Mission to Mars:  The Lithium Lorentz Force Accelerator for High Power Space Propulsion

(Research supported by NASA-JPL's Advanced Propulsion Group and the Princeton Plasma Physics Laboratory's Program in Plasma Science and Technology)

Let's go to Mars!

NASA has identified Mars as the next step in human exploration of the solar system.  Such an ambitious mission provides a major propulsion challenge; carrying people, life support, and equipment almost half the distance between the earth and the sun, and using as little propellant as possible to do it.  The rocket equation tells us that in order for a significant fraction of our initial spacecraft mass to make it to the final destination requires high exhaust velocities.  For our Mars trip we desire exhaust velocities in the 40-50 km/s range whereas the best of today's chemical thrusters give exhaust velocities an order of magnitude lower (4-5 km/s).

To put this in perspective :
(thanks to Lenny Cassady for the following analysis)

The first human crew to travel to Mars will need to carry with them on the order of 60 mT of payload ( based on the numbers NASA uses for its mission studiesassuming that a cargo vessel with many of the supplies the crew will need on the planet was launched earlier and is waiting for them on Mars).  To get the 60 mT ship from Earth orbit to Mars orbit using traditional chemical rockets would require an additional 175,500 kg of fuel payload.  That is enough fuel to fill 6.5 shuttle cargo bays!  And that's only one way!

Now, if we were to trade the chemical rockets for our electric plasma thruster, the same mission would require only about 2/3rds of a shuttle cargo bay of fuel (18,300)!
A huge difference!

That's the reason for all the excitement about high power electric propulsion!
 

Project Description

Current research in the area of high power space propulsion has identified the Lithium Lorentz Force Accelerator (LiLFA) as one of the most promising candidates for planetary exploration and heavy payload orbit raising missions.  Although initial experimental data on the LiLFA obtained at the Moscow Aviation Institute is promising , little is understood concerning the basic physics at play in such devices. Therefore, no systematic optimization of design or operating conditions has been achieved. While the extensive database on gas-fed magnetoplasmadynamic thrusters (MPDTs), the precursor to the LiLFA, offers a starting point, fundamental differences in cathode design, propellant type and injection, and current attachment in the LiLFA require new theoretical models to be developed and tested. As an initial step in this direction, The Jet Propulsion Laboratory (JPL) has proposed and begun work on the necessary testing facilities, the lithium feeding system, and the design of a 0.5 MWe thruster model. Current research at EPPDyL supports these objectives by focusing on the following areas of concern: Fortunately, many of the fundamental processes in MPDTs and LiLFAs have been shown to scale largely independently of power level, depending instead on the ratio of the operational current to a critical current which is dependent upon propellant mass flow rate and thruster geometry.  Therefore, many of our goals can be achieved while operating at low powers (30-100 kWe) using recently implemented facilities at EPPDyL.

Current Research

We currently have two LiLFA experimental  models in our laboratory at Princeton.  The Open Heat Pipe LiLFA  (OHP-LiLFA) designed and built by Thermacore Inc. and EPPDyL and, the workhorse of the present phase of our research, the MAI-LiLFA, designed and built at the Moscow Aviation Institute.

Research efforts to date have been focused on lithium safety and handling issues, the development of a mechanical liquid lithium feeding system, and integration and demonstration of the 30 kWe MAI-LiLFAin support the research objectives outline above.
 

Lithium Safety and Handling

Although less violently reactive than sodium or potassium, lithium, the lightest of the alkalis, presents an interesting experimental challenge.  The soft silvery metal tarnishes quickly in air, reacting with water, oxygen, carbon dioxide, and even nitrogen.  As a liquid (>200 C) lithium will react with most materials and presents an explosion and fire hazard.  The products formed during lithium reactions are strong bases and generally extremely corrosive and/or toxic.  The lithium-water reaction, of particular concern to us, results in the production of lithium hydroxide and hydrogen gas according to the reaction:

One of the major concerns early in our LiLFA research program was the lack of facilities to load and clean the lithium feed systems of the open heat pipe and MAI thrusters.   The glove box facility was obtained and modified to allow for safe loading and cleaning of lithium metal.  For pictures and a description of the glove box and lithium handling and cleaning capabilities currently developed  to support our experimental program, click here.
 

Mechanical Liquid Lithium Feed System

One of the major challenges to experimental lithium thruster programs to date, has been the measurable and controllable feed of lithium to the thruster.    Earlier researchers at Los Alamos and Electro-Optical Systems, in the 60s and 70s, experimented with argon pressure driven bellows and sonic orifice type systems, respectively, with limited success.  More recently, the Moscow Aviation Institute has employed an oil pressure driven piston system in which the lithium flow rate is proportional to the flow rate of the oil over the piston, which is more readily monitored.  Fluctuations in oil density due to unstable temperatures limited the accuracy of this system.

Our solution to this problem has been to design a mechanically driven liquid lithium feeding system.
A mechanically driven piston provides liquid lithium, at flow rates proportional to the piston velocity, to the vaporizer cathode of the MAI-LiLFA

A schematic and detailed description of the system (along with pictures) can be found here.
 

First Firing of the MAI-LiLFA at EPPDyL

On March 13, 2001  the Moscow Aviation Institute 30 kWe LiLFA was successfully fired in the Steady State Low Power Facility at Princeton's Electric Propulsion and Plasma Dynamics Laboratory (EPPDyL).   A 500 A arc discharge (10kWe) was sustained for nearly three minutes at a lithium vapor flow rate of 20 mg/s.  A minor short of one of the feed system heaters ended the test and is currently being repaired.  This marks the first lithium thruster firing in our lab since the successful demonstration of the OHP-LiLFA in August of 1998 and makes us the only facility outside of Russia currently conducting lithium thruster research.
 
 

Check out the firing pictures and video (coming soon)!
 

Pictures from our firing on 12-1-08 can be found here
 

The Lithium LFA Research Team


Currently At Princeton University

Dan Lev               Graduate Student, EPPDyL

Justin Little               Graduate Student, EPPDyL
David Stein           Undergraduate Student, EPPDyL
Prof. Edgar Choueiri  Chief Scientist, EPPDyL
 


Former Researchers At Princeton University

Kamesh Sankaran       Graduate Student, EPPDyL
Andrea Kodys          Graduate Student, EPPDyL
Leonard Cassady       Graduate Student, EPPDyL
                        National Defense Science and Engineering Fellow,
                        Francis Upton Fellow

Former Researcher and Visiting from Ecole Polytechnique

Gregory Emsellem      Graduate Student, Laboratoire de Physique des Milieux Ionisés
 

Sponsors

NASA-JPL's Advanced Propulsion Group
Princeton Plasma Physics Laboratory's Program in Plasma Science and Technology
 

LiLFA Publications

Contact

dlev@princeton.edu
adkodys@princeton.edu