For the obvious reasons, I invariably get too busy to blog exactly when things get interesting...
Powerful, Light, Fast, Efficient
The basic premise of the rules is simple: The teams have to design, build and operate vehicles that can travel up a tether, with the small caveat that they cannot have any energy stored on-board. Since direct solar conversion is specifically not allowed, they must also build a system to transfer power from the ground to the vehicle.
The rules do not call for a laser-based system, but in practical terms, if you want to keep the receiver portion of the system (on the climber) light-weight, the engineering constraints pretty much steer you towards a laser-based system.
So how are the teams scored?
It took a while to arrive a t a formula that captures what we want out of a power beaming system, and doesn’t force the games into a single technical solution, or into a “if you can’t make it good, make it large” sort of situation.
The basic role of a climber is to carry payload. Transport systems are usually rated by throughput – how much you can carry, and how fast – measured in tons per year, for example. A first attempt at a metric would thus be score = payload x speed.
There are two problem with this metric. First, if you buy a larger laser, you can move more payload. We don’t want the games to become a laser-buying competition. Second, since the self-weight of the climber does not count, then for a given amount of generated power, it pays to move as slow as possible, and carry as much payload as possible, thus maximizing the score, but also making the games infinitely long…
The solution to the first problem is to normalize the score by the empty weight of the climber. Thus rather than ask how much mass the climber can move per unit time, we’ll ask how many times its own mass can it carry per unit time. So a 4 lb climber carrying 6 lb of payload at 3 m/s will get exactly the same score as an 8 lb climber carrying 12 lb of payload at the same speed. Thus score = payload mass x average speed / empty mass.
The solution to the second problem is to institute a minimum speed (2 m/s in our case). This way, teams will still try to move as slow as possible (carrying as much payload as possible) but will have to move no slower than 2 m/s.
While conversion efficiency doesn’t factor explicitly into the score, it is very much there – a less efficient climber will need more climber mass to generate the same amount of power, leaving even less room for payload – it will thus be penalized on both the denominator and numerator of the the score formula.
This has worked out well. We know this since the range of entries is wide - the climbers occupy diverse points on this solution space. Some emphasise light weight, some emphasise high efficiency, and none of the teams had to rush out and get more power – they improve their score by making the most efficient use of the existing power source.