Cubesat Deorbit System

CubeSat DeOrbit System

Final Report MAE 434W

December 3rd, 2013

Students:

Joshua Barham

Trevor Jackson

Timothy Lynch

Joseph McNamara

Joe Powell

Troy Tarnacki

Faculty Advisor:

Dr. Robert Ash

Table of Content

Abstract...... ii

Introduction...... 1

Background...... 2

Completed Methods...... 2

Proposed Methods...... 5

Preliminary Results...... 7

Discussion...... 7

Appendices...... 9

References...... 22

List of Figures

Figure 1 - CAD model of baseplate...... 9

Figure 2 - Gasket, baseplate with screws, and baseplate cap...... 3

Figure 3 - Baseplate with membrane, gasket, and cap assembled...... 3

Figure 4 - Cutting wire and battery...... 9

Figure 5 - Enthalpy of Sublimation of Benzoic Acid vs. Temperature...... 10

Figure 6 - Necessary mass of Benzoic Acid vs. Volume of Aerobrake and Pressure inside Aerobrake...... 10

Figure 7 - Necessary Energy for Sublimation of Benzoic Acid vs. Volume of Aerobrake and Pressure inside Aerobrake...... 11

Figure 8 - Baseplate with deployed inflatable...... 6

Table 1 - Saturation Temperature as a function of Pressure with respective Elevations...12

i

Abstract

There has been an urgent need expressed by the United Nations to reduce space debris. It is currently recommended that devices placed in orbit should have the ability to deorbit themselves within 25 years, however, a mandate of this recommendation may be on the horizon. This time constraint restricts CubeSats to a launch altitude of 600 km. By utilizing technology first implemented by NASA's Echo I project a deorbit system is being developed. This paper describes the apparatus that will utilize the sublimation of benzoic acid to inflate a drag device, or aerobrake, that will allow for controlled and expedited deorbit of a CubeSat.

ii

Introduction

Debris from leftover space missions creates a hazardous environment for the International Space Station as well as expensive military and industrial satellites. The United Nations (UN) has published debris mitigation guidelines, which mandatory requirements expected in the near future. To resolve this accumulation of space debris, aerodynamics brakes (aerobrakes) are being developed for CubeSats and satellites to reduce their orbit life [1]. The use of small satellites and cube satellites (CubeSats) is increasing because of their relatively low cost to produce and lower cost to launch due to their low mass. Aerobrakes using mechanical mechanisms are currently being used, but they take up a considerable amount of volume inside the CubeSat while being stored, creating the need for an aerobrake that takes up less space when stowed. The use of benzoic acid to inflate a balloon like aerobrake has been used by the National Aeronautics and Space Administration's (NASA) Project Echo [2]. The use of benzoic acid has not been used on CubeSats for deorbit, creating the potential for a small storage volume aerobrake device.

The design for a benzoic acid driven aerobrake consists of inflating a balloon-like device to create drag, which is attached to a CubeSat. The baseplate encompasses the aerobrake and is pressurized by sealing a membrane material through use of a gasket between the baseplate and cap. A high resistance heating wire is used to cut the membrane and deploy the aerobrake, which is then inflated by the sublimation of benzoic acid. The membrane is responsible for holding the aerobrake inflatable under pressure inside of the baseplate. Without the membrane the benzoic acid within the inflatable would sublimate prematurely, inflating the balloon in the lack of pressure in low earth orbit.

A thin membrane that can be cut by a high resistance heating wire was configured to deploy the aerobrake. A Nichrome (Nickel-Chromium) wire will be subjected to electrical current raising the temperature of the wire. A sufficient power supply is needed in order to supply enough power to heat the Nichrome wire to the melting point of the membrane, cutting it and releasing the aero-brake. Once the aerobrake is released, the vacuum pressure of low earth orbit along with solar radiation energy from the sun and the earth will induce a phase change of the stored solid benzoic acid into a gas inflating the aerobrake. A comprehensive thermal analysis was conducted in order to determine how much energy input is needed to start the sublimation of benzoic acid at a given temperature and pressure.

The baseplate, to encompass the entire aerobrake system, needed to be large enough to contain a folded aerobrake but optimized so that it would not take up any unnecessary amount of space and add unnecessary weight. The baseplate must also be strong enough to prohibit failure due to the internal pressure of the sealed aerobrake. Materials selection for the baseplate will be taken into consideration for cost, ease of manufacture, weight, and overall optimization of the system.

The purpose of the project was to find a low mass, low volume, and low cost solution to anticipated international mandates that provides the ability to deorbit small satellites in a controlled manner. The general increase in the rate of CubeSats being placed into orbit in the past few years, due to their low cost nature, provides a need for a small and low complexity system that is easily attachable to existing CubeSat chassis designs. This will allow CubeSat users to continue placing small satellites into orbit while meeting the requirements for deorbit.

Background

The first successful launch of an inflatable satellite was performed by NASA in 1960. This satellite, known as the Echo I, was constructed to act as a radio relay station and placed into low Earth orbit for the purpose of increasing global communications. Once in orbit, the Echo I was inflated using a sublimating powder known as benzoic acid. Spherical in shape and comprised of a thin metallized polymide, the Echo project demonstrated the low mass, high volume advantages of inflatable payloads used in space operations [2].

It is this research conducted by NASA that is the basis of the Old Dominion University (ODU) CubeSat DeOrbit System. Initiated by Dr. Robert Ash, the initial deorbit design incorporated the use of small gas canisters to inflate a polymide balloon [3]. Since this initial design, previous ODU CubeSat deorbit teams have used the Echo I project as inspiration to utilize benzoic acid as the means of inflation for the aerobrake. The original design also incorporated an aluminum door to protect the aerobrake and gas tanks from any damage from the outside [3]. The design has recently been revised to include the light-weight option of a polymide membrane barrier between the aerobrake and the vacuum of space. A deployment device known as the “ODU Picosatellite Orbital Deployer” or O-POD, was designed by a previous ODU CubeSat team as a means of propelling the CubeSat away from the rocket placing it into orbit. The O-POD remains attached to the rocket and can therefore be reused in future missions. It is the task of the current ODU CubeSat team to incorporate this previous research into the development of a functional deorbit device.

Completed Methods

The first aerobrake capsule design focused on optimization of the volume inside the baseplate so that the maximum amount of space will be available to hold the aerobrake and accompanying components. The first baseplate was designed for testing purposes only. The aerobrake capsule design consists of a baseplate, membrane, gasket, metal cap and are assembled in that order (see Figure 1, Appendix 1).

After the components are put in place screws are inserted through the cap, gasket, membrane, and screwed into the baseplate in order to form an airtight seal between the membrane and the baseplate. The main concepts that were to be tested was the ability of the design to seal the membrane and test the cutting wire. By doing this the baseplate was built overly robust, so that extra time was not needed to do a finite element analysis and the complexity of the design could be simplified so that the machine shop would not have to spend as much time machining a more complex part.

The membrane is responsible for holding the aerobrake inside the baseplate under atmospheric pressure until being release by the cutting wire. The membrane must withstand a vacuum pressure of 10^-4 torr without leaks. Testing consisted of an off the shelf gasket being roughly cut into the shape of the baseplate (See Figure 2).

(Figure 2) - (Top) Gasket, (Left) Baseplate with screws,

(Right) Baseplate cap

The membrane is tensioned over the baseplate between the gasket and the baseplate then the cap is placed on top and screwed down tight (see Figure 3).

(Figure 3) - Baseplate with membrane, gasket, and cap assembled

To test the baseplate a 1 mil Kapton film and a 5 mil Mylar film were used as membranes. The Kapton and Mylar films were put on the baseplate and the gasket was put on top of that. The cap was placed on top of the gasket and membranes and screws were used to screw the cap into the baseplate, compressing the gasket and membrane to create a seal. The baseplate was put into the vacuum chamber and the air was pumped out. The seal held to a small vacuum but leaked out of the spaces between the membrane and baseplate as the vacuum pressure dropped below 500 millitorr. This is evident from negative pressure inside the seal. Several different membrane and gasket materials have been tested without complete sealing success. After doing a failure analysis the result of the leak can be related to the aluminum cap bending where screws are absent.

The cutting wire, which melts the membrane releasing the aerobrake, has also been tested with different materials to ensure cutting of the membrane. The wire fits in-between the membrane material and the gasket to guarantee contact. When powered the wire has been able to melt the membrane; cutting wire vacuum testing has not been completed at this time but is planned.

An electrical analysis of the cutting wire was performed to determine the power required to heat the wire above the melting temperature of a given membrane (see Appendix 3). With the completion of the electrical analysis of the wire, a proof of concept was required to demonstrate the capability of the selected Nichrome (Nickel-Chromium) cutting wire to melt through a 5 mil thickness Mylar membrane. This was accomplished by connecting the Nichrome wire to a 6 Volt, 5 Amp/hour alarm battery (Appendix 1 Figure 4). The wire was connected by soldering the ends to copper leads and manually connecting the lead ends to the positive and negative battery terminals. Immediately after connecting the wire to the battery, the wire temperature began to rise and a second team member pressed the Mylar against the wire. As expected, the wire temperature was greater than the melting temperature of the Mylar and the membrane began to melt.

A thermal analysis of the system was conducted in parallel so that once the capsule system was functioning testing could begin on the inflatable system. The thermal analysis sought to describe the thermodynamic system inside the inflatable as a numerical model where parameters could be changed and the effects on the overall system could be evaluated. The aerobrake can inflate easily in the vacuum environment as long as the internal pressure after sublimation is above the external vacuum of space. Because the aerobrake causes the CubeSat to descend in elevation, internal pressure must be sufficient to maintain aerobrake shape at lower elevations (see Appendix 4). Pressure as a function of elevation can be found in Appendix 2 Table 1 using data published in the Journal of Geophysical Research [4]. Saturation temperature of benzoic acid as a function of pressure at the selected elevation was found using the Antoine equation (see Appendix 5). These values were then used to find the minimum amount of benzoic acid needed for sublimation to attain desired internal pressure at a selected elevation, through use of the Ideal Gas Law (see Appendix 6).

The energy required is based on the enthalpy of sublimation for benzoic acid. A relation for the enthalpy of sublimation based on saturation temperature was derived using microcalorimetry [5] (see Appendix 7). A plot of the relation can be seen in Appendix 1 Figure 5. These values, combined with the mass of benzoic acid required, were used to find the minimum energy required (see Appendix 8). This process was then analyzed over a range of values by varying parameters and analyzing the results. Inflatable volume, desired internal pressure, mass of benzoic acid required, and energy required for sublimation were analyzed over a range of values (see Appendix 1 Figure 6,7).

Proposed Methods

A redesign of the base plate and cap were conducted and a design that optimizes the sealing of a pressure vessel will be used. The baseplate and cap will be a circular design so the stress on the cap and membrane should be evenly distributed. This design will give the best chance for positive sealing of the pressure vessel. Once the baseplate is machined the tests described above will be conducted again. Different gasket materials will also be used to try and optimize the most effective gasket material for sealing the pressure vessel. When proper sealing methods are achieved the cutting wire will be integrated into the test system and debug any unforeseen issues. Once the sealing method and cutting wire are proven to work and are reliable, a prototype baseplate will be designed to reduce weight while still being able to hold up to pressure differences. A thorough finite element analysis will be conducted with different materials and alloys to optimize the system. Stress and deflection in the membrane will be computed using FEA software PaTran (MSC Software, Newport Beach, CA).