Pulsed plasma thruster

Last updated July 15, 2024

A pulsed plasma thruster (PPT), also known as a Pulsed Plasma Rocket (PPR), or as a plasma jet engine (PJE), is a form of electric spacecraft propulsion.^[1] PPTs are generally considered the simplest form of electric spacecraft propulsion and were the first form of electric propulsion to be flown in space, having flown on two Soviet probes (Zond 2 and Zond 3) starting in 1964.^[2] PPTs are generally flown on spacecraft with a surplus of electricity from abundantly available solar energy.

Operation

Schematic layout of a Pulsed Plasma Thruster SchematiclayoutofaPulsedPlasmaThruster.png — Schematic layout of a Pulsed Plasma Thruster

Most PPTs use a solid material (normally PTFE, more commonly known as Teflon) for propellant, although very few use liquid or gaseous propellants. The first stage in PPT operation involves an arc of electricity passing through the fuel, causing ablation and sublimation of the fuel. The heat generated by this arc causes the resultant gas to turn into plasma, thereby creating a charged gas cloud. Due to the force of the ablation, the plasma is propelled at low speed between two charged plates (an anode and cathode). Since the plasma is charged, the fuel effectively completes the circuit between the two plates, allowing a current to flow through the plasma. This flow of electrons generates a strong electromagnetic field which then exerts a Lorentz force on the plasma, accelerating the plasma out of the PPT exhaust at high velocity.^[1] Its mode of operation is similar to a railgun. The pulsing occurs due to the time needed to recharge the plates following each burst of fuel, and the time between each arc. The frequency of pulsing is normally very high and so it generates an almost continuous and smooth thrust. While the thrust is very low, a PPT can operate continuously for extended periods of time, yielding a large final speed.

The energy used in each pulse is stored in a capacitor.^[3] By varying the time between each capacitor discharge, the thrust and power draw of the PPT can be varied allowing versatile use of the system.^[2]

Comparison to chemical propulsion

The equation for the change in velocity of a spacecraft is given by the rocket equation as follows:

\Delta v=v_{\text{e}}\ln {\frac {m_{0}}{m_{1}}}

where:

\Delta v\

is delta-v - the maximum change of speed of the vehicle (with no external forces acting),

v_{\text{e}}

is the effective exhaust velocity (

v_{\text{e}}=I_{\text{sp}}\cdot g_{0}

where

I_{\text{sp}}

is the specific impulse expressed as a time period and

g_{0}

is standard gravity),

\ln

refers to the natural logarithm function,

m_{0}

is the initial total mass, including propellant,

m_{1}

is the final total mass.

PPTs have much higher exhaust velocities than chemical propulsion engines, but have a much smaller fuel flow rate. From the Tsiolkovsky equation stated above, this results in a proportionally higher final velocity of the propelled craft. The exhaust velocity of a PPT is of the order of tens of km/s while conventional chemical propulsion generates thermal velocities in the range of 2–4.5 km/s. Due to this lower thermal velocity, chemical propulsion units become exponentially less effective at higher vehicle velocities, necessitating the use of electric spacecraft propulsion such as PPTs. It is therefore advantageous to use an electric propulsion system such as a PPT to generate high interplanetary speeds in the range 20–70 km/s.

NASA's research PPT (flown in 2000) achieved an exhaust velocity of 13,700 m/s, generated a thrust of 860 μN, and consumed 70 W of electrical power.^[1]

Advantages and disadvantages

PPTs are very robust due to their inherently simple design (relative to other electric spacecraft propulsion techniques). As an electric propulsion system, PPTs benefit from reduced fuel consumption compared to traditional chemical rockets, reducing launch mass and therefore launch costs, as well as high specific impulse improving performance.^[1]

However, due to energy losses caused by late time ablation and rapid conductive heat transfer from the propellant to the rest of the spacecraft, propulsive efficiency (kinetic energy of exhaust / total energy used) is very low compared to other forms of electric propulsion, at around just 10%.

Uses

PPTs are well-suited to uses on relatively small spacecraft with a mass of less than 100 kg (particularly CubeSats) for roles such as attitude control, station keeping, de-orbiting manoeuvres and deep space exploration. Using PPTs could double the life-span of these small satellite missions without significantly increasing complexity or cost due to the inherent simplicity and relatively low cost nature of PPTs.^[3]

The first use of PPTs was on the Soviet Zond 2 space probe which carried six PPTs that served as actuators of the attitude control system. The PPT propulsion system was tested for 70 minutes on the 14 December 1964 when the spacecraft was 4.2 million kilometers from Earth.^[4]

A PPT was flown by NASA in November, 2000, as a flight experiment on the Earth Observing-1 spacecraft. The thrusters successfully demonstrated the ability to perform roll control on the spacecraft and demonstrated that the electromagnetic interference from the pulsed plasma did not affect other spacecraft systems.^[1] Pulsed plasma thrusters are also an avenue of research used by universities for starting experiments with electric propulsion due to the relative simplicity and lower costs involved with PPTs as opposed to other forms of electric propulsion such as Hall-effect ion thrusters.^[2]

Ongoing NASA research

On July 11, 2024 Howe Industries announced that it had contracted with NASA via a $725,000.00 grant to continue research on PPT/PPR propulsion technology. Howe Industries claimed that should PPT/PPR propulsion technology succeed in becoming a fully functional means of propelling space ships to Mars, then PPT/PPR technology should be capable of shortening travel time to Mars, down from the current requirement of approximately 1 year, to a much shorter travel time of only 2 months. Howe Industries further stated that at the current rate of their PPT/PPR research and development program, the technology may not be fully ready to propel a manned space ship to Mars for approximately another 20 years (as of 2024).^[5]

Related Research Articles

In spacecraft propulsion, a Hall-effect thruster (HET) is a type of ion thruster in which the propellant is accelerated by an electric field. Hall-effect thrusters are sometimes referred to as Hall thrusters or Hall-current thrusters. Hall-effect thrusters use a magnetic field to limit the electrons' axial motion and then use them to ionize propellant, efficiently accelerate the ions to produce thrust, and neutralize the ions in the plume. The Hall-effect thruster is classed as a moderate specific impulse space propulsion technology and has benefited from considerable theoretical and experimental research since the 1960s.

An ion thruster, ion drive, or ion engine is a form of electric propulsion used for spacecraft propulsion. An ion thruster creates a cloud of positive ions from a neutral gas by ionizing it to extract some electrons from its atoms. The ions are then accelerated using electricity to create thrust. Ion thrusters are categorized as either electrostatic or electromagnetic.

A magnetoplasmadynamic (MPD) thruster (MPDT) is a form of electrically powered spacecraft propulsion which uses the Lorentz force to generate thrust. It is sometimes referred to as Lorentz Force Accelerator (LFA) or MPD arcjet.

Beam-powered propulsion, also known as directed energy propulsion, is a class of aircraft or spacecraft propulsion that uses energy beamed to the spacecraft from a remote power plant to provide energy. The beam is typically either a microwave or a laser beam, and it is either pulsed or continuous. A continuous beam lends itself to thermal rockets, photonic thrusters, and light sails. In contrast, a pulsed beam lends itself to ablative thrusters and pulse detonation engines.

A fusion rocket is a theoretical design for a rocket driven by fusion propulsion that could provide efficient and sustained acceleration in space without the need to carry a large fuel supply. The design requires fusion power technology beyond current capabilities, and much larger and more complex rockets.

An antimatter rocket is a proposed class of rockets that use antimatter as their power source. There are several designs that attempt to accomplish this goal. The advantage to this class of rocket is that a large fraction of the rest mass of a matter/antimatter mixture may be converted to energy, allowing antimatter rockets to have a far higher energy density and specific impulse than any other proposed class of rocket.

A pulsed inductive thruster (PIT) is a form of ion thruster, used in spacecraft propulsion. It is a plasma propulsion engine using perpendicular electric and magnetic fields to accelerate a propellant with no electrode.

The Variable Specific Impulse Magnetoplasma Rocket (VASIMR) is an electrothermal thruster under development for possible use in spacecraft propulsion. It uses radio waves to ionize and heat an inert propellant, forming a plasma, then a magnetic field to confine and accelerate the expanding plasma, generating thrust. It is a plasma propulsion engine, one of several types of spacecraft electric propulsion systems.

Specific impulse is a measure of how efficiently a reaction mass engine, such as a rocket using propellant or a jet engine using fuel, generates thrust.

Delta-v, symbolized as $and pronounced deltah-vee, as used in spacecraft flight dynamics, is a measure of the impulse per unit of spacecraft mass that is needed to perform a maneuver such as launching from or landing on a planet or moon, or an in-space orbital maneuver. It is a scalar that has the units of speed. As used in this context, it is not the same as the physical change in velocity of said spacecraft.$

A propellant is a mass that is expelled or expanded in such a way as to create a thrust or another motive force in accordance with Newton's third law of motion, and "propel" a vehicle, projectile, or fluid payload. In vehicles, the engine that expels the propellant is called a reaction engine. Although technically a propellant is the reaction mass used to create thrust, the term "propellant" is often used to describe a substance which contains both the reaction mass and the fuel that holds the energy used to accelerate the reaction mass. For example, the term "propellant" is often used in chemical rocket design to describe a combined fuel/propellant, although the propellants should not be confused with the fuel that is used by an engine to produce the energy that expels the propellant. Even though the byproducts of substances used as fuel are also often used as a reaction mass to create the thrust, such as with a chemical rocket engine, propellant and fuel are two distinct concepts.

Laser propulsion is a form of beam-powered propulsion where the energy source is a remote laser system and separate from the reaction mass. This form of propulsion differs from a conventional chemical rocket where both energy and reaction mass come from the solid or liquid propellants carried on board the vehicle.

Jet propulsion is the propulsion of an object in one direction, produced by ejecting a jet of fluid in the opposite direction. By Newton's third law, the moving body is propelled in the opposite direction to the jet. Reaction engines operating on the principle of jet propulsion include the jet engine used for aircraft propulsion, the pump-jet used for marine propulsion, and the rocket engine and plasma thruster used for spacecraft propulsion. Underwater jet propulsion is also used by several marine animals, including cephalopods and salps, with the flying squid even displaying the only known instance of jet-powered aerial flight in the animal kingdom.

A plasma propulsion engine is a type of electric propulsion that generates thrust from a quasi-neutral plasma. This is in contrast with ion thruster engines, which generate thrust through extracting an ion current from the plasma source, which is then accelerated to high velocities using grids/anodes. These exist in many forms. However, in the scientific literature, the term "plasma thruster" sometimes encompasses thrusters usually designated as "ion engines".

Spacecraft electric propulsion is a type of spacecraft propulsion technique that uses electrostatic or electromagnetic fields to accelerate mass to high speed and thus generating thrust to modify the velocity of a spacecraft in orbit. The propulsion system is controlled by power electronics.

A reaction engine is an engine or motor that produces thrust by expelling reaction mass, in accordance with Newton's third law of motion. This law of motion is commonly paraphrased as: "For every action force there is an equal, but opposite, reaction force."

A cold gas thruster is a type of rocket engine which uses the expansion of a pressurized gas to generate thrust. As opposed to traditional rocket engines, a cold gas thruster does not house any combustion and therefore has lower thrust and efficiency compared to conventional monopropellant and bipropellant rocket engines. Cold gas thrusters have been referred to as the "simplest manifestation of a rocket engine" because their design consists only of a fuel tank, a regulating valve, a propelling nozzle, and the little required plumbing. They are the cheapest, simplest, and most reliable propulsion systems available for orbital maintenance, maneuvering and attitude control.

Atmosphere-breathing electric propulsion, or air-breathing electric propulsion, shortly ABEP, is a propulsion technology for spacecraft, which could allow thrust generation in low orbits without the need of on-board propellant, by using residual gases in the atmosphere as propellant. Atmosphere-breathing electric propulsion could make a new class of long-lived, low-orbiting missions feasible.

Microwave electrothermal thruster, also known as MET, is a propulsion device that converts microwave energy into thermal energy. These thrusters are predominantly used in spacecraft propulsion, more specifically to adjust the spacecraft’s position and orbit. A MET sustains and ignites a plasma in a propellant gas. This creates a heated propellant gas which in turn changes into thrust due to the expansion of the gas going through the nozzle. A MET’s heating feature is like one of an arc-jet ; however, due to the free-floating plasma, there are no problems with the erosion of metal electrodes, and therefore the MET is more efficient.

References

1 2 3 4 5 "NASA Glenn Research Center PPT". National Aeronautics & Space Administration (NASA). Retrieved 5 July 2013.
1 2 3 P. Shaw (30 September 2011). "Pulsed Plasma Thrusters for Small Satellites". Doctoral Thesis - University of Surrey. Retrieved 2020-06-27.
1 2 "Plasma thrusters could double the lifetime of mini satellites". The Engineer (UK magazine) . Retrieved 2020-06-27.
↑ Shchepetilov, V. A. (December 2018). "Development of Electrojet Engines at the Kurchatov Institute of Atomic Energy". Physics of Atomic Nuclei. 81 (7): 988–999. Bibcode:2018PAN....81..988S. doi:10.1134/S1063778818070104 . Retrieved 28 February 2024.
↑ NASA is investing in a rocket that could get humans to Mars and back in 2 months — and travel at 100,000 mph By Ellyn Lapointe. Jul 11, 2024. Retrieved July 11, 2024.

External links

"Design of a High-energy, Two-stage Pulsed Plasma Thruster". Princeton University. Archived from the original on 2017-08-08. Retrieved 2020-06-27.
"EO1 Pulsed Plasma Thruster" (PDF). Goddard Space Flight Center. Archived from the original (PDF) on 2011-07-16. Retrieved 2020-06-27.
Ephraim Chen. "Gas-Fed Pulsed Plasma Thrusters: From Sparks to Laser Initiation" (PDF). Princeton University. Archived (PDF) from the original on 2022-10-09. Retrieved 2020-06-27.
Michael Bretti. "AIS-gPPT3-1C Single-Channel Gridded Pulsed Plasma Thruster". Applied Ion Systems. Retrieved 2020-06-27.

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[NASAPPT-1] 1 2 3 4 5 "NASA Glenn Research Center PPT". National Aeronautics & Space Administration (NASA). Retrieved 5 July 2013.

[SURREY_UNI-2] 1 2 3 P. Shaw (30 September 2011). "Pulsed Plasma Thrusters for Small Satellites". Doctoral Thesis - University of Surrey. Retrieved 2020-06-27.

[THE_ENGINEER-3] 1 2 "Plasma thrusters could double the lifetime of mini satellites". The Engineer (UK magazine) . Retrieved 2020-06-27.

[Shchepetilov-4] Shchepetilov, V. A. (December 2018). "Development of Electrojet Engines at the Kurchatov Institute of Atomic Energy". Physics of Atomic Nuclei. 81 (7): 988–999. Bibcode:2018PAN....81..988S. doi:10.1134/S1063778818070104 . Retrieved 28 February 2024.

[5] NASA is investing in a rocket that could get humans to Mars and back in 2 months — and travel at 100,000 mph By Ellyn Lapointe. Jul 11, 2024. Retrieved July 11, 2024.

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