Japan's space program focuses on cost effective solid rocket boosters and does not build ballistic missiles out of them. Liquid engines generally have the advantage in ISP, which is why they dominate large rockets. But solid rockets have substantial savings in manufacturing and operational simplicity that shouldn't be simply handwaved away.
> savings in manufacturing and operational simplicity
Solids are very dangerous and complex to handle. The end to end infrastructure cost absolutely does not make them viable.
Are you seriously suggesting something about operational simplicity when you are dealing with a material that is highly explosive and can easy kill a whole launch crew if badly handled?
In addition, solids require very expensive launch infrastructure to modify the boosters to fly specific routes. Check out the highly complex infrastrucutre the Europeans deploy in Kourou.
There is a reason, non of the new commercial companies even remotely want to touch solids, they are avoid like the plague.
The solid based Pegasus rocket is vastly more expensive then the air-launched liquid fueled Virgin Orbit rocket.
Japan with their solids never were a series competitor in the international launch market and their launch rates were always very low.
And solids are also never practically reusable as the expensive part is the whole fuel manufacture and infrastructure.
Solids are just a bad idea and make no sense when fully consider. Only nation state driven launchers would use them.
Nobody builds SRBs "out of" missile boosters, but they are built by the same manufacturers. Fat contracts for one keep the business afloat for the other, keep the workers for one busy and practiced for the other. It's welfare for defense contractors using the same technology, facilities and expertise as the weapons, NOT part re-use. Part re-use was never the argument, not with the Shuttle and not now.
> Japan doesn't have ballistic missiles.
The Japanese SRBs are built by Mitsubishi Heavy Industries. Do you know what else they manufacture? Missiles of course. Not ICBMs, but they do manufacture missiles, particularly the powerful SM-3 Block II.
Japan focused on solid fuels because they needed to build rockets that were impractical enough to not be usable as ICBMs.
They had to do crazy things like launch them sideways.
This comment makes no sense. Missile and rockets both go sideways. Modern missiles all use solid fuel. Japan's solid fuel rockets are build by Mitsubishi Heavy Industries, which uses the same fuel and manufacturing expertise to also manufacture missiles.
