And once more about landing rovers

[Mihara]

There have been threads about it, I've participated in one, in fact they come up with some regularity... this time, however, my question is quite specific, so I hope it merits a new thread yet again.

So I have a station in munar orbit, three kerbals relaxing on it after several months of kethane mining, and a need to send them a vehicle so they can go and plant flags everywhere they have been already.

I made a nice, little, under-10t lander using the new lander can and a nuclear engine with tanks hanging off it's sides. While I've had some truly bizarre problems placing RCS thrusters on it so that it could safely dock, the ~5000 dV enables it to land in any place on the Mun (or get to lots of other places and land there, but this time I'm only concerned with the Mun) and handle multiple landmarks without refueling while being immune to my suboptimal landings and takeoffs, which was the whole point of using a nuclear engine when it constitutes half of the craft's total mass.

Unfortunately looking for the right place to stick a flag into typically involves more searching, which can take a while on foot and wastes fuel when trying to do it while landing. The obvious solution is to put wheels on, but it obviously can't work if the craft is still oriented vertically, because LV-N. I can put the wheels on sideways, no problem, but then I need to land wheels down. I can't just land on legs and then fold some of them, because the vessel needs to take off on it's own later, and do a repeat landing somewhere else, so I can't drop anything either.

So I can stick side mounted engines on it, keep them disabled, switch to them when I'm near the ground, kill all velocity to zero and use the time that gives me to reorient and land horizontally. And herein's the problem:

If they were the only engines on the ship, I would just set up four of them so that the thrust vector goes through the center of mass and be done with it, SAS would keep it sufficiently stable to land onto the wheels. However, there's also the LV-N, which is also displayed in thrust vector and messes everything up. I can't remove it and still see the correct center of mass, because it is half of the ship's mass.

How do I see the thrust vector of only a certain subset of engines? If I can't see that thrust vector, how do I get the ship balanced to land horizontally?

Edited May 30, 2013 by Mihara

[CookiezFort]

well waht you can do is place the legs (the ones that can retract) and place also some of the landing gears that can retract on the same side, keep the side engines as they are and activate them only when you need to... so what you would do is land with landing struts, open the landing gear, retract the struts and there you go... if oyou tried that already then i do not know anything else

[Mihara]

Like I said, landing vertically and tipping over is not going to be helpful here, I need to take off. For which I will need side-mounted engines anyway. And if they're unbalanced, it'll crash on takeoff anyway.

[Shrike42]

Well, I've had success with landing this:

Derp, sorry, keep hitting quick reply...

Landing that using the rockomax tank and 2 LV-Ns mounted to the side of the landing stage.

It didn't have to take off again, but you're dealing with a smaller system. Let me see if I can knock something together here.

To clarify, here's how I got that onto the Mun:

.

I'll try to put together something that should work based on what you've said so far.

Edited May 30, 2013 by Shrike42

[Mihara]

As much as I appreciate the attempt to help, this is nothing at all like the setup I'm talking about. In my case, the vector of thrust of the nuclear engine has to be perpendicular to the vector of thrust of the side-mounted landing engines. The problem I'm seeking help with is how to determine that the vector of thrust of side-mounted landing engines is actually passing through the center of mass when there's the perpendicular vector of thrust of the nuclear engine throwing the vector display off.

It's not unlike balancing a VTOL plane with a separate set of takeoff/landing engines and no wings -- people have done it, so it's obviously possible, the question is how to do it without spending five hundred iterations on it.

I guess I need a picture with my question, but that has to wait a few hours...

[Shrike42]

There's no way to do that in the VAB, sadly. However, the LV-N and the FL-T400 fuel tank both weigh/mass 2.25.

Just sayin'.

[RoboRay]

Yes, add an equivalent mass in place of the engine. And if you're concerned that the different size of the part might cause the CoM to be a little off... copy the engine, edit the part.cfg to make it a structural piece with the same mass and no thrust capability, then stick that dummy motor on where the LV-N will be. Sort out your balance with the side-mounted engines, then replace the dummy motor with the real LV-N.

Edited May 30, 2013 by RoboRay

[Shrike42]

After thinking about it some more and running some tests, the longer length of the LV-N means its CoM addition to the craft CoM is very slightly larger than that of the FL-T400. You'll need to adjust your takeoff engine placement to compensate, get your center of thrust just slightly behind the CoM the FL-T400 mockup presents.

Sorry for not understanding what you were shooting for the first time, but this will work with far fewer design iterations.

Edit: Or do RoboRay's version of it, since that'll get you the right answer straight away.

[Mihara]

Thanks, a dummy motor sounds like it'll work.

What a pain, though.