Janus58
Valued Senior Member
Your statement about the car is just silly. Of course you can't use car brake in a free fall, I never said you could, in free fall, you use rockets for braking. The point is that even though rockets and cars use different physical means to "apply the brakes", they both would use a method of shedding speed gradually rather than all at once. You completely missed the point in order to concentrate on something totally irrelevant.
The Lunar module had plenty fuel for a landing/takeoff. The descent module carried some 8000 kg of propellant mass, and an engine with an specific impulse of 311 sec. The fuel was in 4 67.4 cubic ft tanks. It is easy to work out that this amount of fuel would fit in those tanks and that those tanks would fit in the descent module. Also knowing the Total mass of the Lunar module and the rocket equation, this is enough to work out just how much velocity change this would produce for the module. Also knowing the mass of the Moon (1/81 that of the Earth), its radius ( between 1/3 and 1/4 of the Earth's) and initial orbital height, you can worked out how much velocity change is needed to land on the Moon. The calculated performance of the Lunar module falls in this range. How can you claim that it couldn't carry enough fuel when it is obvious that you have no clue as to how to figure out how much fuel it would require? (Your "gut feeling" is not sufficient argument).
As already noted the the Moon is much less massive than the Earth and has a much weaker gravity. Its escape velocity ( which determines how hard it is to land on or leave) is about 1/5 that of the Earth's. So it is that much easier to land on the Moon vs. the Earth.
It's not that we couldn't use rockets to land on the Earth, it would just be very wasteful to do so. It would increase the total fuel requirements immensely. Not only do you need to add the extra fuel needed for the landing, you need to add the fuel to lift that fuel in the first place. Something like the Saturn V could, in theory, lift something to orbit by rocket and then bring it back by rocket, but only at the cost of making that something be really small. We make use of the Earth's atmosphere because it is there and we can use it to reduce fuel requirements. It allows us to make more of the initial rocket mass payload vs fuel load. Why use rockets to land on the Earth when its easier and cheaper to use parachutes? It's a matter of using the best method for the given circumstance and taking advantage of what nature has already given us.
Not having an atmosphere for parachute braking would make spaceflight much harder in practice, but not totally impossible.
