While not the topic of the games, as you know setting up the 1-km racetrack has been somewhat of a difficult task… Here’s a brief video showing the way it is done, and perhaps capturing the scale of the climb…

The line, btw, is a 3/16″ steel cable, 4300′ long, and weighs about 300 lbs.

Extra credit goes to Michael Keating (Tetherman), Keith Mackey (Heloman), and our fearless super-pilot, Doug Uttecht of Northwest Helicopters in Olympia, Washington.

More videos coming soon, including laser-tracking videos, which are a lot more exciting since you can see the laser beams that are making it all happen.

To recap, we are offering a prize of $0.9M for a tether sample that has a specific strength of 5 MYuri, and an additional prize of $1.1M for a tether sample that has a specific strength of 7.5 MYuri.

For context, today’s materials perform at 2.5 – 3 MYuri at best, and to build a Space Elevator we need material that is 25 – 30 MYuri.  (A MYuri is the name we gave the SI equivalent of N/Tex, or GPa-cc/g)

A visual guide to the task ahead

Actually, we’ll be more comfortable (and the Space Elevator will function a lot better) with a ~35 MYuri material, but this is the bare minimum that we need. Keep in mind that successive 50% improvements in material strength are very large steps, but that we already know that CNT molecules are measured at ~50 MYuri, and fabricated CNT micro-bundles have been produced by several labs at 10 MYuri, so this challenge is not impossible.

It is important to note that while in order to win the prize we require the core metric of specific strength, we do not require the tether samples to be made in a way that is scalable, profitable, repeatable, or durable. We do not care if it took a whole year of undergrads working around the clock, and the sample is the best of 100 samples that were made. This makes the prize very attractive to CNT research labs, since we’re offering a substantial amount of money at a stage where investors are still (rightfully) shy, since the tether is still far far from being a sellable product.

To date, we’ve had two Carbon Nanotube tether samples at the games.

In the 2009 games, the University of Shizuoka team, led by Yoku Inue, entered a CNT tether loop (our second ever).

The tether sample was made out of Carbon Nanotubes that were grown as an aligned nanotube “forest” on a flat substrate, then pulled into a loosely aligned “sliver” and spun into a thread.

The Carbon Nanotubes themselves are short in everyday terms (a tenth of a millimeter) but still represent an aspect ratio of more than 10,000:1. The tether was then looped around to create a closed flat tape, with cross-over lines similar to Brad Edward’s proposed ribbon construction of a Space Elevator.

Being their first effort at a macroscopic tether, it failed very early, pretty much separating between the micro-fibers.

In the 2007 games, team delta-X representing Nanocomp Inc,  presented a tether sample made out of Carbon Nanotubes that were grown in an aerosol-like phase and spun out directly from this “black smoke” in a way reminiscent of a cotton-candy machine. Delta-X’s tether was a very recent result, and so they did not have the ability to form a closed loop just yet. Instead, the tether was tied in a knot to form a closed loop, and as expected, when pulled, the knot slipped.

Both tethers failed at the macroscopic level, very far from the strengths achieved by the individual CNTs or even the CNT micro-bundles that constitute them

On the one hand, just having these samples and talking with the teams gives us a good indication that the challenge is having its desired effect and is drawing research teams to look into tensile strength of CNTs, which is otherwise one of the harder challenges in the field, and one offering longer-terms rewards.

On the other, we’re hoping that in the next games we’ll be able to at least show performance levels comparable to the Kevlar or Zylon type tethers that are out there today.

Spaceward’s next goal is therefore to aggressively pursue the CNT labs out there – we think that the timing is about right, since CNTs are now produced by an ever larger set of universities, and the production of a 2-gram carbon nanotube tether, while incredibly impressive in last year’s terms, will no longer be a novelty in 2010.

Power density is a measure for how powerful a motor system is in respect to its mass.  In the case of a Space Elevator climber, the system mass must include the motor, the PV array, any cooling systems, and structure mass used to aid locomotion – basically everything but the cargo hold.

For a Carbon Nanotube tether that is 30 MYuri strong, and a characteristic time constant (CTC) of 1 year (Confused? Curious? Read the paper!) the Feasibility Condition requires that the climbers will have a power density of at least 1.0 kWatt/kg.

So where do the competition requirements stand in respect to this?

It is easy to show that when moving straight up, the power density of the climber is directly proportional to its speed (mgv/m), and so a 5 m/s speed in 1 g gravity corresponds to 50 Watt/kg, or about 5% of a real Space Elevator climber.

So how difficult is it to improve this performance by a factor of 20?

Not impossibly so.

The climbers built by the teams are designed to be rugged, and even at 5 m/s are having to deal with significant wind resistance. Even though they are designed to be lightweight, the actual panels on a Space Elevator climber will be much lighter. In space, lacking wind, and lacking cooling air, the PV panels will look more like Saran Wrap or Aluminum Foil than like real “panels”.

The PV panel shown below was manufactured by DLR in Germany, with the intent to be used in space. It is so thin and large (see the people in the back for scale) that it will never survive even the lightest winds on Earth and can fit into the little box at the center (from which it was deployed). In space, however, it would be the ideal building block for a Space Elevator climber, and even today this panel performs at several times the power density we need for a Space Elevator climber.

One of the nice things about this panel is that it is designed for Solar radiation, which means that after the initial laser-boosted stages of the climb, the climber can make the rest of the way (about 80% of it) using sunshine alone, which makes it easy to drive several climbers simultaneously.

Electric motors that operate at the kWatt/kg range exist today (though they are not super efficient), but ironically, the same CNTs that make tethers stronger, stand a very good chance of reducing the weight of electric motors by replacing the metallic windings that are in them.

So to conclude – on the power side of the feasibility condition, the building blocks are there – the solid state lasers, the PV receivers, the advanced motors and power electronics. Not ready to be assembled within a year, of course, but certainly within reach in the 10-year outlook.

Robert Winsor, NSS's Laser man, and Nic DeGrazia, our filmographer in residence, observing the NSS climber during the power test.

NSS's TPV array

Moving to photovoltaics, NSS settled on a PV technology called TPV – Thermal Photovoltaics. These cells are optimized to operate with thermal IR radiation (longer wavelength than TRUMPF’s NIR 1030 nm beam) but have acceptable performance at this wavelength as well. More importantly, these cells can work with high light intensities, which means that you can get more power out of a smaller (and thus lighter) array, if only you can get the transfer the excess heat away from the cells.

What this calls for is a good heat exchanger – and this turned out to be the highlight of the day.

Check out the images of the climber. The TPV cells are completely immersed in acetone (4 ounces) which is vigorously boiling away under the heat load of the beam, completely evaporating every 15 seconds – only to be continuously captured by the bags and dripped back down onto the cells.

Acetone was chosen since it has the lowest boiling temperature, and so will be most effective as the working fluid.  This is a basically a cooling tower (or heat pipe) – something that was used by Centaurus Aerospace back in the 2005 games – using water in vacuum, in their case. The acetone solution is a lot lighter, and yes – more flammable.

We’ve looked into this issue, and we recognize that there are failure modes under which the system can develop a leak, but we feel that a) the acetone is far removed from any spark sources, b) there is only a small amount of acetone in the system, and c) there is no place for leaking acetone to accumulate, and so the consequences of an acetone leak are acceptable. We will also be monitoring the temperature of the PV receiver, and if we see it rising above the boiling point of acetone, we will know that the acetone is depleted and the climb is over.

So after observing the climber operating under full laser power, and with some modifications required, we’ve decided to ok the design, and allow NSS to catch up and participate in this year’s challenge.

One of the nice things about having multiple teams is that you get to see different ideas at work, and NSS is definitely not short on ideas.

Their first climber design featured a thermal (rather than photovoltaic) receiver, based on a Stirling engine. (Stirling engines are high efficiency engines often used for solar power generation) Stirling engines are a difficult proposition for a Space Elevator climber, since they typically weigh a lot more than a PV panel, and so NSS had to design and manufacture their own engine – and it is indeed a beauty. Using Helium as the working fluid, this engine also uses a transparent cylinder head in order to get the laser beam directly into a thermal absorber that is placed inside the cylinder – a perfect way to avoid the latency associated with the thermal mass of a regular absorber plate.

The problem NSS ran into was with properly sealing the engine while keeping the weight down. Anyone who’s ever worked with Helium knows how difficult it is to seal – it is a noble gas, and so is monatomic, which means its molecules are really small, and they get around most seals.

The other problem faced by thermodynamic engines is that while they are able to capture 100% of the energy of the beam (unlike the 30-50% of PV cells) they have to waste a good fraction of it at the heat exhaust side, and this gets worse the hotter the exhaust is. Which means that a thermodynamic engine needs to be coupled to an efficient heat exchanger – something that NSS started to design as well.

As it turned out, NSS was not able to solve the He sealing issue, and started working fast towards a photovoltaic “plan B” climber. However, not all of the effort was wasted – the heat exchanger design turns out to be very important in keeping their PV cells cool – more on that on the next post.

As a side note, Bert Murray and Matt Abrams have vowed that if the prize money is not awarded this year, they will solve the Helium seal issue and be back next year with a working Stirling climber.

Following the successful low-altitude test two weeks ago, we re-assembled this past weekend for another round of testing – this time to full altitude, and integrating all steps of the operation.

Just like last time – everything worked straight out of the box. Set-up was quick (less than an hour) and we were ready for the helicopter. Doug Uttecht was flying for Northwest Helicopters again, and he seems to have been practicing this in his mind over the last two weeks, since we were able to dive right into it.

First flight was a warm-up flight, duplicating last week’s flight, just to make sure we haven’t forgotten anything since then. We additionally rehearsed radio commands so that we will later be comfortable positioning the helicopter.

We then practiced climber pick-up and lay-down, which are now a bit more complicated than they would have been with the winch-based design. This is done with two simple tools that allow us to handle the cable without really getting uncomfortably close to it.

We next performed a series of measurements in order to correlate helicopter positions and lasing angles. The trick is to have the climber within the allowed 15-degree lasing angle throughout the climb, while at the same time maintaining its separation from the helicopter. Not-too-steep, not-too-shallow, and actually, we need to drift the helicopter during the climb since there’s no single position that satisfied all conditions. Given the practice we’ve had, this was almost trivial to do, and what’s more important, since wind conditions  will likely be different during the games, we know we can adjust in real time to different cable sags.

Finally, we did an end-to-end test with battery powered climbers. Only USST and KCSP had climbers ready to go, and KCSP suffered from control related issues and did not have their van full of spare parts with them, so to Brian’s endless misery, they were out of the game. USST was the last climber standing, and on their second try, they put the pedal to the metal and completed the 1 km climb with no problems. Meanwhile, Lasermotive who were out with their beam director, confirmed that tracking was feasible within the 15-degree cone I mentioned.

Not much more to say then – the vertical raceway is now ready and waiting for the teams. More information on the upcoming schedule coming your way soon.

The image was taken from the mock climber, at the climb starting altitude of 100m (330′). The helicopter will be flying at a height of 1300 m (~4500′)

While over the last week (and the next two, most likely) we are pre-occupied with helicopter flights, I do promise to get back to the main business at hand – power beaming – just as soon as possible.

A quick teaser – team NSS are racing against the clock to qualify in time for the games. They were not ready when we held the test flight in Dryden in July, and have been racing to take advantage of our misfortune. I’ll keep you posted on their progress.

Ben

The results of this test flight were nominal – just what we wanted. Happiness is three taut chains!

• GPS position keeping worked flawlessly, with the pilot maintaining a horizontal envelope of <40 m irrespective of flight altitude.
• Virtual Bob, in all configurations, worked exactly as intended, keeping the altitude to within several feet, controlling cable tension, and damping the whole structure.
• Deployment off of the figure 8 was smooth.
• Workload on the pilot was reduced significantly, with heads-in operation proving completely feasible.

In all honesty, this should not have been so difficult to do, but the best laid plans, etc.

A big part of getting it right was finding the right crew:

Doug Uttecht and John Peden of Northwest Helicopters

Our next step is to follow up with a high altitude flight to validate the end-to-end procedures. We need to work on a technique for graciously retreiving the climber when folding the pyramid, and we need to decide on whether cables will be reused once laid on the ground.

If all goes well, we’ll be able to do this flight in two weeks, and then turn our attention to running the games.

Linear Bob, single-stranded

The apex with breakaway link

Single-Strand Bob, full view

Implementing Bob turned out to be very easy, an exercise in “junkyard engineering”. After looking at the weight and strength requirements, we chose to forgo the thick cable in favor of  steel chain, and place it only at the bottom end of the pyramid. We thus connected three 3/16” steel cables (same as the climb cable) directly to the breakaway link, and added 100 feet of 3/8” drag chain at the end of each. The total weight of the chain us 400 lbs.

The dimensions of the chain were chosen based on the rate of mass-accumulation we want to achieve – for example, if the chain weights 2 pound per foot, than as the helicopter rises one foot, it lifts 3 lengths of chains, 1.4 feet each, plus a bit of sag – a total of about 4.5 feet, and so is accumulating mass at a rate of 9 pounds per foot.

Seattle is a good town for finding cheap chain. A few phone calls to used marine equipment stores, and there it was – a barrel of 100 m and 400 pounds of 3/8” chain, weighing about 1.2 pounds per foot. Perfect – we can use it as is, or double it up. This chain has shorter links than a standard trade chain, which means it weighs more per foot. Maybe an old anchor chain. Perfecter.

The point masses for step Bob should weigh about 500 pounds total – 167 pounds each, and should be sturdy enough to be beaten around a little bit, and cheap. Truck tires did the trick, and Tires Inc were happy to donate a few used ones to our cause. We ended up taking only three 75 lbs tires, so were on the light side. (this will show in the video of the flight)

Since the forces at the end of the chains are now very low, we used soft line to tie the ends of the chains to our cars. We deployed it on this beautiful field not far from Northwest’s HQ, hooked up the helicopter, and in no time were ready for the first test flight.

The sequence of images clearly shows how Bob works  - The helicopter picks up the cable from a figure-8 coil we set up, and after the coil is exhausted it picks up the apex of the pyramid. The pyramid “stands up” until the chain begin to rise, at which point the rate of pickup decreases and equilibrium is reached.  The pilot doesn’t have to stop the helicopter – it does it all by itself.

Double Stranded Bob	Double Stranded, Step Bob

Step Bob and Linear Bob

When looking for a softer cable arrestor device, Dryden’s John Kelly came up with the concept of Bob.

Bob is a weight that hangs at the end of the cable, lifted by the helicopter, so that once airborne, the tension in the cable is determined by Bob, independent of the altitude of the flight, which is determined by the helicopter, and can deviate considerably, as long as Bob does not touch the ground.

In order to prevent Bob from potentially becoming, well, a wrecking-bob, we would need to attach slanted stay-lines to him, limiting his motion.  The stay lines must be close to horizontal, so that possible vertical motion of Bob will not be hindered.

Of course the helicopter will now be lifting the stay lines as well, so their weight gets added to Bob’s weight. Actually, if we make the stay lines heavy enough, we don’t really need Bob anymore – the weight will be distributed along the stay lines, and no point mass will be hanging over our heads. Thus was coined the term “Virtual Bob system” – a bobless bob!

Next, we make the heavy stay lines (three of them) slant at a full 45 degrees. Since they are heavy, they will sag, and some portion of their length will lie on the ground. If the helicopter moves upwards, the amount of airborne weight increases, thus pulling the helicopter down. If the helicopter moves downwards, the amount of airborne mass decreases, and the helicopter floats back up. Since this is a very gradual stabilizing force, we call this configuration “Linear Bob”.

If we further place point masses a certain distance up the stay cables, we will give Bob a distinct “notch” for the pilot to pull against – we call this configuration “Step bob”. Ideally the point masses add up to the extra lifting capacity of the helicopter at that altitude, so that the only way it can pull them up is to bounce against the end of travel – thus giving us a very soft, resettable force fuse that is coupled to a fixed altitude – Once the helicopter exhausts its inertia, the weights come back down to the ground, resetting the helicopter’s altitude.

This keeps us with the paradigm of flying constant tension, except the system now has a large, self-correcting sweet-spot.

Following Mike Kapitzke’s suggestion, we also moved the breakaway link to the apex of the stay-line pyramid. Now, since the Virtual Bob system fully determines the position of the apex, if the breakaway link were to pop, everything will fall within the base of the pyramid, and so anyone standing outside the edges is not in the fall zone – very convenient for us.

On the right you can see diagrams of Linear Bob and Step Bob. They work well in theory – all that remains (again) is to try them in real life.