The critical point of hydrogen is 33.2 K, that is Kelvin, or -240 degrees celsius.
Any higher temperature than that, hydrogen is not a liquid, and exerts pressure on the walls of its container.
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so, you have the choice of making a very strong container, or a very well-isolated one, preferably both.
But probably not a lightweight container, which is what you'd prefer.
I thought that writing "liquified hydrogen" would adequately suggest that I was talking about hydrogen in liquid phase; maybe I needed to be clearer, and say that liquified hydrogen really is liquid, and not gas, but I still don't see why.
Aerogel is both very, very light in weight, and also extremely effective at isolating substances of very different temperatures such as LH2 and aircraft parts. Thus, practical LH2 tanks on aircraft would be insulated with aerogel.
Given a tank of LH2 insulated with aerogel, there would be no need for the tank to be "very strong", as it would not need to hold back any more pressure than the (very light) weight of the column of LH2 contained. Liquified-gas tanks are routinely equipped with fart valves to relieve any pressure that arises from heat leaking in and boiling the contained fluid.
