I'd like to contest the idea of a minimum TL on space flight.
Realistically, the Victorians had the materials technology to produce a spacefaring vessel. Of course, this is like saying that the Roman Empire had the technology to produce a railroad.
In FFS and Hard Times, the Resistojet Rocket can be produced at TL4. The Wiley Post Vacc Suit (or a rather more primative version) could be produced at TL 4 as well. It would be more of a heavily modified diving suit that had been adapted to vacc suit use. By modified, I"m referring to the addition of articulated metal bands, similar to a Lorica Segmentata or Visby coat to keep the occupant under pressure.
The resistojet rocket is essentially a big steam cannon designed to hurl a projectile on a ballisic or semi-ballistic trajectory. It could be extrapolated to hurl its payload into an orbital path. At TL4, the technology to produce compressed gasses exixts, so now our voyagers can change their orbital position and even return to their homeworld. Or, with sufficient gasses, they can move to a higher orbit. Also at TL4, the gyroscope and sextant exist, so now our astronauts can stabilize the vessel and plot a course.
Radiation and other forms of life support may still be an issue, but the essentials of a crude form of spacefllight exist. So I"m thinking that the minimum TL for spaceflight might be mid-Tl 4; somewhere around the beginning of WW 1 or even a little earlier, say the 1880s.
Just my thoughts.