Achiving Orbit: Continue burn or wait till ap ?

[ManuxKerb]

Hi

I have a simle question:

When i try to make Kerbin Orbit. I have two ways of doing that:

1. One continueus burn to get me into orbit

Result: very egg shaped orbit, but never loosing m/s due to atmo drag

2. Burn till Ap is hinger than 75 then cut engines restart at ap to circularize.

Result: I lose lots of m/s till im at AP and i would need almost 2000 d/v for circularisation which i need to get up there first (extra).

Are these assumptions correct ?

Thanks!

Edited May 7, 2015 by ManuxKerb

[Phafor]

Mhhhh, good point, never really thought about that.

I guess someone will need to try both the ways with the same exact rocket, and see what is better. This can be easily incorrect information though... Humans make mistakes.

Someone here who has a good explaination on what is the best?

[Taki117]

How are you doing your gravity turn? Flying your rocket improperly can have a huge effect on your initial sub-orbit. Ideally you want to pitch over slightly and let gravity and aerodynamics tilt your rocket toward St he horizon so that you are basically horizontal at about 36km. This should result in a nice gentle slope that goes at least halfway around the planet and requires no more than 1km/s of dV. If you are not performing a gravity turn your parabolic arc will be much steeper and will require more dV to circularize.

[eonmoon]

burn till your ap is up there at 75km+ then you burn half the time before your node to get to orbit, for example, my last burn for orbit to 80 k needed a burn of 62 seconds, so at 32 I hit Z to kick engines to full power for the burn. my worst was 1200 delta v for circilization

[Octa]

Interesting thought. I could test that later this evening.

So.....let me just summarize in my own words: The question is wheather to ascend directly to the planned orbit or to flight to LKO first and do the transfer burn in space at apoapsis?

That's one thing that bothered me as well, especially for high orbits. I guess someone does already know the answer.

[Wanderfound]

Gravity turn 101:

1) Shortly after takeoff, pitch about 5° to the east.

2) Follow the prograde vector from then on. As you ascend, the prograde vector will gradually drift downwards. Pitching over more will make it drop faster, pitching less will pull it back up. Try to keep your nose within 5° of prograde at all times.

3) You want it to be at roughly 45° around about the time you reach 10,000m altitude, but don't worry too much if it's a bit above or below.

4) Once you get above 30,000m the reduced drag means you can afford to pull the nose further from prograde if necessary. What you want to do from here is to keep your apoapsis rising steadily ahead of you. In the map view (and in the main view if you use a flight info mod like Kerbal Engineer Redux, Kerbal Flight Data or Mechjeb) you can see both the height of your apoapsis and your "time to apoapsis". Try to hold the "time to apoapsis"constant. Pulling the nose down will reduce the time to Ap, lifting it up will increase it. Keeping it at around 30 seconds gives a comfortable buffer.

5) Once your apoapsis exceeds 70,000m, look again at your time to apoapsis. If it's still within a minute or so, keep burning and hold the nose down in order to stop the apoapsis from increasing further. If it's raced out to a longer time ahead of you (it probably will, it moves very quickly when you're flying almost flat), cut your engines. Point the nose prograde to minimise drag and coast until you're almost at apoapsis, then light the engines again to raise your periapsis.

Edited May 7, 2015 by Wanderfound

[fjarandag]

2. Burn till Ap is hinger than 75 then cut engines restart at ap to circularize.

Result: I lose lots of m/s till im at AP and i would need almost 2000 d/v for circularisation which i need to get up there first (extra).

2000 dv looks as if you where going straight up and then turning it into an orbit.

There are plenty of videos and tutorials out there much better and comprehensive than a forum reply.

Just google "Kerbal gravity turn".

[Lorunification]

In addition to what has already been said, if your gravity turn isn't too far off the optimum your insertion burn should not take more than 1k dV and usually takes around 600 to 800 dV.

[Octa]

@Wanderfound:

The Question is which ascend method to high orbits is more efficient:

Lets say launch to GSO (2868 km i think)

1:

- launch, start gravity turn

- burn until Ap is at 2868 km, then cut engines

- circularize at Ap

2:

- launch, start gravity turn

- cut engine when Ap is at 75 km

- coast to Ap

- burn prograde until (new) Ap is at 2868 km

- circularize at Ap

[ManuxKerb]

I want to achive orbit, so i need a periapsis of 73 or above. My question is how do i get that efficently, If i pause when my ap is above 70. i lose delta v and if i not pause i get an more eggy orbit. I hope that makes it clearer.

So early turns are not easily made with my rocket designs, they spin out of control easy.

Thanks for testing!! :-)

[Wanderfound]

Octa: assuming a proper gravity turn for both, the first method is more efficient, thanks to the Oberth effect.

Manux: you're fighting against two different fuel thieves during ascent: drag losses and gravity losses.

A flatter ascent with a small circularisation burn maximises drag losses but minimises gravity losses. A steeper ascent with a large circularisation burn minimises drag losses but maximises gravity losses.

In most circumstances drag losses are massively outweighed by gravity losses, so you're better off using the flatter ascent. A gravity turn is designed to get you quickly out of the thick lower atmosphere, but then flatten off afterwards.

However, as you've noticed, one of the exceptions to that is when your craft is too unwieldy to be easily controlled in the lower atmosphere. In that case, it may be worth taking the extra gravity loss in order to reduce the chance of total mission failure. It will cost you more in fuel, though.

Either way, once your apoapsis gets to the altitude you're heading for, point prograde and cut the engines until it's time for the circularisation burn. Unless you have a very unusually built ship [1], drag loss during the coasting phase should only be a kilometre or so of altitude (so if you're aiming for a 73,000m orbit, cut your engines when your apoapsis hits about 74,000m).

Don't forget the "point prograde" bit while coasting. It has a large impact on drag losses.

[1] Either so overpowered that you're still in the lower atmosphere when you cut the engines, or so unaerodynamic that it generates huge amounts of drag even in the thin upper atmosphere.

Edited May 7, 2015 by Wanderfound

[Streetwind]

The continuous burn to orbit is how real rockets do it, but they are computer controlled. It is very, very difficult to do by hand without producing a completely misshapen orbit that would cost a load of extra fuel to boot. Even veteran players and autopilot addons generally don't bother.

So generally, fly the "raise apoapsis to 75km, coast, circularize" style.

And as you get better at the game, you'll discover that something magical is happening: your coasting phases become shorter and shorter, and you spend more and more time burning in the atmosphere and require less and less dV to circularize after the coast. Effectively you coasting trajectory becomes closer and closer to the ideal continuous burn trajectory. You'll never quite get there completely, but you can get so close that the difference in fuel efficiency is a mere footnote.

You need to raise your velocity to about 2200 m/s at a 75km apoapsis in order to lift the periapsis out of the atmosphere. Your circularization burn at apoapsis will be whatever the difference between your current speed and that target speed is. Therefore, the faster you are already going at apoapsis, the smaller your circularization maneuver will be. Going fast at apoapsis requires pitching over early and flying relatively flat (but not too early and flat!). Wanderfound's post above mine in post #6 is excellent on that topic.

You can even use your speed at a 75km apoapsis as a benchmark for how well you are flying. For example, if you are going 1000 m/s (and thus need to burn 1200 m/s at apoapsis), that's not good and leaves a lot of room for improvement. If you are going 1500 m/s, that's decent. If you're going 2000 m/s (and thus need to burn only 200 m/s to circularize), that's very good and you can congratulate yourself on excellent piloting!

By the way, ManuxKerb: what slows you down on your way towards apoapsis is not so much air resistance, which largely stops above 30km, but rather gravity. And you do not lose that speed permanently either. In space, everything you lose on the way "up" to apoapsis, you will always get back on your way "down" to periapsis. You can easily see this effect happen if you for example keep your periapsis at 75km but lift your apoapsis out to beyond the Mun's orbit. You'll be going very fast at periapsis, very slow at apoapsis, then very fast at periapsis again... the more eccentric (elongated) your orbit is, the greater the speed difference between periapsis and apoapsis.

Edited May 7, 2015 by Streetwind

[Octa]

I want to achive orbit, so i need a periapsis of 73 or above. My question is how do i get that efficently, If i pause when my ap is above 70. i lose delta v and if i not pause i get an more eggy orbit. I hope that makes it clearer.

So early turns are not easily made with my rocket designs, they spin out of control easy.

Thanks for testing!! :-)

Ok, then i understood that in the wrong way somehow

I don't even know if it's (practically) possible to have one continous burn until 75 km :X All launches i remember have a phase where they coast to the Ap and have a break until they circularize there.

[Harry Rhodan]

i lose delta v

You'll always lose energy to drag.

[Champ]

@Wanderfound:

The Question is which ascend method to high orbits is more efficient:

Lets say launch to GSO (2868 km i think)

1:

- launch, start gravity turn

- burn until Ap is at 2868 km, then cut engines

- circularize at Ap

2:

- launch, start gravity turn

- cut engine when Ap is at 75 km

- coast to Ap

- burn prograde until (new) Ap is at 2868 km

- circularize at Ap

I really think option 2 is better, but i couldnt explain why

[DJK]

Having a longer burn to your Ap seems more efficient from what i've seen. At least to some extent. That said, it is true & very improbable that you will be burning all the way up in KSP. Like Streetwind said. I usually fly my rockets manually so that they need less than 100 m/s for circularization. Note that you need to throttle down quite a bit in the upper atmosphere for that to work. Coasting starts above 65km altitude. Note that this involves a lot of accuracy.. even the slightest deviation will have a relatively big impact on your trajectory.

[KerikBalm]

Ok, then i understood that in the wrong way somehow

I don't even know if it's (practically) possible to have one continous burn until 75 km :X All launches i remember have a phase where they coast to the Ap and have a break until they circularize there.

Well, it depends on your TWR... if you go all out on a high TWR rocket the whole time, you'll probably have your AP way out there while still 40km in the atmosphere.

If you are doing staging that leaves your upper stage with a low TWR... you may have no choice but to be doing a continuous burn and hoping you have enough time to accelerate before reaching apoapsis (note with low TWRs: its important to have a flat trajectory, so that your mostly parallel to the ground, or else you'll lose a lot due to gravity drag)

[Wanderfound]

Ok, then i understood that in the wrong way somehow

I don't even know if it's (practically) possible to have one continous burn until 75 km :X All launches i remember have a phase where they coast to the Ap and have a break until they circularize there.

It's definitely doable, but takes a fair bit of finesse, and the last few m/s worth of ÃŽâ€V are often spent with the nose pointed radial or anti-radial. Nose a whisker too high and your apoapsis explodes; nose a whisker too low and you'll find yourself falling back into the atmosphere.

I generally only bother with trying for it when I'm flying nuke-based spaceplanes; the low TWR gives you time to react, and as spaceplanes usually have much more control authority than a typical rocket I can whip the nose around quickly.

[Taki117]

Check this video for more information.

[Blu_C]

@Wanderfound:

The Question is which ascend method to high orbits is more efficient:

Lets say launch to GSO (2868 km i think)

1:

- launch, start gravity turn

- burn until Ap is at 2868 km, then cut engines

- circularize at Ap

2:

- launch, start gravity turn

- cut engine when Ap is at 75 km

- coast to Ap

- burn prograde until (new) Ap is at 2868 km

- circularize at Ap

Going direct to your target orbit altitude is almost ALWAYS more efficient. This is because of the Olberth effect, which makes your burns more efficient the deeper into the gravity well you are (that is actually a simplification). The difference isn't enough I would say you should worry about it generally, but if you are really into trying to get the most efficient rocket possible it may be something to take into account.

[mrclucks]

@Wanderfound:

The Question is which ascend method to high orbits is more efficient:

Lets say launch to GSO (2868 km i think)

1:

- launch, start gravity turn

- burn until Ap is at 2868 km, then cut engines

- circularize at Ap

2:

- launch, start gravity turn

- cut engine when Ap is at 75 km

- coast to Ap

- burn prograde until (new) Ap is at 2868 km

- circularize at Ap

Its more efficient to get into the lowest orbit you can first and then go into a higher orbit.

How I get into kso(its keostationary not geostationary ) is get into low kerbol orbit then make a series of small burns at periapsis so you can take advantage of the oberth effect( if you're in an egg shaped orbit, thrust is more effective if you burn when you're closest to what you are orbiting and because you are going faster). I'd usually burn for ten or seconds at a time or some arbitrary number like 100 m/s dv. My launch profile helps with this by me launching to 100km and then setting periapsis at 72km or so.

If you're still confused about the oberth effect then google it and/or watch Scott Manley on youtube explain it. Heck, watch all of Scott Manley's KSP stuff anyways since he explains things rather well and pretty simple

Edit:

@ Blu_C. Well I'm not sure which one of us should feel stupid but at least we agree on the oberth effect

Edited May 7, 2015 by mrclucks

[Old Foxboy]

I think it's good to develop a habit of remembering alt/velocity/pitch at certain points during the ascend. For instance, i try to start my gravity turn when passing 2000 metres, before aerodynamic forces get too strong to make significant corrections. However, if i am flying a fast accelerating rocket, i will pitch down faster. When flying a very sluggish "big barndoor" with loads of drag, i stay almost vertical a little longer and hesitate to let the nose come down too fast.

It's almost always a matter of experience.

Once you have a feeling for it, you will *know* during ascend when and how to adjust your ascend profile.

[Wanderfound]

Edit:

@ Blu_C. Well I'm not sure which one of us should feel stupid but at least we agree on the oberth effect

I'm afraid Blu wins this round...

(don't feel bad, there have been many long threads arguing about Oberth, with experienced players on all sides; it's something that's easy to get confused with)

You're both right in that you get more Oberth effect the deeper you are in the gravity well. The thing to keep in mind is that the burn to apoapsis and the circularisation burn do not each comprise 50% of the total energy. Especially if you do a good gravity turn, 90%+ of the total ÃŽâ€V expenditure will happen during the initial burn to apoapsis, followed by a tiny tap of thrust for circularisation.

Because of this, you want to concentrate your "Oberthyness" where it has the most impact: the initial lift of the apoapsis. Stopping and circularising at 70 gets you maximum Oberth on the circularisation burn, but to do that you need to give up all that tasty tasty low-altitude Oberth that goes past during the coast phase. It's worth losing a tiny benefit on the circularisation in order to gain a substantial benefit on the main lift.

[Octa]

Octa: assuming a proper gravity turn for both, the first method is more efficient, thanks to the Oberth effect.

I really think option 2 is better, but i couldnt explain why

Going direct to your target orbit altitude is almost ALWAYS more efficient. This is because of the Olberth effect, which makes your burns more efficient the deeper into the gravity well you are (that is actually a simplification). The difference isn't enough I would say you should worry about it generally, but if you are really into trying to get the most efficient rocket possible it may be something to take into account.

Its more efficient to get into the lowest orbit you can first and then go into a higher orbit.

How I get into kso(its keostationary not geostationary ) is get into low kerbol orbit then make a series of small burns at periapsis so you can take advantage of the oberth effect( if you're in an egg shaped orbit, thrust is more effective if you burn when you're closest to what you are orbiting and because you are going faster). I'd usually burn for ten or seconds at a time or some arbitrary number like 100 m/s dv. My launch profile helps with this by me launching to 100km and then setting periapsis at 72km or so.

If you're still confused about the oberth effect then google it and/or watch Scott Manley on youtube explain it. Heck, watch all of Scott Manley's KSP stuff anyways since he explains things rather well and pretty simple

Edit:

@ Blu_C. Well I'm not sure which one of us should feel stupid but at least we agree on the oberth effect

I'm afraid Blu wins this round...

(don't feel bad, there have been many long threads arguing about Oberth, with experienced players on all sides; it's something that's easy to get confused with)

You're both right in that you get more Oberth effect the deeper you are in the gravity well. The thing to keep in mind is that the burn to apoapsis and the circularisation burn do not each comprise 50% of the total energy. Especially if you do a good gravity turn, 90%+ of the total ÃŽâ€V expenditure will happen during the initial burn to apoapsis, followed by a tiny tap of thrust for circularisation.

Because of this, you want to concentrate your "Oberthyness" where it has the most impact: the initial lift of the apoapsis. Stopping and circularising at 70 gets you maximum Oberth on the circularisation burn, but to do that you need to give up all that tasty tasty low-altitude Oberth that goes past during the coast phase. It's worth losing a tiny benefit on the circularisation in order to gain a substantial benefit on the main lift.

It's always nice to see 4 people with 5 opinions

Ok, sorry for hijacking this thread, but i just did the quick'n dirty testing. If one should feel to discuss this further, message me and i make i new thread out of it.

Here's what i did:

- Vessel with boosters, mainsail, skipper and poodle

- same mechjeb launch profile (i only changed target orbit)

Try 1: Simply entered "2500km" as target orbit, hit enter and enjoyed the show

Result: 426m/s for final circularization burn, total of 1496m/s left

Try 2: Launched to 100km, but disengaged ascent autopilot during coasting phase and created 2 new nodes to get up to 2500km orbit (change Ap to 2500km at Ap, circularize at Ap)

Result: 419m/s for final circularization burn, total of 1502m/s left

What i saw from the launch, the ascent had potential for tuning, but i'm not sure what the difference will be (since it's the same parameters for both launches).

[Wanderfound]

Those are both variations on "burn straight to apoapsis", however. The competing flight plan was "circularise in LKO, then raise apoapsis, then circularise again".

The difference you're getting is due to the first version (aimed at 2,500km) using a steeper initial ascent, thereby incurring additional gravity losses that outweigh the Oberth lost in the brief coasting phase of the second.