13 ECAPS High Performance Green Propulsion (HPGP) Propulsion Systems In-Space
13 Earth imaging satellites with ECAPS High Performance Green Propulsion (HPGP) systems are being flown commercially in-space. SkySat-3 was launched in June 2016 from the Satish Dhawan Space Centre on Antrix’s PSLV, SkySat-4 through SkySat-7 were launched in September 2016 from the Guiana Space Centre on Arianespace’s Vega, SkySat-8 through 13 were launched in October 2017 from Vandenberg Air Force Base on Orbital ATK’s Minotaur-C and SkySat-14 + SkySat-15 were launched from Vandenberg in December 2018.
To date, the systems have accumulated 21.3 years on-orbit, executed 166 total maneuvers, and delivered a combined 182 m/s of delta-V.
ECAPS, LMP-103S, and HPGP Thrusters
ECAPS designs, manufactures, and tests liquid rocket engines (thrusters) for in space applications. Additionally, ECAPS produces the "green" non-toxic ADN-based propellant used in their engines known as LMP-103S (a comparison of LMP-103S to hydrazine can be found here). LMP-103S is summarized as:
The 1N HPGP thruster used on the SkySat constellation operates with an Isp of 232 seconds at 22 bar(a) inlet pressure and has an adiabatic combustion temperature of about 1600C. The exhaust gases are accelerated through a 100:1 area ratio nozzle and both the chamber & nozzle are made of iridium lined rhenium for high temperature tolerance
HPGP Implementation on SkySat
A critical requirement identified in the evolution of the SkySat constellation was the inclusion of a highly performant propulsion system capable of maximizing available delta-V within tight volume and low power restrictions. The chief capabilities enabled by inclusion of such a propulsion system are:
- Constellation Phase Management
- Launching multiple spacecraft on a single rocket requires the use of propulsion to phase the spacecraft within each orbit plane and then overcome orbit perturbations to maintain their relative spacing
- Mission Flexibility
- A high thrust to spacecraft mass propulsion system enables the constellation to take advantage of a wide range of primary or secondary launch opportunities with established providers and emerging new entrants. The ability to correct for large orbit injection errors or accept injection into a wide range of altitudes and quickly absorb those individual launch differences without significant delay to incorporation of spacecraft into the constellation’s product is a tremendous value
The resultant ECAPS HPGP propulsion subsystem design for SkySat provides approximately 21 kN-s total impulse at a satellite internal volume fraction of approximately 11% and a mass fraction of approximately 10% dry and 19% wet.
SkySat maneuvers are executed via an automated sequence with the maneuver start time, firing duration, and inertial attitude quaternion taken as arguments. Prior to opening the FCVs (flow control valve), the maneuver sequence configures the spacecraft state and enables the required 30 minutes of catalyst bed pre-heating. The pre-heating is started by enabling reactor heater control on the primary heater and thermocouple pair. After 10 minutes, the secondary reactor heater is also enabled for maneuvers that may be occurring when the bus voltage is low and therefore the primary heater does not have sufficient power to reach the pre-heat setpoints.
When the spacecraft time reaches approximately two minutes prior to the desired maneuver start time, the sequence allows the ACS algorithm to slew the spacecraft to the firing attitude. During the burn, the ACS system dynamically controls the individual thruster duty cycles in an off-pulse fashion at 1 Hz command rate to maintain spacecraft orientation throughout the burn (i.e. no body rate buildup). As the controller converges, a steady-state duty cycle in the range of 75% to 100% is held across the four thrusters. After a successful maneuver, the sequence cleans up the spacecraft state, slews the spacecraft back to the nominal cruise attitude.
Below are plots of the thruster performance (Isp) across the SkySat fleet when operating at 100% duty cycle for durations longer than 30 seconds, the current fleet status as relating to the system blowdown curve, and traces of the reactor temperatures