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SixIsNotANumber t1_izk061b wrote

Can someone ELI5/tl;dr for me why/how it could work going one way but not the other?

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Icee777 OP t1_izk38tj wrote

The idea is for the space elevator from Phobos ending about 28 km from the surface of Mars just above the atmosphere to prevent friction.

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borgendurp t1_izkivzf wrote

What's the point then? Isn't the first part the hard part? The first 28 km from Mars I mean

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TerpenesByMS t1_izlcu59 wrote

Yes and no.

Mars atmosphere is much thinner than earth's. Gravity is already the hardest part of getting to orbit from the ground on both planets, on Mars the atmosphere component is smaller.

The bulk of acceleration to reach orbit isn't the up part, it's the sideways part. By having a low-hanging and sub-orbital "docking point" at the base of the elevator, you are still conserving a lot of fuel and delta-V even though it doesn't go "the whole way".

Also, having an asteroid anchor point gives space elevator architects more freedom. Unwinding the inner and outer tether doesn't need to be perfectly synchronized, and tether lengths and counterweights could further be used to adjust Phobos' orbit around Mars.

None of what I just said is fast or easy, but when we're talking about space elevators nothing really is. As described, this might be the "beta version" space elevator that's deployed before any are used on earth - lesser risk, more room to experiment and learn, still has some use if we're jacking around on Mars, etc.

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SixIsNotANumber t1_izkjg6n wrote

Yeah, that's kind of a head-scratcher for me, too. I'm having a hard time wrapping my brain around it.

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borgendurp t1_izkjle7 wrote

I think we get the principality behind it.. just not the point 😆

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SixIsNotANumber t1_izklk1t wrote

Exactly!
I've loved the concept of space elevators ever since I read The Songs of Distant Earth, by Arthur C. Clarke, and I hope I live to see the day one actually gets built...but I don't get this at all. I'm hoping for a good follow-up answer though. Like I said, I want it to be real!

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Icee777 OP t1_izkjvqg wrote

You can halve those 28km by launching from Pavonis Mons - a volcano on Equator. In that way you are out of the dense part of the atmosphere

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borgendurp t1_izkkbvq wrote

Yeah okay.. but why? The atmosphere only goes up to 10.8 km. After that, why do you need 6000km of elevator for?

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manicdee33 t1_izlwy08 wrote

Escape velocity from Mars is about 5km/s. The article explains that the bottom end of the elevator would be travelling at ~770m/s while the outer end of the elevator would be travelling at ~3.25km/s. This means a two-stage escape from Mars (one stage to get to the elevator, second stage to get from the elevator to escape velocity) would only need to provide ~0.8km/s to rendezvous with the elevator, and another ~1.75km/s to escape Mars, saving ~2.4km/s in delta-v overall. This results in significantly lower propellant requirements for cargo moving between Mars and Earth (and thus smaller spacecraft to carry the same payload).

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borgendurp t1_izn1x0a wrote

I'm not too familiar with what you're talking about but from wiki;

In celestial mechanics, escape velocity or escape speed is the minimum speed needed for a free, non-propelled object to escape from the gravitational influence of a primary body

So I'm not entirely sure how that applies?

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manicdee33 t1_iznadnx wrote

In celestial mechanics, escape velocity is the speed an object needs to reach in order for the force of gravity to never reduce the outward velocity to 0. As you travel further away from the primary body the force of gravity gets smaller and smaller, so the deceleration gets smaller and smaller, and the limit at the distance approaches infinity is for the deceleration from the force of gravity to reach 0. If the starting speed of the object was such that by the time it reaches that infinite distance it still has some radial velocity, it has escaped.

Also in celestial mechanics, a "propelled object" is one that can thrust forever (basically a torch ship). A non-propelled object includes an object which has accelerated by burning a rocket engine and has stopped consuming propellant (it's no longer propelled). If that rocket can reach escape velocity, it can coast out of the influence of that primary body. This is how rockets can push space craft from one planet to another: they reach escape velocity to escape the gravity well of one planet, and carefully aim to be caught in the gravity well of the destination.

Hope this helps.

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ItsAConspiracy t1_izwxc4b wrote

Delta-v to launch from the surface and dock with the space elevator, according to the article: 0.52 km/sec.

Delta-v to launch from the surface into Martian low orbit: 3.8 km/sec.

So less than a seventh as much velocity change. Now let's use the rocket equation. A methane rocket has specific impulse of about 370 seconds (that's a measure of how fuel-efficient the rocket is). We'll use a starting mass including fuel of 10,000 kg.

For a delta-v of 520 m/sec, we get a final mass of 8380 kg. We only had to burn 1620 kg of fuel to get 8380 kg of rocket and payload up to the elevator.

For a delta-v of 3800 m/sec, our final mass is only 2748 kg. We had to burn 7250 kg fuel, to put only 2748 kg of rocket and payload into orbit.

Assume in both cases that the rocket is 1000 kg, then with the space elevator we're getting 7380/1620 = 4.55 kg payload per kg fuel, and without the space elevator we're getting 1748/7250= 0.24 kg payload per kg fuel.

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SixIsNotANumber t1_izkk81h wrote

OK, that part of it does make some sense, but I'll echo the other reply to your reply: what's the point?
Isn't the harder part of the process getting off of Mars? (And please don't take my questions the wrong way, I promise I'm not trolling or trying to JAQ-off, I really just want to wrap my brain around why this is meant to be a good/useful thing.)

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Icee777 OP t1_izkyhai wrote

It makes sense once you have an industry large enough to move substantial amounts of mass to and from Mars. Space elevator saves energy. In the proposed form you need only 0.52 km/s of Delta-v to get from Mars' surface to the elevator. And on the other end of the elevator you can release (or catch when importing) that mass at 3.52 km/s of Delta-v. Of course, there will be some energy loss moving up and down the elevator, but you don't need the acceleration that a rocket launch demands.

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