Temperature Estimation for a Plasma-Propelled Rocket Engine: Inferential Measurement Using Optical Emission Spectroscopy Data

The vast majority of rocket engines rely exclusively on chemical combustion to achieve thrust in the vacuum of space. However, a new generation of electrically mediated propulsion systems for spacecraft produce thrust from a fuel propellant using electric energy. For example, in ion thrusters, neutral gas is fiionized, and the ions are accelerated using either the Coulomb force or the Lorentz force, based on electric or magnetic fi elds, respectively. The acceleration of ions away from the spacecraft propels the craft in the opposite direction by Newton’s third law of motion. The variable specific impulse magnetoplasma rocket (VASIMR) is an electric propulsion technology being developed by the Ad Astra Rocket Company at its facilities in Houston, Texas, USA, and in Liberia, Costa Rica. To produce thrust, the VASIMR engine uses a highly ionized plasma (see “What Is a Plasma? ”) accelerated with magnetic fields to produce thrust. In VASIMR, plasma is generated with a helicon antenna using radio frequency waveforms at the frequency 13.56 MHz. Helicon discharges are known to be efficient methods for plasma production [1]. Initially, electromagnetic waves from the antenna energize free electrons present in a neutral gas fuel. These electrons then ionize atoms in the gas through energetic collisions to create a plasma. Charged particles in the plasma follow helical paths along magnetic field lines due to the Lorentz force, which is proportional to the charge on the particle, the velocity of the particle, and the strength of the magnetic field. Carefully designed electromagnets form magnetic field patterns that confine the plasma and move it from the helicon to the ion cyclotron resonant heating (ICRH) section of the rocket. In the ICRH section, a second helicon antenna excites the ions at precisely their gyrofrequency, further energizing the propellant gas [2]. The gyrofrequency of a charged particle is the frequency of rotation of the particle as it spirals in a magnetic field. This frequency is directly proportional to the charge of the particle and the magnetic field strength and inversely proportional to the mass of the particle. In the last portion of the rocket, a final group of magnets, comprising the magnetic nozzle, accelerate the plasma away from the craft along expanding magnetic field lines. The energized plasma physically detaches from the engine and its magnetic field, thus creating thrust. The exhaust velocities from VASIMR are expected to reach as high as 120 km/s [3] in final prototypes. For near-term applications of VASIMR, large solar arrays are expected to generate electric power for the rocket [4]. The VASIMR engine is depicted in Figure 1. Compared to alternative electric propulsion technologies, VASIMR has many advantages as a potential space propulsion system. Plasma is produced inductively in the engine, negating the requirement for electrodes to be in contact with the plasma. This electrodeless design allows greater power densities to be reached for longer periods of time than conventional magnetoplasma or ion engines, without fear of electrodes becoming damaged or wearing out. This design is essential for missions requiring months or years of continuous rocket operation. The propellant gas, which is argon, is inexpensive, chemically inert, and widely available. The greatest advantage of VASIMR is that it produces thrust very efficiently compared to chemical rockets. The efficiency of a jet or rocket engine is measured by the momentum change per unit weight of consumed propellant. This characteristic, which is measured in seconds, is termed the specific impulse of the engine. Used as a measure of economy between rocket engines, specific impulse is comparable to