One of the nice things about Near Infra-Red light is that it gets picked up by regular cameras, showing up as some combination of colors, unique to each camera.  In the case of my camera, it shows up as this pink-purple hue.  To the naked eye, the climber seems unlit.

For more videos from this test session, check out the Space Elevator Blog (”There’s cool, and then there’s Über-Cool”), especially the last video, where Ted’s Maniacal laughter reveals his true nefarious character – who knew.

(The test was conducted at TRUMPF’s facilities in Detroit – they make the cool lasers that put your car together.)

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.

To the point – Team NSS Bert Murray, Captain) has used the delay in the games to their advantage, and have notified us a couple of weeks ago that they are ready to try to qualify. (NSS originally took a hit when sponsorship of a major part of their hardware fell through)

We are expecting to finalize the competition date any day now, and so NSS has really cut it close. We’ll be traveling to the TRUMPF facility in Detroit early next week to witness the qualification, which will involve a demonstration of the transmitting and receiving optics operating at full power, tracking, reflection measurements – 2 days of testing overall. (we did something similar, if you recall, with LaserMotive about 3 months ago.)

Since Chicago is practically next door, both Ted Semon of the Space Elevator Blog and Nic Degrazia of Bitter Jester Creative will come over to watch and report.

Expect some preliminary reporting on Monday, and an Aye or a Nay late Tuesday evening then.

Meanwhile, we’re working hard on finalizing the date for the games – real soon now!  I will of course let you all know just as soon as I find out myself.

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

NSS's Climber

Small is Beautiful

NSS (National Space Society) started its participation in the games pursuing a novel Stirling Engine based design – very different than the rest of the teams. A Stirling engine is a heat engine, which means that the laser beam is converted to heat (with almost perfect efficiency), but the heat is then converted to mechanical energy subject to the laws of thermodynamics, which dictate a certain level of inefficiency that depends on the ability of the engine to dissipate the heat. I will dedicate a separate post to that concept, just as soon as Bert Murray, NSS’s Captain, will allow me to…

Construction of a competitive Stirling engine is very challenging, and even though NSS was able to design around many of the difficulties, time was drawing short and NSS decided to switchto a more conventional PV-based design.

The new PV climber is tiny – a micro-climber. The rules for the competition do not prohibit such a small climber, but at some level going too small is difficult since some components cannot shrink along with the rest of the  climber and so end up being fixed-weight overhead.  Whether such a smaller climber can indeed be made to work remains to be seen.

The bags in the picture are an interesting feature – can you guess when thet do?

As of this moment NSS is still not qualified, and with time drawing short and resource scarce, well, it’s a tight race.  They are very close, as this post testifies, and are working hard to catch up.