Even the simplest rockets flip over!

[giltirn]

I have played KSP for over 300 hours and have never had as difficult a time getting things to orbit as I have today. My rockets just cannot be controlled and tend to fall over.

If I build the simplest possible rocket: small nosecone - mk1 pod - small tank - swivel engine, then launch on half thrust without SAS, it flips over 180 degrees after 450m/s and ends up firing straight downwards. If I add 4 fins symmetrically placed (which look ridiculous and dwarf the entire rocket) it still flips over, at an even lower velocity of 300m/s or so!

I tried deleting and redownloading the game, then starting a new sandbox save without any mods, and it still happens. What on Earth am I doing wrong?

[GoSlash27]

I have played KSP for over 300 hours and have never had as difficult a time getting things to orbit as I have today. My rockets just cannot be controlled and tend to fall over.

If I build the simplest possible rocket: small nosecone - mk1 pod - small tank - swivel engine, then launch on half thrust without SAS, it flips over 180 degrees after 450m/s and ends up firing straight downwards. If I add 4 fins symmetrically placed (which look ridiculous and dwarf the entire rocket) it still flips over, at an even lower velocity of 300m/s or so!

I tried deleting and redownloading the game, then starting a new sandbox save without any mods, and it still happens. What on Earth am I doing wrong?

Probably exceeding terminal velocity and pointing the nose off prograde.

Keep your acceleration under 2 G, make sure the nose is faired- in, and be gentle with the controls. My rockets almost fly themselves into orbit now, and I don't need tailfins.

Best,

-Slashy

[CptRichardson]

I have played KSP for over 300 hours and have never had as difficult a time getting things to orbit as I have today. My rockets just cannot be controlled and tend to fall over.

If I build the simplest possible rocket: small nosecone - mk1 pod - small tank - swivel engine, then launch on half thrust without SAS, it flips over 180 degrees after 450m/s and ends up firing straight downwards. If I add 4 fins symmetrically placed (which look ridiculous and dwarf the entire rocket) it still flips over, at an even lower velocity of 300m/s or so!

I tried deleting and redownloading the game, then starting a new sandbox save without any mods, and it still happens. What on Earth am I doing wrong?

Way too aggressive with your maneuvering. Ease up a bit.

[giltirn]

In my test I don't touch the controls at all, just fly straight up. Try it yourself: small nosecone, mk1 pod, FL-T200 fuel tank, and swivel engine. Don't touch anything, just hit space with the default thrust. If yours acts the same as mine, it will flip over at about 450m/s.

[m4inbrain]

Fins. I use more fins than struts now. Fins everywhere, on every stage.

Looks stupid, works perfect though.

edit: have to be fins with control authority though.

[CptRichardson]

In my test I don't touch the controls at all, just fly straight up. Try it yourself: small nosecone, mk1 pod, FL-T200 fuel tank, and swivel engine. Don't touch anything, just hit space with the default thrust. If yours acts the same as mine, it will flip over at about 450m/s.

Yeah, you're being way too aggressive with your launch. Slow down the thrust, and add a pair of fins, and try to be careful. It's rocket science, not quantum physics!

[giltirn]

Way too aggressive with your maneuvering. Ease up a bit.

I just tried launching at about 30% thrust (minimum to actually launch). I only reached 300m/s because I was spending so much more time in the denser part of the atmosphere. Although it didn't flip under thrust it didn't reach high enough speed to encounter the problem I saw before.

[moogoob]

Fins. I use more fins than struts now. Fins everywhere, on every stage.

Looks stupid, works perfect though.

edit: have to be fins with control authority though.

Not necessarily - any fins that move the centre of mass far enough back will do.

[GoSlash27]

In my test I don't touch the controls at all, just fly straight up. Try it yourself: small nosecone, mk1 pod, FL-T200 fuel tank, and swivel engine. Don't touch anything, just hit space with the default thrust. If yours acts the same as mine, it will flip over at about 450m/s.

Giltirn,

I tried it, throttling back to avoid overspeed.

It handled fine and ran out of fuel before reaching 400 m/sec at 10 km.

Best,

-Slashy

[frizzank]

More boosters now just means you explode faster......

[m4inbrain]

Not necessarily - any fins that move the centre of mass far enough back will do.

Why would you use fins to move the centre of mass? Am i missing something?

edit: you want drag on the bottom of the rocket, fins without control authority actually prevent you from turning the rocket.

[giltirn]

I'm still convinced something is not right. Often when my fuel runs out, the rocket flips over even while continuing to move upwards. I can't imagine any aerodynamic phenomenon that will cause a rocket to flip over like that. Surely if the pointy end is forwards the air streaming around it will keep it closely aligned with the velocity vector??

[m4inbrain]

That's right. It shouldn't happen, theoretically.

edit: nice example would be AP-ammo on tanks. They go straight, pointy bit towards the target. They'd be useless if they wouldn't.

[GoSlash27]

It ran a little hot on launch, so I throttled back. That was probably your problem.

Best,

-Slashy

[giltirn]

Goslash, do you see the same issue as I if you just don't touch any of the controls other than the space bar to launch?

Edit: and fly with SAS off.

[LordFerret]

I'm having the same problem, but as Slash is demonstrating, I'll guess its going to take a little getting used to. One thing I'm finding however is, for example: Mk1 Pod, spent fuel, LV-T30 engine ... on reentry, it wants to nose-in when it shouldn't ..... the engine outweighs the pod. Trying to keep the pod up is nearly impossible. Also, reentry without a heatshield (I used a SC-9001 Science Jr. instead), there's no indication of the overheating progress ... and I would think the SC-9001 would melt out (sacrifice) first ... but that's not the case - the whole thing explodes. :/

[GoSlash27]

giltirn,

Not if I engage SAS, but it tumbles if SAS is off.

If it exceeds terminal velocity, it goes squirrelly.

I'm able to easily keep it in line by keeping the acceleration under 2G.

Best,

-Slashy

- - - Updated - - -

*edit*

It tumbles at about 6km altitude and 400m/sec without any control inputs whatsoever.

If I limit the throttle to keep acceleration under 2 G, it flies fine.

If I engage SAS, it holds on, but struggles a bit.

Your aerodynamics will go hinky if you don't watch your acceleration. Just keep it under 2G and you should be fine.

Best,

-Slashy

[NathanKell]

GoSlash27, how are you calculating terminal velocity?

Almost all KSP rockets are statically unstable, unless you put rather a lot of fins at the bottom. This is because KSP engines are far heavier than real life, thus the CoM is lower than it should be (you want CoM high and center of pressure--i.e. bot lift and drag--low). That means they need to maintain their own stability by constant control movement (from control surfaces or engine vectoring; reaction wheels are probably too weak).

[Holyvision]

Don't underestimate the usefulness of caps-lock key to those gentle control movements.

Edited April 28, 2015 by Holyvision
Typo fix!

[giltirn]

GoSlash27, how are you calculating terminal velocity?

Almost all KSP rockets are statically unstable, unless you put rather a lot of fins at the bottom. This is because KSP engines are far heavier than real life, thus the CoM is lower than it should be (you want CoM high and center of pressure--i.e. bot lift and drag--low). That means they need to maintain their own stability by constant control movement (from control surfaces or engine vectoring; reaction wheels are probably too weak).

How is it possible to move the center of mass towards the nose when the vast majority of the mass is in the fuel, which is typically concentrated towards the bottom of the rocket?

[LordFerret]

Bah. Even keeping it under 2 g's, I'm still flipping all over the place, there's just no control. Everything is fine going straight up under full throttle however, although the whole ship glows (hot) until 30k ... then all seems 'normal'. Still, reentry is a major issue (BOOM!). :/

[GoSlash27]

GoSlash27, how are you calculating terminal velocity?

Almost all KSP rockets are statically unstable, unless you put rather a lot of fins at the bottom. This is because KSP engines are far heavier than real life, thus the CoM is lower than it should be (you want CoM high and center of pressure--i.e. bot lift and drag--low). That means they need to maintain their own stability by constant control movement (from control surfaces or engine vectoring; reaction wheels are probably too weak).

NathanKell,

I'm actually estimating it by the acceleration/ time and visual cues rather than calculating it, since I don't know exactly what my Cd is.

Acceleration/ time: If you exceed 2G acceleration long enough, you will eventually exceed terminal velocity.

Visual cues: As you approach terminal velocity, you see the formation of shock waves from the pressure shift. The more you push it, the heavier the "fog" gets and the more unstable the craft becomes.

I use these cues as a warning. As the fog gets heavier and stability starts to weaken, I throttle back.

I also use the sine of my pitch angle to set my peak acceleration. 2G when vertical, 1.5G at 50* pitch, and .5G at 30* pitch.

Best,

-Slashy

[moogoob]

Why would you use fins to move the centre of mass? Am i missing something?

edit: you want drag on the bottom of the rocket, fins without control authority actually prevent you from turning the rocket.

D'oh meant drag.

[m4inbrain]

How is it possible to move the center of mass towards the nose when the vast majority of the mass is in the fuel, which is typically concentrated towards the bottom of the rocket?

It's in relation, an important information was left out there.

It's not "you want all the weigh at the nose", it's you want the center of mass as far away from the center of pressure, to get a stabilizing effect. You obviously can't put all the weight in the nosecone.

http://www.nar.org/NARTS/TR13.html

That's a good explanation, alot better than i could do.

edit: thought so moogoob

[NathanKell]

GoSlash27: Ah, gotcha. Well, TV will be close to what it is in FAR, I suspect, so it'll be pretty high, and darn hard to catch even at 2Gs. For the aeroFX, unless things have changed that's based on density * velocity^3 which has an extra factor of velocity compared to drag (drag is 1/2 density * velocity^2) so it probably won't be that reliable. However, it should be very reliable for heating intensity, if heating tracks v^3 (as it probably does).