For the purposes of analyzing the characteristics of buoyant spacecraft, we will develop a conceptual mission profile for a lighter-than-air vehicle designed to launch 10,000 kg of payload to low Earth orbit. It will dock with a space station or release satellites and then return to Earth. This payload mass was chosen as a representative value for a medium lift launch vehicle capable of transporting crew and cargo to the International Space Station. Consequently the target orbit is at an altitude between 410km and 420km, included to 51.65 degrees, and an average velocity of 7.66 km/s.
We assume the buoyant spacecraft begins and ends its journey in the stratosphere, at an altitude between 35 and 40 km. To be buoyant in the upper atmosphere and support its own weight, the vehicle must be very light and extremely large, on the order of a kilometer or more in length. Since the vehicle never touches ground, it need not withstand the rigors of the lower atmosphere. The buoyant spacecraft’s journey begins at an altitude at which conventional rockets would have already expended tons of fuel to reach.
Notionally the spacecraft would be tended by high-altitude airships that fly up from the ground to transfer crew and cargo. Although some innovations are required to build and fly airships carrying crew and cargo into the stratosphere, these challenges are reasonably well understood. We will treat high-altitude airships capable of tending the buoyant spacecraft as a solved problem and not consider them further.
Control of the vehicle’s static heaviness relative to the outside air is necessary to change and maintain altitude. This is accomplished by adjusting the volume of lifting gas to increase or decrease the vehicle’s buoyancy. The lifting gas is pumped back and forth between the lifting cells and pressure tanks to adjust its overall density. Filling the pressure tanks increases density and the vehicle becomes heavier. Releasing lifting gas back into the lifting cells decreases density and the vehicle becomes lighter and rises. The system is entirely closed and eliminates any need for venting lifting gas or dropping ballast.
By varying its buoyancy in this manner, the spacecraft can control its altitude and ascend to a maximum altitude of neutral buoyancy, around 50 km. Above this altitude, lift provided by buoyancy will decrease and the spacecraft becomes heavier than air. Above this altitude, it must depend on propulsion to generate aerodynamic lift to support its weight and gain more altitude. As the atmosphere thins at higher altitudes, drag is naturally reduced and the spacecraft can engage its space propulsion system. The vehicle will use high-efficiency electrical propulsion such as VASIMR or other MPD thrusters to accelerate to orbital velocity. The upper surface of the vehicle can be covered with thin-film solar arrays to generate power for these engines.
When returning to Earth, the buoyant spacecraft again uses the atmosphere as an asset rather than an obstacle. Because of its enormous size, the vehicle can take advantage of the vanishingly tenuous atmosphere that exists in LEO by adjusting its attitude to a high stall angle, increasing its drag while still in orbit. The resulting decrease in velocity starts the reentry process. The large surface area to mass ratio makes it easier for the buoyant spacecraft to use aerobraking to shed energy at a much higher altitude than conventional reentry vehicles. The spacecraft descends through the upper atmosphere in a gradual and controlled manner and reaches subsonic velocities in upper stratosphere. Eventually it will come to a stop at the maximum altitude of neutral buoyancy at 50 km. Pumping lifting gas back into the pressure tanks increases it's density and it then sinks down to its minimum altitude, around 35-40 km. The high-altitude airships can again rendezvous with the buoyant spacecraft to tend it and transfer crew and cargo for the final stage back to the ground.