Can't I PULL an asteroid instead of push?

[Shenji]

https://steamcommunity.com/sharedfiles/filedetails/?id=2846281409
I can't pull this asteroid by this mini Nostromo. The asteroid's weight is 113 tons, the thrusters have 4x120kN so this should work. But I can't change the orbit for even 1 meter.
What's wrong here?

[TJ_4874]

I'm thinking that the exhaust of the rocket engines is pushing the shuttle forward but pushing the asteroid the opposite way, evening out the forces the get a net 0 of force in any or all directions

[Shenji]

But I think the ship should at least affect the orbit a bit? I can't even change 0.1 m/s of velocity, that's just feel... wrong...

[Shenji]

https://steamcommunity.com/sharedfiles/filedetails/?id=2846312814
Now it worked. TJ you are right, thanks!

[TJ_4874]

Publicado originalmente por Shenji:

https://steamcommunity.com/sharedfiles/filedetails/?id=2846312814
Now it worked. TJ you are right, thanks!

wait i was?

[Shenji]

Yes. Looks like you should not put boosters too near to another body that block your thrust (although I still didn't figure out how near is too near). I removed the long cross-arms in pic and it still works, so just a bit distances then everything will be fine.
ps- Forgot the pic:
https://steamcommunity.com/sharedfiles/filedetails/?id=2846334635

[koimeiji]

Yes, you can pull asteroids, as you've discovered (exhaust has physics!)

No, it won't make any actual difference. See: Pendulum rocket fallacy.

[Shenji]

I'm not very familiar with the conditions about the 'Pendulum' in the pendulum rocket fallacy, but in my case, an asteroid puller (the Nostromo Jr.) is far easier to control than a pusher. Only need to disable the gimbals and you may noticed I put 3x 2.5m reaction wheels amid the ship. And I don't change directions while accelerating. I think this is, maybe, also the reason why most modern civil market vehicles put engines forward rather than aft/stern.
I'll read something about this in the future. Any suggestions will be appreciated.

[Chibbity]

The pendulum rocket fallacy really applies more to a vehicle operating in gravity trying to leave a planet, doesn't it?

In space (atleast in KSP), pulling a massive object is more stable than pushing it. Whether that's true in real life or not I don't know.

Might just be a symptom of the wobbly rocket physics, but even so; I'd tend to stick with pullers for asteroid retrievals.

[Shenji]

I can't find detailed scientific explanation about this pendulum rocket fallacy, only thing I can find in wiki is a short under-developed section in "Rocket" article.

I think this 'fallacy' only applies when in non-controlled vehicles, when there's any kind of control/SAS, pulling(thruster above CoM) is more stable than pushing. Especially when doing asteroid hauling missions, you can hardly find the exactly CoM of the asteroid.

ps- If you ever haul a trailer by your car then you can easily understand this. Everyone who can drive can pull a trailer easily. But only professional trucker can 'push'(by reverse the trailer head) a trailer into designated position, but much slower and with great caution.

[Chibbity]

The pendulum fallacy is basically the concept that when launching a rocket you could put all the weight at the rear "hanging" and it would self stabilize. (Like a pendulum.)

It's a fallacy because it doesn't actually work that way, a good rocket looks like an arrow not a pendulum.
Here's a decent explanation:
https://handwiki.org/wiki/Astronomy:Pendulum_rocket_fallacy

There is a lot of complicated math behind why, but as I said above, I believe gravity plays a large role in that so in space I wouldn't think the fallacy applies in the same way.

[Manwith Noname]

Not rockets but ultimately the same principle.

https://www.youtube.com/watch?v=OYHCP3-mpxk

[Quizzical]

Publicado originalmente por Shenji:

Yes. Looks like you should not put boosters too near to another body that block your thrust (although I still didn't figure out how near is too near). I removed the long cross-arms in pic and it still works, so just a bit distances then everything will be fine.
ps- Forgot the pic:
https://steamcommunity.com/sharedfiles/filedetails/?id=2846334635

That's partially a question of how exactly the game's air simulation differs from real life.

[Quizzical]

When trying to prevent drag from flipping your rocket, you commonly want to put the drag near the bottom of the rocket if you can. That "if you can" is a huge if, though. If your rocket is shaped roughly like an upside-down T, then if it tilts to one side during its ascent, having the side stick out further on the side that the rocket is pointing away from will mean more downward force on that side, which forces the rocket back upright.

That only works when you're traveling fast enough in a thick enough atmosphere for it to have a meaningful chance of flipping you over, though. That can apply both to ascent from Kerbin (or Eve!) and also to diving into a thick atmosphere. It's the reason why I commonly put inflatable heat shields on the back of a rocket that is trying to land on Eve: to mean that the heat shields in the front actually stay in front rather than flipping over.

[koimeiji]

Publicado originalmente por Chibbity:

The pendulum fallacy is basically the concept that when launching a rocket you could put all the weight at the rear "hanging" and it would self stabilize. (Like a pendulum.)

It's a fallacy because it doesn't actually work that way, a good rocket looks like an arrow not a pendulum.
Here's a decent explanation:
https://handwiki.org/wiki/Astronomy:Pendulum_rocket_fallacy

There is a lot of complicated math behind why, but as I said above, I believe gravity plays a large role in that so in space I wouldn't think the fallacy applies in the same way.

IIRC the fallacy exists because gravity doesn't actually do anything to stabilize the rocket, against common sense.

So there shouldn't be any difference whether you're in free-fall or not; the situations should be the exact same, no? That is to say, there shouldn't be a real difference between pushing and pulling.


Of course, KSP isn't exactly 100% real; little phantom forces and flexing here and there might make pulling slightly more stable than pushing...but I doubt it'd actually matter that much. I don't really ♥♥♥♥ with asteroids anyways; too big, too annoying, not much reward besides "waow i did it!" Though I have experimented with push/pulling interplanetary ships, and I've never noticed a difference.

[Shenji]

Pulling has easier control over pushing especially when target object is:
much heavier, thus much more inertia and/or friction;
can't easily connect with the center of mass;
more than one part and loose connected(no auto strut or other meaning of strut).
I think this is not exactly the same category with the pendulum rocket fallacy.
But still thanks for mentioning it, I have some good reading.