There’s reportedly some technology that uses air-breathing engine, liquefies oxygen during the atmospheric flight phase, and then uses it for the exoatmospheric part. It’s said it could be used for reusable single-stage-to-orbit flight.
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A joke my son told me…
A guy walks into a bar and says, “I’ll have some H2O!” The bartender puts down a glass in front of him, the guy drinks it, pays, and walks out.
Another guy steps up to the bar and says, “I’ll have H2O too!” He drinks the glass the bartender puts in front of him and dies.
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SABRE, right?
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The problem such schemes is that they add enormous complexity in order to merely save a bit of fuel. Enormous complexity adds enormous costs, while fuel is cheap.
Since you need the fuel and oxygen tanks and rockets regardless, it’s simpler - cheaper - just to make your design a bit larger and bring more oxygen to begin with.
And like SpaceX is developing, recover your rockets. (2nd stage recover is apparently coming in a later design.) Using the largely same engines, fuel tanks and other systems you were already using.
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You need to bring your fuel and oxidizer with you, and this makes your craft heavier. It’s not cheap. If the associated components to save on fuel are lighter than the saved fuel, you win out, even if the financial cost seems dire.
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Yep. If you can eliminate half the weight of the craft by not having to carry oxygen, you can go a helluva lot further, or faster. Pack a lot more delta-v in.
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Except that you STILL have to carry oxygen, and almost as much of it. Instead you’re collecting and liquefying it while cruising in the atmosphere at a top speed that’s still only a small fraction of what you need to get to orbit, before lighting the rockets. It’s simper and cheaper just to make your oxygen tank a tad larger than to drag your collection/liquefaction equipment into the air, let alone drag it into orbit.
There’s been much talk of using scramjets to power launchers. Except that you need rockets to get them up to speed, and then rockets to get the remaining majority of the velocity and altitude needed for orbit. And scramjets are sensitive, wanting a constant velocity and altitude to work, while launchers have constantly changing velocity and altitude.
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It’s all fun and games until someone loses a sponge.
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This reminds me of the scene in “Who Framed Roger Rabbit” where the evil judge dissolved the friendly cartoon shoe in the barrel of turpentine.
That poor shoe… 
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Oh yes. One of my stupidest moments in a Chemistry Lab was thinking that as only brief heating of the sulphuric acid was needed, that I could hand hold the test tube to pass it through the bunsen flame. The contents bubbled over fingers and thumb … 
Despite very quickly ditching the test tube and plunging the hand in copious water, I wore a ridged brown stain in the shape of the test tube for weeks.
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HF is quite toxic, because of its ability to permeate well into your body and then bind calcium, but is not a strong acid. One of the reasons it requires significantly more attention when using it than strong acids is that you could be exposed and not know it until many hours later, after the damage is already done.
in other words, it’s bad because it’s poison, not because it’s corrosive.
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I would expect that, if it were seriously concentrated, there would be flame when you poured it on something organic. I’ve certainly seen paper towels go up like that.
With the really concentrated stuff, I was once following all safety procedures (except the one I forgot). Wearing goggles, gloves, a face shield, a lab coat, and a thick rubber apron.
I took a little piece of silicon I had just cleaned off in piranha (concentrated sulphuric acid mixed with hydrogen peroxide. Extremely oxidizing) and had in the tweezers, and took it (barely) out of the bottom of the fume hood to get a better look at it. The fumes from the couple of drops of residual piranha on it burned all the nose hair out of one of my nostrils when I next inhaled.
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Cellulose is neat, but I prefer dissolving sugar with my sulfuric acid:
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You can do something similar with acetone and styrofoam, and acetone isn’t all that bad as far as chemicals go. (It leaves you with that cool and refreshing feeling when you get it on your latex gloves.)
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What’s more, the sulfuric acid itself is a byproduct of seeking copper, the most common ore being copper sulfate. Living in Arizona, I often see sulfuric acid trucks driving away from the nearby mines.
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My first experience with sulfuric acid was playing with an old car battery when I was a kid and then finding that my jeans had turned into lace later in the day.
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