Scrambler C

[pumptato-cat]

What are the merits of having a spring/elastic launcher versus a pulley system launcher?

None, really. At least, for this season. Time isn't worth much so I'd focus on accuracy if I were you.

I suppose the goal would be to maximize the speed at which the vehicle is pulled as the weight falls? Or are spring launchers simply not relevant with such a light falling weight?

I'm not a physics expert or anything but here's my take on it: You should try to maximize the power being transferred into your "spring" so that it's released all at once when the "locking" mechanism is triggered.

I've tried constructing a locking mechanism like the one described on the wikipage, but even after messing around/tuning it for a while, it only makes our vehicle slower. Right now, we just launch it like a "regular" string launcher, just with some rubber loops in the middle of the string. Has anyone else had success with a locking spring launcher?

Yeah, I'd imagine that would cause your car to go slower. I'm assuming you've made a spring launcher with no trigger? Or am I misunderstanding?
If you have, there's why your car's being slow. Again, I'm not sure of the exact science behind this, but think of it this way: you pull a car with a piece of twine(that doesn't stretch) or you pull a car with a piece of stretchy rubber. What's going to move faster? I suppose it depends on the car's weight and friction with axles and all, but in most cases the rubber will want to stretch while pulling the car. Now replace your hand with the 1.5kg weight; instead of all the energy(from the weight dropping) going to propelling the car forward, some of it goes towards stretching the rubber, because it's easier to stretch rubber than to pull the car forwards.)
I HOPE that makes sense, it's the best attempt I have at explaining over text...

I have yet to see another spring/elastic launcher, but I've seen a few pulley launchers.

As someone with a spring launcher, it's not worth it... Spend more time on accuracy and stick with a pulley launcher UNTIL you can hit the target with <5cm accuracy consistently. I spend more time fiddling with the launcher than the car.. Time isn't worth much anyways, so if you can get like 4-5sec in time, you'll have 8-10 points and a decent range for distance score(Anything sub 20 is pretty darn good).
It's not nationals

[knightmoves]

What are the merits of having a spring/elastic launcher versus a pulley system launcher? I have yet to see another spring/elastic launcher, but I've seen a few pulley launchers. I suppose the goal would be to maximize the speed at which the vehicle is pulled as the weight falls? Or are spring launchers simply not relevant with such a light falling weight?

The idea is that if you have the car directly accelerated by the weight with a string, then the mass that is being accelerated is the combination of the weight and the car. So if the car has mass m, and the falling weight has mass M, then your energy equation looks like:

0.5 (M + m) v^2 = M g h

or v = sqrt (2gh M/(M+m))

With a spring launcher, the idea is that you store the energy of the falling weight in a stretched spring, and then transfer all of that energy to the car.

So for stretching the spring, you have 0.5 k x^2 = M g h (x = spring extension, k = spring constant), and then assuming you lock off one end of the spring perfectly and then release the other, you can transfer all that energy to the car:
0.5 m v^2 = 0.5 k x^2 = M g h

and so v = sqrt (2gh M/m)

In the limit of a very heavy car, these are the same, and you very heavy car will travel very slowly whatever launcher you have.
If your car weighs the same 1.5 kg as your falling mass (which is still a rather heavy car) then the spring launcher launches the car faster by a factor of sqrt(2). If the car weighs 500g, then the spring launcher will launch it twice as fast as the direct launcher, and if you could get the weight down to 100g (a bit of a challenge given that more than half of that would be accounted for by the egg), then the spring launcher would be a factor of 4 faster.

Assuming the spring launcher is perfect, which it won't be.

Just for fun, let's put the rest of the numbers in. the falling mass is 1.5 kg, the scrambler height is 50cm, so perhaps you can achieve a practical fall distance of 40cm, if you're careful about it. Picking the 500g vehicle, the pulley launcher might launch it at 2.4 m/s, and the spring launcher might launch it at 4.8 m/s.

The run time is measured over a 7m distance, so in a magic frictionless universe, the pulley launch car takes 2.9s, and the spring launch car takes 1.5s. So having a perfect spring launcher would be worth getting 2.9 cm closer to the target.

You're probably better off spending your time working on accuracy.

[windu34]

Agree with prior comments. Seen multiple devices with 4s times with pulley only. With spring, can probably get to 1.5-2 seconds, but if you're not dead on every time right now, you should definitely focus on accuracy.

[Venomizer]

I also agree with the prior comments. I have seen pulley launchers get as low as 3.5 seconds. However, under 2 seconds even with a spring launcher seems like a stretch. The lowest I have seen with a spring launcher is about 2.3-2.6 seconds. I'm mainly making an educated guess, but since we are dealing with light cars, a ton of friction comes into play (even air friction potentially), especially at the faster speeds, which is probably why getting under 2 seconds with a spring launcher is pretty hard. Another thing, spring launchers will provide a much higher jerk value (the rate of change of acceleration) which will make it harder to produce consistent results (especially with light cars). So if you are doing a spring launcher method, I would highly recommend implementing some rails to guide the car until it exits the launcher to minimize the variation.

[mnoga]

Mtn View scored approx. 7.7 at NorCal states using a spring loaded launcher. Run time was 2.15 seconds, but timing was by hand and generous. Run time was really 2.3s.

[knightmoves]

Mtn View scored approx. 7.7 at NorCal states using a spring loaded launcher. Run time was 2.15 seconds, but timing was by hand and generous. Run time was really 2.3s.

Nice.

I saw a sub-10 with a simple launcher - I think it scored as 9.5, and almost all of those points were run time.