First off: it helps to understand the purpose of the show -- its stealth science education - tricking 10 year old kids into watching an explanation of how a wing works. They sit thru the mini-lectures bcause they get rewarded afterwards with a shot of someone making precision adjustments with sledgehammers. When chosing challenges, its the education that drives the choice. The competition is partly to make it addicting, and partly to give the kids the idea that actually designing and building something might be a lot of fun.
Yes, this is a "rich" junkyard. There are all sorts of neat things to find. And unlike some, there is a lot of stuff that isn't metallic. (usually its construction debris -- the plywood we found had clearly been a concrete form in a prior life) -- Its mostly what you get, when you don't have the yard workers picking over the good bits. The set was a corner of a real working scrap yard. On the other side of the wall, there are cockneys in hydraulic claw loaders, tossing cars thru the air. You have to wear a hard hat when you go to the bathroom. (its out by the truck scales). When stocking the yard between episodes, the random lumps of steel plate are just dumped over the wall from what they have sitting around. But yes, they will add extra stuff to make it possible to complete building a machine.
The basic rule for seeding: If its not possible to safely improvise a part with the time and tools provided, they will provide something that can be pressed into service. It will require some ingenuity to make it work, it will never "just bolt on". If there are specific safety regulations, the relavant parts will always be provided. For example, things like safety valves, regulators, and gas tanks will be planted, and will have their certification paperwork sitting in the directors briefcase. (and if we happen to find such a part that isn't one of the known good ones, they don't let us use it) A good example: The propellor that the navy crew hacked up was provided. Any propellor they could make in the time they had (no time for glue to dry to laminate) would not have been safe to run up to speed. Another example was the tank and regulator used by the Dipsticks submarine - The tank had a current hydro test, and the regulators used were new.
But: Just because they give you a part, that doesn't mean its clear sailing. For example the wheels in the tractor pull. Sure they were there, but none of the differentials in the yard came close to fitting the bolt circle. If you wanted to use them, you had to make it work.
And this brings up another point: That same helpfull crew that hides essential parts, can just as easily remove them. They made sure that there wern't matching differentials for those wheels. In the fire fighting boat episode, there wasn't a pump to be had. Both teams had to make a pump. And not just a wimpy one, the burning shed was supposed to be 50 feet away.
As to engines, yes, there is sample bias in the junkyards you visit. What happens in a conventional junkyard, is that if a car comes in with a running engine, the engine is immediately pulled and stored until it is sold. Only dead engines are put out into the yard. As a junkyard owner, you don't want someone wrecking a $200 engine to get a $2 part. In this yard the teams are taking the place of those yard employee's that have the job of pulling the good ones out. Teams get their engines from the same places people that sell used engines get them -- from cars whose owner has decided to artfully customize his vehicle; with the help of a tree, broadside.
Yes, this is TV, and they do have to make sure at the end of the day, they have two machines, with at least one of them likely to complete the course, and the other at least able to fail in an instructive way. The shows cost close to half a million dollars per episode to make, and the producer is betting that money on half a dozen amateurs. But they do have a surprisingly light touch. We did have ample opportunity to open fire at both feet.
The time limit is pretty real. You get an hour tools down for lunch, and credit for the time that the hosts spend disturbing you. If nothing else, a second day of a film crew adds a lot to the price (there are upwards of 50 people onsite when filming the build). And they may only have the test site for a specific day, so you really do have to finish something like on time.
I can assure you its not scripted, what happens is up to the contestants. The teams really do not find out what they have to build until that morning, on camera. The producer has been very suprised at what the teams made sometimes.
By carefully seeding the yard, they make it possible to have challenges that would be impossible otherwise. If you want remote controlled flight, you have to include the radios, there is no way a team could make a legal system if you don't. To show steam engines, you have to supply a boiler and all the fittings, as improvising a boiler is simply not legal to do.
Given a choce between not covering an interesting topic, and putting (imperfect) readymade bits into the yard, the choice is obvious. Ingenuity is still the most important thing the producers put in that yard. The most used tool is still the brains of the team members. (the welder is in second place).
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