Development of an Arcjet Deorbit Module for Low-Cost Constellations

By Jonathan SKALDEN,1) Manfred EHRESMANN,1) Georg HERDRICH,1)and Stefanos FASOULAS1)

1)Institute of Space Systems, University of Stuttgart, Stuttgart, Germany

(Received for 32nd International Symposium on Space Technology and Science, Fukui, Japan, June 2019)

Mega constellations of small satellites are envisioned to replace standard large GEO satellites currently operated. Not only will thenumber of satellites significantly increase, but also the requirement for flexible, generic platforms optimized to suit individual missiondemands. End-of-Life strategies will become even more important to mitigate further space debris generation. The Institute of SpaceSystems tackles the challenge to develop an adaptable deorbit module based on ammonia fed thermal arcjet thrusters, following thistrend. By applying additive manufacturing, the system shall provide the flexibility and efficiency required by future satellites. Thisstudy presents the first step in this development, by endurance testing of additively manufactured tungsten nozzles. First results showsimilar behavior to conventional designs and proof the general feasibility of the laser melted material in thermal arcjet thrusters.

Key Words: Arcjet, Additive Layer Manufacturing, Tungsten, Satellite Constellations, Deorbit

Nomenclature

I : Current, Ad : Diameter, mmt : Thruster operation time, hm : Mass flow rate, mg/s

SubscriptsTh : Thruster

Con : Constrictor

1. Introduction

Thermal arcjet thrusters (arcjets) have been in developmentat the Institute of Space Systems (IRS) for many years. A va-riety of thrusters in power classes ranging from 100 W–100 kWhave been tested. Highlights were the engineering model of the1 kW hydrazine thruster ARTUS and the flight model ammoniathruster ATOS. Additionally, a large data base of operationalpoints has been built up to allow scaling for different missionrequirements.

The IRS is now working on a new type deorbit system basedon ammonia fed arcjet thrusters. Current trends in the satellitebusiness likely will lead to generic satellite platforms that makeuse of Industry 4.0 methods to allow custom tailoring to per-fectly suit individual mission needs at a low cost. Furthermore,mega constellations featuring such platforms are expected tobecome more important, with the Oneweb constellation pio-neering this development.1) As the number of small satellites inlow Earth orbit is thus expected to increase significantly, End-of-Life (EoL) scenarios, including disposal of the spacecraft,are becoming more important to comply with the Code of Con-duct for space debris mitigation and to self maintain the constel-lation.2, 3) This again requires deorbit modules that are capableof keeping up with the flexibility a generic platform requires.

Using thermal arcjet thrusters for deorbit purposes was previ-ously investigated at IRS as contribution to the European SpaceAgency’s (ESA) CleanSat project.4) Arcjet systems both stand-alone and in dual mode, sharing a common hydrazine tank withchemical thrusters, were designed and evaluated in terms of per-formance, costs, and development effort. This study serves as

base line for further developments of an arcjet deorbit module.The goal is the design of an adaptable deorbit system with

variable nozzle geometry to achieve the best performance forgiven mission requirements using additive layer manufacturing(ALM). As a first step, the IRS investigates additively man-ufactured tungsten nozzles as part of the Integrated ResearchPlatform for Affordable Satellites (IRAS) project. Within thisproject the IRS, German Aerospace Center (DLR), FraunhoferInstitute for Manufacturing Engineering and Automation (IPA),and industry partners investigate the reduction of manufactur-ing costs for small satellites using Industry 4.0 methods.5) Aperformance optimized nozzle design using ALM capabilities,would allow faster deorbit compared to other electric propul-sion systems like gridded ion thrusters. Due to the high thrustdensity that arcjets provide, the time frame to deorbit the satel-lite will be shorter, which reduces operational complexity dur-ing EoL, hence, saving ground operation costs. The shorteningof EoL mission time span also reduces the time for potentialspace debris generation. Furthermore, additional propellant canbe carried to use the system for orbit raising, if required.

This study presents the first experimental test results of ad-ditively manufactured arcjet nozzles made of tungsten. The be-havior of the material during a five hours endurance test is dis-cussed, followed by plans for the next steps in the developmentprogress.

2. Background

2.1. Thermal ArcjetsThermal arcjet nozzles are almost exclusively made of tung-

sten alloys to withstand the heat load of the electrical arc. Dueto the high brittleness and hardness of the material, nozzle de-signs are driven by manufacturing capabilities. Furthermore,the production process becomes exponentially more expensivewith complexity. This limitation leads to the problem that ma-jor efficiency losses by thermal radiation and frozen flow con-ditions cannot be mitigated optimally. The term thermal lossescovers all excess heat, which is generated but not introducedinto the gas acceleration process. For thermal arcjet thrusters,those losses can sum up to 15 %, depending on the propellant.

Frozen flow losses describe the power fraction consumed dur-ing the non-reversed dissociation and ionization process and isaround 50 %, again depending on the propellant.6)

With the use of additive manufacturing, the challenge is tack-led to reduce these major losses to a competitive level. Thermallosses can be accounted for by the use of regenerative cool-ing. This has been achieved before in several thruster designs,at IRS.7) However, ALM allows helix shaped cooling channelsintegrated into the nozzle wall. The cold gas can be exposedlonger to the heat flux and can be brought very close to the hotgas flow inside the nozzle. It is expected to further increase theregenerative cooling capability.

Aside from raising the thrust efficiency, the cooling chan-nels serve another purpose. Currently only pure tungsten canbe manufactured additively, alloys like the usually used thori-ated tungsten or tungsten lanthanum oxide are not possible atthe current state of the art. The reason lies in different coeffi-cients of thermal expansion of the different alloy constituents,which warps the melted spot and heavily reduces accuracy andstructural integrity of the products. Usually the above men-tioned alloys are used for arcjet operation, due to the reducedwork function. Pure tungsten requires more energy to releaseelectrons and, therefore, suffers increased thermal loads. Ex-periments will have to show, if the cooling channels can com-pensate this additional heat load.

However, even if the thermal losses could be completelycompensated, which is physically not possible, the gained effi-ciency would again be halved by the frozen flow losses to 7.5 %.Therefore, it is obvious that a major focus needs to be put on in-creasing the recombination rate in the divergent section of thenozzle. This has been accounted for already in earlier arcjetdesigns, with two primary solutions, the recombination cham-ber and dual-cone nozzle. The general idea is to maintain thegas flow at a higher pressure level to support recombination ofparticles. The dual-cone nozzle geometry balances the gain inrecombination efficiency with losses due to deviation from abell-shaped nozzle.6) Both proof of concepts were experimen-tally produced at IRS, but the maximum efficiency reached withhydrogen was 50 %.7)

With the use of additive manufacturing the efficiency couldbe further increased, by finding a nozzle geometry that isadapted for a certain operating point. Work on developing thisnozzle shape is currently being conducted at IRS.2.2. State of the Art: ALM with Tungsten

Selective laser melting (SLM) with metal is already at an ad-vanced level of development for stainless steel or nickel basedalloys like Inconel. Densities > 99 % can be achieved with veryhigh quality surfaces. With tungsten however, the process is stillin development. According to the Austrian company PlanseeSE, densities of theoretically 96 % are the maximum that canbe achieved at the moment. This is in accordance with researchin the field stating similar results.8)

During the SLM process, melt spreading and solidificationare two critical parameters, which need to be balanced for aproper product with high material density. Due to high thermalconductivity of tungsten, heat is dissipated fast to surroundingpowder and support structures causing a fast solidification pro-cess. The droplets can solidify faster than they can wet the sup-port structure or lower powder layer, and hence, the result is a

rough surface. This effect is called balling and prevents the pro-duction of fully dense/massive structures. A possible solution isto scan for balling droplets and melting them in a second laserscanning process. However, this approach did not yet result indensities close to 100 %.9)

Another issue is the absence of availability of the tungsten al-loys thoriated tungsten (WT20) and tungsten lanthanum oxide(WL10) for SLM production. These are usually used for ther-mal arcjet nozzles. Those alloys have a lower work function,which means less energy is required for thermionic emission ofelectrons. This results in less heating and, hence, reduced wearon the electrodes. The effect is critical for the cathode, but alsorelevant for the anode, which will be subject to increased tem-peratures if it is manufactured from pure tungsten. The higherthermal load needs to be compensated by additional cooling tobe achieved by integrated cooling channels.

3. Experimental

This section provides an overview of the used vacuum fa-cility, power supply, and thruster configuration. Furthermore,the novel nozzle design with integrated cooling channels is pre-sented.

3.1. Vacuum FacilityThe IRS features a vacuum facility, which was optimized for

lifetime test of the flight model arcjet ATOS.10) It consists ofa chamber with a diameter of 1 m and 2 m length and a threestage pumping system listed in Table 1. During operation, anambient pressure of 0.6–0.7 Pa is provided with an argon massflow rate of 20–30 mg/s.

The chamber seen on Figure 1 is equipped with a low-pressure water cooling system. A copper floor with welded oncooling tubes and water cooled towers in the plasma plume al-lows endurance tests over long time periods.

Table 1. Vacuum facility pumps at IRS.

Number Type Suction rateI Rotary vane pump 275 m3/hII Roots pump 2050 m3/hIII Roots pump 12000 m3/h

Fig. 1. Vacuum facility at IRS for arcjet experiments.

3.2. Power SupplyThe VELARC thruster was powered by the engineering

model of the arcjet power processing unit (PPU) flown on theAMSAT-P3-D mission.10) It operates in an ignition mode to ap-ply a high voltage pulse and steady-state mode where the cur-rent is controlled via high-frequency voltage adjustments. Therange of current to be applied to the thruster is ITh =0–10 A. ADelta Electronica SM 3540-D power supply serves as interfacebetween PPU and the low-voltage electrical grid.

At IRS, the thruster voltage UTh and current ITh are measuredby an in-house made multimeter. The current is derived via thevoltage drop over a 100 Ω shunt.3.3. Nozzle Designs

Two nozzle designs were designed and manufactured for thistest campaign. ALM Nozzle A is the additively reproducedcounterpart of the original ARTUS-LM3 nozzle with the pur-pose of material analysis. The second design ALM Nozzle B ismade to investigate the general feasibility of cooling channelsin additively manufactured tungsten and to allow investigationof potential gains in thrust efficiency.

Design limitations were given by the manufacturer PlanseeSE, listed in Table 2. For the nozzle designs, the parameterssmallest cavity and wall thickness were multiplied by a factorof 3–4 to ensure a proper part quality in terms of flat surfaces.This originates from previous experience with metallic ALMproducts designed and utilized at IRS.

Table 2. Geometry limitations and accuracies of tungsten ALM processesat Plansee SE.

Parameter RangePowder layer thickness 20-50 µmMaximum part size 230 x 230 x 150 mmAccuracy ± 0.05 mmSmallest cavity > 0.5 mmWall thickness > 0.1 mm

3.3.1. ALM Nozzle AThe nozzle design depicted in Figure 2 for material testing

is an ALM reproduction of the previously at IRS developedARTUS-LM3 nozzle. It features a 0.4 mm constrictor in diame-ter with a length of 0.3 mm. The convergent part has an angle of90 deg, whereas the divergent part opens by 30 deg. The surfacequality inside the nozzle and at sealing areas was set to Rz= 6.3,as it was the case for the original ARTUS-LM3 nozzle. Thisdegree of surface quality cannot be achieved directly during theALM process, hence, post processing is required. Furthermore,an additional injector is necessary for swirl injection of the pro-pellant, which can be seen in Figure 4.

Fig. 2. CAD view and cross-section of ALM nozzle design A11)

3.3.2. ALM Nozzle BWith nozzle design B, displayed in Figure 3, the first step

has been completed towards a highly function integrated arcjetnozzle. The internal geometry covering convergent and diver-gent angles, as well as, the constrictor dimensions are identicalto ALM Nozzle A to allow performance comparison. Cool-ing channels for regenerative cooling purposes have been real-ized with three helices, which start at the nozzle tip and end asswirl injectors in the discharge chamber. This ensures a longexposure of the propellant to generated heat of the nozzle, asit passes by the whole nozzle length twice. The helix followsa guideline, which again follows the nozzle contour and bringsthe propellant close to the high temperature regions. A mini-mum wall thickness of 1.5 mm is kept to prevent a structuralcollapse during arc operation. The cooling channel profile re-sembles an egg-shaped curve, with the tip in printing direc-tion. Previous investigations at IRS with Inconel ALM struc-tures have shown that this geometry grants the highest successrate for unobstructed internal channels.5)

Since the cooling channels already provide swirl injectionof the propellant, no further injector is necessary. Instead, thethruster configuration features a boron nitride insulator that sta-bilizes the nozzle inside the housing for proper sealing at thethruster exit.

Fig. 3. CAD view and cross-section of ALM nozzle design B11)

3.4. Thruster ConfigurationsTo conduct the experiments, the thruster has two configura-

tions. For the material analysis, the previously at IRS developedARTUS-LM3 (laboratory model) will be utilized, to assess thematerial behavior with argon as propellant.7) The use of argonallows investigation of the material with only minor influenceof chemical reactions. A schematic of the thruster can be seenin Figure 4. However, in this configuration the ALM Nozzle Bcannot be operated. This lead to redesigns in the front sectiondepicted in 5.

Both thrusters have a cathode made of thoriated tungstento lower the work function. The insulation to prevent elec-trical discharges between the cathode and housing is made ofboron nitride and aluminium oxide. Sealing is realised withring shaped Sigraflex elements, a flexible graphite fabric, athigh temperature regions and copper sealings at threaded con-nections. The housing is made of titan-zirconium-molybdenum(TZM), which provides the necessary temperature resistanceand can be properly machined. The higher thermal expan-sion coefficient prevents cracking due to an expanding nozzle.By using Sigraflex sealing elements, the resulting gap betweenhousing and nozzle can be compensated. The distance betweencathode and anode during is set to 0.4 mm.3.5. Test Matrix and Procedure

The thruster was operated at the maximum available currentof 10 A. The mass flow rate was set to 15 mg/s of argon, which

Fig. 4. Arcjet laboratory model ARTUS-LM3 (blue: cathode, red: anode, beige: elec. insulation).11)

Fig. 5. Arcjet laboratory model for testing regenerative cooling with an ALM nozzle (blue: cathode, red: anode, beige: elec. insulation).11)

was found as stable point of operation during a preceding testwith the conventional manufactured counterpart of the nozzle.To comply with the previous material analysis by Bock, thesame test pattern was applied.12) After the time steps definedin Table 3, the thruster was disassembled, the nozzle’s condi-tion photo documented and the constrictor measured with a mi-croscope. Before the images were taken, the microscope wascalibrated with a calibration ruler.

For the following test, the cathode was polished, the seal-ing areas cleaned, and the graphite sealing rings replaced. Atthe nozzle, only the sealing area was cleaned and polished, therest remained untouched. 24 hours before each test, the facil-ity with integrated thruster was evacuated to allow outgassingof the parts and chamber walls. After a test run was concludedand the arc turned off, the thruster was cooled with argon be-low 200 °C. Afterwards the chamber was flooded with argon toa pressure level of 400 Pa to prevent oxidation during the cool-down process.

Table 3. Test parameters.

Parameter Range Measurement deviceITh 10 A In-house multimeterm 15 mg/s Bronkhorst mini CORI-FLOW M13t 1,5,10,30 h Test facility clock

dCon Measured Microscope

4. Results

In this section the results of the material analysis are shown.Due to delays in the measurement campaign, only 5 hours ofthe outlined 30 hours could be covered so far. The influenceon the material is depicted in Figure 7 and 8. Furthermore, the

development of the constrictor’s diameter is shown in Figure 6.The image prior to the test campaign shows a constrictor with

some impurities at the wall, which was to be expected. Theoverall diameter is about 0.01 mm below the designed 0.4 mm,but this is within the manufacturing tolerance. After the firsthour of operation, these impurities were all either melted or dis-posed in the plasma plume. Since there was no intense sputter-ing observed, it is unlikely to be exhausted through the nozzle.The diameter remained nearly constant, but actually the eccen-tricity was reduced. Considering the image shown in Figure 8,the outer surface did not show any signs of wear.

−1 0 1 2 3 4 5 60.2

0.25

0.3

0.35

0.4

0.45

0.5

Thruster operation time t, h

Con

stri

ctor

diam

eter

d Con

,mm

x-axis diametery-axis diameter

Fig. 6. Development of nozzle constrictor diameter against thruster oper-ation time.

After 5 hours of operation, more significant changes can beobserved. The constrictor wall shows a more coarse pattern,

Nozzle throat prior to operation

Nozzle throatafter 1 hour of operation

Nozzle throatafter 5 hours of operation

Fig. 7. Influence on the nozzle constrictor after thruster operation.

Nozzle prior to operationNozzle after

1 hour of operationNozzle after

5 hours of operation

Fig. 8. Nozzle outer surface prior and after thruster operation.

which could be directly related to the tungsten powder size.Also a similar constrictor shrinking process, as it was observedby Bock,12) occurred, reducing the diameter by about 6 %. Alarger fragment was consumed by the arc in the top right cor-ner, which might have been partially melted tungsten powder.However, this is not unusual for an arcjet constrictor after sev-eral hours of operation and has been observed in the referencestudy as well. The outer surface also shows minor changescompared to the previous images. The ALM structure is moreclearly seen with several micro cavities opened on the surface.This most likely happened due to trapped air inside the materialwhich was expanded while heated up during arcjet operation.However, the structural integrity did not suffer from this effect.

Overall, the additively manufactured tungsten withstood theelectric arc environment without major damages. Operationwas stable over the whole 5 hours and no significant anoma-lies were found. So far it can be assumed that ALM tungsten isin general feasible for arcjet operation. However, the followinglong time test have to verify if the described trend continuous.After the campaign is concluded, the same tests will be repeatedwith a conventional tungsten nozzle for a more direct compari-son.

5. Outlook

With the general feasibility of ALM tungsten operated in arc-jet environment being proofed, the next development steps canbe taken. After the material analysis is completed, a perfor-mance characterization of ALM Nozzle A and B will be con-ducted to investigate the efficiency of integrated cooling chan-

nels. The propellant will be switched to hydrogen, since thespecific heat capacity of argon is too low to measure properdifferences in thrust. Also higher power levels up to 1 kW areplanned to be tested to investigate, if the material can withstandthis.

6. Conclusion

Two nozzle designs for thermal arcjet thrusters were manu-factured via selective laser melting of tungsten with the requiredquality. A successful 5 hours, with 4 continuous hours proofedthe general feasibility of the material for arcjet operation. Fur-ther testing of the nozzle at higher power levels and a perfor-mance comparison between the described nozzle designs willgive further insight into the behavior of the material.

Acknowledgments

The support of the IRAS project as well as the Ministryof Economic Affairs Baden-Wurttemberg is greatly acknowl-edged.

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