Solid fuel Pulsed Plasma Thrusters (PPTs) are propulsion systems used by small satellites for space travel. They have demonstrated higher specific thrust through the use of a sulfur fuel cell as compared to the standard Teflon fuel cell. This higher thrust in combination with electrical propulsion is ideal for small satellites on long missions. Measuring the sulfur’s plume density and speed will afford a better understanding of the thruster. Five Langmuir probes were constructed horizontally(x- & y-plane) on an aluminum rod that can move vertically (z-plane), which allows for plume speeds and density to be measured at different positions by adjusting the different probes and vertical rod to reach the position wanted for measure. This vertical rod also saves time, by eliminating the need to bring the chamber up to air. The probes were made by connecting one end of a copper wire to tungsten wire and by ceramic housing. The output of each probe is a Gaussian-like shape, similar to a symmetric “bell curve”, where peak density, time to peak, and Gaussian width are recorded. These values are mapped to create a 3D image of the plasma plume. This image will give a better understanding of the sulfur plasma dynamics and help to prevent damage to the surfaces of the satellite and onboard electrical systems.

Plume Measurements of a Sulfur Pulsed Plasma Thruster

Brandt Monson, Robert Winglee, Ian Johnson, Paige Northway

Advanced Propulsion Laboratory

Background of CubeSat

Pulse Plasma Thruster PPT

Sulfur v Teflon

Langmuir Background

Langmuir Testing

Conclusions

The CubeSat, a small cube shaped satellite, is used by Universities developing space science and exploration programs because of the simplistic design and low cost. Different scientific instruments can be put on the CubeSat to take numerical measurements of the Earth we live on. This will help improve our understanding of the world we live on and the universe we live in.

The probe construction is of a simplistic nature. Consisting of exposed wires inserted into a plasma with a biased voltage potential. Once five Langmuir probes were constructed, they were put in a row of 5, 4 cm apart to measure the coaxial change in density. They were placed on a steel rod so that they could be raised vertically.

We have found that a Pulsed Plasma Thruster fueled by Sulfur produces more thrust than that of commonly used Teflon. In addition to measuring the plasma density we will be testing both Sulfur and Teflon, so that we can continue to analyze the two. Since we’re are the only one ones using Sulfur measuring the plasma density will also be used to find how well Sulfur works as a fuel cell.

Essentially a satellite engine, a pulsed plasma thruster (PPT) is a combination of electromagnetic and electro thermal thrusters. It uses a solid fuel source as propellant. With a large voltage potential difference across the surface of the fuel cell, the thruster emits a plume of charged ions, also known as plasma. The PPT is not very large making it ideal for satellites. A smaller size means there is less weight to be moved into space.

A Solidworks model of a Pulsed Plasma Thruster. 1. Spark Plug, 2. Fuel Cell, 3. Cathode, 4. Anode

A PPT used for testing compared to a regulation sized Rubik's Cube

Invented by Nobel Prize winning Irving Langmuir, Langmuir probes measure current from a plasma plume. With the measured current we can calculate the density of the plasma. The probe tips are immersed in the plasma plume, the current is read as the voltage through the Stangness Transformers.

The circuit diagram for a Langmuir Probe immersed in Plasma

After changing the height of the Langmuir probes as well as the energy put into the system, we were able to model the plasma density according to horizontal and vertical position.

3D representation of the plasma density of Pulsed Plasma Thruster’s plume

Plasma Density for different energy inputs for probes 13cm above the PPT 55 microseconds after firing

Sulfur and Teflon fuel compared side-by-side

With 5 Langmuir probes data was gathered at different axial positions 4 cm apart. These probes showed that plasma density decreased as the radial position from the center increased. By inputting different energy levels across the PPT, a higher plasma density was recorded as we expected. By moving the probes along the vertical axis, data can be represented in another dimension. This 3D model represents the plasma density as a function of time. We can map out the general shape of the plume with this model.

Two Langmuir Probes with a braided wire shield and one with a steel tube shield

The plume from a sulfur fueled pulsed plasma thruster

A 3D printed CubeSat with a sulfur fueled pulsed plasma thruster in the Belljar

A plot for the plasma density versus time for three different energy inputs

The probe tips are made of tungsten wire which are connected through a steel tube to magnet wire. The magnet wire is fed through a steel tube or braided wire which acts as a grounding shield to reduce electrostatic and electromagnetic noise.

Five Langmuir Probes placed on the steel stand