Astra Space: In-orbit validation of the Astra Spacecraft Engine™
By Matthew Gill PhD, Principal Systems Engineer
The first Astra Spacecraft Engine™ reached orbit on June 30, 2021 aboard Spaceflight’s Sherpa-LTE All-Electric Orbital Transfer Vehicle (OTV) as part of the Spaceflight Sherpa-LTE1 SXRS-5 mission. After several weeks of Sherpa-LTE commissioning – spacecraft checks to ensure everything is working as expected – the electric propulsion system fired up on its first attempt on August 20, 2021 and recently passed its 300th burning in orbit. In this article, we will review observed in-orbit data that demonstrates the consistent performance of the in-orbit propulsion system, to date.
Today, in-orbit propulsion and mobility are important components of safe, economical and efficient space services. Whether ascending to a different orbit from the initial insertion, avoiding collisions, or ensuring a constellation is properly equipped for responsible end-of-life operations, an in-orbit propulsion system efficient and reliable is a mission critical system aboard any spacecraft.
The Astra Spacecraft Engine™ (ASE) is a fully integrated electric propulsion system that consists of a Hall Effect Thruster (HET), pneumatic power system, radiation hardened electronics (rad- hard) and an over-wrapped composite pressure system. ship’s tank (COPV) that can be shipped to a fully fueled launch site. Astra manages all aspects of design, integration and testing for shipping and launching a propulsion system. The system uses the same space-qualified components for each customer to meet each customer’s specific mission requirements. Each module can be configured with multiple thrusters if desired and different tank sizes to support a wide range of missions including Low Earth Orbit (LEO) elevation, Earth Orbit elevation mean (MEO), geostationary orbit elevation (GEO), station keeping, cis- lunar transfers and deorbit. Before flight, the propulsion system undergoes a battery of tests on the ground, including vibration, shock, thermal, pressure and leak tests. The complete system is hot-fire tested, with performance measured for comparison with in-orbit operations.
IN-ORBIT DATA FOR SPACEFLIGHT’S Sherpa-LTE1 MISSION
While the Astra Spacecraft Engine™ underwent thousands of hours of component and system-level ground testing prior to its initial flight, Spaceflight’s Sherpa-LTE1 mission was the first time the system was tested in orbit. . Integrating, testing and qualifying a propulsion module has inherent difficulties that go beyond the demonstration of individual subsystems, and this mission gave Astra the opportunity to demonstrate the delivery process, successful launch and operation of an in-orbit integrated propulsion module.
With the aim of quickly demonstrating the in-orbit performance of the Astra Spacecraft Engine™, our team developed the propulsion module from concept to testing to hardware delivery in 6 months. Although Astra’s Spacecraft Engine is designed for standard operation at 400 Watts (W), the rapid development of the Sherpa-LTE1 mission meant that the power management system had to be smaller, as such, the system operated at 340 W for 5 minutes. push times. Working at reduced power still allowed reproducible system demonstrations (more than 300) and progress towards the deorbiting of the satellite.
Average in-orbit performance is within one standard deviation of ground test data – which is a validation of both system performance and a validation of the ground test facilities and processes used to test its systems integrated propulsion. Full-system hot-firing tests were performed before and after the dynamic tests, and ground flight module operating data was used for comparison with in-orbit operations. Due to limited data downlink capacity, data has only been uploaded for a subset of activations as shown below. Altitude change and thruster use were used to derive thrust and specific impulse.
PUSH IN ORBIT OBSERVED
The thrust observed in orbit was calculated using GPS data from Sherpa. Due to the relatively small altitude change from 5-minute thruster operations, the measurement error is large. As such, it is not possible to obtain high precision thrust estimates from individual maneuvers, but the average of several measurements is representative.
In Figure A, the blue line shows the average thruster performance as a function of power, from ground test data. The gray dots indicate the thrust observed in orbit. The red dot indicates the average of the thrust measurements in orbit with one standard deviation. The average in-orbit thrust is within one standard deviation of the expected value, which validates the ground test thrust performance set.
Note: In-orbit thrust has a large standard deviation due to measurement errors when performing short-duration maneuvers, rather than thruster performance variability. Based on thousands of hours of ground testing, there is extremely low variability in propellant performance between multiple ground test firings.
USE OF THRUSTERS IN OBSERVED AND ISP ORBIT
Rad-hard telemetry datasets were used to calculate in-orbit specific impulse (Isp) or thrust produced per gram of propellant used and compared to average ground test data. Due to activation losses (when a certain propellant flows during activation but does not produce thrust until the propellant ignites), the effective Isp for a maneuver depends on the duration of the maneuver . Longer maneuvers have a higher Isp. The black line below shows the impact of activation losses on Isp. Note that due to schedule constraints, the power system used for the Sherpa demonstration is an early engineering (EM) design and exhibits higher turn-on losses compared to our current optimized production designs.
In figure B, the blue line indicates the expected values ISPdepending on the duration of the maneuver. Gray dots show observed in orbit ISPfor 5-minute manoeuvres. The red dot indicates the average orbiting ISPwith one standard deviation. The average in orbit ISPis within one standard deviation of the expected valueconfirming again thethrust performancee put from ground tests.
Note: Isp is lower than the quoted “steady state” Isp due to activation losses which are disproportionately high for short duration maneuvers (some propellants flow upon activation but do not produce thrust until until the thruster starts), between 15 and 20 minutes the Isp approaches the steady state value.
In Figure C, the ssummary table shows in-orbit performance within one standard deviation of ground test data, validating ASE’s in-orbit performance.
Note: System efficiency is total efficiency, taking into consideration trickle circuits, power system wattage, and circuit efficiency.
The Sherpa-LTE1 propulsion system has fired over 300 times and continues to operate in orbit. Operations were consistent, repeatable, and consistent with ground test data. Click here for a full technical overview of the Astra Spacecraft Engine™.