Are true gravity turns possible in KSP?

[Awass]

Without any mods, is it possible to do a gravity turn the way they do it in real life by making a tiny pitch-over right after leaving the pad then just letting gravity and projectile motion kick in to send you into orbit? From what I can tell, the controls are not sensitive enough and the weird aerodynamics seem to make it impossible to get that nice gravity-induced arc.

[Neotician]

I don't get your question, how are you getting into orbit ?

please elaborate

[Awass]

Oh yes, I suppose this is a gameplay question. My threads always get moved.

[Awass]

I don't get your question, how are you getting into orbit ?

please elaborate

My question is, "Is it possible to put yourself on an orbital trajectory by making only one small pitch over right after launch and not touching the controls after that?" From what I gather, that's how it's done in real life so that you're always burning directly prograde, which is most efficient and creates the least stress on the rocket.

[Letum]

Even in reality it's not possible 'right after launch' as aerodynamic forces will line the rocket up with it's direction of travel in the low atmosphere.

In KSP a one-touch altitude turn can be made in the high altitude.

[Awass]

Even in reality it's not possible 'right after launch' as aerodynamic forces will line the rocket up with it's direction of travel in the low atmosphere.

In KSP a one-touch altitude turn can be made in the high altitude.

Well "right after launch" is a relative term. The space shuttle starts its roll program just a few seconds after launch, so I say that counts. BTW, why is it that Kerbin gravity turns have to start at at or past 10k meters to be effective rather than in real life, where they can start within 1000 meters or so? If Kerbin is just a scaled down earth, why do things such as these behave so differently?

[Exposure]

Well "right after launch" is a relative term. The space shuttle starts its roll program just a few seconds after launch, so I say that counts. BTW, why is it that Kerbin gravity turns have to start at at or past 10k meters to be effective rather than in real life, where they can start within 1000 meters or so? If Kerbin is just a scaled down earth, why do things such as these behave so differently?

I believe it has to do with the fact that the stock atmospheres are more like soupospheres.

If you want more realistic gravity turns you can try a mod like FAR, which implements more realistic aerodynamics.

[DMagic]

I think real life launches have safety concerns as well. They want to get the rocket away from the launch pad and any inhabited areas as soon as possible.

[Awass]

I believe it has to do with the fact that the stock atmospheres are more like soupospheres.

If you want more realistic gravity turns you can try a mod like FAR, which implements more realistic aerodynamics.

Well that would explain it, but I have faith that the devs will improve the aerodynamics, so I'll wait.

[stringyquark]

I thought NASA turns right off launch so if things go badly it lands in the ocean and not back on the launch pad, not because it is more efficient. Could be mistaken, just posting out my @$$

[Captain Sierra]

I thought NASA turns right off launch so if things go badly it lands in the ocean and not back on the launch pad, not because it is more efficient. Could be mistaken, just posting out my @$$

Certainly a logical possibility. I know another reason the shuttle turns immediately is that the orbiter's engines are vectored out to help balance it. This also pushes it slightly off straight vertical, effectively forcing an early gravity turn.

This phenomenon is not noticable in rockets, but they do gravity turn at several thousand feet rather than waiting until they get into the stratosphere.

[Awass]

Gravity turns don't have to start past 10k meters, it's best if they start at 0. The Shuttle has to clear the tower before starting the roll program (and so did Saturn) that's the main constraint to the turn start.

I did some launches of the same rocket and wrote down how much ÃŽâ€V MechJeb said it had once it reached a 100km orbit. I varied the start & end of the turn.

MechJeb settings:

100km orbit

0 inclination

Prevent overheats

Limit to terminal velocity

Corrective steering

Turn shape: 100%

[table=width: 100, class: grid]

[tr]

[td]Turn End[/td]

[td]0km[/td]

[td]5km[/td]

[td]10km[/td]

[/tr]

[tr]

[td]35[/td]

[td]648[/td]

[td]663[/td]

[td][/td]

[/tr]

[tr]

[td]40[/td]

[td]713[/td]

[td]682[/td]

[td]608[/td]

[/tr]

[tr]

[td]45[/td]

[td]701[/td]

[td]656[/td]

[td]576[/td]

[/tr]

[tr]

[td]50[/td]

[td]685[/td]

[td]639[/td]

[td][/td]

[/tr]

[tr]

[td]55[/td]

[td][/td]

[td][/td]

[td][/td]

[/tr]

[tr]

[td]60[/td]

[td]617[/td]

[td]565[/td]

[td]493[/td]

[/tr]

[tr]

[td]65[/td]

[td][/td]

[td][/td]

[td][/td]

[/tr]

[tr]

[td]70[/td]

[td][/td]

[td][/td]

[td][/td]

[/tr]

[tr]

[td]75[/td]

[td][/td]

[td][/td]

[td][/td]

[/tr]

[tr]

[td]80[/td]

[td]493[/td]

[td][/td]

[td]380[/td]

[/tr]

[/table]

The left is the turn-end altitude, the columns are turn-start at 0km/5km/10km. Notice the higher you start the turn, the less ÃŽâ€V you have in orbit?

Notice that peak at 40km? That's when the rocket quit thrusting because the apogee hit the 100km orbit. If your rocket's got less T/W and burns longer, you need to change the end of your turn to 55km or whenever it quits the 1st burn. This rocket wanted 40km.

It just seems that if you start a turn too far before 10 km up (with any kind of decent sized rocket), you will start side-slipping rather than following your prograde vector. Seems like this happens because acceleration is so limited up to about 10 km up. It is both inefficient and difficult to to go too far beyond 200 m/s under 10 km with a decent or above sized rocket. Is this just another effect of the kinda messed up aerodynamics? In real life, rockets have a fairly constant acceleration and don't start out accelerating quickly only to begin gradually decelerating until they clear a certain altitude as they do in KSP.

And btw, that's a very helpful graph, but what do you mean by turn-start at 0km/5km/10km? Aren't 0km, 5km, and 10km the turn start altitudes?

[Awass]

I thought NASA turns right off launch so if things go badly it lands in the ocean and not back on the launch pad, not because it is more efficient. Could be mistaken, just posting out my @$$

Well it also turns out to be most efficient because all thrust is directed entirely into the direction of travel since the rocket is always pointed exactly prograde. This also creates minimal stress and drag as the rocket always meets the air head on where it is smallest in cross-section.

[numerobis]

If you put high-drag components at the back and low-drag components at the front, gravity will give you a positive angle of attack, then drag will twist you towards prograde. The main way to do this is with fins at the back, and maybe an airplane cockpit up front.

If you have all 0.2-drag components everywhere, drag won't rotate you at all.

[Awass]

Yes, they are the start altitudes. It took me a couple tries to figure out a useful table layout. I have a LibreOffice spreadsheet with a graph in it.

So what are the numbers inside the graph? Fuel remaining?

BTW, my notions about gravity turns came right from the mouth of Scott Manley himself:

I guess he focused more on people who started gravity turns too late, so I guess there is room for improvement below 10km.

And BTW again, what kind of arc did you end up with during your turn-starts below 10 km? I like to have a nice, long shallow arc before I get to my desired orbital altitude so I don't have to burn too long to circularize.

[stringyquark]

I guess it would be different for a side heavy symmetrical rocket, vs a bottom heavy fully symmetric rocket.

I usually find the bigger the rocket I launch the latter I do my turn, sometimes approaching 30+km. I find a straight up and out of the thick atmosphere to be the most stable in these situations.

[p1t1o]

In KSP, its very difficult to execute an efficient gravity-turn-assisted insertion to a low orbit, and much easier to the higher ones.

In order to "gravity turn" into a ,say 100km orbit, you spend far too much time in the lower atmosphere, and that eats dV

[Moar Boosters]

Certainly a logical possibility. I know another reason the shuttle turns immediately is that the orbiter's engines are vectored out to help balance it. This also pushes it slightly off straight vertical, effectively forcing an early gravity turn.

This phenomenon is not noticable in rockets, but they do gravity turn at several thousand feet rather than waiting until they get into the stratosphere.

I believe they turn earlier in real life because if you build up too much speed early on, it takes way more energy to change direction due to increased atmospheric drag. Even though the atmosphere thins out, the rocket will still be accelerating so the losses from drag won't dissipate until you're a long long way up..... so its better to just point it in the right direction when you're going really slow and won't suffer much drag during the turn.

Launchpad safety is also a big concern. Perhaps there are other possible rocket designs that could do their turns higher up, but seeing as the low-turn is efficient and best for launch safety, that's the method they chose and built their rockets accordingly.

[Awass]

Well there's no single reason why in real life gravity turns start pretty soon after leaving the pad. It's for all the reasons mentioned: safety, minimal drag and stress, and the turn being easiest and least stressful at low speeds.