[jvmboi]

Some "insights" from this mediocre KSP player:

a) for launch you want your rocket to be slender and tall. For a stable landing you want your vehicle to be broad and flat.

b) The engine is at the bottom (moon-wards), by definition. The fuel is above the engine. As you land, the fuel tank depletes significantly which shifts the center of mass towards the top of the vehicle which makes it less stable.

c) The lunar (mun-ar?) surface is really uneven and gravity is low. What you want to land is a steamroller but what you actually have is a springy, ultra-light, top-heavy contraption that's more likely to bounce off of then to flatten moon rocks.

[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."

[dspillett]

WRT b: surely using fuel that is above the engine will bring the centre of mass down, because the engine's mass is still there with less fuel mass above it?

Unless the rest of the vehicle's mass (all the other equipment, and crew if it is a manned mission) has more mass than the engine & landing apparatus of course, which I think (caveat: no deep thinking involved here) is likely for manned missions but less so for others? I'm assuming the mission mass is above the fuel (having the fuel on top would presumably be less safe/reliable/practical/other).

[jvmboi]

I could easily be wrong and I am very open to learning as what I wrote is just my intuition developed from playing KSP.

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.

Obviously, landing on the moon is possible but I do think that the inherent requirement to have engine(s) and fuel tanks below the payload makes landing in a vacuum a bit of a challenge.

[dotnet00]

I'm fairly certain that the effect of point b is just a version of the pendulum rocket fallacy. There is actually no change in stability of a rocket in flight related to if the engines are on top or on the bottom because the tidal forces exerted by gravity are too negligible in that specific case and otherwise gravity is acting equally on the entire body.

Plus, since the engine is typically one of the heaviest parts , and the lander isn't a two-part design like Apollo, the fuel tanks are mostly empty upon landing, and therefore the center of mass is low due to the engine.

[eig]

Does point (b) actually matter? I thought the location of center of mass and center of thrust doesn't matter in a non-gimballed rocket according to the Pendulum Rocket Fallacy (https://en.wikipedia.org/wiki/Rocket#Pendulum_rocket_fallacy).

[pif]

[OT]

In re "mun-ar", maybe you meant "monthly".

Etymology of "lunar": Middle English, from Latin lunaris, from luna moon; akin to Latin lucēre to shine [0]

Etymology of "moon": Middle English mone, from Old English mōna; akin to Old High German māno moon, Latin mensis month, Greek mēn month, mēnē moon [1]

You the Anglophones have this tendency to forget about the noble origins of your languages, just to reinvent it badly ;-)

[0] https://www.merriam-webster.com/dictionary/lunar [1] https://www.merriam-webster.com/dictionary/moon

[vermilingua]

In Kerbal Space Program, the moon is named Mun as opposed to our Luna: lunar -> munar

[pif]

Edit: thanks to commenters for the reference to KSP. I had to ask Wikipedia what it is, and the first sentence is: "Kerbal Space Program (KSP) is a space flight simulation video game developed by Mexican studio Squad".

Mexicans forgetting about Latin is even worse than native English speaker doing the same!

[OkayPhysicist]

It wasn't a mistake. KSP takes place in a solar system that is similar, but not identical to our own, and the planet the the space center is on, Kerbin, has a moon, called Mun.

[deepsun]

They didn't forget anything. Try playing it for 10 minutes, it's fun :)

[dotnet00]

By mun-ar they were just referring to the Moon equivalent in KSP being called the Mun.

[dreamcompiler]

No, CM shifts toward the bottom, especially while the engine is thrusting and the craft is upright in lunar gravity. The liquid methane and O2 slosh toward the bottom of the tanks in those conditions.