This is how I believe they got to Ascension and is probably going to be the best theory there is right now.
I believe that after Kino the gang had ended the zombie mess everyone went back to their normal lives. Takeo and Tank went back to war and if you have ever seen the moon on the original der riese you will remember there was 5 115 spots on it so that's probably where richtofen went and also explains how he got the space suit and then Nikolai went to help the Ascension group. Ok so this part will be kind of unological but remember it's only a theory. So what I think happened was is while the Ascension group was experimenting with the monkeys and 115 the became the cosmic silverbacks and started attacking the Russians and turning them into zombies so Nikolai called up everyone and Richtofen cane on the lunar lander and picked up everyone and went to the launch facility and started killing zombies