How to set up mechjeb for this craft ?

[Numerlor]

It starts cirularizing too early or too late, I've tried different turnshape etc. but I don't understand those numbers too much

http://www.filedropper.com/eternityi

Edited April 9, 2017 by Numerlor

[DStaal]

Honestly, I use this mod instead for launches, it seems to do a better job:

That said - I'm not sure you have enough TWR on your stage 2 there, at least for a gravity turn-style launch.  Your *vacuum* TWR starts at 0.68 - and for a gravity turn you'd still be in the atmosphere at that point, lowering that further most likely.  You'd need that above 1.2 in my experience at the very least.

On the other hand, you've likely got plenty of TWR on your lower stages - putting in another fuel tank on the main lifting engine would allow that stage to burn longer, negating that need.  Even an extra 500-600 m/s might be enough - especially if you do a vertical launch with circulization burn at the top, which this rocket seems more built for.

(And why do you have a decoupler for the engine at the bottom?  You're just shedding it with the tank above - leave it attached to the tank, and save yourself the mass.)

[Numerlor]

I've already got this into orbit using mechjeb and myself but mechjeb alone can't do the job and as I want this for Emulating real life thread it is needed to keep it mostly for mechjeb
and that decoupler on engine is because I want to recover it

Thanks for pointing to that TWR of stage 1, Didin't notice that and I can incerase my  ΔV by quite a bit

Edited April 9, 2017 by Numerlor

[Numerlor]

Now I have noticed that everything goes bad with second stage as seen here (Modified rocket with advice from @DStaal) Any idea why?

 

Edited April 9, 2017 by Numerlor

[DStaal]

Not exactly sure what you mean by goes bad there - but I notice your TWR is still below 1.

[Numerlor]

Mechjeb no longer follows preset curve to pick up speed but stays pointed at the same spot until it reaches 75km apoapsis

[Numerlor]

I kinda fixed this by adding one more tank and 10t more weight, it still goes completly wrong but that wrong is right

[sarbian]

On 4/10/2017 at 6:16 AM, Numerlor said:

Mechjeb no longer follows preset curve to pick up speed but stays pointed at the same spot until it reaches 75km apoapsis

nope

[DStaal]

1 hour ago, sarbian said:

nope

I think he was saying that's the behavior he was seeing with this craft - and he's trying to figure out *why* it's doing that, when that's not what Mechjeb is supposed to do or what he's expecting.

Basically: That's the error.  The solution eludes us so far. 

[Hodo]

Wow that launch TWR is a bit excessive.  Most of my launch vehicles dont have much more than 1.4TWR at launch, and stay around 1.0 till orbit then it drops to about 0.4.   

But it sounds like you are having issues with your launch profile more than the MJ program.

That and you have no real control on the final stage there.  No RCS, no fins, no real SAS of note.  I dont think the Poodle has much gimble on it.

[StahnAileron]

For the kind of TWR profile you have, I think it'd be better if you do a proper gravity turn (like the mod linked earlier) rather than using Mechjeb.

I've been recently using high TWR rockets. I can't can get them to orbit manually using the same methodology that the GravityTurn mod uses: Throttle-limiting to maintain a time-to-apoapsis. MechJeb attempts to get you into a gravity-turn-like profile. In other words:

• GravityTurn controls the throttle, letting gravity naturally "pull down" your trajectory as needed. You just need to keep pointed pro-grade. (If you ever need to adjust pitch manually beyond the pitch-over, you did it wrong or your TWR is too low. I've done both...)
• MechJeb keeps the engine at 100%. You need to keep TWR reasonable for MechJeb to work well in my experience. Otherwise you need to tune MJ's profile to match the TWRs you have.

GravityTurn itself has issues with unorthodox, high TWR, and/or non-throttling (SRB-heavy) designs. After watching and learning from GT, I can do the equivalent myself just about as well. (Just not quite as consistently.) I've been using high TWR designs lately mostly because I'm an impatient stand-up guy. Still, I can usually get into a 75km - 100km orbit using 3400m/s dV or less regularly. I've done this with a High-TWR Lower and Low-TWR Upper stage design. (I had to be a bit more aggressive with the lower stage's climb, I think, to make sure I had enough vertical speed for the low-TWR upper to have enough time to add on horizontal velocity.) GT tries to guess the pitch-over profile for you. Everything else is pretty straight forward. It can be guesswork, so I just fly manual gravity-turns now. I can usually get a feel for when and how hard I need to pitch over for a design.

With MechJeb, since it keeps the throttle at 100%, you get a much steeper climb in the middle section if the TWR is too high. It won't adjust the throttle until you high your target predicted apoapsis height. (MJ is Full/Off only for engines.) Judging from that video you posted, that sees to be the problem you have. I've had the same problem as well with some of my designs and MJ. (Back in 1.0.5... I've long since stopped using MJ for ascent control; MJ was too much abstract guesswork for me in tuning the profile for each launch.)

Try flying the rocket manually and keeping the time to apoapsis somewhere between 40-60 secs by adjusting the throttle on-the-fly. How quickly and aggressively you pitch over is dependent on your TWR. With >2 TWR, you can be fairly aggressive. The low TWR on the last stage may require you to have a steeper climb depending on when you ditch the stages.

Edited April 12, 2017 by StahnAileron
Grammar Correction, 2nd Para.: can't <=> can

[Numerlor]

8 hours ago, StahnAileron said:

For the kind of TWR profile you have, I think it'd be better if you do a proper gravity turn (like the mod linked earlier) rather than using Mechjeb.

Problem is that OP of the thread uses MJ to exclude human error and rockets are flow only with it

@Hodo for hand and higher than ~30km  that gimbal is more than enough, mechjeb doesn't like it but it only affects it out of atmosphere

Edited April 12, 2017 by Numerlor

[StahnAileron]

50 minutes ago, Numerlor said:

Problem is that OP of the thread uses MJ to exclude human error and rockets are flow only with it

Oh, then just use the GravityTurn mod WITH MJ: It has MJ integration for the final circularization burn at apoapsis. Totally automated other than the pre-launch set-up (which the current version tries to guess for you automatically now; override if you wish). Or something goes wrong...

I'd be a little worried about that final stage though: it seems to be carrying a good portion of the launch dV. (I personally seem to prefer about 2000-2500m/s dV in my lower stages before a low-TWR orbital upper takes over for my launch profiles. A proper GT is under low TWR thrust at that point anyway.) TWR isn't TOO much of an issue so much as what the dV spread across those TWRs is. I generally feel a little uncomfortable unless my lowers can get me above about 40km (with a good trajectory) before my upper takes over.

So yeah: Give the GT Mod a try. (See @DStaal's post for the link.) I found it far better/efficient than MJ's Ascent Guidance. (MJ's more brute force, GT is more finesse. I suppose the former is more Kerbal-ly.)

Edited April 12, 2017 by StahnAileron

[Nicias]

On 4/12/2017 at 4:21 AM, StahnAileron said:

Oh, then just use the GravityTurn mod WITH MJ: It has MJ integration for the final circularization burn at apoapsis. Totally automated other than the pre-launch set-up (which the current version tries to guess for you automatically now; override if you wish). Or something goes wrong...

.....

So yeah: Give the GT Mod a try. (See @DStaal's post for the link.) I found it far better/efficient than MJ's Ascent Guidance. (MJ's more brute force, GT is more finesse. I suppose the former is more Kerbal-ly.)

GT for the win.