Why do I suck at orbiting?

[photogineer]

FNG here, trying to get my big ole rocket in to a stable orbit, I've been practicing for a few days now, getting the hang of these and so own.

I've checked out the tutorial videos here and read anything else I can find, and I think I understand the orbit idea in concept. Burn towards prograde to raise the orbit, burn in the other direction to lower the orbit. The problem I'm having is that I can't seem to affect the orbit the way I intend to.

For example, I finish the first stage launch of my rocket, and it's on course for a apoapsis of ~78km, with a periapsis somewhere in the planet's crust. So I drop the stage, warm up the next stage engine and use the RCS thrusters to swing the nose around to face prograde according to the happy fun ball. I then patiently wait until I reach apoapsis at ~78km and fire up the engine. As I expected, the periapsis comes flying out of the planet, I keep the nose pointed prograde and throttled back, when the periapsis got to ~78km, I shut off the engines, and relaxed. Stable orbit achieved.... or so I thought. I now notice that my apoapsis is about 300km out now. Obviously I'm doing something wrong here, I tried using maneuver nodes as a guideline for the next attempt, and the target indicator pulled my below the horizon, which led to violently deorbiting my ship while I remained blissfully unaware in map view, watching the the periapsis do it's thing.

So, where do I start? What do you all think I'm doing wrong?

Edited August 3, 2013 by photogineer
Answered now

[Superfluous J]

The important thing to remember is that you would need, in a perfect world, to do ALL of your burning at apoapsis to raise the periapsis. In the real world (and the Kerboverse) this is impossible because you're only at apoapsis for a millisecond. So what do you do? Burn BEFORE apoapsis! Not much before, but somewhat. The best you can usually do is to divide your burn time in half and start burning when you're that far from apoapsis. So if your burn will take 30 seconds, start 15 before.

That'll do a good job, but it still won't be perfect. For long burns, you may have to burn a bit to push your apoapsis forward, then cut the engines, wait until you're near again, and burn again. This way you only push it out a couple dozen (or hundred, who's counting) meters. The trick here is to stay in front of your apoapsis. Once you pass it, you can't (as easily) keep it near the planet while burning prograde.

And it sounds a bit like you're not using maneuver nodes. Learn them. They're a life (and orbit) saver!

[photogineer]

Alright I think I'll take another look at maneuver nodes, I'm fairly certain I screwed it up the first time I tried it. Will that give me a time estimate on the burn? That seemed to be the case, if so is that only for 100% throttle?

Thanks.

[Lheim]

Burn before you hit Apo.

Not much before.. but the secret is, burn a little below the horizon. Like, I'll start a slow burn at about 45 seconds before Apo and about 10-15 degrees below the horizon. Depending on how your Apo moves.. it will move, up or down, you can raise or lower your nose and adjust it. Do it over several little burns so you can see what's going on. With fine tuning, it's possible to get it nearly circular - you may even end up pushing your apo a minute or a minute and a half out in front of you, then just wait till a bit before it again.

[TaranisElsu]

Another thing to keep in mind is that if your circularization burn at the apoasis is huge, then you aren't doing enough or early enough of a gravity turn. Your orbital speed should be over 1500 m/s when you get to your apoapsis. The recommended gravity turn is: straight up to 5-10 km, then start slowly nudging over towards the East (90 degrees), try to drop the nose 5 degrees for every 1000-2000 meters of altitude. You should finish your gravity turn (nose pointed at the horizon) before your apoapsis reaches 80 km. Of course, don't do too aggressive of a gravity turn or you will spend too much time in the thick atmosphere fighting drag. It is a balance, and you will get the hang of it soon enough .

[CheftonUTD]

Check out this guys series of tutorials:

That should be the first one, a bit of practice after watching those and I'm getting near perfect orbits of the Kerbin and the Mun most of the time now.

[Frederf]

The thing about always burning retro/pro is you don't have to. Go ahead and burn plain horizontal or whatever is needed. Prograde is efficient but efficient doesn't help if you're efficiently getting into the wrong orbit.

Personally I like to end up in something like a 60x22.5km orbit ditching the main stage to be recovered in atmo and then continuing to 80x80. If you are prograde pretty far from horizontal then prograde is going to balloon your orbit with high Ap quite badly. It's a challenge to end your gravity turn with a low Ap low eccentricity orbit from which to make the final burn.

[cpt_nosh]

Play the tutorials ingame.

[Kerbonautical]

Everybody has given solid advice but just a heads up, don't take exact figures too literally other than your speed, apoapsis height and gravity turn angle. Bear in mind that everyone will likely be ditching different stages at different points and that one rocket will hit the magic apoapsis number earlier than others. For instance I have one rocket which can hit the apoapsis at 2240M/s and only requires a small burn to circularize and another that will hit the apoapsis at around 1700M/s leaving me to perform a bigger burn to circularize.

Each ship will fly different but in no time you should have the basics nailed on getting most anything into orbit.

[photogineer]

Thanks for the advice everyone, I've watched the videos and tried practicing all morning, and I am getting better, but I still can't circularize the orbit. In all the video tutorials I see it seems so easy, but when I try to do it my apoapsis just bursts out of control suddenly. It's very frustrating, and I don't even know what I'm doing to cause it. I have tried burning anyone from +/- 10 seconds on the apoapsis to +/- 1 minute!

Also, I've been trying to use maneuver nodes, but no matter how I try to adjust it, I can't even plot a circular orbit, so it's not much use as a guide line. At this point I'm starting to feel like I've got a more fundamental problem, and that is why I can't get my ship in to a circular orbit. I'm going to try one of the stock ones and see if that makes a difference. While I'm doing that, does anybody have any advice on how I may be screwing up my nodes?

[Superfluous J]

Any chance you could record a video and pop it up on YouTube or something? I can't think of a way to help without seeing what's going on.

[photogineer]

Any chance you could record a video and pop it up on YouTube or something? I can't think of a way to help without seeing what's going on.

I'd be more then amenable to that, could you recommend some recording software? I know Fraps is popular, but the free version only allows 30 second clips, and I don't know that I'd use it enough to justify the $37, you know?

[Bunsen]

Thanks for the advice everyone, I've watched the videos and tried practicing all morning, and I am getting better, but I still can't circularize the orbit. In all the video tutorials I see it seems so easy, but when I try to do it my apoapsis just bursts out of control suddenly. It's very frustrating, and I don't even know what I'm doing to cause it. I have tried burning anyone from +/- 10 seconds on the apoapsis to +/- 1 minute!

Also, I've been trying to use maneuver nodes, but no matter how I try to adjust it, I can't even plot a circular orbit, so it's not much use as a guide line. At this point I'm starting to feel like I've got a more fundamental problem, and that is why I can't get my ship in to a circular orbit. I'm going to try one of the stock ones and see if that makes a difference. While I'm doing that, does anybody have any advice on how I may be screwing up my nodes?

I'm not certain what's going wrong, but start by placing a node as close as you can get to the apoapsis marker. Drag the progade burn thingy out until you start to see the apo/periapsis markers on the forecast orbit rotate -- that means the planned orbit is getting close to circular. The closer you placed the node to the apoapsis marker, the faster the apo/peri markers will swing around and the closer to circular the resulting orbit will be. Don't go crazy trying to get it perfect.

If you haven't dropped any stages since your last burn, the maneuver node's burn time will be fairly close to accurate, so use that to time your burn roughly 50% before and 50% after the exact time. The calculated burn time does assume you'll run at full throttle, but you don't have to do that 100% of the time -- it's easier to get the delta-v right if you tail off gently and carefully, and if you did 95% of the burn at full throttle then being a few seconds late with the last 5% won't throw things off by much. This may help with the "apoapsis goes crazy" syndrome, since most of the change there happens near the end of the burn.

Mess around with this a few times, not necessarily right at launch launch -- if you get a 300km apoapsis, try circularizing at 300km just for the practice. You'll find that changes from one orbit to another generally require much shorter burns than it takes to get into orbit in the first place, and that can make it easier to see what's going on and figure out how it all works.

[blizzy78]

If you could precisely detail the steps you're executing (in the order you're doing them), that would help, too.

[SkyRender]

I'd be more then amenable to that, could you recommend some recording software? I know Fraps is popular, but the free version only allows 30 second clips, and I don't know that I'd use it enough to justify the $37, you know?

Try Open Broadcaster Software. It's free, it lets you stream, and it lets you record to disk, all without watermarks or arbitrary limits.

[photogineer]

I'll check out Open Broadcaster, but a friend sent me a copy of Fraps for now, so I'm all set. I made a video of my attempt and posted it

. It shows my launch, staging, circularizing burn and running out of fuel.

Thanks in advance.

[leax256]

I think your problem is that you are confusing a stable orbit and a circular orbit so i'll explain 4 separate orbits.

Stable: a stable orbit is any orbit in which you will never enter the atmosphere of the planet/moon and therefore never crash down.

-This orbit can also be circular or elipical.

Unstable: a degrading orbit in one in which you renter the atmosphere at some point. this orbit will eventually result in full rentry

-Can be eliptical or circular

Circulal: orbit in which the apo and peri are the same or at least close enough

-can be stable or unstable.

Eliptical: Orbit in which the apo and peri are greatly different

-can be stable or unstable.

In short you acheive a stable, but encintric orbit.

P.S. forgive my spelling.

Edited August 3, 2013 by leax256

[GoDores]

You're not doing a gravity turn. You get out of the atmosphere but only at a small percentage of orbital speed, so you have to burn a long time at apoapsis to get into orbit, and as a result a lot of the burn isn't done exactly at Ap, which makes your orbit eccentric. It's even harder because you're using such a low orbit and that gives you little time to circularize before you reenter the atmosphere.

Between 8 and 12 km, pitch over to 45º on the navball. Then as you climb you can roughly track the prograde vector. This will vary a bit from rocket to rocket, but by 40 or 50km you should be burning directly towards the horizon on your navball. You'll want to overshoot your desired Ap by 5-10km because you'll still be in atmosphere and it will slow you down a bit. But once you're out of the atmosphere you should be within a couple hundred m/s of orbital speed. You can do a short burn at Ap to circularize and you won't need a minute-long burn that will mess up your eccentricity.

A couple more efficiency tips: you could make your launcher more efficient by making the first stage side boosters asparagus staged. There's also no reason to have RCS on in the atmosphere.

Edited August 3, 2013 by GoDores

[Zephram Kerman]

Wow, the video really helps understand the situation. So here's what's going on: you did your burn near Apoapsis, like everyone says to do, but we all assumed you were already in a nearly circular orbit. Because your trajectory is very sub-orbital, Apoapsis is way behind you by the time your burn is completed. See that moment moment right around 4:15, where both Pe and Ap are coming up, the music has stopped, and Ap is actually moving the other direction. As it moves farther away, the effect of your burn changes too, so that it's boosting Ap more and more, and Pe less and less.

The cure is to avoid this situation by making your gravity turn lower. The last minute or so should be nearly horizontal. This will boost Pe earlier, and make everything more efficient. Also, start that Ap burn sooner. (Some of my rockets have very low-power payloads, so they don't even have a coast phase.)

You might find it helpful to use MechJeb as a tutor. Do a couple of flights using MJ's Ascent Guidance, to see how an ideal gravity turn should look. Then imitate that in the future.

By the way, for recording, I like MSI Afterburner. It's mainly an overclocking tool, but the video capture feature works very well.

[photogineer]

Thanks everyone, I'm doing much better now. I managed to do a not terrible gravity turn and put myself in to a not super suborbital 300km Ap shot at the end of the first stage burn. I was so focused on the gravity turn I was't watching how high and fast I was going. Whoops, but I was able to pretty easily circularize it, settled with a 292km Pe and a 294km Ap, so I'll take it. I also targeted by deorbit burn with a maneuver node and brought the capsule down sort of near KSC. Unfortunately, KSP crashed just as I was on final descent, so I didn't get to see how close I got.

More practice ahead!

[Diche Bach]

Not sure if you've seen this one, but it helped me a lot to start getting into orbit consistently.

[Superfluous J]

Thanks everyone, I'm doing much better now. I managed to do a not terrible gravity turn and put myself in to a not super suborbital 300km Ap shot at the end of the first stage burn. I was so focused on the gravity turn I was't watching how high and fast I was going. Whoops, but I was able to pretty easily circularize it, settled with a 292km Pe and a 294km Ap, so I'll take it. I also targeted by deorbit burn with a maneuver node and brought the capsule down sort of near KSC. Unfortunately, KSP crashed just as I was on final descent, so I didn't get to see how close I got.

More practice ahead!

Glad we could help! I knew there was some piece of the puzzle we weren't getting