Spaceplane / Mothership integration

[AeroGav]

A question came up recently regarding designs for SSTO landers for atmospheric bodies.   Now, atmosphere immediately has me thinking "spaceplane" but the problem is people often want the lander to fit within a cargo bay of the mothership rather than docking it radially, since it keeps the CoM of the mothership centred.

Having to fit within a mk3 cargo bay means very small wings and high landing speeds for field landings, unless you resort to cheese like clipping or triplane/venetian blind strake wings.

Having to put a docking port on the nose means high drag - low lift and high drag mean generally poor performing spaceplane, though you can still get to orbit with high twr chemical propulsion.

My problem is i've never built a mothership before.  I normally just go with single stage to anywhere spaceplanes, staged spaceplanes, or use IRSU refuelling facilities.

Anyway,  I wondered if  it was possible to do this a third way.  Here is an example of a small SSTO I built,  2 nervs 1 panther.   It reaches 150km orbit on Kerbin with about 1200 dv left over.  With the Panther in Dry mode it could do a lot of flying around on Laythe and still have plenty left to make orbit.    It has a landing speed of about 40m/s fully loaded, and it has two vernier thrusters in the belly to reduce landing speeds on Duna / allow VTOL on low grav airless moons.    It has an inline clamp o tron , reaction wheel and some RCS translation thrusters.

And here is the mothership i started to make for it.       It has two docking ports on the  end of the swept wings that my inline clamp o trons should be able to hook on to.

It has just shed the launcher hardware.   There is a long liquid fuel fuselage in the middle, and a cargo bay at the front , which we can fill up with whatever the mission requires before Kerbin departure.

The problem is that i'm totally out of practice docking stuff, and only ever mastered basic nose-to-nose anyway.   So I gave up and decided to try pre-docking so i could see if the concept at least flies.

It does ! 

Problem 1 - you cannot pre-dock inline docking adapters in the VAB. I found an old thread that said you can attach the subassembly some other way and line the ports up with the offset tool and the two ports will then automatically "dock" once you launch the vessel.   This is no longer the case.  I attached the aircraft with a radial decoupler, lined the two ports up so they appear to be touching, but the ports still don't "stick".   I had set the radial decoupler force percent to 0,  but after firing the decoupler in orbit, the aircraft simply drifts away from the mothership.   From what i understand,  docking ports that aren't  pre-docked , have to spawn at least 10m  from each other for the magnetic attraction to kick in when they reach proximity.  If they are already close when the vessels spawn, the game assumes you just undocked and don't want these ports pulling back together again.   

The other problem is that we're really just kicking the can down the road here.  To be a functional mothership/lander combo,  it needs to be feasible to re-dock after separation.    In fact we need to re-dock sufficiently well aligned that the NERVs of the spaceplanes can push the mothership, like in the picture.  Build guide slots out of modular wing segments or something? Or i just need to improve my skills?

All this re-docking though, is making me realise why i like interplanetary spaceplanes so much.

 

[Aegolius13]

Hmmmm.  Provided you can get the whole thing up in one piece, you can cheat a bit the first time.  You can't use the inline clamp-o-trons (or at least not easily, as you say), but you could attach your landers to those struts with stack separators or something.  This, at least, will guarantee a precise alignment  until you detach / reattach them the first time.  If the lander/mothership combo doesn't need to move again after you get to the destination planet, that would be all you need.

But if you want an exact reattachment, yeah, that's hard.  One old trick I've seen (but not tried) is to put two clamp-o-trons on both ships, so that when both sets attach, they'll be forced into the correct alignment.  Even if that works, though, it's obviously adding extra weight and size that's not ideal for a little lander.  

Personally, when doing stuff like this, I'd probably bite the drag bullet and put the docking port on the front of the lander.  But I just don't find it very fun to fuss around with precise docking (also a reason I don't build those very popular ISS-style modular space stations).  

[fourfa]

The mod Tweakable Everything allows connecting the inline docking ports in VAB/SPH. It has a lot of other effects too, which you may want to deactivate in its config file. But it's convenient for testing stuff like this

I've been working on a concept in this area for a while. Basically: wing modules that undock for stowage, reassembly on orbit, alternate vacuum modules for hopping on low-grav planets. 

Edited April 8, 2017 by fourfa

[AeroGav]

4 hours ago, Aegolius13 said:

mmmm.  Provided you can get the whole thing up in one piece, you can cheat a bit the first time.  You can't use the inline clamp-o-trons (or at least not easily, as you say), but you could attach your landers to those struts with stack separators or something.  This, at least, will guarantee a precise alignment  until you detach / reattach them the first time.  If the lander/mothership combo doesn't need to move again after you get to the destination planet, that would be all you need.

That is what i did on screenshots 3 and 4.   The decouple force of the separators was set to 0%, but when i decoupled, the ships simply drifted apart, the docking ports did not stick.    They had been aligned in the VAB with the offset tool to appear as though they were touching.

The trouble is, when you decouple, the airplane spawns as a separate ship.   If two docking ports spawn in close proximity to each other they do not attract and do not stick, because the game assumes you just undocked from that port and don't want it to immediately reconnect.

What i have to do, after firing the decoupler, is move the aircraft at least 10m away, which resets the two ports in mutual attraction mode again, then move back in and re-dock.  Too much agro !

[Freshmeat]

If you are into mods, Konstruction docking ports allow for locked angles. I would still question whether the coupling was strong enough to prevent the Nervs from twisting, though. If you do not want to install any mods, my best advice is to have engines attached to the mothership, maybe that will make the spaceplanes somewhat lighter. Given that the spaceplanes are capable of reaching orbit, I would launch them separately and dock in orbit. Again using mods (DPAI) makes docking a quite manageable matter. And unless you need two spaceplanes, you could attach a single spaceplane behind the ship with a shielded docking port for less air resistance.

[Spricigo]

My approach wold be:

Design the spaceplane,  the actual specs will depend on what is the landing part of the mission.  The docking port will be in the nose at the centerline. 

To deal with the drag of the docking port an idea it's to design a end cap for it,  actually a small craft composed of probe core,  reaction wheel,  battery,  antenna and some monopropelant packed inside a fairing,  RCS thrusters and docking port outside. 

The core stage will have a docking port at the rear for the spaceplane and one in front for the end cap. Depending on how much it can rely on the spaceplane for propulsion and control it may be just a big fuel tank or an actual functional craft. 

Finally,  I'd figure out some way to put that system in orbit. The fact it's held together by docking ports are a problem,  but using wings with jet engines as Booster stages would be very cool (thus worth the attempt) 

 

[steve_v]

6 hours ago, AeroGav said:

The problem is that i'm totally out of practice docking stuff, and only ever mastered basic nose-to-nose anyway.

The solution to this problem is more practice (or mechjeb ). Docking with inline ports is exactly the same as standard ports, just set the port as the control point.

6 hours ago, AeroGav said:

In fact we need to re-dock sufficiently well aligned that the NERVs of the spaceplanes can push the mothership, like in the picture.

Either use the classic two pairs of ports (perfectly aligned in the editor) method, or a little ModuleManager magic to make the ports angle snap, like so:

Spoiler

// Add angle snapping to Squad Docking ports (12 increments as on textures)
// Author: Psycho_zs
@PART[dockingPortLateral|dockingPort2|dockingPort3|mk2DockingPort|dockingPortLarge|dockingPort1]:FINAL
{
	@MODULE[ModuleDockingNode]
	{
		// If you want X degrees margin, use cos(0.5*X) as captureMinRollDot
		// 0.5 degrees = 0.99999048
		// 1 degree    = 0.99996192
		// 2 degrees   = 0.9998477
		// 3 degrees   = 0.99965732
		captureMinRollDot = 0.99996192
		snapRotation = true
		snapOffset = 30
	}
}

 

This is, IIRC, the same stock mechanism that RDs konstruction ports use.

 

Personally, I tend to put the docking port on the tail of my spaceplanes, between a pair of rapiers.
My inteplanetary (nuke) engines go on the mothership, IMO they're too heavy to send down to the surface and back.
It would be interesting to work out which approach is more efficient though, I like your concept.
 

[Aegolius13]

8 hours ago, AeroGav said:

That is what i did on screenshots 3 and 4.   The decouple force of the separators was set to 0%, but when i decoupled, the ships simply drifted apart, the docking ports did not stick.    They had been aligned in the VAB with the offset tool to appear as though they were touching.

The trouble is, when you decouple, the airplane spawns as a separate ship.   If two docking ports spawn in close proximity to each other they do not attract and do not stick, because the game assumes you just undocked from that port and don't want it to immediately reconnect.

What i have to do, after firing the decoupler, is move the aircraft at least 10m away, which resets the two ports in mutual attraction mode again, then move back in and re-dock.  Too much agro !

I just meant the leave the ships attached via the VAB method, and not even try to use the inline clamp-o-tron until you need to deploy your landers and then return.  Kind of like how you can use a docking port attached to another part as a one-time decoupler.  But as mentioned above, this does not help with reattaching later. 

[bewing]

If you don't want to cheat and use mods then I think the two docking ports per lander method is your best bet. You have always been enthusiastic about making videos, so if you have troubles with docking, make a video of a docking attempt and we will all make lots of nasty (but hopefully educational) critiques of your technique. My first bet is that you are using the navball too much, and your eyes not enough.

 

[Spricigo]

1 hour ago, bewing said:

If you don't want to cheat and use mods... 

Maybe I'm oversensitive buy I find a bit offensive when people seems to imply mods=cheats.  (/end rant) 

1 hour ago, bewing said:

My first bet is that you are using the navball too much, and your eyes not enough.

 

In this regard might be a good idea to add a few parts to help orientation.  Communitron 16 are particularly effective with its red & white pattern. 

[Blaarkies]

On 4/8/2017 at 8:57 PM, Spricigo said:

Maybe I'm oversensitive buy I find a bit offensive when people seems to imply mods=cheats.  (/end rant) 

 

On 4/8/2017 at 5:04 AM, AeroGav said:

 Build guide slots out of modular wing segments or something? Or i just need to improve my skills?

@Spricigo Yes, oversensitive it is 
OP already pointed at a pretty kerbal stock solution to his problem instead of docking mods, I assume they don't want to add mods/cheat unless there is no alternative.

For docking,
-Disable all rotational thrust on the RCS controls'(it is in the context menu on each RCS thruster part)
-Change view to locked. This will lock the camera to your ship, making you that much more aware about how you are moving(you lose some awareness about rotation when both you and target are rotating)
-Use precision mode(capslock). It does a bit of balancing to the RCS thrusters relative to center-of-mass, but mostly it gives you very slow, careful movements.
-Use the "Aim Camera" button in part context menus(might have to enable this in main menu settings first, "advanced tweakables"). Now you can have better sight right over the docking port, or some wing or docking bay(wherever you want)
-If you are going to continue with the guid rail approach, maybe you could try micro landing legs and multiple docking ports. The legs keep the docking ports from snapping together, and the docking ports pull the ships into the correct position...maybe they can easily solve the alignment issue(it's gonna take some design research time on your part though)

Edited April 10, 2017 by Blaarkies

[Wanderfound]

It's a lot simpler if you just put the docking ports on the lateral line.

 

But if you need to make something line up:

[juanml82]

You can counter the drag of a  nose mounted docking port by covering it with a pointed fairing. The problem is that the fairing is single use. But the extra drag of the docking port should only be a problem at Kerbin. It's certainly not a problem in Duna and shouldn't prevent you from reaching orbit in Laythe.

[fourfa]

That's an interesting idea - though it means you're still hauling the mass of the fairing base all over the solar system.  You can also use an ordinary nose cone, and decouple it at main engine cutoff when you're still suborbital but above the vast majority of the drag losses.  Heck, you can have an orbital service station undock and redock aero covers just for trips to and from Kerbin's surface.  Lots of options.