[wcarss]

> As you land, the fuel tank depletes significantly which shifts the center of mass towards the top of the vehicle

Genuinely asking because I think I might learn something: wouldn't a depleting fuel tank above the engines shift the center of mass toward the bottom of the vehicle?

[Out_of_Characte]

Generally, a good spacecraft has the least amount of mass possible on fuel/engines as compared to payload (the useful part of a mission) which is why center of mass either migrates upward or stays mostly neutral.

[dotnet00]

I don't think this is right. My understanding is that the engine is typically the one of the heaviest dry parts of the vehicle and the fuel (at launch) the heaviest individual part overall. Especially nowadays since the electronics and sensors are tiny and antennae are lightweight.

This is especially the case with things like landers and geostationary satellites, where you want as much fuel as you can afford for station keeping, to keep the satellite operable for as long as possible.

Half of Nova-C's mass was fuel (~900kg), payload was 100kg. Starship's payload is ~100t, dry mass is ~150-200t but fuel mass is ~1200t.

[jvmboi]

It makes sense that the engines would be heavy and they are heavy on the launch vehicle. I am not sure how the ratios come out there but I'd still expect the fuel to, by far, take up most of the mass. Then engines for maneuvering in space and to land on the moon don't have to be very big. I looked up the figures for Apollo and found out the following:

"The Apollo's "lunar module descent engine" weighs a mere 180 KG vs the approx. 4200 KG of the rest of the craft (dry mass). Just the fuel for the descent is then roughly 8000 KG."