Realistic solutions to the difficulties of SSTO?

GCNRevenger · 2005-10-05 19:39:25

Yes, I am dismissing it out of hand because of the difficulty of suborbital orbital rendezvous. Do not be decieved by the elegance of the physics, because they do not tell the story of how hard it will be to engineer.
To me, the engineering difficulties of rendezvous are child's play when compared to a 35000+ km climbable 60+ GPa tether - one end of which has to survive Earth's atmosphere, ionosphere, biosphere, etc.
The biggest issue about the rendezvous envelope is the accuracy of the rendezvous: today the closest technology is the ballistic missile defense system(s)
It isn't like the location of the tethers is going to be a surprise. Why isn't it more like orbit matching with the ISS?
And then, how do you actually effect the docking? The window that the vehicle will be close and slow enough will only last a few moments, maybe even less then one second... and thats assuming that its within centimeters of the catchment and the vector is within a few degrees & centimeters per second.
Catchment is an issue, but I think it's less than you're making out - most of the challenge is making it low weight. Once you've matched orbits, you could have multiple opportunities to catch a tether - just wait until it comes around again. The tether could end in a fan, a curtain or even a net of guidewires to give a big target area that's reeled in for docking.

I don't think its going to happen, its just too hard to do on a regular basis with any degree of reliability. If we do have the technology and reasouces to try and devise such a method, then it would be much better spent on just developing a space elevator.
Perhaps it's an issue of resource competition, but I think orbital transfer tethers are a couple of orders of magnitude less expensive than a space elevator. It's a side project by comparison. In fact, isn't it desirable to try our hand at building and operating 350 km 5 GPa tethers before trying for the big guy?
Don't get me wrong, I would love to see surface-to-space elevators happen, but we might not be able to build them for a long time - they could easily become a fusion energy technology: always 25 years away from being realized. To me, orbital exchange tethers look like a good stepping stone with an ROI timeframe that could interest the private sector. And that seems crucial to me so that we can get out of this "the politicians can only handle one project at a time, don't confuse them" mindset.

Nonsense, the engineering associated with the space elevator will be fairly easy, the cable has already been designed and the most complicated part will simply be a sturdy winch at GEO. Climbers should be easy enough, and so the only big engineering hurdle is the power supply for them.

The only thing preventing a space elevator from going up right now is that the CNT composite the cable will be made from isn't yet available, but this is a field that is advancing quite rapidly, and it may just be a matter of a few years. Corrosive oxygen at lower altitudes can easily be defended against with an atomic-thick coating of aluminum or gold, and the cable itself is not conductive enough even with an ultra-thin metal coating to be a lightning problem. The incredible strength of the CNT material, provided there is enough headroom, should be able to resist a little wind trouble too.

And if the cable breaks? No problem, just drop a spare spool of cable from the GEO station, have a helecopter or a boat snag it, and reconnect to the base station. Its expensive plastic, but it is just plastic.

But a rotary tether like the MXER system...

You will know the orbit of the tether, reasonably well: you have to likly get within a few meters of the cable end best-case, which means you not only need to know its location with meter accuracy, but also its vector to within a few degrees and probobly centimeters per second.

It is nothing at all like orbital synchronization with the ISS, because of the extreme constraint on time involved, that not only must you allign your trajectories to within meters or perhaps even centimeters, but you have to do it in only seconds. Maybe even less than one second! This is exponentially harder then simple orbital rendezvous, and will never ever be practical for regular flights.

You aren't going to have multiple runs during the same orbit either, since your launch vehicle won't be in orbit (!). Because it is in a suborbital trajectory, and you will be mating with the tether at apogee, you are only going to get one breif pass before the tether moves away in its eliptical orbit.

Using a rotary tether purely for escape from orbit is not a much better situation, because the tether will be in a considerably higher orbit and hence a much different orbital velocity too. Again, because of the large velocity miss-match, you are only going to get one breif pass before the tether wizzes away from you over the horizon: remember, we aren't talking kilometers here, we're talking meters. Its a very big sky.

Catchment is a much harder issue then you think too. For this to be possible, you can forget about centimeter accuracy rendezvous, so you are going to have to have some method that doesn't require a "direct hit." A "net" is out of the question, all those hanging cables would be death to an unfortunate vehicle that comes in a little too fast. It couldn't get out of the way of the cables without a powerful maneuvering burn, and would impact them pretty hard... consider that the nose of Delta-IV/Ariane/Atlas are made of thin and fragile carbon composite.

On the balence, I think your comparisons between the rotary tether and the space elevator are entirely irrelivent because they are so different. The latter relies on a space vehicle with extreme accuracy in both position and velocity in a very big sky, will never have a high throuput even if it did work, and requires a very large launch investment for what you get.

The space elevator on the other hand is a cinch, the engineering is very simple, infinitely simpler then the rotary tether, the difference there is completly night-and-day. They are entirely different animals in this reguard, and the only thing that is holding back an elevator today is the materials science, which although is not there yet, it is now within sight. And if it works, it will literally open the sky to us, and will provide a true railway to the rest of the solar system. Even if a rotary tether works, it will not, they are a "stepping stone" to nowhere.

I can't stress this enough with only one paragraph, that the space elevator is a "total" solution, which will change the rules for launching payloads all the way from the ground into high orbit and to move freight to the Moon and Mars. Even assuming the tether works, it will only make ground launch a little easier at the expense of very difficult engineering, which makes it so much less attractive then the true elevator that it just isn't worth bothering with.

I also think that you are speaking from a position of ignorance about the advances in CNT composites, we know what material we need now, and we have made large advances in just the past few years. It really is possible that materials of sufficent strength will exsist by October 2015.

Again, you make it sound like the engineering will be pretty easy for a tether when this is clearly not the case, and a space elevator is inherintly simple to engineer, with the only thing holding it back is the insufficent strength of CNT composites, which are improving rapidly.

noosfractal · 2005-10-06 01:41:01

The only thing preventing a space elevator from going up right now is that the CNT composite the cable will be made from isn't yet available.

This is no ordinary composite though. Unprecedented nanoscale perfection is required to provide the required strength. It's like growing a silicon wafer the size of America. Undoubtedly doable one day - but I think it is going to take Drexleresque nanotech.

Corrosive oxygen at lower altitudes can easily be defended against with an atomic-thick coating of aluminum or gold

Perhaps a little thicker because of climbers constantly rumbling by? Again, I'm thinking nanotech self-healing is going to be required. Perhaps every second climber can be a repairbot, but I think that is going to slow your throughput.

the cable itself is not conductive enough even with an ultra-thin metal coating to be a lightning problem.

Unless it's wet? Actually, my understanding is that CNTs are in fact excellent conductors even when dry. Lightning isn't a minor problem.

And if the cable breaks? No problem, just drop a spare spool of cable from the GEO station, have a helecopter or a boat snag it, and reconnect to the base station. Its expensive plastic, but it is just plastic.

At a billion a piece and multi-month deployment time, I'm thinking your insurance bill is going to be impacting your $/lb to GEO.

Climbers should be easy enough, and so the only big engineering hurdle is the power supply for them.

Right now people are talking about MW lasers beaming power to the climbers, but this is not a settled technology. Once again the atmosphere is going to cause you all kinds of trouble.

Things get easier if you start with a 12 mile high tower, but I'm thinking that is going to effect your budget as well.

But a rotary tether like the MXER system...

You will know the orbit of the tether, reasonably well: you have to likly get within a few meters of the cable end best-case

We can improve on this. If you're allowed lightning-proof nanocable, I'm allowed a 100 meter catch zone.

It is nothing at all like orbital synchronization with the ISS, because of the extreme constraint on time involved, that not only must you allign your trajectories to within meters or perhaps even centimeters, but you have to do it in only seconds. Maybe even less than one second!

Again, this time frame can be extended. You're right if we're talking about a bare dumb cable end whipping past at kilometers per second, but the catch mechanism is going to be pro-active and the tether can be lengthened and shortened from the center to make the catch window longer.

You aren't going to have multiple runs during the same orbit either, since your launch vehicle won't be in orbit (!). Because it is in a suborbital trajectory, and you will be mating with the tether at apogee, you are only going to get one breif pass before the tether moves away in its eliptical orbit.

Even if we can't arrange more than one try with a single tether, again we could put tethers in series. Perhaps this could even be used to raise throughput.

A "net" is out of the question, all those hanging cables would be death to an unfortunate vehicle that comes in a little too fast.

We're assuming an attempted rendezvous, so the delta in velocity is going to be managable. I'm thinking lightweight guidewires - just something to assist the main docking. The vehicle is going to have to be designed for catching.

On the balence, I think your comparisons between the rotary tether and the space elevator are entirely irrelivent because they are so different.

They are different except for their crucial stat: $/lb to GEO. Both are targetting $100/lb. This is what will open the sky.

I can't stress this enough with only one paragraph, that the space elevator is a "total" solution, which will change the rules for launching payloads all the way from the ground into high orbit and to move freight to the Moon and Mars. Even assuming the tether works, it will only make ground launch a little easier at the expense of very difficult engineering, which makes it so much less attractive then the true elevator that it just isn't worth bothering with.

I agree that the surface-to-GEO elevator is a better option once it is available - just like I think that fusion is a better energy generation option once it is available. MXER is "only" fission, but that is nothing to sneer at. It's an interesting option if the elevator build date gets pushed to next century.

I also think that you are speaking from a position of ignorance about the advances in CNT composites, we know what material we need now, and we have made large advances in just the past few years. It really is possible that materials of sufficent strength will exsist by October 2015.

I'm not a materials researcher, but this is an area that I follow. The 2015 date is possible (5% probability - lucky breaks all round), but I'd say its even odds that manufacturing difficulty goes up in proportion to length. Everyone is assuming a logrithmic increase in difficulty, but we're at the limits of chemical bonds here. I think the required perfection is going to be hard to achieve and then hard to maintain in atmospheric conditions. The rotating tethers just need a clever catching mechanism. I think it's way more likely we'll have that first.

Austin Stanley · 2005-10-06 02:17:00

Dear God I hope Drexleresque nanotech isn't required, because we aren't having that... ever. I would put good money on current and future impossibility of machine-phase matter or any other such nonsense. Drexler is a smart guy, no doubt, but he doesn't know ^#(@ about chemistry.

I wouldn't worry to much about lighting strikes though. The conductivity of the cable is not necessarily a nano scale phenominon, at least not when you are comparing the macro-cable to macro-lighting. Proper-construction could insure that the cable was non-conductive to lightining strikes. Ancoring the cable in a calm area and a system of lighting rods could also help.

As for mono-atomic oxygen I wouldn't worry to much about the corrosive effect. It's very sparse and so could not eataway at the cable to quickly in any event.

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I think it's important to point out that the Macro-scopic cable can (and should) have very diffrent properties then a Microscopic nanotube. The cable can encoporate diffrent materials into it's structure and can utilise diffrent typse of nanotubes to achive diffrent effects along it's length and cumutivly.

noosfractal · 2005-10-06 04:25:10

I wouldn't worry to much about lighting strikes though. The conductivity of the cable is not necessarily a nano scale phenominon, at least not when you are comparing the macro-cable to macro-lighting. Proper-construction could insure that the cable was non-conductive to lightining strikes.

Even when it's wet?

As for mono-atomic oxygen I wouldn't worry to much about the corrosive effect. It's very sparse and so could not eataway at the cable to quickly in any event.

Perhaps less sparse in the Ozone layer?

I think it's important to point out that the Macro-scopic cable can (and should) have very diffrent properties then a Microscopic nanotube.

The problem is that all the calculations are assuming that the 100+ GPa micro-property can translate into a macro-property. But my understanding is that 3-5% of theoretical performance is considered standard in a composite, with 12-14% being the rarified heights. Here we're asking for somewhere between 50% and 100% of theoretical. The closer you want to 100%, the closer your macro-cable has to be to pure, unbroken CNTs. And in fact, there has to be a good reason for anything else because it adds weight without strength.

Isn't NASA's tether competition coming up soon? It'll be interesting to see how far along we are.

GCNRevenger · 2005-10-06 06:21:23

A 100% contiguous nanotube 36,000km long isn't nessesarry, a cable with tubes perhaps centimeters or meters long should be sufficent because it is fairly easy to join the tubes with covalent bonds to cross-link them chemically. We can do that today right now with shorter nanotubes.

The cable won't be pure nanofiber either, it will be a composite with some other polymer, which will largely interrupt the high conductivity of the cable. Yes it will be much more conductive then just pure matrix polymer, but not compared to copper or aluminum!

A layer of gold atoms a few atoms thick would only be needed at higher altitudes to protect against corrosive non-diatomic oxygen species, so even if it were a conductivity problem (which it shouldn't be) you would only need it "above" where lightning strikes. And if it does rub off after a few weeks or even days? Big deal, send up a climber with a vapor sprayer, problem solved.

As for water, yeah, a little water shouldn't be a conductivity problem. Pure water, like the kind that condenses from clouds, is largely free of ionic impurities and is hence a pretty poor conductor of electricity. 100% pure water hardly conducts at all.

But even if something terrible happend and the cable were to break, nothing would happen because the counterweight is in a stable orbit independantly. If you have launched >100,000km of cable to build the elevators' two main lines and the escape velocity line above GEO, then dropping an extra 30-50km now and then is not a big deal. The insurance people will love the elevator, because if there is a problem with it, it doesn't explode into a pretty fireball just off the Cape' if there is a little problem. Even if they don't trust it, the economics will crush all other forms of launch effortlessly.

I think beamed power from a laser is a sufficently settled technology, its just a matter of scale. The USAF's ABL can fire a laser from its small nose-mounted telescope and keep the beam reasonably confined to ~1m dia. at least 15mi away despite atmospheric scattering. Large FEL lasers are less than an order of magnetude smaller then what is needed. Electricity is pretty cheap compared to rockets.

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No, I don't think you can simply wave your arms and assume you can know an objects' orbit and vector to within meters in a 80,000,000m orbit, wizzing along at 40,000km/hr. This is a fundimental and difficult problem due to the inherint way that radar operates (scans), the variability of the Earth's atmospheric height, and gravitational perturbations.

"this time frame can be extended"

No it can't, because if you go and start changing the length of the cable, you also change its linear velocity. Now you have a two-variable problem, which makes rendezvous harder still. It will also have little lateral give at all either, which leaves one dimension extremely dependant on the vehicle.

"Even if we can't arrange more than one try with a single tether, again we could put tethers in series. Perhaps this could even be used to raise throughput."

$$$, and if you are dealing with a suborbital vehicle, then even though you do have multiple tethers, you will still only get one pass with optimum cargo because apogee should coincide with the perigee of only one cable.

You are also deeply underplaying the difficulty of building a practical catchment mechanism; this is not like an aircraft carrier where you get a solid minute to line up with a carrier only moving 100-200km/hr faster then you, you are only going to get a couple of seconds tops with the cable end and its going to be moving FAST. Oh yeah, and it isn't convienantly sitting flat either. This is a fundimental engineering problem simply because of the constraints involved, which will never be overcome by "cleverness."

$100/lbs to orbit with the MXER system? No way, it could never achieve a high enough throuput for a reasonably investment. This is a wild claim to try and sound competitive to an elevator. Dont' forget the suborbital vehicle cost.

noosfractal · 2005-10-06 18:45:54

A 100% contiguous nanotube 36,000km long isn't nessesarry, a cable with tubes perhaps centimeters or meters long should be sufficent because it is fairly easy to join the tubes with covalent bonds to cross-link them chemically. We can do that today right now with shorter nanotubes.

Yes but every such bond makes makes the material less like 130 GPa CNT and more like charcoal.

The cable won't be pure nanofiber either, it will be a composite with some other polymer

This is not a plus. The other polymer will be less strong than CNTs, but add weight, lowering the effective strength of the material.

A layer of gold atoms a few atoms thick would only be needed at higher altitudes to protect against corrosive non-diatomic oxygen species, so even if it were a conductivity problem (which it shouldn't be) you would only need it "above" where lightning strikes. And if it does rub off after a few weeks or even days? Big deal, send up a climber with a vapor sprayer, problem solved.

Ah yes, the automated repair bots scooting along at 200 km/hr. What? No? 20 km/hr? 10 km/hr? 1 km/hr? As long as it isn't raining? Maybe $200/lb to orbit.

As for water, yeah, a little water shouldn't be a conductivity problem. Pure water, like the kind that condenses from clouds, is largely free of ionic impurities and is hence a pretty poor conductor of electricity. 100% pure water hardly conducts at all.

It'll be interesting to see how the final cable material interacts with water, won't it.

dropping an extra 30-50km now and then is not a big deal.

Another reason for near-zero taper, which makes it hard to get away with anything under the 63 GPa figure.

I think beamed power from a laser is a sufficently settled technology, its just a matter of scale. The USAF's ABL can fire a laser from its small nose-mounted telescope and keep the beam reasonably confined to ~1m dia. at least 15mi away despite atmospheric scattering. Large FEL lasers are less than an order of magnetude smaller then what is needed. Electricity is pretty cheap compared to rockets.

I love the pro/con matrices for laser beamed power. Pro: high power, no added weight. Con: clouds.

No, I don't think you can simply wave your arms and assume you can know an objects' orbit and vector to within meters in a 80,000,000m orbit, wizzing along at 40,000km/hr. This is a fundimental and difficult problem due to the inherint way that radar operates (scans), the variability of the Earth's atmospheric height, and gravitational perturbations.

Can I assume the existence of GPS?

"Even if we can't arrange more than one try with a single tether, again we could put tethers in series. Perhaps this could even be used to raise throughput."
$$$, and if you are dealing with a suborbital vehicle, then even though you do have multiple tethers, you will still only get one pass with optimum cargo because apogee should coincide with the perigee of only one cable.

Actually, Hoyt's July 2000 paper says you get to try repeatedly, but you have to wait 2.6 days between attempts ...

http://www.tethers.com/papers/JPC00LEOGTO.pdf

You are also deeply underplaying the difficulty of building a practical catchment mechanism; this is not like an aircraft carrier where you get a solid minute to line up with a carrier only moving 100-200km/hr faster then you, you are only going to get a couple of seconds tops with the cable end and its going to be moving FAST. Oh yeah, and it isn't convienantly sitting flat either. This is a fundimental engineering problem simply because of the constraints involved, which will never be overcome by "cleverness."

It isn't just me though. NASA July 2003 report says "In order to realize its vision of lower cost access to space, advancements, but not dramatic breakthroughs, are required." Evolutionary vs. revolutionary. Wonder which the business guys will like more?

http://www.inspacepropulsion.com/tech/M … Report.pdf

$100/lbs to orbit with the MXER system? No way, it could never achieve a high enough throuput for a reasonably investment. This is a wild claim to try and sound competitive to an elevator. Dont' forget the suborbital vehicle cost.

You get an immediate 50% - 85% reduction in launch costs, and then, when justified by demand caused by the booming orbital economy, you can put together a purpose-built sub-orbital delivery vehicle that will make you rich at $100/lb. I'd put the IPO at first quarter of 2012. We can use the profits to fund the ongoing space elevator research

GCNRevenger · 2005-10-06 20:56:40

We don't need 100% of the theoretical SWCNT strength to make an elevator, and right now we can't make contiguous tubes of arbitrary length. Putting smaller but macroscopic tubes, applying mid/long saturated alkyl chain crosslinking to them or something, and then drawing them until anisotropic in a high loading matrix polymer gives us a means of trading theoretical strength for using shorter tubes... which in turn, means a sooner and cheaper elevator. We won't need a million tonnes of the stuff either. It is so a plus, it lets us get away with shorter tubes and provides for better wear protection.

The bulk of the cable should be capable of withstanding the rigors of the space environment without much trouble, and so only periodic inspection and replacement of kilometer-scale segments will be required. High speed photography will make such inspection pretty easy. The sections that will get the most wear will probobly be the last ~50km or so down in the atmosphere, which is practical for a reasonably slow moving repair climber to re-spray or drop new cable on occasion. This is simply not a big problem, particularly with the "Hoytether" design and sufficent redundency.

"It'll be interesting to see how the final cable material interacts with water, won't it."

Considering that it will be coated with either an inert matrix polymer, aluminum oxide, or metalic gold in the atmosphere I think we can take a pretty good guess right now: there won't be any interaction with water. You could even make it water-repellant so contiguous (and conductive) sheets won't form. The matrix polymer could also serve as a substrate for any UV-protecting coating too at higher altitudes.

Very recently though, pure SWCNT sheets have been produced with an extremely simple process, which might eliminate the need for a matrix polymer entirely.

"I love the pro/con matrices for laser beamed power. Pro: high power, no added weight. Con: clouds."

Early elevator climbers can be powerd by lasers from the Earth end and still sustain reasonable annual operational rates. Later climbers could of course be powerd by a laser from the GEO end of the cable, with stored power for the short & slow trip through the atmosphere. Obviously.

"Can I assume the existence of GPS?"

GPS has the same constraint as radar does, that it is not a continuous means of location finding, and considering the very short window you will have and the extreme importance of very accuratly alligning your run, GPS will only be able to give you a rough guess between signal packets. Furthermore, the closer you get to the GPS satelites, the less well they work because of the smaller difference in distance. Then there is the trouble of recieveing the signal while you are flying around the ionosphere for ground launch.

Now for the tether again...

Note what the documents say about the catchment mechanism: almost nothing. Nothing except that it will be a "challenge" and has a low technology readiness level. Frankly, I don't think they have any idea how they will make such a difficult system reliable. A space elevator has no such engineering question mark.

The LEO-GEO tether will permit multiple opportunities, but your launch vehicle upper stage will now have to be powerd and maneuverable as well as modified with a catchment mechanism port and overall stiffer structure... which will decrease your payload. Partially defeating the purpose, isn't it? Combine this with a genuine glut of cheap Russian, Indian, and sooner-then-later Chinese rockets and the difference between a LEO launcher and a GEO launcher won't be anywhere near big enough to justify a multibillion dollar tether setup. If you are talking about the difference between $60M Proton or a $30M Soyuz-II for a $500M satellite, this idea just isn't going to fly.

And I strongly disagree and flatly dismiss your foolish notion that a LEO-GEO tether will "cause a booming orbital economy," because this is obviously not the case. The market for satellite launch is already quite small, and the tether system will not achieve a truely radical paradeigm-changing decrease in costs. Cheap rockets are already plentiful and there is no "booming economy," so neither will there be if you reduce prices a little (don't forget the added bus cost & mass).

I think the "business case" set forward in these linked documents are biased to be pro-tether and is either intentionally or incompetantly ignorant of the realities of a globalized GEO launch market. Oh, and they don't seem at all botherd that the payload for Lunar/Mars transfers is so small as to be useless. 500-1,000kg?

Then there is the SO-LEO tether, which I think is much harder then the LEO-GEO, primarily because you only get one shot with a much brefier and harder allignment time, and that shot doesn't come cheap. The advantage here for a space elevator is so large that putting the two side by side rhetorically really doesn't make much sense. Electric climber versus hypersonic jet or suborbital rocket? And say you could put together a super-jet or Falcon-I class SO rocket, that if you can undercut the foreign competition, you won't make enough money to pay off the multibillion dollar tether development/construction.

I think that it is just a (fairly short) matter of time until someone figures out how to make CNTs of sufficent quality to build an elevator. Given the much larger bennefits that an elevator offers versus a tether, that the money spent on the tether would be better spent on CNT composite and large FEL research.

BWhite · 2005-10-06 21:38:33

For what its worth, GCNRevenger's confidence in space elevators make me take their technical feasibility far more seriously.

That said, with six day trips, simply paying back the interest might make it hard to close the business case.

$1 billion construction costs @ 12% interest amortized over 7 years means a $17.5 million per month mortgage payment. With 5 trips per month (6 days / 30) thats $2.9 million per trip. At $100/lb each climber needs to carry 29,000 pounds of net payload.

Are the climbers that big?

Remember, this excludes insurance, wages, energy, ground handling, etc. . .

It excludes the costs of the car/climber as well which needs to be spacecraft pressurized construction.

Go to $10 billion for construction and you need to carry 290,000 pounds every 5 days to amortize at $100/lb

= = =

Oh, the technical challenges of tethers do appear more significant.

= = =

Without a colony "out there" to support, is there sufficient demand to launch enough payloads to repay the bank the construction costs?

TwinBeam · 2005-10-07 01:29:14

Suppose you've got the center of your rotating skyhook at just 220km, and you want to meet it at 20km - a 200km radius rotating arm. Assume you can meet the tether moving at 300m/sec (~1000km/hr), and that the center of the tether is moving at 8km/sec - so the outer end of the tether has to be counter-rotating at 7.7km/sec.

Let's suppose you don't have to grab exactly the end of the tether - let's say you position yourself 1km above the lowest point the tether will go, so you've got the time for the tether to move 1km down and 1km back up past you. How long is that?

You've got twice the time it takes for the tether to rotate R radians, where 200km(1-cos(R))=1km. 200-200cosR = 1, 200cosR = 199, cosR = 199/200 = 0.995, R = 0.1 out of 2pi = 6.283, so ~1/63 of the rotational period.

Rotating at 7.7km/sec around a circumference of 1257km, the rotational period is 163 seconds. So 163 / 63 = 2.6 seconds. Double that time is about 5 seconds.

Five seconds to grab 1km of tether moving vertically at an average of about 400m/sec. And that's ignoring any horizontal relative motions of the cable. Challenging indeed.

Hmm - what if the bottom of the tether were a loop, held open by small remote-controlled wings around the circumference? The jet could tow a hook on a tether, with the hook also being a remote-controlled wing or kite. As the loop passes, have your hook flying off to the side, so your towed tether crosses the loop-tether, and the hook is dragged by it's tether to catch on the loop. I can see some problems with that - e.g. one of the tethers might be cut - but at least it has some chance of catching...

Of course, your jet is then instantly jerked upward at 400m/sec, and thereafter subjected to a continuous 30 G's. That could be moderated by using a longer tether arm - at 1000km radius, it'd be down to 6 G's, and since the center of mass would be much higher, you could make the tether system have mass equal to your jet, which would cut the acceleration down to 3 G's as you drag the tether into a lower orbit and it drags you up.

TwinBeam · 2005-10-07 01:54:28

A variant on the space elevator, to avoid clouds and inclement weather :

Put a huge high altitude zeppelin up above the weather. Directly power an elevator using a CNT loop, with the winch and power systems down on the ground, to take cargo and supplies up to the zeppelin.

Cover the zeppelin with solar cells and store up power to run lasers mounted on the zeppelin to power a normal climber from that point up without any interruption by clouds. You could also deliver power to the zeppelin mechanically, by rotating the CNT loop of the elevator from the ground when you're not pulling an elevator car up - making more efficient use of the ground power systems, and providing power at night to keep climbers moving when solar powered lasers could not.

If a violent storm ever passes below the zeppelin, you can unlatch the lower elevator loop from the ground and pull it up to the zeppelin to keep it out of harm's way. You could also have multiple tether lines to anchor points a hundred miles apart, so that a local squall doesn't mean all anchors have to be cut loose, and climbers on the higher parts of the elevator don't have to stop (assuming enough solar power can be collected to keep it moving).

noosfractal · 2005-10-07 02:09:07

[Polymer embedding] is so a plus, it lets us get away with shorter tubes and provides for better wear protection.

It's a plus if you can still get the strength you need. If the other polymer is just a weak point, then you've got a problem.

The bulk of the cable should be capable of withstanding the rigors of the space environment without much trouble, and so only periodic inspection and replacement of kilometer-scale segments will be required.

I'll give you replacement of kilometer-scale segments in the atmosphere, but higher up you are dreaming. There is a reason that cable has to be so strong. It'd be easier to replace the whole cable. Maybe if there was a continuous unwinding from GEO so that the entire 35000 km is replaced every few years.

Early elevator climbers can be powerd by lasers from the Earth end and still sustain reasonable annual operational rates. Later climbers could of course be powerd by a laser from the GEO end of the cable, with stored power for the short & slow trip through the atmosphere. Obviously.

Hmmm, I wonder what proportion of the power is required for those first 50 km? What with it being deep in a gravity well and all. I'm not sure you'd want to commit to storing that on board your climber.

GPS has the same constraint as radar does, that it is not a continuous means of location finding, and considering the very short window you will have and the extreme importance of very accuratly alligning your run, GPS will only be able to give you a rough guess between signal packets. Furthermore, the closer you get to the GPS satelites, the less well they work because of the smaller difference in distance. Then there is the trouble of recieveing the signal while you are flying around the ionosphere for ground launch.

I'll collect hyper-accurate positioning info at tether control then use lasers for range finding and communication to and from the payload - a positioning phase lock loop.

Note what the documents say about the catchment mechanism: almost nothing.

How about the net in this May 2005 brochure ...

http://www.nasa.gov/centers/marshall/pd … XER_TS.pdf

Nothing except that it will be a "challenge" and has a low technology readiness level. Frankly, I don't think they have any idea how they will make such a difficult system reliable.

This 2003 document also has a picture of a net ...

http://www.tethers.com/papers/MXERJPC2003Paper.pdf

And note that they've broken up their 100 km into more manageable 10 km lengths. That's the sort of thing you do when you're planning for deployment.

There's a 2004 document I won't link to that's marked ...

Distribution C: Distribution authorized to US Government agencies and their contractors; critical technology; March 2004; Other
requests for this document shall be referred to In-Space Propulsion Office, NASA/MSFC, MSFC AL 35812.
This work was supported by NASA/MSFC Phase I SBIR Contract NAS8-03013. Copyright © 2001-2004 Tethers Unlimited, Inc.
WARNING – This document contains technical data whose export is restricted by the Arms Export Control Act (TITLE 22, U.S.C., Sec 2751 et seq.) or Executive Order 12470. Violation of these export laws is subject to severe criminal penalties.

Sounds pretty serious to me. Someone thinks this is worth pursuing.

Combine this with a genuine glut of cheap Russian, Indian, and sooner-then-later Chinese rockets and the difference between a LEO launcher and a GEO launcher won't be anywhere near big enough to justify a multibillion dollar tether setup. If you are talking about the difference between $60M Proton or a $30M Soyuz-II for a $500M satellite, this idea just isn't going to fly.

$30m x 12 x 10 = $3.6b savings over 10 years, and I don't have the billion/year operating costs of the space elevator. I bet I can extract a good chunk of that. Even if I'm near break-even on the first tether, I'll be earning a billion a year for each one thereafter. I'm still calling 1Q2012 for the IPO.

And I strongly disagree and flatly dismiss your foolish notion that a LEO-GEO tether will "cause a booming orbital economy," because this is obviously not the case.

Just like the first major application of computers was to design better computers, the first major application of off-planet industry will be to build better off-planet industry. You'll be hiring whoever launches the MXER tethers to build Earth's first space elevator.

they don't seem at all botherd that the payload for Lunar/Mars transfers is so small as to be useless. 500-1,000kg?

The above documents talk about 2000 kg, but you know why they're starting small? Because this is a real project. They are actually going to have to deliver on their promises. They don't get to pretend that the problems are already solved. They actually have to solve them.

say you could put together a super-jet or Falcon-I class SO rocket, that if you can undercut the foreign competition, you won't make enough money to pay off the multibillion dollar tether development/construction.

Even you are assuming that if we get to $100/lb then demand will outstrip supply. The costs to reach SO can be very low. I hope we have to sell orbital timeshares to make it profitable.

I think that it is just a (fairly short) matter of time until someone figures out how to make CNTs of sufficent quality to build an elevator. Given the much larger bennefits that an elevator offers versus a tether, that the money spent on the tether would be better spent on CNT composite and large FEL research.

I think you've got a few years maximum to make the space elevator such a sure thing that no investor will touch MXERs. If not, they'll be built by private industry.

noosfractal · 2005-10-07 03:13:47

$1 billion construction costs @ 12% interest amortized over 7 years means a $17.5 million per month mortgage payment. With 5 trips per month (6 days / 30) thats $2.9 million per trip. At $100/lb each climber needs to carry 29,000 pounds of net payload.
Are the climbers that big?

Here is one plan targeting 29000 lb payloads every 4 days, but their capital costs are estimated at $20 billion ...

http://isr.us/Downloads/niac_pdf/chapter11.html

Here's an interesting analysis that is probably closer to reality ...

http://www.isr.us/Spaceelevatorconferen … 2_kare.pdf

He also mentions laser launch, which I hadn't heard of before.

GCNRevenger · 2005-10-07 08:41:38

I think that neither an elevator nor a tether system will be built any time soon without government startup investment. This is the kind of investment that government is for, where it opens new possibilities and changes the economics of the whole proposition of spaceflight. A LEO-GEO tether system does not, being that it is only a marginal improvement over exsisting low-cost launch rockets.

I want to re-emphasize, that this talk of a tether system being a "Panama Canal to space" or a "gateway to the solar system" is not simply wrong, its flatly dishonest. It has been a NASA tradition for a very long time to throw a little money at interesting but low/no bennefit projects in order to keep engineers on the payroll, and this is a lovely example of propoganda by engineers who want to stay employed so they can work on their pet projects: its just NASA graft, the same kind responsable for keeping us stuck here for so long.

Now for the technical related stuff...

Six day round trips will only be for the initial cable Bill, that additional cable would be layed to permit two simultainious transits (one up, one down) and support the mass of multiple elevator cars. Then, considering a dozen cars and three-day transits, that would give you a lift opportunity every twelve hours, round' the clock. The "six day" cable will only be the dirt construction path, the twin-rail cable will be the highway. Once the initial cable is in place, it will be quite easy to add additional cables, so the elevator will have the bennefit of radically decreasing the cost of bigger or even subsequent elevators... but a tether will not.

"I'll give you replacement of kilometer-scale segments in the atmosphere, but higher up you are dreaming"

Why? The initial cable probobly won't have to be built in one contiguous piece, and subsequent cables won't have to be either: they will be made in large kilometer-scale sections and joined every so often (100km maybe), and additional sections will be added later to increase the capacity of the elevator. The "heavy duty" elevator will be made of a sandwich of smaller ribbon cables, and if one breaks and wears out, then you cut it loose and attach a new section to replace it. Climbers with high-speed cameras will spot problem areas during normal transit, and on occasion such a repair climber could be sent.

"Hmmm, I wonder what proportion of the power is required for those first 50 km? What with it being deep in a gravity well and all. I'm not sure you'd want to commit to storing that on board your climber."

Considering the extremely high efficiency of electric motors, I think they could probobly manage. Or maybe a compact LOX/LMe turbine, which have exceptionally high specific power. Because adding additional cable to the elevator will be pretty easy and lifts so often, that as long as the CNT material has modest headroom then we can spare a little mass.

"I'll collect hyper-accurate positioning info at tether control then use lasers for range finding and communication to and from the payload"

The magical "tether control" you speak of doesn't have any means of determining its orbit with ultrahigh accuracy any more then a ground radar station or GPS constellation can, this is just arm-waving. And a laser rangefinder might give you quite good vertical distance determination, but not so good horizontal data, and neither one at a long distance that would be preferred for lining up your run. This isn't a silver-bullet solution either.

"Sounds pretty serious to me. Someone thinks this is worth pursuing."

Sure. Yet another one of the cadres of engineers working on pointless pet projects who want to remain being employed. They have to at least give the appearence of usefulness to Congressmen and NASA brass.

"The above documents talk about 2000 kg, but you know why they're starting small? Because this is a real project. They are actually going to have to deliver on their promises."

Nonsense, this is excelent proof that they aren't really serious, and are just tacking on this "bennefit" as a "hey look, we're part of Moon program too, please don't cancel us!" The extremely small payload Moon/Mars is quite simply uselessly small, and the fact that include it in their pitch is just silly. Even 2000kg to LTO isn't worth the bother because of the fuel needed to land.

"Even you are assuming that if we get to $100/lb then demand will outstrip supply. The costs to reach SO can be very low. I hope we have to sell orbital timeshares to make it profitable."

To conclude, I will say it again that a space elevator changes everything, but a tether will not: it will inherintly have a much easier time becomming profitable because it entirely changes the rules, that an elevator will be far more versitile and support new markets (even tourism) that a tether will not.

It will provide a base for a space station of truely Clark/Kubrik proportion that will be suitable for building space craft from smaller payloads, which will enable large and manned missions to be deployed economically from relativly piecemeal construction. A Bigelow-style space hotel now becomes an entirely simple matter, and the traditionally unprofitable space manufacturing of specialty materials suddenly is possible. With this promise of profitable ventures, this will increase demand, and with NASA underwriting initial development & construction will drive per-pound costs far far lower.

You get not only cheap lift, but a destination too! ...A tether does not, it can only fling unmanned payloads into the same orbit.

Because a LEO-GEO tether is only a marginal improvement over direct rocket Earth-GEO launch (payload farings must be stronger/heavier and bus may have to be maneuverable too) even if it did work, and an SO-LEO tether will be much harder with its short and more difficult allignment with only one pass.

This combined with a payload that is already insufficent for its target market (GEO satelites and Lunar payloads), needs to have a reuseable suborbital spaceplane that can easily be captured & released from the tether (rockets won't do, if you miss, you lose your payload!), and the case against tethers and spending the money on elevators looks pretty strong to me.

BWhite · 2005-10-07 09:49:05

Six day round trips will only be for the initial cable Bill, that additional cable would be layed to permit two simultainious transits (one up, one down) and support the mass of multiple elevator cars. Then, considering a dozen cars and three-day transits, that would give you a lift opportunity every twelve hours, round' the clock. The "six day" cable will only be the dirt construction path, the twin-rail cable will be the highway. Once the initial cable is in place, it will be quite easy to add additional cables, so the elevator will have the bennefit of radically decreasing the cost of bigger or even subsequent elevators... but a tether will not.

This is a good answer.

12 hour payload cycles probably can recoup the investment at $100/lb. But again, this also requires potentially staggering levels of demand. 30,000 pounds going up every 12 hours.

2 million pounds of mass per month, per elevator, with multiple elevators likely needed to fully achieve the economies of scale to get to $100/lb.

But yes this will change everything.

= = =

Rather than flying tethers as an intermediate step, a lunar to EML-1 elevator would appear to be a good intermediate step.

= = =

Next up, the geo-politics of all this.

noosfractal · 2005-10-07 16:22:08

I think that neither an elevator nor a tether system will be built any time soon without government startup investment. This is the kind of investment that government is for, where it opens new possibilities and changes the economics of the whole proposition of spaceflight. A LEO-GEO tether system does not, being that it is only a marginal improvement over exsisting low-cost launch rockets.

Certainly the space elevator won’t be built by the private sector, but I agree that if it can be built, then the government should build the first one. However, the cost of a MXER tether system could very soon fall under a billion – especially with all the tether research and the coming glut of cheap launch vehicles. And while a 50% - 80% price reduction is “marginal” in space elevator land, I think there are going to be plenty of customers interested in 2 for 1 and 5 for 1 deals. Murdoch might bankroll it just to get a lock on GEO.

I want to re-emphasize, that this talk of a tether system being a "Panama Canal to space" or a "gateway to the solar system" is not simply wrong, its flatly dishonest.

Actually, the Panama Canal analogy is an excellent one – no new tech, just making the trip cheaper. The space elevator is more like the invention of jet engines.

It has been a NASA tradition for a very long time to throw a little money at interesting but low/no bennefit projects in order to keep engineers on the payroll, and this is a lovely example of propoganda by engineers who want to stay employed so they can work on their pet projects: its just NASA graft, the same kind responsable for keeping us stuck here for so long.

There is advocacy in those papers, but it doesn’t come anywhere near the rapturous certainty coming from converts to the Church of The Elevator. You should hear some of those guys! Every problem, no matter how glaring is dismissed with “easily solved you just use <technology that by itself requires a solid decade of development>.” Cost estimates vanish in classic I-wish-it-were-true fashion: $40 billion, $10 billion, $1 billion, woosh. 100% utilization is assumed. The cable is built and repaired as it is being used. It is immune to the atmosphere and has fleets of tiny robots that pull it to one side so that LEO satellites can pass by safely. Although the cable is a composite, they are positively offended when it is mentioned that no composite has ever achieved the percentage of theoretical performance they require. Papers casting doubt on even the theoretical maximum performance are ignored as a matter of course. They give every sign of being just as successful as the fusion guys – always 25 years away from deployment.

The initial cable probobly won't have to be built in one contiguous piece, and subsequent cables won't have to be either: they will be made in large kilometer-scale sections and joined every so often (100km maybe), and additional sections will be added later to increase the capacity of the elevator.

How will the joins not become weak points, or else be too massive? I haven’t seen any proposals for joins. It does seem necessary though. Would it be some sort of splice or a distinct coupling mechanism?

The "heavy duty" elevator will be made of a sandwich of smaller ribbon cables, and if one breaks and wears out, then you cut it loose and attach a new section to replace it.

If the sandwich is glued together, don’t you have to replace the entire sandwich? How do you incorporate the new layer into the old join? Don’t you have to rejoin when you add a new layer? Are there any existing analogs for this?

"Hmmm, I wonder what proportion of the power is required for those first 50 km? What with it being deep in a gravity well and all. I'm not sure you'd want to commit to storing that on board your climber."
Considering the extremely high efficiency of electric motors, I think they could probobly manage. Or maybe a compact LOX/LMe turbine, which have exceptionally high specific power. Because adding additional cable to the elevator will be pretty easy and lifts so often, that as long as the CNT material has modest headroom then we can spare a little mass.

There goes the payload down to 12000 kg/climber. May be we should just drop it to 10000 kg/climber to account for the in built cable repair system and spare cable sections?

The magical "tether control" you speak of doesn't have any means of determining its orbit with ultrahigh accuracy any more then a ground radar station or GPS constellation can, this is just arm-waving.

The tether has forever to resolve its position using GPS and anything else it needs. The payload vehicle can use the tether’s information until it gets close enough to gather it’s own – if this is even necessary. It looks like they’ve already looked at all this and the net is sufficient.

The extremely small payload Moon/Mars is quite simply uselessly small, and the fact that include it in their pitch is just silly.

Useless for people, but not for robots and supplies.

a space elevator changes everything

Yep. No argument from me on that. Life extension technologies change everything as well. For example, they might let me live long enough to see a space elevator built.

Because a LEO-GEO tether is only a marginal improvement over direct rocket Earth-GEO launch (payload farings must be stronger/heavier and bus may have to be maneuverable too) even if it did work, and an SO-LEO tether will be much harder with its short and more difficult allignment with only one pass.

Yes but the experience gained with the LEO-GEO tether will make it just another evolutionary step. Standard research funding. No Manhattan project required.

reuseable suborbital spaceplane

Oh, you’re right. It won’t be Murdoch that funds the tether, it will be Allen, Bezos or Brandon.

noosfractal · 2005-10-07 19:00:55

It looks like they’ve already looked at all this and the net is sufficient.

Here is another simple, effective idea for a grapple ...

http://www.tethers.com/Movies/CaptureToss.mov

John Creighton · 2005-10-07 19:23:56

It looks like they’ve already looked at all this and the net is sufficient.
Here is another simple, effective idea for a grapple ...
http://www.tethers.com/Movies/CaptureToss.mov

The video looks interesting. How much delta V does the tether contribute, how much does it weigh and how frequently can it be used. I want to get a sense of the economics.

noosfractal · 2005-10-07 20:04:48

The video looks interesting. How much delta V does the tether contribute, how much does it weigh and how frequently can it be used. I want to get a sense of the economics.

Make sure you check out the other videos on this page ...

http://www.tethers.com/OrbitToOrbit.htm

That page also has papers that answer your questions. Briefly: between 2-3 km/s, 10 tons for a 2 ton payload, once a month. It takes a month to reboost because it uses the Earth's magnetic field to generate thrust so that it can be propellantless once deployed.

GCNRevenger · 2005-10-08 16:17:42

"the cost of a MXER tether system could very soon fall under a billion – especially with all the tether research and the coming glut of cheap launch vehicles. And while a 50% - 80% price reduction is “marginal” in space elevator land, I think there are going to be plenty of customers"

I think that a tether system will be quite a bit more expensive then that because of the difficulty of the proposition and the wear on the hardware (batteries, solar array pivots, catchment etc) will need a bit of development, which doesn't come cheap.

The reduced cost of LEO-GEO launch will be offset by the increased cost of the launch vehicle bus (which must now be maneuverable most likly and definatly built stronger), and considering the high risk of the untested technology versus the small decrease (~5-10% for a $500M sat & GEO rocket) in total costs, that the whole contraption is of dubious economic bennefit. Venture capital of sufficent magnetude will thus not be forthcoming.

I have no illusions that a space elevator will be cheap, it will easily cost in the low tens of billions range, but other then the actual cable material I think that there isn't much in the way of a difficult engineering hurdle... like the tethers' orbital rendezvous or the suborbital spaceplane handing off its payload with high reliability.

The cable concepts I've seen don't glue anything together, but use sections joined every so often with either interwoven cable or a metal clamp or something. If it is economical to make 40,000km of cable, then a bit more shouldn't be a big deal. The initial cable might forgo metal clamps and anneal the composite together with prepackaged reactive crosslinking end segments to save on mass. Easy chemistry.

"The tether has forever to resolve its position using GPS and anything else it needs. The payload vehicle can use the tether’s information"

No. The problem is that there is no good method to determine your orbit with extreme accuracy over short time scales, so by the time that you are ready to line up for the capture, your position data will be "out of date" and so will only be a guess. Since the tether end will be dipping into the extreme upper atmosphere (which you can also only guess the height of), and Earth's gravity isn't perfectly symmetric, old data isn't good data for meter accuracy.

The payload can't use the tethers' data alone, because it will need to know its own position with high accuracy too, won't it?

"Yes but the experience gained with the LEO-GEO tether will make it just another evolutionary step. Standard research funding."

No, because the elevator and the tether are so different, that they are not relatives of eachother. It doesn't make sense to spend big money on a tether, and its simply not true that this research would be very useful for an elevator.

Note: edited for clarity

noosfractal · 2005-10-08 17:11:18

I think that a tether system will be quite a bit more expensive then that because of the difficulty of the proposition and the wear on the hardware (batteries, solar array pivots, catchment etc) will need a bit of development, which doesn't come cheap.

Yeah, the lifetime stuff is an issue.

considering the high risk of the untested technology versus the small decrease (~5-10% for a $500M sat & GEO rocket) in total costs, that the whole contraption is of dubious economic bennefit. Venture capital of sufficent magnetude will thus not be forthcoming.

Ideas for cheap satellites for fun and profit is probably for another thread.

The payload can't use the tethers' data alone, because it will need to know its own position with high accuracy too, won't it?

Yes, but that position can be relative to the tether, which it should have plenty of time to work out. But it looks like it isn't necessary.

"Yes but the experience gained with the LEO-GEO tether will make it just another evolutionary step. Standard research funding."
No, because the elevator and the tether are so different, that they are not relatives of eachother. It doesn't make sense to spend big money on a tether, and its simply not true that this research would be very useful for an elevator.

That comment was in reference to the SO-LEO tether, but actually there is a good chunk of tech that the two will share - the lifetime stuff you mentioned above is just one.

John Creighton · 2005-10-08 18:38:42

I don’t think it will be too difficult to identify where the tether is because if you know where it was inertia should dictate where it will be in the future. It should be a fairly basic estimation problem and the research can probably be done pretty cheap if you sponsor a few electrical engineering students to do it for there PHD.

John Creighton · 2005-10-08 19:22:40

For what its worth, GCNRevenger's confidence in space elevators make me take their technical feasibility far more seriously.
That said, with six day trips, simply paying back the interest might make it hard to close the business case.
$1 billion construction costs @ 12% interest amortized over 7 years means a $17.5 million per month mortgage payment.

Why are we amortizing over 7 years. Do we assume the space elevator will on average only last 7 years? If it will last longer then the calculations are wrong because the space elevator will be an asset that can be sold.

With 5 trips per month (6 days / 30) thats $2.9 million per trip. At $100/lb each climber needs to carry 29,000 pounds of net payload.
Are the climbers that big?
Remember, this excludes insurance, wages, energy, ground handling, etc. . . $100/lb. The falcon V is suppose to achieves 1200/lb. Say the first space elevator only marginally beat this and achieved 1000/lb
When considering the economics of a space elevator the first question is what factor determines the price. If the majority of cost is to launch the material then the question is not how much it will cost to build but by what factor will it reduce the cost over the price to lift the required material to build it to orbit. If however, the majority of the cost is to construct the cable then there is a hard limit on the economics of s space elevator.
The falcon-V is expected to achieve $1,200/lb. per pound. At
I don’t think it is necessary to achieve $100/lb for the first space elevator because $1000/lb is still a factor of 10-100 times cheaper then the current launch prices. By your numbers at 1000$ per pound the elevator would only need to lift 3.5 tons per trip or 210 tons per year.
NASA plans to launch at least this much payload per year for the moon missions and that is only to LEO. A space elevator will be able to lift all this mass above GEO. If there is not the market for 210 tons per year (which there is) at 1000$/pound then the remaining payload can be used to lift material to build more space elevators.
As long as the limiting cost of the space elevator is the price to deliever the material to orbit then if the first elevator achieved 1000$ per bound and was built at a launch cost of 1200$ per pound then the second elevator would deliver mass to orbit at 833$ per pound. By the time the 14th elevator is built the price to LEO will have fallen to 93$ per pound. This low cost will be achievable even if the space elevator only lasts for 7 years as you suggest and as long as the price of the material need to lift 1 pound per year does not exceed 100$

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