I had some new ideas and thought I would kick around this idea of what I call a pipelauncher. A pipelauncher is a idea of rocket launch platform which floats on the water and gives subsonic acceleration at lift off. It's "a pipe with one end capped" which if partial filled with enough water will float vertically. The capped end is up. A rocket could be put on top of the capped end, and enough water could be let in the pipelauncher so the top is just above the surface, water is then forced out the bottom end of pipe (by increasing the air pressure inside the pipe) which increases the bouyance which causes the pipelauncher to rise vertically.
A pipelauncher could be quite large [a similar structure could used for base of space elevator {really, really big}]. For purpose of discussion I will propose a modest size pipelauncher which could be used by Soyuz type rockets or larger. In order to maintain vertical balance AND to allow enough distance to accelerate at reasonable gees in order get some speed before the rocket blast off, the pipelauncher has to be quite tall.
Distance equals Accelerate halved times Time squared: D=A1/2Tsquared.
Most of the pipelauncher length will be the aceleration length. The acceleration length can be roughly determined by the total weight of pipelauncher and rocket times [1+gees of accelerate]. For example suppose you had a length of pipe 100' long which if capped at both ends would displace 1000 cu meters. And this pipe weighed 100 tonnes and the rocket weight 100 tonnes. A 1000 tonnes of weight would completely submerge the pipe, 200 tonnes causes 1/5 of pipe to be submerged. If rocket is to accelerate by 1 gee then that force is somewhat the same as doubling the total weight [meaning 2/5 of pipe is submerged]. So remove a cap from end of 100' pipe fill it 1/2 full with water. Now it will float vertically and it has 1/2 of the displacement volume [500 cu meters]. Now, since we only need about 200 cu meter of displacement, the air can be let out using a valve, giving you the 200 cu meter air- this would barely float the Pipelauncher and the rocket.
To accelerate the rocket you would need to increase the volume of air inside the pipe [heat the air and it will expand]. If you double the volume of air, you will get around 1 gee of acceleration- if you only increase volume by 150% then you only get .5 gee of acceleration. Now getting back to acceleration length- if you want a constant acceleration of 1 gee with this 100' long pipe then your acceleration length will be 3/5 of the length of the total length of pipe- 60'. For 1/2 gee acceleration your acceleration length would 70'.
So there is some of the basics. But instead of a single pipe, the pipelauncher will be mostly constructed 3' diameter pipe which is welded together to form a larger pipe. I shall make the total length of pipelauncher a even 1000' in total length. There shall be 12 of these 3' diameter pipe and there 3/8" plate steel "spacers" which are 4' wide. This give a rough circmference of: 12x3 + 4x12= 84' and this gives a diameter of 84 divide by 3.141592= 26.738 feet [320.85"].
The 3' steel pipe will have wall thickness of .312". Per 1' length this pipe weights 118.92 lb. And per sq' of 3/8 steel weighs about 15 lb. So per 1' of this constructed pipe you have 12 times 118.92 lb= 1427 lbs. Plus 48 times 15 = 720lbs which is roughly 2150 lbs.
Now, if I recall correctly cu ft of water weighs 62 lbs. If ends were sealed on this 1' section of constructed pipe it displaces...
There is 561.5 cu' in area which is at point in the pipe where it's welded, then have 1/2 volume of the 12 3' pipes: 42 cu', so give total displacement volume of 603.5 cu'. And this has total displacement of 37,417 lbs. So 100 ft of pipe if ends are sealed has total displacement weight of 3,741,700 lbs. And pipe weighs 215,000 lbs. Or standard tons of 1871 compared to 107.5. So if you were to hold this vertical about 5.75' of 100' would be submerged due to the weight of the pipe.
And of course if 1000' in length 57.5' would be submerged from weight of pipe. And whole pipelauncher would weigh around 1075 tons. And if the total weight of rocket was 1075 tons (big rocket), the weight would submerge the pipelaucher another 57.5', giving total of submerged depth of 115'.
Now this isn't the design yet, but want to get some ballpark numbers. So since every 33' of depth is 1 atm and atm=14.7 psi. The air pressure inside the pipe with a big rocket on it would be: 51.23 psi (roughly- don't take those decimal point numbers seriously). This means that for 1 gee acceleration I would need around 100 psi.
Now, this is in ballpark [it's got to be around 75 psi or less]. But I've run out of time & didn't even get to the new ideas (one reason I started post:) so I will post this then add follow on at later time.