Failing in travelling to Mun

[abickocz]

Hello there felow space explorers. I´m pretty new to this forum however not so new to KSP. Now to my problem. In screenshots below you can see a rocket. By my design (inspired a little byt by Scott Manley). And there are few problems with it. When I launch it in around 4-8 km it is almost uncontrolable. It starts to swing around and so on. When I successfully arrive at orbit I don´t have enough fuel to get me to the Mun and back I always end up with only fuel in lander. And when I land on Mun I don´t have enough fuel to even get to orbit of Mun. So any suggestions ? I know this downstage can get me to the Mun because I used it for launching 2 satellites around Mun with almost same weight as my lander on this rocket. So will you help me with this particulary hard quest for me ?  screens:

1.  https://ctrlv.cz/fFRm
2. https://ctrlv.cz/hOeQ

Edited December 20, 2017 by monstah

[Draalo]

3 hours ago, abickocz said:

When I launch it in around 4-8 km it is almost uncontrolable.

That is because you are maybe too fast at that altitude. Just guessing it from MJ readouts. And the fins in the middle stage are bad - remove them. Mount the lower ones as low as possible.

Reduce the mainengine to 70% when reaching 3km - maybe more. Dont go over 350m/s while below 15km. Dont forget to increase thrust again after decoupling the booster.

Front is draggy, avoid hard steering. Go straight up till you reach 50m/s. Then steer east, about 5-8° and follow the progrademarker all the time till you are over 25km.

If you get visual effekt you are too fast - reduce thrust (more).

 

PS: standard parachute is enough, remove the XL (also draggy) and consider using an adapter/cargo bay at the top.

One more hint: remove all monoprop from the lander can - 60kg less weight, more dV

Edited December 20, 2017 by Draalo

[Zeiss Ikon]

First thing I'd recommend is to take those fins off the (I presume) second stage; move them down to the base of the booster core.  With all that side area up front from the lander's side tanks your aerodynamics are overcoming your reaction wheel(s) and gimbals when you get up close to the speed of sound (300-350 m/s).  With the fins at the bottom of the core, they'll improve things instead of adding to the problem.

If your center engine is a Reliant (as the liquid fuel booster engines seem to be), swap it out for a Swivel -- the steering force from the gimballed engine is many times that of your reaction wheel(s).  Also, if you're not already, start your gravity turn about 100 m/s at 5 to 10 degrees (the exact figure depends heavily on your thrust to weight ratio), and once it's started, use SAS set to Prograde -- that'll keep your nose from getting far enough off your velocity vector to turn your ship over (and following the gravity turn will also save you fuel, building up horizontal velocity as you climb).

If you can get to Mun orbit without using any of the lander fuel, that lander should have plenty to get down and back up to Munar orbit, but you'll do better to put an FL-T400 tank in the center, set flow priority so the side tanks drain first, and drop them when they're empty.  That will give you plenty of fuel to get back to Kerbin, maybe even enough to circularize.  That lander isn't going to stand up to reentry, though, especially not at Munar return velocity.  You may need to make multiple high aerobrake passes, and you're likely to still have trouble with the lander can -- those things are made of cardboard and aluminum foil.

[Fearless Son]

What @Zeiss Ikon said.  Looking at your pictures, the problem seems to be that your center of drag ends up too far toward the front of your rocket compared to your center of mass.  

There are three things I see here that are contributing to your troubled ascent:

1. As your lower stage tanks drain, your center of mass gradually rises toward the nose of the rocket.
2. As your speed increases, the drag forces on your rocket get stronger, even if the altitude is slightly thinner.
3. The fins in the middle of the rocket raise your center of drag up toward the front up until the point your jettison the lower stage.

None of these things would give you a problem early in the flight, and by the time you are high up drag is so minor that it would be of minimal concern, but right in that middle ground is where all these factors are working the most against you and that is the point where things start going wrong.  The good news is the fact that you can get to those higher altitudes despite these problems means that your rocket is not a flawed design, it just needs a few tweaks.  

Primarily, I would advise you to get rid of the fins in the middle of the craft.  The ones on the bottom of the craft should be good enough.  It looks like you have a pair of tall liquid fuel boosters flanking your main booster, is that correct?  Then add a pair of fins to those as well, they certainly could not hurt.  After the boosters have expended their fuel, they will lose the fins on them but by the time you are high enough to detach them the air will probably be thin enough that you do not need as many fins anymore.  

[Superfluous J]

And if you're wondering why that rocket worked for Scott Manley and it's not working for you, the video you got it from may have been an old one from before the change to aerodynamics. In the deep dark past, you could have massively draggy things at the fronts of your rockets and not affect how they flied.

[foamyesque]

Put your lander in a fairing. The difference it will make will be extraordinary.

[HebaruSan]

1 hour ago, Fearless Son said:

1. As your lower stage tanks drain, your center of mass gradually rises toward the nose of the rocket.

That's good, though. Rockets should be heavy at the front and draggy at the back.

[Zeiss Ikon]

If this is a career, the player almost certainly doesn't yet have a fairing large enough to cover that lander.  With those side tanks, it's 3.75 m across, and the biggest a 1.25 m base fairing can expand is 2.5 m diameter.  Still, if there is a 2.5 m fairing available, that's a very good option (though with the landing legs on the outside of the tanks, you may still have a Kraken problem where they clip through the fairing).

[Aegolius13]

13 hours ago, foamyesque said:

Put your lander in a fairing. The difference it will make will be extraordinary.

This.  You have a lot of draggy parts on that lander.  The Mk 1 lander can is about as aerodynamic as it looks (i.e., not very).  And the form factor changes on top from 1.25m parachute, to small probe core, to lander can add a bunch of drag to the front.  

I suspect your're also having difficulty because those SRBs get you going very fast as they burn out and get light.  You might try to play around with thrust settings on those, as well as throttle on your main engine.  

If all else fails, try a more vertical gravity turn.  

As for the low fuel issue, improving your aerodynamics will likely help.  But you probably need (wait for it) more fuel.  I would look at maybe a Poodle-powered stage to finish your climb to orbit, circulize, transfer to the Mun and start your descent.  A bigger launch stage is probably necessary too.  If you have unlocked it, you might try to sub out the entire launch rig for a Skipper and some 2.5m LFO tankage.  That should give you low drag and a lot of throttle and attitude control.  Adjust fuel levels and add SRBs to taste.

 

[Zhetaan]

@abickocz:

There are a lot of suggestions about fixing the drag issues on your rocket, and you should consider that advice, but your problem is not really a draggy rocket; you don't have enough fuel for the mission and no amount of aerodynamic redesign can fix that.

Your atmospheric delta-V for the entire rocket is only about 4300 m/s.  If we assume you're using the Terrier (stage 2) exclusively in space, then you have roughly 3800 m/s (atmospheric) to use to get to orbit.  You need about 3500 m/s to get to orbit, so that should happen fairly reliably, except that this is where all the extra drag becomes a problem.  It's probably costing you more to make orbit and it definitely is not stable, but you are getting there, so let's keep looking for other problems.

Let's assume that you have only the 2222 m/s (vacuum delta-V now) in the Terrier for your Mun mission:  You need 1200 to get from low Kerbin orbit to low Mun orbit.  Add the 600 or so that you need to land, 600 more to take off, and 300 or so to return from Mun orbit to Kerbin atmosphere, and that requires 2700 m/s; you simply do not have enough rocket to finish the mission with your current design.  Specifically, you have enough to land on the Mun but not take off again, which is exactly your problem.

Therefore, you need to either:

1. Add more fuel and engine to your rocket.
2. Remove payload.

I'd consider switching the drop tanks from FL-T200 to FL-T400 tanks (you have enough thrust-to-weight to do this without too much trouble), but you're probably best off by adding a dedicated transfer stage in between the orbital launcher and the lander.  If you can get the lander to Mun orbit fully fuelled, then it can both land and get back to Kerbin.  But you have to deliver it to the Mun with full tanks.

Edited December 21, 2017 by Zhetaan

[Draalo]

12 hours ago, Zhetaan said:

Your atmospheric delta-V for the entire rocket is only about 4300 m/s

Why looking at atmo-dV? That only matters for the lift off stage. More precise: only the atmo TWR matters. Assuming the first stage has minimum ~1600m/s dV its enough to reach the upper atmo, where engines like terrier or poodle working at 99% vac-isp.

 

Booster + Swivel have roughly 2000m/s atmo-dV (note that the swivels isp gets better too at higher atmo)

S4 has 2400m/s - enough for low Kerbin orbit circularisation + Mun transfer

Lander has 2222m/s, needed is:

310m/s for capture

580m/s to land (ideal value)

580m/s to reach orbit again and

~340m/s for a Kerbin PE about 25km

total: 1510m/s. That means (2222-1510=) 712m/s reserve for landing and adjusting - more than enough.

 

@abickocz:

assuming that you have striped off the middle fins, got rid of the monopropellant and exchanged the chute with the smaller/lighter one you get even more dV because of the reduced mass and drag.

 

Just saying in case: In the VAB you can adjust the thrust of solid fuel boosters - lower it a bit, you want an atmo-TWR of 1.5 for the lift off.

Edited December 22, 2017 by Draalo
typo