The following is a brief description of the legacy and new LTTT suite optimization tools.  CHEBYTOP, VARITOP, and SEPTOP were not part of the LTTT development effort, but have been used extensively in the past for mission design and trades.  The LTTT development effort has yielding higher fidelity tools that are extensively documented with an increased ease of use.  The tools have all been thoroughly compared to one another for a large set of missions, but have yet to be formally validated by flight data.  The LTTT suite produces consistent results when used for their appropriate mission types.

CHEBYTOP (Not part of the LTTT)
CHEBYTOP is a pseudo-acronym for the Chebyshev Trajectory Optimization Program.  The tool was originally written by Forrester Johnson et al. at The Boeing Company in 1969, and was later updated by Boeing, the Jet Propulsion Laboratory (JPL) and analysts at the Glenn Research Center (GRC).  CHEBYTOP uses Chebychev polynomials to represent state variables.  These polynomials are then differentiated and integrated in closed form to solve a variable thrust trajectory.  This solution can then be used to approximate the performance of the constant thrust trajectory.  CHEBYTOP is not capable of analyzing multi-leg missions, i.e. round trip flights, intermediate flybys, multi-body trajectories, etc.  CHEBYTOP is also limited to interplanetary missions with only the Sun’s gravity field.  CHEBYTOP is considered a low-fidelity program by today’s standards, but has been highly valued for its capability to rapidly assess large trade spaces.  CHEBYTOP is the only tool available without any user restrictions.  CHEBYTOP is considered appropriate for collegiate level analysis and can be downloaded directly from this website. 
+ CLICK HERE TO DOWNLOAD CHEBYTOP

VARITOP, SEPTOP, and NEWSEP (Not part of the LTTT)
VARITOP is the Variational calculus Trajectory Optimization Program developed by Carl Sauer at JPL.  SEPTOP and NEWSEP are updates to the original VARITOP.  VARITOP is the most general of the tools, handling nuclear electric propulsion (NEP) as well as solar electric propulsion (SEP) and solar sail trajectories.  The three tools are all based on the same mathematical formulation sharing many common subroutines.  The calculus of variations is used in the formulation of state and co-state equations integrated numerically to solve a two-point boundary value program.  Optimization uses transversality conditions associated with the variational calculus, primer vector theory, and Pontryagin’s maximum principle.

SEPTOP is the Solar Electric Propulsion Trajectory Optimization Program that can simulate thruster throttling and staging.  SEPTOP can use thrust and propellant flow rate polynomials to represent specific thruster options.  SEPTOP can also use polynomials to represent solar array performance as the spacecraft changes its distance from the sun.  

NEWSEP is another variation of SEPTOP that can accept discreet values of a thruster’s throttle table rather than estimating the infinite throttle point performance using a polynomial.  NEWSEP was used to provide trajectory support for the Deep Space 1 mission.  The VARITOP and derived legacy tools are considered medium fidelity.  These tools are available for NASA and academic use only and are available directly through JPL.  These tools are no longer maintained and their availability may be limited.  Due to the availability of MALTO, SEPTOP is not recommended for new users.

MALTO
The Mission Analysis Low-Thrust Optimization (MALTO) tool was specifically developed as a more “user friendly” low-thrust optimization tool with relatively easy convergence especially for missions with multiple gravity assists.  MALTO uses many impulsive burns to simulate a continuous burn trajectory about a single gravitational source.  The mission setup, parametric trades, and post processing can be performed with a MATLAB based graphical user interface (GUI).    The thruster and power system modeling is comparable to the VARITOP programs.  Optimization in MALTO is calculated using and requires the SNOPT code developed independently by Dr. Philip Gill at the University of California San Diego.  MALTO is considered a medium fidelity tool and is freely available to NASA contractors and civil service and academia directly through the JPL website: https://download.jpl.nasa.gov/.  Commercial licenses can be obtained through the Caltech Office of Technology Transfer for a fee: http://www.ott.caltech.edu/; please submit requests through Karina Edmonds.

COPERNICUS
Copernicus was originally developed by the University of Texas at Austin under the technical direction from the Johnson Space Center.  Copernicus is a generalized trajectory design and optimization program that allows the user to model simple to complex missions using many objective functions, optimization variables and constraint options.  With Copernicus, one can model simple impulsive maneuvers about a point mass to multiple spacecraft with multiple finite and impulse maneuvers in complex gravitational fields.  The tool uses a graphical output for real time feedback during the optimization process.  Copernicus is an n-body tool and is considered high fidelity.  Copernicus has been transferred to an in-house development effort specifically for the Constellation program.  The updates to Copernicus are expected to be freely available to NASA and NASA contractors only, and requests can be submitted by contacting the ISPT project. The preferred method to obtain Copernicus is to send an e-mail request to Larry.Kos@nasa.gov.

OTIS
The Optimal Trajectories by Implicit Simulation (OTIS) program was developed by GRC and Boeing.  Earlier versions of OTIS have primarily been launch vehicle trajectory and analysis programs, but have since been updated for robust and accurate interplanetary mission analyses, including low-thrust trajectories.  The tool is named for its original implicit integration method, but also includes capabilities for explicit integration and analytic propagation.  Vehicle models can be very sophisticated and can be simulated through six degrees of freedom.  OTIS uses SLSQP and SNOPT to solve the nonlinear programming problem associated with the solution of the implicit integration method.  OTIS is a high fidelity optimization and simulation program.  OTIS is freely available to anybody in government, academia, and industry through the GRC technology transfer office at: http://technology.grc.nasa.gov/ , but is subject to export control regulations.

Mystic
Mystic was developed by Dr. Greg Whiffen and others at the JPL.  The tool uses a Static/Dynamic optimal control (SDC) method to perform nonlinear optimization.  Mystic is an n-body tool and can analyze interplanetary missions as well as planet-centered missions in complex gravity fields.  One of Mystic’s strengths is its ability to automatically find and use gravity assists.  Mystic also allows the user to plan for spacecraft operation and navigation activities.  The mission input and post processing can be performed using a MATLAB based GUI.  Mystic is currently used on the Dawn mission, and considered a high fidelity optimization and simulation program.  The use of Mystic on Dawn will later serve at validation of the LTTT suite.  Mystic is currently available to NASA only and requires considering tool specific expertise.  Requests for Mystic should be made directly through the JPL website: https://download.jpl.nasa.gov/.

SNAP
The Spacecraft N-body Analysis Program, SNAP, was developed at GRC with help from Mike Martini of Analex Corporation.  It is a high fidelity trajectory propagation program that can be used for planet-centered trajectories such as atmospheric grad, shadowing, solar radiation pressure, and high order gravity models.  SNAP uses a Runge-Kutta Fehlberg method of order 7-8 to propagate trajectories.  SNAP does not contain a trajectory optimizer but can use control laws.  SNAP is freely available to anybody in government, academia, and industry through the GRC technology transfer office at: http://technology.grc.nasa.gov/ , but is subject to export control regulations.

Additional Information:
As new information is obtained updates may be available below:

MALTO users wishing to model the BPT-4000, NEXT, and HiVHAC thrusters can download the .m files below and replace the existing files in the MALTO GUI “callback” directory.

Link 1 (engine_list.m)
Link 2 (soleng_panel.m)