Achieving Perisapsis After Launch Without Increasing Apoapsis

[Apollo13]

Still having trouble with this after hundreds of launches. Time to get this right. I've searched the forum and YouTube videos; no joy.

When I launch and hit my apoapsis, I cannot achieve periapsis without significantly increasing apoapsis. Prior to apoapsis, I ensure the rocket is tangential to the surface curve (navbal is half brown (right side), half blue (left side). Just before apoapsis, I kick on the engines to full burn. Everything goes well, the periapsis increases (from negative) with minimal disturbance to apoapsis. Immediately after the periapsis goes positive, suddenly the apoapsis skyrockets into a very elliptical orbit. During the burn, I ensure the rocket is constantly tangential to the surface.

WTF am I doing wrong? For learning purposes, I've used MechJeb to see how it accomplishes this task without increasing periapsis in hopes of discovering the secret. It appears to be doing as I do; rocket tangential...full power burn. So, no help there.

Any hints and guidance will be appreciated. If you know of a video or tutorial or forum thread, a URL would really be appreciated.

Edited May 20, 2014 by Apollo13

[Scarecrow88]

Your periapsis will only remain as periapsis all the time it is lower than your apoapsis. As soon as Pe > Ap it actually becomes Ap, and if you continue your burn at this point, your Ap will just continue getting higher.

[Apollo13]

Your periapsis will only remain as periapsis all the time it is lower than your apoapsis. As soon as Pe > Ap it actually becomes Ap, and if you continue your burn at this point, your Ap will just continue getting higher.

When I have this problem, my periapsis is NOT higher than the apoapsis. As I mentioned, the problem occurs just after the periapsis goes positive and is way below the apoapsis. For instance in a launch just a few minutes ago, my apoapsis was 240k, my periapsis was was -504k. As soon as the periapsis climbed upto 0, the apoapsis changed dramatically.

[Pecan]

Burn calculations assume all the thrust is delivered instantaneously. You can approximate that for a more accurate manouevre by starting the burn half the burn-time before you reach the node; so at T:-2s for a 4s burn.

[Apollo13]

Pecan: thanks. I do that as well. Given that achieving periapsis requires a very long burn, this solution hasn't helped me.

[Laie]

You do know about maneuver nodes, do you?

http://wiki.kerbalspaceprogram.com/wiki/Maneuver_node

They make it much easier to plan and time your burns. Just click anywhere on your flight path, then click-and-drag any of the six colourful markers on that thing that just popped up. You'll get quickly get the hang of it. Also notice how this places a countdown timer and some more info next to your navball.

[cantab]

Approach 1: Pitch down a bit to keep your apoapsis under control while still burning to raise periapsis.

Approach 2: Stop burning when you see your apoapsis starts running away, restart when you get nearer to it.

[Taki117]

Pecan: thanks. I do that as well. Given that achieving periapsis requires a very long burn, this solution hasn't helped me.

You need a higher TWR. If your orbital insertion burn is more than about a minute that's too long. What times are we talking here? (bold text)

[Kasuha]

Do your burns in map mode. Make sure your ship is sitting on the apoapsis marker all through the burn. If it slips behind, burn above prograde. If it slips ahead, burn below prograde.

But in general, if you prepare a maneuver and that maneuver has some reasonable periapsis and apoapsis and you burn through that maneuver, you should not end up with any significantly different values for periapsis and apoapsis. If you do, some pictures (from map mode) might help.

Edited May 19, 2014 by Kasuha

[allmhuran]

Could you take a screenshot (im map view) at the start of your burn and at the point where your apoapsis has skyrocketed? (but I expect cantab has the solution you need.)

[Apollo13]

You do know about maneuver nodes, do you?

http://wiki.kerbalspaceprogram.com/wiki/Maneuver_node

Yes, I use those nodes constantly.

Forgot to mention in my OP, I use the nodes and it takes two or three maneuvers to achieve circular orbit. I'm looking to do it all in one maneuver, like Mechjeb.

Edited May 19, 2014 by Apollo13

[Nedal]

Approach 1: Pitch down a bit to keep your apoapsis under control while still burning to raise periapsis.

Approach 2: Stop burning when you see your apoapsis starts running away, restart when you get nearer to it.

Do your burns in map mode. Make sure your ship is sitting on the apoapsis marker all through the burn. If it slips behind, burn above prograde. If it slips ahead, burn below prograde.

But in general, if you prepare a maneuver and that maneuver has some reasonable periapsis and apoapsis and you burn through that maneuver, you should not end up with any significantly different values for periapsis and apoapsis. If you do, some pictures (from map mode) might help.

These are your answers.

[zarakon]

Do your burns in map mode. Make sure your ship is sitting on the apoapsis marker all through the burn. If it slips behind, burn above prograde. If it slips ahead, burn below prograde.

This is what I do. You can also adjust your thrust to keep with it. I find this the easiest way to get near-perfect circular orbits

[LethalDose]

Everyone here has made really good suggestions. If your burn to circularize is taking a long time, then increasing your upper stage's TWR will lead to a shorter, and hopefully more precise, burn.

Cantab mentioned pitching down. More precisely, if you pitch down before apoapsis, and pitch up after apoapsis, it will help control your apoapsis during circularizing.

Since you mentioned watching your periapsis go from negative to positive, it sounds like you have a mod providing you with orbital information, and it's good you're using one of these (If you're not, I like VOID, most players seem to prefer MJ or KER). Anyway with the mod or if you're using map mode like Kashua recommended, you can also try to keep your time to apoapsis at about 1 minute during your ascent. This is a trick that Scott Manley recommends. He has a good video

that you may find useful.

[HeadHunter67]

One of the other things I've discovered is that sometimes, it's good to tweak the Radial In/Out while plotting your circularization burn. This helps to prevent an eccentric or elliptical orbit. You don't need to adjust it much, just enough to keep the periapsis at the point of the node. Done properly, it results in a very circular orbit!

[Mr Shifty]

Note that you don't want to necessarily be pointed tangent to your orbit when you burn. You should aim to keep your vessel pointed at the maneuver node marker on the Navball, which means you'll be pointed below the horizon at the beginning of the burn and above the horizon near the end of it.

[Redshift OTF]

Well MechJeb just makes a manoeuvre node at apoapsis and performs half the burn before it gets to apoapsis and half after so the manoeuvre point is hit in the middle of the burn. How long does the burn take? Are you using LV-N engines to perform this burn?

[KvickFlygarn87]

It would be really helpful if you provided a video of your problem

[SpeedDaemon]

Try starting & finishing your gravity turn a little earlier. This should help to start raising your PE sooner, so you'll have a shorter burn near AP. Makes things easier, especially if you have a lower TWR.

Cut off your ascent burn earlier and leave your AP a few km below where you actually want it, this will let you start your PE burn sooner without overshooting your AP.

Use a mod like Kerbal Engineer Redux that will show your AP, PE, and time to AP in the same place. Try to get your burn done before you hit AP, as close to AP as possible, without going over. If you modulate the throttle and have a good enough TWR, you can usually point at the horizon and ride the time-to-AP at 5-10 seconds, which should get a circular orbit pretty easily. If you see the time to AP growing, lower the throttle, and wait to catch up.

[MockKnizzle]

You need to launch more sideways and less up. With a proper launch you shouldn't need more than 500-ish m/s to circularize at 75km.

Other than that, just pay attention to your apoapsis altitude and don't let it run away from you. Burning tangent to the ground only works when you are exactly at apoapsis, you need to aim a little below the horizon if you're early and a little above the horizon after you pass it.

Also, why on earth does it take you more than one maneuver to circularize?

[godefroi]

Try starting & finishing your gravity turn a little earlier. This should help to start raising your PE sooner, so you'll have a shorter burn near AP. Makes things easier, especially if you have a lower TWR.

This. If you're doing "really long" burns at Ap to circularize, your ascent was probably too steep. Pitch over earlier, or pitch over farther. Your "time to apoapsis" shouldn't ever get really big (over a minute or two...).

[Apollo13]

Thanks to all for taking the time to answer. Lot of good ideas here, which I'll implement.

[HeadHunter67]

I was always advised that, on the ascent, time to Ap should be between 30-60 seconds - pitch down or up as needed to keep it in that range. When I started, I used to do the "straight up to 10k, 45 degrees until desired Ap, begin circularization burn at T minus 1/2 of the burn time"... that's a good way to start, but it invariably leads to a really elliptical orbit with an apoapsis far above the desired altitude.

So now, I tend to ascend at a shallower angle - straight up to get out of the troposphere, and then I slowly chase the prograde marker down to the horizon, keeping an eye on the time to Ap as I go. All you really need to do is get high enough to minimize drag and stay at terminal velocity, and a "flat" ascent will naturally increase your altitude as well as your orbital speed. Now I find I'm going much faster when my Ap marker gets to the desired altitude, and then I can start to dip below the horizon to continue increasing speed as I ascend. When I finally reach the Ap, I only need a short burn to bring up my Pe to the same altitude. This method also gives me a lot more time to work on keeping my orbit nice and round - I'm not frantically trying to set up a node in time to start the burn.