NASA_AF-M315E

NASA – Internship Final Report

Characterization of AF-M315E in a 1N Thruster

Clay Blanchard

Marshall Space Flight Center

August 8, 2014

Reviewed by NASA MentorKevin Pedersen

ER – 23

1Summer 2014 Session


Characterization of AF-M315E in a 1N Thruster

Clay Blanchard1

Louisiana State University, Baton Rouge, LA 70820

NASA Marshall Space Flight Center, Huntsville, Alabama, 35812

AF-M315E is a “green” monopropellant developed by the USAF. This propellant offers advantages over industry standard solutions such as hydrazine because of the simplified launch processing associated with propellant toxicity. The motivation for the development of “green” monopropellants is to find alternative solutions to Hydrazine. Hydrazine is a widely used propellant for many RCS thruster applications and is highly toxic, expensive, and hard to store. The propellant is widely used because of its high reliability on orbit. The scope of this project is to characterize the propellant AF-M315E. This propellant is an ionic liquid, which is decomposed and combusted by catalysis in the thruster head end. The green monopropellant to be studied and characterized will be the Air Force developed AF-M315E (a Hydroxyl Ammonium Nitrate based propellant). Analyses will be performed to predict the performance of the thrusters, prepare and test the ground test system, prepare and lead design and test reviews, and test and analyze the thruster systems.

I. Introduction

The use of hydrazine has been dominant in monopropellant propulsion systems for many years. Problems

with this propellant include expensive storage, handling, disposal procedures, and most importantly propellant toxicity. Traditional green propellants are used for low specific impulse and thrust applications. The purpose of AF-M315E is to provide low toxicity, while having specific impulse comparable to that of Hydrazine. The 1N thruster is an ionic liquid monopropellant thruster that, in this project, utilizes AF-M315E, an Air Force developed propellant. AF-M315E reacts with an Iridium catalyst bed that is contained inside the 1N thruster. The decomposition of AF-M315E requires the catalyst bed to be preheated. The monopropellant decomposes into different gases, which provide thrust from expansion through the nozzle. This decomposition is hypothesized to result in higher thruster performance than hydrazine. The propellant is pressurized at the top of the Propellant Tank using gaseous nitrogen. The main goal of this project is to test and observe the performance of AF-M315E. In conjunction to the aforementioned goal, the catalyst performance and life cycle will be sought after.

II. DesignA. Test Stand

The test stand described in this section was built to accomplish the goal of characterizing AF-M315E. This stand will be used as a means of providing a secure structure, and a low pressure. All of the supporting structures for the test stand were manufactured in MSFC building 4205 labs 104, 110, 111 and 106. This included fabricating a test stand and designing and constructing plumbing for the fluid system. The fluid system includes the high and low-pressure systems along with the cooling and propellant feed system. Precautions were taken to avoid over pressurization of the tubing that can arise from misuse of the system. These precautions included following proper operating procedures and safety measurements. To assure proper pressure rating in the tubing, hydrostatic testing was done with all test stand plumbing. The pressure was set to twice the operating pressures. The test stand and fluids system were designed such that steady state firing and pulse firing could be tested. A series of instrumentation were integrated into the system in order to monitor the pressure and temperature of the fluid system and certain thruster performance parameters, such as chamber pressure and temperature, mass flow rate, and thrust. Once all of the test stand components were fabricated, every wetted article including valves, fittings, tubing, and instrumentation were deemed compatible with AF-M315E according to safety regulations.

1 Research Assistant, Space Propulsion Systems, Marshall Space Flight Center



1. Fluid System

The system is broken down into 5 sub-systems: High Pressure, Low Pressure, Propellant Feed, Vacuum and Cooling systems. The high-pressure system is used to pressurize the propellant tank and also provide a gas feed for a GN2 purge to the vacuum chamber. The low-pressure system is used to provide inlet pressure to the electronic controller that controls the pneumatic regulator on the high-pressure system. The propellant feed system is used to deliver the propellant into the thruster. This system starts at the propellant tank and ends with the thruster. The propellant tank is pressurized by the high-pressure system, which causes the propellant to flow to the thruster. Along the way to the thruster, a pressure transducer and flow meter show the properties of the propellant respectively. The last sub-system is the coolant system. This system was created to cool the vacuum chamber, load cells, and exhaust gas to the vacuum pump. Figure 1 shows the schematic of the entire fluid system.

A more in-depth description of the 5 sub-systems are shown below:

1a. High Pressure System

The propellant system is pressurized and purged with nitrogen supplied by a nitrogen storage bottle (K bottle). The gas storage bottle supplies nitrogen at 4000 psig to two pressure regulators. These regulators are connected in parallel with isolation valves (hand operated) to provide the system with pressure. The purpose of the hand-operated regulator is for the system propellant purge, and the pneumatic regulator (remote operated) will be used during testing to assure safety. These regulators flow nitrogen through either a solenoid valve or hand valve, connected in parallel. Then through a series of two check valves. Then finally flows into the top of the propellant tank. A third pressure-reducing regulator is tapped off of the K-bottle to provide low pressure purging of the vacuum chamber. This is part of the cooling system as well. Relief valves and a burst disk are installed for safety precautions incase of a system over-pressurization. Pressure gauges and pressure transducers are located within the system to observe the regulated pressure in the system at various locations.

1b. Low Pressure System

Low-pressure nitrogen is supplied from a facility line regulator, located inside the test cell (building 4205; 104). The nitrogen supplies pressure to a current driven pressure transducer. The transducer will sense the pressure, which will help provide feedback on regulating pressure to the pneumatic regulator on the high-pressure system. Another important function of this system is to provide pressure to the pneumatic normally open valve (NOV). In case of over-pressurization in the system, the NOV will vent the gas to the atmosphere.

1c. Propellant Feed System

The propellant feed system starts with the propellant tank and ends with the thruster. The propellant tank consists of a short SST tubing, 6” long, with an outer diameter of ¾”. The system includes a filter, turbine flowmeter, solenoid shutoff valve, hand operated valve, and the thruster. The propellant tank will be filled with propellant by removing the cap off the top of the tube and distributing propellant into the top of the tube using a syringe or pipet. The propellant tank will be pressurized up to 400 psig during test operations in order to flow propellant through the system After the test, the remaining propellant will be drained out through the hand-operated valve or thruster. The system will then be flushed with distilled water both through the thruster and out the valve. A camera will monitor the propellant system for leaks, and also the pressure gauges during the testing operations.

1d. Vacuum System

The importance of the vacuum system is so that the thruster can be tested at low pressures. Testing at low pressures allows more accurate results. The 1-Newton thruster that is being tested is located inside a small vacuum chamber, with a volume of 2.62ft3. Hot gas from the vacuum chamber flows through a shut off valve (hand valve) outside the chamber, then through a cross-flow water-jacketed heat exchanger, through the rotary vacuum pump, and finally discharges into the top of a drum located outside the walls of building 4205. The heat exchanger also functions to cool the exhaust gas to prevent damage to the vacuum pump. The SST drum is used to collect and condense fluids instead of exhausting directly to the atmosphere. A vacuum gauge (in bunker) and vacuum



transducer (on chamber) will be used to monitor the vacuum chamber pressure at all times. An Infrared camera will be used to monitor the internals of the vacuum chamber during the test.

1e. Coolant System

The purpose of the coolant system is to do just that; cool the hot exhaust from the thruster before it enters the vacuum pump. It is also used to cool the load cells (within the chamber) with the nitrogen purge. In order to effectively cool the thruster exhaust, three water-cooled heat exchangers are added in series. A flat plate heat exchanger is located below the thruster, a copper coil below the thruster (on top of the flat plate), and a cross-flow plate heat exchanger (outside the chamber). The hot exhaust gas will fire directly into the coil and hit the flat plate. After this, the gas will get sucked through the vacuum tube and through the cross-flow heat exchanger outside the chamber. The cross-flow heat exchanger will cool the gas even more before it enters the pump to prevent pump damage. Water is supplied from a potable water source inside the test bunker. Water is flowed through these three heat exchangers and ejected into a sink drain within the lab.

Figure 1. Fluid System Schematic

2. Part Servicing

The components of the fluids system were ultrasonically cleaned and passivated. The cleaning process involved degreasing the components with a 15-minute soak in the ultrasonic cleaner. An employee at NASA completed all of the passivation process. After these processes, all of the components were sealed in clean bags to ensure the parts remained uncontaminated. When assembling the fluid system, all of the parts were handled with gloves, to prevent contamination.



3. Fabrication

Prior to beginning construction of the test stand, a CAD model was created. The vacuum chamber and test stand were modeled in PRO-E. The height of the stand was determined based on the constraint of the viewport of the vacuum chamber. The thruster needed to be seen through the viewport in the center of the chamber during testing. The width of the stand was already determined based off the physical limitations of the chamber. The depth was built such that a coil and cold plate heat exchanger could fit under the stand. The stand was composed of four T-Slotted aluminum legs and a flat SST plate on top, which is where the thruster assembly hangs. Many of the actual thruster assembly parts were water-jet cut out of SST plates. The heat shield on the assembly was created by hand pressing a ball bearing into a thin SST shim. The outcome was a half spherical shape with holes around the perimeter to mount to a flange. Other parts were machined at the component development area (CDA), by lab technicians. A pulley system was manufactured for a one point in-vacuum calibration of the load cells. The support for the pulleys were water-jet cut out of SST and spot welded to the top of the thrust stand.

Figure 2 shows the thrust stand mounted inside the vacuum chamber. Note that the thruster will be fired directly down into the copper coil heat exchanger shown in the picture.

Figure 2. Thrust Stand

The plumbing for the fluid systems was manufactured out of 0.035” and 0.049” thick SST tubing. The tubes were hand bent and flared using a tube-flaring machine. The entire fluid system was mounted to an aluminum panel for organization. Figure 3 shows the fluid system including HOR, gauges, pressure transducers, and solenoid valves.



Figure 3. Pressure System

B. Thruster

1. Injector

The injector was custom built by Plasma Processes. The injector consists of a 1/8” SST tube for the propellant inlet and a flat face with 100micron diameters holes on the injector face. The injector is threaded so that it can connect to the nozzle. On the perimeter of the injector is a flange. This flange is used to attach the thruster to the thrust stand. Figure 4 shows the thruster, catalyst, injector, and load cell mount starting from the left respectively.

Figure 4. Thruster Assembly

The load cell mount was manufactured with a water-jet cutter in-house. The mount is made of 1/8” SST plate. The triangular shape was designed in order to shed weight of the part. Figure 5 shows the load cells, load cell mount, and flange spacer from top to bottom respectively.



Figure 5. Load Cell Mount

Figure 6 shows the scale of the full thruster assembly. This assembly is mounted above the copper coil as described above.

Figure 6. Thruster on stand



2. Catalyst Bed

For monopropellant thrust applications of AF-M315E, a catalyst must be present to start the decomposition reaction. Plasma Processes also manufactured the catalyst that this system used and therefore is proprietary. Figure 7 shows the monolithic catalyst bed at various magnifications. The catalyst is composed of Iridium. In order for the reaction to occur, the catalyst bed must be preheated to about 340°C. In the testing section below, catalyst heater verification was done. The results are shown in Fig. 9.

Figure 7. Iridium Catalyst

3. Nozzle

Plasma Processes manufactured the nozzle used in this system as well. The manufacturing of the nozzle consisted of electroforming, with the substrate being Rhenium and the coating being Iridium. The Iridium coating is used to protect the part from oxidation. The nozzle can be seen in Fig. 4 and 6 above.

C. Heat Exchanger

Since the thruster will be expending very hot gasses during the test, a series of heat exchangers will be used to absorb heat from the gas. The decision was made to have the thruster fire directly down onto a plate heat exchanger. The copper coil was added in order to cool and contain the surrounding gas. Both the plate and coil heat exchanger were located in the vacuum chamber. The cross flow heat exchanger was mounted outside of the chamber to further cool the gases before they enter the vacuum pump. If the pump were to expel these hot gasses, it would quickly burn up. This is why the heat exchangers are very important. The copper coil heat exchanger was created by hand bending 3/8” copper tubing around a solid aluminum cylinder of the diameter needed. The result was 10 loops of coil. Wire was used in between the loops of the copper in order to tightly pack the copper coil. The cross-flow and plate heat exchangers were purchased. The three heat exchangers were assembled in series. The inlet of the heat exchanger assembly was hooked to a water tap located in the lab. The exit was attached to plastic tubing and discharged into a sink. The three heat exchangers can be seen in Fig. 8 below.



Figure 8. Heat Exchanger

III. System Testing and Safety A. Testing

1. ObjectivesThe primary goal of this development was to characterize the performance of the monopropellant AF-M315E. A

secondary goal for this project was to observe the degrading of the catalyst bed over time.

Performance of a 1N AF-M315E thruster will be assessed by:

• Flowing AF-M315E propellant through the thruster while measuring flow rate, thruster inlet pressure, and thrust

• Perform tests at different inlet pressures• Measure thruster’s chamber pressure and combustion temperature• Measure the thruster’s outer surface temperature using an IR camera• Vary test firing duration and time between firings• Repeat a baseline firing periodically to determine if performance has changed• Measure temperature at thruster valve interface• Measure temperature near injector tube supply O-ring

As mentioned above, the catalyst bed must be preheated in order to combust the monopropellant. The bed must be heated to 340°C. The test performed was to make sure the heater functioned properly. The heater was only heated to around 130°C to test functionality. Figure 9 shows the thrust assembly with the preheater turned on.



Figure 9. Catalyst heater with Infrared Camera

D. Test Matrix

The different testing plans that are to be completed are listed in the following table:

Test DescriptionTrim/SS tests Dial in test pressures and developed steady state response data

The following test matrix will be performed:Test Description Inlet Pressure [psig] Duration [sec]Trim Facility checkout 300 5SS SS 400 10SS SS 350 10SS SS 300 10SS SS 250 10SS SS 200 10SS SS 150 10

Table 1: AF-M315E Monopropellant Test Matrix

E. Safety and Hazards Analysis

The following safety analysis was performed by this project’s team leader, Gustavo Martinez. Since this project used high pressure and a class 1.3 explosive, several measures of safety had to be taken into considerations in order to run the planned tests safely. The high pressure comes form a 5000psi K-Bottle. In accordance with NASA safety standards, a Hazards Analysis had to be addressed in order to complete the Test Readiness Review. The Hazards Analysis or Assessment was performed to enhance safety of the AF-M315 1N Thruster testing and to ensure compliance with NASA, OSHA, and national consensus standards. The assessment also serves as a summary of identified hazards to be used by decision makers when examining the risk level for the test.

Each safety hazard has been assessed to identify severity and probability of the hazard and assigned a Risk Assessment Code (RAC). Definitions used for Severity and Probability in this assessment are as follows:

Initial RAC: The RAC is expressed as a combination of the severity and the probability classifications with the severity classification listed first such as IA, IIA, IID, etc.



Residual RAC: Indicates the residual hazard probability and severity after all the identified controls are incorporated.

Status: Indicates whether the identified hazard is “Open” or “Closed.”Open - Corrective action to eliminate or control the hazard has NOT been completed. The hazard will remain

open pending completion of corrective action and verification.Closed - Corrective action to eliminate or control the hazard has been implemented. Closure of a hazard will

be based on Industrial Safety Office acceptance and approval.Risk Assessment Classification (RAC): The RAC indicates the risk associated with each individual hazard. It

is derived by considering both the severity and probability of a hazard.Hazard Severity: An assessment of the worst-case potential injury or system damage if an identified hazard

were to result in an accident defines the hazard severity. Severity categories are listed below.Hazard Probability: The likelihood that an identified hazard will result in a mishap defines the hazard

probability.

Figure 10. RAC Definitions [1]



Figure 11. Severity and Probability Definitions [1]



The following table provides a summary of the identified hazards for the AF-M315E 1N Thruster test.

Concern No.

ConcernInitial

RACResidua

l RACResidu

al RiskStatus

1 Rupture of High Pressure Tubing System 3C 3E Low Closed

2 Rupture of Low Pressure (<150 psig MEOP) Tubing System

3C 3E Low Closed

3 Rupture of K-Bottle due to over-pressurization results in damage to test facility and/or injury to personnel

3C 3E Low Closed

4 Rupture of water coolant system due to over pressurization of water

3C 3E Low Closed

5 Electrical shock due to exposure to water 1C 1E Low Closed

6 Loss of power causes unsafe conditions 3C 3E Low Closed

7 Unplanned ignition or detonation of propellant

2C 2E Low Closed

8 Asphyxiation due to venting of GN2 in an enclosed room results in death or injury to personnel

1C 1E Low Closed

9 Slips, trips, and falls due to water spillage 3C 3E Low Closed

10 Injury to test personnel or the environment due to exposure to propellant

3C 3E Low Closed

Table 2. Identified Hazards

IV. Conclusions and Future workIn conclusion of the project, a sophisticated propellant testing system was designed and built. Even

though the thrust stand and all necessary systems were complete, the system could not be operated for total project completion. Due to time constraints of this internship, a hot test fire was not conducted for this thruster. Some problems that came up during this project included the manufacturing of the radiation shield, and safety requirements. Using a thinner shim to properly deform the spherical shape solved the radiation shield problem. The safety requirements inhibited the process of the project. The team was unable to test the completed testing system because the NASA safety team had to inspect and improve the lab setup via a TRR. This issue was not resolved because of the internship time constraint.

In order to complete this work, the thrust stand will be tested and the propellant characterized. The future work for this project will consist of the testing article listed in the section above. The first test will consist of a steady state thrust test. After this test, catalyst will be inspected and results written. Once the steady state data is acquired, the team will move on to pulse testing. Once testing data is acquired, the propellant AF-M315E can be characterized.



V. AcknowledgmentsThe author would like to thank the following people for their invaluable assistance with our project:

Chris BurnsideKevin PedersenTyrone DanielCharles PierceTony YarbroughJohn Wiley

Darrel GaddyBoyd McNuttBrad BullardErik KrausJeremy KennyLuke ScharberLogan Kennedy

Jonathon JonesTexas Space GrantLouisiana Space GrantOhio Space GrantAlabama Space Grant

VI. References

1. MSFC NASA Safety Code, MWI 8715.15 Rev E


NASA_AF-M315E

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