I was going to use my slimeblock launcher and ran up to it, but I lost most of my forward momentum, so I almost just went straight up.
I tried several times. When you run onto a slimeblock launcher and get launched up you almost stop for a split second, and then you go up. Earlier you went much further forward. Now you loose most of your momentum.
1. Make a slimeblock launcher. Piston with a slimeblock on it, and pressureplates that sends a redstone signal to the piston, just by the slimeblock.
2. Run onto the slimeblock and touch the pressureplate. Try to touch both at the same time. It's easier. If you run diagonal it should work.
3. You now see that you loose most of your forward momentum.
4. Fix it.