Advantages of Stratospheric Launch
Make HTHL reusable launch vehicle more efficient
Reduce steering loss and gravity drag
Increase thrust and specific impulse, alleviate over-expansion issue
Lower aerodynamic drag loss and reduce Max-Q
Allow a relatively lower thrust/weight ratio at ignition
Avoid the destructive acoustical energy reflected by ground
Both launch platform and SLSS are fully reusable. Helium lifting gas inside launch platform can be recovered and reused. After launches SLSS, the launch platform returns to its launch pad directly. SLSS slows down by aerodynamic braking. After re-entry, it is finally recovered by means of conventional wheel landing. Depends on mission type, the upper stage can be one-time-only conventional upper stage or a reusable space plane.
Stratospheric-airship-assisted launch platform provides altitude redundancy. Stratospheric-launched suborbital shuttle (SLSS) owns payload recovery capacity. If engine failure occurs shortly after launch, the SLSS has sufficient altitude redundancy, which is provided by the launch platform, to safely dump all its fuel and glide to an airfield nearby. If there is a serious lifting gas leakage issue in platform's ascent stage that the mission has to be aborted, the SLSS can be launched and rockets at low thrust along a lofted trajectory to burn out its fuel, and subsequently glides back to the space port. Besides, the platform owns large amount of lifting gas reserve in its LH2 tank and helium tank, which can guarantee a successful launch mission in the scenario of minor lifting gas leaking.
Due to highly inflammable nature of hydrogen on the ground, the following measures will be taken to boost a safe implementation of hydrogen lifting gas:
- Hydrogen is used as lifting gas only in ascent and cruise stages
- High altitude and corresponding low pressure can grant more safety to hydrogen usage
- The venting of hydrogen lifting gas occurs at high altitude
- Helium ballonet serves as semi-isolation layer between hydrogen and electronics
- Conductive materials included in hydrogen ballonet envelope to remove static charge
How to offload extra buoyancy after stratospheric launch platform launches SLSS?
Vent hydrogen lifting gas. Hydrogen venting not only offload buoyancy, but also can boost safety in platform's return stage.
Advantages over conventional air-launch approaches?
Conventional air-launch, specifically, launching a rocket/vehicle from an airplane, is greatly constrained by the maximum payload capacity and available volume of its carrier airplane. Launching a mainstream communications satellite or spaceship using this approach is not feasible since there is no operational airplane with the required payload and volume capacity. The design, manufacture and operating of a new super-large airplane with required volume would be costly, and launching a rocket/vehicle from such airplane can also be risky.
Meanwhile, the launching-from-balloon approach also suffers a critical drawback that the balloon is lack of steerability. As such, the delta-v penalty would be large to compensate positional and azimuth error that brought by balloon. The unpredicted free falling of rocket/vehicle debris additionally raises safety issues. Moreover, the balloon is not reusable and hence increases the cost.
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