Propulsion Subsystem

The propulsion system passes compressed air through nozzles to generate thrust to allow the spacecraft platform to translate and rotate on the table. Air at 80 psi, delivered from the plumbing system discussed here, is expelled through air nozzles. At this pressure, the air nozzles are rated to produce 1.0 N of thrust. However, in practice it appears that this value is near 0.25 N. An experimental setup to accurately determine the thrust produced by each thruster is being developed by another student. The components specific to the propulsion system are listed in the table below

Name	Manufacturer (Supplier)	Part Number	Quantity
Air Nozzle	Silvent	MJ4	8
Solenoid Valve	Peter Paul (Sempress Pneumatics)	582W19DGM 12VDC	8

The nozzle is attached to the output of the solenoid valve, using some spare tubing. The figure below shows the solenoid valve and nozzle assembly.

It is desirable to have the solenoid valve very close to the nozzle, such that when the valve is closed, only a small amount of air remains in the lines that is expelled as residual thrust. The thrusters are mounted with hot glue to the underside of the bus deck. Small cutouts have been machined into the bus deck where the nozzles expel their exhaust, as shown below.

The plumbing used to deliver the 80 psi air to the nozzles is discussed in the Plumbing section. The electronics used to actuate the solenoid valves is discussed in the Solenoid Valve Actuation section.

Plumbing

Compressed air at 4,500 psi is stored in small air tank that are typically used for paintball/air gun applications (First Strike Hero 2's carbon fiber air tank - 88/4500). The method for filling the tanks is discussed here. This section, however, discusses how the air from the tank is regulated and delivered to the air bearings and thrusters. The components used are listed in the Table below.

Name	Manufacturer (Supplier)	Part Number	Quantity
Air Tank	First Strike (ansgear.com)	First Strike Hero 2 (88/4500)	1
Air Tank Adapter	ansgear.com	EMPIREUFABLACK	1
Steel Tubing to M 1/8" NPT	MMC	52215K439	1
1/4” Steel Tubing	MMC	89895K411	2"
Steel Tubing to M 1/8” NPT Elbow	MMC	52245K545	1
Teflon Tape	MMC	6802K11	1
High Pressure Regulator	Swagelok	KCP1FLC2A2P10000	1
Pressure Relief Valve (100 psi)	MMC	5784T11	1
Pressure Relief Valve Adapter	MMC	4464K351	1
Low Pressure Regulator	MMC	3834T51	1
Secondary Air Filter	MMC	41575K61	1
Pressure Gauge 0-160 psi (Nozzle line)	MMC	4000K718-4000K575	1
Pressure Gauge 0-100 psi (Air Bearing line)	MMC	4000K718-4000K574	1
Clear PVC Tubing	MMC	1487T1	50 ft
1/8” PVC Tubing Tee with 1/8” M NPT	MMC	5121K941	1
1/8” PVC Tubing to 1/8” M NPT	MMC	2808K22	2
1/8” PVC Tubing Elbow	MMC	2808K115	13
1/8” PVC Tubing Tee	MMC	5117K13	9
1/8” PVC Tubing to 1/8” F NPT	MMC	5121K511	2
1/8” PVC Tubing to M 10-32	MMC	5117K83	20
1/8” PVC Tubing Cross	MMC	5463K93	1
Steel Tubing Cutter	MMC	2751A12	1

Air is released from the air tank through the tank adapter, and flows through stainless steel tubing at 800 psi to the high pressure regulator, which regulates it to 80 psi, and is shown on the bottom pressure gauge. After the high pressure regulator, the air flows past a pressure relief valve that vents the lines if the pressure exceeds 100 psi (in the event that the high pressure regulator fails). Following this, the line is split. One line goes to the air bearings, and the other goes to the thrusters. The air bearing branch passes through a low pressure regulator which brings the pressure to 60 psi for the air bearings. The air then passes through an air filter to remove any 'last minute' contaminants. Following this, the air passes through the air bearing solenoid valve and then branches off to the bearings themselves. The thruster branch immediately goes to the underside of the bus deck, where it branches off to all 8 solenoid valves. A schematic of the air flow and corresponding pressures is shown below.

Many fittings are used. The following figure shows the air flow from the top of the bus deck, and the subsequent figure shows the air flow from the underside of the bus deck.

Using PVC Fittings

The flexible PVC tubing is attached to the various components using barbed fittings. Barbed fittings have a small raised edge that prevents the hose from coming detached from the fitting, shown below.

The procedure for attaching the PVC tubing to a fitting is as follows:

1. Select the fitting you wish to use.

2. Cut a segment of tubing to the length required, using the wire cutters.

3. Apply some PVC Sealent to the fitting, making sure to apply this to only the exterior of the fitting, as shown below.

4. Manually push the segment of tubing onto the fitting. Use pliers if necessary, as shown in the following two figures.

5. Allow the glue to cure for 24 hours before supplying air.

6. To remove a tube from a fitting, the hose will be destroyed. Use the wire cutters to cut away at the hose, without damaging the fitting.

Note: Use teflon tape for all threaded fittings.

Solenoid Valve Actuation

To actuate the solendoid valves, MOSFET power transistors are used. MOSFETs have an extremely fast response time, and are very inexpensive. One MOSFET is used for each solenoid valve. The following table lists the components used in the design of the solenoid control board, and the following figure shows the solenoid control board.

Name	Manufacturer (Supplier)	Part Number	Use
MOSFET (N-channel)	Nexperia USA Inc. (Digikey)	568-7512-5-ND 1727-5893-ND	Onboard
Push Terminals	TE Connectivity (Digikey)	A104596-ND A104586-ND	Onboard
Signal Cable F	Tensility (Digikey)	839-1103-ND (obsolete) 839-1075-ND (obsolete)	Onboard
Signal Cable M	Tensility (Digikey)	839-1054-ND (obsolete) 839-1075-ND (obsolete)	Onboard
Current limiting resistors			Using 1kΩ
Pulldown resistors			Using 22 kΩ
Solderless Breadboard			Onboard

The digital signals from the on-board computer come to the transistor board through the signal cable on the right. The multicoloured wires run to the gate of each transistor (and pass through a current-limiting resistor first). When the on-board computer applies a +2.7 V signal to the gate (the ground of the computer sits at the same potential as the MOSFET source), the Gate-Source Threshold voltage (1.7 V) is passed, and the MOSFET allows current to flow with nearly zero resistance. Through this mechanism, the MOSFET acts as a switch that is activated by the on-board computer.

In case of a MOSFET failure, it is desirable to protect the on-board computer from the 12 V line. Therefore, a resistor (1 kΩ) is placed in series with the signal cable. Pulldown resistors of 22 kΩ are placed between the gate and ground in order to allow the electrons to flow to ground when the on-board computer sends a Low signal. In a previous iteration, the pulldown resistors were placed between the gate and source of the MOSFET, but we found that this was the source of major problems when paired with the Jetson Xavier NX.

Power flows through the solenoid and transistor circuitry as follows:

1. +5 V and GND leave the 5 V regulator and power the on-board computer (Arduino Pro Mini).

2. +12 V leaves the 12 V regulator and immediately goes to all solenoids and the on-board computer (Jetson Xavier NX).

3. The MOSFETs are downstream of the solenoids. Therefore, when current is not allowed to flow, there is a 12 V drop across the MOSFET. However, when current is allowed to flow, the 12 V drop is across the solenoid valve which causes it to open.

4. The source pin of the MOSFETs is connect to GND (kind of, see below note).

5. The on-board computer supplies a 0 or +3.3 V signal with respect to GND, which is used to actuate the Gate of the MOSFET. When the High signal is supplied, the MOSFET allows current to flow, and when the Low signal is applied, the MOSFET acts as a break in the circuit.

It should be noted that the source pins of all the MOSFETs used for switching the solenoids pass through another MOSFET before reaching the GND terminal on the 12 V regulator. This additional MOSFET is used for emergency stop purposes, discussed in Emergency Stop System. The air bearing MOSFET is exempt from the emergency stop circuit.