KSP: Laythe Colony Part 4, Drop Ships and Lonely Rovers

After the second Jool launch window, I still had 196 days to get a few extra ships off Kerbin before its destruction on Year 3, Day 0. They couldn't transfer to Jool until the third launch window - around Year 3, Day 260 - but they could still get out of harm's way. I hadn't specified exactly how Kerbin is destroyed, but since this entire scenario is based on Seveneves, I think it was reasonable to say that these ships should not sit in cismunar orbit. So I decided to send them out to Minmus for parking.

Colony ship #11 or #12 - I lost count. Parked at Minmus for a front row seat to the end of the world.

By this point I was getting pretty tired of building colony ships. Each one takes about a dozen launches to assemble, crew, and fuel in low Kerbin orbit. But I managed to get two more built and parked at Minmus. I also realized that there would be a little bit of a housing shortage on Laythe with the extra 72 Kerbals these colony ships carry, so I sent up one more HAB1 transfer ship as well. But parking ships in Minmus orbit isn't exactly efficient, and I am running a pretty tight Δv budget. A perfect opportunity, then, to create one last piece of hardware for this mission.

The Drop Ships

The DS1 lander, a last-minute mining platform and fuel tanker for the fleet.

Until now, the only ships in my fleet with mining capabilities were the LR1 rovers, which can refuel space planes on the surface of Laythe. The planes can then climb into Laythe orbit and transfer any spare fuel to the colony ships. But it would take quite a few launches to fully refuel the colony ships this way. Better to mine on a moon with a shallow gravity well, like Pol, and net a bunch more fuel. So I designed a drop ship mining platform/tanker to do just that. Refueling the ships parked at Minmus before the third Jool transfer window would be a good test.

I've done space planes and straightforward powered descent, but never a true VTOL in the sense of a ship that is designed to hover and translate horizontally looking for flat ground or good mining prospects. Most of my knowledge about drop ships comes from watching Cupcake Landers videos. I just tried to make it symmetric, place the C.G. properly, and set up the fuel tanks so that the C.G. doesn't shift much as they drain.

Drop ship mining practice on Minmus.

Even though they're essentially flying fuel tanks, cruising through mountain ranges in the low gravity of Minmus in them is easy and actually kind-of fun. Normally I'm trying to time suicide burns just right or not stall out my space planes, both of which are more stressful technical tasks. Piloting a drop ship is closer to a sci-fi landing experience.  Which reminds me: if you're looking for a quick diversion from the brutally technical challenge of KSP, Outer Wilds is a beautiful (and creepy) exploration/mystery game with some incredible open-world storytelling. Absolutely worth going in blind and playing through.

Back to Minmus mining, though. I only had time to build two of these drop ships. They can operate autonomously, but they also have room for a pilot, for navigation in frontier areas with poor relay coverage, and an engineer, for more efficient mining. I realized while building these final few ships that I neglected to put relay antennas on the colony ships, something that is required for remote piloting rovers, space planes, or drop ships. Since I might need to do a lot of remote piloting in the Jool system, I decided to steal a couple relay satellites from Kerbin orbit.

Stealing a satellite with the grabby claw I knew would come in handy.

Once these ships leave Kerbin orbit, there won't be any need for a Kerbin comms network anymore, so I (literally) grabbed some of Kerbin's relay satellites with the last two colony ships. It is possible to create a remote piloting connection through a relay satellite in a grabby claw, something I find satisfyingly appropriate for Kerbal-style mission "planning". Anyway, I made a few round trips to Minmus surface to refuel the ships of the third wave and then that was it for Kerbin.

0 Days Remain

On Year 3, Day 0, time was up for the Kerbal home planet. The remaining population (of 432 Kerbals) was in flight, either on the way to Jool or at Minmus awaiting the third transfer window. No more hardware would be launched and the roughly four kilotons of ship and propellant in the fleet would have to become the Laythe colony. But it would still be almost another two years before the first colony ship arrives in the Jool system. Before that, the robotic fleet would have to lay the groundwork.

The Lonely Rovers

The 18 ships of the first Jool launch window arrived at their destination during the second half of Year 3. I set up the transfers such that the relay satellites would arrive first, since having a working comms net in the Jool system would be crucial to the rest of the mission. The RS3 ships and especially the ion engine satellites themselves have plenty of Δv to spare, so I just brute-forced them into useful coverage orbits around Jool and Laythe.

The first relay satellites arrive at Jool. I'm definitely guilty of setting up the WiFi before unpacking...

For the remainder of the ships, though, the Δv budget was tight enough that I definitely wanted to grab Tylo gravity assists on the way in. This created a bit of traffic as several ships would hit the Tylo gateway within days, or sometimes hours, of each other. To get captured using a gravity assist, I aimed to pass "in front of" Tylo, so that its gravity mostly pulls in a direction opposite my orbit and I feed it some of my kinetic energy. After some refinement, I also was able to target a captured orbit with a periapsis similar to the orbital radius of Laythe. From there, it's easy to get a low-energy intercept on the next orbit with just a couple small correction burns at periapsis and apoapsis.

Busy airspace (or, spacespace?) around the Tylo gateway.

Using gravity assist captures off Tylo, or in a few cases off Laythe itself, my average Δv from low Kerbin orbit to low Laythe orbit was about 3475m/s, with a tolerance of about ±350m/s. This is quite a bit below the 4360m/s you get from the subway map, which would have been cutting it very close for some of my ships. As it is, all of the robotic fleet made it to low Kerbin orbit with fuel to spare and without having to do any aerobraking. Assuming all the Δv saved went into accelerating Tylo (and it wasn't on rails), its apoapsis would be raised by about 1nm.

Getting to Laythe is not the same as landing on Laythe, though. It's a water world with only a few islands to target. I've landed there before, using a custom deorbit burn tool to target the island on the equator with the flattest terrain. To hit that island, it makes sense to burn over the small island that's about 90º west of there. I set up each ship in a near-circular 100km equatorial orbit and then start a burn just as the ship passes over the coast of that island:

Laythe deorbit burn over the small island on the equator, to hit the flat island about 90º to the east.

After the burn, the lander can ditch its propulsion module (which is mostly empty now and will burn up separately) and prep for entry. For the first phase of the landing, an inflatable heat shield protects the descent package from the initial atmospheric heating.

Landing Phase 1: Using an inflatable heat shield to protect the payload while bleeding off some speed.

As the air gets thicker, the drag on the heat shield overcomes the ability of the reaction wheels to keep it facing forward, so the lander flips around. The fairing still provides thermal and aerodynamic protection for the payload, and the heat shield now becomes more of an air brake, bleeding off even more speed in preparation for the final descent.

Landing Phase 2: The craft flips around, with the heat shield now acting as an air brake.

At about 3km AGL, the speed is low enough to jettison the fairing and deploy the main parachutes. The heat shield stays attached until the main chutes deploy, at which point it can be jettisoned in a controlled orientation so it doesn't crash back into the ship.

Landing Phase 3: Fairing jettisoned, main chutes deployed, heat shield dropped.

Finally, at about 300m AGL, the descent engines kick in and bleed off the final bit of vertical velocity. They don't have much fuel, so the burn has to be timed pretty well. I use the AeroGUI's AGL indicator and the lander's shadow to judge it.

Landing Phase 4: Powered descent. Kicks up a good amount of sand.

That's how things should go. But the first two landings were not quite perfect. I nearly overshot the landing zone on the first try, coming down less than 1km from the eastern shore. This is almost exactly where I landed my first Laythe mission, and I knew it was on a major slope. In the process of preparing for a potentially harrowing post-landing slide into the ocean, I forgot a few steps of the landing checklist and the descent engines didn't start up. The resulting ~15m/s impact was enough to break off the mining rig and fuel tank from the first LR1 rover down. But the drivetrain survived, so it could still act as a scout if it could get up the hill.

The first (hard) landing on Laythe in this mission, dangerously close to the shore.

Having nearly overshot the landing zone into the ocean, I tweaked the deorbit burn a little (from 104m/s to 110m/s). However, this was a little too much tweaking and lander #2 wound up heading straight for the lake in the middle of this island. Luckily, this was a HAB1 lander, which has a little more fuel on board for the powered final descent. I managed to just barely hover-translate to the cliff edge overlooking the lake's eastern shore with no fuel to spare.

Landing #2 involved some last-second piloting to steer away from the lake to the edge of a cliff.

Those two landings gave me the upper and lower limits for the deorbit burn. I used 108m/s as the burn for the remaining 12 landers, and they all touched down safely on the relatively flat land between the lake and the eastern shore.

Typical landing zone after dialing in the exact deorbit burn.

I say relatively flat because it's still filled with sand dunes. They're no problem for the 6- and 8-wheeled rovers, but I need a 1-2km stretch of actually flat terrain to use as a space plane runway. I scouted for a while before settling on the strip marked out by the pink markers in the landing photo above. It's about 1.5km long and 300m wide, near the equator, and aligned well for west-to-east landings. It's completely flat in the crosswind direction and slightly sloped upward in the "upwind" landing direction. I'd prefer something flat in all directions, but this is the next best thing.

By the end of the first wave, I could place the landers with about ±2km accuracy from orbit. But they are rovers, so it's easy to reposition them as needed. The LR1s all grouped together to form the corners of the runway, acting as visible markers for the space planes on approach. They're needed at the runway for refueling anyway, so this seems like the best place for them. In order to avoid excessive part counts in one location, I decided to move the HAB1s, the colony habitats, away from the runway and toward the lake. There, they could be assembled into housing groups.

Setting up some modular housing on the dunes.

It's not a metropolis, but having a mobile and reconfigurable colony seems ideal on the sand dunes of an otherwise pretty desolate water planet. In total, 13.5 of the 14 rovers in the robotic fleet made it to the surface, and all 14 were able to find their way to each other and remotely set up the infrastructure for a colony. It'll be another year before the colony ships arrive in the second wave, but when they do, they'll have a place to stay - with a nice view.

KSP: Laythe Colony Part 3, The Colony Ships

Jool Launch Window #2 is all about getting as many Kerbals in transit to Laythe as possible, and that means building a fleet of colony ships. This was actually the first ship designed for this mission, but I only built one as a proof-of-concept before committing to the Robotic Fleet for Launch Window #1. Those habitats, rovers, and relays will arrive first to pave the way for the colony ships.

The colony ships are built in orbit, with each part launched separately on the same heavy-lift boosters that sent up the Robotic Fleet. The core of each ship, around which the rest of the ship is built, is a Passenger Module:

The Passenger Module

The Passenger Module has room for 18 Kerbals (half the crew of each ship), with two main living compartments on each end, a central stack of general purpose seating, and two observation domes. It's meant to be the "comfortable" portion of the ship, to make the multi-year journey more bearable than would be possible in a lander cockpit. Not that Kerbals really care.

One of the main quality of life considerations for the colony ships is the ability to spin to generate artificial gravity in some of the living quarters. For this reason, the rest of the ship is built along an axis passing through the center of the passenger module. Forward, the next part is the Docking Module:

The Docking Module

While it has space for another six Kerbals, the Docking Module is more of a working space than a living space. Since it's on the central axis, there's no artificial gravity. But it has a large science lab and common area for the crew. Most importantly, it serves as the docking interface for the Space Planes, which shuttle crew to and from the colony ships.

The Space Planes

The Space Planes are really the key to this entire mission, providing a way to get hundreds of Kerbals down to Laythe without having to exactly target flat landing sites from orbit. I tweaked and tested the design to the point where getting to orbit, docking with a colony ship, and returning to Kerbin for a runway landing was utterly routine. Each colony ship required four Space Plane round-trips and two one-way trips to fully crew. The two one-ways go with the ship to Laythe, where they will be used to ferry Kerbals down to the surface.

The back-to-back docking configuration for the Space Planes minimizes the moment of inertia along the spin axis. The two planes have to be exactly symmetric, so each interfaces with two medium-size docking ports for alignment. It is possible, with careful flying, to get both ports to engage at the same time. In addition to enforcing symmetry, this makes the final structure much more rigid. Finding parts that are exactly the right spacing on both sides to make this possible was the trickiest part of the design.

I could write an entire post about the Space Plane design, but I think I'll just post some pictures and videos of it kicking ass instead:

The last picture has a story that goes with it: For some reason, after dozens of clean flights, I botched a take-off and slammed back into the KSC runway with the gear still down, breaking off both wings, the outer engines and fuel tanks, the vertical stabilizers, and all but the two inner horizontal control surfaces. The fraction of a plane that was left was somehow still able to gain altitude, do a wide 180º turn, and make a water landing just off shore.

Anyway, back to the colony ships. Behind the Passenger Module is a truss structure I just call The Node:

The Node

This is the lightest and simplest part of the colony ship, primarily serving as a connector between the crew stack and the propulsion. It also carries the large solar panels, some battery storage, extra reaction control systems, and side ports for docking other modules, such as for refueling. Altogether a small but important building block.

To push all this, three Propulsion Modules are launched separately and docked to the back of the Node. These are the full four-engine versions of the propulsion modules used for the Robotic Fleet.

The Propulsion Modules

With 12 engines in total, the colony ships actually have a higher thrust-to-weight ratio than the robotic landers. The entire colony ship, including the two Space Planes, comes in at just under 300 tons and has a fully-fueled Delta-V of about 4400m/s, which should be enough to get to Laythe orbit with just a tiny bit of help from gravity assists off Tylo (or Laythe itself).

The process of building a single colony ship takes 13 separate launches: six for assembly, six crew shuttles (including two permanent ones), and one refueling run. (While the propulsion modules get to orbit fully-fueled, the total Delta-V counts on topping off the two Space Planes.) It's without a doubt the most ambitious in-orbit construction project I've attempted in KSP.

Oh, and I built 10 of these for Launch Window #2. That's about 3,000 tons of hardware, including 20 Space Planes and 360 Kerbals, on the way to Laythe.

196 Days Remain

I had intended for the second launch window to be the last ships out, but my arbitrary deadline of Year 3, Day 0 for the destruction of Kerbin leaves some time to send up a few more. They'll have to wait for the third launch window, possibly in the relative safety of a Minmus orbit, but I can think of a few extra pieces of hardware that would be useful to the colony.

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...

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...