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K-35 development program (or, How it All Went Sideways)


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From the GAP (Give Aircraft a Purpose) contract pack:

GAP contract: Speed 100
(or, How it started out so well)




tl6KYEz.png The tricky bit is "maximum airspeed of 100" which actually means "do not exceed 100" before reaching target altitude and levelling off.

GmnrplN.png We also have a contract to test the Wheesley jet engine at 4000m. Which means we need two of them: one to carry the plane up, and a second to activate at target speed and altitude.  Clearly the best tool for this job is SPACEPANTS, and the best way to use them is one leg at a time.  Kick the lower leg to get airborne, then the upper leg for the inflight ignition test.

KPQ02D0.png Time to get a leg up!



KPQ02D0.png Now to put the other foot down!


xvxmnOF.png There’s Mach 2 and some bucking...and here’s a fun bit of tumbling...


sZvDa0M.png Can you bring ‘er back in one piece?

tl6KYEz.png I am on the runway, on the wheels, and stopped.

sZvDa0M.png So you were also able to land it?

tl6KYEz.png I parachuted onto the runway, does that count?

DQ32olJ.png Well done Jeb.  Two contracts in one flight!




Craft: K-35-100 Flounder (KerbalX link)


  • Stage to ignite lower engine, which has plenty of power for takeoff and supercruise, and is probably the safest and most efficient way to fly the plane.
  • Stage again to ignite the upper engine and fly it the Kerbal way.
  • Conventional landings are possible, but dangerous.  Due to the high thrust of the engines, they must be completely throttled down before enough airspeed can be bled off to safely touch the ground.  And then the plane tends to do donuts on its wheels before coming to rest.

Facilities: Level 1

Crew: Pilot Kerman is recommended for SAS.

Mass 7.66t

Cost 12,322.0

No. of Stages 3

Crew Capacity 1

Part Count 16

Built in KSP 1.1.3
Size 8.99 x 3.88 x 9.06


Edit: I really wish there was a way to resize images in this editor.

Edited by Torquimedes
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GAP contract: Build and test a submarine!
(or, How it began to go sideways)





GmnrplN.png We have a contract to build and test a submarine.  My calculations show the easiest way to do this is add full ore tanks to a small, simple craft to make it sink, and empty the ore tanks to return to the surface.


BobDown   What's an ore tank?  I don't think we have those.


tl6KYEz.png Well we have a plane named Flounder.  Let's flip it sideways like a flounder and see if it swims.  


GmnrplN.png I’ll add more SPACEPANTS to accommodate a passenger cabin for tourists. And a probe core for SAS since we have no experience flying underwater.


BobUp    We can use a Galvanic Isolation Liquid Layer (GILL) to separate oxygen from the water for combustion, so the jet engines will work underwater!


GmnrplN.png Now we have a single plane/submarine that can perform underwater missions, shoreline passenger tours, and I'm sure we'll find a way to make it hover later for search & rescue and navbuoy deployment missions.  Everyone knows the best craft is designed by committee to do everything!


JebDown  So.... that's it?  It doesn't look like it will fly anymore.


The K-35-300 PLUBMARANE (don't ask what happened to model 200)

Since it isn't intended to fly, an Engineer should be able to handle it.  Bill insisted he take the first Kerbal sub on its maiden voyage and drove it down to the beach.  He had to skim the surface at moderate speed, pitch down with the diveplanes, and then apply full jet power to overcome the buoyancy of the Flounder without proper ballast.  As long as he stayed level at full throttle, the diveplanes had enough authority to descend slowly.

Bill's view at the maximum tested depth of 437 meters:



He decided it was time to return to the surface, and thought 37 m/s was a perfectly reasonable speed to do so.

tkt5_FpW_GEE_7B2bfHvnxxojYnelSGcm6JBp4uINWzLTg59UBufqZRxVivc6wQlgTCfvKWzP_Hggz2Oc-quiZaEVfMRNJ5qvS9v375eTWHV-xjTTMDLnGHOWgWmmOuXYobqoC04 unnamed.png


Bill had forgotten that PLUBMARANE still contains PLANE.  The Flounder broke the surface and rapidly accelerated into the sky under full jet power.



unnamed2.pngSince it specified that the craft must surface safely, not necessarily return to base in one piece, the contract has been fulfilled.

Bill panicked for a bit, since he was now definitely flying something that wasn't intended to fly and didn't do it well.

Then he remembered the chute leftover from the Flounder's plane days, managed to flounder his way over to KSC, cut the engines and deployed the chute.  One hard bounce later he was safely down.


BobUp I declare this a successful test of the plubmarane's multirole capability and designated Bill its official pilot! unnamed3.png






Craft: The K-35-300 Flounder Plubmarane (KerbalX link)


Submerged Operation:

  1. Throttle to zero, engage SAS
  2. Taxi very carefully down to the beach.
  3. Roll very slowly into the surf before throttling up.
  4. Skim the waves at moderate speed before pitching down.
  5. Try to stay level for maximum pitch authority to remain submerged.

Flight Operation:

  1. Oh boy...
  2. Engage SAS.  Make doublesure SAS is engaged.
  3. Throttle full and stage.
  4. Pull up around 50 m/s, carefully to avoid overstressing the aft gear.
  5. Do not try to turn with rudder controls.  The rudder is nearly useless in the air, but the torque can spin it right round (baby) in flight and then it will fall like a brick.
  6. Do not attempt to land on the wheels, use the chute.



Crew: Any Kerman.  There is a probe core inside the passenger compartment of this model.

Mass 10.85t

Cost 17,021.0

Crew 1

Passengers 2

Part Count 24

Built in KSP 1.1.3
Size 4.99 x 2.69 x 8.0

Edited by Torquimedes
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GAP contract: Barnstorm Jeb's Shed!
(or, How it ONLY goes sideways because it now struggles with up or down)




Jeb We need an aerobatic aircraft I can fly low precise maneuvers with, so I don't splatter into the ground or my own toolshed.

Bill Rather than risk all the unknowns inherent in designing a new aircraft from scratch, clearly the best solution is to add a hover engine to our proven plane/boat/sub.  Surely a submersible hoverplane would be the perfect all-purpose multi-mission committee-approved craft to complete all of our short-range Kerbin-based contracts.  Does short-range imply a range limitation?  We'll address that in the next requirements revision.

The K-35-400 Flounder PLUBMOVERANE

Bill  In this configuration, it can't fly like a plane without falling over and crashing.  It lacks the power to hover stationary like a VTOL.  It can't submerge without filling its SpacePants with OX ballast and careful piloting. But it can lift off the runway at 15m/s, fulfilling Jeb's requirement for a low-speed aircraft.  Wait, did he say low ALTITUDE? Surely speed and altitude are interchangeable. I'm sure this will work just fine. 


Jeb's objective is to fly through that teeny building under the marker.


It's still pretty small when he's 3 seconds away and slewing toward it in a craft that flies like a boat.


Despite all this, Jeb sailed his flying hoverboat through with flying colors.  


After barnstorming the toolshed, Jeb aimed for a slight climb without increasing throttle, and the Flounder instead settled itself into a very gentle landing.  




Craft: K-35-400 Flounder Plubmoverane (KerbalX link)

This is actually the safest VSTOL I've built in KSP, since it can't build up enough speed to break itself as long as you stay mostly level and yaw into prograde when it (often) bounces off the ground.

VSTOL operation:

  1. Engage SAS decisively, and do your best to keep the hover engine pointed down.
  2. Do not hold the brakes and attempt a vertical takeoff.  This model Flounder needs some forward speed to gulp enough air to feed the aft engines, even at their low throttle limit, while the hover engine runs at full throttle.
  3. Do not panic if the aft engines flame out during takeoff, or flight, or landing, or whenever you slow or turn.  They provide slight forward thrust to give a general sense of direction to your endeavor, and put some airflow over your fins.
  4. Due to the ungainly posture of the landing gear to clear the hover engine, this Flounder is likely to tip over during a conventional landing.  The parachute is recommended.

Maritime operation:

  1. In the hangar, set the thrust limiter of the hover engine to 0 and the aft engines to 100.  Fill the oxidizer tanks as ballast.
  2. The Flounder can fly in this confiiguration, but you really don’t want to.
  3. Carefully taxi down to the water and very slowly roll in.
  4. Scoot across the surface to at least 20 m/s.
  5. Gently descend and stay as level as possible to maintain pitch control.

Crew: Pilot Kerman.

Mass 10.15t

Cost 18,374.0

Crew 1

Passengers 2

Part Count 27

Built in KSP 1.1.3

Size 4.99 x 2.79 x 7.96

Edited by Torquimedes
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GAP contract: Deploy navigation buoys
(or, When bits started falling off)






GNB5STv.png I didn't see the buoy subassembly mentioned, so I built my own and mounted two on the K-35 Flounder's new ordnance racks.


K-35-500 Flounder Plubmoverane with navbouys

GNB5STv.png In order to accommodate a pilot and three passengers for the current mission requirements, I replaced the cockpit with another cabin and installed a probe core.

BobDown In testing, it couldn't taxi off the runway to the water without shearing off its hover engine, and it's now too heavy to splashdown on its parachute. This Flounder can't swim!


GNB5STv.png Minor details to be revisited in the next round of revised requirements revisions.

b44cB77.png We don’t have a pilot. Jeb is stuck in orbit and Val is suspended from flight status, recovering from the... Mun clone... incident.


unnamed.png My suspension only applies to cockpits. I can fly the probe core with my kPad sliding around on this table.  While facing backwards!

This could explain why everyone else is screaming.



Buoys were dropped at each navpoint, and transmitted SCIENCE afterward.


Val kPadded the Flounder and crew safely back to KSC.




Craft: K-35-500 Flounder Plubmoverane (KerbalX link)


(Not rated for maritime operation.)
Flight Operation:

  1. Engage SAS.
  2. Throttle full and stage.
  3. The K-35-500 should float off the runway about 50m/s.  Max level speed is 160m/s.
  4. Gentle turns with the rudder are recommended.  This model has a more effective rudder.
  5. Never forget that the hover engine is your lift, so don’t bank too far.
  6. Stage to drop a “nav bouy” that can transmit barometer readings. Stage again to drop the second bouy.
  7. To land, on final approach throttle down to 1/3 to reduce the hover lift, and hope SAS can overpower its tendency to tip over after touching down.  Or just use the chute.

Mass 10.62t

Cost 27,629.0

Crew: Any Kerman.  There is a probe core inside the passenger compartment of this model.

Passengers 3

Part Count 41

Built in KSP 1.1.3

Size 5.08 x 2.72 x 7.96

Edited by Torquimedes
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GAP contract: Give an airplane ride
(or, The prototype we actually used more than once)





Val has been cleared for full flight status, so she fired up the K-35-400 Flounder and loaded two passengers for a tour of the island airfield.  Rather than risk landing on the old gravel runway, she popped the chute at full throttle to parasail the Flounder on top of a hangar.





The tourists settled down once they saw the great view out their windows from this vantage point.






CraftAforementioned K-35-400 Flounder Plubmoverane (KerbalX link)

Edited by Torquimedes
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GAP contract: Build a seaplane
GAP contract: Submarine tour
(or, It now does everything! (Just barely.) )






Brochure photo of the wreck of the Kiomi:


BobUp Two contracts for very different missions, seaplane and submarine.  Clearly the best solution is a single multimission hypercompromise craft that has already been modified far past its original design specs.  

BillUp Speaking of modifying specs, the missions call for a Pilot, an Engineer, and 6 passengers.  So let's slap a couple more crew cabins into the engine section, and an airscoop to feed the hover engine at full capacity.  Then we can reverse the hover engine to pull us down into the water and make submarining much easier!  



The K-35-613 Flounder Plubmoverane

BobDownYou do know the “S” in VSTOL means short?  The Flounder can now takeoff from a runway, land on the water, submerge and surface, takeoff from the water and land on its wheels again.  But, none of those operations are short anymore.  It takes the full runway to takeoff or land.  And it doesn’t operate vertically anymore, even falling on its parachute, because the chute wasn’t upgraded to compensate for all the added mass!

JebDownPassWell we still have three letters left, so let’s use ‘em before they get away from us.

BillDown  As it turns out, reversing the hover engine did not make it easier to stay underwater, it just robbed power from the rear engines which normally drive the craft forward (and down).  Features of the new design include an increased tendency to yaw, which is difficult to counter now that the rudder is closer to the center of mass.  The increased mass makes the torque wheels less effective at turning or stabilizing the craft underwater, and the lack of batteries means power can't be stored for when torque is needed most.  As a result, underwater operation sometimes devolves into tumbling and bobbing to the surface like a cork.  And sometimes leaping into the air like a flying fish, except flying fish don't explode on returning to the water.

JebUp  So there’s no margin of error, but it can work.  If everything goes perfectly.

Jeb and Bill loaded the six tourists aboard and took off from the runway.  The Flounder is now so heavy that it stalled and bounced a couple times before finally floundering into the air.

Jeb performed a "seaplane landing" by stalling the Flounder just over the water, and hoping the impact wouldn't be fatal.



Bill took over the controls for the submarine portion of the trip and headed down.


Approaching the Kiomi:


View of the Kiomi from a passenger window:


Bill resurfaced without damage.


Jeb took the controls to return to KSC and took off from the water.  Since this craft is not rated for a runway landing he approached KSC and planned to use the chute.  


Bill remembered that he forgot to remember to adjust the chute for the added mass, and advised Jeb to go back out and land on the water.


Bill then drove the plubmoverane up the beach and onto the KSC runway to complete both contracted missions.


Six more satisfied customers, and two more contracts complete!


CraftK-35-613 Flounder Plubmoverane (KerbalX link)

Crew: Any Kerman.

Mass 14.38t

Cost 18,794.0

Crew 1

Passengers 7

Part Count 32

Built in KSP 1.1.3

Size 6.23 x 3.05 x 9.83

Flight Operation:

  1. Engage SAS, throttle full, and stage to begin rolling.
  2. At about 60 knots, try to raise the nose, and if you manage to get all the wheels off the gound, immediately level off and try to slowly climb away from the ground.
  3. DON’T PANIC if you fail to miss the ground, you may bounce a few times before you run out of runway and it becomes easier to miss.
  4. Turning with the rudder is recommended, since this model needs ALL its hover thrust to keep from falling out of the air.  Try to avoid banking.
  5. Conventional landings are preferred, since the chute is inadequate to fully cushion the landing.  Throttle back to 2/3 and adjust speed with pitch until you touch down, then apply brakes.  This model has less tendency to tip over while landing.

Maritime Operation:

  1. Set thrust limiter on hover engine to 0.  Set thrust limiters on aft engines to 100.
  2. Taxi... carefully... down into the water.
  3. Throttle up and skim across the surface to about 30m/s then pitch down to submerge.
Edited by Torquimedes
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