How to limit apoapsis without wasting energy?

[Orc]

Hi all,

 

A question from a hopeless noob.

 

I'm trying to build a reliable orbital vehicle, where reliable means gets to orbit and back without killing the pilot.

 

I am using the Demo version, and am mostly trying to use a stack of FL-T200 fuel tanks and the LV-T30 engine. I've played around with multiple configurations like a straight up stack with two stages, a 9 tank, 1 engine, no Decouplers, and a short-ish central tank with two engines on small adjacent tanks. 

 

ANYWAY, as I understand things the Optimal use of fuel is burn at maximum thrust until you run out of atmosphere to push against (roughly 50km ?). Unfortunately with most of my designs burning the engines that long leaves an apoapsis usually over 130Km. Some times it gets as high as 250km, and once I got over a 1000km. Making orbit from that altitude is difficult, and often my Periapsis ends up so close to the surface of Kerbal that the trajectory can not be described as an orbit. I have achieved a Periapsis of 7,500 m, which led to bad things happening to Jeb Kerman. 

 

If you cut back the throttle earlier in the flight, like after 10km the apoapsis ends up at around 80 to 90 km but then there isn't enough fuel to make a circularizing burn.

 

Help me, what am I doing wrong or missing? My kerbal-nauts are bound to start refusing to fly ;-)

 

Take care, and may your rockets end up where you want them to, or at least provide a pretty explosion if they don't.

 

Regards and thanks in advance,

Orc

 

 

[Aethon]

I haven't played the demo in a long time but this may help.  

Try and keep your time to apoapsis between 45-55 seconds.  I think you can view that in the map screen by hovering the cursor over AP.  As time to AP increases you can decrease the throttle or turn further beyond prograde.  I try to hit 43 second as I turn through 45 degrees on the navball.  As my projected apoapsis reaches about 55km I want to be pointing directly at the horizon.  To keep AP from running away from you point the nose below the horizon into the brown/orange area on the navball.  I go as low as 10 to 15 degrees below.  This will keep your AP from racing away while bringing your periapse up.  I rarely need more than a 4 second burn to circularize.  

Hope that helps.  Welcome to the forums.  Another suggestion, buy the game.  I've gotten over 2600 hours of enjoyment out of it.

After rereading your OP, you should kill the throttle once your AP is slightly above where you want it (atmospheric drag will lower the final AP somewhat), coast to near your AP, and do a prograde burn there to bring your PE up.

Edited December 5, 2015 by Aethon

[Venusgate]

Actually, the fullburn rule has to do with your angle of attack relative to the gravitational body you're escaping more than the atmosphere

It is difficult to eyeball for those who haven't done it hundreds of times (AND YOU WILL! :D) but it's just a matter of getting a good ascent profile.
Honestly, if you are horizontal between 25 and 35km, and still climbing, that's totally fine, your sideways velocity will eventually raise your Ap to 70km and you'll have much better control doing so. And when you eventually reach the Ap, you wont need as much fuel to circularize, as you're's already likely going 1600m/s+, on your way to the ~2200m/s needed for LKO

Edited December 5, 2015 by Venusgate

[Claw]

2 hours ago, Orc said:

ANYWAY, as I understand things the Optimal use of fuel is burn at maximum thrust until you run out of atmosphere to push against (roughly 50km ?).

So that's kinda true, but there's definitely more things happening besides "burn fast and hard." Also, the atmosphere on Kerbin essentially ends at 70km. So you really want to get above that.

It's not entirely clear to me, but it sounds like you're burning the engines and just heading straight up. If that's the case, what you really want to do is slowly tip your rocket over as it's climbing. So initially you launch straight up, but over time your rocket eventually ends up firing it's engines while horizontal. If you shoot straight up until you're above the atmosphere, then turn sideways and fire the engines to make an orbit, you'll definitely run into some problems. Likely fuel issues and lack of time to run the engines long enough before falling back into the atmosphere again.

 

My suggestion would be to take a look at some videos. This one, for instance, is from one of the more popular KSP video makers (it's roughly 10 minutes). You might have to be a bit careful with videos, because the aerodynamics changed in KSP v1.0. So some of the methods for getting to space before version 1.0 don't work so well anymore.

 

Good luck, and welcome to the forums!  

 

 

 

 

[Snark]

Broadly speaking:  All problems getting to orbit are either rocket-design problems or piloting problems (or both).  To diagnose the former, we'd need to see a picture of your ship.  From your post, it sounds as though there may be an element of the latter, so I'll address that here:

+1 to Claw's response.  It sounds like you may be launching "too vertical."

In order to get to orbit, you need to accomplish two things:

• Get way up high in the sky.
• Go really really really fast, sideways.

Both of those (climbing up, and accelerating horizontally) use a lot of rocket power, i.e. fuel. The important thing to remember is that the second one is far more expensive than the first.  By far the lion's share of your dV is building up that horizontal velocity, much more than the dV you need to climb to orbital altitude.

Ideally, you'd like to be thrusting horizontally (or only slightly above it) the whole way.  In fact, that is what you would do, if that pesky atmosphere weren't in the way.  For example, if you're taking off in a vacuum (e.g. from the Mun), that's exactly what you do: upon liftoff, you immediately crank it over to almost perfectly horizontal-- just the tiniest bit higher than horizontal to keep you from scraping your toes. Then you burn hard until you're going at orbital speed just above the ground, and keep burning until you've raised your apoapsis to where you want it.  Then you coast up to apoapsis and need only a very small burn to circularize, since you're already in a near-circular orbit.

On Kerbin, you can't do that when you're down on the ground, because drag.  But it's still the ideal trajectory, so you want to get into a horizontal mode as soon as possible, i.e. as soon as you've climbed enough for atmosphere to not be such a big deal.

So you need to start tipping eastwards as soon as you're off the pad.  Gently at first.  By the time you reach 10-12 km in altitude, you should already be tipped at around 45 degrees, moving at 300+ m/s, and accelerating hard.  By the time you're at 25 km, you should be only 20-30 degrees above the horizontal, and still accelerating hard.

Then just burn until you raise your apoapsis to something reasonable, like 80 km.  Coast up to apoapsis, then do a burn to circularize.  The circularization burn won't be too huge, because you're already going sideways very fast.  Depending on your launch profile, it'll be anywhere from 800 to 1200 m/s.

So I guess the best question for us to ask you is:  how many m/s are you needing for your circularization burn?  If it's a lot higher than 1200 m/s, then you're doing something wrong (I'd guess not going horizontal enough on the ascent).

Edited December 5, 2015 by Snark

[Red Iron Crown]

16 hours ago, Orc said:

ANYWAY, as I understand things the Optimal use of fuel is burn at maximum thrust until you run out of atmosphere to push against (roughly 50km ?).

A correction here: Rocket engines are less efficient in atmosphere, they generate less thrust for the same amount of fuel used (sometimes dramatically so).

Aside from that, the video Claw linked shows how to do an efficient gravity turn for a smooth ascent.

[Orc]

Hi all 

 

Thank you all for the replies. Sorry for asking questions about the out-of-date demo. It's all I've got for the forseeable future, for financial reasons. I can't watch the videos (yet) because my home internet connection is hideous slow and prone to disconnecting. I'll take a look next time I am somewhere with free Wi-Fi.

 

From the replies I deduce that I am waiting entirely to long to roll the rocket horizontal and start getting horizontal speed.

 

I was following a 'rule' that said full power to 70km, turn to 45degree at 10km, and roll over to to 65% at 50/55 kms. Looks like I need far more aggressive leveling out.

How does one upload images to these forums? I haven't figures it out yet. 

 

Many thanks, take care and may your rockets end up exactly where you wanted them to go.

Regards

Orc

[Hupf]

3 hours ago, Orc said:

How does one upload images to these forums? I haven't figures it out yet. 

You upload them at an image hosting website like http://imgur.com/ (preferred in these forums?) or http://www.directupload.net/ and then you click on the "Insert other media" button at the bottom right of the editor and select "Image from URL".

Like this:

[Ljas]

5 hours ago, Orc said:

I was following a 'rule' that said full power to 70km, turn to 45degree at 10km, and roll over to to 65% at 50/55 kms. Looks like I need far more aggressive leveling out.

Another rule worth heeding is that you should not follow the advice of any KSP guide made before May 2015. That method would've worked fine in the beta, but 1.0 changed everything.

[pandaman]

On 06/12/2015 at 8:07 AM, Orc said:

I was following a 'rule' that said full power to 70km, turn to 45degree at 10km, and roll over to to 65% at 50/55 kms.

 

Yes, that piece of advice is very out of date.

 The advice given by others above is right, watch film of actual launches if you can, if it's from the right angle you will see the curve towards horizontal start as soon as they clear the launch tower.

But above all, enjoy learning and playing this great game.

Edited January 2, 2016 by pandaman

[MaxL_1023]

A rough rule of thumb:

1. Straight up until you reach 100m/s

2. Turn ~15 degrees eastwards 

3. Keep your nose pointed prograde (in the direction of rocket motion)

4. If you are below 45 degrees at 350m/s, turn up, If you are above ~60 degrees, turn down. Anything else shouldn't cause too much trouble.

5. If you get below 15 degrees before 30km, pitch up a bit until you get to 40km.

5. Once you pass ~40km, turn horizontal until your AP gets to where you want it. 

 

Basically, you don't want to need too much fuel to circularize (the 60 degree rule) but also don't want to have to plow through air below 40km when your velocity approaches orbital (rule 5). The 15 degree turn and prograde alignment is a gravity turn - it basically makes your rocket turn itself into orbit.

[Plusck]

Agree absolutely with everyone above.

I have played the demo fairly recently. When you get round to buying the full version (and you will... !) you'll find that the really most efficient way of getting to orbit is to start turning as soon as you leave the launchpad. A couple of degrees east (90° on the navball) straight away, then basically follow prograde and if you did it right you'll be at around 350m/s at 10km altitude and inclined about 45° to the horizon. If you head towards the horizon a touch earlier, you'll start to get very hot around 30km altitude but it will be even more efficient in the end.

The demo, however, is not quite up-to-date for the atmospheric effects. That means that it may be a touch more efficient to make the start of your gravity turn later. The old (old, old) rule of 10km straight up then turn 45° is NOT valid for the demo or the full version, but the basic idea is certainly more valid for the demo: head straight up to start with, but above all remember that it is horizontal speed that you need and make it happen as soon as you feel that you're not just going to plough through (and get heated by) heavy low-level atmosphere.

[Fearless Son]

Once you are up above 20 or 30 km, I recommend you switch to the map view ("M"), then pull up the nav-ball (click the little tab at the bottom of the screen) and watch your apoapsis climb.  Your exact ascent angle will depend on your thrust-to-weight ratio, but you should be in the middle of your gravity turn by this point, climbing roughly forty-five degrees relative to the surface.  As your apoapsis gets closer to 70 km you can progressively turn your rocket to more of a horizontal vector and keep thrusting.  As you approach your target altitude, you can start easing off on the throttle a bit and cut it entirely when your apoapsis is exactly where you want it to be.  Once there, you can drift up toward your apoapsis and just let the laws of motion carry you there.  Again, exactly where you do this next step and for how long will depend on your thrust-to-weight ratio, but you should wait until you are close to the apoapsis and then thrust prograde when you get near it.  The closer you are to the apoapsis as you thrust, the less you will push the apoapsis out and the more of that energy will go toward raising the periapsis.  Just be careful not to leave it too late, or you will not have enough time to complete your orbital stabilizing maneuver and will fall back into the atmosphere.  Like earlier, start easing off the throttle as your target periapsis gets near where you want it to be, then cut it when it is exactly where you want it.  For a standard circular orbit, you will probably want to cut the throttle entirely just as your apoapsis and periapsis start to "flip", as that will be the point at which they are closest to each other.  

Edited January 5, 2016 by Fearless Son