Launching really big stuff

[Kimberly]

This is my spacedock:

I need to get it into orbit. It only weighs in at a modest 20 tons, but the problem is, it's huge. And as you may have noticed, its backbone is a single strut, so it can't be split into multiple sections. (I'm actually going to place a few inhabitable modules on the far side later.) I have no real idea how to launch it to the target height of 300 kilometers over Kerbin...attaching big rockets with big engines isn't a problem, but it doesn't seem to be remotely controllable if I place rockets on the sides--the center of thrust is directly below the center of mass, but as soon as I launch whole thing tips over.

Any ideas?

[Johnno]

The crazy part of my brain whispered 'skycrane'. I mulled it over for a minute... a tank/structure attached to the top of it with plenty of engines, then fuel tanks below, potentially even in asparagus style, with the last fuel line(s) leading through the payload and to the engine assembly.

sort of like <<C=====

Engines - payload - fueltanks.

My brain is chugging along at orbit velocity now, gotta go test this crazy idea. I'll see if I can pop up a screenshot in 10-15 minutes.

Edit: Well after a bit of testing I've come to the usual conclusion, keep it simple. I ended up scrapping my skycrane idea and simply sticking jumbo64 + x200-32 + mainsail under the center, then two boosters on the sides of the center stack with the same fuel tank combos but with skippers instead of mainsail, both feeding the center tank. 5112 m/s delta-v but as my dock clone got really wobbly at the end I didn't get a non-decaying orbit, AP did end up over 500km tho (PE at 66km).

You should be ok with a strong center stack below the center and boosters around/on the sides of it, just strut the payload properly and you'll be fine.

Like so:

Edit 2: added screenshot

Edited July 15, 2013 by Johnno

[AlexanderB]

You got a craft file? I'll figure a nice booster to fly it up there..

1 tip already: its big, light and draggy, it wants to be the rear of the rocket, so the thing will tip over without a bunch of tail fins or some other way of redistributing the mass/drag.

[Kimberly]

Craft file: https://docs.google.com/file/d/0B-90GQp7TMiRS3BnREI4c1p1N2M/edit?usp=sharing

Thanks for taking the effort to test it, Johno. Your set-up looks interesting and I'm going to try it out when it's not 4:40 AM. But I should note that your dock recreation is symmetrical, while my dock (intentionally) isn't. I guess I could make it symmetrical, if I reconfigure the intended habitable modules, but I'd prefer to keep the design as-is if possible.

Edit: Hold on, that download site seems to have a lot of pop-ups (a few are getting through my Adblock), link changed.

Edited July 15, 2013 by Kimberly

[Johnno]

But I should note that your dock recreation is symmetrical, while my dock (intentionally) isn't. I guess I could make it symmetrical, if I reconfigure the intended habitable modules, but I'd prefer to keep the design as-is if possible.

Ah right, wasn't sure how the final version would look. But I'd suggest the same approach just with attaching the center stack below your dock's center of mass. Symmetry isn't the real issue, it's center of mass. If you can get an attachment point right below it, you should be fine sticking any stack of boosters below it. Problem with having rockets on the sides / around it is as you've noticed it tipping over if it's not symmetrical/CoM centered between boosters.

A single attachment point below your CoM would likely be the easiest way, then a lot of struts to secure the payload to the lifter.