What TWR/DeltaV do I need

[Pinchy]

I know I can test this tonight when I am home, but have just been thinking about it on way to work.

I want to know if using (4 or 6) RCS thrusters and 2 ROUND8 fuel tanks mounted on a very simple rover (rovemate probe/wheels/Batteries/Solar panels/ laser experiment, Antenna and separator) will take me from low minmus orbit to surface - do some science and transmit that back to Kerbin) I don't plan on returning the rover home

[Wemb]

Not sure - I can land on minmus in my 'grasshopper' light lander on RCS alone - but I can't get back into orbit, even with the normal fuel tanks empty. (One science Jnr, landercan, 4x round mono tanks, 4x RCS blocks and 3 oscar fuel tanks and a spark engine). You probably can, but it'd be tight. The limited thrust may be a bigger problem than dV.

Wemb

[Pinchy]

How much does your grasshopper weigh wemb.. BTW loving the name of that 'rover'

[Wemb]

How much does your grasshopper weigh wemb.. BTW loving the name of that 'rover'

Bit less than 1.4 tons empty, I think - mostly built to shuttle science too and from low minmus orbit - but I way over did it in terms of the amount of mono I took with me. I'll have another play landng her on RCS tonight I think.

Wemb

[Warzouz]

You could use RCS only and the RCS engine in addition to RCS packs.

[GoSlash27]

Pinchy,

RCS thrusters aren't gonna work with a round-8. It carries LF&O instead of monopropellant.

To answer your question, yes you can do that. You need a minimum of 180 m/sec DV and 1.2:1 t/w on Minmus (.06G on Kerbin). I recommend bumping it up to 360 m/sec for flexibility in how you land.

A design using RCS for a 1.4t payload would need 1 R1 tank and a single linear RCS port. It would weigh 1.74 tonnes fully-loaded. This would not be the lightest or cheapest solution, though. An LV-1 Ant and round-8 would get the job done for 1.64 tonnes and approx $260.

Best,

-Slashy

Edited July 23, 2015 by GoSlash27

[Pinchy]

Thanks Slashy - I was thinking of RCS rather than Ant so that when I land on minmus I can move around using the RCS thrusters - if I use ANT is that not just one way direction i.e I wouldn't be able to land the rover on its wheels and then move it. I haven't used ANT engines before - so can't confirm my understanding, but I built a rover on minmus using 2 twitch engines that I mounted to the back of the rover with 2 Round8's mounted on top of each other, but I accidentally blew it up and messed up the quick save accidentally hitting F5 rather than F9 - so now I have my ship on minmus as planned but no rover - so my plan was simply to put another orbital type station for another contract in minmus and send down the rover to replace my busted one!! I wouldn't be able to use Twitch engines to bring me down as again my rover would be pointing backwards all the way to the surface?

[Sharpy]

Pinchy,

RCS thrusters aren't gonna work with a round-8. It carries LF&O instead of monopropellant.

Unless they are Vernors. I did a couple Minmus landings on Vernors alone.

The big problem though is that they *are* quite powerful for light vessels and SAS likes to go wild trying to stabilize the craft, firing them rapidly wobbling it so and fro, if the craft is light - a way around that is to have only one thruster per side (at CoM) so SAS won't try to use them for rotation. And yes, for 1.4 tons a single Vernor is a plenty to take off from Minmus.

[Snark]

Thanks Slashy - I was thinking of RCS rather than Ant so that when I land on minmus I can move around using the RCS thrusters - if I use ANT is that not just one way direction i.e I wouldn't be able to land the rover on its wheels and then move it.... I wouldn't be able to use Twitch engines to bring me down as again my rover would be pointing backwards all the way to the surface?

Actually, I find that the simplest & most flexible rover design is to do exactly that: Build the rover vertically, like a rocket: probe core on top, cylindrical body, engine on the bottom. Then just attach wheels to the sides. You can fly it like a rocket; you can land it like a rocket, gently, tail-first. (Don't need landing legs, as long as you set it down under 7 m/s.)

Then, once you're down on the surface, just let the rover rotate forward and settle on its wheels. At this point, it's like a rocket lying down on the surface. You can drive it around like a rover, the probe core even lines up nicely on the nav ball to give an intuitive view for navigating. The engine is pointing backwards, so if you need a little boost to go up a hill, it's perfectly situated. If you ever want to take off again (e.g. to make a suborbital hop to another biome), just rotate up to the vertical and take off.

(This design does require putting some reasonably strong reaction wheels on the rover, but you'd probably want that anyway to avoid flips.)

A picture's worth a thousand words, the below gives the general idea. (Ginned up for illustration purposes, didn't bother to launch, put science instruments on it, etc., but you get the idea.)

[cantab]

Keep in mind that RCS is an on/off system. It can actually be tricky to land on since you can't do a nice steady descent, you have to keep tapping the keys.

[Pinchy]

Yes but I could adjust the maximum thrust or just disable SAS?

my plan was to attach 4 just by each corner of the rovemate and then if I still didn't have enough thrust then to put another 2 in the middle on the side like a 4 or 6 on a dice

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That's a great little rover there Snark - liking the design - I haven't unlocked those bigger type wheels in career mode yet - I only have the S2 and M1's at the moment.

[GoSlash27]

Pinchy,

For a 1.4t vehicle, just 1 would provide adequate thrust. 4 will be way more than you need.

FWIW when I build self-landing rovers, I make the fuel tank the core of the structure. For example...

Best,

-Slashy

[Pinchy]

Interesting comment re the reaction wheels - on the one I built on Minmus (using KIS) - maybe that's why it was so twitchy before I broke it - because I didn't put any reaction wheels on it....

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Thanks Slashy - do you have a better picture - I can't work out where the fuel tank is - do you not need some sort of probe-core to make it controllable - otherwise wouldn't it just effectively be a piece of space junk?

[Sharpy]

Keep in mind that RCS is an on/off system. It can actually be tricky to land on since you can't do a nice steady descent, you have to keep tapping the keys.

I never found it a problem. I can easily estimate that one tap of "I" reduces my speed by e.g. 0.7 m/s, so during the final descent I just tap it when my speed exceeds 1m/s and I land lightly like a feather.

[GoSlash27]

Pinchy,

The fuel tank pretty much is the rover. Let me see if I can find a better shot...

*edit*

This one was designed to operate on Tylo. Just an example of the design philosophy where the fuel tank doubles as the chassis.

Best,

-Slashy

Edited July 23, 2015 by GoSlash27

[Pinchy]

so that looks like a rockomax 200 with 4 docking ports 4 twitch engines wheels and solar panels that looks something that would do the job more than fine to land - but then I would need to put 2 others (maybe ants) on the back to fire me fwds when I land?

[Sharpy]

The wheels have their own propulsion (electric), if not a very high top speed, so an extra engine (Spark maybe; Ant with its minuscule thrust won't do any better than the wheels) is welcome but completely optional.

I wonder why the 4 docking ports, also on Tylo that solar battery won't be doing much good.

(note if you don't place the probe core facing the direction you go, a Jr. docking port is a decent substitute for a reasonably facing "control from here" point.

[Pinchy]

thanks - looks like I have some options to try when I get home - thanks again for all the input

[GoSlash27]

so that looks like a rockomax 200 with 4 docking ports 4 twitch engines wheels and solar panels that looks something that would do the job more than fine to land - but then I would need to put 2 others (maybe ants) on the back to fire me fwds when I land?

I wouldn't use that specific design for Minmus; it's way too big for that job. It's just to show what I'm talking about when I say "fuel tank as the chassis".

Best,

-Slashy

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The wheels have their own propulsion (electric), if not a very high top speed, so an extra engine (Spark maybe; Ant with its minuscule thrust won't do any better than the wheels) is welcome but completely optional.

I wonder why the 4 docking ports, also on Tylo that solar battery won't be doing much good.

(note if you don't place the probe core facing the direction you go, a Jr. docking port is a decent substitute for a reasonably facing "control from here" point.

Sharpy,

We're getting off-subject. This was a .90 design from back when a solar panel was fine on Tylo. The docking ports were for carrying and manipulating loads.

I'm not suggesting festooning a Minmus lander with docking ports, I'm just showing what a tank- as- chassis design looks like.

Best,

-Slashy