For endo-atmospheric flying hypersonic gliding craft, the dead weight of expended stage is the carrier of kinetic energy, energy that is required/consumed to overcome drag while maintaining cruise altitude. Reducing mass fraction with lighter casing will result in higher burnout velocity, that takes it to higher mach number and higher thermal regime. Lower Mach number allows this missile to continue to use material that can sustain the lower temperature, (I.e. lesser Max-Q pressure, and Max-Q temperature) beyond which one will require greater thermal protection (nose and glide-body) like a long range ballistic missile). I guess it is at a sweet spot for mass fraction for this type of trajectory.
BTW making a rocket stage with Maraging steel casing is more difficult than using composite, as maraging steel is extremely hard to work. IOW composites is not the zenith of ultimate engineering, maraging steel is equal if not more difficult.
As an aside a friend asked why the booster stage of K-15 so tiny that takes it to a only 5 km altitude. It is counter to conventional space rocket / ballistic missile design principals to use staging with progressively smaller stages to obtain higher effective mass fraction. I was similarly puzzled 4 years ago. The significant of this choice is shown on page 14 of the PPT, and also in the Shourya article I wrote on Indian Defense Review 4 years ago ( [url="http://www.indiandefencereview.com/news/indias-multifunction-missile-for-credible-deterrent/"]India's Multifunction Missile for Credible Deterrent: IDR[/url] ). IOW stealth. Any expended stage wobbles after separation and that makes it most visible to a long range radar that is scanning the search space. When the single stage craft rises above the radar horizon at an inclined attitude, it presents a very small RCS for the rest of the flight.
BTW making a rocket stage with Maraging steel casing is more difficult than using composite, as maraging steel is extremely hard to work. IOW composites is not the zenith of ultimate engineering, maraging steel is equal if not more difficult.
As an aside a friend asked why the booster stage of K-15 so tiny that takes it to a only 5 km altitude. It is counter to conventional space rocket / ballistic missile design principals to use staging with progressively smaller stages to obtain higher effective mass fraction. I was similarly puzzled 4 years ago. The significant of this choice is shown on page 14 of the PPT, and also in the Shourya article I wrote on Indian Defense Review 4 years ago ( [url="http://www.indiandefencereview.com/news/indias-multifunction-missile-for-credible-deterrent/"]India's Multifunction Missile for Credible Deterrent: IDR[/url] ). IOW stealth. Any expended stage wobbles after separation and that makes it most visible to a long range radar that is scanning the search space. When the single stage craft rises above the radar horizon at an inclined attitude, it presents a very small RCS for the rest of the flight.