Both https://en.wikipedia.org/wiki/Ullage_motor https://en.wikipedia.org/wiki/Ullage_motor and https://en.wikipedia.org/wiki/Multistage_rocket#Hot-staging https

Both https://en.wikipedia.org/wiki/Ullage_motor https://en.wikipedia.org/wiki/Ullage_motor and https://en.wikipedia.org/wiki/Multistage_rocket#Hot-staging https://en.wikipedia.org/wiki/Multistage_rocket#Hot-staging can eliminate the "Ignite in free fall" requirement. At the cost of introducing other complexities in the rocket's design.