Why are Space Elevators not a horrible idea, as bad as gunpowder cannons to space?

[tater]

Thruster fails on normal lander. Boom.

The surface would be prepared, it's sort of a runway.

Computer fail would kill any lander.

[PB666]

5 hours ago, tater said:

Crew? Why would you even use a crew? It's not like the guy would be micromanaging the slide, the computer would do it.

As I recall from the class, as well as all the stuff at various Space conferences (Space '90, '91, ... etc) they were all concerned with the fact that mining doesn't actually supply excess propellants. In fact, it can only really offset landing costs. Ehricke's goal was to try and make it possible (from a propellant balance standpoint) to deliver net propellant to LMO.

I'm not saying it's even possible, but it's certainly an interesting concept that is outside the box.

 

Moon is for metals, but consider that aluminum is also a component of propellants, other metals could also be used to make solid rocket cores.

[tater]

Yeah, I know, I wasn't gonna type out all the components of regolith on my phone

 

 

[cantab]

1 hour ago, tater said:

Thruster fails on normal lander. Boom.

The surface would be prepared, it's sort of a runway.

Computer fail would kill any lander.

A normal lander can have multiple thrusters, indeed the only manned Moon lander to date did sort of (though an ascent engine failure would still have killed the astronauts). And it can be piloted manually. A sudden failure might affect the lander attitude, but there's space and time to bring it back under control. That's not so with the slide landing concept, which if it goes wrong is liable to go catastrophically wrong near-instantly.

And a runway dozens of kilometres long that needs to be kept at a high degree of flatness and free of significant rocks, on the dusty, rocky, bumpy Moon with all the difficulties of working outside there. While every time something lands the very *design* means the lander trashes said runway. That's going to cost an absolute bomb to build and maintain, it probably makes disposable rockets look cheap.

[tater]

1 minute ago, cantab said:

A normal lander can have multiple thrusters, indeed the only manned Moon lander to date did sort of (though an ascent engine failure would still have killed the astronauts). And it can be piloted manually. A sudden failure might affect the lander attitude, but there's space and time to bring it back under control. That's not so with the slide landing concept, which if it goes wrong is liable to go catastrophically wrong near-instantly.

And a runway dozens of kilometres long that needs to be kept at a high degree of flatness and free of significant rocks, on the dusty, rocky, bumpy Moon with all the difficulties of working outside there. While every time something lands the very *design* means the lander trashes said runway. That's going to cost an absolute bomb to build and maintain, it probably makes disposable rockets look cheap.

A few things, the slider shown is just the art in the book. You could just as well have multiple thrusters on that as well for failures. I think this technique was proposed exclusively as a lifter for lunar-mined propellant transfer to orbit, so the whole thing is an ascent stage (so if it lands partially full, it has an abort mode built-in). As such a cargo vehicle, it has no need of a crew, so no crew is at risk. Honestly, any reusable vehicle designed to lift ISRU propellants should certainly be unmanned anyway, as every kg of crew compartment hurts efficiency. Clearly you'd arrange this runway such that any failures would result in the debris missing the base. For this forum it seems pretty relevant, though, as it's kinda kerbal.

As for the slideway maintenance, yeah, that's an issue to groom it, but it's predicated on a lunar mining base that requires robotic bulldozers that scoop stuff up, anyway. Perhaps they dump the tailings as a layer? Again, it might be not worth looking into, I just remembered reading it, and thought it might be interesting to discuss,  and it has been.

[Stargate525]

6 hours ago, cantab said:

It's still risky. What if a thruster fails? What if a skid hits a bump or rock? What if the computer control has a glitch?

Pretty much my point. And you're going ten times faster than the shuttle (again, twice the speed of the concorde's cruising speed). Time to correct an error is nil.

[tater]

Yeah, it's incredibly tight, I agree. 

Of course Kraft Ehricke was no slouch, so it's fair to say he considered the obvious drawbacks.

There is a 2d volume as well (Dynamics).

[PB666]

I agree that the flight dynamics are very tricky, I'm just remembering that the turn diameter for a 747 at cruising altitude is around 25km, getting a precision course at 7 times that speed and 45 times the kinetic energy is  . . . . .

A runway to get a airplane to take off of about  160 kts is about 1.5 km, if you wanted about the same G forces would have to be about 60 km long, but there are no theoretical limitations.

If one then set up a railway with electromagnets on that track and had superconducting magnets on the ship you could Maglev the ship in.

The problem is this take the moon, a minimal eccentricity to land, lets say we found high ground so it avoids many low spots, but still we are talking about an apo/peri difference of at least a few kilometers, Ok so no we a 60 km long strip and the greatest impact forces are going to be at contact, which means to say the magnetic repulsion is going to have to be greatest as the ship reaches the track.

E_diff = 1/2 (V_diff (Sin _{diff theta}))² So that the part of the track which the ship joins has to be exactly the same shape and position as the ship orbits (which places the track necessarily on the equator). But if we consider that the apo/pe difference is likely to be 10 or 20 km minimum and the track is 45 miles long, then that end of the track is going to be way off the ground.

I agree though, I think this is a better solution than a space elevator. The problem with a lunar space elevator is that the orbit is so slow, that the counter mass somewhere 100000 kilometer away from the moon to balance the weight of the wire really makes this is not feasable.

OTOH accelerating a ship even if from 0 to say 300 m/s on a track saves removes a considerable amount of the most expensive dV, the dV spent while the ship is verticle fighting  gravity  while it is hoovering over the ground trying to gain altitude to clear down stream obstacles. If you can give a ship 50 to 100 m/s vertical ascent velocity you can spend the vertical ascent velocity as the sin of a small angle along a horizontal acceleration vector, which means very little is wasted.

 

9 hours ago, Stargate525 said:

Pretty much my point. And you're going ten times faster than the shuttle (again, twice the speed of the concorde's cruising speed). Time to correct an error is nil.

The space shuttle had to correct for problems that a moon slider would not.

1. The shape of the outer atmosphere
2. Winds aloft
3. Interactions of the airfoil that differed from trip to trip (weight changes, heat shield degradation and replacements, etc)

These do not occur in the vacuum of the moon.
 

[fredinno]

 

19 hours ago, tater said:

A few things, the slider shown is just the art in the book. You could just as well have multiple thrusters on that as well for failures. I think this technique was proposed exclusively as a lifter for lunar-mined propellant transfer to orbit, so the whole thing is an ascent stage (so if it lands partially full, it has an abort mode built-in). As such a cargo vehicle, it has no need of a crew, so no crew is at risk. Honestly, any reusable vehicle designed to lift ISRU propellants should certainly be unmanned anyway, as every kg of crew compartment hurts efficiency. Clearly you'd arrange this runway such that any failures would result in the debris missing the base. For this forum it seems pretty relevant, though, as it's kinda kerbal.

As for the slideway maintenance, yeah, that's an issue to groom it, but it's predicated on a lunar mining base that requires robotic bulldozers that scoop stuff up, anyway. Perhaps they dump the tailings as a layer? Again, it might be not worth looking into, I just remembered reading it, and thought it might be interesting to discuss,  and it has been.

tailings would have to be crushed.

5 hours ago, PB666 said:

I agree that the flight dynamics are very tricky, I'm just remembering that the turn diameter for a 747 at cruising altitude is around 25km, getting a precision course at 7 times that speed and 45 times the kinetic energy is  . . . . .

A runway to get a airplane to take off of about  160 kts is about 1.5 km, if you wanted about the same G forces would have to be about 60 km long, but there are no theoretical limitations.

If one then set up a railway with electromagnets on that track and had superconducting magnets on the ship you could Maglev the ship in.

The problem is this take the moon, a minimal eccentricity to land, lets say we found high ground so it avoids many low spots, but still we are talking about an apo/peri difference of at least a few kilometers, Ok so no we a 60 km long strip and the greatest impact forces are going to be at contact, which means to say the magnetic repulsion is going to have to be greatest as the ship reaches the track.

E_diff = 1/2 (V_diff (Sin _{diff theta}))² So that the part of the track which the ship joins has to be exactly the same shape and position as the ship orbits (which places the track necessarily on the equator). But if we consider that the apo/pe difference is likely to be 10 or 20 km minimum and the track is 45 miles long, then that end of the track is going to be way off the ground.

I agree though, I think this is a better solution than a space elevator. The problem with a lunar space elevator is that the orbit is so slow, that the counter mass somewhere 100000 kilometer away from the moon to balance the weight of the wire really makes this is not feasable.

OTOH accelerating a ship even if from 0 to say 300 m/s on a track saves removes a considerable amount of the most expensive dV, the dV spent while the ship is verticle fighting  gravity  while it is hoovering over the ground trying to gain altitude to clear down stream obstacles. If you can give a ship 50 to 100 m/s vertical ascent velocity you can spend the vertical ascent velocity as the sin of a small angle along a horizontal acceleration vector, which means very little is wasted.

 

The space shuttle had to correct for problems that a moon slider would not.

1. The shape of the outer atmosphere
2. Winds aloft
3. Interactions of the airfoil that differed from trip to trip (weight changes, heat shield degradation and replacements, etc)

These do not occur in the vacuum of the moon.
 

You can still use your engines to correct your trajectory.

[PB666]

35 minutes ago, fredinno said:

 

tailings would have to be crushed.

You can still use your engines to correct your trajectory.

corrections are not easy, close to the landing site.

[GoSlash27]

I didn't see anybody mention this, so I'll put it out there:

 The Moon is tidally locked to Earth. There is therefore no synchronous orbit within it's sphere of influence, so a lunar elevator is impossible.

Best,

-Slashy

[tater]

Slashy, the Lagrange points L1 and L2 work for the counterweight.

[FleshJeb]

On 2/11/2016 at 4:03 PM, cantab said:

And a runway dozens of kilometres long that needs to be kept at a high degree of flatness and free of significant rocks, on the dusty, rocky, bumpy Moon with all the difficulties of working outside there. While every time something lands the very *design* means the lander trashes said runway. That's going to cost an absolute bomb to build and maintain, it probably makes disposable rockets look cheap.

I agree that the rest of the idea is far-fetched. However, I'm a land surveyor, and this is entirely possible. I could design the systems and procedures necessary to make it happen. We already do machine-controlled grading to a very high degree of accuracy here on Earth. In an airless environment, laser-leveling would be much easier.

[cantab]

Possible, sure. But you give me an estimate of the cost to grade 40 kilometres (and whatever width you think is appropriate) here on Earth, never mind on the Moon. That's the distance you need to stop from lunar orbital speed with a deceleration of ten gees.

[sgt_flyer]

rechecked a bit the last posts @tater - in vacuum, there's one thing you want to avoid at almost all costs - High friction. (due to using wheels + mechanical brakes or skids) - because friction transforms kinetic energy into heat - without an atmosphere, you don't have convection to help you dissipate the heat.

so that would leave mostly magnetic based braking.

still - even a very long maglev surface (even longer than 40km - there are several Maglev projects here on earth that are 200km+ long) would surely cost much less than a space elevator hanging from the Earth - moon lagrangian point  - besides, under the moon's gravity, you could build the thing on top of bridge pillars much more spaced and high than what's possible here on earth. 

Edited February 12, 2016 by sgt_flyer

[tater]

The paper I have is a general paper about his vision of a lunar mining base. Given the robotic earth movers, grading the strip is not a big deal, it only takes time (as the equipment needs to be there anyway, heck, the flat surface could be the first long strip actually mined.

[Rakaydos]

5 hours ago, sgt_flyer said:

5 hours ago, sgt_flyer said:

@tater

@tater

 

still - even a very long maglev surface (even longer than 40km - there are several Maglev projects here on earth that are 200km+ long) would surely cost much less than a space elevator hanging from the Earth - moon lagrangian point  - besides, under the moon's gravity, you could build the thing on top of bridge pillars much more spaced and high than what's possible here on earth. 

For a single ribbon cable, 100 tons of Spectra fiber is enough to span the entire length, with enough left over to minimise the nessisary counterweight. Two MCT launches could put up a double ribbon for redundancy, and away you go.

[PB666]

17 hours ago, GoSlash27 said:

I didn't see anybody mention this, so I'll put it out there:

 The Moon is tidally locked to Earth. There is therefore no synchronous orbit within it's sphere of influence, so a lunar elevator is impossible.

Best,

-Slashy

There is an L2 point on the far side of the moon, problem is that you have to get really far from the moon to have enough inertia to keep a line from falling into the moon essentially you would need to be on the edge of earths SOI. Its just a bad idea. If you want a space elevator you need it on a fast spinning small moon, even then it is questionable.

 

[Rakaydos]

9 minutes ago, PB666 said:

There is an L2 point on the far side of the moon, problem is that you have to get really far from the moon to have enough inertia to keep a line from falling into the moon essentially you would need to be on the edge of earths SOI. Its just a bad idea. If you want a space elevator you need it on a fast spinning small moon, even then it is questionable.

 

L1 point is closer. 100 tons of narrow ribbon, basted on the liftport numbers.

[FleshJeb]

20 hours ago, cantab said:

Possible, sure. But you give me an estimate of the cost to grade 40 kilometres (and whatever width you think is appropriate) here on Earth, never mind on the Moon. That's the distance you need to stop from lunar orbital speed with a deceleration of ten gees.

On Earth, $80-160 million, (assuming costs of $1-2/cubic meter). Maybe double it for the ultra-high accuracy required. I'm sure we could find a good spot: http://wms.lroc.asu.edu/lroc/view_rdr/WAC_CSHADE The civil engineering is the easy part. However, I can only see it handling about one landing a month, given the time it would take to re-tune the surface.

Terminal guidance and control would certainly be challenging, but doable with laser reference stations on the ground.

Finding the flipping UNOBTANIUM for the skids is where this ALL falls apart.

Better off just carrying the fuel and doing a standard landing.

[fredinno]

19 hours ago, FleshJeb said:

On Earth, $80-160 million, (assuming costs of $1-2/cubic meter). Maybe double it for the ultra-high accuracy required. I'm sure we could find a good spot: http://wms.lroc.asu.edu/lroc/view_rdr/WAC_CSHADE The civil engineering is the easy part. However, I can only see it handling about one landing a month, given the time it would take to re-tune the surface.

Terminal guidance and control would certainly be challenging, but doable with laser reference stations on the ground.

Finding the flipping UNOBTANIUM for the skids is where this ALL falls apart.

Better off just carrying the fuel and doing a standard landing.

Maybe use Carbon fiber covered titanium?

Edited February 13, 2016 by fredinno

[SomeGuy123]

What exactly is wrong with maglev?  You can even fine tune your control circuitry to make up for small variances in the position of the maglev track segments.  No physical contact, you can even store the energy of orbit spaceflight or use it for something.  (if you were really clever and used all superconductors, you could dump the energy of decelerating one spacecraft to launch another.)

 

 

 

[sgt_flyer]

2 hours ago, SomeGuy123 said:

What exactly is wrong with maglev?  You can even fine tune your control circuitry to make up for small variances in the position of the maglev track segments.  No physical contact, you can even store the energy of orbit spaceflight or use it for something.  (if you were really clever and used all superconductors, you could dump the energy of decelerating one spacecraft to launch another.)

 

 

 

thought of of maglev deceleration  too (at least,a variation of it, with a 'maglev car' that has a deployable docking system , which would have to match the incoming spacecraft speed's, catch it then slow both of them down - simply to give a wider clearance for the spacecraft -can work in reverse too - and the maglev car itself could be used for rapid transit between the two extremities of the very long track))

basically, the interrogation i was given is this :

Can your spacecraft finetune his orbit with enough precision (with all N-Body problems) to get his periapsis just above your maglev track without risks of hitting the moon (because slightly too sideways or too low)   

at least, if you can have a precise enough orbit, if the maglev's track malfunction during the spacecraft approach, you would simply let the spacecraft continue on it's original orbit .

[SomeGuy123]

4 hours ago, sgt_flyer said:

thought of of maglev deceleration  too (at least,a variation of it, with a 'maglev car' that has a deployable docking system , which would have to match the incoming spacecraft speed's, catch it then slow both of them down - simply to give a wider clearance for the spacecraft -can work in reverse too - and the maglev car itself could be used for rapid transit between the two extremities of the very long track))

basically, the interrogation i was given is this :

Can your spacecraft finetune his orbit with enough precision (with all N-Body problems) to get his periapsis just above your maglev track without risks of hitting the moon (because slightly too sideways or too low)   

at least, if you can have a precise enough orbit, if the maglev's track malfunction during the spacecraft approach, you would simply let the spacecraft continue on it's original orbit .

Remember you do have a little leeway.  On approach you'll have extremely exact positioning.  Laser beams from near the mouth of the maglev track, phase detection of multiple encoded beams, markers on the lunar terrain leading up to the track - there are a lot of methods, and you could use several methods to make sure you're "in the pipe" on approach.  An abort would just be an RCS firing so you miss the track by flying a few 10s of meters above it.  

You don't have to have a perfect solution of the N-body problem, merely one good enough that you can correct any small errors on approach before you actually have to make contact with the receiver track's magnetic fields.  And it doesn't have to be perfect - unlike the ski example, where you probably have to touch the ground with a tolerance of less than a millimeter or risk destroying your skis or bouncing into space - you have a little more tolerance with magnets.  

And even doing this routinely is possible.  You would make the digital systems that do this all run on single solid state chips.  You'd use FPGAs for perfect, clock by clock timing.  You'd have 3 or more parallel digital systems all wired to the sensors.  (and majority gate decisions)  You'd have enough parallel sensor systems that if several fail to give accurate or consistent data the faulty data would be rejected automatically.  (it's as simple as calculating the position and velocity reported by each system and rejecting the ones that don't agree precisely with the majority)

All this would physically fit on a single circuit board with a digital bus that connects to the sensors.  

Making computers this reliable is expensive but routinely doable.  The glitchy computers you are used to using had their software made in a rush, they have vastly more features, they may use a wide variety of hardware, they update the hardware chips in use every couple years, and so on.  

I'd say the technical challenge is the magnetics (a maglev railway able to receive something at over a kilometer a second?  Wow. )  and getting enough construction machinery to the Moon and operating it for years to build something this major.

You'd basically need self replicating machinery and a massive industrial base on the Moon itself.

 

Edited February 14, 2016 by SomeGuy123

[SomeGuy123]

Anyways, I do think the magnet railway is a great idea.  Having something leave the Moon at orbital velocity (just too low for sustained flight, there would have to be a correction burn after launch to fix your apo and periapsis) without a puff of propellant consumed is how you do something long term.  Only n00bs burn propellant - master space engineers avoid it whenever possible.

  There are far less points of failure - a track segment failure on the magnetic railway could result in some nasty wrecks, but most of them probably will leave most of the railway intact.  You can do maintenance on it.  Unlike an elevator ribbon, a shut down railway with faults can be taken apart whenever you want - it isn't under perpetual tension.