Designing A Resuable Munar Rover With Crew...?

[Titler]

Hello all,

A little description of what I'm trying to do; I have a large station in orbit around Mun to refuel landers on the surface, and I can get designs down and drive around with them... but what I can't do is ever seem to get enough fuel down to return the lander with wheels to the same orbit again, much less refuel and re-use it. To give you an idea of the kind of designs I've been using, here's a screenshot...

You should be able to see I've used a single tank of fuel, and the two radials on the first ever design, and the tiny engines and larger fuel loads as my latest, but no matter what I seem to do I can't get enough fuel to the surface to allow me to return to the station; Is there a trick to the descent/ascent I need to start using, do I have to give up and use even smaller, single Kerbal crewed vessels, or something else? Are rovers expected to be abandoned, and crewed landers to be stationary on the surface... some design tips if not would be desperately appreciated, thanks!

[Black_Tomcat]

The best solution would probably be to use nuclear engines mounted on radial fuel tanks instead of the normal radial ones.

You can get a much higher efficiency and the thrust is enough to land on the Mun.

Unfortunately you'd have to place the engines pretty high, so your rover might easily tip over.

You also should rather use the lander cockpit since you can mount parts on its side more easily.

[Jason Patterson]

To land and return to orbit you'll need 1200-1400 m/s of delta-v with a reasonable thrust to weight ratio. Your first design has about 1050 m/s, and the second has approximately 1500 m/s. Ideally the second should be able to get back into orbit, if not necessarily back to the refueling ship.

Your second design has quite a high TWR (6.5 or so on the Mun) so you could easily add fuel to get a bit more delta-v if you wanted.

Your best bet is to build a rover that uses a smaller command module. Losing a ton or two by switching to the Lander Can will give you a few hundred more m/s without adding any fuel at all.

[Titler]

Thank you for the comments so far, I've experimented with the nuclear engines, and they may be part of the solution, but I'm not quite there yet I think. Here's the Mark VII docking at Munbase Gamma to transfer the remaining insertion fuel and RCS, with what I thought would be a nice all round idea... the engines can also be used on the surface if I'm feeling crazy! And the Monoprop tanks etc moved to the front rebalanced it correctly. The lander legs were to point it at roughly 45 degrees and give it a low, flat rise to a low Mun orbit...

BUT! Mechjeb found this impossible to fly, and locked into a series of strange sweeps after the decouple burn, off target along the vertical at first, and then trying to correct this ended up sweeping way beyond the target again and back into a circular orbit...?

So I decided to place the craft vertical, to see if he was confused about an off centre-line centre of thrust, and use lander legs to slowly come down from, and re-raise the lander into the Vertical. However... whilst working on that (it's still in the "Bouncing up and down at the landing pad to test the wheel-survivability" stage!) I had a look at what Mechjeb reported for all the previous versions; and the information on Delta-V was interesting, and brings me to what may be the issue?

The Mk 1, the one on the left of the first image;

17.57 Tonnes

1366 m/s Acceleration

240 Maximum Thrust

1.39 Surface TVR

755.811 Vacuum Delta-V

52 Seconds Flight Time

That's even lower than your estimate, so no wonder if floundered on the surface. However Delta V clearly isn't the solution by itself, as when I looked at an alternative lander I'd experimented with...

... it has 2357m/s Delta-V if I'm reading that correctly? Much more than the 1500m/s which should be able to get to orbit as suggested by Mr Patterson; but even this failed to do so. So I'm wondering if it's the mentioned Thrust To Weight problem, which would eat up the fuel? In this case, it's hard to see how I can estimate that, and what I should be aiming for... I'll have a look in the ISS Mapsat and Mechjeb mods to try and find more information, but is there any way to see an efficiency rating for vacuum flight in the hanger? Because I've yet to find one...

[automcdonough]

In my opinion it's too much weight, I think you over-built. Try something a little lighter. Set a goal and build to it, nothing more.

1 man rover with jets to land and take off?

bam. 5.7t with ~1200 d/v of fuel. Add a little more juice for take-off and you're golden. That thing on mun has TWR >1 on RCS alone.

craft file in here: http://forum.kerbalspaceprogram.com/showthread.php/25339-stock-99-isplore-rover

Can't help it, sorry. I love these things.

I'll modify one of these tonight and try it out to see what sort of d/v it needs. Are you losing a lot in setting up docking? maybe try one of those with the ions on it..

[magnemoe]

I made one of this who can land on Mun and return to orbit.

It use an home 3.75 meter fuel tank and two radial engines. Doing it stock I would use the flat 2.5 meter tank and some extra 1.25 to get required dV

Either the small orange radial engines or two 909 under small tanks.

The lander can is lighter than the 3 man pod so you should use it.

Downside it that this will not be an nimble rover, for long trips you might want an skycrane with docking port so you can drive under and raise the legs to lover it on rover, doing this you want an extra set of landing legs or struts to avoid it from tipping over if you miss. You can even take the skycrane empty to orbit, refuel and land it at a pickup point.

This depend on use, is this mainly an lander with an rover option so you don't have to walk, or an rover who can be lifted to orbit and reused.

Edited May 14, 2013 by magnemoe

[Titler]

Hmmm... Yes, I'm trying to use only Stock parts as KSP is appallingly optimised under Windows XP and just adding the Space 1999 Eagle mod has choked it to the point of death; I crash after trying to launch a second craft as it is unfortunately...

Regarding a target goal, the goal is a fully re-usable Munar Landar, that can carry at least 1 kerbal, either automated and to pick up 1, or flown by 1 to total 2 or more; It's both a team visit to Munoliths, or a rescuer of Kerbals I don't feel like abandoning. It has to get to the surface and back to station orbit above Kerbal, leaving nothing behind. At the moment the station is at 100km, but can be dropped lower. It's carrying 3636 fuel at the moment, storage space for 9720 (and 6750 monoprop), has 6 crew in various compartments looking out of windows for nice screenshots or to take over a shift on Mun, and has docking ports ready for a lander arriving on site and refuelling from Kerbil at the same time.

The problem I have is that KSP takes about 2 hours to get from Kerbil to the station, where it chokes again and drops to single figure frames per second due to horrible engine optimisation, and then a few minutes away from the item count to surface and decent performance again. I am hoping in the long run this will be fixed, because I like to build the support framework before missions in general, rather than an Apollo style "First!" achievement then advancement dead end. Thus the station, which I will later try and fling from Mun to Minimus orbit... where it can pick up newer rovers for that environment.

But I don't understand yet how to predict Mun-performance before I get there. That's what I'm trying to tease out, how to get a sense of the hidden thrust-to-weight which is eating ability to return to orbit... for instance my last rover, using fuel tanks as superstructure actually beats by a fair way the Vacuum Time given for Magnemoe's rover, but it performs much much worse in practice it seems? I'm trying to figure out why... and why the one which is 3 tonnes smaller than all of them (17t, the first) actually had the worst fuel-time of all.

[automcdonough]

I started tweaking one of my rovers for this last night, it's packing twice the fuel and still seems fairly nimble. But then I looked up this chart and decided I need even more.

According to this, going from LKO to mun landing is 1710m/s each way. So a capable rover for your mission must pack at least 3420d/v of fuel. It's a decent amount of weight, possibly too much to slap onto my base isplore design and still be offroading at 20+m/s. Maybe. Only one way to find out but I probly won't have a chance to mess with it again for a few days.

Chances are either move up to bulkier slower design like others have been posting or consider a larger lander to carry all that fuel that the rover can detach from.

[DMagic]

According to this, going from LKO to mun landing is 1710m/s each way.

Not each way. Coming back from the Mun does not take another 860 m/s. It should take only a little more than what's necessary to break Mun orbit. I don't know the value off the top of my head for the Mun, but from Minmus it only takes about 150-175 m/s to get from a 25km orbit back to Kerbin. If you're just getting back to Mun orbit, like the OP, that doesn't matter, but if you're planning to return to Kerbin you shouldn't need that much delta-v.

[Titler]

Ok, I think I've made a bit of progress; I was able to get the tonnage down, and design a rover along similar lines to the ones shown here, but using my very first chassis as the basis. Who knows if it drives well, right now I'm just trying to solve the fuel problem! I left quite a graveyard of abandoned Kerbals on Mun, and discovered an interesting bug where if KSP selects the same crew again, one half of them disappears when they get too close to each other, and I ended up with 1 Kerbal of 3 in my landing craft, and 2 of 3 in the nearby previous lander, how odd!

As you can see, I arrive on Mun now with 1573 m/s Delta V for the return. But here's where it gets hard again. That's enough to put me into a Munar orbit, but I can't get back to the station. Although the station is orbiting around the equator at 100km height, and I've tried setting it as a rendezvous target and launching into it's alignment with the Ascent Guidance, it then tries to do an hour+ burn to align, which obviously is just not going to happen on that fuel. So... what's the next trick please, what is it I'm doing wrong and how can I get back to a meeting with the station? Dropping the station to a lower height, or changing inclination won't be a problem (I think!) as I have enormous fuel and rcs reserves (assuming it doesn't rip apart moving!) aboard that...

Edited May 21, 2013 by Titler

[GoDores]

Have you tried flying it manually? Mechjeb doesn't always know quite what it's doing, and if you're taking off from the Munar surface with something that has 1500m/s dV you should have plenty of fuel to rendezvous with something in Munar orbit, assuming you're relatively coplanar (I.e. you're not launching from the pole to rendezvous with a module in an equatorial orbit).

Another question: why relaunch your rovers? That's a lot of mass that does you no good once you're off the surface. It would be more efficient for you to land a rover and a lander separately. Leave the rover on the surface once your crew is ready to return, and you save the launch mass and have a rover that can be used for future exploration. I also put a probe module on all my rovers so I can do some uncrewed driving around places I land even if I don't plan to send a crewed mission back anytime soon.

[Titler]

Well, the idea is to learn by challenging tasks; copying Apollo landers may be historical but I'm more interested in learning to do something new for myself, or at least discover the limitations of space travel by pushing at the boundaries... my goal then is a reusable lander, as part of an imagined permanent future Kerbal Colony around Mun, before Chinese Kerman gets there first!

I don't think Munbase Gamma is going to be used beyond that though; I'd hoped to maybe move it from planet to planet with future upgrades, but attempting to change the orbital height down from 100km to 30km above Mun with the Mainsail engines I'd left aboard proved to be rather disastrous, with the G-forces just tearing it apart even with a tiny 27m/s burn... Fortunately there were a range of small little cute wee orange engines aboard that I'd placed to make a small escape pod, and even though the whole station still concertinaed so alarmingly I had to pull all my solar panels in to avoid them smashing off against it's own hull, I was able to drop it to the required height (although it's a degree off elliptical now due to the incredibly poor station handling, I'll send one last part up to try and balance it before that final burn).

I think the main hurdle, looking at it the overall setting tonight is that Mechjeb (currently the 2.0.7 pre-release) wants to launch in the opposite orbital motion to the space station; leaving Kerbal you go East, then exit to Mun orbit from the West side, which means you are captured on the opposite of the figure 8 at Mun, which puts you in a West spinning orbit around Mun. But Mechjeb wants to launch East from the Mun's surface still, using default launch profile, thus the next burn has to completely reverse the lander's orbit to catch the Station, which it doesn't have the fuel for of course.

The solution was to use 180 degrees as the Orbit Inclination when launching. You'll still need a corrective burn as East/West will be tilted depending on how far North/South you've gone (You're pointing straight up from the surface of a ball of course) but the corrections are now into single seconds as you're in the right frame of motion.... And finally, Success!

Billy-Bobfrey Kerman, who is best dodgy named Kerman, gets his crew back to the station from landing and returning at 10 degrees or so off equator with 505m/s of Delta V left!

With a manual, more careful launch that may be enough to get anywhere on Mun without needing further lander designs; all I did in the end was change the engines and add the small round fuel tanks. Interesting; I wonder if I can get the original back to the station with it's worse fuel load, or if it's Thrust/Weight ratio is too poor? But now the basic lander and flight plan is solved, dropping from the sky onto Munoliths proceeds! After one last shot of Billy-Bobfrey's triumphant return to his Munar Home...