Advantages of Stratospheric Launch

Make HTHL reusable launch vehicle more efficient

Horizontal takeoff, horizontal landing (HTHL) reusable launch rocket/vehicle will be more efficient if the rocket/vehicle is launched above the dense part of atmosphere. HTHL Single-Stage-To-Orbit (SSTO) will be feasible using existing technologies and hardware, albeit it may not be the most efficient approach.

Reduce steering loss and gravity drag

High altitude makes it permissible to launch a rocket/vehicle at a pitch much closer to horizontal. This grant flexibilities to trajectory optimization and reduces steering loss and gravity drag, especially immediately after the launch. Moreover, the rocket/vehicle can take aerodynamic lifting to counter the gravity, which makes HTHL winged vehicles favorable.

Increase thrust and specific impulse, alleviate over-expansion issue

Ambient pressure at high altitude is much lower than the sea level. As such, back-pressure is greatly reduced. Rocket engine thrust and specific impulse can both receive a substantial gain, especially for cryogenic engines. Besides, the low ambient pressure greatly alleviates over-expansion problem. Rocket engine designers thereby own a larger flexibility to choose a larger expansion ratio, and further increase thrust and specific impulse.

Lower aerodynamic drag loss and reduce Max-Q

High altitude launch avoids flying through the dense portion of atmosphere and thereby reduces the amount of fuel wasted by aerodynamic drag, especially for winged HTHL vehicles. This also can minimize the maximum dynamic pressure and lower the structural weight.

Allow a relatively lower thrust/weight ratio at ignition

Thrust-to-weight ratio of stratospheric-launched rockets or rocket-propelled vehicles doesn't need to be larger than one at ignition.

Avoid the destructive acoustical energy reflected by ground

Avoid the destructive acoustical energy reflected by ground in the conventional ground lift-off.

Reusability

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.

Reliability

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 scenarios of minor lifting gas leaking.

Hydrogen Safety

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

QA:


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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