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Old 22-02-2018, 07:40 PM   #7
skulb
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Join Date: Apr 2011
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Well, in vacuum they use electrical engines rather than the chemical ones they use for liftoff on earth. https://en.wikipedia.org/wiki/Electr...aft_propulsion
As far as I know these engines are just powered by a battery inside the craft and expel matter to propel the craft forward. Or to reverse thrust when you want to stop. Not much thrust but that obviously works in vacuum and uses almost no energy.
You also have more modern ion drives and mass drivers that also do not depend on igniting kerosene in the vacuum of space. Only the launch rocket requires air to run. It's obvious when you think about it of course, but no mission, real or fake, has ever tried to get to the moon with its launch rocket. That would just be silly.

The basic idea of space travel is that speed and fuel might as well be conserved because you have to spend some to slow down anyway when you get to wherever you're going. The faster you're going the more fuel you will need to stop again. This is the big challenge with the existing theories of space travel. There isn't necessarily a limit to how fast a space craft can travel, within reason. But how do you stop? The only way I know of is a retrograde burn to slow you down enough to enter an orbit, or using the atmosphere itself to gradually slow down over the course of many orbits. But you still have to slow down a lot with burns before this even becomes possible. Without it you either crash into something eventually or skip off the atmosphere of the planet you're trying to get to and disappear in space. And they calculate fuel costs for this with a formula called delta-V. https://en.wikipedia.org/wiki/Delta-v
This is a measure of the momentum you can generate with the craft in question and the fuel available. The optimal delta V figure +/- 25% must be worked out for a mission and then the craft can be designed with enough fuel for the worst case scenario.
And they did all this for the Apollo movies. They just didn't do any of it manned because that's not possible even today. They probably did them unmanned though, because they had proved theoretically that it was possible. And if not in the 60s and 70s almost certainly later when the robotics and automation technology improved.

As for intersecting with an orbiting object around the Moon, this would indeed be far more exact than I would be willing to try in real life. But I have done it once on a simulator. But it was amazingly fiddly and I was able to save my progress before trying. Otherwise I would never have made it. But clearly it is possible, and doesn't even really require much fuel. Just millimeter precision over huge distances. Personally I give NASA employees the benefit of the doubt that they probably are a bit smarter than me. And that years of training might even make it easier to dock things in space for them than it is for me.

And yes, parachute to land on earth. The reason they come in almost horizontally is to use the air resistance in the atmosphere on earth to gradually slow down. If you don't do that you will explode because you're going too fast. The condensed atmosphere would make the craft catch fire you see. But not if you slow down enough first, which does not necessarily require fuel. Depending on the speed you can use the aerobraking technique I mentioned and spend several orbits dipping in and out of the atmosphere, to gradually slow down before landing at an angle. And no, all of that obviously doesn't work on the Moon. But you can land on an object with low gravity by reverse thrusting because you fall so slowly even in free fall. If you are accurate it is possible to stay vertical while slowly reducing the speed as you descend. This too is very tricky at first though, as I know because I have also tried this in a simulator. The slightest mistake and the craft can flip or speed off in unwanted directions. But again, they are allegedly professionals and I am not. And even I was finally able to land on the Moon without crashing to my death in the simulator. If I remember it right you should get below 20 m/s before you land. If you are going faster than that you will either crash, break landing struts or tip over. Either outcome would be very, very bad in real life. But you're not going to fall faster than 75 m/s or so even in free fall, so not much fuel needed there to land. Or take off. It is theoretically possible to reverse thrust land on earth as well. But this would require huge quantities of fuel, and that would not be possible for an earth based mission that has to spend so much fuel to get out of the gravity well of the planet first. But theoretically you could launch a craft from another planet, like the Moon, and instead spend all your fuel landing with reverse thrusting on earth. It would be silly and pointless when you can just use a parachute that costs 25 dollars. But it is possible. It's a perfectly good way to land under the right circumstances. Like when landing on the Moon.

Anyway, I would strongly recommend that you give a space flight simulator a go. Several good ones, but I am a big baby and presently prefer Kerbal Space Program. It has funny cartoon characters but the physics are accurate, albeit on a 1/4th scale to save some time. But it can give you a good idea of how orbits and delta V works in a fun way. God knows I would know nothing about it if it hadn't been for simulators.



And for the record, I don't pretend to know what happened one way or the other with the Apollo missions. But as far as I'm concerned everything the US government says or does is almost bound to be fishy. If they say something I automatically assume that they are lying. And they said that they sent four Freemasons to the Moon in 1969. So that is probably a lie. But that doesn't mean unmanned space flight is a lie. And certainly not the underlying theories. If I can do it with existing theories and technology (and save games) I think that pretty much proves it. It's not as hard as people think. It's just fiddly and mistakes get you killed most of the time. That's why no one does this with manned missions. Unless they are crazy. Or lying in some mad Cold War propaganda stunt.

PS: KSP does not include radiation, which is why you can send Kerbals to the Moon in it, unlike in the real world. They also have unlimited air and don't need to eat, so you can just come back and get them later if they get stuck without enough fuel to get home. This does not work in real life obviously. But other than that it's accurate enough.

Last edited by skulb; 22-02-2018 at 09:12 PM.
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