Gravity Turn ascent trouble.

[antipro]

hi, after several launches Gravity Turn gave me this values:




This is the probes launcher:




These are two videos that show the same thing:
as soon as the first stage detaches, the launcher no longer can maintain the alignment with the prograde direction
and for some seconds it pull up its nose in a non-aerodynamic fly.
I just would like to understand why of this behavior? have any ideas?
I don't know... too long fairing?

Full launch: https://www.dropbox.com/s/69kj8n4zjilhxyy/launch.mkv?dl=0
Short launch: https://www.dropbox.com/s/ey9wt7hvsla5qcz/short launch.mkv?dl=0

Screenshot: (look navball aligment)

[AHHans]

Your time to apoapsis (AP time) is dropping below the 50s that you told the mod to keep. So the mod points the rocket more to upward to increase the AP time. The first two stages had enough TWR to to the AP time at 50s even with this flat trajectory, but the third stage has lower TWR so it struggles to keep the AP time up.

1 hour ago, Superfluous J said:

You seem to have a very beefy, squat launcher for a very large, light payload.

The yaw control input isn't maxed out, so it isn't a problem with aerodynamic instability.

[AHHans]

5 hours ago, antipro said:

I know what you mean, the 2nd stage has no fins to help the rocket keeping direction.

As I wrote in my other answer: the problem is not that the rocket has a problem keeping the orientation that the autopilot wants it to have, it is that in order to keep the time to apoapsis from falling to autopilot steers the rocket further up.

Have a look at the thrust setting at the navball and the yaw indicator in the bottom left: while stage 6 is running the autopilot throttles down in order to not increase the time to apoapsis above 50s while still pointing prograde. After staging it first increases the throttle to max, and when that still isn't enough it yaws to the left to raise the nose and increase the time to apoapsis again.

[Spricigo]

2 minutes ago, Superfluous J said:

Of course not.

I'm a bit surprised that the rocket doesn't have aerodynamic instabilities, and I'm sure of the OP continues making rockets like this there will be issues eventually.

But more on point, the high TWR is why the mod didn't work as advertised. Gravity Turn is designed to work with efficient rockets.

Higher TWR don't cause aerodynamic instabilities, it may cause overheating  and it certainly increase the drag losses (gravity losses on the other hand, will decrease. Net effect is "depends"). However this craft is not overheating and is quite capable of reaching the desired orbit, how adding fuel  will improve it? Exta fuel don't seem to be necessary or useful. Maybe you are trying to fix what is not broken (and only making the craft more expensive in the process). 

As for why the mod supposedly didn't work as advertised, @AHHans,  already nailed it: it works exactly how it is set to work. I'd try to increase the [Hold AP Time Start] to 80, if I understood the parameter correct, that will keep the upper stage pointed prograde while the time to Ap drops from 80s to 50s.

 

1 hour ago, Superfluous J said:

Gravity Turn is designed to work with efficient rockets.

IMHO efficiency is an overused word by KSP players. *shrugs*

[AHHans]

40 minutes ago, antipro said:

sure, I've reproduced your version:

Thanks!
. o O (hmmm.... interesting...)
It pitches over more aggressively than I do. (Which is not surprising, I like getting out of the atmosphere early!) What I didn't understand at first is that it doesn't throttle up after staging: apparently it doesn't know about the TWR of the stages and just keeps the throttle setting the same until the AP time decreases to the lower limit, then it increases the throttle to what is needed to keep the AP time constant.

Again we see that the difference between a "perfect" gravity turn and a "meh, good enough" gravity turn is very small. So there is IMHO little use in trying to get ones strategy perfect. (In contrast a way "too flat" or way "too steep" ascent will cost you significant dV!)
And as already discussed: getting of the launchpad with a too high TWR will cost you dV, either in gravity losses (too steep ascent) or in atmospheric drag (matched gravity turn that then stays too long in the lower atmosphere) or by lugging around an oversized engine that you aren't using (if you throttle down from the very beginning).

2 hours ago, antipro said:

So you won!

Well IMHO I won^(TM) if you got a better understanding how the launching a rocket works and what to look out for to improve your rockets and ascent profiles.

[Serenity]

Try putting the majority of your SAS modules closer to the engine rather than split apart.

Maybe that will be enough to hold it?

I think separating your rocket at 30k altitude  for that second stage design is way too optimistic to hold prograde.

It needs like 50k maybe at least.

The more top heavy you are the worse things you make with such a long fairing.

Edited November 17, 2020 by Boyster

[Superfluous J]

You seem to have a very beefy, squat launcher for a very large, light payload.

What is the TWR at launch?

you would likely benefit from dropping the side boosters and going for a more standard staged launcher, so you keep some mass higher up the rocket while still in atmosphere.

[Zhetaan]

7 hours ago, antipro said:

These are two videos that show the same thing:
as soon as the first stage detaches, the launcher no longer can maintain the alignment with the prograde direction
and for some seconds it pull up its nose in a non-aerodynamic fly.
I just would like to understand why of this behavior? have any ideas?
I don't know... too long fairing?

The launcher absolutely can maintain the alignment; it is instead choosing not to do so.  @AHHans has the right of it; pay close attention to the attitude control needles in the lower left corner of the screen.  They barely move; a rocket that tries and fails to overcome aerodynamic instability would have one or all of those needles hard against the edge of the gauge, indicating that even full-output control authority is not enough.

Instead, to flesh out what @AHHans was telling you, is that after your first two stages burn out, your third stage has a much weaker (I assume vacuum-rated) engine that cannot push the apoapsis out far enough ahead of the rocket to keep the set time-to-apoapsis.  Therefore, it has to make use of mathematical trickery and use some radial-out thrust to shift the orbit (and the apoapsis with it).  Typically, this also lowers your periapsis, but since you're still in a powered ascent and the geometry of the orbit is right, the actual effect is only to slow the periapsis's rate of increase (that is only barely perceptible; although what your rocket is doing is inefficient, it's obviously not going to jeopardise the launch).

If you want to stop this behaviour, then you're going to need a different time-to-apoapsis target that allows you to get a bit higher before your stage separation.  Whether that means unlocking the finish time field or changing your launch profile completely, I don't know.  I do notice that you're achieving orbit, unstable though it may be, at only 40,000 metres, which I admit that I don't prefer (I dislike risking that much atmospheric heating, and that goes more so for fighting drag losses the way I saw your rocket doing every few seconds), but your exceptionally flat ascent trajectory is a product of your high-powered initial launch stages.  Your vacuum engine simply doesn't have the power to keep up at that altitude while also maintaining a fully prograde orientation, so it doesn't; another way of describing the radial-out burn is that it increases the eccentricity (at least in this case; it's not a fixed rule) and raises the apoapsis (and the ascent path) that way.

Edited November 17, 2020 by Zhetaan

[swjr-swis]

10 hours ago, antipro said:

I just would like to understand why of this behavior? have any ideas?
I don't know... too long fairing?

I think @AHHans has it right, but would like to also point out that fairings can generate an enormous amount of drag -and thus torque- when deviating even slightly away from directly into the airflow (usually surface prograde). So a very short propulsion stub with a huge fairing on top that is still in quite thick atmosphere needs to stay pointing very close into the airflow to avoid getting overwhelmed by sheer drag forces. If your autopilot is trying to fly otherwise due to its programmed flight path, it's going to have trouble.

Edited November 17, 2020 by swjr-swis
torgue? ::looks at keyboard, then at fingers:: seriously?

[N_Danger]

 I noticed that your initial settings are for start turn at 12.5 m/s and with a 32.48 degree angle. 

I generally go with a 50 m/s start and a 20 degree angle depending on T/W.  You need to zoom out of the thicker atmosphere or you will end up going too fast with to much aerodynamic pressure on that large fairing when the autopilot tries to correct for your hold to AP time as others have noted. Check the T/W for each stage at the altitude of separation, for me keeping a T/W of at least 1.5 through out the ascent is best for my designs. 

 Don't always take the recommended initial numbers from AG as gospel. I always experiment with every new vehicle playing with the start m/s and turn angle to find the best profile. And for me the best profile is defined as a 20 to 50 m/s burn to circularize. 

[Snark]

9 hours ago, Boyster said:

Try putting the majority of your SAS modules closer to the engine rather than split apart.

Maybe that will be enough to hold it?

That's... not how it works.  The location of reaction wheels has no effect on their effectiveness.  Detailed discussion here, for anyone who's interested.  TL;DR is that it doesn't matter where you put the reaction wheels.

9 hours ago, Boyster said:

The more top heavy you are the worse things you make with such a long fairing.

Actually, it's the other way around, in terms of aerodynamic stability:  top-heavy is good for stability.  It's being bottom heavy that can be a problem, though it's not clear to me whether the OP's problem here is aerodynamic instability versus just having an autopilot mod that has issues with the TWR of the craft.  I kinda get the impression that it's more about mod behavior than aero stability per se.

 

9 hours ago, antipro said:

I know what you mean, the 2nd stage has no fins to help the rocket keeping direction.

Where's the CoM of the second stage?  It looks pretty bottom-heavy to me.  If the CoM is way down near the bottom, then adding fins there won't help much-- they won't have much lever arm to work with because they'll be too close to the CoM.

[AHHans]

10 hours ago, antipro said:

This is the craft file if maybe someone is interested or thinks to do it better:

Of course I can do it better! You all are doing it wrong all the time! (*)

Well, I couldn't resist, so I downloaded the craft and after some fiddling around (a skipper engine is on the weak side) got to the following: on the rocket I filled the ADPT-2-3 tank full again, but removed the two SRBs. That brought the TWR on the launchpad down to 1.66 (and is cheaper).  For the launch I did (manually flown, I don't have any autopilot mod installed):

• start at full throttle
• pitch over 10 deg when it gets to 40-50 m/s
• set SAS to prograde when the prograde marker gets to the 10 deg (and then keep that for the full ascent until the circularization burn)
• throttle down as needed to keep the TWR at about 2
• throttle back to full at/after staging
• ditch the fairing when passing 60 km altitude
• keep burning until the AP reaches 120 km, the coast and circularize

That resulted in a significantly steeper ascent than your examples, it passes roughly through the marker of 45 deg pitch at 10 km altitude. And it had 1900 m/s dV left after circularization.

@antiproI would really like to see what the Gravity Turn mod does with the craft as I suggested. (Will it be better than my ascent strategy?) So you could do me a favor and post a description (video?) of that.

[Edit:] Two images, craft in VAB, and in orbit after circularization

P.S. (*) Was that enough over the top to make the irony clear?

Edited November 18, 2020 by AHHans

[antipro]

28 minutes ago, Boyster said:

Try putting the majority of your SAS modules closer to the engine rather than split apart.

I'll try to add a big SAS and reduce the fairing height.

 

21 minutes ago, Superfluous J said:

What is the TWR at launch?

 

28 minutes ago, Superfluous J said:

you would likely benefit from dropping the side boosters and going for a more standard staged launcher, so you keep some mass higher up the rocket while still in atmosphere.

I know what you mean, the 2nd stage has no fins to help the rocket keeping direction.
I'll try some modification.

[Superfluous J]

2.16 TWR is about 0.9 more than I would usually use, and 0.7 above what most people seem to like.

Try to shoot for 1.2-1.5 TWR at launch. (S7 in your image). S6, which I believe is the main stack after you drop the SRBs, should be about the same.

I suggest doubling the fuel and seeing what the TWR is. If it's still over 2, add another tank. Repeat until you're between 1.2 and 1.5 TWR. Then see what your vacuum dV is. If it's 3300 or so you're good to go. If it's less, add (smaller) SRBs to give you some extra oomph off the pad.

[vv3k70r]

3 hours ago, antipro said:

I just would like to understand why of this behavior? have any ideas?

You are chaning COM in the atmosphere and this last stage is not the most flyable object.

Ist here any purpouse of liquid fuel engine in first stage? Meybe try to use only one liquid fuel engine and detach tanks?

First stage of boosters only? And ignition of liquid fuel on the top of the atmosphere? Even around periapsis? Boosters are dirt cheap to power in comparision to liquid fuel expandable stage.

 

[Spricigo]

2 hours ago, Superfluous J said:

2.16 TWR is about 0.9 more than I would usually use, and 0.7 above what most people seem to like.

Yet, seems to be quite effective if maybe not the most efficient design. 

Which brings the question: what is the purpose of your suggestions other than to bring TWR close to some arbitrary value? To make the rocket more how you, rather than the OP, like it?

 

[Superfluous J]

1 hour ago, Spricigo said:

Yet, seems to be quite effective if maybe not the most efficient design. 

Which brings the question: what is the purpose of your suggestions other than to bring TWR close to some arbitrary value? To make the rocket more how you, rather than the OP, like it?

 

Of course not.

I'm a bit surprised that the rocket doesn't have aerodynamic instabilities, and I'm sure of the OP continues making rockets like this there will be issues eventually.

But more on point, the high TWR is why the mod didn't work as advertised. Gravity Turn is designed to work with efficient rockets.

[Serenity]

8 minutes ago, Snark said:

That's... not how it works.  The location of reaction wheels has no effect on their effectiveness.  Detailed discussion here, for anyone who's interested.  TL;DR is that it doesn't matter where you put the reaction wheels.

I always though if i put  SAS is closer to the engine i can maximize the overall effect and responsiveness of the engine's gimbal, maybe it doesn't make a difference i guess.

Edited November 17, 2020 by Boyster

[Snark]

4 minutes ago, Boyster said:

I always though if i put  SAS is closer to the engine i can maximize the overall effect and responsiveness of the engine's gimbal, maybe it doesn't make a difference i guess.

It does not.

Where the engine is, relative to the CoM, does make a big difference to how effective engine gimal is at holding bearing-- that's because it has a bigger lever arm to work with if it's farther from the CoM.

But it doesn't matter where reaction wheels are.  At all.

[Superfluous J]

4 hours ago, Spricigo said:

. *snip well reasoned analysis that I don't agree with the conclusions of*

This is way out of scope for this question and is also a conversation I've had too many times over the years. I'm bowing out.

[Spricigo]

1 hour ago, Superfluous J said:

This is way out of scope for this question and is also a conversation I've had too many times over the years. I'm bowing out.

Oh well. If we can't have both, I'd rather prefer respect than agreement. Thanks for the conversation.

 

[antipro]

In primis, many thanks to @AHHans @Spricigo @Boyster @Superfluous J @Zhetaan @swjr-swis @N_Danger  and @Snark
for your valuable advices.
 

In short and basically, what @Spricigo and @AHHans said, works for that rocket:

8 hours ago, Spricigo said:

I'd try to increase the [Hold AP Time Start] to 80, if I understood the parameter correct, that will keep the upper stage pointed prograde while the time to Ap drops from 80s to 50s.

 

15 hours ago, AHHans said:

Your time to apoapsis (AP time) is dropping below the 50s that you told the mod to keep. So the mod points the rocket more to upward to increase the AP time. The first two stages had enough TWR to to the AP time at 50s even with this flat trajectory, but the third stage has lower TWR so it struggles to keep the AP time up.

Indeed, set up a [Hold AP Time Start] to 80s or more has eliminated the misalignment issue.

However, I decided to make some little rocket modifications in order to release the 2nd stage later and have a smoother ascent:
I've added a 9t fuel tank in the 1st stage and I reduced the 2nd stage ADPT-2-3 amount of fuel from 1080 to 540, for a total of 6t.

Now I can launch this new rocket version with these TWR values and GT settings to a 120km orbit with the necessary fuel to fly by eve:






This is the craft file if maybe someone is interested or thinks to do it better:
Craft File: https://www.dropbox.com/s/87kd58llg4hddb5/eve probes launcher.craft?dl=0

This is the launch video accelerated at 4x:
https://www.dropbox.com/s/zv1coe8hivugcq0/launch x4.mkv?dl=0


Ad maiora.
 

Edited November 18, 2020 by antipro

[vv3k70r]

19 hours ago, Spricigo said:

suggestions other than to bring TWR close to some arbitrary value?

If it has TWR under 1 so it taking off after wasting some fuel by burning it on the ground to fit over 1. Just do not tank this fuel.

14 hours ago, Snark said:

But it doesn't matter where reaction wheels are.  At all.

When I realise it I started to put all this things in the rovers and landers  - rockets are just attached. If I'm sending them another core rocket they can cheaply take off.

[antipro]

6 hours ago, AHHans said:

@antiproI would really like to see what the Gravity Turn mod does with the craft as I suggested. (Will it be better than my ascent strategy?) So you could do me a favor and post a description (video?) of that.

sure, I've reproduced your version:

Then I used GT to automatically find the best "Start m/s" and "Turn Angle",
subsequently I gradually increased the "Hold AP Time Start" until the second stage has no longer pulled up its nose during circularization.


Here's the GT values:

Result: ∆v left in orbit = 1964ms that compared with your 1922ms is pretty the same.

 

Now, if I compare the @AHHansversion ∆v left: 1964ms
with my version ∆v left: 1537ms ( that anyway is enough to perform an Astrogator two-nodes maneuver, to flyby eve and aerobrake into an equatorial orbit)




Finally I can say that your version costs 4.064 less, it weighs 6,8t less but with a gain of about 427ms ∆v.
So you won!

Note: According to KER data, the ∆v difference is 3504ms - 2856ms = 648ms.
I don't understand why KER gives such higher ∆v values. Seriously, I can't get it.

Here's the @AHHans version launch video, 5m14s : https://www.dropbox.com/s/fknd9b963tlsn7z/eve probe launcher AH.mkv?dl=0
Here's the same video speeded up at 4x, 1m18s: https://www.dropbox.com/s/i0zrhlw9ckwplb5/eve probe launcher AH 4x.mkv?dl=0

 

Edited November 18, 2020 by antipro

[Spricigo]

12 hours ago, vv3k70r said:

On 11/17/2020 at 10:09 AM, Spricigo said:

suggestions other than to bring TWR close to some arbitrary value?

If it has TWR under 1 so it taking off after wasting some fuel by burning it on the ground to fit over 1. Just do not tank this fuel.

I wonder two thing: 1.Why did you quoted half of my sentence? . 2how it relates with what you posted just after it?

[vv3k70r]

11 hours ago, Spricigo said:

Why did you quoted half of my sentence?

Nothing personal - just quoted intresting text, not the whole meaning.