Rocket from troposphere - Why not?

[mrxyz]

Why aren't helium balloons used to carry rockets or payloads to the troposphere which is about 25km above sea level before they are launched?

To reach actual outer space is 100km above sea level. The above will in turn save 25km worth of fuel and the overall mass of the rocket which needs to be pushed would also decrease.

Why isn't this done in launches?

 

[mfb]

If you want to reach space, you usually want to reach an orbit, which requires a speed of roughly ~8km/s. This is the tricky part - the height is quite "easy" to reach. In terms of the total energy, you can save something like ~1-2%, so you still need a massive rocket. And how do you launch such a massive rocket in a reliable way from a balloon?
This launch would reduce atmospheric drag significantly, but the tricky launch situation and the required gigantic balloon are a big downside.

There is some concept to reach a height of ~20km with a static structure, but it is purely hypothetical.

 

[berkeman]

Why aren't helium balloons used to carry rockets or payloads to the troposphere which is about 25km above sea level before they are launched?

To reach actual outer space is 100km above sea level. The above will in turn save 25km worth of fuel and the overall mass of the rocket which needs to be pushed would also decrease.

Why isn't this done in launches?

This was already discussed in this forum this month:

https://www.physicsforums.com/showthread.php?t=658399

If after reading that thread you still have questions, please post in that thread for continuity.