Sita (Duna/Laythe lander) - refinements requested

[Commander Zoom]

I've put together a two-kerbal lander and tested it on both Duna and Laythe (with the help of HyperEdit to put it in orbit around each), but while I'm mostly satisfied with its performance, I could use some help with the details.

Sita.craft - Download link

(EDIT: Revised version uploaded, see below!)

In the VAB

Testing in the Great Western Desert. (Note that while this version had MechJeb installed, the .craft linked above is entirely stock.)

The main issue is that while the performance of the aerospike engines is excellent at all altitudes, they are unlike other engines in that they don't have nodes underneath. This requires some truly creative methods to mount the lander in the middle of a stack. I'd like to replace at least the center engine with a more conventional one, allowing me to stack the lander as I would any other stage... but how to do that, while retaining enough delta-V to comfortably land on and return from Laythe? (Duna's easier, but I'd like to use one lander for both.)

Any and all comments/suggestions are appreciated. Thanks in advance.

Edited August 8, 2013 by Commander Zoom

[Synapse]

Do you need that many engines?

[Captain Sierra]

I'm not quite well versed in what it takes to return to orbit on Duna but if your descent is (almost) entirely unpowered, you only need lv-909s and probably less fuel.

[prysmatik]

If that mount point is the worst of your concerns, maybe try this solution?

https://dl.dropboxusercontent.com/u/12011994/sita.craft

Once you add a decoupler or what-have-you you'll have enough depth to not bang into the other spikes.

A couple other refinement ideas if you're interested:

you could replace your RCS thrusters with 1 set of 4 if you center them based on the center of mass. (Maybe you thought of that and took the change caused by fuel reduction into account, idk.)

If you plan on keeping the 4 outboard engines, you could attach them straight to the fuel tank without the radial decouplers. Otherwise... If you DO plan on dumping them at some point, maybe rethink your parachute placement.

Good luck.

Edited August 7, 2013 by prysmatik

[prysmatik]

Alternative:

Replaced Aerospike w/a nuke. Reduced RCS thrusters.

sita_nuke.craft

It occurred to me, you may not plan on landing your ascent stage again at all, just going up to dock w/ship; which explains the lack of chutes/legs on the ascent craft. I get it now.

I dunno how to calc dV and all; but I think if it gets to orbit, it probably has enough to get home from Laythe. Gravity slingshots are your friend. You *may* need to add fuel lines to burn those aerospikes a little longer @ Laythe. But I'm not sure, my lander had more mass; you might get away w/it. I would, just in case; then you could use have the X200 can of fuel on aerospikes if you wanted, or drop them early and be no worse for wear. It leaves it as an available option.

[sojourner]

Don't forget to test Kerbal ingress/egress. That ladder looks to short.

[Commander Zoom]

Don't forget to test Kerbal ingress/egress. That ladder looks to short.

I have, actually; it is Just Right. (That's why the legs are mounted high on the side tanks, rather than at the bottom - to "lower" the craft.)

Do you need that many engines?

Maybe? You tell me. In my experience, engine weights being what they are in KSP vs. real life, there's little reason not to put one under every tank, especially if you're going to be dropping it at some point.

I'm not quite well versed in what it takes to return to orbit on Duna but if your descent is (almost) entirely unpowered, you only need lv-909s and probably less fuel.

Duna is not so much the issue; Laythe is. I tested it on Duna first, and could get back to orbit with just the core stage; but Laythe wound up needing everything that I didn't use for landing. (~2800 m/s, according to the wiki ; Duna, only 1500 to 2000.)

Slapping MechJeb back on for a moment gives me the following stats for the .craft I posted:

Ascent: 2906 / 2921 m/s ; TWR 1.17

Descent: 1099 / 1104 m/s ; TWR 2.24

(note that these are the numbers *after* I replaced all the engines with aerospikes, which is why the atmo and vac delta-Vs are nigh identical; my first attempt did use 909s, as I recall, possibly with a Poodle for the core, and they turned out to be inadequate.)

You're right that I could probably get by with less fuel in the descent stage, though - four small tanks (or, I suppose, two large ones at the sides - less stable, though) would probably suffice.

prysmatik:

You're probably right on reducing the number of RCS thrusters. I tend to go with "more is better" and alternate between "at the CoM" and "out at the ends, for more lever-arm torque". But on the craft this short and stubby, I probably don't need to worry about the latter. This isn't an orange jumbo we're moving around.

That central girder is terribly unaesthetic to me, but you're right, it works. *chuckle* What I have now, deleted from the posted .craft, is a set of FOUR of those (well, cubic struts, but close enough) with decouplers, fitting into the gaps between the RCS tanks.

As noted above, the original idea was for the four outboards to be, collectively, the descent stage - thus, the parachute placement. However, see above regarding the need for *all* the oomph to get off Laythe.

Nukes - I don't like to use nukes on anything that lands. Not just for safety reasons, but because it seems like almost anything is more efficient, thrust per weight, in atmosphere.

And it doesn't have to get home from Laythe (or Duna) by itself; this is going to be one component of a multi-part ship. So if that's why you put it there - thanks, but no.

Edited August 8, 2013 by Commander Zoom

[Commander Zoom]

Second draft posted, after initial revisions:

Download: Sita II

Cut down on the RCS thrusters and replaced the FL-T400 outer tanks and aerospikes with 200s and 909s. Note that this cuts down the descent stage's delta-V to just 557 / 724 m/s, but that should be enough (as long as the chutes don't rip off)...

EDIT: Speaking of, have just remembered to re-add reinforcing struts between main stage and pods - now in Sita II download, but not in above image.

Edited August 8, 2013 by Commander Zoom

[zak233]

do you just right click save link as to download these craft files?

sry for the noob question

nice lander btw

[Commander Zoom]

That should be all you have to do, yes. (And then put them in your own KSP VAB directory, of course.)

I decided not to bother with Google Docs or the like just for a small file like this.

Thanks.

[Commander Zoom]

Final bump, probably:

My remaining "attachment issues", so to speak, have been sorted out - I've retained the central aerospike, but have been able to replace my previous hack job of a baseplate (that never did work entirely to my satisfaction) with the new large quad-coupler, a part reduction on the order of 16 to 1. So I'm pleased with that, and also the other small refinements that were suggested here. Once more, thank you all.