How do you design your spacecraft for docking

[gilflo]

Hi guys

I was wondering what design for a spacecraft would give an easy way to dock it.

I have built a perfect balanced space plane with 2 rapier engines. CG and lift center are just in the middle of the rear part between the 2 delta wings.

Where to put a docking port and how to put the RCS quad to be able to manoeuvre it with RCS without side effects, I mean how to get a good RCS translation on 3 axes without perturbation on other axes?

Easy on a rocket, but on a plane?

The best way should be to put the docking port on CG, but it looks impossible, even with the MK2 docking port....

[helaeon]

Location of the docking port actually isn't super critical. It only gets annoying if you get up to dock and decide you'd like to orient the spacecraft a little differently, then when you rotate it you'll need to translate it because the ship rotates around its center of mass.

Location of the RCS ports absolutely is critical, each port should act on the center of mass equivalently. On space planes I find the linear ports are really handy as you can put them directly through the center of mass and only worry about one translation axis. Or equally spaced away from it to get an even push against the center of mass. Same principle as balancing multiple engines on a space plane to not get rotation.

The RCS build aid mod is super super useful here as it will show you your rotation on an axis if you click to place a RCS part at a certain place.

[el_coyoto]

I usually use an inline docking port (and use "Control from here" when docking) for my space planes or put a docking port in front of the nose when it's not too ugly.

In both cases, RCS thrust balance might be an issue, but I usually eyeball it and put the RCS quads roughly balanced against the CoM (usually 2 groups of 4 quads, each group equally spaced away from the CoM).

If I realize that RCS thrust unbalance is too much of a problem during docking, I may tweak the RCS placement a bit but I usually add one more reaction wheels to compensate for off axis RCS thrust : it's an ugly hack, but inline docking ports are kinda hard to use on a space plane because of the shifting CoM.

In fact, this is one of the reasons I try to minimize CoM shifting on my spaceplanes (but putting fuel tanks around the CoM) : they fly better of course, but RCS becomes way easier to place.

A more refined method would be using RCS Build Aid mod...

EDIT : ninja'd by helaeon

Edited November 7, 2014 by el_coyoto

[Scarecrow88]

For space planes I find it a lot easier to use the inline RCS ports. Placing quad ports in the right place for one axis usually means the other axis is off, resulting in unwanted rotation. Inline ports can be placed individually for each axis in just the right place.

[Alshain]

I take all different strategies in my designs. Inline is one of those, but certainly not the only way. The main thing to be careful of is clearance, don't hit the tail on the station. That's mostly a problem for inline ports. If you your engines can be moved, Dream Chaser style works well too. Or you could just go brute force and put it on the front.

For RCS, I prefer the single port RCS as you can see in all these image. It's just easier to tweak out the rotation that way, you just have to add a lot of them. RCS Build Aid is a must.

Edited November 7, 2014 by Alshain

[capi3101]

The RCS build aid mod is super super useful here as it will show you your rotation on an axis if you click to place a RCS part at a certain place.

I find the RCS Build Aid mod useful for spaceplane construction in general because it will show you your "dry center of mass" (i.e. where the CoM will go when the plane runs out of fuel). That definitely is helpful for when you're looking to determine where the CoL should be.

Going to have to try the linear ports; I've been using the standard RCS blocks for my (few) planes that included RCS capabilities.

(That's now four endorsements for the RCS Build Aid mod on this thread alone...)

[gilflo]

Thank's all of you. I am going to try the single port RCS.

[Skorpychan]

Generally, I just slap Sr docking ports on both ends of what I want to dock. This is tricky for landers, because they need chutes at the top and an engine underneath. I've compromised by sticking dual drogue chutes stuck out at the sides, to make room for a docking port on top. And maybe a decoupler-enabled docking port underneath for use when in a stack of craft being hauled interplanetary.

I don't tend to use spaceplanes, because I'm not good at building them, and prefer to much about with spacecraft designed to be spacecraft, rather than having lots of atmospheric stuff dangling off them adding weight.

[Hodo]

Hi guys

I was wondering what design for a spacecraft would give an easy way to dock it.

I have built a perfect balanced space plane with 2 rapier engines. CG and lift center are just in the middle of the rear part between the 2 delta wings.

Where to put a docking port and how to put the RCS quad to be able to manoeuvre it with RCS without side effects, I mean how to get a good RCS translation on 3 axes without perturbation on other axes?

Easy on a rocket, but on a plane?

The best way should be to put the docking port on CG, but it looks impossible, even with the MK2 docking port....

Most of the time I try to place an inline docking port as close as I can to the CoM of the craft so I don't have to deal with to much added movement from transition movements with RCS. But sometimes that isn't possible. Here is one of my larger albums and I think my last one with a picture of a SSTO docked to a station. Granted it was my second largest SSTO in service and my most used.

Javascript is disabled. View full album

[Wanderfound]

The key points for any docking ship is that it needs to be able to rapidly adjust velocity in all directions while maintaining heading. So, two choices:

1) A small ship with enough torque to spin on its own length in a few seconds.

or...

2) Something with enough balanced RCS that it can accelerate, decelerate and translate without changing heading.

Spaceplanes are a little tricky due to their uneven weight distribution, but it's not too hard. You can set up your RCS many different ways, but what I usually go for is 4-way blocks, placed at 45° angle snap, in symettrical pairs above and below. On a small ship, it's one set of thrusters directly on top of CoM. On a large ship, it's two sets of thrusters, balanced either side of CoM.

You can see the top half of the RCS setup here:

While you're still getting the hang of it, you'll probably find nose/tail mounted shielded ports the easiest to use. Once you get that down, give the Mk1/Mk2 inline ports a go.

From a manouevring point of view, the position of the docking port relative to CoM is irrelevant. The CoM/RCS relationship is the important thing. However, inline ports do tend to cause some navball funkyness, as the game tends to get a bit confused about exactly where the docking port is (it tries to steer your CoM into the target port instead; this isn't a problem when port and CoM are in line, but if they're offset...).

You can overcome the navball issues by just eyeballing the last few metres. Mods like RPM make that easier to do:

Edited November 7, 2014 by Wanderfound

[gilflo]

Hi guys

Thank you for all your advices.I built my own spacecraft and docked it on a big tank in orbit 110km around Kerbin.

Docking was very easy.

Take off with 4 rapier engines. Leave 2 small tanks at 30000m when Rapier change on rocket cycle. Then Leave 2 Rapier with their tanks just before orbiting at 100km. Still 1000DV left.

Re-entering and Landing is not on the runway, but not far from it.

Here's some pics