An Elevated Evacuated Tube (EET) is a lightweight, vacuum tube segment that extends the protected zero-drag flight corridor through the densest part of the atmosphere so the vehicle exits into the free atmosphere at an altitude of 15 km. This configuration reduces the peak aerodynamic load and heating on the vehicle during its transition to open air and moves the resulting sonic boom to a much higher altitude, greatly reducing its impact on the ground and minimizing disturbance to populated areas.

The EET does not support the weight of the spacecraft, rather the EET is designed to keep itself centered around the spacecraft's ballistic flight path. The section of the EET nearest to the ramp is engineered to support the weight of the launch sled and its guideway, as the launch sled detaches from the spacecraft at the end of the ramp and then decelerates against the guideway while it is inside the first part of the elevated evacuated tube.

The EET operates as an aeronautically supported structure, held aloft by numerous lift nacelles powered from the ground through high-voltage direct-current (HVDC) transmission. Its internal radius is set by the vehicle radius plus a some additional margin for station-keeping:

$R_{outer} = R_{spacecraft} + R_{margin} + R_{ribs}= 1.2 + 2.0 + 0.1 = 3.3\ \text{m}$

For reference, the diameter of an Airbus A300 is 5.64 m, giving it a radius of 2.82 m.

The evacuated interior provides a zero-drag path for the spacecraft, while the external shell must withstand atmospheric pressure varying from approximately 62 kPa at 4 km to 12 kPa at 15 km. To resist this differential pressure efficiently, the tube is constructed as a stiffened shell using frames, formers, and stringers similar to those used in modern aircraft fuselages.

Assuming an average skin thickness of $t_{skin} = 4\ \text{mm}$, aluminum-lithium alloy density $\rho = 2700\ \text{kg/m}^3$, and an external radius $R_{outer} = 3.3\ \text{m}$, the skin mass per meter is estimated as:

$m_{skin/m} = (2\pi R_{outer}) t_{skin} \rho = 2\pi(3.3)(0.004)(2700) \approx 224\ \text{kg/m}.$

Including internal stiffeners and joint structures adds roughly 50% giving an estimated total structural mass per meter of:

$m_{tube/m} \approx 224(1.5) = 336\ \text{kg/m}$

This design can maintain structural stability against buckling at the maximum external pressure of 62 kPa while maintaining a safety margin suitable for operational gust loads.

Because the tube is evacuated, it experiences a small buoyant lift equal to the weight of the displaced air:

$m_{b,eq} = \rho_{air} A_x = \rho_{air} \pi R_{outer}^2$

At 15 km, where $\rho_{air} = 0.192\ \text{kg/m}^3$, this corresponds to about $m_{b,eq}=\rho_{air}\pi R_{outer}^2 = 0.192\cdot\pi\cdot 3.3^2 \approx 6.58\ \text{kg/m}$ of buoyant relief, whereas at 4 km, where $\rho_{air} = 0.819\ \text{kg/m}^3$, this corresponds to about $0.819\cdot\pi\cdot 3.3^2 \approx 28.03\ \text{kg/m}$. While this offsets only a small fraction of the structural mass, it is nevertheless included for completeness.

The tube is supported aeronautically by gimballed lift nacelles which are attached to to tube by struts, each housing an electric motor driving a high-altitude propeller optimized for low-density operation. The nacelles provide both vertical lift and attitude control through thrust vectoring. Power for the lift fans is transmitted from the ground through HVDC cables integrated along the structure.

Lift Generation

The lift fans performance is characterized by their thrust to weight ratio (in N/kg) and by the power consumed per Newton of thrust generated (in W/N). As the individual lift fans will spend most of their operational time on station, their rotors will be optimized for maximum efficiency at their on-station altitude. In other words, air density changes are handled by varying the rotor sizing rather than by using a single rotor design and accepting non-uniform performance with operational altitude. For example, the rotors designed to operate at the lowest altitudes will be engineered more like the rotors of an eVTOL aircraft, while the rotors optimized for the highest altitudes will tend towards resembling the large rotors used on the Mars Ingenuity helicopter.

For example, the Ingenuity helicopter draws 350 Watts when flying and generates more than enough thrust to lift itself off the surface of Mars. Its mass is 1.8kg and on Mars the acceleration of gravity is 3.73 m/s²; therefore, its propulsion system generates 1.8kg×3.73m/s² = 6.7N of thrust. It's power per unit of thrust is 350/6.7 = 52 W/N. A modern drone motor, such as the A45 brushless drone motor can generate a maximum pulling force of 52 kg while drawing 4100 W, which is only 4100/(52*9.81) = 8 W/N. From public databases we can determine the relationship between power and thrust.

If we assume four lift fans per meter, and that each meter of tube weighs 336kg plus 100 kg for the pylons, gimballed motors, and propellers, then each lift fan would need to produce support 436/4=109 kg of thrust. From the image above, we can determine that the power per fan will be roughly 11 kW. This gives us a power-per-unit-thrust of 11,000/(109*9.81)=10.28 W/N.

The total power requirements will depend on which power-over-thrust value we use. The entire elevated evacuated tube is 102.4 km in length, then its total mass will be 436*102400 = 44.6 million kg. The power needed to keep it aloft will be between 0.5 to 2.2 GW.

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Electrical power is delivered from the ground through HVDC conductors sized according to:

$I = \dfrac{P_{tot}}{V_{HV}}, \quad A_c = \dfrac{I}{J}, \quad d = 2\sqrt{\dfrac{A_c}{\pi}},$

where $V_{HV}$ is the line voltage and $J$ the allowable current density. Using $P_{per\,m} = 120\ \text{kW/m}$, $V_{HV} = 600\ \text{kV}$, and $J = 2\times10^6\ \text{A/m}^2$, the resulting conductor area is $A_c = 1.0\times10^{-7}\ \text{m}^2$, and the aluminum conductor mass per meter (with tapering and dual lines) is:

$m_{HVDC/m} = 0.5 \times 2700 \times 2 \times 1.0\times10^{-7} = 0.00027\ \text{kg/m}.$

Even when including insulation and structural supports, the cable mass remains negligible compared to the tube mass.

In summary, the Elevated Evacuated Tube provides a mechanically efficient, electrically supported platform that extends the zero-drag corridor to 15 km altitude. Its structural mass per meter is on the order of 430 kg, its lift requirement is roughly 4.2 kN/m, and its power draw is approximately 44 kW/m at operating altitude. HVDC transmission from the ground minimizes onboard mass and energy storage needs, enabling sustained operation without propellant or consumables.

A camera drone filming the summit of Mt Everest, where the air density is roughly one-third what it is at sea level