Nice MD Phil .
I bought one from an auction a couple of months ago and I prefer them to the Moon Explorer.
Have you let it go over some rough terrain ,,,,, man made in the house of course. You may be surprised at its ability.
I don't know how to post direct links but here's the number 331338063181.
Don't forget that the 3.5 inches doesn't give a great launch height but its matter of compromise really. Test it out yourself and see what your happy with.
I'm going to put a bit of epoxy on the bottom of the spring once I'm happy with the performance
Good luck chilli .
Got some springs off the bay and got him firing eventually .
They came with a 6.5 mm diameter and as the original hole was only 6mm I just had to run a 6.5 mm drill down the rocket to get them in.
You can cut the springs to what length you want and the longer you have the more power you get,,, but,,, if you have them much longer than David Kirk said then they don't sit correctly on the robot in the unlocked position so they want to be about 3.5 inches long but this of course means power loss,, dohhh ! The other point is that if you have them longer then the pressure on the spring puts a lot of load on the plastic lock tabs on the rocket and the launching post . Also it makes it hard work on the cam to push the rocket round to release it.
So in this case shorter is better and that's something you don't say everyday.
I hope the picture works as I've not posted a pic for a long time,, so it may be to big or small.