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

[sgt_flyer]

@cantab

quick note on the moon quench gun - if you are shipping stuff from the moon back to earth, shouldn't it be possible to directly send it to earth without needing to circularise around the moon (or even needing to spend any dV for the spacecraft outside of RCS attitude) ? as the moon is tidally locked to earth, you only have to wait for the correct time on earth to fire the quench gun, and accelerate the payload directly on a path towards atmospheric reentry. (or if you just want to ship to leo, limit a bit the gun power, so the payload aerobrake , then once you lowered you orbit enough, circularise in leo - for much less dV than having to do the transfer manoeuvers.)

 

[YNM]

21 hours ago, peadar1987 said:

Not really. A bridge like that would be primarily loaded by bending moments. A space elevator would be loaded in tension. If you're going to have a big structure, tension is pretty much the best loading mode to have. That's why the space elevator is hovering around the edges of possibility, while a single-span Atlantic bridge is outright impossible.

Any bridge is also getting tension. Bending do occur, but so your space elevator will be (guess some sort of coriolis effect/force on the lifting cars ?).

20 hours ago, sgt_flyer said:

An active structure, that's another story though  (You basically have a giant maglev continuously accelerating mass so it has a ballistic trajectory 'higher' than the track's curve - so such a structure would be held through tension too (as the accelerated mass would tend to 'press' upwards onto the magnetic track). You'll just need tethers on the side of the track to prevent sideway movements - that's the launch loop concept. (Because a launch loop could be also used as a bridge as well as an orbital launch system  - start acceleratng on a separate track, match the speed and 'latch' onto one of the outgoing maglev elements - the maglev element would transport you at very high speeds towards the other end - where you 'unlatch' the maglev to get onto a slow down separate track.

No matter how you have birds flying in your semi-trailer, the weighing station would know whether you carried too much bird or not.

[peadar1987]

46 minutes ago, YNM said:

Any bridge is also getting tension. Bending do occur, but so your space elevator will be (guess some sort of coriolis effect/force on the lifting cars ?).

In fact, the bridge is more likely to be experiencing a net compressive load, if it is in any way arch shaped. In reality the bending moment will lead to it having a compressive force on the top face and a tensile force on the bottom face.

In any case, I was talking about the primary loading. Of course a bridge will have elements of compressive or tensile forces on it, thermal expansion of the deck, and so on, but its main form of loading is in bending, as you have the self-weight of the bridge and whatever you put on it, acting over a huge lever arm because of distance to the piles. That's why things like suspension bridges have towers, it's essentially to create another long lever arm to provide the resisting moment and reduce the stress on the deck (among other things).

Likewise, a space elevator will experience some bending because of things like wind loading, radiation pressure, or even gravitational interactions with the moon, but the vast majority of the loading on the structure will be in line with its support, leading to a tensile load.

The tensile load will scale with the length of the cable, due to the cable's self-weight. A bending load will scale with the square of the length of the cantilever, due to the self-weight of the beam increasing with length, and also due to the bending moment increasing with the length of the lever arm.

[Laie]

7 hours ago, sgt_flyer said:

if you are shipping stuff from the moon back to earth, shouldn't it be possible to directly send it to earth [...]?

Yes, but.

I don't think a cannon from the moon could deliver a payload to a precise location. The landing area will probably be +-10 degrees latitude and +-40 longitude. If that's not good enough for your purposes, you'll have to do a midcourse correction. This doesn't need much fuel, 50m/s will be plenty, but it does require a full-featured spacecraft that can get and change it's bearings.

[SomeGuy123]

5 hours ago, Laie said:

Yes, but.

I don't think a cannon from the moon could deliver a payload to a precise location. The landing area will probably be +-10 degrees latitude and +-40 longitude. If that's not good enough for your purposes, you'll have to do a midcourse correction. This doesn't need much fuel, 50m/s will be plenty, but it does require a full-featured spacecraft that can get and change it's bearings.

You could in principle make this really really simple.  A package of RCS nozzles pointed different directions, a couple of pressurized tanks, a bank of valves, a valve controller, and the guts of an iphone.  (a camera to see the starfield for position references, a radio, IMUs)

Just saying a "full featured spacecraft" makes it sound a lot worse than it is.  It wouldn't be cheap, but it's not that bad.

[fredinno]

 

18 hours ago, Rakaydos said:

Less practical than an earth based elevator? What problems does an earth elevator NOT have, that a lunar elevator has, that makes up for material tech difficulties and space trash around earth?

The Moon has less demand to get stuff on and off of.

 

[Rakaydos]

1 hour ago, fredinno said:

 

The Moon has less demand to get stuff on and off of.

 

I believe that, like orbital refueling infrastructure, an elevator is a "if you build it they will come" type endevor. Low traffic makes low capacity reasonable, and the cost reduction on that low capacity can be gamechanging.

[PB666]

2 hours ago, fredinno said:

 

The Moon has less demand to get stuff on and off of.

 

What is the lunasynchronous altitude, i dont think one exists. You can use earth-moon lagrangian points however. Since the moon is tidally locked to earth.

However consider this, the moon has no atmosphere, surface gravity = 1.62 m/s2 the gravitational accel. u is 4.90E12 and radius is  1738 km, orbital velocity is thus SQRT(u/r) = 1678 m/s. It doesn't take much to accelerate to orbit. If you has a craft with a decent thrust to weight ratio (say 0.45g) all you need to do is take off on a ramp of say 20% 100 meters and you have enough vertical velocity to make orbit before it falls back to the moon.  You don't loose much energy returning to lunar orbit because you don't have drag. Building a ramp 100 meters long 40 meter high and 10 meters wide is alot easier than building a rope to L2. If one could have precision ascent one could literally create a electromagnetic track that accelerates the craft at 3 g followed by an acceleration pretty much along a shallow trajectory 10 to 20% at 0.5 g, very little energy wasted in the accent fighting gravity.

 

[Rakaydos]

1 hour ago, PB666 said:

What is the lunasynchronous altitude, i dont think one exists. You can use earth-moon lagrangian points however. Since the moon is tidally locked to earth.

However consider this, the moon has no atmosphere, surface gravity = 1.62 m/s2 the gravitational accel. u is 4.90E12 and radius is  1738 km, orbital velocity is thus SQRT(u/r) = 1678 m/s. It doesn't take much to accelerate to orbit. If you has a craft with a decent thrust to weight ratio (say 0.45g) all you need to do is take off on a ramp of say 20% 100 meters and you have enough vertical velocity to make orbit before it falls back to the moon.  You don't loose much energy returning to lunar orbit because you don't have drag. Building a ramp 100 meters long 40 meter high and 10 meters wide is alot easier than building a rope to L2. If one could have precision ascent one could literally create a electromagnetic track that accelerates the craft at 3 g followed by an acceleration pretty much along a shallow trajectory 10 to 20% at 0.5 g, very little energy wasted in the accent fighting gravity.

 

Again, it's not liftoff that is the hard part, it's landing. An elevator does that for you too.

[PB666]

Landing is the reverse of liftoff but with a finite end-point, and if you get it wrong it will be the endpoint. Chinese show you how to land on the moon, thats the way to do it, no problem, microprocessors and GPS. But if you are seriously mining the moon, then lift off is going to be more costly if the mats are intended for orbital use.

[magnemoe]

11 hours ago, PB666 said:

Landing is the reverse of liftoff but with a finite end-point, and if you get it wrong it will be the endpoint. Chinese show you how to land on the moon, thats the way to do it, no problem, microprocessors and GPS. But if you are seriously mining the moon, then lift off is going to be more costly if the mats are intended for orbital use.

Finite endpoint as in fresh crater
GPS will have a few issues on Moon and will give an fresh crater. 
US and Soviet landed on moon with probes 4 decades before China, yes something like the Chinese software will work better, my guess they used radar or image for positioning. 

[YNM]

On 02/09/2016 at 6:51 PM, peadar1987 said:

In fact, the bridge is more likely to be experiencing a net compressive load, if it is in any way arch shaped. In reality the bending moment will lead to it having a compressive force on the top face and a tensile force on the bottom face.

In any case, I was talking about the primary loading. Of course a bridge will have elements of compressive or tensile forces on it, thermal expansion of the deck, and so on, but its main form of loading is in bending, as you have the self-weight of the bridge and whatever you put on it, acting over a huge lever arm because of distance to the piles. That's why things like suspension bridges have towers, it's essentially to create another long lever arm to provide the resisting moment and reduce the stress on the deck (among other things).

Likewise, a space elevator will experience some bending because of things like wind loading, radiation pressure, or even gravitational interactions with the moon, but the vast majority of the loading on the structure will be in line with its support, leading to a tensile load.

The tensile load will scale with the length of the cable, due to the cable's self-weight. A bending load will scale with the square of the length of the cantilever, due to the nself-weight of the beam increasing with length, and also due to the bending moment increasing with the length of the lever arm.

 

Yes, I know. But my point is also in the cables. In a suspension bridge, actually the deck only carries little of the strain - the cable does. Using carbon nanotubes in great length can be applied on bridges and space elevator. So, the question is, will a carbon nanotube that long be viable ? It needs to be a single strand all way long... And most rough tensile force estimates for theters puts it outside nanotube's ultimate tensile strength. Now you can add swaying motions, collision... Not a great idea IMO.

[tater]

Krafft Ehricke proposed a Lunar Slide Lander. they'd prepare a long landing strip on the surface (robotic bulldozers, basically) and land via a very shallow approach using a tiny fraction of the dv required for normal landings on the Moon. They'd then, to borrow from KSP, lithobrake, using skids and sliding along the surface. The craft would have apply radial thrust to control the slide, he figured about 120 m/s worth of dv over about 90 seconds.

It's kind of kooky, but a novel idea. Note that the point of this lander is I think to land empty tanks to haul off ISRU derived propellants. It's in the last chapter of the textbook from a class I took in the 90s (Lunar Bases and Space Activities of the 21st Century).

Edited February 10, 2016 by tater

[magnemoe]

1 minute ago, tater said:

Krafft Ehricke proposed a Lunar Slide Lander. they'd prepare a long landing strip on the surface (robotic bulldozers, basically) and land via a very shallow approach using a tiny fraction of the dv required for normal landings on the Moon. They'd then, to borrow from KSP, lithobrake, using skids and sliding along the surface. The craft would have apply downward thrust to control the slide, he figured about 120 m/s worth of dv over about 90 seconds.

 

It's kind of kooky, but a novel idea. Note that the point of this lander is I think to land empty tanks to haul off ISRU derived propellants. It's in the last chapter of the textbook from a class I took in the 90s (Lunar Bases and Space Activities of the 21st Century).

Interesting but sounds a bit unsafe
Note that if you mine and process fuel landing cut into you output it has no other effects. You need to have enough fuel to land and you have to lift this up to orbit first. 
1.72 km/s for takeoff and landing, runway landings would be round two, yes you could use with without coming inn at 1.5 km/s but slower. 

[Stargate525]

I think you'd have a very hard time convincing anyone, including seasoned astronauts, to land a craft at over a kilometer a second. To put that in perspective, you are LANDING at twice the speed of the Concorde's CRUISING speed.

[tater]

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.

 

Edited February 10, 2016 by tater

[wumpus]

I've seen KSP Minmus landers that did this (land on wheels and set the brakes to minimal.  Presumably only useful for landing on the Great Flats).  Unfortunately when I tried it myself the hanger insisted I wanted VAB symmetry.

As far as "outside the box", it looks like a carrier-style landing with a loooong deck (think railroad, not carrier deck).  Not manned*, and the tailhook isn't going anywhere (unlike on a carrier), so maybe it might work.  For extra bonus kerbality, add magnets and try to generate power from the tailhook and use it for a railgun launcher (no way you can store that power).

* Note that considering the huge infrastructure costs, accounting probably wouldn't notice the loss of life.  You still don't want to cause the second great space mutiny.

[tater]

This particular idea was proposed in the context of a regolith mining base. Such a facility would be scraping off large amounts of regolith anyway to separate the Oxides that form most of the regolith. The idea was an 80km long runway of fine sand. Again, pretty kooky, but not entirely impossible, I suppose.

[fredinno]

19 hours ago, Rakaydos said:

I believe that, like orbital refueling infrastructure, an elevator is a "if you build it they will come" type endevor. Low traffic makes low capacity reasonable, and the cost reduction on that low capacity can be gamechanging.

Orbital refueling can be used to refuel space tugs- and that's something we already kind of need for GEO transport. It's also nowhere as expensive as a Space elevator.

1 hour 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.

 

Those landers are likely still not cheap.

[tater]

No, none of it is cheap. It's just that seeing the talk about ramps, raglans, and other novel ideas instead of an elevator made me remember the slider, and I thought people might be interested in it. Dr. Ehricke died shortly after that paper was accepted, so the idea sort of died with him.

[Stargate525]

2 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.

Okay, then you can chalk 'is usable by people without killing them horribly' onto the list of benefits of an elevator.

[tater]

14 minutes ago, Stargate525 said:

Okay, then you can chalk 'is usable by people without killing them horribly' onto the list of benefits of an elevator.

A lunar elevator would involve trips of possibly weeks.

[Stargate525]

1 hour ago, tater said:

A lunar elevator would involve trips of possibly weeks.

And as we all know the human body is perfectly capable of surviving a 1km/s landing onto regolith, but has an unfortunate lifespan of five days...?

[tater]

11 minutes ago, Stargate525 said:

And as we all know the human body is perfectly capable of surviving a 1km/s landing onto regolith, but has an unfortunate lifespan of five days...?

You seem to be under the impression that the vehicle impacts normal to the surface at that velocity. The idea he posited was that the craft could impact the surface at a tiny vertical velocity because it's at such a shallow angle. The same can be said of, say, the Space Shuttle. Shuttle landed at about 100 m/s, and did just fine. If it had done so in a vertical dive, the outcome would've been somewhat different.

The idea was in fact a rolling landing, minus the wheels, which would be replaced with large skids. Whereas an aircraft uses lift combined with throttle to control sink rate, this craft would use rockets to control the sink rate, and would use them continuously during the skid to keep the nose up, and the skids optimally contacted to the surface.

I'm not even suggesting it works, we have no idea. It was a novel suggestion, however, and interesting to consider.

Edited February 10, 2016 by tater

[cantab]

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?