13 May, 2013

Thoughts on air-assisted versus ground launch for LEO missions

Most of what I read on this topic is focused on energy considerations -- i.e. how much delta-v can an air launch save versus a "simpler" ground-based multi-stage system. And when you look at it this way, the answer -- somewhat surprisingly and disappointing to my ever hopeful mind -- is not as much as you might imagine or wish for. You will find a good treatment of the situation here:


The bottom line is, from a purely energetics standpoint, air launch can be viable for relatively small mass payloads (assuming they are within the capabilities of the launcher/carrier aircraft), but it's not anything to write home about. You can compare the perfromance and cost of Orbital Science's Pegasus (air-launched) versus its Taurus (conventional multi-stage ground launch) to get an idea of what I mean.

But I'm thinking that energetics is not the sole or even the primary consideration to be addressed in trades of air versus ground first stages. I see these other factors as being significant drivers to choosing an integrated solution:

  • Options for reuse, especially of high value components
  • Reasonable and survivable (I'm talking of hardware here since I'm not focusing on a human-rated system at this point) abort modes
  • Opportunity for gradual checkout of all vehicle systems prior to commit to flight
  • Enhanced dispatch reliability and launch phasing with respect to an orbital target
  • Minimized time and consumables dedicated to orbital target rendezvous
  • Use of existing hardware to drive down NRE
  • Options for "non-conventional" approaches to system architecture and CONOPS. Things like:
    1. Tow, rather than carry of the flight vehicle
    2. "Leave behind" gear (very massive tires, wheels, brakes, steering actuators, struts, structure)
    3. Transfer of oxidizer and/or fuel from the launcher aircraft while in flight
    4. On-board oxygen generation within the launcher aircraft
    5. Towed UAV that returns to takeoff runway autonomously after releasing the flight vehicle

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