Why does the periapsis open up so slow when I try to orbit?

[Chamandepastel]

It only happens with certain rockets, but what I do is I burn 45 degrees east at 10km and then when the apoapsis is at 90km I stop and wait. I burn horizontally, but the periapsis opens up really really slow and I end up burning a whole lot of fuel and the periapsis ends up at around 60km at most. The apoapsis on the other hand gets huge to like 500km and sometimes shoots me into the suns orbit. What am I doing wrong here?

[boomerdog2000]

If you are burning too early, your current point will become your new periapsis. You're starting your gravity turn at the right height but you generally want it to be more shallow then just 0 degrees, then 45 degrees, then 90 degrees. It should be more of an arc. Then, at or a little before apoapsis you should burn horizontally/prograde.

[Kosmo-not]

You're not pitching up to maintain altitude.

[Kasuha]

True gravity turn means burning prograde (exactly) after initial kick to the side and letting the gravity to curve the trajectory. By burning prograde you are adding the most energy to the orbit but you are raising both apoapsis and periapsis. But such maneuver is very hard to execute in a way which leads to circular orbit - in most cases you end up either on suborbital trajectory or start raising apoapsis above all limits. In such cases you need to apply corrections.

If you want to raise just periapsis and you are not at apoapsis yet, you need to burn slightly below the prograde marker. If you are behind apoapsis, you do it by burning slightly above the prograde marker. To raise apoapsis, burn above prograde when ascending. You don't need to raise apoapsis when descending.

If you have already raised your apoapsis to sufficient height, place a maneuver at the AP marker and pull the prograde handle till you get a circular orbit. Then you can start burning half the time estimated to burn before you reach the maneuver node and by burning at the maneuver marker you will be raising just the periapsis. If you manage to execute the whole burn you should get an orbit sufficiently similar to what you planned by the maneuver.

Oh and regarding the question why it takes so long - in map view, move the cursor to look at Kerbin from above and zoom out. You'll be able to see the whole ellipse of your trajectory. In general, your periapsis starts somewhere at -330 kilometers near the center of the planet and you need to pull it through all that height before it even becomes visible.

Edited November 2, 2013 by Kasuha

[Alistone]

I'm picturing a suborbital apoapsis at 90,000meters. Then you burn along the horizon and instead of simply raising your periapsis (which is what a horizon burn should do when you are at apoapsis) You are saying it becomes a highly elliptical orbit with periapsis still below atmosphere...

Having a high T/W ratio couldn't cause this unless you weren't burning horizontally. But having a low T/W ratio might if you ended up far beyond apoapsis while still trying to burn along the horizon to reach orbital velocity.

I suggest two possible solutions:

1. After 30,000 meters in elevation begin burning horizontally while you maintain a heading due east. (If you have a very low T/W ratio this will help you build orbital speed so that your burn at apoapsis will require less time.

2. About 30s before apoapsis turn horizontally and keep apoapsis 30 seconds ahead of you by adjusting the throttle. If it is get to be more than 60s away then throttle back so you can catch up to it. If it gets closer than 30 seconds throttle up to keep it in front of you. If you accidentally get past apoapsis then pitch upward to 45 degrees and set engines to maximum until it gets back in front of you. (This works for almost any craft, but if you have too low of a T/W ratio you won't be able to keep apoapsis in front of you and will fall back to kerbin. But at least it would identify a problem... low T/W ratio. If that is the problem then you can either add more thrust or do more of number 1 to try to add orbital velocity sooner).

Either way, make sure you have been keeping your nose along the horizon. As you circle the planet it tends to be constantly pulling up.

Edited November 2, 2013 by Alistone

[Sirrobert]

Are you using manouver nodes?

It'll tell you how much deltaV you need. A 90km orbit needs a total orbital velocity of 2400m/s or so

[EtherDragon]

Hi Chamandepastel,

There are a few reasons this could be the case:

If your rocket has low power (not much thrust) then that could be one reason - it may just not have enough thrust to affect your orbit enough. Best to use a Maneuver (at the Apoapsis) to plan the burn, then see how long the burn will take. If you find, after starting up the rocket, that the burn will take more than about 2 minutes, your rocket engine is too weak (or you have too much extra mass).

Whenever you are at a node (periapsis or apoapsis) any thrust you apply only effects the opposite node. In other words, if you thrust at the periapsis, it will never raise your periapsis - only the apoapsis will be changed. And vice-versa: thrusting at the apoapsis only changes your periapsis. The further you are from either node, the smaller (and weirder) the change.

There is a point, even at the apoapsis, that it will "flip" and become the periapsis.

Check out this video to see if it gives you any hints:

Here's another good one, although the topic covers many different things. Skip to 12:25 to watch the Launching bit...

Edited November 2, 2013 by EtherDragon

[capi3101]

First off, welcome to the forums, Chamandapastel.

Here's a procedure for a gravity turn in simple terms:

1) Launch. Burn straight up to 10k; this part you have down.

2) Turn to course 090 at 45 degrees elevation; again, this is something you're already doing, which is good.

3) Assuming you're playing stock, you'll occasionally need to switch over to map mode while your launch is ongoing. That's fine; spend as little time there as necessary and switch back to staging mode when ready.

4) You want to stay at 45 degrees elevation until your time to apoapsis gets to be about 35 seconds or so. Once that's reached, turn to follow the prograde marker.

5) Continue to follow the prograde marker unless your time to apoapsis falls below 30 seconds. If it does, return to 45 degrees elevation for a while.

6) Once your time to apoapsis reaches one minute, steer along the horizon - zero elevation on course 090. You're still coasting upwards so your apoapsis is fine; what you're doing now is applying more thrust to start raising your periapsis.

7) Continue to watch your altitude and your projected apoapsis altitude. If you reach your desired apoapsis altitude and you're still below 50k in the atmosphere, continue burning for a little bit, about another 10-15k or so. Then kill your burn. You'll lose some altitude due to atmospheric drag; this compensates for it.

8) Once you're above 70k, set up a maneuver node at apoapsis. You should only need 200-300 m/s of delta-V, tops (and that's assuming something somewhere was screwed up).

Hope that helps.

[Moonfrog]

I bet the simple answer is he may be burning after the highest peak of his orbit. When that happens you get the problem he is describing, I had it many times. He needs ot burn earlier.

if your node has a burn time of 1 minute, u should burn at 30 secs. split the time in half and burn that much early. I'm guessing your burning at exactly node time 0. You should never burn at 0 because you will overshoot, then need more fuel to compensate.

THe advice about a longer arc also makes this easier. Instead of going up to 90k, in stead aim for 71 km. That is roughly the minimum u need to orbit. after much practice you notice that during your burn the peak will move to the right, u can actually control it with your thrust. There will be a time when your trhust can equal the rototation of the new peak. Only if your a perfectionist.