batting the aerospace ball around for something the regular rules forbid...

[Cannonshop]

I want to build something the rules generally don't cover and forbid, but some fluff supports.  I want to build an amphibian dropship INTENDED to land on water.

and survive the experience.

and Take Off Again.

the initial functional purpose would begin with colonist delivery where there are no spaceports, but there ARE large amounts of standing water.

so...how would that work as rules?

we would have to begin, with construction rules.  Said dropship would have to have specific 'landing gear' that aren't...well, conventional landing gear.

It would also need a 'hull' configuration that is hydrodynamic.

THIS MUST COST SOMETHING, over-and-above the normal construction costs for a dropship.

not talking C-bills here, I'm talking mass and/or volume costs...but to be useful, it can't be excessive.

so let's get cracking. 

There are a few conditionals; 

Spheroid versions should cost more than Aerodyne.  Why? stability on landing.  A sphere is going to be less stable on open water, even with the doors shut (Witness Russia's experiments with circular battleships in the late 19th and early 20th centuries).  Naturally to make this useful, we need doors opening above the theoretical water-line.  We also need to mount the engines high-and an Aerodyne works for that-you can mount the engines OVER the wings (as was done with jet-powered flying boats in the fifties.)

it's going to have expensive sensors.  electronics and avionics are vulnerable to wet environments and MORE vulnerable in wet-and-salty environments (coastal waters on worlds with oceans.)

since it doesn't require a paved landing area, but DOES require open water bodies of a certain size (think:Large) we can extend the 'take off run' to the distance of overthrust times a percentage of the mass times two for aerodyne models (and no VTOL takeoff capability for Aerodynes.)

Users would obviously be Periphery factions with wet planets, an emphasis on aerospace capability and a lot of 'pioneer' work. 

this suggests we can pretty much forget about carrying 'mech bays on any but the largest versions, and maybe limit it to light vehicle and/or aerospace bays (to release hovercraft, wiges and fighters, which do NOT need a ground level ramp.)

it's a reasonable reason why nobody is building mass quantities of these in the Inner Sphere or Clan worlds.

some configurations might be different.  obviously if it's meant to deploy wet-naval assets, it would require a well-deck.  This should have mass and take up space on the sheet.

add a 'desalination plant' or electrolysis plant and it can refuel pretty much anywhere-this should certainly add to the mass of fuel system in some significant way.

this should also allow superior cooling when 'grounded' (on water) and firing on-board weapons emplacements.  (using the ocean or lake or river as a massive heat-sink).

it should not be submersible.

so, any other add-on ideas, and what kind of percentages of mass should the specialized "Amphibian" dropship sub-class have to deal with?

[The_Caveman]

A spheroid ship landing on water doesn't necessarily have to be unstable.

Fit it with outrigger floats that deploy some distance from the hull. Imagine Sputnik with deployable pontoons instead of antennas. When the ship begins to roll, the buoyancy of the floats on the descending side (and the weight of the floats on the ascending side) will neutralize its motion. The floats will also create drag that will slow the ship's tendency to spin.

As for the engines, there are already canon designs with engines mounted in pods outside the hull. I don't see any reason this should be difficult to arrange for an amphibious vessel.

[Hptm. Streiger]

Don't think that the engines need to be above the waterline. You might need some gas thrusters that push the water from the nozzle but after that the drive will vaporise any water coming close.

Don't have the numbers available (GURPS vehicle builder) yet, but extra sealing, pressure proofing for the structure, add submersible drives might cost 25-50% additional mass.

[Alsadius]

TBH, water is probably a better environment for landing a DropShip than the ground is. It's a bit more forgiving of small speed and location differences, and the ship is already designed to be watertight, since it needs to be airtight for vacuum travel. Mass distribution won't be a huge problem either, since the ship is already designed to have all of its weight "supported by" its engine while it's thrusting.

The biggest problem would be that rocket engines are very sensitive to the density outside the engine bell - IRL, the Space Shuttle's engines generate 23% more thrust in vacuum than they do at sea level, because of the density change between air and vacuum. Water is about a thousand times worse than air(literally). If you can't clear the water out of the engine nozzle, you're probably not going to space today. So you'll either need to have engines that are capable of getting you out of the water (which likely requires very different design specs than an air/vacuum engine), something like a JATO to give you a sudden kick to clear the water, or engines mounted above the waterline.

For BT design, I'd want to keep this a bit simple. Let's say a minimum thrust that's one point higher than the body's gravity would normally require(so you need 3/5 or better to lift off from Earth, not 2/3), and raise the cost and mass of the engine by 20% or something to represent the difficulties of designing/mounting the engines to avoid the water. The other systems are either not affected(because they're inside an airtight vessel), or they're so cheap that it's not worth tracking them (doors and cargo handling equipment).

[Daryk]

David Drake's RCN series used water landings by preference, and might provide some inspiration as to how to make that happen (outriggers FTW!)…  8)

[The_Caveman]

A fusion rocket isn't even going to notice that it's underwater. Sure water is incompressible, but that only matters in a fixed volume. It will be blown clear of the nozzles by the multi-megaton energy of the rocket exhaust.

Any water anywhere near the ship is going to become steam in milliseconds if you light up the fusion drive. You're not going to make friends with the local fishermen.

[RifleMech]

As has been mentioned some Spheroid dropships have their engines mounted in pods on the outside of the engines. I can see giving them an Amphibious quirk. For aerodynes I can see using the Combat Vehicle Flotation Hull for those built for water landings. (I'd apply that to ASF as well.) For these dropships I don't think there should be added cost since they're built to land on water from the beginning. They'd just be really large Amphibious planes. https://en.wikipedia.org/wiki/Amphibious_aircraft   Although I do think that the Spheroid Dropships engines would be more vulnerable to attack.

For Aerospace units that are adapted for water landings I'd use the Partially Amphibious or Fully Amphibious Chassis Mods. Partially Amphibious being inflatable and not having a drag penalty and used for more on Spheroids. https://www.scalemates.com/kits/slingshotmodels-48003-helicopter-inflatable-pontoons--1004413   Fully Amphibious would be replacing the normal landing gear with Floats. https://en.wikipedia.org/wiki/Floatplane These would have a drag penalty in flight, as if carrying bombs.

There's rules in Strategic Operations allowing Dropships to partially submerge themselves do that takes care of launching and recovering units under water, page 74. I'd double and triple the time it takes Dropships that were adapted to water landings to be able to raise and lower themselves in water.

As for the damage for taking off from water landings. I would think that being made, or adapted, to land in water would negate that penalty.

[Cannonshop]

David Drake's RCN series used water landings by preference, and might provide some inspiration as to how to make that happen (outriggers FTW!)…  8)

that's one of the series that inspired this, the other being the shuttles from Niven and Pournelle's "Legacy of Heorot" series-those were effectively early dropships that ONLY landed on water bodies.

[idea weenie]

I'd expect that the engines would use pressurized hydrogen gas to clear the water out of the engine bells, followed by slow and steady ramp up in power to you don't shock the engine from dealing with relatively incompressible water in the exhaust stream (air is much more compressible than water).

The real fun is the onboard quartermaster, who has to make sure that the heavy stuff is located below the waterline, or as close to the center of the hull as possible.  You would also have issues with any shifting cargo, as once some of it starts to shift, the ship tilts slightly, causing more cargo to shift, causing more tilting, aso.  You can handle this with more small cargo holds instead of a few large ones.  You would also have more small fluid tanks, so the ship's engineer can try and shift fluids around to keep the center of mass steady.  Cargo transfer will be interesting.

You might want to go with a dedicated crawler (like the Tactical Operations mobile structure developed to move a Dropship) to take the Dropship out of the water and onto a dry surface.  The sold surface makes cargo transferal easier, and the crawler can help tell which side of the Dropship has too much weight on it.  When it is time for the Dropship to launch, the crawler carries it back to the water, a tug pushes it farther away, and the Dropship takes off.

[The_Caveman]

The real fun is the onboard quartermaster, who has to make sure that the heavy stuff is located below the waterline, or as close to the center of the hull as possible.  You would also have issues with any shifting cargo, as once some of it starts to shift, the ship tilts slightly, causing more cargo to shift, causing more tilting, aso.  You can handle this with more small cargo holds instead of a few large ones.  You would also have more small fluid tanks, so the ship's engineer can try and shift fluids around to keep the center of mass steady.  Cargo transfer will be interesting.

This is true even of spheroid DropShips that are land-based. Air is even less forgiving of attitude displacements than water, and while there are multiple thrusters that should be able to compensate, once an unstable ship starts rolling it's going to be hard to restore. If you have thousands of tons of cargo break free and start sliding around the deck, that is a LOT of momentum to arrest.

Being water-based just makes the loading and unloading more interesting.

[Alsadius]

It seems as though I may have been unfair to the idea of water-launched rockets - they've been designed IRL(though never built). https://en.wikipedia.org/wiki/Sea_Dragon_(rocket)

Basically, they planned to use some nitrogen to purge water from the rocket engine before ignition, and ballast tanks to ensure it was vertical. It'd be less efficient than a traditional land-fired rocket, but not so badly so that it couldn't be over-engineered to compensate(or so the theory went). And of course, efficiency isn't an issue with BT DropShip engines, so that problem would disappear.

So yeah, perhaps I was overly skeptical.

[Easy]

cleanup

[grimlock1]

TBH, water is probably a better environment for landing a DropShip than the ground is. It's a bit more forgiving of small speed and location differences, and the ship is already designed to be watertight, since it needs to be airtight for vacuum travel. Mass distribution won't be a huge problem either, since the ship is already designed to have all of its weight "supported by" its engine while it's thrusting.

Ships sealed against vacuum are designed to keep air in.  That means their seals all face outwards.  The pressure of the air in the cabin pushes the hatch outward, against its frame, maintaining a seal.

Watertight seals are the opposite. Pressure of the water pushing inward against the frame is what gives you a seal.

Also, just because the habitable space is airtight, doesn't mean the rest of the ship isn't exposed to vacuum.  All the volume around the engines, near weapons and sensors, etc.  That could be a lot of volume that's not contributing to buoyancy.

An aerodyne will have a minimum landing speed, perhaps, if it can't land on its tail. The materials of the inflatable floatation component, like a floatplane, will need to resist tearing and deflation.

Interior cells inside the floats may help to minimize the risk of flooding a float. This brings up the question of what happens if a floating spheroid has its floats critted out. How fast does it sink like a bathysphere? How far down could a Dropship like a Union go before it reached a crush depth?

For that matter, what is the crush depth of a Union-class Dropship?

Cellular construction is a must, otherwise this thing would be a deathtrap!
I doubt a dropship could go very deep before Bad Things^tm happen.  Space ships are designed for a pressure differential of 14.7 psi.  Atmospheric pressure versus vacuum. You only need to go down 33 feet for water pressure to equal what we are feeling from 300 miles of atmosphere. Sure, dropships are armored, but armor and pressure hull are two different animals.

If a deck that is normally above the waterline goes below Depth 1, I would consider that deck and everything below it a mission kill. Salvageable, but out of the fight.  Even something that goes to the bottom could be recovered because it will take on water and flood, preventing an implosion.

Does TO/SO have a rule for how hard a sinking ship hits the bottom?  For context, Titanic was moving at several tens of MPH when she hit the bottom. 
Union dropship performs a 4MP charge against the bedrock.  Automatic hit.  Roll for damage.

[Easy]

cleanup

[Cannonshop]

The viability of inflatable floatation components seems like a materials issue. It has to be flexible enough to collapse against the main hull at or near the landed, settled waterline.

If it's circular, for a spheroid Dropship, it might need to have gates and/or ramps, so hovercraft and wet navy vehicles can deploy from hangars out to open water.

It also has to be tear resistant enough to take the hit if they are deployed for the landing touchdown.

For a spheroid, it would need to be able to take the full weight of the Dropship as it settles and shuts off the torches. Water will backfill in against the steam and vapor cloud. I imagine the spheroid will bob in the water somewhat due to this.

An aerodyne will have a minimum landing speed, perhaps, if it can't land on its tail.

Interior cells inside the floats may help to minimize the risk of flooding a float. This brings up the question of what happens if a floating spheroid has its floats critted out. How fast does it sink like a bathysphere? What is the crush depth of a Union-class Dropship?

Non-inflatable floats, seaplane-style, will still need those upper limits on landing speed. Maybe a spheroid could hit the water and even completely submerge before bobbing back up to the surface and deploying floats like outriggers. An aerodyne could perhaps just dive right into the water like a seabird and plane back up to the surface and deploy floats, again, like outriggers.

The design spec here seems to call for a non-submersible that can deploy submersibles. Do temporary landing washovers break that requirement? It might bear on the question of equipment weight if the Dropship will not float on the surface of its own buoyancy like a battleship. If it will, it might require floodable compartments on the bottom in order to get it better settled in the water. Float components could be designed to compensate, in order to work in a complimentary way with ballast compartments. If it will not, then how is it floating? What would it take to sink it? Only a section of SI, or would there be a specific crit location?

Ballast would definitely be a crit location on a 'water lander', especially if it's spheroid, but also for Aerodynes (you don't want to flip over on landing, right? or, say, while you're being towed to the dock/taxiing.)

I think you'd also need a higher SI than ground-based/ground landing ships and hatches/doors on the exterior would DEFINITELY cost more in mass than they do for standard droppers (because of exterior pressure pushing in, as you noted already).

say...50% higher mass cost for those?

suggests there's also a 'maximum' that's lower for armoring.  Why? because of bouyancy requirements (Surface area vs. mass)  Armor is heavy for the surface area, even magitech Battletech armor, so maybe lowering the % of mass you can use for Armoring, while increasing the proportion of SI (minimum SI per ton, maximum armor per ton).

basically it would end up requiring an SI of around 50% higher, and have an 'armor cap' that works out to a lower theoretical maximum.  (reflecting pressure doors, compartmentalization, and the need to maintain bouyancy and float.  keeping in mind also that these would be 'balancing features' since the game's CR already results in soap-bubble levels of bouyancy, while the fluff insists these are heavier than water, never mind that the rules tend to result in things like the Dictator's mass/volume being lighter than air...)

[The_Caveman]

Ballast would definitely be a crit location on a 'water lander', especially if it's spheroid, but also for Aerodynes (you don't want to flip over on landing, right? or, say, while you're being towed to the dock/taxiing.)

I think you'd also need a higher SI than ground-based/ground landing ships and hatches/doors on the exterior would DEFINITELY cost more in mass than they do for standard droppers (because of exterior pressure pushing in, as you noted already).

say...50% higher mass cost for those?

Doors don't weigh anything. Most DropShips already have way, way more SI than they need so existing designs are good here.

suggests there's also a 'maximum' that's lower for armoring.  Why? because of bouyancy requirements (Surface area vs. mass)  Armor is heavy for the surface area, even magitech Battletech armor, so maybe lowering the % of mass you can use for Armoring, while increasing the proportion of SI (minimum SI per ton, maximum armor per ton).

No, this just doesn't make any sense. Buoyancy has nothing to do with surface area, it's computed from volume. And EVERY BT spacecraft, regardless of the amount of armor it packs, has more than enough volume to float.  Stop trying to sabotage the designs' efficiency because reasons.

I don't see any reason whatsoever for these ships to have to be "balanced" by making them suck. They're going to be rare anyway just because a seaport isn't the best place for a spacefaring society to conduct trade and even if all your cities are close to the water you can still build ports for land-based DropShips--and a seagoing DropShip is pointless for any planet that doesn't have oceans or big lakes.

Give them a 5% of overall tonnage allotment for waterproofing and leave it at that.

[grimlock1]

Suppose that while floating, the Dropship requires a minimum of Depth 2, or even 3. In Depth 1, or 2, water, the ship is incapable of doing anything under the waterline and requires tugboats. Alternately, it couldn't enter Depth 1 water at all. In Depth 3 water, it can maneuver on its own. The Dropship would require some kind of propulsion to use while floating, even if it's only capable of station-keeping. Would the Dropship require a strong anchor? It seems like it would.

Time to consult the large naval vessel construction rules.  How much engine does it take to move 3,600 tons of Union around?

Heavy seas might put decks on the waterline awash, but won't necessarily flood the Dropship. This might put an upper limit on just how heavy those seas could be before above-the-waterline decks would flood internally. Including pressure seals on hatches above the waterline would be necessary.

  I'm not too worried about wave action flooding upper decks.  Sealing that kind of water out is relatively easy. You could chain the average car to the deck of a crab boat from Deadliest Catch and the interior would stay pretty dang dry, assuming you remembered to roll the windows up.

Depth 1 is not very deep, so properly constructed pressure hatches would seem to be the norm for the whole ship. If the landed Dropship has tall mast-like structures, these might be vulnerable to heavy seas, heavy winds and damage from submersion, and might suggest that a floating aerodyne have a deck more like a carrier, with one or two bigger 'bumps' for elevators and vehicle deployment. There might be 'pads' on the surface for landing and deploying WiGE and VTOLs. Maybe even a runway for fighters or Small Craft, although a Dropship designer might run out of space for a 'do-everything' model. The ability to deploy large wet naval vessels could fill many of those roles, as VTOLs and WiGEs could be deployed and serviced on them, instead of having to land directly on the Dropship.

Depth 1 is 6 meters, or about 19 feet.  That's 8.5 psi.  That's helpful for flotation but for things like hatches and seals, its already getting up there.  8.5 psi is 1,224 lbs per square foot.  A 4'x4' hatch is 16 square feet. That's 19,580 lbs of water trying to push your hatch in. That's like have a Minotaur standing on the hood of your car.

A spheroid's flight deck might be near the top and require very low stall speeds in order for a fighter to trap into double-ended hangar decks. In space, there is no stall speed, so it isn't an issue. An aerodyne with a flat top might be much better for recovering ASF, as the landing roll would be longer. Some sort of capture system could give ASF with landing rolls longer than the length of the hull a safe way to land.

Uh, how do spheroid or aerodynes for that matter recover fighters normally?

[Elmoth]

Uh, how do spheroid or aerodynes for that matter recover fighters normally?

A question I have asked myself a lot of times. The Seeker has this feature that it can recover fighters, but only if they match the speed and bearing of the dropship http://www.sarna.net/wiki/Seeker) so I would assume that they have some kind of tractor beam or crane system that works along with the VSTOL properties of ASF. . And that same VSTOL status allows the ASF to take off and land in less than 25 metres (the width of a Leopard being 51 metres).

[Daryk]

There are no tractor beams in BattleTech...

[RifleMech]

Strategic Operations says that dropships in water can't move. Although I do believe it can move with the currents.

When in water they sit at a depth 1. When flooded they can go down to a depth 4.

A Depth is 3 meters. A mech is only partially submerged in depth 1 and fully submerged in depth 2.

Rules for Fighter Recovery are in TW page 85. It doesn't really say how they get into the dropship though. I would imagine that they either fly in or some device reaches out and grabs the craft.

[idea weenie]

When in water they sit at a depth 1. When flooded they can go down to a depth 4.

A Depth is 3 meters. A mech is only partially submerged in depth 1 and fully submerged in depth 2.

Rules for Fighter Recovery are in TW page 85. It doesn't really say how they get into the dropship though. I would imagine that they either fly in or some device reaches out and grabs the craft.

I thought Depth 1 was 6 meters?

Still, let's run some numbers about how deep the water would be for a Union Dropship to be buoyant.

Data:
Union Dropship - best approximated by a 80 meter wide sphere, massing 3500-3600 tons
Water = 1 ton per cubic meter
Equation:
Volume of a Sphere cap = 1/3 * pi() * h^2 * (3r-h)
Where h is the height from the outside of the sphere, and r is the radius of the sphere

I cheated, and used a spreadsheet to calculate these values:
3500 tons = 5.4 meters depth before the Union is displacing 3500 tons of water.
3600 tons = 5.479 meters depth before the Union is displacing 3600 tons of water.

So just under 6 meters of water is enough to let a Union Dropship float.  While floating, the top ~74 meters of it are out of water.  The Union will get more movement out of air and sail than with water currents.

[Cannonshop]

I thought Depth 1 was 6 meters?

Still, let's run some numbers about how deep the water would be for a Union Dropship to be buoyant.

Data:
Union Dropship - best approximated by a 80 meter wide sphere, massing 3500-3600 tons
Water = 1 ton per cubic meter
Equation:
Volume of a Sphere cap = 1/3 * pi() * h^2 * (3r-h)
Where h is the height from the outside of the sphere, and r is the radius of the sphere

I cheated, and used a spreadsheet to calculate these values:
3500 tons = 5.4 meters depth before the Union is displacing 3500 tons of water.
3600 tons = 5.479 meters depth before the Union is displacing 3600 tons of water.

So just under 6 meters of water is enough to let a Union Dropship float.  While floating, the top ~74 meters of it are out of water.  The Union will get more movement out of air and sail than with water currents.

weight distribution also plays a role here as to why conventional droppers crash on water-esp. your spheroid models.  basically there's a darn good chance your average Union class is going to end up sideways or inverted before sinking, even on still water.

[RifleMech]

I thought Depth 1 was 6 meters?

Still, let's run some numbers about how deep the water would be for a Union Dropship to be buoyant.

Data:
Union Dropship - best approximated by a 80 meter wide sphere, massing 3500-3600 tons
Water = 1 ton per cubic meter
Equation:
Volume of a Sphere cap = 1/3 * pi() * h^2 * (3r-h)
Where h is the height from the outside of the sphere, and r is the radius of the sphere

I cheated, and used a spreadsheet to calculate these values:
3500 tons = 5.4 meters depth before the Union is displacing 3500 tons of water.
3600 tons = 5.479 meters depth before the Union is displacing 3600 tons of water.

So just under 6 meters of water is enough to let a Union Dropship float.  While floating, the top ~74 meters of it are out of water.  The Union will get more movement out of air and sail than with water currents.

Depth 1 will only cover a mechs legs. So 3 meters.

I imagine the air will push it around.

[Sabelkatten]

Depth 1 will only cover a mechs legs. So 3 meters.

I imagine the air will push it around.

Unless it's inside a building a mech is typically assumed to be 12 meters tall. 3 meter are about up to the knees.

[RifleMech]

Hmm? :-\  I'll go look at TW again but I'm sure that's what it said.  :(