Trouble getting LKO with less than 6k dV.

[qoonpooka]

My dV map says LKO, in stock, is 4,550 dV. I'm trying to efficiently park a command module in orbit to some early-career science. I've parked satelite contracts around the Mun, landed on the Mun (didn't have enough fuel to escape again, though. R.I.P. Jebediah Kerman), etc. I'm guessing I usually make up for my craptastic launch piloting by my ability to put every maneuver node I fly to 0.0 m/sec without overshoot.

So the current vehicle consists of two vertically stacked SRBs (BACC initial stage, RT-10 2nd stage) and then 3.5tons of final stage including a -909, T200+T100 fuel, science jr, quad landing pods, capsule, mk-16, one PV panel, and a 2HOT.

KER says 4,878 dV. By my understanding of the dV map, this should give me 328 more dV than I need to attain a 75km orbit.

I launch, @2km alt I tip about 2 degrees to start the gravity turn. The rocket does not keep tipping. @~10km I ditch the BACC and light the RT-10. Now I'm tipping the rocket manually towards 45-deg as I accelerate. RT-10 quits @~25km and I light a full-throttle -909. At this point my Apoapsis is nearing 75km so it doesn't need to burn for long. I set a maneuver node for a prograde burn at the Apoapsis to circularize. Burn time comes up: 1m 52s. I have barely that much fuel full. As I climb to apo- my velocity is dropping into the 600m/sec range, and I end running out of fuel before I'm anywhere /near/ circularized. 4,878 dV becomes a suborbital flight.

I'm pretty sure I'm flying the ascent wrong, but I can't find any guidance for getting to orbit efficiently - just people saying 'add moar boosters.' Is the dV map wrong?

EDIT: Great analysis has helped me adjust the thrust on the solid stages, and change how I'm gravity turning. I was, apparently, making the turn 'too perfect' for stock's drag model as one of my errors. The rest was just the fact that solid boosters are crap and throw off the delta-v calculation in KER.

Edited January 13, 2015 by qoonpooka

[orangexception]

If you're using the stock aerodynamics, then you need to limit your speed to terminal velocity on ascent. There should be a readout on KER for terminal velocity.

[qoonpooka]

This suggests, since I'm using solids, that I'm just gonna lose a ton of dV to either drag or gravity, since I can't throttle them?

[Keras]

Sounds like your gravity turn is way too high. I believe a general guideline is ~45° at 10km and at about 30km you should be burning almost horizontally. This is a rough estimate since I myself am playing with NEAR and that forces me to automatically do a good gravity turn or I`ll have to burn straight up. If you did a good gravity turn your periapsis should be relatively close to 0 or even above by the time your apoapsis reaches 80km.

PS: You can throttle your boosters in the VAB (thrust limiter option when right clicking them). I usually set boosters so I have about 1.5 TWR at launch.

[Slam_Jones]

Also, one of the things I do to keep dV low is to keep my TWR at the right amount. I believe it's 1.65 TWR from launch, and 1.45 TWR at higher altitude. If you have KER (Kerbal Engineer Redux) it will display these figures.

Edit: And yes, solid boosters are very inefficient, especially at higher altitudes.

[Janos1986]

DeltaV alone won't do, watch your TWR.

It should be around 1.7 to 2 for lifting stages that swim in the souposphere and 1.3 or more for upper stages and thinner air.

Obviously staying in terminal velocity is the key to avoid wasting fuel.

Also, you should care about your ascending trajectory If it's too steep circularizing will be a pain, if you level too early air will kill your speed.

You'll find your balance.

[qoonpooka]

I'll give the thing a tweak to 1.7 TWR for the BACC and 1.5 for the RT-10 and report back. Thanks for the advice!

EDIT: Did so. Results were uninspiring.

Sounds like your gravity turn is way too high. I believe a general guideline is ~45° at 10km and at about 30km you should be burning almost horizontally. This is a rough estimate since I myself am playing with NEAR and that forces me to automatically do a good gravity turn or I`ll have to burn straight up. If you did a good gravity turn your periapsis should be relatively close to 0 or even above by the time your apoapsis reaches 80km.

PS: You can throttle your boosters in the VAB (thrust limiter option when right clicking them). I usually set boosters so I have about 1.5 TWR at launch.

Also, one of the things I do to keep dV low is to keep my TWR at the right amount. I believe it's 1.65 TWR from launch, and 1.45 TWR at higher altitude. If you have KER (Kerbal Engineer Redux) it will display these figures.

Edit: And yes, solid boosters are very inefficient, especially at higher altitudes.

So I set the boosters to 1.7 and 1.5 TWR and started my gravity turn gently at 500m. (It takes forever for this thing to pitch over, no fins on it, so it's all the capsule's torque). 45 Deg @ 10k, basically flat @30k....

Apoapsis never got above 55km. Ran out of gas @ 32km and drag did the rest.

DeltaV alone won't do, watch your TWR.

It should be around 1.7 to 2 for lifting stages that swim in the souposphere and 1.3 or more for upper stages and thinner air.

Obviously staying in terminal velocity is the key to avoid wasting fuel.

Also, you should care about your ascending trajectory If it's too steep circularizing will be a pain, if you level too early air will kill your speed.

You'll find your balance.

Trying a stronger 'souposphere' stage and toning down the 2nd solid.

2nd EDIT: Same results, except this time, as I'm burning horizontal @30km, my apo is falling very slowly. My prograde direction of flight is horizontal when my thrust is 45-degrees. am I misinterpreting instructions about 'flying horizontal' here, and I should be parking my prograde indicator there? Or my nose there?

Edited January 13, 2015 by qoonpooka

[Streetwind]

Orbital velocity is about 2200 m/s (give or take some depending on where your Ap and Pe are). That's about half the budget. The other half is consumed in order to offset four sources of losses.

1.) Steering losses. Anytime your rocket is not pointing fully prograde, your ascent is inefficient because you have a radial or normal component in the thrust vector that isn't helping you accelerate. This includes gimbaling the rocket engines to keep yourself pointing prograde, because that causes them to point away from prograde. The closer to prograde you stay, and the less control input you need for it, the smaller your steering losses will be.

2.) Gravity losses. Anytime your rocket is not burning flat towards the horizon, your ascent is inefficient because a part of the thrust is wasted simply on keeping you from losing height... "maintaining the status quo", if you will. Only the remaining thrust will actually contribute to accelerate you. The closer you come to burning flat towards the horizon, the smaller your gravity losses will be.

3.) Aerodynamic losses. Any objet moving through an atmosphere is slowed down by drag, which is a force that depends on the thickness of the atmosphere and increases with the square of your velocity. The slower you move through the atmosphere and the thinner the atmosphere is, the smaller your aerodynamic losses get.

4.) Oberth Effect, and the Rule of Constant Speed. The former says, simplified: the faster you go, the less effort you need to go even faster. The latter says: if you keep your speed constant, then for the same amount of effort you will fly farther and get there quicker if your speed is higher. Or, in other words: if at any point your engines are off and your speed is going down, you're doing it wrong, because when you re-light your engines, you will be much slower than you were when you turned off your engines, and thus you will get much less out of the effort you expend! Should have just kept your engines on!

Now, you'll quickly notice that these points contradict each other. You gotta stay slow - but you need to reach orbital velocity! And the slower you are, the less you get out of your engines! And you also need to burn towards the horizon - but you'll fall back down to the ground if you do that. Unless you go really fast. But you gotta stay slow! Also, how are you supposed to fly without steering?

This is why the optimal way to orbit is a compromise between all four of these factors. It is called a "gravity turn": you touch your controls exactly once for a split second shortly after launch, creating a tiny flick to the side, and then zoom hands-off all the way to orbit, with the rocket turning itself for you through gravity pulling the nose down at exactly the same rate the prograde marker drops towards the horizon, ensuring you stay fully prograde at all times. And of course, you never stop burning for a single second except for staging. By the time you turn your engines off, you are already circularized.

This is, of course, a fairly difficult feat to pull off without computerized aid - it's the crown jewel of rocket piloting. And also of rocket construction too, because not all vehicles even lend themselves to this profile. Many of the creations possible in KSP simply can't do it, because your engine TWR curve needs to be exactly right and your vehicle needs to be shaped in a certain way. As such, pretty much everyone "fakes" the turn by steering manually.

The 4500 dV figure quoted by dV maps is a figure that's valid for a pretty well-flown fake gravity turn into a fairly low orbit. If you need more to get there, then you need to look for where you have losses that exceed those expected from an optimal profile. Based on your description, I'd say the following are your issues:

- You go too fast too early, where the atmosphere is still thick and soupy (as orangexception already said)

- Your steering is not close enough to a smooth, gradual pitchover (but this is probably by far the smallest problem)

- Because you burn too hard too early, and don't turn naturally, your apoapsis rises too quickly and you need to turn your engines off, causing you to be really slow upon reaching apoapsis and needing a huge amount of fuel to accelerate again

- Also, due to your coasting you're spending a longer time until completing orbit insertion, thus losing more speed to gravity than you would normally do

Edited January 13, 2015 by Streetwind

[WanderingKid]

Any chance you can post your craft file, qoonpooka? Want to take it for a spin and see if I can tell anything obvious about it.

[qoonpooka]

This is why the optimal way to orbit is a compromise between all four of these factors. It is called a "gravity turn": you touch your controls exactly once for a split second shortly after launch, creating a tiny flick to the side, and then zoom hands-off all the way to orbit, with the rocket turning itself for you through gravity pulling the nose down at exactly the same rate the prograde marker drops towards the horizon, ensuring you stay fully prograde at all times. And of course, you never stop burning for a single second except for staging. By the time you turn your engines off, you are already circularized.

This is, of course, a fairly difficult feat to pull off without computerized aid - it's the crown jewel of rocket piloting. And also of rocket construction too, because not all vehicles even lend themselves to this profile. Many of the creations possible in KSP simply can't do it, because your engine TWR curve needs to be exactly right and your vehicle needs to be shaped in a certain way. As such, pretty much everyone "fakes" the turn by steering manually.

Yeah, I've been doing the 2-degree tip at liftoff since I read that this is how you gravity turn, but then non of my rockets have ever kept tipping. I try to do it slowly, to keep my thrust aligned with my prograde vector, but this seems to involve me not turning fast enough because I wind up with a very steep rise to apo, and exactly the long coasting period you describe as Verrah Bad.

I wonder if turning off SAS would let the thing tip on it's own better? But you also say that this is probably the least of my problems so I'm trying to focus elsewhere. It sounds like I may need to just swallow the (much) higher pricetag of liquid stages for lifting and save solids exclusively for kick-starting.

- - - Updated - - -

Any chance you can post your craft file, qoonpooka? Want to take it for a spin and see if I can tell anything obvious about it.

https://drive.google.com/file/d/0B7Rn2EgE0DrDYnBUbG9uaU1WcW8/view?usp=sharing

Not sure if google drive is the best way, but Dropbox wants money.

[Starhawk]

I would advise you to watch your time to apoapsis carefully, along with your apoapsis itself. These are helpful in determining whether your rate of turn is too sharp. If you tip far too early, the atmo kills your velocity.

As you do your gravity turn, watch your apoapsis to make sure it is increasing as well as moving farther out from your craft, but not too far. Once you see your apoapsis at about 75 to 80 km you can kill your engines and coast until you get fairly close to the apo and burn for circularization.

As I understand it, the aerodynamic model currently employed in stock does not lend itself to true gravity turns at all. I guess what I do is a simulated gravity turn - the KSP way.

[RocketScientistsSon]

This is why the optimal way to orbit is a compromise between all four of these factors. It is called a "gravity turn": you touch your controls exactly once for a split second shortly after launch, creating a tiny flick to the side, and then zoom hands-off all the way to orbit, with the rocket turning itself for you through gravity pulling the nose down at exactly the same rate the prograde marker drops towards the horizon, ensuring you stay fully prograde at all times. And of course, you never stop burning for a single second except for staging. By the time you turn your engines off, you are already circularized.

This is, of course, a fairly difficult feat to pull off without computerized aid - it's the crown jewel of rocket piloting. And also of rocket construction too, because not all vehicles even lend themselves to this profile. Many of the creations possible in KSP simply can't do it, because your engine TWR curve needs to be exactly right and your vehicle needs to be shaped in a certain way. As such, pretty much everyone "fakes" the turn by steering manually.

This explains everything I have noticed empirically. I almost cried reading this, it clears up so many questions i felt too stupid to ask. thank you!

[Streetwind]

If you have any significant reaction wheel torque, you are going to have to turn SAS off to let the rocket turn naturally, yes.

However, KSP has its own quirk regarding the gravity turn: because the atmosphere is modeled poorly, turning too early actually hurts you. Try starting your turn at 5 to 6 km.

Also, don't blindly obey rules like "point towards the horizon at 30km". Those are rules of the thumb which people use to orient themselves, not laws of physics. If your apoapsis is not climbing anymore, you should under no circumstance be burning flat towards the horizon unless your apoapsis is already outside the atmosphere at that point. Your actual ascent profile will vary somewhat with each different vehicle.

If your prograde marker is pointing towards the horizon, and your're burning at 45 degree, you incur major losses for not pointing prograde. This is a result of not enough upwards kick (the opposite problem to having long coast phases, which are the result of too much upwards kick), and usually happens when you turn too early in stock KSP, which is why I'm suggesting you wait a little longer above.

Edited January 13, 2015 by Streetwind

[Red Iron Crown]

Oberth Effect...says, simplified: the faster you go, the less effort you need to go even faster.

That's not a very accurate explanation of Oberth, even in simplified form. I wrote a little summary of the Oberth effect and some of the misconceptions around it that might be worth a read.

The rest of your post is a great explanation, well done.

[Streetwind]

Oh, an addition:

This is an excellent video on flying gravity turns. Scott here is mostly complaining about people turning too late, but he also demonstrates how turning too early also doesn't help.

Pay attention especially to the very first flight, which he pilots manually. Observe his control input, the point at which he starts turning, how he has SAS off and how the rocket follows the prograde marker. It's not an ideal, natural gravity turn, but for a manually faked one, it's quite decent - better than many players routinely do. He has a bit of a coast phase, but then again, he is aiming for a 100km apoapsis, and coasting through space doesn't hurt as much as coasting through the atmosphere. Also note his speed at the point he starts the circularization burn. He'll have something to say on that later in the video.

EDIT: Oh come on Red, you can't math-slap newcomers like that and you know it Ascending into orbit ultimately is a change in the size of your orbit, because you're technically in orbit around the virtual center of Kerbin while your periapsis is below the surface. Thus you can make this simplification with a little bit of squinting.

Edited January 13, 2015 by Streetwind

[Slam_Jones]

Gravity turns don't need to be perfect, but the rate at which you do them makes a huge difference.

Here's my typical ascent profile (I usually use about 4,700dV to get orbit):

(My eyes area all but glues to the TWR readout btw, never letting it rise about 2.0)

Straight up at launch.

Point at 5 degrees at 1000m

Point at 35 degrees at 9km (typically due to TWR etc, the actual prograde will be dragged to about 45 degrees @ 10km)

Maintain until apoapsis starts to approach 50km

At each 5km past 50 (55km, 60km, 65km) I drop another 15 degrees

(So basically 60 degrees at 55km, 75 @ 60km, 90 @ 65km)

When my apoapsis reaches 70km, I drop almost all my thrust ( down to about 10% throttle usually). My goal at this point is to reach my apoapsis at the same time I finish circularizing. I typically keep my eye on vertical speed at this point.

I'm usually not flattened out til about 50km, or when my apo is at 70km or above.

Edited January 13, 2015 by Slam_Jones

[Keras]

2nd EDIT: Same results, except this time, as I'm burning horizontal @30km, my apo is falling very slowly. My prograde direction of flight is horizontal when my thrust is 45-degrees. am I misinterpreting instructions about 'flying horizontal' here, and I should be parking my prograde indicator there? Or my nose there?

I guess my estimate was a little too far off, I have been playing with NEAR for a long time and don`t remember too well how thick the stock souposphere actually is

What the others posted should help you better with stock atmosphere. A more general advise would be to just watch your time to apoapsis. As long as it still rises you are good. If it rises fast you should lower your nose, if it rises slowly keep it steady and if it decreases you have to rise your nose. You should always keep that above 30 sec and not much higher than 1 min, if you can`t do that you need more thrust or a higher gravity turn.

[WanderingKid]

https://drive.google.com/file/d/0B7Rn2EgE0DrDYnBUbG9uaU1WcW8/view?usp=sharing

Not sure if google drive is the best way, but Dropbox wants money.

Since when?

Anyway, first note. Craft file has 2.0 TWR on first booster, and 1.21 on second. Adjusted to 1.6 starting and 1.5 starting, and test flight:

BACC burnout: ~6,400 and a little over term velocity. Began slow turn towards 45 degree here.

RT-10 burn: Had to fight to get it down near 45 degrees reasonably by 10k. Drag's a bit feisty. 45 degree pitch gets craft ~25 degree prograde burn.

RT-10 burnout: ~16k. Speed 513.7, just barely over Term Velocity. You WILL fight drag on this thing. Add in a small reaction wheel or something to help. Apo was at 19.1k when burnout concluded, 200.02 vertical and 313.44 horizontal.

-909 Burn: Starts at a TWR of ~1.6, good. Kept 45 degrees until 20k then started usual slow pitch down. 20k height - 35 degree pitch. 24k height - 25 degree pitch. Around 29k Vertical 'pops' to speed again. Lowered to 10 degree pitch (basically prograde).

-909 Burnout: 38k up, 1710.71 m/s horizontal, 201.64 m/s vertical. Apo height 48,914.8m.

Craft is ~500 m/s short, but not according to KER. The RT-10 stage is costing you a ton in drag. I barely broke term velocity with an engine that, at the end, is a 2.71 TWR. This is your heaviest loss of dV, as far as I can tell at first glance.

Second launch: Straight up until 35k. Pretty sure drag is the issue for dV loss. RT-10 cuts out at 25k. 2,150 m/s left for the -909 at this stage. That's not enough to achieve orbital speed.

It's not you, and it's not your piloting. Someone else may be able to give you the specifics, but you've got a massive drag issue you need to compensate for. It looks like you're trying to do this as an 18t orbital lifter. I'm not sure you can go orbital without liquid fuel here. 2 -400's with a LV-T45 and two radial RT-10s with thrust limiting for initial liftoff would get you where you need to be instead of the double SRB only.

[Streetwind]

RT-10 burn: Had to fight to get it down near 45 degrees reasonably by 10k. Drag's a bit feisty. 45 degree pitch gets craft ~25 degree prograde burn.

Not drag losses, but rather steering losses - if I understand you correctly here, you were far off of pointing prograde. You should have turned less hard and kept closer to the prograde marker. Then more of the RT-10's thrust would have gone towards accelerating the craft rather than keeping it hovering while it shimmied sideways unenthusiastically.

I'll download this right now and give it a try.

[qoonpooka]

Guys, you're all awesome. I managed to wrestle this thing into orbit with ~100m/sec of dV left for the de-orbit. (I was tempted to try having the pilot get out and push but I'm crap with RCS controls.)

Given the losses described above, I'm happy with only losing ~200m/sec of dV on the ascent. This thing can park capsules in orbit, with science on board, and it involves - as described by a couple of people - tacking on a flight computer and watching the apo and time to apo. The analysis of what that RT-10 was doing is super useful for diagnosing future problems too. It is comforting and frustrating that it's not my piloting. My piloting I can fix. Swapping out to an LV-30 stage is expensive, but now I can at least make more informed decisions about it. Changing to 'Answered.'

Two things struck me about this craft, though:

1) I need to make sure the PV panel is facing the sun before I get broadcast happy. Ran it out of power so my de-orbit was a hail-mary because my retrograde marker was at all visible on the navball.

2) The legs are too high. I wind up landing on the engine and the whole craft comes apart, losing all the scientific equipment... oops.

[numerobis]

SRBs have 50% more drag than most rocket parts. When you say "terminal velocity", are you taking that into account? In any case, if the hypothesis is drag, the easiest way to determine the cost of drag losses is to throw on a MechJeb node and look at the recorder.

For a gravity turn in stock aerodynamics, if all your parts are the same drag coefficient, your spacecraft won't tilt to follow the prograde marker -- there's no torque! If you have SRBs at the back and normal rocket parts at the top, you get some torque, not always enough. With more advanced SAS you can set it to follow prograde directly.

Used to be, the optimal (for a particular craft that was largely reasonable) was to start turning at 7 or 8 km, and finish only around 40 km, so the advice to be horizontal at 30km seems low to me. It depends on your TWR though; the lower the TWR, the more you want to pitch up.

[WanderingKid]

Term Velocity as per KER, nothing more extravagant.

Yeah. That particular craft has really good top-end TWR. Starts at 1.6, which is high for my usual upper stages. The reason I switched to near horizontal at 30k was I had 202+ m/s vertical, and figured a spaceplane vector with so much thrust would carry it.

I've never said I'm an expert pilot, was just looking for the obvious reasons it wasn't working.

[UmbralRaptor]

Since no one has mentioned it, in stock, boosters are draggy, suffering 50% more drag per unit mass than most other parts. As such, needing somewhat more ÃŽâ€V that those charts (which tend to assume all liquid designs due to their age) is unsurprising.

After some playing around, I was able to get what I think is your ship into orbit.

That said, what are your goals? Replacing the BACC with a pair of RT-10s that use explosive staging would likely reduce costs and enhance performance. If you're merely acquiring science from near/far kerbin space, I would lean towards an all-solid suborbital design. If you want a long term science return sat, I would skip on the materials bay and if crewed thermometer. Crew reports are adequate.

Edit: ninja'd by numerbois.

Edited January 13, 2015 by UmbralRaptor

[Streetwind]

Yeah, managed to get this thing into orbit on try 2. Geez, I haven't flown stock aero in ages, and it shows!

Had the BACC set to 1.6 ignition TWR, the RT-10 to 1.3. This turned out to be very little, especially on the RT-10, and I had to do a super slow turn to avoid the prograde marker dropping like a rock. I was barely even at 45 degrees by the time the RT-10 burned out at ~26km or so.

A third try with the BACC at 1.64 (77.5%)and the RT-10 at 1.49 (42.5%) went much better, I was able to fly a flatter turn and got into orbit with 276 m/s left.

Glad to hear you worked it out in the meantime, too!

[qoonpooka]

That said, what are your goals? Replacing the BACC with a pair of RT-10s that use explosive staging would likely reduce costs and enhance performance. If you're merely acquiring science from near/far kerbin space, I would lean towards an all-solid suborbital design. If you want a long term science return sat, I would skip on the materials bay and if crewed thermometer. Crew reports are adequate.

Goal was to pick up materials science from upper atmo on ascent, grab a full-value temp reading before I park a probe in Kerbin orbit to do it for me, later, and do EVA flyovers in orbit - then de-orbit and recover everything.

Settled for a partial success.