In principle liquid breathing is still an option. It's not great, but certainly better than oxygen + hydrogen.
Yeah, it's hydrogen, hazardous to mix and handle, but so is acetylene, and that can explode all by itself if it's under too much pressure. Mess up the mix and you die, but if you mess up the mix for any breathing gas, you're pretty effed (the wrong N2/O2 mix can cause any of hypoxia, narcosis, seizures at various depths).
Not many people have done a pure hydrogen/oxygen dive (vs. hydrogen/helium/oxygen), but from one who has on a rebreather:
> “The first cautious sip of hydrogen just to activate the ADV was satisfying,” [Harris] said. Gas density was not subjectively improved, but Harris noticed an obvious benefit—the HPNS-induced hand tremors he typically experienced after 180 meters/590 feet disappeared. [1]
[1]: https://indepthmag.com/n1-the-inside-story-of-the-first-ever...
It's used on a somewhat regular basis in clinical settings (e.g. liquid ventilation). I'm not aware of any diving experiments, though. There might have been some classified military tests, but nothing official AFAIK.
https://doi.org/10.5001%2Fomj.2011.02
I think liquid breathing for human divers is going to remain science fiction forever. There just aren't any circumstances where it would make sense to do that instead of using an atmospheric diving suit (or an ROV).
The main character wanted to get out to the asteroid belt in an accelerated time frame (not relatively accelerated but rather faster than the current slow methods)
> "Sorry, sir," said Mary, the perky nose on her image twitching under her large glasses as she thought. "I'd be glad to turn over all the cable we've been making for the Mars rotovator, but by the time Bull's division could turn it into a cable catapult, you'd be there using the existing system."
> "Can we push the gee limit higher?" asked Randy.
> "Well... yes..." admitted Bull. "We have small express pods we use to send emergency cargo. We can accelerate those at ten times the normal three-gee launch acceleration and get them up to three hundred kilometers a second."
> "How soon would that get me there?" asked Randy.
> Bull, his fingers too large to operate a cuff-comp, pulled an hp pseudocray out of his shirt pocket, and did a short calculation. "Three weeks," he said. "But those capsules accelerate at thirty gees! You'd be squashed flat!"
> Randy paused for a while as he thought. "I've read about deep-sea divers who survived at high pressures by breathing an oxygen-carrying liquid," he said. "If I floated in a tank of that, I could handle thirty gees easily."
> Tony, Mary, and Bull each thought for a while; then all three nodded, although reluctantly.
> "If I were you, I'd check with some medical and diving experts first," said Tony. "It could be hard on your lungs."
---
Granted, that's science fiction and there are some real problems with that approach too...
https://en.wikipedia.org/wiki/Liquid_breathing#Space_travel
> Acceleration protection by liquid immersion is limited by the differential density of body tissues and immersion fluid, limiting the utility of this method to about 15g to 20g. Extending acceleration protection beyond 20g requires filling the lungs with fluid of density similar to water. An astronaut totally immersed in liquid, with liquid inside all body cavities, will feel little effect from extreme G forces because the forces on a liquid are distributed equally, and in all directions simultaneously. Effects will still be felt because of density differences between different body tissues, so an upper acceleration limit still exists. However, it can likely be higher than hundreds of G.
> Liquid breathing for acceleration protection may never be practical because of the difficulty of finding a suitable breathing medium of similar density to water that is compatible with lung tissue. Perfluorocarbon fluids are twice as dense as water, hence unsuitable for this application.
We're not at the point where we can launch at hundreds of Gs yet (and I'm not sure I would want them to be centripetal Gs initially either - that sounds very not fun) ... but when we get to the point were we want to launch a person into space in the direction of another planet at 10 or 20 or 30 Gs of initial acceleration, this is likely something to be revisited.
Robert Forward (the author of Timemaster) was a physicist and I would do a check of his anatomy related topics before getting into one... but I would be confident of his math.
He wasn't unfamiliar with the technology being used.
https://en.wikipedia.org/wiki/Robert_L._Forward
> He then went to work at the research labs of Hughes Aircraft, where he continued his research on gravity measurement and received 18 patents. He took early retirement in 1987, to focus on his fiction writing and consulting for such clients as NASA and the U.S. Air Force. In 1994, he co-founded the company Tethers Unlimited, Inc. with Robert P. Hoyt, where he served as chief scientist and chairman until 2002.
https://en.wikipedia.org/wiki/Tethers_Unlimited,_Inc.
> Founded in 1994 by Robert P. Hoyt and Robert L. Forward, Tethers Unlimited began developing products based on space tether technologies, including concepts for removal of space debris and momentum exchange tethers for launching payloads into higher orbits. TUI has since broadened its suite of technologies to include power, propulsion, actuation, and communications systems for small satellites, robotic technologies for on-orbit fabrication and assembly, optical fiber winding and deployment, software defined radio communications, and 3D printed radiation shielding.
...
And so, on his word, I'd suspect that it would not be unreasonable to do a fluid breathing 30g ... given the right fluid breathing setup. The rotovator setups are looking at significant fractions of a day for earth capture to space release (6 or 12 hours). I don't think there would be human significant jerk, snap, crackle, and pops either. Though again, I'd be curious to see that acceleration profile (not that I'd ever be doing it myself).
That first bit, where you breathe in the fluid, must be horrendous. No thanks!
I understand the practice GP is describing pretty well but when I try to simulate what is going on in my mind, it's not like these gasses are in gaseous form once they are in the blood. Are we just talking about the concentration that are reached because the pressure affects the diffusion gradient?