It's not just the center of gravity. I think people really underestimate how hard landing on the Moon is, because counter-intuitively, the lack of an atmosphere makes things harder, not easier. Landing on Mars (or even Venus (!!) - which is the first other planet we ever landed on) is easy mode by comparison.
With the lack of atmosphere is that there is no 'natural' attitude/orientation correction. If you're tilted 5 degrees then you'll stay that way. With an atmosphere drag and aerodynamic forces can be used to ensure a proper orientation.
'Just make sure you come down straight' isn't so easy because when you enter the Moon's 'orbit' (not necessary, speaking colloquially) you're traveling extremely fast. And so to land you need to zero out your horizontal and vertical velocities. You do this by literally turning around the opposite direction and thrusting. And then you need to simultaneously also ensure your vertical velocity stays near zero as you approach the surface.
And then finally you need to come down with your vertical velocity at near zero, your horizontal velocity at zero, and perfectly orientated. This is really friggin hard. If you have even a hair of velocity you're going to bounce, skid, and otherwise do nasty things - which is why so many landers end up on their side, if not upside down. And then there's the Moon's surface itself. Come in on an even slightly unlevel terrain and again you're in for a wild ride.
A bit of Kerbal Space Program really makes you appreciate what an ordeal a satellite landing is. And that is a much simplified setting with e.g., no control lag (even on unmanned crafts), a much closer satellite, etc.
Even just designing a craft that will not topple once landed is punishing.
Yes. SpaceX absolutely depends on the v^2 automatic deceleration effect the atmosphere provides. They also depend on GPS which is not available on the moon. SpaceX also takes advantage of 50 years of improvement in orientation sensors, INS, image analysis, thrust vectoring servos, and thrust control.
As another post points out, the Apollo landings had real pilots who could look out the window. It's a testament to their skills that they all made it home.
Mongol spacecraft do tend to have a rather high center of gravity, but they land with sufficient velocity to imbed the landing spire of their spear-like ships deep into the crust for stability and dramatic effect.
To paraphrase one of my favorite historians - Every sweeping historical generalization has a "probably doesn't apply to the Monguls" exception. If you don't expect your professor to waste chalk writing this over and over, then he won't grade you down for omitting it from your test answers and term paper.
Yes, this is why they did a similar thing with Ukraine (Feb/everything is frozen - for those who haven't been in frozen countries in the Winter, the earth/ground/dirt (pick your word) becomes hard like cement)).
What are you referring to? They had a failed invasion of Korea; they had a very successful invasion of China that ended when they were defeated in a naval war well outside of Asia.
The invasion of China wasn't successful. It was a quagmire. Japan didn't have the men or resources to conquer China and Chiang could just retreat whenever he had to.
I recommend reading Eri Hotta's Japan 1941: Countdown to Infamy
It's a book about how Japan decided to go to war with the united states. It details how China was a quagmire for Japan that was sucking up all the empire's resources. They could conquer land but they couldn't hold it. They could achieve tactical victories against Chiang but could not erode his ability to stay in the field and fight.
The story of how the leaders in Tokyo decided to double down on a war they were slowly losing in China, by starting a war they would more quickly and apocalypticly lose against the United States is a fascinating one.
Fuel will be a huge percentage of the weight, and won't that be mostly exhausted after taking off and then doing a powered descent? The center of gravity will move quite a bit upwards.
Yes and no. For any vehicle that is carrying humans you'll want sufficient fuel to compensate for error or unexpected complications. And depending on the situation for a lunar landing that may require sufficient fuel to abort a landing well into an attempt so that a second landing (or return) can be attempted (instead of committing and hoping the crew just doesn't die). This doubly so if there's a capacity to refuel in orbit for a second attempt.
Also you'll need to assume for at least the foreseeable future that any Starship HLS landing will require sufficient fuel to return to orbit.
With enough margin left you can very reasonably adjust the center of balance by using multiple tanks and pumping all the fuel into rear tanks.
That should be enough given that the Starship HLS is estimated to have around 1.5 million kg of fuel at max capacity and only 100k kg of payload to the lunar surface (200k kg payload max for non-landing starship). That makes the payload mass to lunar surface only 6-7% of the total fuel capacity.
So outside of an extremely risky "attempt landing with no fuel left for an abort or return to orbit", you'll have at least double to quadruple the weight of the payload in just fuel alone.
Now for an earth landing of course the calculus here is different, especially since Starship's earth landing strategy explicitly requires throwing the vessel on it's side during the "bellyflop" and only pulling out of that fall with a powered landing at the last possible second.
TLDR No. Where center of mass will be able to make a difference fuel will make up significantly more mass than the payload.
I’m a be honest: Jeb and Bill have done great work for us, but when it comes to getting a tipped over rocket upright… they haven’t always come through for me.
> 2. Make sure your robotic lunar lander has a low center of gravity.
Also, make sure your robotic private spacecraft doesn't land on the edge of a crater. Or partly on a big rock. Or where a rock or ledge is high enough between the legs to reach the rest of the craft.
Adding to the sibling @mathgeek's comment, that's only true when there are no outside forces other than gravity. You can see that by taking the counter-argument to its extreme: with gravity of epsilon, even a gentle prod at the top of the object will topple it over.
It's been a while, but IIRC when we assume all other variables as constant, a lower center of mass (a) will decrease the denominator and thus increase the resulting necessary tipping force.
Physics 1A: the center of mass does not change. It is irrelevent. The tendancy to tip over is how wide the base is: look at the LEM. Big wide base. Did not tip over. None of them tipped over. They tip over faster on earth due to larger Earth's gravity and mass.
That is exactly a description of why center of mass is the important variable. If the center of mass wanders outside the support point, it tips over. That's the whole deal right there!
Magnitude of gravity changes nothing as long as it’s not 0. Your CG is either inside the area covered by the hull of your leg contact points or it’s not. It won’t be stable in other regimes just because gravity is lower.
I think it does change the situation when you're potentially dealing with lateral movement. In 1/6th of a g it takes less lateral velocity to topple over.
1. Never invade Russia in winter.
2. Make sure your robotic lunar lander has a low center of gravity.