Aerial vehicle with a volatile fluid

1 . An aerial vehicle comprising:

a first inflatable element;

a first mass of lifting gas contained within the first inflatable element;

a second inflatable element coupled to the first inflatable element; and

a second mass of a volatile fluid:

contained within the second inflatable element;

characterized by a saturation-vapor-pressure curve that intersects an atmospheric temperature-pressure profile within an operational altitude range of the aerial vehicle; and

configured to:

condense during ascent of the aerial vehicle across a first phase-transition altitude, in the operational altitude range, to passively reduce a net buoyancy of the aerial vehicle; and

evaporate during descent of the aerial vehicle across a second phase-transition altitude, in the operational altitude range, to passively increase the net buoyancy of the aerial vehicle.

2 . The aerial vehicle of claim 1 :

wherein the second mass of the volatile fluid comprises butane:

characterized by the saturation-vapor-pressure curve that intersects the atmospheric temperature-pressure profile of Earth atmosphere between 12,000 feet and 22,000 feet altitude; and

exhibiting phase change between liquid and gas under ambient atmospheric conditions between 8,000 feet and 25,000 feet altitude.

3 . The aerial vehicle of claim 1 :

wherein, in a first configuration of the aerial vehicle configured to operate within the operational altitude range, the second mass of the volatile fluid comprises butane:

characterized by the saturation-vapor-pressure curve that intersects the atmospheric temperature-pressure profile of Earth atmosphere between 12,000 feet and 22,000 feet altitude; and

wherein, in a second configuration of the aerial vehicle configured to operate within a second altitude range exceeding the operational altitude range, the second mass of the volatile fluid comprises a mixture of butane and a secondary volatile fluid:

characterized by a second saturation-vapor-pressure curve that intersects the atmospheric temperature-pressure profile of Earth atmosphere between 15,000 feet and 30,000 feet altitude.

4 . The aerial vehicle of claim 1 :

wherein the second inflatable element is nested inside of the first inflatable element;

wherein the first mass of lifting gas occupies a region of the first inflatable element between the first inflatable element and the second inflatable element;

wherein the first inflatable element and the first mass of lifting gas cooperate to thermally insulate the second inflatable element and the second mass of the volatile fluid from local transient changes in ambient temperature of air outside of the first inflatable element:

to slow a first rate of reduction of buoyancy by the second mass of the volatile fluid due to condensation during ascent of the aerial vehicle above the first phase-transition altitude; and

to slow a second rate of reduction of the net buoyancy of the aerial vehicle during ascent of the aerial vehicle above the first phase-transition altitude; and

wherein the second mass of the volatile fluid is configured to release thermal energy into the first mass of the lifting gas, via the second inflatable element, during condensation resulting from ascent of the aerial vehicle:

to slow a third rate of reduction of buoyancy by the first mass of the lifting gas due to ambient temperature reduction during ascent of the aerial vehicle; and

to slow the second rate of reduction of the net buoyancy of the aerial vehicle during ascent of the aerial vehicle above the first phase-transition altitude.

5 . The aerial vehicle of claim 1 :

wherein the second inflatable element:

is located outside of the first inflatable element; and

is suspended from the first inflatable element; and

further comprising a reflective shroud:

interposed between the first inflatable element and the second inflatable element; and

configured to:

shield the second inflatable element from direct solar radiation; and

reduce a rate of evaporation of the second mass of the volatile fluid due to solar heating of the second inflatable element.

6 . The aerial vehicle of claim 1 , wherein the second inflatable element:

is located outside of the first inflatable element;

is suspended from the first inflatable element; and

comprises a set of pleats that cooperate to:

present an expanded exterior surface area of the second inflatable element to ambient air; and

promote transfer of thermal energy between ambient air and the second mass of the volatile fluid.

7 . The aerial vehicle of claim 1 :

wherein the second inflatable element:

defines a closed bottom that forms a sump; and

comprises a film arranged over and enclosing the sump; and

is configured to funnel liquid volatile fluid, in the second mass of the volatile fluid, toward the sump; and

further comprising:

a passive check valve arranged on the film and configured to pass liquid volatile fluid, in the second mass of the volatile fluid, into the sump;

an active valve:

coupled to the sump;

operable in a closed position:

to retain volatile fluid, in the second mass of the volatile fluid, within the sump to limit increase in the net buoyancy of the aerial vehicle during descent of the aerial vehicle below the second phase-transition altitude; and

operable in an open position:

to release gaseous volatile fluid, in the second mass of the volatile fluid, from the sump; and

a nozzle:

arranged within the second inflatable element;

coupled to the active valve; and

configured to atomize liquid volatile fluid, in the second mass of the volatile fluid, expanding from the sump, through the active valve, and into the nozzle in response to the active valve transitioning from the closed position to the open position:

to increase the net buoyancy of the aerial vehicle below the second phase-transition altitude.

8 . The aerial vehicle of claim 1 :

wherein the second inflatable element defines a closed bottom that forms a sump; and

further comprising a pump:

coupled to the second inflatable element; and

configured to spray liquid volatile fluid, in the second mass of the volatile fluid, occupying the sump toward an inner surface of the second inflatable element:

to increase a rate of heat transfer from ambient air into liquid volatile fluid, in the second mass of the volatile fluid, via the second inflatable element;

to increase an evaporation rate of liquid volatile fluid in the second mass of the volatile fluid; and

to increase a rate of net buoyancy increase of the aerial vehicle below the second phase-transition altitude.

9 . The aerial vehicle of claim 1 , further comprising:

an inflatable control envelope encompassing the second inflatable element; and

an air pump:

coupled to the inflatable control envelope; and

configured to pump ambient air into the inflatable control envelope:

to increase pressure within the inflatable control envelope above a local ambient air pressure;

to increase pressure applied to the second inflatable element above the local ambient air pressure;

to induce condensation of gaseous volatile fluid, in the second mass of the volatile fluid, within the second inflatable element; and

to reduce the net buoyancy of the aerial vehicle; and

a vent:

coupled to the inflatable control envelope; and

configured to release ambient air from the inflatable control envelope:

to decrease pressure within the inflatable control envelope toward the local ambient air pressure;

to decrease pressure applied to the second inflatable element toward the local ambient air pressure;

to increase evaporation of liquid volatile fluid, in the second mass of the volatile fluid, within the second inflatable element; and

to increase the net buoyancy of the aerial vehicle.

10 . The aerial vehicle of claim 1 , further comprising:

a lifting gas vent:

coupled to the first inflatable element; and

configured to vent lifting gas, in the first mass of lifting gas, from the first inflatable element to decrease the net buoyancy of the aerial vehicle;

a ballast vessel;

a third mass of a ballast material contained within the ballast vessel;

a ballast valve:

coupled to ballast vessel; and

configured to release ballast material, in the third mass of ballast material, from the ballast vessel to increase the net buoyancy of the aerial vehicle; and

a controller configured to:

access a target maximum altitude and a target minimum altitude specified for the aerial vehicle;

during an ascent of the aerial vehicle:

before the aerial vehicle reaches the target maximum altitude:

calculate a first ascent rate of the aerial vehicle; and

estimate a predicted maximum altitude of the aerial vehicle during the ascent based on the first ascent rate; and

in response to the predicted maximum altitude exceeding the target maximum altitude:

calculate a first vent duration based on a first difference between the predicted maximum altitude and the target maximum altitude; and

activate the lifting gas vent for the first vent duration:

 to vent lifting gas, in the first mass of lifting gas, from the first inflatable element;

 to decrease the net buoyancy of the aerial vehicle; and

 to reduce the predicted maximum altitude toward the target maximum altitude; and

during a descent of the aerial vehicle:

before the aerial vehicle reaches the target minimum altitude:

calculate a first descent rate of the aerial vehicle; and

estimate a predicted minimum altitude of the aerial vehicle during the first descent based on the first descent rate; and

in response to the predicted minimum altitude falling below the target minimum altitude:

calculate a second vent duration based on a second difference between the predicted minimum altitude and the target minimum altitude; and

activate the ballast valve for the second vent duration:

 to release ballast material, in the third mass of ballast material, from the ballast vessel;

 to increase the net buoyancy of the aerial vehicle; and

 to increase the predicted minimum altitude toward the target minimum altitude.

11 . The aerial vehicle of claim 1 :

further comprising a condensate vessel fluidly coupled to the second inflatable element;

wherein the second inflatable element is configured to funnel liquid volatile fluid, in the second mass of the volatile fluid, into the condensate vessel; and

further comprising:

a heatsink coupled to the condensate vessel;

a fan configured to direct ambient air across the heatsink; and

a controller configured to activate the fan during descent of the aerial vehicle below the second phase-transition altitude:

to accelerate transfer of thermal energy from ambient air into liquid volatile fluid, in the second mass of the volatile fluid, occupying the condensate vessel;

to increase an evaporation rate of liquid volatile fluid, in the second mass of the volatile fluid, occupying the condensate vessel; and

to increase a rate of net buoyancy increase of the aerial vehicle below the second phase-transition altitude.

12 . The aerial vehicle of claim 11 :

wherein the fan is configured to direct ambient air across the second inflatable element; and

wherein the controller is further configured to activate the fan during ascent of the aerial vehicle above the first phase-transition altitude:

to accelerate transfer of thermal energy from gaseous volatile fluid, in the second mass of the volatile fluid, into ambient air via the second inflatable element and via the condensate vessel;

to increase a condensation rate of gaseous volatile fluid in the second mass of the volatile fluid; and

to increase a rate of net buoyancy decrease of the aerial vehicle above the first phase-transition altitude.

13 . The aerial vehicle of claim 1 , further comprising:

a condensate vessel:

fluidly coupled to the second inflatable element; and

a heating element:

coupled to the condensate vessel;

configured to heat liquid volatile fluid, in the second mass of the volatile fluid, occupying the condensate vessel:

to increase an evaporation rate of liquid volatile fluid, in the second mass of the volatile fluid, occupying the condensate vessel; and

to increase a rate of net buoyancy increase of the aerial vehicle below the second phase-transition altitude.

14 . The aerial vehicle of claim 13 , further comprising a controller configured to:

during descent of the aerial vehicle below the second phase-transition altitude:

calculate a descent rate of the aerial vehicle;

access a target minimum altitude, below the second phase-transition altitude, specified for the aerial vehicle; and

estimate a first transition altitude of the aerial vehicle, from descent to ascent, based on the descent rate; and

in response to the first transition altitude falling below the target minimum altitude:

activate the heating element:

to heat liquid volatile fluid, in the second mass of the volatile fluid, occupying the condensate vessel;

to increase the evaporation rate of liquid volatile fluid, in the second mass of the volatile fluid, occupying the condensate vessel;

to increase the rate of net buoyancy increase of the aerial vehicle; and

to transition the aerial vehicle, from descent to ascent, above the first transition altitude; and

during ascent of the aerial vehicle:

calculate an ascent rate of the aerial vehicle;

access a target ascent duration, between a descent-ascent transition altitude and an ascent-descent transition altitude, specified for the aerial vehicle;

estimate a real ascent duration of the aerial vehicle, between the descent-ascent transition altitude and the ascent-descent transition altitude, based on the ascent rate; and

in response to the real ascent duration exceeding the target ascent duration:

activate the heating element:

to heat liquid volatile fluid, in the second mass of the volatile fluid, occupying the condensate vessel;

to increase the evaporation rate of liquid volatile fluid, in the second mass of the volatile fluid, occupying the condensate vessel;

to increase the rate of net buoyancy increase of the aerial vehicle; and

to decrease the real ascent duration.

15 . The aerial vehicle of claim 1 , further comprising:

a lifting gas vent:

coupled to the first inflatable element; and

configured to vent lifting gas, in the first mass of lifting gas, from the first inflatable element to decrease the net buoyancy of the aerial vehicle;

a ballast vessel;

a third mass of a ballast material contained within the ballast vessel;

a ballast valve:

coupled to ballast vessel; and

configured to release ballast material, in the third mass of ballast material, from the ballast vessel to increase the net buoyancy of the aerial vehicle; and

a controller configured to:

access a target maximum altitude and a target minimum altitude specified for the aerial vehicle;

during a first ascent of the aerial vehicle:

before the aerial vehicle reaches the target maximum altitude:

calculate a first ascent rate of the aerial vehicle; and

estimate a predicted maximum altitude of the aerial vehicle during the first ascent based on the first ascent rate; and

in response to the predicted maximum altitude falling below the target maximum altitude:

withhold activation of the lifting gas vent to withhold venting of lifting gas, in the first mass of lifting gas, from the first inflatable element; and

during a first descent of the aerial vehicle:

before the aerial vehicle reaches the target minimum altitude:

calculate a first descent rate of the aerial vehicle; and

estimate a predicted minimum altitude of the aerial vehicle during the first descent based on the first descent rate; and

in response to the predicted minimum altitude exceeding the target minimum altitude:

withhold activation of the ballast valve to withhold release of ballast material, in the third mass of ballast material, from the ballast vessel.

16 . The aerial vehicle of claim 15 , wherein the controller is configured to:

during the first ascent of the aerial vehicle:

access a first current altitude of the aerial vehicle; and

estimate the predicted maximum altitude of the aerial vehicle during the first ascent by:

accessing a set of ascent curves from memory, each ascent curve in the set of ascent curves representing an ascent rate versus altitude up to an ascent-descent transition altitude;

identifying a first ascent curve, in the set of ascent curves, representing the first ascent rate versus the first current altitude; and

estimating the predicted maximum altitude of the aerial vehicle during the first ascent based on a first ascent-descent transition altitude represented in the first ascent curve; and

during the first descent of the aerial vehicle:

access a second current altitude of the aerial vehicle; and

estimate the predicted minimum altitude of the aerial vehicle during the first descent by:

accessing a set of descent curves from memory, each descent curve in the set of descent curves representing a descent rate versus altitude up to a descent-ascent transition altitude;

identifying a first descent curve, in the set of descent curves, representing the first descent rate versus the second current altitude; and

estimating the predicted minimum altitude of the aerial vehicle during the first descent based on a first descent-ascent transition altitude represented in the first descent curve.

17 . The aerial vehicle of claim 1 , further comprising:

a condensate vessel:

fluidly coupled to the second inflatable element; and

a valve:

interposed between the second inflatable element and the condensate vessel;

operable in a closed position:

to retain volatile fluid, in the second mass of the volatile fluid, within the condensate vessel to limit increase in the net buoyancy of the aerial vehicle during descent of the aerial vehicle below the second phase-transition altitude; and

operable in an open position:

to release gaseous volatile fluid, in the second mass of the volatile fluid, from the condensate vessel into the second inflatable element to increase the net buoyancy of the aerial vehicle during descent of the aerial vehicle below the second phase-transition altitude.

18 . The aerial vehicle of claim 17 , further comprising a controller configured to:

during a first descent of the aerial vehicle:

in response to the aerial vehicle occupying a first altitude below the second phase-transition altitude at a first time:

trigger the valve to transition from the closed position to the open position:

to release evaporated volatile fluid, in the second mass of the volatile fluid, from the condensate vessel into the second inflatable element;

to increase the net buoyancy of the aerial vehicle; and

to transition the aerial vehicle from descent to ascent below the first altitude at a second time succeeding the first time;

calculate a first descent rate of the aerial vehicle between the first time and the second time;

access a target minimum altitude, below the second phase-transition altitude, specified for the aerial vehicle; and

during a second descent of the aerial vehicle succeeding the first descent:

trigger the valve to transition from the open position to the closed position;

based on the first descent rate, calculate a trigger altitude at which release of the second mass of the volatile fluid into the second inflatable element is predicted to transition the aerial vehicle from descent to ascent at the target minimum altitude; and

in response to the aerial vehicle occupying the trigger altitude at a third time:

trigger the valve to transition from the closed position to the open position:

to release evaporated volatile fluid, in the second mass of the volatile fluid, from the condensate vessel into the second inflatable element;

to increase the net buoyancy of the aerial vehicle; and

to transition the aerial vehicle from descent to ascent proximal the target minimum altitude at a fourth time succeeding the third time.

19 . The aerial vehicle of claim 17 :

wherein the valve, in the closed position, is configured to hold liquid volatile fluid, in the second mass of the volatile fluid, in the condensate vessel under pressure exceeding ambient pressure during descent of the aerial vehicle below the second phase-transition altitude;

further comprising a nozzle:

coupled to the second inflatable element and to the condensate vessel; and

configured to atomize liquid volatile fluid, in the second mass of the volatile fluid, expanding from the condensate vessel, through the valve, and into the nozzle in response to the valve transitioning from the closed position to the open position.

20 . An aerial vehicle comprising:

a first inflatable element;

a first mass of lifting gas contained within the first inflatable element;

a lifting gas vent:

coupled to the first inflatable element; and

configured to vent lifting gas, in the first mass of lifting gas, from the first inflatable element to decrease a net buoyancy of the aerial vehicle;

a second inflatable element coupled to the first inflatable element;

a second mass of a volatile fluid:

contained within the second inflatable element;

characterized by a saturation-vapor-pressure curve that intersects an atmospheric temperature-pressure profile within an operational altitude range of the aerial vehicle; and

configured to condense during an ascent of the aerial vehicle across a phase-transition altitude, in the operational altitude range, to passively reduce a net buoyancy of the aerial vehicle; and

a controller configured to:

access a target maximum altitude specified for the aerial vehicle;

during the ascent of the aerial vehicle:

before the aerial vehicle reaches the target maximum altitude:

calculate a first ascent rate of the aerial vehicle; and

estimate a predicted maximum altitude of the aerial vehicle during the ascent based on the first ascent rate; and

in response to the predicted maximum altitude falling below the target maximum altitude:

withhold activation of the lifting gas vent to withhold venting of lifting gas, in the first mass of lifting gas, from the first inflatable element.