[Steely Tom]

Squirt a wee bit of liquid Nitrogen into that hot air stream, and it immediately expands to 10,000x it’s intial volume - that’s thrust.

Imagine what would happen if they squirted a wee bit of liquid hydrogen into the hot air stream.

[Hodar]

Imagine what would happen if they squirted a wee bit of liquid hydrogen into the hot air stream.

Hydrogen is flammable, Nitrogen is not. If you have a non-flammable fuel, you have no need to fear extreme heat. Plus, Hydrogen is very chemically active, Nitrogen is practially inert.