Launching issues

[Kickn4fun]

     So, I'm having a TON of issues launching the last few days. New player here, so I've been watching a lot of videos and reading guides on this forum.

My issue is with breaking into Kerbal orbit with my larger rockets. I've tried many many combinations of engines, fins, staging and fuel setups. I've tried taking a whole section of the external boosters off for less weight, but lose fuel too fast. I've tried 'asparagus' staging, or feeding all into the center. My rocket KEEPS wanting to flip BACKWARDS..? 

     My best launches I've tried is this: Full throttle on takeoff, SAS on. At about 20-30 meters per sec, rotate 5-10 degrees, just until the pro-grade marker starts to move; turn SAS off. I keep following the pro-grade marker during acceleration (My rocket seems to require manual intervention to stay on course. Very little, though). On my best launches, at around 8000 meters, 60* and 260 m/s, my rocket likes to flip BACKWARDS...

     Help, please? Thanks all.

 

[Kickn4fun]

Hers is a pic of what it's doing. The rocket is turning West on it's own.

[FullMetalMachinist]

15 minutes ago, Kickn4fun said:

Help, please? Thanks all.

The short answer is that you're trying to throw a dart backwards, with the fins in front and then being surprised when it flips around. 

All those fuel tanks and engines at the back are very heavy, and your payload is big and light. This makes your CoM close to the back, and lots of drag at the front. Like a backwards dart. So it's most stable orientation is for the engines to be at the front. 

The reason that you get up to around 8km up before it flips is likely a combination of that being the first air density boundary and how fast you're going. In the thick lower air your fins at the back have enough 'bite' to counter the drag at the front, and you're going slower so the drag at the front isn't so bad. Once you get higher and faster those two things switch, and you flip. 

You can fix it a few ways. Personally I'd get rid of the structural girders that are holding the landing legs, and then wrap the whole payload in a fairing. 

Edit: I always forget this part. Welcome to forums; and to the wide, wonderful, frustrating, and rewarding world of KSP! 

Edited April 14, 2017 by FullMetalMachinist

[bewing]

The classic passive stability problem: Too much drag at the front, not enough drag at the very back. Along with too much weight at the back, and not enough at the front.

Now, it is not necessary for a rocket to be passively stable. You can build a slightly less-than-stable rocket, and still use SAS and control surfaces to keep it flying straight.

There are several things you can do about it. Put the upper bit into a fairing. Move those side-mounted boosters down. Take their nosecones off. Add a lot of fins and draggy stuff at the back. Clean up the aerodynamics at the front. Add some mass to the front. etc.

[Kickn4fun]

4 minutes ago, FullMetalMachinist said:

You can fix it a few ways. Personally I'd get rid of the structural girders that are holding the landing legs, and then wrap the whole payload in a fairing. 

Thank you for your answer. 

Heh, believe it or not, that's the 3rd design I came up with for the lander, and by far the most compact. I'll try removing the girders. I know the engine will clear, but I was going for lots of 'Balance'... maybe too much. 

Follow up question. Would I be better off putting the fins towards the top then?

 

Oh yeah, and I haven't unlocked fairings yet.

Edited April 14, 2017 by Kickn4fun

[Dman979]

1 minute ago, Kickn4fun said:

Follow up question. Would I be better off putting the fins towards the top then?

No. Fins should stay at the back, to create drag there. Ideally, they'd be discarded once you're about 30km up, because they are less useful there.

[bewing]

1 minute ago, Kickn4fun said:

Thank you for your answer. 

Heh, believe it or not, that's the 3rd design I came up with for the lander, and by far the most compact. I'll try removing the girders. I know the engine will clear, but I was going for lots of 'Balance'... maybe too much. 

Follow up question. Would I be better off putting the fins towards the top then?

 

Oh yeah, and I haven't unlocked fairings yet.

Then the best thing to do is to understand that launching straight up -- with SAS on, and lots of control surfaces -- can save an unstable design. It's not as efficient as gravity turning, but without a fairing you will never get this thing to gravity turn successfully anyway. Just launch it dead straight up and turn when you are above the atmosphere. Or you don't even need to turn. If you aim it right, you can just go straight up all the way to the Mun or Minmus. I do it all the time.

[GoSlash27]

Kickn4fun,

 KSP is all about "launch issues". That's it's charm.

 Imagine yourself watching a NASA launch on TV. They come to the 2 minute hold and show a pic of the rocket that's about to launch, and it looks exactly like the rocket you've posted here. Would you expect that rocket to make orbit, or would you expect it to lose control around Max Q?

My best advice is this: Aerodynamics matters in KSP. Your rocket should look like something that would be successful IRL.

HTHs,
-Slashy

 

[Kickn4fun]

7 minutes ago, GoSlash27 said:

Kickn4fun,

 KSP is all about "launch issues". That's it's charm.

 Imagine yourself watching a NASA launch on TV. They come to the 2 minute hold and show a pic of the rocket that's about to launch, and it looks exactly like the rocket you've posted here. Would you expect that rocket to make orbit, or would you expect it to lose control around Max Q?

My best advice is this: Aerodynamics matters in KSP. Your rocket should look like something that would be successful IRL.

HTHs,
-Slashy

 

Hahaha! I'd probably be rolling on the ground if NASA ever flew anything I've built!!!

I came across some of Scott Manley's videos, that's what got me interested in trying out this game. So some of his earlier videos, he suggested going for science a lot at first, and I think it's come to bite me a bit. As an update, I added 6 of the smaller SRBs, and was able to launch straight up to 100,000m with very little issue, and tons of fuel to spare. Minmus, here come my Kerbals!

[FullMetalMachinist]

10 minutes ago, Kickn4fun said:

I came across some of Scott Manley's videos, that's what got me interested in trying out this game.

Another quick word of friendly advice, be careful about KSP videos on YouTube,especially older ones. Almost 2 years ago the game got a big update and drastically changed the aerodynamic model to be more realistic. 

Videos from before then can be very misleading about what is currently possible (or just way harder now) in the game, especially 'how-to' videos. 

[Aegolius13]

Lots of good design tips above,  but one flying tip:  SAS hold Prograde is your friend for stability.   Ideally if you do your pitchover right,  you can leave it on Prograde the rest of the way.  This promotes stability and also keeps drag low.

But if your rocket is not designed for aerodynamic stability,  it's like trying to balance a bowling ball on a pin - things can go awry even with the SAS trying is best to keep them stable. 

[GoSlash27]

7 hours ago, Kickn4fun said:

I came across some of Scott Manley's videos, that's what got me interested in trying out this game. So some of his earlier videos, he suggested going for science a lot at first, and I think it's come to bite me a bit. As an update, I added 6 of the smaller SRBs, and was able to launch straight up to 100,000m with very little issue, and tons of fuel to spare. Minmus, here come my Kerbals!

Kickn4fun,

 I concur with Scott Manley on that point; you should get lots of science early. What's biting you in the butt isn't the science, it's your design. Too light, lumpy, and draggy in the nose. That sort of thing used to work fine before KSP 1.0, but it doesn't anymore. That's why FullMetalMachinist has cautioned you about old "how-to" videos.

Your rocket needs to be streamlined and have the center of mass high. You no longer do that "climb to 10km and go thataway" procedure, Asparagus staged "space pancakes" are no longer ideal.

Your rockets now need to look like real rockets and your launches need to look like real launches.

Have a look at this one:

 

Best,
-Slashy

[Kryxal]

The thing that really bothers me about that rocket is the Rockomax decoupler ... why did you feel the need to go to a 2.5m part?

On closer examination, you're using a science lab.  Don't try to go back to the smaller parts below that, just stick to 2.5m and it should help.  You do have the larger fuel tanks and engines unlocked, right?  If not, you might be able to save the design with some BIG fins near the bottom, since you'll probably be going till 20+ km with the outer ring there.

[Kickn4fun]

2 hours ago, Kryxal said:

The thing that really bothers me about that rocket is the Rockomax decoupler ... why did you feel the need to go to a 2.5m part?

I was having structural issues, going between 1.25m and 1.5m sections, and the larger decoupler solved that.

 

Well, I listened to your guy's advice, and researched the fairing. Removed the girders around the lab, and reduced the struts to a total of 4 instead of 8. I added an intermediate rocket stage below the lander to help get into orbit. I also added a 2nd group of fins at the bottom of the rocket, and changed my fuel tank layout slightly to utilize the larger tanks. (I still need to get the larger engines.) This design actually flew very well. I left SAS on, but just nudged the nose over every so often, and it tended to gently fall over on it's own as well.

[bewing]

PS. you can often get fairing parts to test on contracts, and then you don't have to research them.