Rocket ascend profile with NEAR/FAR

[Wjolcz]

So I have this one problem. I just CAN'T FLY rockets with NEAR installed. What I usually do is I build a spaceplane, and it works perfectly fine. Every single aircraft of mine flies perfectly. I can fly it to the orbit, do whatever I need to do there, come back, land on the runway and recover. However, that doesn't work very well for bigger payloads. Simply don't know how fast to go and when to start the gravity turn. The rockets usually make it to the orbit, though. They just tumble they way out of the atmosphere, doing flips like some anti-gravity propelled olimpic diver.

So if anyone using FAR or NEAR could post some tutorial, or just tell me when to turn, how fast to go, or any other tips I would very happily give some rep.

And yes, I use fairings.

The thread is titled FAR/NEAR because if something works in FAR it should work in NEAR, right?

Edited September 10, 2014 by Veeltch

[ola]

I haven't used NEAR, only FAR, so bear with me. The usual problems with rockets tend to be either the centre of mass, drag/angle of attack (AoA) or wobbly rockets - especially payloads. I tend to RP a bit - those poor ground crew kerbals probably don't want spent SRBs raining down on them. Probably.

I try to design my rockets so I start the gravity turn at about 500 metres. I nudge the rocket just slightly, slightly towards the east and I try to keep my AoA down to a minimum, at least way past max Q which usually occurs around 7500 metres or so. Max Q is usually very close to mach one. Having a very wide fairing creates drag, it's almost like having a drag chute attached at the top! This can be avoided by using control surfaces (aka movable fins), but you'll have lots of other problems with those, so try to avoid them. Keep your AoA down and your TWR at reasonable rates, around 1.50 is usually good. I find that egg-shaped fairings tend to give less drag.

Centre of mass (CoM) changes during ascent. As the first stage fuel is burned, the CoM starts climbing upward. To me, this tends to be a problem around the 20km mark, give or take 20%. Also, it depends on your ascent profile as well, if you're at about 80 deg pitch at 20km you won't have this problem. You're not making a very efficient ascent either, but that's beside the point. If you're at, say, 40 deg pitch at 20km you may very well run into this problem while pitching. The AoA is too big for the reaction wheels and the engine gimballing, so your rocket keels over. You might save the situation if you shut the engines down when you're pointing the wrong way, enable SAS and try to kill the spin and wait for the atmosphere to get thinner so you won't have any external forces pushing your crate around.

Wobbly payloads are a bane. That's why I use Kerbal Joint Reinforcement, which I really recommend. When your payload starts to sway you try to counteract, which makes the payload sway even more the other way, and presto! You're wobbling.

I try to do the following: fairing diameter should always be within twice the size of the rocket diameter. Min TWR at launch should be around 1.50, TWR through 20-25km should not be over 2.50 TWR. Throttling back or capping the engine isn't really good, since it lowers the efficiency of gimballing. Try to use an engine with a large gimbal range. The vernier engine added in 0.24 is really nifty, but expensive! Adding it to a tiny rocket tends to make it oversteer, and adding it to a 5 metre 2000 ton monster won't help you much. Make your rockets long and sleek. Look at examples in real life, like the SpaceX Falcon 9 and other rockets of similar size. Do any of them have fins? Stuff protruding? What kind of fairing shapes do they have? Etc.

Oh, and about the ascent profile: about 80-75 degrees until 12-14km, then I slowly start my pitching until I hit 0 about at 50km or so. -10 degrees at 65km depending on where I'm going, my delta-V budget and what my staging layout looks like.

Edited September 10, 2014 by ola

[Wjolcz]

Thank you very much! Very helpful. Will try to do all this next time I launch a rocket.

[Wanderfound]

First: streamline. Nosecones are essential, pancakes are to be avoided.

Second: start nudging the nose to the east the second you leave the launchpad, but do it gradually. Keep the nose touching the edge of the prograde marker. But keep doing it; it's a process, not an event. Get it to about 45° by 8,000m or so, and keep going. You aren't aiming for "climb then turn"; you want a smooth curve.

By the time you hit 40,000m, you want to be nearly horizontal. Watch your climb rate, and hold it below 100m/s in the upper atmosphere. Pitch down to reduce it, but not so far that you start dropping. Do it right and by the time your apoapsis hits 70,000m your periapsis will be well above ground level. You'll eventually need to shut the engines down briefly, but your final circularisation burn should only take a few seconds.

[Jouni]

FAR is generally very forgiving with rockets. In a typical launch, I turn about 5 degrees to east once the climb rate has reached 70 m/s, and then keep the rocket pointing more or less prograde, until the apoapsis is high enough. If the rocket has low TWR, or I just want to do a quick ascent, I may point a bit above prograde, but almost never outside the prograde marker. Sometimes I turn too quickly and end up flying horizontal at 30 km, but that just means that the ascent is going to take more time and marginally more delta-v.

Nosecones are useful and easy to add, so I always use them. Fairings are rarely necessary and often annoying to work with in VAB, but I still usually use them for aesthetics. Fins and winglets are mostly useless. Almost any rocket will fly nicely without them, as long as you don't stray too far from prograde.

[ola]

You guys aren't using Deadly Reentry, are you?

[Wanderfound]

You guys aren't using Deadly Reentry, are you?

I am.

Streamline, streamline, streamline, and fly smooth. You can survive ridiculous speed at surprisingly low altitudes if you do it right.

Edited September 11, 2014 by Wanderfound