system
Given the following post, respond as if you're commenting on Reddit:
human
How does NASA liquefy such large amounts of Hydrogen?

I understand that you need lots and lots of pressure to lower the temperature enough to liquefy hydrogen. In fact I figured for a 20 Liter tank you would need around 50atm of pressure, or 722 PSI. How does NASA achieve such large amounts of pressure for even greater fuel tank sizes?
gpt
Cryogenic refrigerators don't (for many reasons including the reason you stated) liquefy in one stage (even when it is possible). Generally, the working gas is repeatedly compressed and cooled (5 -10 times). The compression raises the temperature of the gas, making it easier to extract its thermal energy, and when it's cooled the gas pressure drops again. Repeated cycles get you down to the liquefaction temperature. 

While I've looked for schematics that demonstrate this, most of them are quite complex. The basic plant cycle goes like this:

1) Fill the warm system with hydrogen gas and start running the compressors. 

2) As the plant runs, the gas on the input side gets colder and the gas coming up the output side (which is cold) is used to cool the input side gas. This is generally assisted with turbo expanders (basically backwards compressors). 

3) This process boot straps itself down in temperature until you start making liquid. 

4) As you make liquid, the pressure in the system drops, and you have to keep topping it off with warm gas. 

5) With this process, you'll make liquid at some rate until you have as much as you want.

This is the basic idea behind any large scale cryogenic plant for hydrogen, helium, nitrogen, etc. From the volume that NASA needs, I'd bet they run their own cryogenics plant for this purpose.