Hybrid propulsion systems

1. A system comprising:

a plurality of series propulsion units positionally mirrored across a centerline of an aircraft, each series propulsion unit of the plurality of series propulsion units comprising a respective propulsor of a plurality of propulsors and a respective electrical machine of a plurality of electrical machines, each respective electrical machine configured to drive a respective propulsor of the plurality of propulsors using electrical energy received from one or more electrical busses;

a parallel propulsion unit mounted on the centerline of the aircraft, the parallel propulsion unit comprising: a propulsor, a gas turbine engine configured to drive the propulsor, and an electrical machine selectively configurable to:

generate, for output via the one or more electrical busses, electrical energy using mechanical energy derived from the propulsor or the gas turbine engine; and

drive the propulsor of the parallel propulsion unit using electrical energy received via the one or more electrical busses; and

one or more electrical energy storage devices operably coupled to at least one of the one or more electrical busses, wherein the one or more electrical energy storage devices are configured to both:

charge using electrical energy sourced via the at least one of the one or more electrical busses; and

discharge to provide electrical energy to the at least one of the one or more electrical busses.

2. The system of claim 1 , wherein a particular electrical machine of the plurality of machines included in a particular series propulsion unit of the plurality of series propulsion units is configured to:

generate electrical energy using mechanical energy derived from a particular propulsor of the plurality of propulsors included in the particular series propulsion unit; and

output the generated electrical energy onto the one or more electrical busses.

3. The system of claim 1 , wherein the one or more electrical busses comprise direct current (DC) electrical busses.

4. The system of claim 1 , wherein the plurality of propulsors comprise variable pitch propellers.

5. The system of claim 1 , further comprising a full authority digital engine controller (FADEC) configured to control operation of the parallel propulsion unit.

6. The system of claim 1 , further comprising a thermal management system configured to manage a temperature of the one or more electrical energy storage devices.

7. The system of claim 1 , wherein the system does not include an auxiliary power unit (APU).

8. An airframe comprising:

a plurality of series propulsion units positionally mirrored across a centerline of an airframe, each series propulsion unit of the plurality of series propulsion units comprising a respective propulsor of a plurality of propulsors and a respective electrical machine of a plurality of electrical machines, each respective electrical machine configured to drive a respective propulsor of the plurality of propulsors using electrical energy received from at least one of one or more electrical busses;

a parallel propulsion unit mounted on the centerline of the airframe, the parallel propulsion unit comprising: a propulsor, a gas turbine engine configured to drive the propulsor, and an electrical machine selectively configurable to:

generate, for output via the one or more electrical busses, electrical energy using mechanical energy derived from the propulsor or the gas turbine engine; and

drive the propulsor of the parallel propulsion unit using electrical energy received via the one or more electrical busses; and

one or more electrical energy storage devices operably coupled to at least one of the one or more electrical busses, wherein the one or more electrical energy storage devices are configured to both:

charge using electrical energy sourced via the at least one of the one or more electrical busses; and

discharge to provide electrical energy to the at least one of the one or more electrical busses.

9. The airframe of claim 8 , wherein a particular electrical machine of the plurality of machines included in a particular series propulsion unit of the plurality of series propulsion units is configured to:

generate electrical energy using mechanical energy derived from a particular propulsor of the plurality of propulsors included in the particular series propulsion unit; and

output the generated electrical energy onto the one or more electrical busses.

10. A method comprising:

propelling, by a plurality of series propulsion units positionally mirrored across a centerline of an aircraft, the aircraft, each series propulsion unit of the plurality of series propulsion units comprising a respective propulsor of a plurality of propulsors and a respective electrical machine of a plurality of electrical machines, each respective electrical machine configured to drive a respective propulsor of the plurality of propulsors using electrical energy received from one or more electrical busses;

selectively, by an electrical machine of a parallel propulsion unit mounted on the centerline of the aircraft:

generating, for output via the one or more electrical busses, electrical energy using mechanical energy derived from a propulsor of the parallel propulsion unit or a gas turbine engine of the parallel propulsion unit configured to drive the propulsor of the parallel propulsion unit; and

driving the propulsor of the parallel propulsion unit using electrical energy received via the one or more electrical busses;

charging, by one or more electrical energy storage devices of the aircraft that are operably coupled to at least one of the one or more electrical busses, using electrical energy sourced via the at least one of the one or more electrical busses; and

discharging, by the one or more electrical energy storage devices, to provide electrical energy to the at least one of the one or more electrical busses.