I presume that if H2 were good for airplanes it would have been used by now. But I am curious as to why H2 seems to be a preferred fuel for space rockets while hydrocarbons are preferred for cars, trucks, and airplanes. Why is liquid H2 better than jet fuel for rockets?
Because
specific impulse (which is related to exhaust velocity) matters for rockets. Rockets burn fuel and throw the burnt gases overboard, and the thrust comes from throwing the hot, expanding gases overboard. So you want something that burns not and leaves the nozzle at high velocity; the higher velocity it leaves the nozzle, the more thrust it produces. You could throw a ton of material out the back of that nozzle but if it only leaves the nozzle at 0.5 m/s, it's barely going to produce any thrust, whereas if that same ton of material leaves the nozzle at 100 m/s, it produces a lot more thrust. And LOX/LH2 has the highest specific impulse, at 381s:
Basics of Space Flight: Rocket Propellants The RS-25 (space shuttle main engines) rocket engines on the space shuttle that burned LOX/LH2 from the external tank actually had exhaust velocities of in excess of 4,000 m/s! The solid rocket boosters didn't use LOX/LH2 of course, they used a fuel and oxidizer that was a lot more dense but had a much
lower specific impulse. Their job was to get the shuttle out of the atmosphere as quickly as possible, while representing as little atmospheric drag (which relates to volume more than exhaust velocity) as possible, and then detach and let the RS-25 engines do their job.
With a rocket, you want as little mass (fuel and oxidizer) as possible to produce as much thrust as possible, because
carrying more fuel and oxidizer makes the rocket more massive, which requires
even more fuel and oxidizer to carry the extra fuel and oxidizer.
None of these constraints apply to air-breathing jet engines. Why? Because air-breathing jet engines can throw stuff out the back that
didn't come from what was being carried onboard the vehicle. You've probably noticed that engine manufacturers have been making bigger and bigger
turbofans, with larger and larger
bypass ratios. This is why the A320neo has a larger engine nacelle than the original A320, and why the Boeing 737 MAX has larger nacelles than the original Boeing 737 and the 737-800 and 737-900. Actually, since the 737 was developed, the nacelles on the 738 and 739 had gotten so large that they had to be made in a sort of oblong shape that looks smashed on the bottom so as not to get too close to the ground. When Boeing tried to use an even higher bypass ratio in the 737 MAX, the engine nacelles were so large that they could no longer be mounted in the original position, which required them to be repositioned on the wing. This in turn caused the aircraft to pitch up when power was applied, and Boeing created a system called the Maneuvering Characteristics Augmentation System (MCAS) to push the nose back down in case this happened. You're probably familiar with what happened after Boeing made multiple mistakes in how this system was implemented. Anyway, air-breathing jet engines use a fan attached to the turbine to grab onto atmospheric air and force it backward, creating additional thrust. This is something that rocket engines, which operate outside of an atmosphere, cannot do, because if they had spinning fans, the fans would have nothing to grab onto. In a rocket, all of your thrust comes from the burnt gases, whereas in an air-breathing jet engine, most of the thrust actually comes from the fan.
Personally, I think they're paying too much attention to how much carbon emissions come out of the engines and not focusing on the entire system as a whole. People have burned wood for centuries for energy and this didn't create a problem because the carbon in the wood originally came from the atmosphere. Likewise, burning jet fuel isn't a problem so long as the jet fuel's carbon comes from the atmosphere. As long as you used renewable energy sources to make the jet fuel (like solar and wind power), and the carbon in the jet fuel is processed from organic material that took its carbon from the atmosphere (biogas, etc.), then you can burn carbon neutral jet fuel.
