1.0.2 Ascension curve and flight trouble..

[MoridinUK]

I have a rocket, with a 4100 atmos dv (KER). It is a tall thing, nothing out the sides at all, just two stages, the 45 gimboled engines on both stages.

I can not get to orbit... Obviously I'm keeping atmos eff below or at 100% and I'm trying to use a gravity turn, but I find i'm having to be way to careful in turning slowly and letting the prograde marker on the nav ball stay near where I'm pointing or we go flipping out. My next try I think I'm going to go straight up and turn right once at 25km and out of the thickest bit of atmosphere.

Any advice at all? When should I start to turn? I saw HMV (5th Horseman) started a turn almost off the pad but I couldn't keep the rocket straight!

I tried reconfiguring with an extra two engines on the side and some extra fuel, but that was way harder to keep on target...

[eddiew]

In 1.0.2 a gravity turn means just that. Nudge your prograde about 5 degrees east soon after launch and then just go hands off and let gravity turn you on the way up

And make sure you have fins on the back end

(Pics will help, it may be your design or your piloting that needs adjustment.)

[Moonfrog]

I have run into this problem as well. From what I've read, the problem seems to be the centre of mass. When the centre of mass, due to fuel usage moves closer to the middle of your rocket, the more unstable your craft becomes. With my rockets, i am fine for a while, but then a slight nudge and it goes out of control. For this reason, i plan on building much heavier bottoms and taller rockets, to try and get the centre of mass as far down as i can. Might even add structural fuesale to act as a blocker to maintain the mass at the bottom as the fuel dissapears to make it more stable.

Not tested these ideas out, but, hopefully we can find a solution together. Having to build my launch rockets much differently now.

[Poynting]

You want to keep your centre of mass high not low. Then try to keep your centre of drag near your centre of mass. Having the centre of drag behind the centre of mass (with respect to velocity direction) will ensure the rocket is stable regardless of its attitude but if it is too far back you won't be able to steer at all. It is possible to maintain forward flight even in an unstable configuration so long as you keep the too aligned or have the attitude authority to overcome the drag torque. But if you turn too hard the drag applies a larger and larger torque (positive re-enforcement) and the rocket flips. Its a balance between the manuverability you want and the positive feedback.

[eddiew]

You want to keep your centre of mass high not low.

This! ^^^ Remember that darts have the heavy bit forward and the feathers at the back; the fact that you have a self propelled dart doesn't change anything about the aerodynamics of it

One common issue during flight is if you've used many short fuel tanks to build a long rocket. The top tank drains first, then the next, then the next, until all the fuel is down at the bottom end - as of course, is the heavy engine. This drags the CoM back behind the CoL and all of a sudden your rocket wants to fly backwards because that's the way round darts with a heavy base would fly.

[MoridinUK]

Ah some of this is seriously awesome! Makes Staging a great idea, pity asparagus staging is going to create way too much drag... (and I haven't unlocked fuel pipes yet either). I can put some of the first fins on (they are all I have unlocked) and I have managed to get it into orbit, but only just... Gravity turn.... hands off? Pilot SaS off then too?

[Methusalem]

This! ^^^ Remember that darts have the heavy bit forward and the feathers at the back; the fact that you have a self propelled dart doesn't change anything about the aerodynamics of it

One common issue during flight is if you've used many short fuel tanks to build a long rocket. The top tank drains first, then the next, then the next, until all the fuel is down at the bottom end - as of course, is the heavy engine. This drags the CoM back behind the CoL and all of a sudden your rocket wants to fly backwards because that's the way round darts with a heavy base would fly.

Thar's actually an interesting point. On the next rocket I'll try fuel lines. Pumping from the bottom tank into the tank above, from there into the tank above, all the way up into the top tank. The payload on top is normally the heaviest piece, so that should keep CoM and CoL pretty close together and improve stability a lot. At least it's worth a try.

[pandaman]

Try balancing a sweeping brush on your finger.

With the heavy bit (bristles) at the top and the end of the handle on your finger it's far easier to balance than the other way up.

Same principle applies to rockets. (It's no coincide Jeb used to be a janitor).

[Moonfrog]

heavy bit on top? feels awkward since, top of a rocket is usually designed to be lighter than the bottom of the rocket, for delta V reasons. Its worth a shot, fidlding time. If anyone has any design suggestions on how to push the centre of mass upwards, plz post them.

[Old Foxboy]

heavy bit on top? feels awkward since, top of a rocket is usually designed to be lighter than the bottom of the rocket, for delta V reasons.

Nope. Only the upper stages usually will be smaller, but that doesn't mean they have to be lighter compared to the relative size of the 1st stage.

However, you only need a top heavy rocket until you are out of the dense lower atmosphere. So, have big enough fins on the bottom of the 1st stage, and don't deviate too much from the flight vector when punching through maxQ.

[MoridinUK]

So, have big enough fins on the bottom of the 1st stage, and don't deviate too much from the flight vector when punching through maxQ.

Sorry I'm too dumb to understand this I fear... maxQ? and how big fins, and you mean the first stage (i.e lift off) not 1 stage, (the second from last?) as KSP numbers them in descending order?

[EtherDragon]

maxQ is another way of saying terminal velocity. Basically it refers to the fastest speed you should go at a given altitude for best efficiency.

Here is a handy guide that explains why rocket flipping occurs.

Here are your remedies:

1. Add large enough tail-fins that they counteract a rocket's tendency to nose over making it inherently stable. (best solution)

2. Start your gravity turn pretty early (before reaching 100m/s velocity) and make sure you don't let your nose deviate too far from the prograde marker. (workable, but takes a deft touch).

Here, have a video that demonstrates #2:

[MoridinUK]

Thanks for that! I'm managing LKO now, but I'm in fear of what I'll have to design to get heavy loads into space, I guess I'll have to actually build my space stations in a modular form.. lol..

[EtherDragon]

Thanks for that! I'm managing LKO now, but I'm in fear of what I'll have to design to get heavy loads into space, I guess I'll have to actually build my space stations in a modular form.. lol..

Yes! This exactly. Huge projects, like space stations, are best assembled in space. But if you use larger tail-fins you can actually get pretty big loads into orbit in a single launch.

If you want to assemble in orbit, you'll need to master rendezvous. Luckily Scott Manley has a

on docking. It's from an older pre-release build, so don't pay attention to the ascent profile (or the graphics). But everything after reaching orbit is relevant to rendezvous and docking.

[MoridinUK]

Yes! This exactly. Huge projects, like space stations, are best assembled in space. But if you use larger tail-fins you can actually get pretty big loads into orbit in a single launch.

If you want to assemble in orbit, you'll need to master rendezvous. Luckily Scott Manley has a

on docking. It's from an older pre-release build, so don't pay attention to the ascent profile (or the graphics). But everything after reaching orbit is relevant to rendezvous and docking.

Thanks for the advice, I found actually docking not too taxing, it was getting the Rendezvous that drove my head nuts. Back before I installed TAC LS I used to just leave em for weeks to meet! My attempts never seemed to hook up as neatly as Scott's or HMV's.

Edit: Then there was that one time they got real close and as I tried to pull it off I realised they were orbiting in reverse directions... *facepalm*