Seaplanes: Searching for Consensus

[Geschosskopf]

I've been dinking around with seaplanes ever since 1.05 came out and have begun to form opinions of what works and what doesn't when it comes to general design characteristics.  I'm wondering if my thoughts are generally in line with those of others so here goes....

Part Breakage:  What causes part destruction in the water is a huge mystery to me, and this unknown lurks under all my observations, casting doubt on my conclusions.  Horizontal speed seems irrelevant.  I've had boats reach 250m/s without damage but on various runs, sometimes parts (even those apparently above water) die at lower speeds.  I'd really like to know what it is that destroys parts in the water these days.

Water Takeoffs:  Provided your CoM is slightly aft of the center of the parts that are touching the water, taking off from the water will eventually happen or become possible with elevator input once you build up sufficient speed for your wings to lift you.  If your CoM is forward of the longitudinal center of the wet parts, you probably won't achieve flying speed.  Achieving high speed on the water seems to depend on natural planing, the whole plane lifting slightly and the nose lifting more than the tail, thereby reducing the surface area in contact with the water, and this only happens if the CoM is far enough aft.  While up pitch can force the nose up into a planing attitude if it won't come up by itself, this almost always results in a decrease in speed due to deeper immersion of the tail plus the drag of the deflected control surfaces.  But all in all, water takeoffs are much easier than water landings.

Water Landings:  At any reasonably SANE landing speed (say less than 100m/s), part breakage is rarely if ever an issue.  What IS an issue is fliipping over.  This is apparently caused vastly increase drag happening at the tail upon contact with the water, with the plane's forward momentum causing the nose to drop sharply.  The further below the plane's CoM this point of 1st contact is, the more leverage the sudden drag has to flip you.  So you want to land as slowly as possible to minimize your momentum, and you don't want too much vertical space between the part that hits the water first and the CoM.  Also, once the nose hits the water, its buoyancy acts against the nose-down moment, and the longer the nose is, and the wider it is, the more buoyancy and leverage it has to keep you from flipping.  But even designing the wet parts to conteract the flipping, I still find about 50m/s to be the safest possible landing speed---slower is better.

General Design:  There are 2 main options here:  A realtively conventional plane design with pontoons of some sort stuck on below, or a flying boat where the main fuselage itself is in the water and small pontoons under the wingtips.  Taking the above points into consideration, I see some general guidelines for each type.

• Pontoons:  The design of the plane itself is pretty much the same as normal, which simplifies balancing lift, CoM, and thrust.  However, if the pontoons are too far below the actual plane, they cause severe flipping tendencies on landing, to reduce which they should extend out ahead of the main fuselage nose.  The pontoons also need to be relatively large diameter compared to the fuselage to provide the necessary buoyancy to float at all, and also for the all-important counter to the flipping tendency.
• Flying Boat:  You need to keep as many parts out of the water as possible to minimize water drag, so you can gain take-off speed.  This typically means a shoulder wing configuration, intakes on top of the fuselage, etc.  All this complicates proper balancing of CoM, CoL, drag, and thrust, which can cause problems in flight both in the air and in space (if this is an SSTO).  Also, the main fuselage should be pretty long (as in zero or few radial fuel tanks), so the buoyancy of the nose has the most leverage to prevent flipping on landing.  Mk2 fueselage makes the best flying boat IMHO due to its width and built-in lift, both of which tend to prevent flips on landng.

Given the need for some sort of pontoons, either to hold up the whole plane or just the wingtips, all seaplanes will have more aerodynamic drag than landplanes so will need more thrust to do the same jobs.  They also have more mass so need more wing to stay in the air at the same speeds, so need more thrust for that to reach the same speed as well.  And they also need enough wing and/or flaps to land and take off at 50m/s or less, which again means more thrust.  Acceleration during water takeoffs is slow, which means a longer, less efficient burn just to get airborne, so more fuel is needed, which means more mass, which means more wing, which means yet more thrust.  So any sort of seaplane will therefore be bigger and more expensive than a landplane to do the same flight, and somewhat more difficult to make SSTO.

Comparing pontoon planes and flying boats, it seems to me that pontoon planes have less drag in the water (due to less wet surface area) so are easier to get airborne (shorter takeoff time and distance with less thrust needed) but more overall mass and more drag in the air due to have essentially 3 fuselages.  Plus they're harder to keep from flipping on landing.  However, they';re easier to design and fly.  The flying boat has fewer extra parts added to the bare necessities it needs to fly so is lighter and has less drag in the air, meaning it's easier to SSTO (at least on paper).  It's also safer to land.  But it can be a bear to get airborne so needs the most thrust of the 2 types, and present some tricky design and possibly flying challenges.  Overall, therefore, there doesn't seem much to choose between the concepts.

What do you all think?

 

[jwbrase]

2 hours ago, Geschosskopf said:

Water Landings:  At any reasonably SANE landing speed (say less than 100m/s), part breakage is rarely if ever an issue.  What IS an issue is fliipping over.  This is apparently caused vastly increase drag happening at the tail upon contact with the water, with the plane's forward momentum causing the nose to drop sharply.  The further below the plane's CoM this point of 1st contact is, the more leverage the sudden drag has to flip you.  So you want to land as slowly as possible to minimize your momentum, and you don't want too much vertical space between the part that hits the water first and the CoM.  Also, once the nose hits the water, its buoyancy acts against the nose-down moment, and the longer the nose is, and the wider it is, the more buoyancy and leverage it has to keep you from flipping.  But even designing the wet parts to conteract the flipping, I still find about 50m/s to be the safest possible landing speed---slower is better.

 

To my understanding, flipping is a very real issue that real seaplanes have to deal with, and for a real life seaplane, 50 m/s is quite a high speed to be contacting the water at.
 

[worir4]

The breaking at low speeds you mention is i believe caused by the vertical speed not horizontal. Parts break at their impact tolerance which can be seen in the SPH/VAB. Some parts have quite low impact tolerances and so break easily compared to others. 

A solution would be to keep those parts from coming into contact with the water and only use high impact tolorance parts for the floats.

[selfish_meme]

I have an SSTO mk3 flying boat that uses hydroplanes.

 

Also a Juno powered mk1 seaplane with short pontoons.

Both have many similarities with your assesment and a couple of differences.

 

Also you forgot high engine placement requires care as the thrust is assymetrical and can push the plane further down in the water.

Edited December 2, 2015 by selfish_meme

[problemecium]

In my experience what kills (and in fact what has always killed) parts hitting water is the vertical speed on contact. Once in the water, parts can zoom around as fast as they want, but during the moment they hit it, if their speed perpendicular to the surface of the water (i.e. vertical) is above their collision tolerances, they'll explode.

[steve_v]

Having mucked about with the seaplane thing a little the last hour or so, I can say that the whole buoyancy thing seems rather screwey to me... I'm getting phantom rotation forces in the water, and if anything other than outboard pontoons touches the surface it's like a brick wall - insane deceleration.

It also seems to me that there's some witchcraft going on with water drag vs. portion of float part submerged... even a fraction more than 50% under the surface and boom - brick wall effect. I'm guessing it's this + a little bobbing that's causing my weird spinning while taxiing issues.

Anyone got designs that are consistently landable?

[selfish_meme]

Yeah, this made Spacecraft Fridays from Azimech so it should be pretty good http://kerbalx.com/SelfishMeme/S-EX-1

[steve_v]

Heh, looks like an alarmingly similar layout to my current test vehicle, having abandoned the traditional seaplane for a more flying-boat setup.

Having flown yours (v nice BTW, though a tad unstable at low speed, at least in FAR), and insta rolled it while moving at ~20m/s, I'm even more convinced there's something non-linear about the buoyancy calculation - I recon there's a step change in drag with relation to submerged volume in there somewhere. This "water" still doesn't behave very much like water.

Anyone know how the code actually works?

Edited December 2, 2015 by steve_v

[selfish_meme]

Was not set up for FAR, and I didn't claim it was, it works very well in stock. I do not experience low speed instability, but it has a hidden wing, which FAR probably ignores, in FAR it might need more lift area.

[steve_v]

No offence intended, I kinda assumed it was a stock design. It flies surprisingly well in FAR, considering it wasn't designed for it.

[Mikki]

1 hour ago, parameciumkid said:

In my experience what kills (and in fact what has always killed) parts hitting water is the vertical speed on contact. Once in the water, parts can zoom around as fast as they want, but during the moment they hit it, if their speed perpendicular to the surface of the water (i.e. vertical) is above their collision tolerances, they'll explode.

Same opinion here, had not much time to many experiment fzrther, but it is mostlikely exceeding vertical speed crash tolerance of ANY part of the vessel...

Good luck watering!

[selfish_meme]

1 hour ago, steve_v said:

No offence intended, I kinda assumed it was a stock design. It flies surprisingly well in FAR, considering it wasn't designed for it.

Nah not offended just didn't want you to mistake it for something it wasn't.

[NathanKell]

A part breaks when:

1. It is transiting from no water contact at all to some water contact, and

2. speed * Max(dot product of velocity * down axis, 0.1) > crash tolerance * 1.2

 

Note that both constants are configurable in Physics.cfg.

 

tl;dr max impact speed varies between 1.2 x crash tolerance and 12 x crash tolerance, depending on angle.

[kiwi1960]

I have built sea planes.... I have built submarines...

 

I now want to combine the two... its been done before....

[regex]

16 hours ago, Geschosskopf said:

If your CoM is forward of the longitudinal center of the wet parts, you probably won't achieve flying speed.

Use wing and engine AOA to overcome sloppy COM and COBouyancy issues (the plane below tilts forward in the water but lifts off at @60m/s).

Quote

Given the need for some sort of pontoons, either to hold up the whole plane or just the wingtips, all seaplanes will have more aerodynamic drag than landplanes so will need more thrust to do the same jobs.

Pontoons aren't entirely needed because of the stupidly idiotically OP reaction wheels.  The plane below can easily go without.  I've since revised it to have over-wing intakes because I simply can't stand KSP not modelling water intake but I have no updated screenshots because I haven't touched KSP in weeks.

Edited December 2, 2015 by regex

[Geschosskopf]

3 hours ago, NathanKell said:

A part breaks when:

1. It is transiting from no water contact at all to some water contact, and

2. speed * Max(dot product of velocity * down axis, 0.1) > crash tolerance * 1.2

Note that both constants are configurable in Physics.cfg.

tl;dr max impact speed varies between 1.2 x crash tolerance and 12 x crash tolerance, depending on angle.

Yeah, I only just noticed the other day that the impact tolerance of parts is NOT in m/s as I'd thought all along but in kN/s (IIRC, it's kN something anyway).  So it's hard to relate that to any specific speed just by looking at it.  And this brings up my problem with it figuring out how it works.  On my speedboats, parts break when the boat has no perceptible downwards motion at all.  It's going along at a given speed, holding a certain attitude rock solid thanks to SAS, and a part might just vanish for no apparent reason.

And BTW, it would be nice if just going along in the water created wake fx, and if parts destroyed by water exploded with visual and audible fx :).

2 hours ago, regex said:

Use wing and engine AOA to overcome sloppy COM and COBouyancy issues (the plane below tilts forward in the water but lifts off at @60m/s).

Pontoons aren't entirely needed because of the stupidly idiotically OP reaction wheels.  The plane below can easily go without.  I've since revised it to have over-wing intakes because I simply can't stand KSP not modelling water intake but I have no updated screenshots because I haven't touched KSP in weeks.

Wing and engine AoA work fine for subsonic airplanes but become quite problematic for SSSTOs. (single stage seaplanes to orbit).

I wouldn't do without at least minimal wingtip pontoons on a flying boat.  You don't want to drag a wingtip accidentially when you 1st touch down due to having a slight amount of bank on.  Either the wing might break or you might cartwheel the whole plane.  OTOH, a small wingtip pontoon might break off saving the wing and you can continue the mission.  So I look at them as a form of insurance.

[NathanKell]

It's in m/s when testing ground and water collisions. The formula is exactly as I stated, since I wrote it.

 

And yes I agree that wakes would be good, and better fx.

Edited December 3, 2015 by NathanKell

[Mister Dilsby]

Well, I don't know about max speed and collision physics as I'm keeping everything under 70 m/s--but I did just seaplane to orbit and back. Pontoon design, Mk2 parts, and now fully qualified as my Laythe lander

[arsenal3185]

On 12/2/2015, 12:10:21, Geschosskopf said:

Part Breakage:  What causes part destruction in the water is a huge mystery to me, and this unknown lurks under all my observations, casting doubt on my conclusions.  Horizontal speed seems irrelevant.  I've had boats reach 250m/s without damage but on various runs, sometimes parts (even those apparently above water) die at lower speeds.  I'd really like to know what it is that destroys parts in the water these days.

 

It's the true Kraken. The space Kraken was just a wanna be.

On 12/2/2015, 4:31:35, kiwi1960 said:

I have built sea planes.... I have built submarines...

 

I now want to combine the two... its been done before....

Those kooky Japanese! (Is that an I-400?)

[Geschosskopf]

2 hours ago, arsenal3185 said:

Those kooky Japanese! (Is that an I-400?)

Actually, several nations built at least 1 sub that could carry aircraft internally and others carried aircraft in external pods on normal subs.  For most, this was for the same reasons that they all put aircraft on their cruisers, to provide long-range recon to the ship in the days before radar, to help it hunt for prey in commerce warfare.  The Japanese different mainly in doctrine, viewing their submarines as scouts and attrition weapons of their main battlefleet, instead of lone raiders.

[KerikBalm]

Making a seaplane into a submarine is easy

This is a sub:

(I've modified it a little since then, to have the hydroplanes lower on the pontoons).

That was a bit hard to takeoff, but I was able to make an action group to deflect hydroplanes and the canard to get it to lift off of the water.

If I inverted the deployment of the hydroplanes, it would sink as I went faster, and once the main wing went under the water...

it became a dynamic diving submersible

 

Also, to avoid the problem of flippage, the action group for the canard and hydroplanes helped (the forward hydroplane would have a more positive AoA to help it from going under, and overall it acted like flaps), and that was coupled with a pair of braking chutes that I feply at 10 meters above the water.

Then of course, if I do manage to flip it, I can dive under water, roll, and surface again

Edited December 4, 2015 by KerikBalm

[Fendleton]

6 hours ago, arsenal3185 said:

On 12/3/2015, 11:31:35, kiwi1960 said:

I have built sea planes.... I have built submarines...

 

I now want to combine the two... its been done before....

Those kooky Japanese! (Is that an I-400?)

Actually, that's HMS M2. First ever submarine aircraft carrier and a pretty good example of why the idea never caught on. Designed as a K-class steam powered submarine (yeah, that went well. The K-class were insane deathtraps even by the standard of World War One submarines. And that's saying something.) but converted to a regular diesel-electric design.....with a 12-inch gun mounted in the front of the conning tower. The idea was that because torpedoes were terrible back then you'd surface off the side of a merchant ship and blast it with the massive gun the instant you surfaced. This had some rather massive issues as a concept, but they built three of the darned things anyway.

After realising how bad an idea this was (also after M1 sank because a merchant ship accidentally rammed it....ripped the big gun off, and the sub flooded through the loading system), M2 got converted in 1925 or so into what it looked like in that picture....the gun got removed and the space where it had been fitted became a hanger. Trouble is, having a really big door leading into your submarine (which from that photo looks like it might even open inwards?) is a really bad idea. Especially as, since it was built in the same way M1 was, that hanger bay wasn't that well sealed off from the rest of the submarine. One day the hanger flooded during a launch and the sub just went straight down. As you'd imagine, the Royal Navy didn't really pursue the idea after that.

[/end history lesson]

 

In KSP however we don't have pesky things like water pressure, tensile and compression strain on materials, or having to worry about the wellbeing of our crews at all. So a submarine aircraft carrier should be perfectly viable ingame. Though it might be easier to make one that launches and recovers aircraft from a proper flightdeck rather than using seaplanes.

 

------

 

In terms of actual seaplanes, I tend to build flying-boats. Much easier to build and control, and I'm used to high-wing designs anyway since that's what I normally make anyway. Really big planes also tend to flip a lot less. I modified the Mallard that comes as a stock plane recently....buffed it up from the weedy stock engine setup to having a 4XPanther setup in the wing roots (also more fuel, obviously). Using my standard test for long-range stable flying (turning on SAS, taking off with full afterburner, then having the crew bail out so I can fly it until the engines run out of fuel using only the flap-deployment menus that don't shut down in an uncrewed vessel) I made it almost halfway around the planet *on afterburner*. And then landed it in the dark using only flap deployments. So you can get pretty good range out of a big flying boat even if you do something really stupid with it.

Edited December 4, 2015 by Fendleton
Must not get dates wrong on unimportant history lesson

[arsenal3185]

On 12/3/2015, 11:58:53, Geschosskopf said:

Actually, several nations built at least 1 sub that could carry aircraft internally and others carried aircraft in external pods on normal subs.  For most, this was for the same reasons that they all put aircraft on their cruisers, to provide long-range recon to the ship in the days before radar, to help it hunt for prey in commerce warfare.  The Japanese different mainly in doctrine, viewing their submarines as scouts and attrition weapons of their main battlefleet, instead of lone raiders.

But is it an I-400?

[Geschosskopf]

On ‎12‎/‎5‎/‎2015‎ ‎5‎:‎32‎:‎02‎, arsenal3185 said:

But is it an I-400?

No, As @Fendleton noted in the post above, it's HMS M2.  If you're into weird submarines, you should follow up by reading about the K class he mentioned, probably the most notorious things even put in the water.  There are a number of books devoted to them and the horrors they inflicted on poor matelots.  Here's an example:

https://www.goodreads.com/book/show/6308933-k-boats

 

 

 

[bewing]

If you build a seaplane with submarine capabilities and a small positive buoyancy, then takeoff is not an issue. You just dive, then bring the nose up -- and let buoyancy do the rest. This allows you to avoid all the issues with having parts be off the centerline, or of needing a long takeoff run.

And there are definitely still bugs in the water crash calculations. The worst one being that when a kerbal boards a floating module, there are random sudden huge forces that can instantly destroy a craft and literally send parts 1000 meters into the air (probably an uninitialized variable bug). Beyond that, I have seen craft take water damage and be destroyed on leaving the water, into the air. Also, once a craft is in the water, it rotates around a different point than the CoM, but that new rotation point does not seem to be properly taken into account when calculating the impact velocities of the individual parts. I have seen rockets splash down very safely into the water, then the top slowly rotates over, and some fuel tank or science lab touches the water at .05 m/s -- and BOOM! Half the rocket turns to shrapnel.