Sirrus 50 Summit Blue â XS/S. The Stratos an Sirrus Series offer unparalleled design and features in ventilated daypacks and backpacks. The proven Osprey AirSpeed suspension is now offered in a 50-liter volume for overnight to weeklong ventilated backpacking comfort. A full quiver of bells and whistles make these styles a great choice for first time to casual day hikers and backpackers at an excellent price.