high altitude for each of these steps. Remember that attaining orbit is not about going up so much as it is about going sideways very fast. With the path you are taking you will be going way too slow by the time you get your apoapsis where you want it and you will have to do a much larger burn to circularize.

The only reason to point straight up at all is because of the atmosphere and this is only a big problem at low altitudes (<10km). A more efficient turn will start no later than about 10km, and possibly even lower. Don't allow your ship to exceed terminal velocity (see wiki for an altitude chart). I would try to be at a 45 degree angle around 15-17km, hold it for a few seconds and then very gradually lower it all the way to the horizon. You can even point below the horizon a few degrees toward the end to help with the circularization (though this is most useful with an apoapsis of 100km or higher). The best way I've found to judge the success of your gravity turn is to see how much DV you need to complete a circular orbit once you get out of the atmosphere at your apoapsis. The lower this is, the better in general.

Another thing you might try is to use MechJeb's ascent autopilot tool. Even if you don't plan to use MechJeb for this regularly, it can be well worth the time to try it a few times and see what the ascent profile looks like. It may not be perfect, but I find that it yields reasonable results and you have several options to fine tune it. I highly recommend it for testing this stuff out, and then afterwards you can go back to doing manual ascents.

[Mahnarch]

At this point; I love watching the launch but, I don't feel a need to be at the buttons at every second, anymore.

Now, I just hit "Engage Autopilot" and sip a tasty, tasty beer Pepsi.

[Northstar1989]

Oddible,

Are you using FAR or stock aerodynamics?

If you're still using stock aerodynamics, I suggest installing FAR and forgetting everything you know about optimal gravity-turns. Gravity-turns should be performed much sooner in FAR, and become limited by how aerodynamically stable your rocket is (try to turn too fast and you'll spin out of control). Ideal TWR (to minimize Delta-V to orbit) also becomes much higher when you get rid of the unrealistic stock soup-o-sphere...

Stock aero is getting replaced with a more realistic and FAR-like aero model in 1.0, so I strongly suggest getting used to realistic aerodynamics now before SQUAD forces you to do so... Who knows, you might even like FAR better, since in a few ways it looks like FAR will actually be easier than stock aero (it has body-lift, which will be absent from the new stock aero AFAIK, for instance...)

Regards,

Northstar

Edited March 9, 2015 by Northstar1989

[EtherDragon]

Also, you'll have to relearn everything when 1.0 comes out again. So be ready to completely unlearn the optimal stuff of today.

[Aragosnat]

And remember to have fun....

[RatchetinSpace]

It takes a trained eye to know when to turn over, however people have done the maths on the issue. Ideally you want to turn slowly and gradually along your entire launch however that's not always possible, especially with experimental craft.

My advice would be to throttle up to max until your spacecraft reaches 150m/s, then throttle down to about 2/3rds or slow enough so you don't accellerate away from 150m/s too fast. Now wait till you reach 200m/s and then throttle back to stop going any faster. Once you reach about 9km then begin to slowly tilt over, once you reach 10km you should be at around 15 degrees. At 12 km roughly 20~30 degrees, now throttle up to max and slowly pitch over to past the 45 degree mark. I personally like to stay in around the 65 degree mark but your craft might not have the TTWR to sustain that so just keep your craft accelerating sideways and gaining apogee. Once you've reached your desired altitude for your obit, cut the engines.

Of course, this is just what I would do, many people have different opinions and its a debated topic. I would say that if your craft can get in orbit, its a successful launch and if you don't have enough fuel just add more boosters. Scott Manley has a great video on youtube discussing this topic. He recommends that as a rule of thumb, start turning at 10km.

Hope this was helpful!

[Montag]

The wiki has terminal velocity versus altitude for Kerbin, Eve, Laythe, and Duna. You want to follow that curve for your velocity. Scott Manley has a video speaking to gravity turns. The easy for Kerbin is roll to 45 at 10km then gradually decrease the angle after that. The advice from GoSlash just seems wrong. Once you get past the atmosphere penalty you want to throttle up to get in orbit as fast as possible to not fight gravity anymore. I am guessing that Max Q should typically occur at 10-12.5 km and throttle up could be 15k. Above 20k, I think you would have to be at extreme (re-entry speeds) to not want your max acceleration.

[Jikahn]

What you want to do is be thrusting along the horizon at the earliest opportunity with a low amount of atmospheric drag. The sooner you start moving sideways the less Delta V you are going to lose to gravity, but you don't want to lose it all to drag either.