Putting a transfer stage in LKO?

[Rylant]

Hey all, so I have finally got a nice design for a lander ship with about 5k vacuum delta v with ISRU and mining capabilities.  It also has enough thrust to weight to get off any body including Laythe, except Eve.  It’s big; like 700 tons. I can get it into LKO and from there it can go pretty much anywhere. Moho is tight though.  I find that I don’t have a lot of wiggle room for wasted delta v if I am doing a Moho return.  And I tend to waste delta v; lol I am sure there are players out there who will say “You can’t land in Moho from LKO with 5k delta v? I can get there and back with that much!!!”. Sadly, that’s not me. 
 

So, I was wondering if I could launch my 700 ton lander into LKO and then also launch a transfer stage into LKO as well. They could dock up? Then I could use the transfer stage at least for that initial 1600 k burn and ditch the transfer stage. Then I should have ample delta v for the Moho land and return. 
 

Is that feasible? Can I dock the main lander to a transfer stage head first, initial burn to Moho, and undock and ditch the transfer stage?

 

 It’s probably easier to get that size of ship into LKO with 2 trips, rather than just adding the transfer stage to the 700 ton ship for the initial launch?

 Rylant
 

 

[Streetwind]

10 hours ago, Rylant said:

Is that feasible? Can I dock the main lander to a transfer stage head first, initial burn to Moho, and undock and ditch the transfer stage?

I don't know. Can you?

In all seriousness, the answer is "yes, if you have a docking port, and the skill to perform a rendezvous and docking". It's a perfectly valid strategy, and people have built entire motherships out of 10+ individual launches in orbit.

The main difficulty is the inherent wobbliness of docking connections. If you want to move a 700 ton vessel, you have to bring either a lot of thrust or a lot of patience, and if you pick the thrust option, that poor docking port is going to struggle. So my recommendation is: bring an engineer and an inventory full of struts. Once docked, the two vessels are considered a single spacecraft, which means you can strut from one to the other in EVA Construction Mode to stiffen it up. The struts will later automatically disconnect upon undocking.

Edited June 5, 2021 by Streetwind

[AHHans]

@Streetwind already said most of it, but some additional remarks from me:
- You will probably have to manually switch off the engines on your lander ship. You really don't want to have one set of engines pushing in one direction and another set of engines pushing in the opposite direction. Feel free to re-arrange the staging order in orbit, but don't trust it too much, instead make sure that exactly the engines that you want to use are active before hitting the throttle.
- If you want to make it (IMHO) more elegant then you can also use the the transfer stage to only push you into a highly eccentric orbit around Kerbin, and then use your main lander ship for the rest. That way you can either land and re-use the transfer stage or safely dispose of it in Kerbins atmosphere. This means you can only gain about 900 m/s from the transfer stage, but it may be fun nevertheless.
- I'm not sure if you are aware that long burns actually waste dV. If you have a high dV burn (e.g. the 1600 m/s) at low TWR, that will take so long that it extends over a significant (angular) fraction of the current orbit, then you'll need quite a bit more dV than the nominal amount. In this case you can save dV by splitting up the burn over two or more orbits (as long as you stay within the SOI of the orbited body).

[king of nowhere]

13 hours ago, Rylant said:

Hey all, so I have finally got a nice design for a lander ship with about 5k vacuum delta v with ISRU and mining capabilities.  It also has enough thrust to weight to get off any body including Laythe, except Eve.  It’s big; like 700 tons. I can get it into LKO and from there it can go pretty much anywhere. Moho is tight though.  I find that I don’t have a lot of wiggle room for wasted delta v if I am doing a Moho return.  And I tend to waste delta v; lol I am sure there are players out there who will say “You can’t land in Moho from LKO with 5k delta v? I can get there and back with that much!!!”. Sadly, that’s not me.

 

You can go to Gilly first and refuel there. Once you are on Gilly, doing Gilly-Moho with 4 km/s - with an additional km/s for landing on moho and refueling again - is not hard.

Same for going away from Moho.

That said, if you are set on needing a transfer stage, streetwind said it all

[Snark]

19 hours ago, Rylant said:

Is that feasible? Can I dock the main lander to a transfer stage head first, initial burn to Moho, and undock and ditch the transfer stage?

Absolutely!

My own suggestion is that since you already have plenty of engines on the lander, then you may want to consider a "transfer stage" that's basically just a fuel tank, to save mass (i.e. don't lug around a second set of engines).

Dock it to the lander, then stage away any engines on the transfer stage so that it's just a fuel tank docked to the lander.  (Or it could have some small engines on it that don't stage away; just remember to deactivate them before you start firing your lander's engines.

8 hours ago, Streetwind said:

The main difficulty is the inherent wobbliness of docking connections.

Wobbliness can be an issue, yes.  Autostruts solve this pretty straightforwardly, though-- just take the main tank and set it to "autostrut to root", then it should hold itself in position just fine.

8 hours ago, Streetwind said:

If you want to move a 700 ton vessel, you have to bring either a lot of thrust or a lot of patience

Well, he already mention that he's got plenty of thrust on the lander, so I think the engine aspect is covered.  Just need to autostrut the transfer stage so that it doesn't wobble.

(Or, alternatively, if the lander has a docking port centered underneath, he could dock the transfer stage under the lander, so that it's pulling the docked tank rather than pushing it.)

7 hours ago, AHHans said:

You will probably have to manually switch off the engines on your lander ship.

If he's got enough engines to give a 700 ton ship significant thrust, that's a whole lot of engine mass.  I'd suggest doing the opposite, and switching off the engines on the transfer stage (or, indeed, just eject them before doing the transfer burn).  No point in lugging along a whole second set of engines, unless the Isp is so much higher that it's worth the extra mass.

7 hours ago, AHHans said:

If you want to make it (IMHO) more elegant then you can also use the the transfer stage to only push you into a highly eccentric orbit around Kerbin, and then use your main lander ship for the rest. That way you can either land and re-use the transfer stage or safely dispose of it in Kerbins atmosphere. This means you can only gain about 900 m/s from the transfer stage, but it may be fun nevertheless.

Actually, there's another variation of this that can allow getting by with a somewhat smaller transfer stage:

1. Your lander and transfer stage boost to a highly elliptical orbit around Kerbin (oriented the correct way for your intended interplanetary burn), with low periapsis.
2. Then use refueling tugs to dock to it at Pe and top off all the tanks before disconnecting.
3. Thus, when you start your interplanetary burn (with full tanks), you're already going ~3000 m/s at Kerbin periapsis, so you don't need as much boost to do the burn.