Stable rockets and general questions

[lemonhands]

I'm currently still playing the 0.13 demo untill I can get a new debit card to buy the full game, so excuse me if some of these questions seem a little obvious. I've only been playing for two days.

What causes a rocket to divert from a path perpendicular to the launch pad? Asymmetry? Unstable parts parts? I always use the symmetry button and last launch noticed my rockets engines had a little wobble in them, and I figured if I could get them stable that would stay straight and I would get a better line into space... however it didn't seem to change much. I always need advanced SAS on my ships, the regular ones don't cut it.

Also, how does SAS work? Does on module cover the movement for an entire ship? Will multiple SAS systems do anything? Does the layer order have any effect on SAS? Does layering order have any effect on struts, fuel lines, fuel tanks, wings... anything that isn't an engine or a decoupler?

I've watched a few video on youtube on how to time orbital slingshots to mun, and I've gotten out there and gotten an orbit around it, but how do I (theoretically) get to planets past that? I could estimate an average velocity of my craft, but how could I measure distance to make sure my rocket ends up where it's supposed to?

Also, what are prograde and retrograde... I'm guessing prograde is pointing the direction you're traveling and retrograde is pointing the opposite?

[Festivefire]

1.) what causes ships to wobble on the pad, or even fall over, is when they either are asymmetrical or when they are to heavy for the bottom stage to hold up. to fix that, use other parts as a launch clamp, that decouple when the main engine ignites.

2.) SAS uses a tourqe wheel that exerts i think 5 kerbal force units around the axis of were it is placed to point the rocket in the direction it was when it was turned on. more sas units will exert more force, but the scale on which they increase and decrease force based on the amount of steering error often causes it to over correct on larger rockets.

3.) for planetary transfers i recommend using either a mod like Protractor-Rendez which is only compatible with the full version, but how would you use it without any where to go in the demo anywho, or using this online calculator and a protractor on your screen, or mechjeb (super awesome auto-pilot) will tell you your angles as well.

4.)you are correct about prograde and retrograde, prograde is marked on the navball by a yellow circle with three ticks pointing out of it, and retrograde is marked by a yellow circle with an x through it.

[Vanamonde]

Rockets wobble for a number of reasons. That's realistic and unremarkable, so the important question is not what causes it, but how to control it.

For one thing, the longer a rocket is, the more it flexes along its length, so when designing a ship, it's best to keep it as short as possible by building outward. Right now, there's not much penalty for that because air resistance is not fully implemented in this version of the game.

SAS is supposed to be the more simple version, and is good at stopping unwanted rotation around the long axis, but not so good at stabilizing the other two axes. ASAS is supposed to be the advanced version. It's a little weaker at stopping lengthwise rotations, but better at stabilizing the other two axes. SAS exerts a force, and more than one exerts more force. ASAS exerts a lesser force, but is amplified by contol devices such as fins and RCS thusters. More than one ASAS does not exert noticeable additional control. In practice, one ASAS and no SAS is enough to control all but the largest rockets.

The two means of effecting control are control surfaces (fins, elevators, canards) and the "thust vectoring"-capable engines. Surfaces only work in atmosphere, while vectoring engines work anywhere, though they are slightly less powerful than static engines. If your ship doesn't have some of these things, it's essentially an unguided, ballistic missile that just goes where it's pointed, and can't be steered.

Also, look at your ship as it flies. Are the parts flexing with respect to each other? If so, that can contribute to wobbliness. You want a rigid rocket (who doesn't?), so attaching the flexing parts to other things with struts may help stabilize the whole ship.

Getting to other planets is almost exactly the same problem as getting two ships to rendezvous in Kerbin orbit. Have you tried that? Practice that for a while, and planet-voyaging will seem familiar to you when you get the full game. Just pretend the planet is the target ship for the rendezvous.

"Prograde" is indeed the direction of the ship's travel, but it can also refer to the direction a planet is rotating, or to the direction the planet is orbiting the sun. "Retrograde" is 180 degress away from whatever you're calling prograde in your current frame of reference.

Edited October 4, 2012 by Vanamonde

[Temstar]

A rocket is like a barely controlled sustained explosion. It's an inherently unstable machine that require active steering to stay on track, as Robert Goddard found out after launching his. It's impossible both on real life and in KSP to build a rocket that goes straight without active steering. Since manual steering is quite hard you'll generally need ASAS or other autopilot on your rocket.

SAS is a reaction wheel, it can exert force to make your spacecraft rotate on any of the three rotational axis. The command pods have a built in miniture SAS which is sufficient for small ships. ASAS is completely different, ASAS is a computer that uses any tools that can provide steering force (SAS, command pod SAS, control surfaces, gimbling engine, RCS system) to keep the rocket on the same heading when toggled on.

[Vanamonde]

in KSP to build a rocket that goes straight without active steering

Not to be nitpicky, but it is possible to make a gyroscopically stabilized rocket, but then ALL it can do is go in a straight line.

[vexx32]

Yeah, I've seen one of those... Tough to build, but they work quite well.

[lemonhands]

Fantastic information, thanks a bunch. Learned alot about SAS and now I can apply it to my rockets.

I usually have a final stage with side mounted fuel tanks, and off of those I would make longer fuel tanks for exiting atmo, but if you have two fuel tanks linked up to a horizontal decoupler it will not stop you fuel flow, so I didn't know what to block it with. I was putting RCS fuel under them to block the flow and keep it for the final stage, now I'll put SAS under it since I know my main ASAS will tap into them to better control my ship. I also strutted the hell out of the thing, I have to have the struts believable and symmetrical. It irks me when I see people on youtube just slap a strut on sideways, even though it's symmetrical because of the symmetry tool it's just like... damnit man, you're ruining it!

For some reason I just don't like the idea of an autopilot, I want to do it all myself. And I always use thrust vector capable rockets, I try to stay away from solid boosters all together.

In my thanks I give you my flag ship: Wings of the Lemon God

[Temstar]

but it is possible to make a gyroscopically stabilized rocket

You mean like the V-2? But the gyroscopes still control stuff to keep the rocket on track. In the case of V-2 it's rudders and graphite exhaust vanes.

You can build spin stabilized rockets I suppose, but that's still diverting some of the thrust into spinning the rocket so as to keep it under control rather than an uncontrolled rocket going straight by itself.

[Festivefire]

he doesn't mean a gyroscopic nav system that uses gyroscopes to detect acceleration. he means a spin-stabalized rocket. same concept as a football.

[lemonhands]

With all this talk about a straight flying rocket: I ask because I think it's what's attributing to the erratic behavior of my rocket, I try and launch at a very, very slight incline until I past the first layer of atmo, light blue, and then for each layer curve more into a circular orbit. I've got this down and am now able to nail a near perfect equatorial Kerban orbit every time. Thing is, if I push my rocket too much in terms of angle, it will lost control and lean way too far despite disabling SAS and counter steer, it will lean back the other way and go out of control. I have to cut engines and prematurely eject the stage, and then parachute into the ocean. I can't make up all that delta V.

[Festivefire]

a solution to a rocket that leans out of control if you over tilt it is to try to keep weight at the top, not at the bottom, distribute tanks radialy on launch and core stages, not verticly, and your orbital craft should be very small compared to the launch vehicle, also you can add upper canards to help with it tilting over, and put them on radial decouplers to shed deadweight as they are useless out of atmosphere.

edit: if your first stage flies straight up, you could attach radial engines at an angle for spin stabalization, and have them on each core stage as well, so you can shed a stage, then tilt, then activate the spin stabilizing moters, and decouple them with the entire stage, or decouple them early for the next tilt over depending on timing.

[Nibb31]

Some Russian ICBMs used a mass that moved inside to modify the center of mass to steer the rocket. Wacky, but it worked !

[fenderzilla]

just add plenty of SAS modules and make sure you press "t" to turn them on before takeoff.

[foamyesque]

just add plenty of SAS modules and make sure you press "t" to turn them on before takeoff.

*wince*

That was terrible advice even back when SAS modules and pods were literally the only way to steer a rocket.

[stupid_chris]

wow I noticed the necroing at "playing the .13 demo"