Saturday, May 24, 2008

Let's Get Around

Moving right along from the 16th century (not good smoke signal, kemosabe), let's put world building aside for the moment to discuss getting around the dozen or so worlds that are in easy reach, for various values of world and generous but not completely outrageous values of easy reach. In other words good old plausible interplanetary space travel, which oddly enough was the major background preoccupation of the true, original rocketpunk. Given that subject, here is the inevitable general reference to Winch Chung's Atomic Rockets site.

First of all, where do we want to go? The tourist guide book has grown enormously in richness and detail since the 1950s, but not much in overall layout. The big loss is Venus. No jungles or tropical world-sea, and certainly neither variety of Heinlein Venusians, the little people in most of his early books or the dragons from Between Planets. Real Venus turns out to be so absurdly Dantesque that it's become like the giant planets; you don't even think of landing there.

Mercury is kind of cool - well, maybe that is not the word I was looking for; let's say interesting, but with a very rugged climate.

The Moon, except that it is more eroded-looking than the old Chesley Bonestall Moon, is pretty much what it always has been: the most common sort of real estate in the universe, pretty worthless except that it is close to a couple of critical freeway junctions.

Mars? We almost lost Mars in 1964, and any native Martian civilization has gone the way of Dejah Thoris, but when all is said and done, real Mars is still the essential rocketpunk Mars. It looks like rocketpunk Mars, which is to to say it looks like the American Southwest, only more so. As far as we can tell, liquid water still occasionally flows there. How can Mars still be anything but the leading attraction, the interplanetary trip you make if you can only make one?

The asteroids. Ceres Juno gets some spiff as a dwarf planet, Pluto's boo-hooey loss her gain. The old asteroid mining mother lode may be played out, except for eventual industries serving deep space customers. My argument here, one of my favorite heresies along with Pelagianism, is that any assumptions about Earth-to-orbit launch cost that make it cheap enough for large numbers of people to go up there also make it cheap enough to send up food, structural fabrications, and so on, at least to destinations in near-Earth space, rather than building a vast industrial infrastructure in the asteroid belt and shipping stuff back.

That said, the asteroid belt is surely full of surprises, and relatively easy to get at - the smaller ones, which are most of them, you can orbit at walking to freeway speed, or even just pull up alongside.

Jupiter's moons, big and small, are way beyond anything we used to imagine, but the big ones are all fairly deep in Jupiter's very nasty radiation belts, which could limit their mass tourism appeal. Saturn's moons also have much new coolness, and probably less radiation. The rings, what can you say? They're the crown jewel of the Solar System and have been since they were discovered, but no one dreamed of their full beauty and complexity till we got close-ups. Shouldn't this gorgeously bedecked planet really have been Juno, empress of the heavens?

Alas, the rings of Saturn are probably murder to drive through, no matter how carefully you try.

Past Saturn we've only scouted a bit, but when we lost Pluto we gained the Kuiper Belt - in fact, we lost Pluto as a "real" planet because we gained the Kuiper Belt. Thanks to Eris, the (partly) known Solar System now extends nearly 100 AU from the sun - the AU, or astronomical unit, equal to the Earth-Sun distance, being the standard tourist measurement used to avoiding having to pack so many zeros with our bags.

In very round numbers, Mercury and Venus are a third and two-thirds of an AU from the Sun, Earth (shock!) one AU, Mars about one and a half AU, the asteroids mostly 2-4 AU, and that is the inner Solar System. Jupiter is near 5 AU, Saturn 10 AU, Uranus 20 AU, Neptune 30 AU, and then the Kuiper Belt.

All of these tourist paradises go around the Sun (or around something that goes around the Sun), and pretty fast toward the center. Mercury books along at around 50 km/s and orbits in a third of a year, Earth at 30 km/s making it in a year, you'll be glad to know, while objects in the outer reaches of the Kuiper Belt, 100 AU from the Sun, mosey along at around 3 km/s (about 6000 mph, a mere crawl), and take a thousand years to make it around the track.

Astronomy 101, yes - consider it a tourist refresher. For travel purposes a pretty good way to visualize it all is as a vast whirlpool. Interplanetary ships - and their destinations - do not travel through empty "flat" space, they travel through the Sun's gravitational whirlpool and are swept along with it. Ships can't just go with the flow, however - if they did, they'd just go round and round and never get anywhere, because they' be in nearly the same orbit as the planet they left.

To get anywhere you have to cut across the flow of the solar gravitational whirlpool, and to get anywhere quickly you have to cut across it pretty sharply, on a "steep" orbit. Cutting across the flow at all takes fuel, or technically propellant. (Some kind of rocket engine is also helpful.) Cutting across it sharply takes lots of fuel, or else an engine with great gas mileage, for which the rocket science jargon is specific impulse.

All of this makes interplanetary navigation endless fun, at least if you're very good at math or have access to nifty software. But let's start with a general question - how long does it take to get around this Solar System of ours?

By way of gross simplification, forget the whole solar gravitational whirlpool and just think of speed and distance the old fashioned flat way. A chemical fuel rocket engine of the kind we've been using for 50 years, with ample fuel tankage, will send you off into the empyrean at about 5 km/s. To slow down again you'll need a lot more fuel tankage, but doable. Since one AU is just about 150 million km, at 5 km/s you will go one AU in 30 million seconds - just about a year.

So, the first-approximation speed of space travel with chemfuel rockets is about 1 AU/year - which, in fact, is a decent broad range for present-day interplanetary mission speeds. Mars probes get there in a year or so, Jupiter probes take a few years, Saturn probes several years, probes to the outer system take decades.

This is too slow for convenient human travel, even in the inner system. How much can we improve it, without invoking too much magic? For speed we don't need more thrust, we need better gas mileage, i.e. higher specific impulse. Classic atomic rockets, of the NERVA type ground-tested in the 1960s, can roughly double chemfuel performance - still dreadfully sluggish.

Various electric space drives, based on well-demonstrated principles (some tested on a lab scale or even in actual service) can improve on chemfuel performance by up to about tenfold. Ion drive is the most famous of these, but it is a very low-power drive, suited to small probes or satellite station-keeping rather than big interplanetary ships. For a big ship we'll need one of the others, but for once we can legitimately handwave the details and simply say that our ship has a nuclear-electric drive, of a fundamentally conservative type that we should be able to build in this century.

Small probes can use solar-electric drive, out to about Mars, but any big ship will need lots of juice, meaning some form of nuclear power. Fusion drive is cool, but actually achieving a fusion space drive verges on technomagic, and it isn't necessary for this speed range. Fission is the stone ax of nuclear energy, but we know how to make a stone ax, whereas we haven't really figured out copper yet, let alone bronze.

A nuclear-electric drive might boot us along at about 25 km/s, five times chemfuel speed, with fuel to slow down again. This comes to 5 AU/year - and now the inner Solar System starts to look manageable, with Mars and the asteroids a few months from Earth, and even Jupiter's moons only about one year away. Beyond that the highway stretches pretty long and lonesome still, two years to Saturn, one way; a decade to the heart of the Kuiper Belt.

The boot our drive gives us will be so gentle we won't feel it, since the acceleration of this drive is only, let us say, one milligee. The ship will take a month to reach cruising speed and another month to slow down, but since it isn't standing still during that time, it comes out to only one month lost to sluggish pickup, on a trip of typically a few months.

Why not save a couple or three weeks by specifying a more powerful drive engine? Because even that gentle 1-milligee drive has a fairly spectacular power output. If the exhaust velocity is 50 km/s, and the drive is capable of pushing a 4000-ton ship at 1 milligee, the required thrust power is a nice round 1 gigawatt, about as much as a large power-station reactor.

In round numbers, let's say that our ship has a "dry" mass of 1500 tons, and carries a 500 ton payload plus 4000 tons of propellant. (For a ship like this, "fuel" and "propellant" are entirely separate. The nuclear fuel is inside the reactor and stays there. The propellant supplies no energy, but is scooted out the back to push the ship along.) At full load our ship doesn't even make one milligee, but as it scoots off propellant it loses mass, and it is down to 2000 tons and an agile 2 milligees as propellant runs out.

The ship's drive engine, including reactor, electric generator, and waste-heat radiators, might come to about 1000 tons, two-thirds of the "dry" mass of the ship, the rest being connecting structure, fuel tankage, and payload bay or cargo pods.

Getting around the real Solar System of planets swirling in the gravitational whirlpool is more complicated that the simple model of speed in "flat" space, and the actual travel time for a ship like this, say from Earth to Mars, is wretchedly hard to estimate short of doing full-on orbit simulation. Slower chemfuel spacecraft tend to use Hohmann or near-Hohmann orbits, with well-known rules. Ships with uber-powerful torch drives can take orbits so steep that a straight-line approximation will do. These nuclear-electric ships can cut fairly steeply across the solar gravitational whirlpool, but they cannot ignore it.

At a guess, a ship like this might be able to make two trips to Mars per Earth-Mars orbital cycle of about 18 months. One would likely be a fast passenger-express run near opposition (i.e., when Earth and Mars are closest, lined up on the same side of the Sun). The other would be a slow freight run, with less propellant and more payload - the positions of Earth and Mars in this part of their orbital cycle are such that "fast" steep orbits use up gas without saving much time.

Interplanetary ships like these are very unsuited to local service, such as the Earth-Moon run - the distances are too short for them to get up to speed, so local missions remain the province of chemfuel or nuke-thermal ships. In fact these ships take a few days just to spiral through the Van Allen belts, meaning that passengers probably take ferries to get out to them.

Now it's time to put on the green eyeshades. How much do these ships cost - and, what we really want to know - how much does a ticket aboard one cost?

I'm going to say a billion dollars, or Euros/whatever, in round numbers, to buy a new interplanetary ship, and cheap at the price - only four times the cost of a 747. Once someone ponies up for it, taking out a loan or the economic equivalent, they have to make payments each year, and also pay the various operating costs of the ship - maintenance, fuel and propellant, and a host of miscellanies including salaries for crew, Mission Control, or both.

Let's optimistically say that the ship has to earn its operators a couple of hundred million a year, and it makes a couple of one-way trips a year. Thus the cost of a one-way cargo mission to Mars, Mercury, or somewhere in the asteroid belt is about $100 million. A trip to Jupiter costs about twice as much. Happily that is not your ticket price, unless you are a 22nd century robber baron and the ship is your personal space yacht. If the ship can carry 100 passengers, the cost of a ticket is a nice round $1 million - what people a couple of years ago were getting for nondescript houses in Southern California 'burbs.

A hundred passengers does not seem like much for ships several times the size of jumbo jets, but interplanetary travellers can't live for a few months in an airline coach seat. They'll need something like train roomettes, plus diner and lounge car, and also a hotel crew of stewards. Hence my round-number estimate of 100 passengers and million-dollar tickets. Of course there will be ships carrying research teams and work crews long before there is commercial service that you can buy a ticket for at any price. But once we have ships carrying hundreds of people around the Solar System on a routine basis, some sort of commercial service seems likely to arise.

Next time we'll look more at these ships, their destinations, and the sort of human Solar System they might produce. Unless I decide to write about something else.


Anonymous said...

Interesting post. Your spaceship concept seems reasonable. As a kind of general purpose transport, you might be able to reduce ticket costs even more by hualing cargo every trip; I'm sure that colonies, outposts, and touriest hotels will need items that can't be supplied locally. By the way, I'm sure that when you begain writing about the asteroids, you ment to said tha Ceres, not Juno, was a dwarf planet. Thinking about the next paragraph while writing is always a good way to mistype something. Heaven knows I've done it enough times!
Something that you mention, about having different types of rocket engines, I really have to agree with. So often, during discussions online or where ever, people just assume that there is only going to be one-size-fits-all rocket motor and that's it. It annoys me; here and now, we don't use just one type of engine to propel vehicles, why should rocketships be any different? Anyway, good post. I hope you decide to continue the theme next time.

Rick said...

Ferrell - Ceres should be miffed at me! Fortunately she must be the gentlest of the Olympians, the goddess of grain, so with any luck she won't turn me into a bowl of cornflakes.

(It wasn't a thinking-ahead mistake but a total brain fade!)

Hauling cargo may not hold ticket prices down, because it takes up payload mass, reducing passenger capacity. (Passengers themselves don't weigh much, but the cabin structure for them does.)

On engine types, there's a tendency to think only of the most spectacular vehicles, which may tend to a single engine type, e.g., all big modern airliners have turbofans, though plenty of smaller ones are turboprops, and piston planes are still flying passengers.

Electric space drives are especially ill-suited to most local service. So in a setting like this, even if big nuclear-electric interplanetary ships are the queens of the spaceways, other rocket types will remain conspicuous, doing everything but interplanetary missions, and sometimes those.

Barring a vastly sweeping and unforeseeable tech revolution, plain old chemfuel rockets will be ubiquitous as far into the future as the eye can see, because nearly all spacecraft, whatever their main drive, need attitude/maneuvering thrusters, and most local service spacecraft will be all-chemfuel.

Nuke-thermal and solar-electric rockets (the latter with very gentle thrust) have niches, while Isaac Kuo at SFConsim-l makes a case for laser-driven rockets for freight service on heavy-traffic routes.

Anonymous said...

"Hauling cargo may not hold ticket prices down, because it takes up payload mass, reducing passenger capacity. (Passengers themselves don't weigh much, but the cabin structure for them does.)"
Maybe, but that supposes that the ship can carry either passangers or cargo; Passanger cabins might be contained in a removable habatat module, but most likely it will be a permanent part of the ship's structure. The cargo, on the other hand, would most likely be bolted onto the ship's structure, with only those items that are sensitive to tempoture, pressure, and/or radiation would be inside the hull. The added cost would be in propellent and I'm not sure how much that would cost per kilogram. You'd have to factor in finding it, purifying it, shipping it, and the labor involved in filling up the tanks. However, the more people that buy tickets, the lower the cost. Up to a point, anyway.

Rick said...

Transportation economics can be wonderfully weird, mainly because it costs nearly as much to run an empty bus as a full one. The space line is eager to sell a ticket for the last empty roomette on an interplanetary liner, for a lot less than a million dollars. One more passenger, plus baggage, and food and air for the trip, is no more than a ton, so the cost to the space line is only a couple of tons of propellant, or a very slightly reduced speed, easily absorbed in the schedule.

If you have one ship making alternate fast passenger and slow freight runs, you might want a semi-modular design where you can disconnect the whole passenger compartment, with all its life support, etc. The slow freight run is probably robotic.

But it's equally possible that the best economics will turn out to be quite different ship classes, fast passenger/express ships, and slow freighters. I'll explore this a lot more in a front pager, but on near-Hohmann orbits even 0.1 milligee is acceptable, meaning a much bigger payload relative to ship operating cost, even with slower turnaround.

Passengers demand speed, because a year is a long time to twiddle thumbs in a roomette, plus the hotel expenses pile up.

Anonymous said...

Too true. The best way may well be to either go totally modular or have multiple ship classes, each with their own sets of capabilities.

Jim Baerg said...

"any assumptions about Earth-to-orbit launch cost that make it cheap enough for large numbers of people to go up there also make it cheap enough to send up food, structural fabrications, and so on, at least to destinations in near-Earth space, rather than building a vast industrial infrastructure in the asteroid belt and shipping stuff back"

I'll quibble with that. If we get something as cheap as what the space elevator people are hoping for, I think you are right, but I think there are Earth to orbit price ranges where mass drivers or the lunar space elevator for lunar launch or ion drives from near earth asteroids would compete for radiation shielding & raw materials for zero gee manufacturing. Note that using slag for Apollo style heat shields makes sending stuff down cheap even with present technology.

My favorite scenario for asteroid mining involves 'siderophile' elements Ie: the elements that 'like' to go into metallic iron rather that silicate rock. Most of earth's supply of these elements is sitting in the core where they are a little hard to get at :^) So it might be economic to mine the nickel iron asteroids for these elements (platinum group, gold etc)& bring them back to earth. The iron etc would be kept for use in the asteroid belt.

I've also seen the suggestion that electromagnets could be used to pull out small fragments of nickel-iron meteorite from the lunar regolith. Those could be processed to extract platinum group elements.

Re: jupiter's moons - IINM Callisto is outside the radiation belts, unlike the other big 3.

An interesting possibility for interplanetary travel is Robert Winglee's Mini-Magnetosphere Plasma Propulsion (M2P2), which is a sort of magnetic sail for catching the solar wind & rotating planetary magnetic fields. IINM there would be some common components in an M2P2 drive & some sorts of electric drive, so using M2P2 when you want to accelerate away from the sun & ion drove otherwise could have some advantages.

I think space elevators for getting on & off bodies smaller than earth will be important even if we never get materials strong enough for an earth to space elevator. Many asteroids & moons have gravity greater than the acceleration an ion drive can generate, & current materials can be used for a space elevator for these bodies. The interplanetary nuclear-electric drive ships can rendevous with the tether, or just carry the tether to drop to the surface of a newly visited body.

Rick said...

Jim - your quibble could be right, especially since launch cost is one of the most intractable problems of space access. It is now about $10 million per ton or person. In the past I've used $100,000 or so as the level needed for extensive space travel - two orders of magnitude being an awful lot to ask for.

Even a tenfold reduction to $1 million requires major progress, but could be enough for significant numbers of people to go up on a regular basis, though the orbital tourist hotel business will be high end to say the least. On the other hand it is a lot to pay to ship stuff up.

The first thing we'll want to manufacture in space is surely propellant, since spacecraft use so much, probably burning off more than their whole "dry" mass each mission.

I've had a longstanding bias against space manufacturing, because of the image of huge industrial plants in the asteroid belt, rather outrageous for a long time to come. But useful space manufacturing could be on a tiny scale by Earth standards. An operation producing three tons per day of structural material is enough to build several spaceships per year, a pretty impressive space shipyard.

So once we have reliable interplanetary shipping, it might become worth looking at modest scale space manufacturing. And what is gold worth per ton?

Jim Baerg said...

I've tended to see space manufacturing as something done in the earth-moon region rather than the asteroid belt. It would be either to make stuff for use in space or to make stuff that is much easier to do in zero gee or ultra-high vacuum ( or impossible to do without one or the other).

Space manufacturing would use materials brought up from earth if the quantities involved are tiny or if launch costs drop by a few orders of magnitude. Otherwise lunar or near earth asteroid materials would be used. Since the moon is closer, asteroid material would be used only for elements that are scarce on the moon, ie: Carbon Hydrogen & Nitrogen. Maybe also those siderophile elements, but metallic meteor fragments in the lunar regolith might be cheaper than going to the asteroids.

If space mining does result in much cheaper siderophile elements, it might depress the price of gold severely, but the platinum group elements are so useful for catalysts, their price would likely remain high enough for a lot of mining to remain economic.

Rick said...

I have not been a fan of the Moon, because it is not all that easy to get stuff off of. (Compared to an asteroid you can all but pull up alongside of.) It also lacks the stuff I'd imagine us wanting first in bulk quantities, volatiles for rocket propellant.

But all of this depends hugely on what we are doing out there, and what techs turn out to work well for doing it. If (as I imagine) the lunar surface is a good source of aluminum - however spelled - and mass drivers work nicely in service, that's an obvious source for fabricating stuff.

Which is a way of saying I'm gonna kick this down the road a bit to discussion of midfuture space development and how it might go.

Jim Baerg said...

I don't see any reason this wouldn't work to get stuff on & off the moon cheaply.

Given lunar material to work with you can extract oxygen with is most of the mass for hydrogen-oxygen fuel, or you can use an aluminum-oxygen rocket which needs no hydrogen.

Note: I put a linefeed in the above URL otherwise the end got lost.

Going to the near-earth asteroids for volatiles would be good, but there is a lot you can do with lunar resources.

Rick said...

Stupidly, I thought a lunar elevator was out due to the slow rotation. I forgot about Lagrange points. The thing is embarrassingly long, but can be built out of nonmagical materials, a major engineering advantage.

Nor did I think of cracking oxygen out of lunar rocks. This is what I get for not paying much attention to the Moon.

Anonymous said...

The Interplanetary Space Tourism Industry would probably be extremely hazardous in its early decades, if not centuries, compared to the Earth Orbit Space Tourism Industry what with either the lack of an atmosphere that would protect said tourists and employees from space radiation or too much of the stuff that makes it too hot to survive any length of time. The Science Channel program Exodus Earth has some ideas on how that could be countered, but I have a feeling that said features aren't going to be easy to implement or even wide spread until the industry is profitable enough to warrant them.

Interplanetary travel would be akin to early twentieth century if one thinks about it. The fastest craft available still takes days to cross vast distances and the more economical and slower craft would take weeks, if not months to cross the same distance and would primarily cater to the freight and cargo industries. A Cycler Station would be extremely slow compared to either type of craft, but they could be similar to cruise ships of today and are basically Space Hotels but orbit two planets. It may take two years or so for a full round trip, but one might as well do it in style and luxury since delta V isn't that much of a concern except for getting off to your destination.

- Sabersonic

Rick Robinson said...

Sabersonic - Deep space radiation, especially, may be a very serious barrier to any regular human activity; in the worst case a deal breaker.

I have tended to think of passenger travel as relying on speed, both to minimize radiation exposure and to save on thumb twiddling. But cycler stations are an interesting alternative, especially since even 'fast' orbits (absent magitech drives) take weeks, requiring cabins, galleys, lounges, etc.

And a good many people could do their work en route, e.g. writers. (!) They are out of telephone range, but email only takes minutes in the inner system.

Citizen Joe said...

In the same setting in which I spoke of the 5 mile long xray laser ship and the sheepdog style fighters, I also helped with a lot of the rationales for intrasystem travel.

The whole thing boiled down to the fusion economy. For some reason (that I never found out) fossil fuels suddenly stop being usable (I think it was some sort of terrorist attack). After the initial grab for resources war, the Earth had to solve their energy problems with fusion power. Initially, the Deuterium-Tritium fusion was used, but there were concerns about high energy neutrons, plus getting all that deuterium and tritium was not terribly easy. That lead to Helium-3 fusion, but He3 was even more rare. Earth struck out for the Moon pulling those He3 reserves. Fusion technology advanced bringing online the D-D breeder reactor that made Tritium and Helium-3. But again, there were radiation concerns and as far as we knew, Earth was the only habitable planet in the universe.

Earth's demand for power was never ending and they wanted more and more He3 for the terran power plants. Note that He3-He3 is the cleanest fusion but it also requires tremendous energy input and containment. The magnetic fields needed for He3-He3 fusion proved unfeasible for space vessels, but the D-T fusion rocket was very usable and the waste wasn't an issue in space.

So, Earth expanded outward, looking for resources. First the moon, then Mars (orbit) as a shipyard for the belt miners. If you can get material from the asteroid belt, you don't have to lift it from Earth surface. Several Earth-Mars cyclers were eventually set up at the various Sol-Mars LaGrange Points.

The initial Jupiter Class tankers for scooping Jupiter's atmosphere for Deuterium and Helium-3 were fission rockets. When the Human FTL was invented, Jupiter became largely forgotten in favor of Uranus's Moons Oberon and Titania. The fission powered Jupiter class tankers were retasked to Saturn, specifically Titan for its Nitrogen reserves. New D-T fusion tankers continued operation around Jupiter, but instead operated ion mining from cosmic spallation within Jupiter's radiation belt. Three times as many of the new D-T Jupiter Class ships went to Uranus to act as tugs. The new class of vessel, the Uranus class refinery ship needed those tugs for a slingshot maneuver around Uranus, bound for a 5 year trip to one of the Mars Cycler orbits. During that time, it refined the heavy water ice using the D-D breeder reactors, eventually reaching the cycler with enough He3 to service Earth for about 6 months. It also had a significant amount of Tritium, which it used for its Fusion rockets and also traded it in to the Asteroid Miners to recharge their Tritium batteries, which decays to He3. Eventually, all this Helium-3 goes planetside to Earth where people can appreciate clean plentiful energy...

Until the aliens show up and start zapping them from space.

Rick Robinson said...

There are plenty of reasons, though not quite 'sudden' ones, for shifting from fossil fuels to fusion! He3 is the most attractive fuel, and scarce as it is, a little of it goes a long ways.

In practice, though (as distinct from a story setting!) I doubt we'd have to go all over the Solar System looking for it. I believe that it can be brewed up in a D-T reactor, and in fact the T can also be brewed up. And on the moon, dirty breeder reactors are not a problem!

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Canageek said...

One think about is what we can do in space that we can't on earth. My understanding is that manufacturing certain items might be a lot easier without gravity (I've heard speculation about ball bearings, and crystal growth are two I've heard.) Once you have a product that you want to make in orbit, the economics of pulling things out of the gravity well fall into place. Once you are already lifting things out of the well, we can presume economy of scale will make things cheaper, and the cost of leaving the gravity well is a big chunk of the total cost, right?

The other thing that strikes me, is that there are people looking into mining space now, so evidently some people think it might be profitable: