Thursday, December 27, 2018

Can Sheridan make it?

Of the SF TV series of the past century, Babylon 5 was probably the one that came closest to accuracy in its physics. Unlike the ubiquitous and unexplained fake gravity in Star Trek and Star Wars, B5 was a space station modelled on an O'Neill colony, that rotated for pseudogravity.
In one of the pivotal episodes, station commander Sheridan is forced to jump from a shuttle running along tracks near the center of the station to escape a bomb:
In the episode, Ambassador Kosh is forced to reveal a secret to save Sheridan. But Sheridan wasn't jumping from an airplane over a planet; he was jumping from near the center of a space station, where the acceleration from the rotation is quite small. How close could he have gotten to getting down by himself, and what's the smallest amount of equipment he would have needed to make it?
We can make estimates of the parameters: He's about 150m (500 feet) above "ground", which is moving at 25m/s (60mph). He probably jumped at 5 m/s as he was trying to escape the bomb. And the shuttle was probably moving in the neighborhood of 25 m/s itself.
So he has an axial velocity of 25, a radial velocity of 5, and needs to pick up a circumferential velocity of 25 to match the ground when he gets there.
Let's say he can hit the ground at 5, his jumping speed (think of it as 10 mph), and make it.
What does he have to work with?  First and foremost, the 60 mph wind of the shuttle's speed. That's nearly terminal velocity for a human in a draggy attitude. He could easily use skydiver technique to move back up near the 0-G centerline with that much wind to work with. Then he could just hang there and await rescue (which was 2 minutes away.)
But he didn't. Maybe the explosion blew him the wrong way, or stunned him, or something. In about 10 seconds, the wind will bring his axial velocity down to about 5 m/s, survivable when he hits something; he will have traveled about 150m axially at that point, but he appears to have plenty of room in that direction. His radial speed of 5 will only have taken him 1/3 of the way "down" by then, so he could land on one of the big struts you see and hang there against a gravity equivalent to the surface of the moon.
What if he just keeps going all the down? Remember, there is no force accelerating him outward. Velocities do not accumulate the way they do falling toward a planet; he is accelerated by the wind of the turning station, and that is always in a tangential direction. Here's the path he takes:
When he hits the "ground" he has a radial speed of about 7m/s and a tangential speed about 8 slower than the station is turning. So he hits about equivalently to jumping off the top of a truck doing 19 mph. He might break a leg, but would very likely live. If he hits flat, soft ground, and rolls, he has a decent chance of walking away.
Buildings and trees, on the other hand, are not his friends...
However, if his uniform had Rocky the Flying Squirrel webbing between arms and legs, he could very likely make a perfect two-point landing.

Wednesday, December 12, 2018

Is the heliopause shrinking?

Wait a minute, what's the heliopause?

There is a region of space, which may be thought of as the solar system, in which the effects of the Sun dominate the average conditions of the interstellar medium. Inside, there is the solar wind, a flux of charged particles emanating from the Sun; outside, there is a flux of galactic cosmic rays which is held at bay by the Sun's magnetic field. The heliopause is simply the name we give to the boundary.

Why might it be shrinking? Over the past several (11-year) solar cycles, it has been noted that the activity levels of the sun, measured by things like sunspots, have been waning.

That means that the Sun's magnetic field is getting weaker, and not shielding us from cosmic rays with the efficacy that it used to. And in fact, cosmic ray readings have been rising at observatories where such things are measured.

It occurred to me that we might have a direct measurement, however. The latest thing in the science news is that Voyager 2 has just crossed the heliopause, as detected by its measurements of its environment, notably the levels of cosmic rays. Voyager --> 2 <--.

Turns out that Voyager 1 crossed the heliopause some 6 years back. It was 121 AU from home. Voyager 2 just crossed the heliopause at 118 AU. So guess what, it does look like the heliopause is shrinking.

Maybe yes, maybe no. Turns out that having only two datapoints in a cyclic process, like the solar cycle, doesn't necessarily tell you a lot. We are at the bottom of a solar cycle, and Voyager 1 came out at the top of one:

So it definitely did shrink, but that may be part of a cycle.  On the other hand, it does seem to have reached interstellar space somewhat before scientists expected it to. So who knows.

Saturday, October 27, 2018

Autogyro as flying car -- part V

Let me sum up the argument and add a few observations.

In Part I we introduced the gyro and related its history and why it was seen as the flying car of the future in the 30s.

In Part II we contrasted the gyro with the helicopter and showed why it could be substantially less expensive.

In Part III we looked at what would be needed to make a gyro easy enough to fly as a car used by a substantial number of people, and the technological innovations since the 30s that might make that possible.

In Part IV we looked at the economics again, this time from the demand side, and what a widely used airspace traffic system might look like.

In Travel Theory we went over the economics of Part IV again in great detail. By far the biggest flaw in most of the flying car proposals or prognostications we have seen over the years has been the lack of a serious cost-benefit study. Quite frankly, there has always been a gap between the value of a flying car and its cost. But now that we have some numbers, we can see that it might just be possible that it could close in the foreseeable future.

Going back over the series, I see that I have left out one of the more important issues in the argument for the gyro as flying car -- the weather. Weather is the major limiting factor for flying light aircraft.

Unless you are Home on the Range, where the skies are not cloudy all day, the most common problem is clouds. Under visual flight rules, which is what most of us think of when we imagine being in a flying car, they often form a ceiling that limits your altitude. Even if you are flying on instruments, you have to be able to pop out into clear air at least 500 feet over the airport and find the runway, which is a fairly nerve-wracking (e.g. dangerous) experience.

The rotorcraft really shines here compared to a fixed-wing aircraft. Not only can you come out going much slower, but you are enormously more maneuverable, safely, near the ground.  Add that to a high-precision GPS and peer-to-peer navigation system as in Part IV, and the cloud problem begins to clear up quite a bit. In fact, with a few more sensors and transponders between the vehicle and a well-equipped landing pad, completely automatic landings would be fairly straightforward in a heavy fog.

The number 2 weather problem is wind. There are two aspects to this: first is the added difficulty of takeoff and landing, and the second is the fact that turbulence gives you a queasy and uncomfortable flight. This turns out to be another area in which the gyro (or helicopter) shines.

The aerodynamic reason is wing loading. If you have a small wing going fast, it is less perturbed by any given wind because the wind is a smaller percentage of the airspeed it sees. This is why big airliners (wing loading 150 pounds/square foot) are much steadier than light small planes (15) in gusty winds. A Piper Cub hit by a 25-knot gust can see its lift double or nearly disappear; an airliner just gets a small bump.

The rotor on a gyro (or heli) is moving at airliner speeds and has a loading (e.g. 75) that is a lot more like the jet. This means that not only is the gyro a more comfortable ride in gusty winds, but it's a lot more controllable in crosswinds for takeoff and landing.

There are two flight schools at Bay Bridge Airport, just across the Chesapeake from Annapolis. One teaches gyros and the other fixed-wing. Usually it's very busy there, with several planes going around the pattern practicing takeoffs and landings as well as transient traffic.

One day last week, we had a blustery day of 15 gusting to 20 knot quartering crosswinds. This is not at all unusual; as I write from a different place in the Bay, the winds here now are 15 gusting to 25. Anyway, that day at Bay Bridge the gyros (and a couple of transient helicopters) had the place completely to themselves.

The implications for flying cars are left to the student as an exercise.

Wednesday, October 17, 2018

Travel theory

It is somewhat surprising, but to a certain extent we can calculate just how much it would be worth to have flying cars of various types. There is a research literature in travel theory, which is the study of how much people travel in various environments in different modes. It is mostly used in road and transit planning, but it gives us data and a point of departure. Here is the kind of thing you will find: this graph shows that as long-distance travel becomes more convenient, people do more of it:

Andreas Schäfer, Regularities in Travel Demand: An International Perspective, J. Transportation and Statistics, Dec. 2000

One would expect that if flying cars were available, people would make even more, even longer trips. But the surprising, unexpected empirical finding from travel studies is that people in all these societies spend about an hour a day travelling, whether they are in Zambia walking barefoot or in the US riding in an air-conditioned car. Some people travel a lot more than others within a given society, of course, but the average across a given a society is just over an hour—apparently a human universal.

To get a handle on how much people might travel under situations more advanced than ours, we can fit a curve to the data and extrapolate to longer trips and use it in calculations for faster transport modes:

For Americans, this is almost entirely by car. It turns out that there is another more-or-less surprising universal: your car does 40 MPH. For virtually any trip, of any length, the effective speed of a car as measured by the time taken to go the point-to-point distance as the crow flies is 40. You might think that you could do better for a long trip where you can get on the highway and go a long way fast; but again, the road system is essentially fractal. The big highways, on the average, take you out of your way by an amount that is proportional to the distance you are trying to go:

 Now the really interesting thing is that we can combine these two functions and derive the value to the average American, as measured in the amount of time they are willing to spend, of getting to whatever destinations there may be at a given distance from where they live:

There are two features of this graph that have at least intuitive explanations. The first is the peak in trips under ten miles. This is due to a combination of the low (time) cost of such trips, and a shadowing effect. If there is a McDonalds 5 miles from your house, you aren't going to go to one 10 miles away. To the extent that many destinations are alike, the near ones “shadow” the value of the far ones. The other interesting feature is the sort of hump going out to 50 miles. This is probably a daytrip phenomenon, together with the kinds of destinations for which people will make a trip of that length: a ballpark, a hospital, a restaurant fancy enough for an anniversary, etc. But in any case this is the empirically determined value to people, as revealed by their willingness to make the trip, of destinations at various distances.

Now we can design some flying cars—at least specify how close to home they can take off and how fast they can go. Let's take three designs to cover the spectrum: a helicopter-like one that lands in the driveway but can only do 100 knots; a convertible airplane that can do 200 and land on a short private strip or straight stretch of road; and a jetcar that can do 400 but has to be flown out of a full-fledged airport. Here's how they compare with a car, noticing that for many short trips with the latter two you never take off at all but just use them in car mode:

The next step is to substitute these travel times back into the value equations and find out how people would travel differently with flying cars of these types, and how much it would be worth to them. At short distances, the helicopter (or pure VTOL) dominates:

(Note that for these distances, you never actually fly the jetcar!)

This is the obvious advantage to a flying car, and the one most people are thinking of when they imagine a VTOL type: the ability to make the kinds of trips you normally do make, faster. But perhaps surprisingly, the numbers show that that's not where the major value would actually be:

When you look at longer distances, the jetcar dominates. The difference is that you would make a lot more long trips than you do now. Jevons rules. These are higher value trips but are too expensive in time to make very often with a ground car. Note that the value of having the given vehicle, as compared with a car, can be determined by taking the total area under the curves. The reason that the curves for the flying cars appear to represent less value for trips less than 20 miles or so is that you'd be taking fewer trips under 20 miles if you had a flying car—you'd be taking longer ones that were of more value to you instead.

We are now in a position to evaluate the value of any flying car, at least as specified by a latency number (the number of minutes added to a trip by having to go to an airport, convert car to plane, etc.) and a speed. The result is expressed as a multiple of the value of having a ground car:

Value of a flying car represented as a multiple of the value of a ground car, as a function of speed and latency (overhead time per trip).

The same chart, in a more easily read-out-able form.

A jetcar that was a VTOL you could fly from your driveway and make long trips at 400 MPH would be worth 7 times as much as an ordinary groundcar. The jetcar in the previous graphs, the convertible that did 400 but had an overhead of an hour per trip, has about half that value, 3.5 times that of a car. A fast prop-driven convertible (250 mph) would be about 2.5 times as valuable as a car, and a slow one (100 mph) only about 1.4, if they had to be flown from airports. That's roughly the same increment to the value of a car that you get from being able to drive to an airport and fly commercially at 400 knots, but incur a three-hour overhead in addition to actual flying time.

In theory if you would pay today's average of $35,000 for a groundcar, a fast VTOL jet car you could fly from your driveway would be worth $245,000, as far as pure travel time value is concerned.

Its value as a status symbol remains to be seen.

Tuesday, October 16, 2018

Crystallized space-time

There's an interesting article over at Nautilus about a new theory of black holes, or rather a theory in which there aren't black holes at all, but a phenomenon based on a bunch of quantum effects instead of relativistic ones.
...George Chapline, a physicist at the Lawrence Berkeley National Laboratory, doesn’t expect to see a black hole. He doesn’t believe they’re real. In 2005, he told Nature that “it’s a near certainty that black holes don’t exist” and—building on previous work he’d done with physics Nobel laureate Robert Laughlin—introduced an alternative model that he dubbed “dark energy stars.” Dark energy is a term physicists use to describe a peculiar kind of energy that appears to permeate the entire universe. It expands the fabric of spacetime itself, even as gravity attempts to bring objects closer together. Chapline believes that the immense energies in a collapsing star cause its protons and neutrons to decay into a gas of photons and other elementary particles, along with what he refers to as “droplets of vacuum energy.” These form a “condensed” phase of spacetime—much like a gas under enough pressure transitions to liquid—that has a much higher density of dark energy than the spacetime surrounding the star. This provides the pressure necessary to hold gravity at bay and prevent a singularity from forming. Without a singularity in spacetime, there is no black hole.
The theory is far from mainstream, but if supported by actual data, has the potential to upend a whole lot of astrophysics. But what caught my eye, or rather mind, when I read it was its similarity to a passage in a science fiction story from 1930:
"It is not matter at all, in the ordinary sense of the word. It is almost pure crystallized energy. You have, of course, noticed that it looks transparent, but that it is not. You cannot see into its substance a millionth of a micron—the illusion of transparency being purely a surface phenomenon, and peculiar to this one form of substance. I have told you that the ether is a fourth-order substance—this also is a fourth-order substance, but it is crystalline, whereas the ether is probably fluid and amorphous. You might call this faidon crystallized ether without being far wrong."
"But it should weigh tons, and it is hardly heavier than air—or no, wait a minute. Gravitation is also a fourth-order phenomenon, so it might not weigh anything at all—but it would have terrific mass—or would it, not having protons? Crystallized ether would displace fluid ether, so it might—I'll give up! It's too deep for me!" said Seaton.
"Its theory is abstruse, and I cannot explain it to you any more fully than I have, until after we have given you a knowledge of the fourth and fifth orders. Pure fourth-order material would be without weight and without mass; but these crystals as they are found are not absolutely pure. In crystallizing from the magma, they entrapped sufficient numbers of particles of the higher orders to give them the characteristics which you have observed. The impurities, however, are not sufficient in quantity to offer a point of attack to any ordinary reagent."
"But how could such material possibly be formed?"
"It could be formed only in some such gigantic cosmic body as this, our green system, formed incalculable ages ago, when all the mass comprising it existed as one colossal sun. Picture for yourself the condition in the center of that sun. It has attained the theoretical maximum of temperature—some seventy million of your centigrade degrees—the electrons have been stripped from the protons until the entire central core is one solid ball of neutronium and can be compressed no more without destruction of the protons themselves. Still the pressure increases. ..."
That's from Skylark Three, by E. E. "Doc" Smith. All the details are different, of course, and people still referred to the stuff of empty space as the luminiferous ether instead of "space-time," but you get the idea.

Thursday, October 11, 2018

Ammonia, the fuel of the future

(You may, if you wish, sing that to the tune of Columbia, the Gem of the Ocean.)

Back in 2009, at Foresight, I wrote a post on the prospect of ammonia as a fuel, basically as a hydrogen carrier:
What will your car run on in 2020 or 2030? What form of energy storage and transmission will allow intermittent energy sources, such as wind and solar, to be a viable input to the economy?
There’s a good chance, of course, that cars will still run on gasoline — its demise has been predicted early and often — but there are also lots of reasons that petroleum will not be a sound basis for a rapidly-expanding economy. We’ll want to save the hydrocarbons as a feedstock to our nanofactories…
Why not batteries? For cars, in particular, batteries are heavy but they are also inefficient. You lose a lot of energy by storing it in a battery and taking it out again. Almost certainly, nanotech will allow us to build lighter, more efficient batteries, or their equivalent, such as ultracapacitors. But that comes with a major drawback: the higher the energy density of a closed-cycle battery, and the more quickly you can charge it, the more quickly it can release its energy. In simple terms, a really good battery would be a bomb.
The answer to both weight and safety concerns is an air-breathing battery. Only storing one of the two reagents saves weight, and means that the potential energy isn’t in the battery, but in the battery and a large volume of air.
The answer has always been hydrogen. (I assume hydrogen in Nanofuture, for example.) Hydrogen is very light. It’s fairly safe — since it’s lighter than air, a leak dissipates rather than forming an explosive mixture. It can be generated from water by electrolysis and used in fuel cells with decent efficiency (about twice that of internal combustion engines). It seems likely that nanotech will give us ways to separate and recombine hydrogen that produce a very high-efficiency energy-storage cycle.
But there are some drawbacks: hydrogen is bulky, though light, and it needs to be stored at very low temperatures. It’s a real pain to deal with, and that means it’s expensive to deal with. Storage and transportation quadruple the cost of hydrogen as a fuel (see the bottom of the last page here).
Turns out the best way to deal with hydrogen is to use some highly hydrogen-bearing compound, such as methane. This is in wide use as a fuel, in the form of natural gas. (Alternative forms are the alcohols, such as methanol and ethanol). But we are still stuck with that carbon, and not only does it produce CO2 emissions but intermediate products such as CO tend to poison present-day fuel cells.
 An often-overlooked alternative is (anhydrous) ammonia, NH3. Because it’s a polar molecule, it’s easier to liquefy than methane (-33C as opposed to -162C). A liter of ammonia contains more hydrogen than a liter of hydrogen. It’s easy to crack into hydrogen and nitrogen — nitrogen can be emitted without worry since it is already 78% of the atmosphere. You could burn ammonia in a big, powerful, fuel cell, or even a big, powerful internal combustion engine, indoors without the kind of problems you’d get with carbon-bearing fuels. There’s a fairly easy path to using ammonia as a fuel. It’s already produced in major industrial quantity and there are thousands of miles of ammonia pipelines. It’s a good fuel for current-day fuel cells, and near-term nanotech is likely to improve the catalysts for all parts of the cycle.
 Looks like I may have gotten one right, especially that last sentence. We note the following from Rice:
HOUSTON — (Oct. 4, 2018) — Rice University nanoscientists have demonstrated a new catalyst that can convert ammonia into hydrogen fuel at ambient pressure using only light energy, mainly due to a plasmonic effect that makes the catalyst more efficient.
A study from Rice’s Laboratory for Nanophotonics (LANP) in this week’s issue of Science describes the new catalytic nanoparticles, which are made mostly of copper with trace amounts of ruthenium metal. Tests showed the catalyst benefited from a light-induced electronic process that significantly lowered the “activation barrier,” or minimum energy needed, for the ruthenium to break apart ammonia molecules.
The research comes as governments and industry are investing billions of dollars to develop infrastructure and markets for carbon-free liquid ammonia fuel that will not contribute to greenhouse warming. But the researchers say the plasmonic effect could have implications beyond the “ammonia economy.”
The other development in the interim is that the technology for hydrogen fuel cells is getting within hailing distance of piston internal combustion engine power/weights. (But they ain't cheap!) So come 2030, who knows, you might not need that turbine after all.

Wednesday, October 10, 2018


Chapter 13 of WIMFlyC begins thusly:

Travel theory tells us that if we can make a flying car that only has five minutes of overhead and can do 250 knots once in the air, it would be worth about 5 times as much as a ground car. (250 knots is a speed limit in US airspace below 10,000 feet, so we will use it as a point of departure for speculation. You can't fly much over 10,000 feet without needing oxygen or pressurization.) What are the chances of building such a thing? How much power do we need, for example, to get 250 knots out of our flying car? Here's a graph of a wide range of 20th-century propeller aircraft, showing power vs speed. There's a huge variation in speed at any given power (and vice versa), but it seems possible to design one that gets 250 kts if you have more than 500 HP available:
This graph has an odd structure, apparently; there are two separate bands for single-engine planes, which are interleaved with two bands of twins. What's happening is that the high-power, fast singles are WWII fighters and the like, built with very different design goals from the small light private planes which make up most of the low end. The variation in power has a lot to do with aircraft weight, of course. Using the same engine, you would get much more speed in a single-seater with barely room for your flight bag than you would in a car-like plane that carries a family of four and their luggage. We can finesse the issue by dividing by the weight of the loaded aircraft. Here are power-to-weight ratios by airspeed:

You can see that the designs are much better mixed, all falling along the same “main sequence”.

A speed of 250 kts means that we need a horsepower number somewhere between 0.15 and 0.25 times the weight in pounds. You can see that the graph curves toward higher speeds at the same power loading over about 150 knots. This is due to the fact you use less and less power to produce a pound of lift the faster you go, so faster airplanes tend to be bigger and heavier. Let's estimate that with high-tech materials we can make the vehicle 2000 pounds and have it carry 1000 pounds of payload. That means we need somewhere on the order of 600 horsepower. And that's a problem: a piston engine producing 600 horsepower weighs on the order of 900 pounds. Airplanes with those statistics were produced, in the Thirties and Forties, but they were fighters and racers, more than half engine. We would be much better off with a turbine. The Lycoming/Honeywell LTS101-650C, for example, packs 675 HP in only 241 pounds.

I then proceed to argue that the flying car you would like to have, one that would get up into the range of airliner speeds while being able to land on your driveway, would have to have a turbine in it.

However, for the cheapo build-it-today autogyro version, we can probably get away with something different. One of the reasons that piston engines are so heavy in airplanes (my 0-360 gets 180 HP out of 250 lbs) is that they are directly coupled to the prop, and the cubic inches are to provide torque at a relatively low speed. The way to get high power at low weight is to have lower torque at high speed, and then gear it down. The Rotax engines used in most current gyros works that way, for example.

You could also get better power-to-weight by using a 2-cycle engine; Rotax makes a snowmobile engine that gets 150 HP out of 100 lbs. The major problem with 2-cyclers for aviation is that they do have a tendency to announce their end-of-useful-life by suddenly seizing up.

So what we would do in the gyro is to have two such engines, and no gearboxes. Instead couple the engines directly to generators, which feed electric propulsion and the rotor motors. (Today's electrics have a remarkable power to weight, getting up to several HP/lb.) You have some batteries, which provide you with a hole card; but you have an even bigger one: you're an autogyro. Just run your rotor motors in regenerative braking mode on the way down, and you can regain quite a bit of the energy of your altitude. That and your second engine give you a decent margin of comfort.

So, yes, the flying car of the future will feature a turbine, or maybe something like a fuel cell, after a bit more technological progress. But we can still build a usable one today with what we have.