Why hydrolox fuel isn’t really that good for high energy payloads as people think it is (a thread)

Please RT this; I want as many people to read this as possible, because this is very important.

Pic credit: @ErcXspace

Many people who are familiar with rockets think that hydrolox fuel is always the best option for high energy missions, due to the high specific impulse of these engines. For example, the New Horizons probe, Parker Solar Probe, and the Perseverance Rover...

...were all delivered to their destinations with an efficient hydrolox engine, the RL10. But in this thread, I will show you why hydrolox fuel for high energy injections isn’t really as superior as people say it is. Most times, it isn’t even the best choice.

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Hydrolox fuel gets engines to very high specific impulses, but due to the fuel being very lightweight, the dry mass of the rocket will cancel out all of the benefits of hydrolox fuel.

...Even through kerolox and methalox engines are not as efficient, the fuels are significantly denser, which makes it up for a lower Isp.

The dry mass to fuel ratio with hydrolox is around 1:10 (SLS core stage), and can get a specific impulse of up to 470s (RS25 vac; the design I made)...

For comparison, kerolox fuel can get a dry mass to fuel ratio of 1:30 (F9 stage 2) and an Isp of 365s (E2 engine); and methalox at 1:20-25 (Superheavy booster) and 390s (Raptor Vac with extended nozzle).

470s Isp, 10t dry mass, 100t propellant mass: 11052m/s

390s Isp, 5t, 4t dry mass, 100t propellant mass: 11644m/s, 12460m/s

365s Isp, 3.3t dry mass, 100t propellant mass: 12291m/s

See what happened there? Hydrolox gets the LEAST amount of delta V.

It’s the same thing as getting to orbit. Why does the Delta IV rocket have a lower payload/liftoff mass ratio than Falcon 9 and Starship?

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Now, you’re probably wondering, how does hydrolox engines have a higher performance than kerolox engines for deep-space missions, if it isn’t because of the Isp?

Well, you see how the core stage of the Atlas V and Delta IV burn for such a long time, and the upper stage only needs to burn a little bit to reach orbit? That is the reason...

At LEO, the upper stage still has a pretty nice propellant mass ratio, whereas other stages, like the Falcon 9 stage 2, burns for a longer time to reach orbit, thus having a lower propellant mass ratio.