I vaguely remember reading about some supposed advantages of launching very large rockets (Saturn size and larger) from underwater in an ocean or similar large body of water. I was recently reminded of this by a short scene in a popular show (no spoilers) showing a rocket larger than Saturn emerging from the sea in a fiery launch.

What are the advantages of a submerged launch, if any? Has any serious work been done in this area? Or is this idea just imaginative science fiction?

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    $\begingroup$ Interesting that small space satellites have been launched by submarines' ballistic missiles: en.wikipedia.org/wiki/Submarine-launched_satellite. $\endgroup$ – Heopps Jan 15 '20 at 8:21
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    $\begingroup$ 284 pages of Sea Dragon coolness here: neverworld.net/truax/Sea_Dragon_Concept_Volume_1.pdf $\endgroup$ – Organic Marble Jan 15 '20 at 22:19
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    $\begingroup$ @leftaroundabout What naïveté in particular did TRW’s review of the Sea Dragon proposal overlook? $\endgroup$ – Russell Borogove Jan 16 '20 at 17:23
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    $\begingroup$ As I've noted in the answer to a related question, I think "one of the primary advantages of sea based development testing is that it permits early experimental evaluation of combustion stability on a full scale basis without an exorbitant outlay for facilities" translates to "if we're gonna blow one up, let's blow it up in the middle of the ocean". $\endgroup$ – Russell Borogove Jan 17 '20 at 1:31
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    $\begingroup$ What's the name of the show? Use '>!' (a spoiler paragraph) $\endgroup$ – Mazura Jan 18 '20 at 0:36

Sea Dragon

The very large rocket was probably Sea dragon and the advantages were more on allowing a massive vehicle to be built at all rather than inherent advantages in starting underwater.

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(image credits)

Building the launch vehicle on a slip way and floating it to the launch site bypasses a number of size constraints in building and moving large assemblies (possibly mythical example) on land. It also potentially allows optimal launch site selection and simple range clearance. Flooding tanks to get the vehicle upright can be simpler to organise than very large cranes or vertical integration.

Starting a rocket while submerged technically allows the vehicle to start moving upwards from the positive buoyancy, by dumping ballast getting lift for 'free'. On the other hand a vehicle starting on land doesn't need to do anything to reach sea level to start with.

Actually starting the rocket while submerged would technically get increased thrust from the ground effect, but the pressure effects and water hammer potential would be complicated to manage and notably submarine launched ballistic missiles generally cold launch, floating to the surface and only firing once clear.

The downside of submerged launch is that the rocket also needs to be strong enough to withstand positive pressures, wave motion and salt water which look to far outweigh the advantages other than the special case of firing from a submarine. It also by definition is starting at low altitude in thicker atmosphere where some minor advantages can be had launching from high ground.

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    $\begingroup$ Don't forget noise abatement. Much of the initial sound energy will be absorbed by the ocean, and the launch itself will be out at sea far away from land. $\endgroup$ – RonJohn Jan 15 '20 at 21:14
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    $\begingroup$ @RonJohn Sealife might not agree with that point on noise abatement. $\endgroup$ – Basil Bourque Jan 16 '20 at 0:25
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    $\begingroup$ Lake Titicata at 3812 m elevation should make a great launch site. At 15° south it’s also relatively close to the equator. No need to launch in salt water from sea level ;) $\endgroup$ – Michael Jan 16 '20 at 9:46
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    $\begingroup$ @Michael: However, it suffers slightly from the fact that you can't build your rocket in a convenient port and then tow it to Lake Titicata. $\endgroup$ – MSalters Jan 16 '20 at 13:26
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    $\begingroup$ Reminds me of the Futurama episode where the protagonists' spaceship is dragged to the bottom of the ocean. As they descend, someone nervously asks how many atmospheres of pressure the ship is designed to withstand, to which the Professor replies, "It's a space ship, so anywhere between 0 and 1." $\endgroup$ – Nuclear Hoagie Jan 16 '20 at 14:10

You might be thinking of the Sea Dragon project, although this never got past the conceptual / early planning stages.

Some of the advantages of a sea launch are that you can be far away from habitation and the water can provide cooling and acoustic damping during launch.

But the disadvantages are also serious. You are even more at the mercy of the weather than a traditional launch and sea water is both corrosive to rocket engines and contains all manner of organisms that would like to encrust the rocket and turn it into their home.

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    $\begingroup$ Plus of course...for every 30-odd feet down you go the ambient pressure increases by 13PSI / every 10M by approaching 100kPa so the bottom of your rocket has to be built increasingly crush resistant and it's all got to drain any water very fast once it's launched. Any gain from hydrostatic lift would be eaten up by carrying tonnes of water into the atmosphere $\endgroup$ – houninym Jan 17 '20 at 8:50

Early in the development of the Polaris missile system, there was a lot of work on launching a missile from underwater.

Polaris was a nuclear deterrent to rapidly launch multiple missiles from a fully submerged submarine. Staying submerged until a boat-load of launches were complete was a key goal: the boat was to be very difficult to track and destroy even as multiple missiles were rising from it.

Direct launch from the missile tubes was known to be infeasible without destroying the boat due to the heat and pressure of the launch. Unlike travel in air, where the plume can rapidly expand away from the missile and platform into the air, the missile tube and surrounding water would confine the exhaust gasses so that pressure and temperature would rapidly rise toward the rocket engines chamber pressure and temperature.

The project attempted to launch using a much smaller “extractor” motor, but this was also not successful. Designing a tube (to protect the sub) and aft missile end (to protect the missile during launch, which was the point after all) was not compatible with an extractor motor that could lift the missile to the surface.

In the end, they went with an air-powered push to the surface, followed by motor ignition when the missile has cleared the surface.

Early Polaris launch video

There are persistent rumors that certain Russian torpedoes use rocket motors for propulsion. Torpedoes are small, tough, and their weight isn’t so important.

Polaris and it’s follow-ons are ballistic launchers; they don’t go to orbital speeds. It seems hard to believe that a rocket for orbital use could be built tough enough yet light enough to launch from under water.

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    $\begingroup$ Interesting stuff, but the intent of my question is about very large rockets buoyant in open water, not about operating at depth such as torpedoes and submarines. Sorry for the confusion. I edited the title to be more clear. $\endgroup$ – Basil Bourque Jan 16 '20 at 0:18
  • $\begingroup$ I think you’d have the same problems with a rocket ‘buoyant’ to the extent that the nozzles are underwater. (And they don’t float) $\endgroup$ – Bob Jacobsen Jan 16 '20 at 0:50
  • $\begingroup$ No need for a torpedo - while incomparable to a Saturn V, Russia's Bulava missile has been launched from submerged submarines. Although it's debatable why such a thing would be desirable, I'm curious whether a submarine launched rocket could reach orbit. $\endgroup$ – Roundel Jan 16 '20 at 13:11
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    $\begingroup$ @Roundel the Russians have done it en.wikipedia.org/wiki/Tubsat-N $\endgroup$ – llama Jan 16 '20 at 17:19

Launching from international waters.

In addition to the factors mentioned in other posts, there's an additional benefit from launching from the ocean: you can launch from international waters. This could be handy if you're launching a rocket that uses some form of material or process that is illegal or heavily regulated for civilian use in your home country. For instance, you could potentially use the legal shelter of international waters to launch an Orion-drive/nuclear pulse propulsion rocket, since the Nuclear Test Ban Treaty bans its signatory nations from allowing the civilian use of nuclear explosives (though in this specific case, you'd probably want a conventional, chemical first stage to get it free from the water before you activate the main engine).

  • $\begingroup$ Never mind where you launch from or what you launch, once beyond the atmosphere you may not even carry (or detonate!) A-bombs. space.stackexchange.com/a/12961/1235 $\endgroup$ – Camille Goudeseune Jan 18 '20 at 20:23
  • $\begingroup$ @CamilleGoudeseune If you’re launching from international waters, you don’t need to care about treaties like that since they’re binding on countries, not private individuals who haven’t signed them. $\endgroup$ – nick012000 Jan 18 '20 at 20:45
  • $\begingroup$ Huh. I wonder if any individual billionaire or corporation owns enough bombs for this... $\endgroup$ – Camille Goudeseune Jan 19 '20 at 19:53
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    $\begingroup$ @nick012000 These individuals and their companies are still subject to the laws of their countries. $\endgroup$ – Martin Schröder Jan 20 '20 at 21:44

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