This rocket will not fly, why?

[Reactordrone]

EDIT: If it makes a difference, I'm using clustered 909s instead of a 2.5m engine on the payload. How gracefully does stock handle that?

You'd be better off with a poodle, at least a quarter of a tonne lighter than the 4 LV-909s and with more thrust and better Isp.

[Captain Sierra]

Craft file and i will fix it for you. I can easily see your rocket is top heavy and you have no controll but those mainsails. You didn't tell when you start your gravity turn. I generally start my gravity turn at 20k and keep my velocity fairly low untill that point comes. If you are trying to turn while those 2 boosters are attached its probably causing uneven drag.

I'm judging by the remark about the gravity turn that you did not actually read the rest of the thread past my original post. Second page, fourth post down I state my gravity turn profile.

You'd be better off with a poodle, at least a quarter of a tonne lighter than the 4 LV-909s and with more thrust and better Isp.

Probably, but that takes up way more space for not that much benefit. quarter ton less mass, 20 extra thrust, and 5 extra vac Isp at the cost of a lot of ground clearance.

Why do people complain about a rocket being top heavy? Top heavy is GOOD for stability.

Heck if I know. Thats not even anywhere close to as top-heavy as that launch vehicle can be. I've thrown jumbos in space with this launcher (albeit in a slightly different configuration not geared for TMI).

Edited May 14, 2015 by Captain Sierra

[mrclucks]

-Tip over a bit around 75m/s and then follow prograde(fins enable you to turn SAS off and follow prograde until 40km or so)

-Slow down. I get to my tip over speed as fast as I can and then after that keep it under 2 twr, reducing throttle even more when I see mach effects occurring

- disable SAS for launch and only use your main/center engine gimbal for the turn and minor corrections

After I launch, I rarely correction direction and pretty much only use throttle control. It may take a little while to understand the bew gravity turn, but after a few launches on a couple rockets, you'll get a feel for how a rocket should generally go

[Bev7787]

Lower your thrust before hitting the transonic barrier. That might help

[Rickenbacker]

It looks very top heavy to me. Add a few fins at the back of the rocket and try again.

If it was top heavy it would fly just fine, it's probably bottom heavy, and very draggy at the front. Fins might help, along with a VERY gradual gravity turn.

[SRV Ron]

You have the steering authority in the first stage. However, the new aerodynamic physics are going to raise havoc if you are going too fast in the lower atmosphere. Be sure the booster pair is properly braced as well.

[RandomRyan]

no fairings,no fins,not enough DeltaV. dude, this is KSP. just add more boosters and try not to blow up

[RAINCRAFTER]

Why do people complain about a rocket being top heavy? Top heavy is GOOD for stability.

Um... the Pendulum Fallacy. You said the Pendulum Fallacy. Sorry, but it's just not true.

[Harry Rhodan]

Um... the Pendulum Fallacy. You said the Pendulum Fallacy. Sorry, but it's just not true.

That's about having your CoT in front of your CoM.

[Captain Sierra]

You have the steering authority in the first stage. However, the new aerodynamic physics are going to raise havoc if you are going too fast in the lower atmosphere. Be sure the booster pair is properly braced as well.

Boosters each have a decoupler, two struts, and a fuel line holding them on. No instabilities there. After actually taking her to space, problem appeared to be me rushing to hit Mach per pre-1.0 ascent logic.

[cicatrix]

My purely amateurish view on this problem is as follows.

Nearly all problems with lifters start at the moment of gravity turn. You want to steer this thing very gently and few degrees at a time. Basically, if you don't have fins the only way to steer your rocket is the engine gimbal which is usually not enough. You can enhance your steering authority by adding RCSs, reaction wheels or aerodynamic control surfaces (fins). What happens is as you start steering your rocket it has too much momentum to immediately change its heading so it continues flying upwards but this time the aerodynamic pressure on the top of your rocket forces it downward which results in increasing the 'angle of attack' which in turn exposes more area to the airflow. It snowballs after that.

To avoid that you should have something that will oppose the increasing aerodynamic pressure on the side of your rocket. As I said it can be RCS, reaction wheels or fins. The heavier your rocket the more steering authority you should have in order not to flip.

Also, keep in mind that TWR greater than 1.3 is bad. If it's ~2 or more then the chances of your rocket to flip increase up to nearly 100%.

Many who used stock aero before 1.0 still do the same thing - up to 10 km then 45 degree east and up till the desired height. This doesn't work this way now. I used FAR before and it it taught me the proper way of doing that.

1. Have fins + reaction wheels. Always.

2. Have TWR less than 1.2 (max 1.5)

3. Start turning earlier (6-7 km).

4. Turn slowly, wait your prograde marker to respond to your steering before continuing.

Edited May 14, 2015 by cicatrix

[sgt_flyer]

Depending on the staging and TWR, you may want to do a pitchover manoeuver now ex : with my chaos star series, i make a pitchover turn of 5-6° from vertical as soon as i reach 35 - 40 m/s of speed with a twr between 1.5 and 1.7 at this time - so only a few seconds after liftoff. At these speeds, it should still be pretty easy to do a pitchover then, wait for the prograde marker to catch up to your attitude, and gently follow the marker (AKA : Gravity turn) Once the rocket loses some TWR (due to throttling down or stage separation), the speed at which the gravity turn is occuring will increase (inversely, as your TWR goes up due to fuel consumption, the turning rate slows) - i go for the horizon only above 35000 / 40000m - and by then, i'm already only 20° above the horizon - all that thanks to the initial 5-6° pitchover a few hundred meters above the launch pad. I make it to orbit using around 3400 / 3450 m/s of vacuum delta-V (though with efficient sea level engines in the first stage)

I still don't have lost any of those chaos stars due to a rocket backflip

But basically, once you reach high speeds you don't want to wander too far from your prograde marker

How soon and how much you do your pitchover manoeuver entirely depends on your rocket

Edit : here's the chaos star 500 album - you'll be able to see how tilted it is and how much TWR it has in various phases of the flight. Keep in mind that my heading marker basically only leaves the prograde marker's circle on two occasions : during the initial pitchover, and after i fire the upper stage (where atmo doesn't count anymore)

Javascript is disabled. View full album

Here's the thread for those chaos star rockets, alongside the recommended ascent profiles

http://forum.kerbalspaceprogram.com/threads/115732-V1-02-Chaos-Star-Series-1500-1200-and-500-tons-to-LKO-launcher-maximum-fairing-diameter/page3

Now, keep in mind : the objective is to mostly keep your attitude marker within the prograde marker's circle throughout the high speed while the atmosphere can affect you.

You can use the small freedom of the prograde marker to also slow down / accelerate the prograde's marker turn, but you can also play on the throttle to do so.

No need for insane amounts of control authority - if you keep an equilibred rocket attitude within the prograde marker, it will remain naturally stable.

Edited May 14, 2015 by sgt_flyer

[Captain Sierra]

Depending on the staging and TWR, you may want to do a pitchover manoeuver now ex : with my chaos star series, i make a pitchover turn of 5-6° from vertical as soon as i reach 35 - 40 m/s of speed with a twr between 1.5 and 1.7 at this time - so only a few seconds after liftoff. At these speeds, it should still be pretty easy to do a pitchover then, wait for the prograde marker to catch up to your attitude, and gently follow the marker (AKA : Gravity turn) Once the rocket loses some TWR (due to throttling down or stage separation), the speed at which the gravity turn is occuring will increase (inversely, as your TWR goes up due to fuel consumption, the turning rate slows) - i go for the horizon only above 35000 / 40000m - and by then, i'm already only 20° above the horizon - all that thanks to the initial 5-6° pitchover a few hundred meters above the launch pad. I make it to orbit using around 3400 / 3450 m/s of vacuum delta-V (though with efficient sea level engines in the first stage)

I still don't have lost any of those chaos stars due to a rocket backflip

But basically, once you reach high speeds you don't want to wander too far from your prograde marker

How soon and how much you do your pitchover manoeuver entirely depends on your rocket

Edit : here's the chaos star 500 album - you'll be able to see how tilted it is and how much TWR it has in various phases of the flight. Keep in mind that my heading marker basically only leaves the prograde marker's circle on two occasions : during the initial pitchover, and after i fire the upper stage (where atmo doesn't count anymore)

http://imgur.com/a/IeQAd

Thx. I did some research into the aero model and it definitely models the trans-sonic drag spike. When that hits me, the drag on the payload becomes so great my CD is shifting ahead of my CM, so I'm flipping out.

Far as the ascent profile, I kick it over about 5 degrees at 2km alt (doing it on launch sometimes makes the turn too aggressive) and then kick off SAS. Until I hit Mach, the thing will do the gravity turn by itself. Then it takes a bit of wrestling (SAS seems different, dunno why) but I can keep it in the turn, ending at around 50-60 degrees over when I break 30km. At this point I throw it over to the horizon line to extend the orbit.

[Motokid600]

Two things no ones mentioned.

Make the root part of your entire rocket the one closest to its CoM. Greatly increases stability.

Take the offset tool and move the engines up into the tanks so only the bell that swivels is showing. This way your fins can be closer to the rear of the rocket and again increases stability. Also I wouldn't use fins with controls on them. Only static ones unless your really hurting on control authority.

[Fishslap]

Oh please! No reaction wheel, no struts and no fins and you blame Squad?

[Motokid600]

@Fishslap - Please read the thread before you post. It's page 4.

[moronwrocket]

Why do people complain about a rocket being top heavy? Top heavy is GOOD for stability.

Yeah, it took me a while to understand why this is true. Here's the moron explanation:

Think of a stick with a heavy weight near one end. Now, if you wanted to balance that stick + weight in your hand, which was is easier, with the weight low, or with the weight high? With the weight high, because small imperfections in your location translate to smaller, slower movements of the weight - it's on the end of a long lever arm, so it "rotates" down slower, than if it was close to your hand on a short lever arm. Those slower movements allow more time for your changes to happen before flipping. Actually try it if you can - it really helps to give a sense of what's going on.

Second, imagine a flying rocket as a simple tube filled with fluid. Now, if it is fully filled and moving straight up, but tilted at an angle - the two halves have the same drag force (to first order), and because they both have the same mass/inertia they both experience the same deceleration - there isn't a net torque making the rocket turn. Now, if it is half filled and moving straight up, but tilted at an angle - the two halves still have the same drag, but the front half has much less inertia and so decelerates much faster, causing torque, flipping your rocket. Simple way to physically see this is to take a paper towel roll and weight one end of it, and toss it around - the heavy end naturally wants to end up in front. What took a long time for me to understand was this has 0 to do with gravity - same effect would occur in weightlessness. I find the "dart" analogy a little misleading, because people attribute too much to the fins - they help accelerate the process, but aren't necessary for it to happen - look at Nerf darts, perfect example!

I think part of the reason this is unnatural is that most of our encounters with stability involve 2, 3, or more points of contact in a static situation, where lower == more stable. With an aerodynamic situation and a single point of contact, higher == more stable.

[Anglave]

Also the mainsails give pretty good control until I get 20+ degrees off prograde (by which point I'm already losing control to begin with)

At what altitude are you turning by 20° or more? Post 1.x, you simply cannot do that below about 20k altitude.

Go straight up (or 5° east directly off the pad) and keep that line until 20km. Then do your turn as normal, the mainsails should be more than enough control authority at that point.

Secondly, you're lifting a relatively light payload and not a lot of fuel with three mainsails. I suspect your TWR gets huge as you ascend, and your speed might be getting crazy high for your altitude. Try to stay under 100m/s until 10k, and not over 250m/s until 20k, then turn, then go full throttle.

Almost all rockets are going to be aerodynamically unstable, lots of drag at the front, most of the mass at the back. This only gets worse as they ascend, burning fuel moves the CoM toward the rear, and higher speed puts more drag forces on the nose. The faster you go, the worse it gets, and if you try to turn at all in the thick stuff, you're not going to space today.

[Snark]

A note on torque and reaction wheels:

Also, you can put as many reaction wheels way out on the end of the rocket as you want, but it won't actually help. Reaction wheels do not apply linear force like a lever; they apply torque like a power drill. Thus they will be much more effective if they are clustered near the center of mass, which as I mentioned might be surprisingly low on the rocket.

The second reaction wheel I just added is actually between the sustainer and the upper stage, so its not far off the CoM.

Actually, it doesn't matter where you put them. They work equally well anywhere on your vessel, they don't have to be near CoM. That's what physics says, and that's what the game does. It's a common misconception that it matters.

The reason it doesn't matter is that for a free-floating body, the center of rotation (which determines the moment of angular inertia) is going to be the center of mass regardless of where the torque is applied.

That is, if you have a long skinny rocket, and you put a reaction wheel on it and use that to, say, flip the rocket end for end, it's going to rotate around the middle of the rocket, regardless of whether the reaction wheel is in the center of the rocket or out at one of the ends. That's because a given amount of torque will create a given amount of angular acceleration.

By the way, that's not just what the physics equations say. It's also what happens in-game: I just now tried a test to verify. Built two otherwise-identical rockets, put them floating next to each other, one has a reaction wheel in the middle and the other one out at one end, try to rotate both. Result: same performance. So the game is modeling the physics correctly in this case.

(That said: The general mass distribution of your rocket does make a difference how fast the reaction wheels can turn it, and reaction wheels have mass, so moving them can affect the moment of inertia of your ship. But assuming that the mass of your reaction wheels is only a very tiny fraction of your ship mass, this ought to be negligible. And of course, Kerbal ships aren't perfectly rigid, so if you have a ship that's ....e to flexing under torque, then reaction wheel placement can matter for that.)

[Edit: LOL. The forum software thinks that the word I used meaning 'having a tendency to', spelled "p" followed by "rone", is a swear word followed by an e. Sheesh.]

The fact that reaction wheel placement doesn't matter goes against most folks' "common sense," thus the misconception. After all, say you have a golf club, and try waggling it back and forth if you hold it in the middle versus holding it at one end. Lots easier from the middle, right? But that's a false comparison with the KSP case, since in this case the center of rotation is where you're applying the torque. When you waggle the golf club from one end, you're applying the forces externally and you're making it pivot about that end. A long rod rotating about its center has a much lower moment of inertia than the same rod rotating around one end.

Moral of the story: Put your reaction wheels wherever is most convenient for your design. If you don't believe me, try it out for yourself in-game.

Edited May 15, 2015 by Snark

[Eric S]

At what altitude are you turning by 20° or more? Post 1.x, you simply cannot do that below about 20k altitude.

I think I'm going to disagree with that, though it really depends on your TWR. Most of my designs work fine if I start a gradual turn at 35 m/s velocity, gradual enough that I'm at about 10 degrees off vertical at 3500-4000m and 35 degrees off vertical at 10-12km. If you mean a 20 degree turn all at once, I'll agree, because I never turn more than five degrees off of prograde except for right at launch where I stay vertical and my prograde isn't always vertical until I've built up some speed.

[ghostbuzzer7]

Grav turns with the Delta IV heavy design consequently results in wobble with the new aerodynamics by default. Strut the two side boosters at the top and bottom. Another key factor is that adding fins wont help your rocket go straight, the best way to fly bilateral symmetry is by not doing a grav turn until you reach thinner atmosphere such as 25,000 meters. We aren't playing in .90 anymore... or .25 or .18 nope good ole physics.

[cicatrix]

When you ask yourself why rockets flip over try this:

Exactly the same reason why the stick on your palm is unstable is 'what's wrong' with all the rockets (both in KSP and in real life).

[AaronLS]

The thing flew better like this than it did when it had a fairing on it. Fairings are more harm than good when your payload is basically a 2.5m stack with a few radial bits.

Maybe so, but make sure the tip of your fairing is as sharp as possible. It greatly reduces drag, which not only lets you accelerate more in atmosphere, but makes the rocket more stable by reducing drag at the tip.

Make sure you pop your fairing once out of atmosphere, as that does shed weight so that your orbital burns are more efficient.

http://forum.kerbalspaceprogram.com/threads/119198-Nose-Cone-Experiments

[MarvinKitFox]

... which looks like it should fly

Study up a bit more about aerodynamics, and re-evaluate this statement of yours, please.

[cybersol]

I'm just too stubborn to let the air win, plus going faster saves dV to orbit if you can stay in control. So I have just been learning to design rockets that can punch through the transonic region with enough stability and control to not spin out. At a fundamental level "cicatrix" is right that is seems to be about enough control authority, especially for heavy rockets. However, he is wrong that TWR needs to remain low to make it work. My most effecient rockets to orbit tend to have TWR in the 1.7-2.2 range, but I admit it can be a challenge to keep them flying true. Challenged Accepted!