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Underwater thrust


Stewcumber
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I'm making a submarine to pilot in kerbin's seas. I've tried jet engines coupled to liquid + oxidizer tanks but the jet engines did fire due to a lack of oxygen(?). I've tried liquid fuel rocket engines but they're not efficient and by the time you've got enough fuel you're very buoyant due to the fuel tanks.

I've tried propellors and they don't interact with the water it seems.

I'm currently trying rotors coupled with wings at about a 45 degree angle (like a water based propellor) but they spin very slowly with standard rotors.  So I'm currently trying a heavy rotor with huge torque and blades spinning slowly.

Has anyone got a underwater propulsion working on a stock game?

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Unfortunately Stock is not very accommodating for underwater activity.

If you are able to use a mod then the closest to stock is @ColdJ's Working Underwater Lite mod. It alters the configs of stock parts to make underwater travel more accommodating. Get it from SpaceDock or CKAN

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  • Jet engines work as normal in atmosphere. They need an air intake to work. Yes it's unrealistic.
  • Breaking Ground props were explicitly coded not to work underwater. Yes, I think that's a stupid decision.
  • Normal wing/control surface pieces WILL work as props when attached to Breaking Ground rotors. I's pretty fiddly, so most people avoid it. (I've never done it, and I LIKE fiddling with details.)

Tips:

  • A submarine is just an airplane in a much denser medium, so minimizing drag should be your primary design constraint if you want any speed at all.
  • A jet sub that can do 20m/s seems to be a good average target, although I've gotten up to 40m/s on more optimized (but not outlandish) designs.
  • Very few parts are natively denser than water, so making a sub sink is hard. The most commonly used one is LOTS of full ore tanks. I've ended up building a few subs in the 600-1000 ton range.
  • I've not been able to maintain approximate neutral buoyancy with more than about an hour's worth of fuel. Once that gets burnt, you're headed to the surface without active control.
  • There are design tricks/exploits you can do to have variable buoyancy. The primary one is clipping a lot of the smallest ore containers into a cargo bay. It works in reverse of normal physics, since the mass stays the same, but when you open the bay, suddenly the volume of all those ore containers is in the water and you float.
  • I recommend using the Vessel Mover mod to get to the water from the runway, although trying to get there with wheels is a fun challenge.
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I used some elevons to make underwater propellers. They can function very much like the breaking ground propellers.

Jet engines do work under water, Just make sure to include air intakes. 

Here is a sub that I made recently.

 

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Posted (edited)

Thanks for the answers everyone. I think I reached a similar conclusion to everyone else and had the most success with old-school props using wing parts.

KSP-x64-2022-04-27-23-11-25-968.jpg

KSP-x64-2022-04-27-23-10-27-886.jpg

I steer by adding a hindge to the rear prop so the thrust is angled (like a jetboat I guess) rather than a rudder but that joint in particular is absolute kraken-bait. I'll probably revise it to have a normal rudder and (canards if they're called that underwater) all over it to get it to change direction. Don't mind the delivery system going nuts in the background that's all fine!

Edited by Stewcumber
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