Sunday, March 31, 2019

TinyCross: Chassis Build

Just hit print...
This winter build season is coming to a close: almost all of the mechanical work for TinyCross is done! Here's a recap how the rolling chassis came together:

The frame and suspension is mostly a kit of aluminum plates and 80/20 extrusion, with almost no post-machining required, so it went together very quickly. After the main box frame assembly and seat mounting, I started with a test build of a single corner of A-arm geometry to make sure I hadn't missed any clearance issues. I also wanted to get a first impression of the stiffness in real life, since that's probably the biggest risk of this new design.


I had no trouble at all with the front-right corner. Everything fit together as planned and the stiffness felt adequate, largely thanks to the zero-slop QA1 ball joints. I was actually a little surprised at how well-behaved it felt. Once all six ball joints and the air shock pins were tightened down, it really did have only the degrees of freedom it should have: one for steering and one for suspension travel. There's no rattling or play at all. With high confidence from the test corner, I went into production line mode for the other three.

PTFE-lined QA-1 1/4-28 rod ends (CMR4TS) are the real stars of this build. It would not be possible with McMaster's selection of ball joints, which are either cheap and overly loose or expensive and overly tight.
I say there was almost no post-machining, but just tapping all the 80/20 ends was a whole day of work.
About here is where the perfect build ended, though, because when I went to attach the front-left corner, I discovered that there was a slight interference between the A-arms and the air shock valve stems. The parts I designed, all 2D plates, are 100% symmetric, so I didn't bother to model the other corners. But the shocks themselves are not symmetric, so it wasn't exactly correct to assume that things would fit together the same in the mirrored configuration. Since the interference was minimal, I debated cutting notches in the A-arms for the valve stems. But I was able to find a more satisfying solution.

Can you spot it?
Not modeling the mirrored parts was a semi-legitimate time-saving strategy, but not modeling the rear corners was just laziness. And of course there was a major interference there: the brake calipers would not have cleared the corners of the frame. (In the front, it's no problem since there is extra space for steering travel.) It was nothing that couldn't be solved with a hacksaw and some improvisation, though. I actually like the final outcome better than the original design...

...he justifies, in post.
Minor issues aside, I am pleased with how the chassis turned out. It's much stiffer than tinyKart, thanks to a slight excursion into the third dimension, but still very light. And I went from 50% to 99% confidence on the suspension design after getting hands on the assembled corners.

So far, so light.
Most of the machining for this build was for turned parts within the four drive modules. The spindle shafts for the wheels were made from 7071 aluminum and support the wheel bearings (6902-2RS). Unlike on tinyKart, the shafts are doubly-supported within a box structure built around the wheel, which should be much more impact tolerant. The large drive pulleys got some weight reduction and a custom bolt pattern to interface with the wheel hubs.


My favorite bit of packaging is the brake caliper occupying the volume inside the belt loop, with the brake disk flush against one side of the wheel pulley. Torque is sourced and sunk from the same side of the wheel - in fact from the same metal plate.


The motor shaft and motor pulley also required some custom machining. This was a weak link on tinyKart: The original design used set screws on shaft flats, but it was prone to loosening over time (or, in one case, completely shearing off the 10mm shaft at the flat). After switching to keyed shafts (via Alien Power Systems motors), those problems mostly went away. But there was still axial play, and the torque was still being transmitted through a 3mm key into an aluminum keyway.

For TinyCross, I wanted to have a clamping and keyed shaft adapter so the torque would be primarily transmitted through friction, with the key as back-up. There's not a lot of room to work within the 15-tooth drive pulley, so that just gets bored out as much as possible and then pressed like hell, with retaining compound, onto a 7071 adapter. This adapter then gets the 10mm bore with a 3mm keyway. But it also gets slotted, turning it into a clamp. Finally, an off-the-shelf 0.75in aluminum clamping collar tightens the whole assembly down onto the motor shaft, with the key in place.


Additionally, the outboard side of the shaft interfaces with another bearing, for double support, and has a pocket for a shaft rotation sense magnet, to be picked up by a rotary encoder IC.

Not messing around.
For brakes, I opted for the same disks, calipers, cables, and levers as on tinyKart. I briefly debated going hydraulic, but the plumbing for four wheel disk brakes seemed like an unnecessary nightmare. tinyKart never had a problem with braking torque; it could easily lock up both front wheels. It just had so little weight on the front wheels that braking and steering were often mutually exclusive activities. With four wheel disk brakes, TinyCross should be much more controllable under braking. The TerraTrike dual pull levers are key to making this work: they have a fulcrum between the lever and two cable ends that ensures both cables get pulled with equal force. I have one such lever for the two front discs and one for the rears.

Independent rear brake lever...what could go wrong?
The last piece of the mechanical puzzle is the steering. At each wheel, there's a steering arm that terminates in a place to mount yet another ball joint, using a T-nut. This is driven by a link comprising a threaded rod with an aluminum stiffener, another trick carried over from tinyKart. The aluminum stiffener is compressed and the threaded rod is stretched by two nuts, creating a link that's stiffer than either part by itself.



For the rear wheels, all that's required are two fixed mounting points for the other end of these rods. Rear toe angle is set by adjustment with the threaded rod.

The toe-setting plate doubles as the TinyCross badge.
The front requires an actual steering mechanism. In lieu of a rack-and-pinion, I used a simple four-bar, driven by the steering column through a universal joint buried in the middle of the front suspension support tower. Each link in the four-bar has its own set of thrust bearings and radial bushings, to minimize the extra linkage slop.



This sort of setup works since the steering throw is very short: ±45º of travel is all it needs. Amazingly, it all clears over the full suspension travel and there doesn't seem to be much bump steer. (I shouldn't be amazed, since it works in CAD, but I am anyway.)

And just like that, it rolls.


Unfortunately, although the frame and suspension were on-target, the four power modules are way over weight budget. The wheels themselves are annoyingly heavy. I can't do much about that, but I can probably take some weight out of the surrounding assembly. A lot of the design is driven around the off-the-shelf cast aluminum rims. If I am willing to chop down the rim and re-machine its outer bearing bore, I can probably save a little weight and a lot of width. I can maybe even get it below 34in, which would help with getting through doorways.

But for now I will shift focus to the electronics. Preliminary bring-up of the motor drives has been uneventful (that's a good thing), but I'll need to actually hook them up to LiPos next, which could always become interesting in fiery ways.

To be continued...

Thursday, November 29, 2018

KSP: Laythe Colony Part 2, The Robotic Fleet and Launch Window #1

In honor of the successful Mars InSight landing this week, I thought I'd do a progress report on my long-term KSP mission to get as many Kerbals off Kerbin by Year 2, Day 0 as possible. Part 1 sets up the premise and the main strategy. In this Part 2, I throw about 1,000 tons of robotic hardware at Jool during the first available launch window, with hopes that at least some of it winds up in a single spot on the surface of Laythe as the seed for a colony.

The busy 1000m/s on-ramp to Jool Transfer Orbit.

The Robotic Fleet

For the first launch window, I decided to send only uncrewed vehicles to feel out the Jool transfer orbit, the details of maneuvering within the Jool system, and the landing procedures at Laythe. The robotic fleet consists of three types of ship: Triple Relay Satellites (RS3), Laythe Rovers (LR1), and Habitats (HAB1). Each one has a different function crucial to settling a remote colony.

RS3

Does this thing get HBO?
These are the smallest and lightest ships, but critical to this first remote-controlled mission phase. One of the relatively new realism additions to KSP is that uncrewed vehicles need to have a line-of-sight communication path back to Kerbin, or to a ship with a crew, in order to maneuver. To achieve this requires lining up a bunch of relay satellites around Kerbin and at other useful locations in the system. 

Each RS3 assembly carries three small relay satellites with their own ion drives. In addition to the ones already parked around Kerbin, two sets of three are on their way out to Kerbin L4 and L5 stations and then eventually other equally spaced points in the orbit. Three more sets are heading out to an intermediate orbit between Kerbin and Jool. And four sets of three are in the fleet heading for Jool, to set up a network around Laythe.

The start of the mission's comms network.

LR1

Practice driving on Kerbin.
The Laythe Rovers are giant 30 ton workhorses. The main function of these 8WD crawlers is to seek out ore to mine and make fuel on Laythe. They each have two large drills, a refinery, and a huge fuel storage tank. They can dock with a parked space plane to refuel it, which is critical for sustaining a link between the Laythe surface and hardware/habitats in orbit.

Landing the rovers is a four-step process. They come packaged in an aero shell with a heat shield, so the initial descent involves just surviving with the heat shield pointed in the right direction. After some time, the drag on the large heat shield flips the package around and the heat shield itself becomes a supersonic air brake, with the aero shell protecting the rover. Once subsonic, the heat shield and fairing are discarded and set of parachutes further slows and rights the rover. Lastly, a set of four rockets slows it to a safe velocity just in time for touchdown.

Step 5 is to quickly deploy the solar panels and drive out of the way of falling fairing debris.

HAB1

Home, sweet home.
The Kerbals can live for extended periods of time in orbit, but having a home base on the Laythe surface will be important for long-term survival. In order to facilitate construction, the surface habitats are themselves rovers with roughly the same chassis as the LR1s. They land the same way and, once on the surface, can drive to each other. This will be important, since the landing target might hundreds of square kilometers.

The habitats are extremely modular. They can be individual homes for a single Kerbal family, including single-passenger mini-rover parked in front. Or, they can be docked together indefinitely to form a larger base, thanks to a central hallway section with docking ports on either end. The slight angle of the hallways allows them to fit inside the aero shell.

How much fuel to bring?

The LR1 and HAB1 landed payloads are both around 28 tons, about half the mass of my first Laythe lander. (That lander had to be heavy in order to have enough fuel to get back off of Laythe, a task to be handled by space planes this time around.) In that mission, two identical ships flew independently to Laythe with an average of about 2500m/s of fuel-burning Δv. But, they also made heavy use of aerocapture at both Jool and Laythe. Without that, it would take something more like 4360m/s to get from low Kerbin orbit to low Laythe orbit, according to the amazing KSP Subway Map.

With a known Δv requirement, figuring out how much fuel to bring is simple. With the 800s specific impulse of the LV-Ns, the minimum wet to dry mass ratio is:
So, the ships need to carry about 3/4 ton of fuel for every one ton of dry mass. Not too bad, since the same heavy lifter that carries up the packaged landers can carry up an equivalent mass of LV-N engine, fuel tank, and liquid fuel. Thus, each robotic lander requires two separate launches:

First, a heavy lift booster hauls the lander payload in its aero shell into orbit.
These are the unsung heroes of the mission, relentlessly hauling all the more exciting hardware into orbit.
Next, a second booster brings up a propulsion module, with LV-Ns and a lot of liquid fuel.
Just remember to check yo' staging...
The two meet in orbit and create a transport ship with a wet to dry mass ratio of about 1.775, for a Δv of about 4500m/s.

Rendezvous between an LR1 lander package and its propulsion module.
A total Δv of 4500m/s is cutting it a bit close, but they would only need a small amount of aerobraking or Tylo gravity assist to gain back a comfortable margin. There's also a good amount of RCS fuel on board that can be dumped (in a prograde or retrograde fashion) near the end of the trip if it's not needed. Additionally, the RS3 ships have a lot of fuel to spare if their propulsion modules can be swapped onto the more thirsty landers nearer to Laythe. The wet to dry mass ratio of the fleet as a whole has a comfortable margin.

Launch Window #1

The first Jool launch window happens around Day 190 in-game. (The simple and more complex online calculators both agree to within a few days).  Up to that point, I spent time refining the landers and practicing the landings on Kerbin. But once the designs were locked, the push began to assemble the fleet in orbit. 

The practical limit on fleet size is how many ships can be juggled during the actual launch window. In order to boost the wet to dry mass ratio, these ships have the two-engine version of the propulsion module, which gives them a somewhat low thrust to weight ratio. The ~2000m/s ejection burn had to be split into two parts: one into a 10-day elliptical orbit and a second to escape onto the final Jool transfer. Even still, the burns were 10 minutes each, so the ships had to be spaced out so they would reach their final periapsis burn at reasonable intervals.

Also, it would be nice if they didn't hit the Mun on their way in.
After the final burn, the transfer takes over two Kerbin years, meaning Kerbin will have been destroyed by the time the first ship even arrives at Jool. The lander designs can never be tweaked, and the crewed fleet will have to set out with no guarantee that there will be a base waiting. No pressure.

Lots of empty space to cross now.

661 Days Remain

With the first 18 ships on their way to Jool and then Laythe to set up base, the priority shifts to getting Kerbals off-planet. This means mass-producing and then filling the immense colony ships, which are the most intricate builds I have attempted in KSP yet.

More to come...

Monday, September 24, 2018

TinyCross: Electron Herding Unit

My ultralight electric crosskart kit has arrived:


By "kit" I mean the water jet-cut plates and the 80/20 extrusion. There's still some custom machining to do, but most of the kart is designed as plate-and-standoff structure, so it does feel a lot like a kit build. I want to start putting it together, but also realize I am way behind on the electrical side and should get some PCBs on order before I really dive in to the build.

Since it's 4WD with independent motors, I want two dual controllers, front and rear. One of my first three-phase controllers was a dual layout, using IXYS GWM100-01X1 three-phase bridge modules. These modules have excellent cooling, thanks to their huge isolated heat sink interface that can be mounted directly to the chassis (with thermal paste). And while the GWM100-01X1 is undersized for this project, the newer MTI200WX75GD looks spectacular. So let's begin.

3ph v2.2: MTI200WX75GD High-Power Upgra...crap.

They're not available. Despite being in stock for a while, they seem to have disappeared. I mean if you want, you can still get one, but the big distributors have stopped stocking them and the little distributors are pains in the ass. Some day they will return, and I'll be ready for that day, but until then I need some other way of herding electrons.

Can I Just Stuff Some FETs Somewhere?


Sometimes having an extra constraint, even if silly, is good for making progress on things. It makes optimization easier by removing degrees of freedom. So in this case my silly constraint is that any alternative MOSFET configuration must fit entirely within the footprint of the MTI, so they could be swapped without changing the board design. 

I played with some DirectFET and Power SO8 layout ideas before settling on just one monstrous FDMT80080DC per leg. They're similar in many ways to the MTI, with comparable resistance, gate charge, and current ratings. They also have top-side cooling that, although smaller in surface area and not isolated, could sink heat to the chassis fairly well. Using just one per leg yields a tight and tidy layout that fits within the 12mm span of the eight MTI pads allotted to each phase. I've also left solderable tracks for adding extra conductors if needed.

Extending this constraint further, I decided the 12mm strips should fit an entire phase: FETs, gate drive, local capacitance, current sense, and output.


Copy-paste six total for a dual controller. A header in the middle eats current/voltage sensor signals and spits back gate drive inputs. The gate drive is the same one I use on everything: anti-parallel optos with dead-time RC. It's low part-count, easy to bootstrap the high side, tri-stateable, and guarantees dead-time in hardware. And as far as the microcontroller knows, it's just driving some LEDs. What more could you ask for?

Cross Karts, Not Current

For current sense, the ACS781 offers an impressively tiny 100A (150A peak) surface-mount Hall effect current sensor. The current sensing range can be extended by diverting some of the current around the sensor. I've had issues with nearby large currents coupling into small Hall effect sensors before, so in this case I've taken extra care to keep non-sense currents flowing perpendicular to the sensitive axis of the ACS781:


The sensors are aligned so that their sensitive axis is along the board X axis (left to right). Except for the sense leg, all nearby current is routed along the board Y axis (top to bottom). This is especially important for the sense and bypass current flowing immediately adjacent to the next sensor over, since cross-coupling is harder to deal with than a slight change in sensitivity on a single phase. Similarly, phase wires exiting vertically (Z axis) should not produce much field in the sensitive direction. Hopefully.

The Rest of the Panel


I've done separate logic/power boards for a while, but this is my first controller designed as a multi-board panel. In this case I wanted the power board, logic board, and two motor sensor boards all in one. OSH Park has a four-layer prototyping service by the square inch (no panelizing penalty), and some of the most beautiful-looking boards I've ever gotten have come from them. They do quantity three for everything, which is perfect since it gives me two controllers and a spare.

The logic board sits on the gate drive / sensor interface, with a single STM32 running the show. It turned out a lot denser than I thought. While it's more common to do a full schematic first and then layout and route, I tend to make nicer boards if I route as I go, doing a few nets at a time and planning the placement and pinout with the routing in mind. And sometimes you just get lucky and find an open path on the layer you need.

It's All So Clean Until You Add Wires

Since the dual controllers are on the kart centerline and the motors are in the corners, I need to run about 700mm of motor phase and sensor signal wires together to each corner, and I know how that story usually ends. I'll use shielded cable for the signal wires and try to keep them separated as much as possible, but I have a new trick to try for making the Hall sensor signals more robust. For starters, they will run through a buffer on the sensor side, to decrease the impedance. (Normally, they are just open-drain with pull-up resistors.) The real trick is on the logic board side, though, where they will be sent through this:


The signals are fed into a three-channel optocoupler, which means noise would have to be able to inject mA-level current into the signal to change its state. But the input side of the optocoupler also references only the phase signals, not a local or even remote ground. Common-mode noise, even on the order of Volts, can't change the state. It's a sort-of three-phase differential signal.

The Big Picture


It's been interesting to revist the idea of a dual controller. I'm honestly not sure how my first one worked at all, although I think the robustness of the IXYS bricks and opto gate drive helped me out a lot there. This one feels a lot more routine, and I'll consider it a success if it just works without any fuss, like Twitch X's drive. Speaking of Twitch X, there's a really neat consequence of independent front-wheel drive that might make for some interesting software parallels down the road...

Back to the build for now.

Saturday, August 25, 2018

TinyCross: An Ultralight Electric Crosskart

It's time for a new go-kart!

I'm going for something a little bigger and more capable than tinyKart, but with the same ultralight genetics. This time, the inspiration is from something I didn't even know existed until recently, called a crosskart. The best way to understand the appeal of a crosskart is just to watch this video:


Now, there's no way an ultralight electric version will be as exciting as that, but if I can get to an end result that's about 33% crosskart, 33% downhill racer, and 33% electric scooter, I'll be more than satisfied. So, here's the concept:

TinyCross chassis and single corner module.
The design follows the water jet-cut plate and 80/20 construction that's been my go-to for rapid and adjustable construction. There are still a few machined parts, mostly turned shafts, but a large portion of the chassis can just be "printed" on a couple sheets of aluminum.

The 0.25in aluminum plate components to be cut.
Long-time readers may recognize that I've switched over to new CAD software. The days of free SolidWorks are over, so I'm using Fusion 360 now. Rather than arguing with people about the best ECAD/MCAD software, I tend to just try the different packages until I understand their fundamental strengths and weaknesses. I've got maybe 100 hours in Fusion 360 now and the biggest difference to SolidWorks I've noticed is the assembly constraint framework: SolidWorks "mates" are refined, purposeful, and user-friendly. The "Width Mate", for example, is sublime. Fusion 360 "joints" are raw and don't inherently guide you to the good practice of constraining your CAD the same way as your real assembly. But, they have a general flexibility that I find very powerful and time-saving.

As I've done more and more projects, I spend way more time in the CAD than actually building. This design has been almost a year in the works before I felt confident enough to order plates. I'm still not sure it'll turn into exactly what I want, but here are some of the details that I have been working on:

Drive Module x4

Drive block with motor and brakes.
The powertrain is entirely contained in a ~4kg block that is repeated in each of the four corners. It uses the same motors as tinyKart, Alien Power System 6374-170 outrunners, but with different gearing for the larger 12.5in scooter wheels. With four of them, I expect around 8-10kW peak output. I've carried over the offset bearing  belt tensioner that worked so well on tinyKart. The brakes are the same as well, but I managed to fit the caliper in the middle of the belt loop to keep everything in-line.

Suspension and Steering

Front suspension and steering geometry.
Suspension separates a crosskart from a go-kart in my mind. I wanted something that can handle speed bumps, potholes, gravel, dirt, snow, and other surfaces that would (and did) tear tinyKart apart. Now, I don't have any experience designing vehicle suspension, so I took a reasonable first guess on the geometry but left room for tweaking later. The shocks are mountain bike air shocks, which are both very light and easily adjustable. The total suspension travel is about 5in.

Because the suspension takes up so much space, I wound up moving the steering linkage to the very front of the kart to make more legroom. This turned out to be a major headache since the Ackermann geometry is backwards. I was able to get back to a reasonable geometry by using a slightly more complex linkage. Minimizing bump steer from suspension travel was also an interesting new challenge for me.

Chassis

Box chassis to add a little stiffness.
tinyKart's chassis was truly flat-packable, but as a result it twisted a lot. This actually worked in its favor somewhat, keeping all four wheels on the ground in lieu of suspension. But here, I was willing to go vertical and make a bit more of a box structure to keep the chassis stiff. Note that there are many stiffening spacers between the thinner bottom plates that I've left out of the model to keep the assembly smaller.

I still want to be able to separate the chassis into front an rear sections for transport with as few fasteners as possible. Removing the two side rails and then sliding the rear chassis out of the front chassis plate sandwich seems reasonable.

Electronics

I've settled on motors and batteries (12S/20Ah LiPo), but haven't started any of the controller work yet. I'd like to do an update version of the 3ph "Duo" v2.1 controller, one in front and one in back, but based on the newer IXYS MTI200WX75GD-SMD, which is a pretty awesome little part. I need to run the math though, since the original design was never meant for these power levels.

I'm also planning a simple board for the steering wheel, to host a display, trigger throttle, and some knobs or switches for quick setup. Not sure how far I will dive into independent torque control, but I do plan to link everything together with CAN to leave that possibility open.

Power/Weight Budget

I'm shooting for about 34kg / 75lb for the kart without batteries. With the full 12S/20Ah it will be about 40kg / 88lbs. This is about 50% heavier than tinyKart, but still light enough to manipulate up onto a table to work on, or into a vehicle for transport. Plus, most of the weight of a go-kart is the rider, so with double the power it should have plenty of acceleration to spare.

Parts should be coming in this week so next post will have some real pictures!

Sunday, August 6, 2017

KSP: Laythe Colony, Part 1

A while back, I read a book called Seveneves, by Neal Stephenson, and found it to be immensely entertaining. The premise is that, for unknown reasons, the moon explodes and humanity has two years to get off the surface of the planet before the pieces crash into it. What I particularly enjoyed is that it's set in the near future, so we only have the tools we have now to work with. One of those tools is the International Space Station, which becomes the hub of a permanent space habitat. With no budget constraints and no point in risk aversion, a convincing amount of hacking manages to throw lots of hardware into space in a short period of time. And it falls on a few people to figure out what to do with all of it, since no viable long-term mission plan is proposed by ground leadership, who have other concerns. (Just go read it!)

You know who else has no budget constraint and no concept of risk aversion? The Kerbals! I decided a fun challenge would be to see how many Kerbals I can get off-planet in two (Kerbin) years, with the ultimate goal being to land them on Laythe and set up a colony. Laythe's atmosphere, while not directly breathable by Kerbals, has enough oxygen for jet combustion. And there is plenty of water.

Like, seriously a lot of water.
It's been a while since my Kerbals first carried out a mission to Laythe and back. Making a precision landing on Laythe's sparse, sand dune-covered islands was a challenge, especially with a top-heavy vertical lander. In order to establish a colony, Kerbals and equipment will have to meet at a single location on the surface of Laythe, so a better approach is needed. For the passengers, this means riding down on one of these:


This space plane uses four CR-7 R.A.P.I.E.R engines, hybrid engines that switch from air-breathing to closed-cycle at altitude. So, it takes off like a jet, builds up as much speed and altitude as it can, then switches to rocket propulsion to reach orbit. KSP's aero model was entirely redone for v1.0 and onward, making atmospheric flight both more realistic and more difficult. But this plane can still ferry six Kerbals into Low Kerbin Orbit (LKO) with a bit of fuel to spare.

It's also surprisingly sporty.
The Kerbals can't ride space planes all the way to Laythe, though. Well, they can if you build one like this. But I can barely get my six-passenger one into orbit so I'll need a more suitable living space for the passengers on the long cruise to Laythe. This led me to build my first true space station in LKO:

Finally a place to park all my space planes...
I wanted it to be modular and symmetric so it would be easier to add a propulsion stage for the long journey. This meant docking two space planes, and after some thought I decided they should be back-to-back to minimize the inertia on the long axis. (Any guesses as to why I would want to do that?) The planes are docked with two ports each for precise alignment and more rigidity.

The T-shaped station is actually launched into orbit in two pieces. The docking module attaches to the space planes, while the crossbar of the T is the main passenger module, where the Kerbals will ride out the long trip. The passenger module will become the hub of a much larger ship once propulsion is added on opposite the docking module.

Getting pieces this large into orbit is the work of a new two-stage heavy lifter.

No, the first stage is not recoverable.
The first stage is powered by five RE-M3 Mainsail engines, and includes all eight side-mounted fuel tanks and the bottom central fuel tank. The second stage is just a single tank and Mainsail. Together, they can hurl about 50t into orbit. Except for the space planes, every piece of hardware that goes to Laythe will start out mounted to the top of one of these ascent stages.

And there are many more pieces of hardware required to make the trip. The main strategy will be to send a crapload of unmanned stuff (rovers, habitats, and utility vehicles) to Laythe during the first launch window (~184d in) and then follow up with a fleet of passenger ships, based on the station above, during the second launch window (~1yr, 225d in). But, since KSP has added an element of realism in that unmanned craft need communications to be remote controlled, a network of relay satellites is also needed. All of this also needs efficient propulsion for the long leg of the trip.

More to come...