Docking Woes

[capi3101]

Hello all, welcome back to the forums. Alas that my account was created just last month; I get to start completely over...

Anyway, I've been playing through the awesome historical tutorials on the wiki and have completed the Gemini 6A/7 tutorial (the one that teaches docking). I've been playing the game for a while now and I've been looking for a good way to get to any target I choose, and I realized that docking might be the way to go. So, I came up with a design for each of the modules of the craft I'd need, designed a booster that would get them into orbit, and began launches.

I'd show you guys a picture of my problem, but as this is my "first" post, the forum won't let me. So here it is: the design calls for five engines in a cross-shaped pattern. I can't get the damn things to line up properly. Any suggestions? I realize that's nothing to go off of; can't post the damn craft file either.

One thing real quick - if I have two modules docked together, one of them has a fuel tank and the other one has a fuel tank and an engine, will the engine draw fuel from the first tank without me having to pump it around manually?

Thanks y'all.

[vencaslac]

in order to be able to answer your first question: "how to get the damn things to lign up correctly" we would need some sort of picture showing what you've got... I realise you don't currently have the possibility of showing one but i think updating this post once you are able to shouldn't be a problem.

to answer your second question reguarding fuel flow, the transfer stage of all my interplanetary bound modules has very little fuel, in fact it can only take 4 tons from low kerbin orbit to duna 100km orbit so i employ an extra orange tank for anything heavier... fuel does automatically crossfeed from it to the transfer stage and it drains from the extra tank first

[Pyotor Gagarin]

Second question first: I believe that engines consider all connected parts to be a "whole ship"; and they will draw from the farthest tank first, unless cross-feed (or the tank itself) has been disabled.

First question: Lining up multiple parallel tanks can be surprisingly difficult sometimes. You might try making one tank with struts and all, and THEN turning on quad symmetry to get four identical sides. Or you could use the alt key to replicate identical sections.

hope that helps!

[capi3101]

I'll be sure to put a picture up when I get a chance. Craft file too.

The fact that you can cross-feed an engine is encouraging; it means I could put together the engine assembly on the ground and just add tanks; that's essentially what I was planning anyway but that means that I don't have to have a tank with the actual engines, right?

The bitch'll be to get it the whole thing under the weight capacity of the booster...

(BTW Pyotor Gagarin, I wasn't putting the ship together in the VAB. Thanks, though).

[capi3101]

Still can't post attachments...this is getting annoying.

Quick update; I've gotten rid of the ship I was using in favor of a single piece thruster stage. I wasn't sure it would make it to orbit due to the load; my original booster design had failed with that much weight attached originally. Three tries and more struts later, though, I finally got it up into orbit safely.

That's not to say I don't want advice on how to do this sort of "sideways docking" maneuver in the future, though...

[capi3101]

Still no attachments...okay, this is getting ridiculous. I'll post some links to some pics of the problem I put up on a different forum:

Ugly SOBs, aren't they? I added more RCS thrusters to help with the steering in the next iteration. Might be part of why I was having so much trouble in this iteration. Anyway...

[sal_vager]

Yeah attachments don't work properly anyway, so I recommend you use Imgur

As for engines in a cross? Try fitting 4 of those radial attachment points to the bottom and an engine on each plus one in the middle, like this:

[capi3101]

Hmm...interesting idea. Wouldn't that reduce the potential delta-V, though? I mean, you'd have five engines burning off of one tank, so they'd each have only about 20% of the delta-V that a single engine would have for the whole tank, right?

Definitely going to have to take another look at those BZ-52s; I didn't realize you could pull stuff like that with those things...

[Sovek]

Actually.... no. I have a booster that uses KW rocketry engines, 4x 120kn engines and 1x 230kn engine on a short and medium sized 2.5m fuel tank. I get more dV due to them being lighter and better Isp. This is good if you need something slightly more powerful than your current options, but not overly powerful where the weight and lower Isp are a problem. If we go by stock options, say we have a rocket and you need about 750kn of thrust for a good T:W. Your current options are either not enough power, or way too much power. The weight of the mainsail, coupled with lower Isp make it a very unappealing option.

[Jason Patterson]

The delta-v for a given amount of fuel being moved by a particular number and set of engines is going to be a constant. If you were to install 4 LV-N's in the pattern that sal_vager showed, and provided them with the same amount of fuel as 4 LV-N's installed on 4 separate (smaller) tanks, it would go just as much.

Perhaps you meant that 4 LV-N's vs 1 LV-N would get less delta-v from a given amount of fuel. In that case you would be correct, but not for the reason that you list. The only reason that it would get less delta-v is that it would have more non-fuel mass. Burning the fuel faster (or through four engines, however you want to think about it) would have no effect. The same amount of fuel mass will be expended at the same relative velocity in either case, so the same change in momentum (and thereby change in velocity) will occur.

There are only three things that determine delta-v - fuel mass, non-fuel mass, and Isp of the engine(s).

You'll have a much easier time steering your ships with two sets of RCS blocks, one set fore and one aft.

[capi3101]

Perhaps you meant that 4 LV-N's vs 1 LV-N would get less delta-v from a given amount of fuel...

That is what I meant; sorry if that wasn't clear.

Did the math on one LV-N drawing off of one X200-32 tank versus five LV-Ns drawing off of it. Average Isp for both in vaccum is 800s, of course.

Wet mass of an X200-32 is 18t, dry mass is 2t. Mass of an LV-N is 2.25t, mass of five LV-Ns is therefore 11.25t.

Given dV=ln(m0/m)*9.81*Isp, the delta-V for one LV-N is 12,252.58 m/s. The delta-V for five LV-Ns is 6,214.69 m/s. That's assuming no payload.

I need the higher delta-V worse than I need the thrust for my purposes. I may play around a little more with the design, though.

You'll have a much easier time steering your ships with two sets of RCS blocks, one set fore and one aft.

Yeah, I figured that one one pretty quick. I've pretty much abandoned my earlier design though I've still got it in orbit. I was thinking about re-purposing it.

The next iteration of "flying gas can" had 12 RCS thrusters on it, one set fore, one set aft and one set along the center of mass; handled like a charm.

I should really get more pics up. Too bad I still can't post attachments...

[RoboRay]

I should really get more pics up. Too bad I still can't post attachments...

Try imgur.com

[capi3101]

Well now, strike all that...I'm doing the math again.

[capi3101]

You know what, I'm going to give sal_vager's design a shot; the math's saying the design will give me comparable amounts of delta-V with more gas cans (lesser amounts but still acceptable based on the mission parameters) with more acceleration. Fewer launches/rendevous/damn docking maneuvers for longer-range destinations too...

Now I'm hoping I haven't screwed up the math somewhere.

[capi3101]

Built sal_vager's design this evening; have yet to test it out but I hope to get to that tomorrow. Had one big problem; I couldn't add any parts for the booster without the game adding one of those LV-N fairings to the center engine (which naturally would've blown two of the engines right off the ship when it decoupled). Solution? Turned the whole damn thing upside down...

[capi3101]

Well, tonight I finally got a chance to dock a probe lander to the combined engine/supply modules and try out the design. Damn thing wobbles. A lot.

Any idea what I might be able to do about that? It's going to be no good to me on interplanetary missions if I can't steer during the burns; it was hard enough boosting my orbit to 250k.