Projects

Resistojet

The hardware development of a resistojet heater commenced with the award of a UK Space Agency grant to support the manufacture, test and evaluation of a high temperature heater.

The resistojet design, procurement, assembly and test was completed successfully within the short timescale of the programme with argon and nitrogen propellants demonstrated, but site safety concerns prevented operation with ammonia propellant.

Whilst the design enabled the specific impulse of argon and nitrogen to be increased by around 2.5 times that at 20°C, the specific impulse of ammonia would be increased by at least 4 times to over 400s. This is because the operating temperature of the resistojet (1,600°C) significantly exceeds the ammonia dissociation temperature, which creates a lower average molecular weight of the exhaust gases, resulting in a higher exhaust velocity.

All the wetted materials used in the resistojet are compatible with inert and reducing gases, and the heater is designed for operation at 28Vdc, with the heater resistance, inlet pressure and nozzle diameter adjusted to accommodate a range of propellant types, power availability and thrust requirements. The anticipated performance of the development unit is shown in the table below for several propellants.

In the design of the development resistojet, in order to minimise radiative losses, the heater dimensions were kept to a minimum. As a result, the overall diameter of the resistojet was 20mm, which included a multi-layer heat shield, and 45mm long, including the mounting flange. The resistojet weighs in at 55gm without the flow control valve and with 100mm of heater lead.

The status of the resistojet development is currently on hold. There are several design improvements ready to be incorporated into the assembly of the next build, but the holding factor is the availability of a vacuum test facility capable of supporting the use of ammonia propellant.

Cathode Thruster

The Cathode Thruster is an electric thruster design based on a combination of a hollow cathode, resistojet and Hall Effect thruster. The design is aimed at smallsat applications, with a nominal thrust level of 10mN, power of 150W, and specific impulse greater than 1,000s at 28Vdc operating voltage using ammonia propellant.

The selection of ammonia propellant was made as a result of a desire to keep the system cost low, so operation at bus voltage, assumed to be 28Vdc, and the use of a low cost, low molecular weight propellant which can be readily stored and regulated to the thruster offered a way to achieve high performance and low cost. With the low energy, low mass exhaust products, the possibility of solid state thrust vector control is also being investigated, as a low cost alternative to a thruster pointing mechanism.

Operating at bus voltage minimises the complexity of the thruster power supply, allowing a simple isolating drive amplifier, or in the simplest case, leaving the anode live permanently. In this case, current would only be drawn when propellant is present, so the operation of the flow control valve allows current to be drawn and effectively switches the anode supply on.

The properties of ammonia as an alternative propellant to xenon are listed in Table 1.

Electron collisions with an ammonia molecule results in a range of products, as listed in Table 2.

The exhaust velocity of the thruster is proportional to the square root of the accelerating potential over the mass of the ion. This relationship is shown for ammonia and xenon over a voltage range from 20 to 50Vdc in Figure 1.

An estimate was made for the thruster specific impulse based on the assumed particles generated as a result of electron collisions and thermal dissociation, shown in Table 3.

The prospect of an electric thruster capable of producing a specific impulse of 1,000s at 15W/mN using low cost, storable propellant at bus voltage, would be interesting.