Development and testing of an Additive Manufactured Propulsion System (AMPS)

Development and testing of an Additive Manufactured Propulsion System (AMPS)
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Development and testing of an Additive Manufactured Propulsion System (AMPS) for use as a high impulse thruster for small to micro Cubical Satellites.

Overview

Experimental Propulsion Laboratories (EPL) was faced with the challenge of producing a high-pressure, liquid oxidizer tank, combustion chamber, and integrated fuel grain composed of a material that could act as both the structure and fuel for the propulsion systems. The EPL began the process of evaluating technologies that could produce a hybrid propulsion design that could meet the rigid mechanical properties for space applications while avoiding traditional tooling challenges. The final part would need to be designed for multiple restarts and tested at a peak thrust level of 6.2 lbf for duration of 16 seconds.

The result was the Additive Manufactured Propulsion Systems (AMPS) for use as a high impulse thruster for small to micro Cubical Satellites.  The first successful test of the system occurred in August of 2010.

The following case study illustrates the development and testing of the Additive Manufactured Propulsion Systems (AMPS) for use as a high impulse thruster for small to micro Cubical Satellites.


Development and Testing of a Propulsion Systems

Designing and manufacturing a complex and functional hybrid, single-part propulsion system provided challenges. Understanding the limitations with traditional subtractive manufacturing technologies, EPL saw the opportunity to develop a unique propulsion system based on the use of Additive Manufacturing (AM) technologies.

Additive Manufacturing technologies in Aerospace applications has presented both opportunity and challenges to engineers and scientist in the field.  The ability to produce parts and components using additive manufacturing technologies hold promise in both metals and plastics.  Where as, traditional subtractive manufacturing technologies can be restrictive in design development and material selection.

AMPS-H motorAMPS-H Motor

The AMPS-H (Additive Manufactured Propulsion System – Hybrid) motor from EPL is the first known functional additive manufactured rocket motor system (see image 1).  It was designed specifically for the small satellite market as a multi-start thruster that could deliver significant deltaV to a small cubesat spacecraft while maintaining a 10 cm3 form factor.

First the AMPS-H motor was designed with the combustion chamber located inside the oxidizer tank (see image 3).This is only possible if the motor is additively manufactured.  Additive manufacturing allows for internal cavities and channels to be produced inside the part maintaining a single component, unlike traditional manufacturing processes that require the part to be produced in multiple sections and then fused together.  The objective was to eliminate as many components as possible and incorporate them into a single part.
Making the part complex by design but simple since the part could be produced straight from a 3D CAD file, with no tooling required (see image 2).  By utilizing additive manufacturing technology, a 3D CAD model is designed on the computer, then “printed” in 3D.  To optimize the part for end-use, design changes can be made to the part and produced (or printed) again and again with little concern for casting, as tooling and secondary machinery is not necessary.  Using additive technology allowed the hybrid design to be very compact and energy dense.

3D cad file of AMPS3D cad file of AMPS

While the challenge of designing and manufacturing a hybrid, single-part was addressed with the use of additive technology, the next step was to find a material that could meet the rigid mechanical properties required for space applications.  A variety of additive technologies available on the market can produce the AMPS design, however, the material properties available provided to be too weak and inconsistent density of the fussed structured. After producing several CAD design concepts of the AMPS, EPL determined it was time to move forward and produce a prototype of the AMPS for testing and demonstration.

EPL contacted several service providers that utilize additive manufacturing technology and received parts made with various additive manufacturing methods and materials.  The parts received by EPL were dimensionally correct but lacked the needed physical properties required.  “First, the models didn’t come in a functional state straight from the service provider,” Mathew Dushku, EPL.  “It is critical the design holds high-pressure, and only a fully fussed structure would be able to do that.  All the models received had voids in the structure providing leak paths for the high-pressure liquid to escape.  Second, the build material properties the service providers offered did not meet the high tensile strength values and over time would become brittle in a vacuum.  Due to the lack of success, there was consideration on our part to revise the concept and possibly pursue another avenue for the initial testing.”

During the evaluation process and design review, EPL learned of a strong, highly functional alternative material, called Windform, specifically designed for the additive manufacturing laser sintering technology.  “A colleague of mine, who knew about the production issues and problems I was experiencing, suggested I look at a new product on the market called Windform XT 2.0,” continued Mathew Dushku.  “My colleague explained that he used the Windform material in a project involving a small satellite and that the material performed above expectations.”

Manufactured by CRP Technology, Windform exhibited high tensile strength and could be fully fussed, thus creating a solid structure that could hold the high pressure.  EPL determined the Windform material would make the ideal material to design the oxidizer tank and combustion chamber as a single piece for the AMPS-H motor.  To further simplify the hybrid design, EPL used Windform as the fuel core to the hybrid motor.

Stewart Davis, Director of Operations at CRP USA, describes the approach and process to the EPL concept.  “When EPL contacted us with their idea, we knew the combination of CRP’s additive laser sintering technology and Windform would be the perfect combination to meet the strict design and functionality requirements. The goal was to avoid the pitfalls Mathew experienced with other service providers, and help him achieve a product that would perform to his expectations.”  Utilizing additive manufacturing technology, Windform provided several advantages, including no leak paths due to joints or voids from casting in a fuel. The combustion chamber port geometry was built directly into the fuel and the design could be changed without any tooling changes. The forward and aft combustion chambers were all integrated as a single piece with the fuel grain.  Internal oxidizer feed lines, igniter port, injector port, nozzle port, nozzle retraining ring groove, and pressure transducer ports were all incorporated and were all part of the single additive manufactured part.  Having the ability to incorporate all these features into a single part made a very complex motor design very simple to manufacture, where otherwise was un-manufactureable.

Internal side view of AMPSInternal side view of AMPS
Designed as a single-part for production
Patent-pending

The EPL project represented clear challenges for CRP and the additive manufacturing technology.  Challenges included: pressures of 0-2,000 psig, temperatures both High (4,000 degree F) and Low (-80 degree F).

After hours of analysis and design, the final CAD model was ready to be prototyped. Designed using the Windform XT 2.0 material properties, and with the integration of the internal tubing, the CAD file was sent to CRP USA and the part produced.
The result was a laser sintered additive manufactured, prototyped, single-part that was sent to EPL for testing.

AMPS on Test StandAMPS on Test Stand

When EPL received the first prototype rocket motor, they immediately noticed the difference in the quality of construction.  Other prototypes produced by other service providers, utilizing various processes and materials had visible porosity in the structure and were not dimensionally correct.  However, the Windform XT 2.0 prototype from CRP USA was solid and had a smooth outer finish.  Dimensionally, the part was within all specified tolerances and had well defined features.  EPL was impressed with the fact that the prototyped motor came as a ready-to-use part, no additional manufacturing required.  This saved EPL both time and money, which further increased profitability, and reliability, and allowed them to get to market faster.  After evaluation of the part the assembly and integration process began.  The EPL team had a plan to introduce the AMPS-H at the Small Satellite Conference in Logan, Utah.  EPL not only planned to show the concept, but also demonstrate the feasibility of producing a high performance rocket motor with the additive manufacturing process.

Mathew Dushku describes the challenges faced when preparing for the ‘live fire’ test.  “Preparations went very smooth.  Straight out of the box the motor was hydro pressure tested to insure that a margin of safety >2.0 had been met.  Next, the motor was integrated on the test stand for cold flow testing where the injector and the high-pressure oxidizer tank would undergo thermodynamic cycling and flow rate calibration.  After test stand integration was complete, several safety measures were taken to make the test stand ready for live firing.”

On October 8, 2010, at a test site near the Logan Airport in Logan, Utah, a small group of observers from the Small Satellite Conference gathered.  Among the attendees were several government agencies (ORS, AFRL, NASA), industry leaders (JPL-Jet Propulsion Lab and SDL-Space Dynamics Lab), and several university professors. 


Conclusion

Applying theoretical and practical knowledge were key to the design and production of the AMPS production.  The combination of the additive laser sintering technology, Windform, and CRP resulted in a single-part, high-performance hybrid thruster that meet the rigid material properties required for space applications.  The final part met the requirements of multiple restarts and tested at a peak thrust level of 6.2 lbf for duration of 16 seconds.
The AMPS system is currently being integrated into several concepts for integrated boosters in Cube Satellites.