EMI/EMC, Lightning, Radiation Shielding Design Approach for the Dragon COTS Spacecraft: Part I W. Elkman, J. Trinh, P. McCaughey, W. Chen Space Exploration Technologies 1 Rocket Road, Hawthorne, California 90250, United States of America william.elkman@spacex.com [email protected] [email protected] [email protected]

Abstract—Designing the Dragon COTS (Commercial Orbital Transfer System) for EMI/EMC interface compliance to the II. EMI/EMC PHILOSOPHY AND CONTROL PROGRAM International Space Station (ISS) is a complex task. It involves The Dragon Spacecraft is exposed to combined ground, designing Dragon for self-compatibility, compatibility with the launch and Space environments that depart from the EMI Falcon 9 Launch Vehicle, designing to the LEO Space radiation environment, as well as interface compatibility to the ISS environment for Falcon 9. Regarding the EMI environment, mechanical, electrical and RF interfaces. This paper describes SpaceX has implemented an EMI/EMC Control Program the Space Exploration Technologies tailored approach to achieve tailored to the requirements of the COTS CRS program. Dragon spacecraft self-compatibility, compatibility with the Electromagnetic Compatibility for the Dragon Spacecraft is Eastern Launch Range RF requirements and interface verified to ISS RF and conducted power interface compatibility to SSP 30237, 30238 and 30243, respectively, the requirements, as well as to the Range, ground environments, ISS requirements for EMC emissions and susceptibility, and test EGSE (electrical ground support equipment) and internal methods for verification and overall ISS EMC compatibility. The Dragon Avionics systems electromagnetic environments, by detailed design analysis methods used to predict design analysis, tailored, limited component/unit test, subsystem test performance are described, as well as the detailed tailored test methods used to verify functional performance and margins to and system level test in keeping with the COTS reliable, low the Falcon 9 LV, Eastern Launch Range RF requirements, and cost, implementation strategy. The EMISMs (electromagnetic the ISS suite of EMC and survivability requirements. interference susceptibility margin) for interfaces internal to Dragon are, by test, 6dB for non-critical and 20dB for critical interfaces (12dB and 34dB respectively, by analysis). External I. INTRODUCTION interfaces between Dragon and the ISS comply with EMISMs, The SpaceX Dragon Spacecraft is part of the COTS/CRS by test, of 20dB for critical and 6dB for non-critical circuits (Commercial Orbital Transfer System/Commercial Resupply (34dB for EEDs and FTS circuits, by analysis). Additionally, Service) system. The system is comprised of the Falcon 9 Dragon must comply with the ISS Space environment launch vehicle (LV) and the pressurized Dragon Spacecraft. requirements for Crew radiation safety, Space plasma, bulk The Falcon 9 EMC and lightning mitigation design is charging, ESD, total ionizing radiation dose, cosmic ray SEE described in a companion paper for this conference. The (single event effects) and MMOD (micrometeoroid/orbital Dragon vehicle is re-usable. It is launched with the Falcon 9 debris). The design leverages common elements of mitigation LV, uses its internal propulsion system to reach the ISS orbit, between these environments, to achieve compliance with the dock with ISS (International Space Station), de-mate, re-enter, ISS requirements and verify Mission/Safety critical descend with the aid of parachutes, achieve a water landing, performance compliance. then serviced and re-launched. Certain electronic units are modular and employed on both Falcon and Dragon for III. SYSTEM EMI, PLASMA AND BULK CHARGING DESIGN efficiency. Because Dragon must be EMI qualified for APPROACH compliance with the ISS interface requirements, it is efficient In order to meet the combined requirements for Dragon to qualify both the Dragon and the Falcon 9 LV Avionics to self-compatibility, launch Range environments and, common tailored MIL-STD-461, MIL-STD-1541, SSP-30237 compatibility with the LV, and ISS environments, the system and SSP-30243, as well as required aspects of the Range requirements are convolved from the detailed functional Safety requirements, EWR-127-1. There are specific requirements for ISS operation, as well as with the launch exceptions regarding the RF and conducted power interfaces Range environments, including lightning and tribo-electric to ISS that require a design tailoring to Dragon avionics. charging. In a manner common to the Falcon LV design, the Design Verification section, Part II of this paper, shows the

978-1-4577-0811-4/11/$26.00 ©2011 IEEE 641 detailed methods of analysis and test verification for the EMI, exception of the Solar Array harness extending outboard from lightning, Plasma grounding, corona and bulk charging the Actuator, the harness is fully shielded. Signal lines are requirements. double shielded and power single shielded with 360 degree fully circumferential EMI back-shell circular connectors and IV. DRAGON SPACECRAFT ARCHITECTURE EMI shield banded boots. The Claw mechanism, providing the The Dragon spacecraft uses a core solid aluminum pressure PWB (printed wiring board) harness connection between the section, averaging more than 80 mils aluminum thickness. Trunk and Dragon, is also fully shielded. Depending upon the configuration for C1 or C2 Dragon, the pressure contains redundant Avionics. Below the Pressure A. Requirements Identification and Flow-Down section, there is an annular un-pressurized section formed by a The modular electronics units used in Dragon, share a series of radial bulkheads. The remaining Avionics is situated common architecture to the RIO based Avionics in Falcon 9, in the radial bulkhead section. The Pressure section and radial as indicated in the companion paper for Falcon 9. Therefore, bulkhead section are thermally isolated from the external the EMI requirements are tailored from a convolution of the vehicle environment by a composite honeycomb TPS (thermal functional requirements of the LV, as well as the contractually protection system). On the lower heat stress non-ablative required specifications for Range and ISS use. The Range surfaces, the TPS supports a thermal insulating foam (SPAM). requirements are specified from MIL-STD-461E, while the On the ablative re-entry surfaces, the TPS supports an ablative ISS requirements, due to their heritage, are primarily tailored PICA insulating foam. The forward bulkhead is encased in a from MIL-STD-461C. The source requirement documents are composite Nosecone, jettisoned on-orbit. The aluminum shown in Table I below. The specific EMI tests required are pressure section and un-pressurized radial bulkhead section indicated in Table III. are interconnected with joint resistances less than 10 Table IV, EMC Compliance by Mission, shows the milliohms across multiple joints and 2.5 milliohms per single breakdown of the EMC requirements by Dragon Mission. joint. These two assemblies form the power distribution single Engineering level, DragonForce and IronMaiden simulator point ground surface. All Avionics units are bonded to this level tests, and also unit and subsystem analysis, were used to ground plane. The Trunk section fittings form an ESD bleed, verify the Dragon C1 compliance as indicated in the table. 106 to 109 ohms, to the Dragon structure. The Solar Array Dragon C2 is optionally scheduled to dock with the ISS. As composite frame structure is static bonded (0.1 ohms) to the part of the verification for Safety critical interfaces to the ISS, support fitting extending to the Actuator/Deployment qualification tests (QTP) are required for the IPCU (120 Volt mechanism. The PCBM (passive berthing mechanism) forms to 28 Volt power converter), for C2 radiated emissions (RE02) part of the Pressure section ground plane. The external TPS and for C2 radiated susceptibility (RS03). The requirements structure elements are static bonded to mitigate tribo-electric for Plasma grounding, ESD, bonding, corona mitigation, Crew charging. All of the external structure elements participate in safety and Pyro safety, are specified in Part II of this paper, the Plasma voltage balance equilibrium on-orbit. With the Verification.

Fig. 2. Dragon C2 Transparent Architectural View.

Fig. 1. Dragon C2 Architectural Layout with Surface Materials

642 TABLE I TABLE III ISS REQUIREMENT FLOW-DOWN DOCUMENTS COTS EMC CONTROL PLAN TEST REQUIREMENT Referenced REFERENCED DOCUMENT TITLE TEST, ISS TEST, Range DESCRIPTION Document Interfaces FTS Interfaces SSP 30238C Space Station Electromagnetic Techniques, 31 CE01 Power CE102 CE01 conducted May 1996 Line emissions, DC power, SSP 30240F Space Station Grounding Requirements, 30 Jul low frequency, 30 Hz to 2005 15 kHz, SSP 30243H Space Station Requirements for Electromagnetic CE03 Power CE102 CE03 conducted Compatibility, 30 Jul 2005 Line emissions, DC power, 15 SSP 30237H Space Station Electromagnetic Emission and kHz to 50 MHz, (10k- Susceptibility Requirements, 15 Mar 2005 10MHz) MIL-STD- Requirements for the Control of Electromagnetic Signal Line CE N/A Signal dependent 461, Rev C Interference Characteristics of Subsystems and measurement to select and E Equipment, test methods tailored by SSP 30238 TD, FD to achieve SSP30243 compliance RE02 RE102 RE02 radiated emissions, SSP 50808 International Space Station to Commercial Emissions electric field, 14 kHz to Orbital Transportation Services (COTS) 18 GHz; (18 GHz to 40 Interface Requirements Document (IRD), April GHz by analysis), (10k- 2008 18GHz) CE07 N/A CE07 conducted TABLE II Inrush/Outrush emissions, time domain UNIT ACRONYM LIST (TD), DC power leads, Acronym Description transient ACRA ACRA Controls Telemetry Unit CS01 Power CS101 CS01, conducted ECS Environmental Control System Line susceptibility, power EED (NSI) NASA Standard Initiator (Pyro) leads, 30 Hz to 50 kHz, FC Flight Computer (30Hz-150kHz) GNC Guidance Navigation & Control CS02 Power CS101 CS02, conducted GPS GPS Receiver Line susceptibility, power IMU Inertial Measurement Unit leads, 50 kHz to 50 MHz, NS Network Switch (30Hz-150kHz) PDB Power Distribution Box CS06 Transient CS115, 116 CS06, conducted Pyro RIO Pyrotechnic (Electro-Explosive) Device susceptibility, spikes, Controller RIO (same as Stage Controller power leads, (damped RIO) sine, 30ns pulse) RIO Remote Input Output Unit RS02 Transient CS115, 116 RS02, radiated Signal Line susceptibility, magnetic S Band S Band Receiver induction, (damped sine, SC Stage Controller RIO 30ns pulse) B. Functional Design Requirements RS03 RS103 RS03, radiated Susceptibility susceptibility, electric The Dragon functional design constraints have been used to field, 14 kHz to 18 GHz determine the grounding, conducted emissions/susceptibility DC Bonding N/A Joint, connector bond, and radiated emissions/susceptibility design approach. The Measurements chassis seam resistance, functional basis for the design requirements is described (resistance type below. dependent R, S) CS114 Current CS114 CS114 alternate to RS03 1) SPG Grounding Design for Primary Power Probe Injection low frequency, CW bulk The PDB (Power Distribution Box) is the SPG (single point cable injection, signal and ground) point for Dragon and the Trunk. All units use paired power lines power and return wiring for the 28V battery bus. Typically 18AWG to 20AWG wire gauge is used for power. The wire During functional integration at the factory and at the gauge has been selected to comply with the COTS avionics launch pad, SLC-40 at CCAFS, the Dragon avionics ground is equipment operating voltage range from 18V to 36V, tied to the ground system of the GSE (ground support including harness drop, inrush/outrush transients and GSE equipment) and Dragon support rack (DSR). As indicated in ground bounce. the Falcon 9 companion paper, the ground system is not segregated into separate technical and flight Earth grounds. As a result, ground bounce inrush/outrush voltage transients and repetitive ground equipment switched load voltage transients are present on the Earth ground system. As distinct from the Falcon 9 distributed grounding arrangement, Dragon has a

643 single low impedance ground plane for both PDB’s, with the The Dragon receivers have a nominal threshold sensitivity exception of the Trunk RIO. In addition, the power harness of -110dBm. Because Dragon is geometrically more compact lengths are significantly shorter than those for Falcon. than Falcon, the isolation between S band receivers, LNA’s Therefore, Dragon does not experience significant voltage and antennas, to the S band transmitters and antennas, is more drop in the power harness. The release fittings connecting the of a challenge with regard to interference margin. To provide Trunk and Dragon are satisfactory for ESD, tribo-electric adequate margin for spurious emissions, the modular avionics charging and lightning mitigation purposes, but not as a low unit main board and personality module circuits were impedance ground for the Trunk RIO. The composite Solar evaluated for fundamental clock frequencies, switched power Array fairing and Nosecone structures have a bulk resistivity frequencies and RS422/485 data rate harmonics, as for Falcon. of, nominally, 10-1 to 10-3 ohm-cm, bulk resistance, greater Additionally, the spurious OOB (out-of-band) performance of than 1 ohm end-end, and the release fittings (Trunk to Dragon) the transmitters was evaluated. The manufacturer’s EMC are nominally 105 to 107 ohms. Due to these considerations, spurious performance data was used to predict the margins, and common mode rejection considerations for internal then the spurious performance tested in the SpaceX EMC lab. grounding of the avionics boards, a true single point ground Based on the unit-level tested and predicted spurious (SPG) approach is used. performance, the balanced connector, harness and chassis shielding effectiveness was selected to provide at least 20dB 2) Common Mode Rejection SPG Grounding for Secondary self-compatibility margin to the receiver threshold sensitivity Power, Signal Lines levels. However, it was determined that modular Faraday Due to the presence of common architecture for Falcon and Cage chassis would be required to restrict OOB spurious Dragon RIO based units, the common mode rejection design transmitter leakage, and relocation of the transmit antennas of Falcon has been adopted. Secondary power converters relative to the receive antennas, would be needed to prevent maintain transformer isolation from primary power ground. saturation. This is discussed further in Part II, Verification. Although this approach minimizes ground loops, it contributes to ground bounce due to switched power conducted spurs, GSE load switching and lightning transient common mode on 4) RF and Lightning Radiated Susceptibility Design the signal lines and secondary power lines. To mitigate these As is the case for Falcon, Dragon is exposed to the CCAFS effects, RC filters have been implemented to clamp the signal Range radar environment, and NASA X and C band imaging and secondary power grounds to chassis. Transient voltage radars during ascent (10.48-10.499 GHz and 5.4-5.9 GHz suppression (TVS) diodes have been used at unit external respectively). Additionally, the ISS radars generate S, Ku and interfaces to protect against lightning and ESD damage. The Ka band fields of 161 V/m, 250 V/m and 125 V/m. The TVS devices range from 300W to 1.5kW depending on the Avionics is analysed and tested to verify immunity to these interface circuit location relative to the stage umbilical harness. levels. Also, the receivers and LNA saturation and damage There is a nuance to the SPG approach that requires attention levels are determined from the manufacturer’s data and to detail for the RS422/485 circuits used as the signal interface verified by test at SpaceX. Pad SLC-40 operations are for all Avionics units. Because these circuits can be damaged regularly exposed to lightning, particularly during the May when the common mode voltage excursion of more than +/-10 through October period. The avionics is designed with double Volts occur between either line and the power rail, TVS shielded harness with 360 degree shield terminations to protection diodes are used to clamp the RS422 lines to chassis mitigate spurious emissions, but also, reciprocally, to limit the and the secondary power lines are RC filtered or protected induced pin stress from lightning and launch Range RF. There with diodes to chassis ground to control this rail differential is a minimum of 34dB margin to the 1Amp, 1Watt, 5 minute voltage for transient conditions, including lightning. An no-fire and 20dBm limit for RF no-fire. overview of the interface circuit protection design is provided in the Verification section, Part II of this paper. 5) Tribo-Electric Charging Mitigation Design 3) Radiated Emissions Self-Compatibility Design Specific upper atmospheric cloudy weather conditions are Dragon employs S band for TLM (telemetry) and video known to cause tribo-electric charge buildup and damage level downlink (2216 and 2231.5 MHz), S band CMD (command) discharge voltage stress to LV interface circuits. The CCAFS uplink (2202 MHz), UHF band (414.2 MHz) for TLM/CMD Range Safety document (EW 127-1) specifies the following to the ISS, and GPS navigation (1575.420 MHz). The ISS weather condition restriction: SSRMS (robotic arm) RF susceptibility levels set additional constraints on the intentional radiated emissions from Dragon. “Do not launch if a vehicle has not been treated for surface The levels for S band and UHF band, are, respectively, 80.5 electrification and the flight path will go through any clouds V/m and 10 V/m. The susceptibility levels outside these above the -10 degree Celsius level up to the altitude at which notches range from 10 V/m to 125 V/m and are discussed in the vehicle's velocity exceeds 3000 ft/sec.” more detail in Part II of this paper. This condition is considered mitigated if all vehicle surface materials are grounded to LV electrical ground structure by less than 109 ohms/square. As discussed below, the Falcon 9

644 and Dragon external surface materials have been treated with SpaceX has implemented the JSC-29129 “Electrical Auxiliary static dissipative paint and electrically bonded to vehicle Power Unit Corona Design Guideline”, maintaining a structure electrical ground, with surface resistivity less than minimum spacing of 30 mils for the 120 Volt trace and the required threshold level. This has been verified by test contacts, a margin of a factor of 5 to the NASA 20 Volt/mil during final vehicle integration at pad SLC-40. guideline. Additionally, the PCB’s are conformal coated to prevent corona effects on the circuit traces and contacts. 6) Plasma Interaction Design The Plasma environment is described in NASA-STD-4005 8) Crew Radiation Protection Design and NASA-HDBK-4006, the LEO Spacecraft Charging SSP-50808, the COTS-ISS ICD, and SSP-50005, the ISS Standard and Handbook, respectively. The ram facing Flight Crew Integration Standard, specify the safe limits of surfaces are exposed to a nominal electron current of 15 crew exposure to non-ionizing and ionizing radiation. The milliamps/m2 and AO (atomic oxygen) ram current of 1 requirements are based on IEEE C95.1 for RF exposure and milliamp/m2. The wake side current is specified as less than ANSI Z136.1 for laser exposure. Additionally, the SSP 10-3 of the ram side electron current, or 15 uamps/m2 or less. documents specify the crew maximum safe levels of ionizing radiation. The exposure analysis for RF, laser and ionizing 7) Bulk Charging and Corona Design radiation dose are discussed in the Verification section, Part II SSP-50808, the COTS-ISS ICD, requires that Dragon of this paper. Avionics are designed to preclude damaging effects due to corona. To prevent corona at the 120 Volt ISS interface,

TABLE IV EMI COMPLIANCE BY MISSION COTS/CRS Mission Dragon Capsule (C1 Dragon Capsule & Dragon Capsule & CRS Missions Verification Mission) Trunk (C2 Mission) Trunk (C3 Mission) Provisions

ISS Conducted Engineering level tests of 1. Engineering level tests 1. Engineering level tests 1. Engineering level Emissions: all C1 avionics units on including all new C2- including all C3-specific tests including all CE01, CE03, CE07 Dragon Force simulator; specific units on Dragon units (including IPCU) CRS-specific units Engineering level tests of Force simulator on Dragon Force (including Powered 120V, 28V IPCU 2. Tests performed on simulator Cargo) on Dragon interfaces integrated flight capsule 2. Tests performed on Force simulator and integrated flight trunk integrated flight capsule 2. Tests performed on integrated flight capsule ISS Conducted Engineering level tests of Engineering level tests of Engineering level tests of N/A Susceptibility: Dragon Force simulator; Dragon Force simulator; Dragon Force simulator; CS01, CS02, CS06 Engineering level tests of Engineering level tests of Engineering level tests of 120V, 28V IPCU 120V, 28V IPCU interfaces 120V, 28V IPCU interfaces interfaces

ISS Radiated Emissions: Limited engineering level Tests performed on N/A N/A RE02, Operational RE unit tests of EM units integrated flight capsule (including IPCU) in EMI and integrated flight trunk chamber; including all new C2- Limited engineering level specific units tests of Dragon Force simulator ISS Radiated Limited engineering level Tests performed on Tests performed on N/A Susceptibility: unit tests of components integrated flight capsule integrated flight capsule RS02, RS03, RFS (including IPCU) in EMI and integrated flight trunk; and integrated flight Analysis chamber Add unit tests for new trunk; items Add unit tests for new items

645 REFERENCES V. CONCLUSION [1] ‘Eastern and Western Range Safety Manual’, EWR 127-1, The Dragon spacecraft is part of the SpaceX COTS Dec 1999 Commercial Re-supply Services launch system in conjunction [2] ‘Space Exploration Technologies COTS CRS EMC with the Falcon 9 LV. The design requirements for both Control Plan’, Rev G, March, 2010 Dragon and Falcon have been synthesized to a single [3] ‘IEEE Standard for safety Levels with Respect to Human enveloping set of EMI and Space Survivability design Exposure to RF Fields’, IEEE C95.1-2005, 2005 requirements to achieve a reliable and cost effective CRS [4] ‘ANSI Standard for Safe Use of Lasers’, ANSI Z136.1- system. These requirements have been implemented and 2007, 2007 verified by analysis and test, to show compliance with the [5] ‘STS Electrical Auxiliary Power Unit (EAPU) Corona Launch Range safety requirements and the ISS interface Design Guideline’, JSC-29129, June 2000 safety requirements for EMI, Plasma effects, bulk charging, [6] ‘LEO Spacecraft Charging Design Standard’, NASA- corona and Crew safety ionizing and non-ionizing radiation. STD-4005, June 2007 The verification methods are discussed in Part II of this paper. [7] ‘LEO Spacecraft Charging Design Handbook’, NASA- HDBK-4006, June 2007

[8] ‘Avoiding Problems Caused by S/C On-orbit Internal Charging Effects’, NASA-HDBK-4002, Feb 1999

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