Existing engineering materials allow the construction of towers to heights of many kilometers. Orbital launch from a high altitude has significant advantages over sea-level launch due to the reduced atmospheric pressure, resulting in lower atmospheric drag on the vehicle and allowing higher rocket engine performance. High-altitude launch sites are particularly advantageous for single-stage to orbit (SSTO) vehicles, where the payload is typically 2% of the initial launch mass. An earlier paper enumerated some of the advantages of high altitude launch of SSTO vehicles. In this paper, we calculate launch trajectories for a candidate SSTO vehicle, and calculate the advantage of launch at launch altitudes 5 to 25 kilometer altitudes above sea level. The performance increase can be directly translated into increased payload capability to orbit, ranging from 5 to 20% increase in the mass to orbit. For a candidate vehicle with an initial payload fraction of 2% of gross lift-off weight, this corresponds to 31% increase in payload (for 5-km launch altitude) to 122% additional payload (for 25-km launch altitude).
Example calculations of mass required for a fifteen-km tower sized to support a 2000 ton launch weight:
Structural material: Graphite epoxy:
LL = 107.5 km No taper needed
tower mass 280 tons
LL = 15.4 km taper required
tower mass 5300 tons
(area taper ratio 2.6)
The improvement in performance is primarily due to lower air density. By starting at a lower atmospheric pressure,
the vehicle has several design advantages that result in a reduced delta-V required to reach orbit. As well as the
reduced drag, the aerodynamic advantages include:
1. Reduced atmospheric drag loss
2. Vehicle can be designed with less attention to aerodynamics.
3. More optimum trajectory curves toward horizontal faster
4. Maximum aerodynamic stress (“Max-Q”) occurs at a much lower pressure; lower aerodynamic stress
5. Aerodynamic vibrations lower; allows less robust (lighter) payload
6. Wind loads on vehicle in flight much lower
7. Acoustic loads much lower
8. Cryogenic storage easier (lower conduction and convective heating)
9. Aero-shroud jettison (for vehicles which jettison non-essential parts) can occur earlier in the trajectory
Data on engine performance improvement with the change from near sea-level to near-vacuum conditions has been tabulated by Isakowitz, Hopkins, and Hopkins (1999). For example, the Rocketdyne Atlas MA-5 sustainer engines produce a specific impulse 309 sec operating in vacuum; while the MA-SA booster (essentially the same engine with a nozzle reoptimized for low-altitude operation) produces a specific impulse of 253 seconds at sea level. High altitude operation results in a 22.1% increase in performance.