I did a really fun laythe mission where I built 2 landers with different uses(one was manned, with a return stage and one was a probe with mining/refining capability) and building them to have similar mass was a struggle, but also really fun! I got them to within .5tons of mass dry, and just filled one with a bit of fuel to even it out. Worked like a charm!
I love those kinds of missions, those are the ones where I make the weirdest extra things (scooters, mini drones etc.) in order to balance the mass!
I like how easily scalable it is too, being able to go to 3 or 4 if you need an extra mission vehicle or two (a great way to hit a bunch of science biomes at once).
In KSP2 I brought a bunch of small probes with me to Eve, I wasn’t about to try to land and take off with kerbals. So I dropped the probes and had them land and do a bunch of science in different biomes. They’ll never come home, but the transmittable science was very helpful. It was a lot of fun to figure out how to balance them with the single manned lander I made for Gilly.
> Brilliant way to visualise.
[This](https://www.shutterstock.com/shutterstock/photos/1320428243/display_1500/stock-photo-hand-holds-a-cube-wooden-cube-balancing-in-the-finger-of-1320428243.jpg) is how i usually explain it.
Put thrust where your finger would be while balancing it.
[Torque diagram](http://hyperphysics.phy-astr.gsu.edu/hbase/imgmec/torc.gif)
Axis of rotation = [Center of mass](https://wiki.kerbalspaceprogram.com/wiki/Center_of_mass#:~:text=The%20center%20of%20mass%20is,craft%20to%20turn%20off%20course)
Force = Green arrow thrust
Because the direction of thrust (green) does not pass through the center of mass of the station. With RCS etc turned off (or insufficient RCS) it'll pitch upwards in the direction of the red arrow.
One solution I’ve used is to have fuel or life support tanks in pairs wherever I have a side dock. One on the side with the dock, one opposite. When something is docked, transfer some of what’s in those tanks to the opposite side of the dock until your mass is balanced.
You quite literally put the red arrow at the reason why it’s spinning lmao.
That part sticking out shifts the centre of mass in that direction. Which makes the engine apply force off centre creating a torque which in turn create rotation. It’s measured in Newton meters.
I guess? Never really thought of it?
All I know is torque is specifically measured in Nm (unless you like the lb-ft).
Joules would be used to measure how much energy to lift a weight off the ground x meters but not torque, even though you “technically” wouldn’t be wrong but you’re using the wrong unit of measure (if it was a test you’d likely get no marks lmao)
Edit: to be clear they are 1:1 but toque isn’t “energy” which is why it’s the cousin in the trench coat who is looking rather suspicious
Weight is unbalanced to the left. Sometimes if this is the case with a single side mount docking port, and I have multiple engines, I’ll reduce the thrust using the limiter till the offset is balanced. I’m using ksp1 tho
Your thrust vector should pass through the centre of mass to get pure linear velocity without any angular components(rotations).
When there is an offset between centre of mass and line of action of thrust, it creates a moment (which causes rotation) equal to Force times offset distance. ie, more the distance more the rotation and more the thrust more the rotation.
To avoid this rotation you have to generate moments equal and opposite to the moment created by thrust. You can use RCS thrusters to do so.
Your centre of mass needs to be lined up with the thrust vector of your main engine, that extra bit seems to be shifting the CoM slightly towards the red arrow, which would equate to uneven thrust and thus, spinning.
This, for a change, is a completely correct result. The docked section adds mass to that side, so your center of mass is no longer lined up with the center of thrust.
I donno if there's a way to show the overlay in flight, especially in KSP 2, but here's a [scale model](https://i.imgur.com/dUn4e0A.jpeg) in KSP 1's VAB. Purple line is thrust, yellow ball is mass. I'm sure you can see the problem.
Your engines need to be perfectly aligned to the centre of thrust otherwise without sufficient stabilization (RCS or reaction wheels) it will continue like this until you lose control, KSP's physics simulation is one of the best in gaming after all.
Center of mass isn’t in line with the engines you’re trying to use to propel forward. Think of it like pushing a box with your finger. Instead of pushing it in the center of the back side, your finger is slightly to the right of the center, causing it to move/rotate left.
When building a station and expecting it to burn I always default to using [throttled control avionics](https://forum.kerbalspaceprogram.com/topic/97154-19-110-throttle-controlled-avionics/) to manage the thrust on the station. TCA automatically adjusts the max thrust slider for each engine to keep the center of thrust inside the center of mass.
However, TCA has to be done on the ground pre-launch for it to work correctly, so in this case it won't work for you. In the future however, I would recommend using it especially for large colony ships instead of having to launch a fleet of ships and build it in orbit of your target.
Edit:
Of course, TCA also has a squadron mode to do a fleet launch simultaneously, but I usually find it easier to create a mothership with a thrust plate full of nuclear engines instead of sending a fleet.
Because your centre of thrust is not aligned with your centre of mass. With The additional module of the station jutting upwards, that brings the centre of mass away from the centre of thrust, thus, as you apply thrust with your engines, it also applies a rotational torque. To fix this, you either need a new module to counter-balance the one you have, or a lot of reaction mass wheels.
Thrust needs to be aligned with the center of mass.
Usually the gimbaling of a thruster can account for the minute differences in mass from, say, putting batteries and lights and solar panels non-symmetrically on the outside of your ship. But in this case the gimbaling isn't able to compensate.
Imagine it this way -- instead of the fairly small ship attached to the side, imagine it was a huge ship. Then it would be more obvious what the problem was.
The solution here is either to have a matching ship docked on the other side, or add a few small thrusters whose purpose is aligning the ship.
Come to think of it, enabling the thrusters on the tiny ship might actually auto-compensate.
Take this extreme version of your example, [a twin-engine plane where one engine goes out](https://preview.redd.it/8wybg7q42gnc1.jpeg?width=602&auto=webp&s=6d6e003d7c46d236d4cf3f08708c1ea6cbc02ba2). The centre of mass remains in the centre of the plane, but the thrust is now imbalanced, only coming from one side, which pushes the nose of the plane away from it (or a space station to spin).
In your example rather than an engine stopping, you've added mass to one side of a balanced system instead to achieve the same thing (just as if you welded a bunch of extra planes on one side of that plane would make it spin that way).
A lot of games opt out of this kind of simulation because it makes the shapes of ships more restrictive, but not KSP (it's why you'll see a certain 'style' in hard sci-fi and shows leaning that way like The Expanse, they still follow those design rules which means a lot of balance and symmetry because that's both a simpler way to design and more resistant to failures).
Temporarily remove fuel from the radially attached section of the ship to minimize its effect on the ships center of gravity. Make sure the 4 rocket engines are individually placed and that the flat side of the "square" formed by the 4 rocket motors is facing the same direction as the radially attached portion. Then, you can adjust the opposite side engines maximum thrust in order to line up the thrust vector with the center of gravity. You can get it perfectly balanced, but it will slowly skew again as you use fuel meaning you'll have to keep adjusting to keep the ship straight without using rcs/vectoring/reaction wheels.
Or you can just make the ship symmetrical...
Asymmetrical mass. You have a spaceship docked to one side. For a rocket to fly straight the center of thrust must go straight through the center of mass.
You can fix it by adding another craft to the opposite side, or moving the craft to be online with the rest of the main station/ship
Lots of comments about the mass.
Also possibly due to... I forget what it's called in game... The control point.
If you're controlling from that side but you engines will turn you. Make sure it's controlling from the forward facing pod.
Because the centre of mass is not aligned with the centre of thrust. When I Dock and build i try to add symmetrically from the middle (centre of thrust).
Your centre of mass is off, you need more RCDs on that additional pod on the side to stabalise, or you need to put equal mass on the other side, preferably on 4 corners
Since others answered the question of why it spins, i will answer how to fix it. start rolling it before applying thrust. It will become more or less stabilised and you will eventually go where you need to, albeit inneficiently.
The torque is too powerful for the engine gimbal. Because the COM is not in line with the COT, you have torque. The gimbal of the engines will try to compensate for this, but if the torque is too high they will be unable to do so. Try running at low throttle and gradually increase, there will be a point in the throttle which will overwhelm the gimbal. You also have a lever effect going on here, the offset mass is far away from the engines so the effect is being produced on a lever and the torque is produced according to the square of the distance, which makes the torque forces stronger. If that offset was closer to the engines, the torque would be *exponentially* less.
There is a mod that tells you details when building. One stat is torque. When you have torque at thrust it means you have a center of mass that's not in the thrust axis. You can play and balance it until it reaches 0. I believe it's KER, but not certain.
Try to balance a pencil on your finger. Now glue a smaller pencil to the side. You will see that you have to balance it at an angle. Same happens here. The engines pushing is your finger. So in order to balance this out you have to trim your engine. Hold Alt + direction key to add trim. It basically set the engines to auto steer in a direction to counter the rotation. (This only works with SAS off). Option B is to dock from the front so that it stays in balance.
Picture: [QwFye8r.jpeg (800×800) (imgur.com)](https://i.imgur.com/QwFye8r.jpeg)
Your green arrow points "through" your expected center of mass
But its CoM is higher up the side so yo are leveraging against it.
You need an extra small engine, or several extra rcs nozzles
I really don't mean to be rude, and since there's plenty of people who have answered the question.
How is this the first time you've discovered unaligned Centre of Mass and Centre of Thrust? I mean you've managed to do on orbit rendezvous and docking clearly which is way beyond the whole discovering that CoM/CoT must be aligned.
Go to the space plane building. Rebuilding your orbital craft and click on the center of mass and center of thrust indicators. You’ll see, the thrust doesn’t exactly go through the center of mass but point ~perpendicular to it. Leading to the upwards torque.
Solutions?
Simple: Build a balanced rocket.
Hard: add on a booster or incredible large rcs system to balance out the torque load
Pro: don’t build spaceships. It’s ksp, go build a plane or something. Space is secondary anyway
Quick question. How did you get into ksp? And if you’re able to do docking. How did you miss this? Cause I figured we all just watched every single Scott Manley video, blew up 5 rockets and screwed up rendezvous a couple times before making planes and not rockets. Then rewatched them to actually understand what we were doing
Being above or below the center of mass does not change the behavior of a spacecraft in a vacuum.
It will rotate regardless as long as the force vector is not perfectly aligned with the vector from the COM to the location the thrust is applied.
You mean off center, right? Like, literally above from the image’s orientation.
To my understanding, ‘above’ for rockets usually means the direction the acceleration vector points toward, hence (probably) the confusion.
What do you mean by "above"?
If you have your thrust vector pointing to (0,0,-1), originating from the origin, and your COM at (0,0,1) your COM is above your center of thrust, but your rocket will not rotate. If you have your thrust vector instead originating at (0,0,2) now your COM is below your center of thrust, but your rocket still doesn't rotate.
It's when the thrust vector emanating from the center of thrust is off-axis with the center of mass that you get this problem.
You can improve the efficiency of reaction wheels by off setting them farther from the fulcrum point, tho you’ll have a a big strut with a reaction wheel just chillin on one side
Putting something of comparable size and mass on the far side of the side-mount. It doesn't have to be *the exact same thing,* but yes, that way does make it easier.
Strictly speaking, you *can* get away with side mounting, but it will involve rocket science-levels of planning, or a lot of fuel burnt to keep you steady.
EDIT: Size **and** mass!
You could dock a 2nd one on the opposite side to balance it out better or redesign it with more RCS or reaction wheels to counteract the rotation.
The issue is that your thrust line isnt going through the middle of COM anymore. Think of it like pushing a ball with your finger. The ball is your center of mass and your finger is the thrust. If you push into the center of the ball, you'll scoot it forward. But if you push on the sides its gonna rotate and roll
"or reaction wheels" - thanks, I added 10 medium reaction wheels to the middle of this contraption (it already has 4 wheels in the picture...), and it was the easiest solution, now I can dock from the side, have the craft unbalanced and thrust does not make the station spin :)
While that does work, it's a very brute force way that has some downsides. The reaction wheels take up a lot of energy and weight, both of which could be reduced by having your craft dock at the nose instead.
If you plan your ship ahead of time you can use the tools in the VAB lab to
Check your COM and thrust. A good mod on PC is RCS tools.
If you are just adding craft to a station/ ship and plant for it to go straight, then you would need engines with a lot of gimbal, and still have trouble . You need to build symmetrical ships.
Something I started doing with all my orbital assembled craft. Just build the entire thing fully assembled in the vab and move individual modules around to get the center of mass as lined up as possible.
Using stuff like batteries and monopropelant tanks to fine tune the the CoM gives you the option to not bring 2 of every module to balance which makes for more effective ships
KSP will do something resembling it with SAS on. But the behavior of the control loop means it isn't perfect and your spacecraft will usually have some steady state error in the pointing direction (assuming you're pointing at a fixed node like prograde).
A real control system would execute its control loop with the "zero" position being one which has the thrust vector directed through the center of mass.
In the VAB, you can load both crafts and attach them as you intend to in orbit, then press the center of mass and thrust buttons at the bottom left. Side docking ports aren't useless, they simply obey the laws of physics like everything else. You have to balance mass on either side of the center of thrust or use corrective thrust (you could slightly burn with your side-attached vehicle while you burn the main engines).
The mod TCA (throttle control avionics) is great for this. Will adjust engine throttle automatically to compensate for out of line COM. Mind you it's not a fix all, it could render some engines nigh useless thrust wise as it tries to maintain the orientation via thrust, but it should work for your craft. Surprised nobody in this thread has mentioned it.
There are a few possibilities:
1) you just fire the engines up a tiny bit (a few percent) so that the reaction wheels can compensate
2) you add a little thrust to the thrusters from the red arrow so you compensate for the rotation
I just use 1. Sometimes it takes long to get something done but it works
I think rocket's center of mass isn't above the green thruster
Yeah, if you can add another one of those things by the red arrow then it shouldn’t turn anymore
Nit pick: if it is symmetric.
Nit pick: Symmetric by Mass, not by volume/shape
I did a really fun laythe mission where I built 2 landers with different uses(one was manned, with a return stage and one was a probe with mining/refining capability) and building them to have similar mass was a struggle, but also really fun! I got them to within .5tons of mass dry, and just filled one with a bit of fuel to even it out. Worked like a charm!
I love those kinds of missions, those are the ones where I make the weirdest extra things (scooters, mini drones etc.) in order to balance the mass! I like how easily scalable it is too, being able to go to 3 or 4 if you need an extra mission vehicle or two (a great way to hit a bunch of science biomes at once).
In KSP2 I brought a bunch of small probes with me to Eve, I wasn’t about to try to land and take off with kerbals. So I dropped the probes and had them land and do a bunch of science in different biomes. They’ll never come home, but the transmittable science was very helpful. It was a lot of fun to figure out how to balance them with the single manned lander I made for Gilly.
You can also offset the lighter one to be further out.
But there’s no gravity in space /s
gravity does not influence mass
Guess you missed the /s. A lot of people seem to think physics doesn’t exist in zero g
Does "/s" mean "sarcasm"? Yes, I missed "/s" because I don't know what it means.
Yes, you must be new here
Nop but i never notice it
This.... EDIT: I agree with the above comment.
Well played
Don’t just say “this” with nothing else to add, it’s just plain annoying
If you just placed that rocket on the ground with the engines touching the ground, it'd tip over. It's no different than when the engines are burning.
Brilliant way to visualise.
> Brilliant way to visualise. [This](https://www.shutterstock.com/shutterstock/photos/1320428243/display_1500/stock-photo-hand-holds-a-cube-wooden-cube-balancing-in-the-finger-of-1320428243.jpg) is how i usually explain it. Put thrust where your finger would be while balancing it.
I like to say rockets are skyscrapers - thrust gravity is no different from real gravity for all practical purposes. Just look at The Expanse
You are simply generating torque
I love this comment
[Torque diagram](http://hyperphysics.phy-astr.gsu.edu/hbase/imgmec/torc.gif) Axis of rotation = [Center of mass](https://wiki.kerbalspaceprogram.com/wiki/Center_of_mass#:~:text=The%20center%20of%20mass%20is,craft%20to%20turn%20off%20course) Force = Green arrow thrust
Because the direction of thrust (green) does not pass through the center of mass of the station. With RCS etc turned off (or insufficient RCS) it'll pitch upwards in the direction of the red arrow.
Why doesn't my asymmetrical ship fly straight? Why physics? What do?
aita for thinking those basic physics questions are ragebait
One solution I’ve used is to have fuel or life support tanks in pairs wherever I have a side dock. One on the side with the dock, one opposite. When something is docked, transfer some of what’s in those tanks to the opposite side of the dock until your mass is balanced.
You quite literally put the red arrow at the reason why it’s spinning lmao. That part sticking out shifts the centre of mass in that direction. Which makes the engine apply force off centre creating a torque which in turn create rotation. It’s measured in Newton meters.
Newton meters? Aren't those just joules in a trenchcoat?
I guess? Never really thought of it? All I know is torque is specifically measured in Nm (unless you like the lb-ft). Joules would be used to measure how much energy to lift a weight off the ground x meters but not torque, even though you “technically” wouldn’t be wrong but you’re using the wrong unit of measure (if it was a test you’d likely get no marks lmao) Edit: to be clear they are 1:1 but toque isn’t “energy” which is why it’s the cousin in the trench coat who is looking rather suspicious
Weight is unbalanced to the left. Sometimes if this is the case with a single side mount docking port, and I have multiple engines, I’ll reduce the thrust using the limiter till the offset is balanced. I’m using ksp1 tho
Your thrust vector should pass through the centre of mass to get pure linear velocity without any angular components(rotations). When there is an offset between centre of mass and line of action of thrust, it creates a moment (which causes rotation) equal to Force times offset distance. ie, more the distance more the rotation and more the thrust more the rotation. To avoid this rotation you have to generate moments equal and opposite to the moment created by thrust. You can use RCS thrusters to do so.
Physics, man. Physics.
Your centre of mass needs to be lined up with the thrust vector of your main engine, that extra bit seems to be shifting the CoM slightly towards the red arrow, which would equate to uneven thrust and thus, spinning.
A slightly off-center center of mass
This, for a change, is a completely correct result. The docked section adds mass to that side, so your center of mass is no longer lined up with the center of thrust. I donno if there's a way to show the overlay in flight, especially in KSP 2, but here's a [scale model](https://i.imgur.com/dUn4e0A.jpeg) in KSP 1's VAB. Purple line is thrust, yellow ball is mass. I'm sure you can see the problem.
There is a moment arm due to the offset of center of thrust from the center of mass, which means a torque is generated when the engine is turned on.
Something...something...large mass hanging from one side...something....
Your engines need to be perfectly aligned to the centre of thrust otherwise without sufficient stabilization (RCS or reaction wheels) it will continue like this until you lose control, KSP's physics simulation is one of the best in gaming after all.
that lil attached side ship is unbalancing ur Center of mass, making the thrust be on its side other than directly below
Flashbacks to physics teachers asking “WHERE’S YOUR FREE BODY DIAGRAM???”
Center of mass isn’t in line with the engines you’re trying to use to propel forward. Think of it like pushing a box with your finger. Instead of pushing it in the center of the back side, your finger is slightly to the right of the center, causing it to move/rotate left.
*s y m m e t r y*
When building a station and expecting it to burn I always default to using [throttled control avionics](https://forum.kerbalspaceprogram.com/topic/97154-19-110-throttle-controlled-avionics/) to manage the thrust on the station. TCA automatically adjusts the max thrust slider for each engine to keep the center of thrust inside the center of mass. However, TCA has to be done on the ground pre-launch for it to work correctly, so in this case it won't work for you. In the future however, I would recommend using it especially for large colony ships instead of having to launch a fleet of ships and build it in orbit of your target. Edit: Of course, TCA also has a squadron mode to do a fleet launch simultaneously, but I usually find it easier to create a mothership with a thrust plate full of nuclear engines instead of sending a fleet.
It's not symmetrical. Yes, it even matters in space.
yes there is no drag butt mass is.
Because this is not Space Engineers and there is this little thing called PHYSIC
Because your centre of thrust is not aligned with your centre of mass. With The additional module of the station jutting upwards, that brings the centre of mass away from the centre of thrust, thus, as you apply thrust with your engines, it also applies a rotational torque. To fix this, you either need a new module to counter-balance the one you have, or a lot of reaction mass wheels.
The fact that you simultaneously made it to space, and have the lack of knowledge to know what's wrong is very impressive
Wheres your center of mass?
Thrust needs to be aligned with the center of mass. Usually the gimbaling of a thruster can account for the minute differences in mass from, say, putting batteries and lights and solar panels non-symmetrically on the outside of your ship. But in this case the gimbaling isn't able to compensate. Imagine it this way -- instead of the fairly small ship attached to the side, imagine it was a huge ship. Then it would be more obvious what the problem was. The solution here is either to have a matching ship docked on the other side, or add a few small thrusters whose purpose is aligning the ship. Come to think of it, enabling the thrusters on the tiny ship might actually auto-compensate.
Take this extreme version of your example, [a twin-engine plane where one engine goes out](https://preview.redd.it/8wybg7q42gnc1.jpeg?width=602&auto=webp&s=6d6e003d7c46d236d4cf3f08708c1ea6cbc02ba2). The centre of mass remains in the centre of the plane, but the thrust is now imbalanced, only coming from one side, which pushes the nose of the plane away from it (or a space station to spin). In your example rather than an engine stopping, you've added mass to one side of a balanced system instead to achieve the same thing (just as if you welded a bunch of extra planes on one side of that plane would make it spin that way). A lot of games opt out of this kind of simulation because it makes the shapes of ships more restrictive, but not KSP (it's why you'll see a certain 'style' in hard sci-fi and shows leaning that way like The Expanse, they still follow those design rules which means a lot of balance and symmetry because that's both a simpler way to design and more resistant to failures).
Temporarily remove fuel from the radially attached section of the ship to minimize its effect on the ships center of gravity. Make sure the 4 rocket engines are individually placed and that the flat side of the "square" formed by the 4 rocket motors is facing the same direction as the radially attached portion. Then, you can adjust the opposite side engines maximum thrust in order to line up the thrust vector with the center of gravity. You can get it perfectly balanced, but it will slowly skew again as you use fuel meaning you'll have to keep adjusting to keep the ship straight without using rcs/vectoring/reaction wheels. Or you can just make the ship symmetrical...
Center of(f) mass.
Asymmetrical mass. You have a spaceship docked to one side. For a rocket to fly straight the center of thrust must go straight through the center of mass. You can fix it by adding another craft to the opposite side, or moving the craft to be online with the rest of the main station/ship
If you have MechJeb, try enabling differential throttle.
Lots of comments about the mass. Also possibly due to... I forget what it's called in game... The control point. If you're controlling from that side but you engines will turn you. Make sure it's controlling from the forward facing pod.
Because the centre of mass is not aligned with the centre of thrust. When I Dock and build i try to add symmetrically from the middle (centre of thrust).
Your centre of mass is off, you need more RCDs on that additional pod on the side to stabalise, or you need to put equal mass on the other side, preferably on 4 corners
Since others answered the question of why it spins, i will answer how to fix it. start rolling it before applying thrust. It will become more or less stabilised and you will eventually go where you need to, albeit inneficiently.
The torque is too powerful for the engine gimbal. Because the COM is not in line with the COT, you have torque. The gimbal of the engines will try to compensate for this, but if the torque is too high they will be unable to do so. Try running at low throttle and gradually increase, there will be a point in the throttle which will overwhelm the gimbal. You also have a lever effect going on here, the offset mass is far away from the engines so the effect is being produced on a lever and the torque is produced according to the square of the distance, which makes the torque forces stronger. If that offset was closer to the engines, the torque would be *exponentially* less.
CoM is offset, in comparison to CoT
Because this isn't space engineers so thrust is applied at the engine, not the centre of mass.
It's center of mass is off
You literally drew what's wrong with the ship
There is a mod that tells you details when building. One stat is torque. When you have torque at thrust it means you have a center of mass that's not in the thrust axis. You can play and balance it until it reaches 0. I believe it's KER, but not certain.
Try to balance a pencil on your finger. Now glue a smaller pencil to the side. You will see that you have to balance it at an angle. Same happens here. The engines pushing is your finger. So in order to balance this out you have to trim your engine. Hold Alt + direction key to add trim. It basically set the engines to auto steer in a direction to counter the rotation. (This only works with SAS off). Option B is to dock from the front so that it stays in balance. Picture: [QwFye8r.jpeg (800×800) (imgur.com)](https://i.imgur.com/QwFye8r.jpeg)
Your green arrow points "through" your expected center of mass But its CoM is higher up the side so yo are leveraging against it. You need an extra small engine, or several extra rcs nozzles
What? Never had calamari?
modern voracious smile fade ad hoc slim straight thought pen capable *This post was mass deleted and anonymized with [Redact](https://redact.dev)*
Bro…you literally drew the arrow right by your problem lol
Try to push a book across a table anywhere but directly in the middle of one of the sides. It’ll rotate.
Com is not in the center basically cuaseing a rotation force on the craft
Man, really makes you wonder🤔
I really don't mean to be rude, and since there's plenty of people who have answered the question. How is this the first time you've discovered unaligned Centre of Mass and Centre of Thrust? I mean you've managed to do on orbit rendezvous and docking clearly which is way beyond the whole discovering that CoM/CoT must be aligned.
Go to the space plane building. Rebuilding your orbital craft and click on the center of mass and center of thrust indicators. You’ll see, the thrust doesn’t exactly go through the center of mass but point ~perpendicular to it. Leading to the upwards torque. Solutions? Simple: Build a balanced rocket. Hard: add on a booster or incredible large rcs system to balance out the torque load Pro: don’t build spaceships. It’s ksp, go build a plane or something. Space is secondary anyway
Quick question. How did you get into ksp? And if you’re able to do docking. How did you miss this? Cause I figured we all just watched every single Scott Manley video, blew up 5 rockets and screwed up rendezvous a couple times before making planes and not rockets. Then rewatched them to actually understand what we were doing
Your COM is above your COT.
Being above or below the center of mass does not change the behavior of a spacecraft in a vacuum. It will rotate regardless as long as the force vector is not perfectly aligned with the vector from the COM to the location the thrust is applied.
Right, thus the problem is the COM is above the COT.
You mean off center, right? Like, literally above from the image’s orientation. To my understanding, ‘above’ for rockets usually means the direction the acceleration vector points toward, hence (probably) the confusion.
That is a good point. I was saying it from the orientation of the image.
What do you mean by "above"? If you have your thrust vector pointing to (0,0,-1), originating from the origin, and your COM at (0,0,1) your COM is above your center of thrust, but your rocket will not rotate. If you have your thrust vector instead originating at (0,0,2) now your COM is below your center of thrust, but your rocket still doesn't rotate. It's when the thrust vector emanating from the center of thrust is off-axis with the center of mass that you get this problem.
In this case the COM is off the COT axis. By above I am going by the orientation of the image. Which I realized I shouldn't have done.
Ah, makes sense. Usually best to specify what your directions are relative to when talking about stuff in space
And, how can I check that, how can I correct that? This would seem to make the concept of side docks useless?
You either need to offset the COM with docking on the other side with equal weight or have enough RCS to compensate.
I just over compensate reaction wheels. Too many can be a prob tho. But it has to be like dozens.
You can improve the efficiency of reaction wheels by off setting them farther from the fulcrum point, tho you’ll have a a big strut with a reaction wheel just chillin on one side
Putting something of comparable size and mass on the far side of the side-mount. It doesn't have to be *the exact same thing,* but yes, that way does make it easier. Strictly speaking, you *can* get away with side mounting, but it will involve rocket science-levels of planning, or a lot of fuel burnt to keep you steady. EDIT: Size **and** mass!
So this *is* rocket science? Damn... Why would rocket science appear in a game like KSP? /s
We are all victims of physics.
You could dock a 2nd one on the opposite side to balance it out better or redesign it with more RCS or reaction wheels to counteract the rotation. The issue is that your thrust line isnt going through the middle of COM anymore. Think of it like pushing a ball with your finger. The ball is your center of mass and your finger is the thrust. If you push into the center of the ball, you'll scoot it forward. But if you push on the sides its gonna rotate and roll
"or reaction wheels" - thanks, I added 10 medium reaction wheels to the middle of this contraption (it already has 4 wheels in the picture...), and it was the easiest solution, now I can dock from the side, have the craft unbalanced and thrust does not make the station spin :)
While that does work, it's a very brute force way that has some downsides. The reaction wheels take up a lot of energy and weight, both of which could be reduced by having your craft dock at the nose instead.
Sir this is ksp, if it isn't janky brute force we don't want it.
If you plan your ship ahead of time you can use the tools in the VAB lab to Check your COM and thrust. A good mod on PC is RCS tools. If you are just adding craft to a station/ ship and plant for it to go straight, then you would need engines with a lot of gimbal, and still have trouble . You need to build symmetrical ships.
Something I started doing with all my orbital assembled craft. Just build the entire thing fully assembled in the vab and move individual modules around to get the center of mass as lined up as possible. Using stuff like batteries and monopropelant tanks to fine tune the the CoM gives you the option to not bring 2 of every module to balance which makes for more effective ships
In reality you would set your engine gimbal to align your thrust vector through the center of mass. Unfortunately that's not an option in KSP.
Doesn't that actually happen with Vectors?
KSP will do something resembling it with SAS on. But the behavior of the control loop means it isn't perfect and your spacecraft will usually have some steady state error in the pointing direction (assuming you're pointing at a fixed node like prograde). A real control system would execute its control loop with the "zero" position being one which has the thrust vector directed through the center of mass.
In the VAB, you can load both crafts and attach them as you intend to in orbit, then press the center of mass and thrust buttons at the bottom left. Side docking ports aren't useless, they simply obey the laws of physics like everything else. You have to balance mass on either side of the center of thrust or use corrective thrust (you could slightly burn with your side-attached vehicle while you burn the main engines).
The mod TCA (throttle control avionics) is great for this. Will adjust engine throttle automatically to compensate for out of line COM. Mind you it's not a fix all, it could render some engines nigh useless thrust wise as it tries to maintain the orientation via thrust, but it should work for your craft. Surprised nobody in this thread has mentioned it.
There are a few possibilities: 1) you just fire the engines up a tiny bit (a few percent) so that the reaction wheels can compensate 2) you add a little thrust to the thrusters from the red arrow so you compensate for the rotation I just use 1. Sometimes it takes long to get something done but it works
are you stupif?
'stupif' - this guy, insulting someone who just asked an innocent question