Qualitative and Quantitative Assessment of
Optimal Trajectories by Implicit Simulation
(OTIS) and Program to Optimize Simulated
Trajectories (POST)
Author(s)
Nelson, Doug
Advisor(s)
Olds, John R.
Editor(s)
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Abstract
There is an ongoing debate among aerospace professionals about which trajectory
optimization program calculates the best results. The two programs that are most often
included in the debate are Optimal Trajectories by Implicit Simulation (OTIS) written by The
Boeing Corporation in conjunction with NASA-Glenn Research Center (GRC) and the Program
to Optimize Simulated Trajectories (POST) written by Lockheed Martin Astronautics and
NASA-Langley Research Center (LaRC).
The main difference between the two programs is the way that they represent the physics of
the problem. POST uses the more traditional direct shooting approach that calculates the state
variables as a function of time throughout the entire trajectory. This guarantees that the physics
of the problem are accurate at all times during the simulation. On the other hand, OTIS has the
capability to solve the trajectory problem in more than one way. In addition to explicitly
calculating the trajectory with direct shooting when operating in Mode 3, OTIS can also solve
the problem implicitly when run in Mode 4. The implicit method used in Mode 4 is known as
the collocation method and uses a series of polynomials to represent the state variables. During
the actual process of solving the problem, there is no guarantee that the problem satisfies all of
the physics of the problem until it is solved.
The impetus of this project was the fact that both programs must satisfy the same laws of
physics in the end and should therefore not predict widely differing results. Instead, the
expectation was that each program would arrive at very nearly the same optimum trajectory
with minor differences attributable to different optimizers and solution techniques. To prove
this theory, both programs were used to optimize the trajectories of two separate launch
vehicles:
1) ACRE-92 − A VTHL rocket-powered single stage to orbit vehicle
2) Hyperion − An HTHL rocket-based combined cycle vehicle
3) Hyperion − An HTHL rocket-based combined cycle vehicle
The results of the work support the initial theory. In both cases, the mass ratio (MR)
calculated using OTIS differed from that calculated using POST by less than one half of one
percent. The real differences in the two programs were more qualitative than quantitative as
each requires slightly different styles of inputs and presents its own unique challenges to the
user.
optimization program calculates the best results. The two programs that are most often
included in the debate are Optimal Trajectories by Implicit Simulation (OTIS) written by The
Boeing Corporation in conjunction with NASA-Glenn Research Center (GRC) and the Program
to Optimize Simulated Trajectories (POST) written by Lockheed Martin Astronautics and
NASA-Langley Research Center (LaRC).
The main difference between the two programs is the way that they represent the physics of
the problem. POST uses the more traditional direct shooting approach that calculates the state
variables as a function of time throughout the entire trajectory. This guarantees that the physics
of the problem are accurate at all times during the simulation. On the other hand, OTIS has the
capability to solve the trajectory problem in more than one way. In addition to explicitly
calculating the trajectory with direct shooting when operating in Mode 3, OTIS can also solve
the problem implicitly when run in Mode 4. The implicit method used in Mode 4 is known as
the collocation method and uses a series of polynomials to represent the state variables. During
the actual process of solving the problem, there is no guarantee that the problem satisfies all of
the physics of the problem until it is solved.
The impetus of this project was the fact that both programs must satisfy the same laws of
physics in the end and should therefore not predict widely differing results. Instead, the
expectation was that each program would arrive at very nearly the same optimum trajectory
with minor differences attributable to different optimizers and solution techniques. To prove
this theory, both programs were used to optimize the trajectories of two separate launch
vehicles:
1) ACRE-92 − A VTHL rocket-powered single stage to orbit vehicle
2) Hyperion − An HTHL rocket-based combined cycle vehicle
3) Hyperion − An HTHL rocket-based combined cycle vehicle
The results of the work support the initial theory. In both cases, the mass ratio (MR)
calculated using OTIS differed from that calculated using POST by less than one half of one
percent. The real differences in the two programs were more qualitative than quantitative as
each requires slightly different styles of inputs and presents its own unique challenges to the
user
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Date
2001-04-26
Extent
Resource Type
Text
Resource Subtype
Masters Project
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