"Grab" Docking?

[dharak1]

So I had a thought, if something was in a geostationary orbit and you lunched something up to dock to it would you need to get an orbit? I mean you would have to go slightly west to account for the slow rotation. But would this be possible?

[Dizzle]

I'd think that without enough lateral velocity to get an orbit, the target would have a lot of lateral velocity relative to your craft.

[dharak1]

Hmmmm... I'm not sure if you keep your lateral ground velocity 0 and since its in a geostationary should they not have the same velocity? maybe I'm wrong but I am about to go to sleep and have no time to test it.

[Fel]

First, I would say Geo-Stationary adds zero value... you might as well just go out to the edge of Kerbin's Sphere of Influence to get an orbital velocity as slow as possible.

But, of course, the further out you are, the longer it takes to get there. Kerbin's geo is around 2.5Mm? which isn't a cake walk to get to. (All the while the position keeps moving meaning you'll end up having the same rendezvous issues.)

And of course, you'll change the orbit rather significantly (with the magical magnetic docking ports) if you do manage to dock.... only if the game doesn't tear the space station apart ;p

Hmmmm... I'm not sure if you keep your lateral ground velocity 0 and since its in a geostationary should they not have the same velocity? maybe I'm wrong but I am about to go to sleep and have no time to test it.

No, the velocity vector due to rotation is constantly changing. If you throw an object up in the air, it will move relative to the *CHANGE* in the earth's "Instantaneous Velocity Vector at a moment in the earth's rotation"

*Yeah, that is confusing.

Edited June 6, 2013 by Fel
fixing minor error

[Punk]

Despite being stationary relative to a point on the surface of a planet, the object in geo-sync orbit is still travelling at high speeds (for geo-synch orbit the speed is simply equal to the rotational rate of the parent body). If you could fire the rocket to intersect the orbiting craft perfectly you would find that the orbiting ship would still shoot by at several Km's a second. If you could launch perfectly and somehow dock without everything getting torn to shreds in an instant, you would find the initial orbit of the ship in geo-synch would have decayed significantly, as its total velocity is now spread over more mass with no equal increase in kinetic energy.

[Punk]

No, the velocity vector due to rotation is constantly changing. If you throw an object up in the air, it will move relative to the earth's rotation.

Close, as soon as the object leaves your hand it is completely independent to the Earth. It's trajectory is set in stone as soon as it leaves your hand, air resistance does factor in but only usually marginally for low speeds. If the Earth were to instantly stop rotating as soon as you let go, the ball would fly away at something like 25 Km's a second (as would everything else attached to its surface, like mountains and stuff).

I'm not sure I've got that right. I've forgotten too much of my school physics...

Edited June 6, 2013 by Punk

[Vanamonde]

An object in geostationary orbit is still in an orbit. To dock with something, you need to match orbits. Therefore, anything that docks with a geostationary object will, whatever method you use of putting it there, end up in a geostationary orbit. So to answer your question, you could fly straight to an intersect point from the surface, assuming you lead the target enough to allow for its motion while you're en route, but you'd just have to burn a lake's worth of fuel to match speeds once you got there, because you'd arrive almost stationary while the target was zipping past you 90 degrees from your path.

[rodion_herrera]

Why not try it? I mean, if KSP didn't exist, this would be one of those "theory" threads. But we do have something like KSP, so why not try it?

[Temstar]

So I had a thought, if something was in a geostationary orbit and you lunched something up to dock to it would you need to get an orbit? I mean you would have to go slightly west to account for the slow rotation. But would this be possible?

Your misconception comes from thinking that orbital velocity at KSO is the same as your "orbital" velocity on the surface of Kerbin just standing still due to Kerbain's rotation because an object in KSO is just hanging motionless overhead. In reality there's some 780m/s difference between the two.

Imagine a small circle, a dot traces this circle and goes around it once every 6 hours. This is a person standing on the surface of Kerbin at the equator and he is rotation around the centre of Kerbin at 174m/s. This is Kerbin's rotational speed.

Then imagine a much bigger circle. Another dot traces this circle and also goes around it every 6 hours. This is a Keostationary satellite. This satellite is most definitely not flying around at 174m/s because it makes the circle in 6 hours and its circle is much bigger than the person. This is satellite's orbital velocity is much higher than an stationary object on the equator.

If you shoot a spacecraft directly up to KSO it will be still at 174m/s. If you do this in a way so that the Keostationary satellite and the spacecraft arrive at the same point in space a the same time what you'll see is the satellite slamming into the spacecraft at 780m/s, or 2808km/h, or about mach 2.3. This kind of velocity is about as fast as a bullet from a handgun and the "grab" docking is going to look pretty violent.

[loppnessmonsta]

Dharak1, what you describe is called a missile.

[rodion_herrera]

Ok let's do this the right way. From what I can ascertain, based on the OP's description, the initial situation is like this, yes?

I'll be modifying this based on what I can understand with some of your replies...

So basically what the OP wants is to fly his rocket directly to the station because it's geosynched, yes?

Now according to Temstar, the green guy's speed is 174 m/s and the yellow station is moving at 780 m/s...correct?

Edited June 6, 2013 by rodion_herrera

[Temstar]

No the station is moving at (someone correct me if I'm wrong) 1009m/s, there is a difference of 835m/s (did my maths wrong the first time, sorry).

To get an idea how powerful an impact at 835m/s is, have a look at this video:

www.youtube.com/watch?v=aSVfYwdGSsQ

See that, but three times faster. Or alternatively two rocket sleds going 50% faster than the one in the video in a head on collision.

Edited June 6, 2013 by Temstar

[rodion_herrera]

No the station is moving at (someone correct me if I'm wrong) 1009m/s, there is a difference of 835m/s (did my maths wrong the first time, sorry).

Yes, but my video is correct in the depiction? You agree with this?

[rodion_herrera]

UPDATE: Added a flight path (white dotted line) and the rocket (red dot)...

So the red dot, is what the user expects his rocket is doing, but in reality, the path of the rocket is the dotted white line. But I'm curious, is this what really happens, and so what the OP says, can still happen, right?

[Sathurn]

Ever notice when you burn straight up, you fall strait down and land WEST of KSC?

You would not have the lateral speed needed to dock, And if you manager your apogee perfectly and dock, the new mass would pull the Geostationary objected out of orbit.

[Sma]

Challenge accepted. Well...sort of. I originally thought this was about just docking with a geostationary object but I guess it's talking about a launch going straight up with no gravity turn? There's been some debate about gravity turns and straight up launches. Even straight up launches do get somewhat of an orbit because of the rotation of the planet. I guess it could be possible to do this but you would have to take into account the rotation of Kerbin, and the time it would take to go from launch to geostationary.

I may still try it though, so long as I can make a rocket that goes straight up without too much lag.

[Zapmunk]

Unless I've forgotten everything I've learned in physics, you have to be moving at the same velocity as the satellite in order to dock.

The satellite is moving with large tangential velocity relative to the planet (ignoring rotation), and the straight-up rocket is moving with small tangential velocity relative to the planet (ignoring rotation).

Thus, in order to dock, you would have to burn to match your velocities once you rendezvoused, which would accomplish the same thing as establishing an orbit in the conventional manner.

Once your rocket leaves the launchpad, Kerbin no longer applies any force to the rocket (disregarding air resistance), so the effect of the rotation of the planet is negligible after you launch.

You can test this quite easily by launching a rocket straight up and letting it fall back down- it won't land on the launchpad, but to the west of it, because the planet rotates underneath your trajectory.

In regards to the original proposition- I don't believe such a thing is possible. The magnets on the docking ports aren't strong enough to produce the ÃŽâ€v required to match their velocities, and that's assuming that you actually line them up properly and they don't just smash into each other.

[aven17]

You don't gain any additional orbital speed launching straight up. As you fly further away from the surface, you must increase orbital speed to stay directly over the point you launched from. It is possible you could launch at a specific angle in order to reach the object in geostationary orbit, but straight up won't get it done.

[The Jedi Master]

If I had a geosynchronous object in orbit, I would certainly try it. But I don't think you would have time to dock, since you would be on a suborbital trajectory.

[Motokid600]

What's the benefit of a station in geosync? Even if you do manage to dock wouldn't you have burned more fuel then its worth? Circularizing a 2.5mkm orbit would take just as much fuel as killing your relative velocity if you were to try it Rodion's way... No?

[Sma]

I've put up the first part of testing this, though it still needs to be circularized. Hopefully the 2nd launch will have the same inclination. As other's have said it doesn't really accomplish much other than maybe docking faster, and maybe bragging rights, if you could somehow manage it. You'd likely be better off just doing a normal launch but planning it so that both crafts meet up before the 2nd craft makes 1 orbit. It would sorta be like the 6hr transfer from launch to dock with the ISS that they've been doing lately. Pretty cool stuff.

[honolululu]

Let's say the space station is right above KSC in a geostationary orbit.

If you want to launch 'straight up' to the station you are going to have to consistently raise your lateral speed as you get up to the station, and further away from KSC. Resulting in perfectly matching orbits when you reach the station, provided you remained on the line between the space station and KSC all the way up.

EDIT: Just like rodion_herrera's frame2 video above. Nice, would be cool if the dots spread out equally over time, and the station laid them down also.

Edited June 7, 2013 by honolululu

[Ruedii]

You would have to burn forward with Kerbin's rotation. Otherwise your lateral speed would remain the same as Kerbin's rotation speed, but you would be in a higher radius, thus moving you west.

So, no the burn wouldn't be straight up, unless you had a tether to convert the forces. (a.k.a. a Space elevator). There is however one advantage to synchronous orbit, you can always predict where the object will be, so you don't need to move forward or backwards in your orbit a whole lot. However, it takes a lot of fuel to get there in a single arch. You will likely get some really bad gravity losses if you try to go straight to synchronous orbit.

[dharak1]

Thanks for all the replies guys! It helped a lot.