Why does my rocket keep flipping?

[Ikkjot]

I made a rocket and when I launched it, it kept flipping so I thought it was a issue with the rocket and then when I used another rocket that was in my saved that never crashed and flipped just flipped near 20,000-25,000m in the air. Is it a problem with the game or mass? Because I know that the saved rocket never flipped.

also I was in science

Edited December 5, 2020 by Ikkjot

[Vanamonde]

As a rocket rises it consumes fuel, which causes the center of balance to shift. Also, the air thins which means fins help less with attitude, but airflow also tries to twist the ship less. All sorts of stuff is going at once, as a matter of fact. How about sharing a picture of your ship? 

[foamyesque]

Rockets flip for one of (usually) two reasons:

 

1. A center of mass that has moved below the center of aerodynamic pressure, causing aerodynamic instability;

2. A center of mass that has moved out of alignment with the center of thrust, which will induce a torque, and that torque becomes larger than your ability to counteract it.

 

A picture of the rocket would be useful in figuring out the cause. It's usually aerodynamics if you're in an atmosphere, since engines are heavy and so fuel draining will tend to move the CoM down over a given stage's burn if you have them at the back (as is usual). One of the advantages of a staged design is that the mass of the next stage up helps to counteract that.

[Ikkjot]

2 hours ago, Vanamonde said:

How about sharing a picture of your ship? 

 

1 hour ago, foamyesque said:

picture of the rocket

I cant send pictures but I can tell you what it had

saved(not the newer one): It had the mk1 cockpit and parachute and 2 RT-10 Hammers. For the other one I had parachute, mk1, pressure and temp log thing, and a heat shield 1.25m then a medbay and antenna and then it was one liquid fuel and then 2 rt-10hammers and one rt-5 flea

Edited December 5, 2020 by Ikkjot

[Vanamonde]

You can share pictures through a sharing site like Imgur. Console players can use their phones to take pics of their screens and do it that way. 

As a guess, it sounds like part of the problem is that you're relying on the solid fuel engines (Hammer and Flea), and they do not help with steering. 

[Spricigo]

It may be the case that your craft is so that SAS can barely keep it stable following prograde, in previous iteration you got away with it, this time you just steered to much. 

In any case, there are many possible causes for the issue and, without picture are kind of blindly throwing some ideas. Maybe we get lucky and find some solution, but it is a whole lot more likely if we can take a look at the craft.

[Ikkjot]

3 hours ago, Spricigo said:

It may be the case that your craft is so that SAS can barely keep it stable following prograde, in previous iteration you got away with it, this time you just steered to much. 

In any case, there are many possible causes for the issue and, without picture are kind of blindly throwing some ideas. Maybe we get lucky and find some solution, but it is a whole lot more likely if we can take a look at the craft.

 

3 hours ago, Vanamonde said:

You can share pictures through a sharing site like Imgur. Console players can use their phones to take pics of their screens and do it that way. 

As a guess, it sounds like part of the problem is that you're relying on the solid fuel engines (Hammer and Flea), and they do not help with steering. 

I found out how to add images using discord media so heres the rocket:

note: the av-t1 winglets arent there anymore 

[Ikkjot]

heres a updated one:

[jimmymcgoochie]

No wonder it’s flipping- you took off the only thing that was keeping it stable, the fins. You need to keep the weight as close to the front and the drag as close to the back, and fins will both add a little drag and passively correct your course if it drifts out of line. Without any fins on it, any deviation from prograde will be amplified by thrust and the heavy SRBs on the bottom will make it very unstable, causing flipping.

Put some fins on both the Hammer SRBs- basic fins will probably do- and if necessary on the LF stage too. Get rid of the radiators and heat shield below the capsule as just by looking at the rocket I don’t expect it to get much further than LKO where re-entry heating is minimal, but put a decoupler between the last stage and the pod or things could get a bit dicey on the descent.

I don’t see the point of using that many SRBs in a stack, you’d probably be better off with a liquid fuel engine with radial SRBs if necessary to boost its launch TWR to help it off the pad. LF engines have better fuel efficiency than solids and can also be extended by adding more fuel tanks, plus you can add steerable fins to the bottom to make it even easier to control it.

[Ikkjot]

7 minutes ago, jimmymcgoochie said:

No wonder it’s flipping- you took off the only thing that was keeping it stable, the fins. You need to keep the weight as close to the front and the drag as close to the back, and fins will both add a little drag and passively correct your course if it drifts out of line. Without any fins on it, any deviation from prograde will be amplified by thrust and the heavy SRBs on the bottom will make it very unstable, causing flipping.

Put some fins on both the Hammer SRBs- basic fins will probably do- and if necessary on the LF stage too. Get rid of the radiators and heat shield below the capsule as just by looking at the rocket I don’t expect it to get much further than LKO where re-entry heating is minimal, but put a decoupler between the last stage and the pod or things could get a bit dicey on the descent.

I don’t see the point of using that many SRBs in a stack, you’d probably be better off with a liquid fuel engine with radial SRBs if necessary to boost its launch TWR to help it off the pad. LF engines have better fuel efficiency than solids and can also be extended by adding more fuel tanks, plus you can add steerable fins to the bottom to make it even easier to control it.

Okay Thanks! I am new to the game and I don't understand much.

[Vanamonde]

The normal rocket engines pivot a little to help you steer the ship. The engines designated SRB are simple, cheap, but strong rockets, but they do not help with steering. Generally the idea is to use SRBs to give a ship a hard initial push at takeoff, when you don't to steer much, then eject those and use normal liquid-fuel rockets for upper stages. Fins also help, but less and less as you rise and the air gets thinner. Capsules inherently help with steering, but you can add to their assist with the "reaction wheel" parts, too. 

[Spricigo]

23 minutes ago, Vanamonde said:

The engines designated SRB are simple, cheap, but strong rockets, but they do not help with steering. Generally the idea is to use SRBs to give a ship a hard initial push at takeoff, when you don't to steer much, then eject those and use normal liquid-fuel rockets for upper stages.

Well, there is alternatives* for the lack of steering. e.g: the overpowered reaction wheels we have in KSP. 

The real issue with  SRB on later stages is that they are quite heavy.  That extra mass will cause a cascading effect with every stage below it requiring more fuel and more power. For the first stage , specialty one that will not be kept for long, the extra mass is a much lesser concern.

 

*not to mention  launch vehicles that don't require such "complex procedures" like steering the craft during launch. A preference not many players share with me. 

[foamyesque]

Your issue is the addition of the radiator and the removal of the fins. Radially attached parts have significant drag (and, if asymmetric, can also cause a torque), and because you've put it at the top of your rocket, you've moved your center of aerodynamic force upwards. At the same time, you've eliminated the fins at the bottom that would've moved the center of aerodynamic forces back down. For a rocket of that size the capsule would provide entirely adequate steering otherwise. The heat shield's probably also unnecessary for an LKO machine like this appears to be, but if it's in the stack it shouldn't be causing aero issues.

My suggestion would be to eliminate the radiator -- I don't see anything on the rocket that would require it -- and, if that's not enough, putting the smallest fins on the second of the solid stages. First one won't get you going fast enough for aero instability to be an issue, but the first Hammer will.

Note: The above is trying to work with your extant rocket. My own preference for using solids for launchpad kick is to radially mount them and co-fire them with a gimballing liquid engine; the liquid will give you a bunch of control authority and with its likely higher efficiency is better at sustaining your flight, but the solids will shove you up to working speed much faster and cut back on your gravity losses significantly since you don't have to painfully crawl up at 1.1 TWR or what not. By the time they fall off you'll have burned enough liquid fuel, and gained thrust by getting into thinner air, that the liquid engine can now propel you just fine on its own. In addition, because they're a radial attachment at the bottom of a rocket, they can -- it depends on the weight distribution of the rest of the rocket -- act to stabilize you aerodynamically as they burn.

[vv3k70r]

On 12/5/2020 at 1:52 AM, Ikkjot said:

flipped near 20,000-25,000m in the air. Is it a problem with the game or mass? Because I know that the saved rocket never flipped.

If You add control surfaces it will help, specialy if You add some on the nose too.

How is Your TWR and its dynamic?

Do not put so many draging stuff on the top of the rockeet if You are about to loose propulsion in atmosphere.

Rocket was suported by air. On 25km there is not much of it and You have to depend on thrust to grab this air faster.

Rocket You show can be stable only on very high air speed. Rockets are made for high speed. There is nothing wrong with it. Just keep prograde when You are get high and keep it. And keep building speed. When You stop and You in atmosphere it will flip.

 

 

[jimmymcgoochie]

Three things that make rockets more stable during launch:

1. Balance the mass carefully. The heaviest part of any moving object will naturally try to get to the ‘front’ and unless you can correct that with other forces e.g. aerodynamics, that’s what will happen. Set your fuel tanks to drain from bottom to top in each stage, drop spent boosters as soon as possible and avoid using big, heavy engines when you don’t need to. Solid boosters are fairly powerful for their size and cost, but are also heavy and so you shouldn’t use those in your core stages; stick them on radially to give the rocket an extra kick off the launchpad then drop them once they’re spent.
2. Watch your aerodynamics. A big blunt fairing on the front of your rocket can add a lot of drag and make it unstable, so try and keep your payload sizes in line with the size of the booster and add fins on the bottom of your first stage if necessary to keep it pointing towards space. You shouldn’t need controllable fins unless you have a really huge rocket, and avoid adding fins to upper stages as they’re dead weight in space.
3. Engine gimballing. Many of KSP’s liquid fuelled engines have some degree of gimbal (exceptions include the Reliant and Terrier) which means they can point their thrust to one side and correct their course while the engine is running., however these don’t provide roll control unless you have more than one engine nozzle e.g. the Mammoth, or the Making History Bobcat. Radially mounted engines such as the Thud, Twitch and the Making History Cub tend to have particularly high gimballing angles to be used as vernier engines, providing less thrust but a lot of control; verniers were used on the Atlas and Soyuz rockets as the main engines were fixed, whereas the engines on the Black Arrow and Space Shuttle had a high gimbal angle to provide control. Most real rockets didn’t need fins at all as the gimbals or verniers provided enough control.