Liquid hydrogen is ideal as a propellant for manned interplanetary missions to Mars, etc.
eg. Use the liquid hydrogen with a high-temperature nuclear rocket motor [temp: 3,200K]
and you get an exhaust velocity of about 10 kilometers per second [22,400 mph]....
Add that velocity to orbital speed [7.8 kilometers per second] and you've got an impressive Earth departure velocity, assuming the [say] 600 ton manned Mars spacecraft
in LEO has a mass ratio of about 2.718; meaning more than 400 tons of LH2 would be required.

VASIMIR is better. Uses electricity to propel the exhaust plasma to higher velocities still.
A smaller mass-ratio is possible and desirable.
But liquid hydrogen is still the ideal fuel for such a propulsion system.
Problem?
How to keep it cold and a liquid longer in orbit.
Why is that a problem?
Even with VASIMIR propulsion, you'll need to park and assemble hardware and propellant
in orbit with a mass over 200 tons.
No HLV will be built to orbit 200 tons in one shot; you'd need a HLV with a thrust
over 9 million pounds to do so...It would shatter windows in Tampa, Florida; it ain't going to be built.

So?
So keep liquid hydrogen colder longer in orbit while the orbital assembly takes place over
several days prior to a Mars shot.

How?
Use liquid helium, circulating in pipes through a tank containing liquid hydrogen
to chill the LH2, keeping it colder and a liquid longer.
Example: Use one or two tons of liquid helium, which would gradually boil away, vented into space, to keep [say] 100 tons of LH2 chilled longer, taking the feeble heat energy absorbed from the liquid hydrogen with it.
HNY