STK Astrogator Earth Flyby

TLDR: How do you set up an EEV maneuver, specifically, how do you go from an earth orbit to a heliocentric orbit with a periapsis within Venus’s orbit in astrogator? I know the theory but astrogator doesn’t want to cooperate.

I’m trying to model the upcoming Dragonfly mission in STK which uses an Earth gravity assist to change direction to Venus and beyond. Basically I’m trying to model a burn from an Earth parking orbit to a heliocentric orbit with a perigee inside Venus’s orbit. After a small TCM at the heliocentric perigee it should result in an Earth gravity assist after a total of about one year of travel time. You can google the mission which will show the rough outline for the orbit—I’m only interested in its approach to Venus, nothing beyond that.

In astrogator, After launch there is a target sequence, within which I propagate the LEO, setting the duration stopping condition to 0 and making it an independent variable for the targeter. I then have an impulsive maneuver I set it to thrust vector and the coordinates to “VNCSun”. I made the VNC components independent variables and gave “V” an initial condition of -6000 m/s. I then set the results for this maneuver to: “eccentricity” wrt Earth 1.2 +/- .2and “perigee” wrt the Sun > 80000000 +/- 1e6 km. Not the best, but I was trying to force a result close to that in the mission profile. I then set up the differential corrector.

My thinking is that when you’re in an Earth orbit, if you conduct a burn in the direction opposite to the Earth’s heliocentric velocity it will decrease your perigee wrt the Sun, hence the negative velocity thrust component. However, the satellite just follows its launch trajectory back to Earth. When I give it a positive velocity, it become hyperbolic wrt the Sun. In between and it doesn’t escape earth’s SOI.

If someone could explain how to set up an EEV maneuver in astrogator I would be eternally grateful. Additionally, I went through a new horizon’s tutorial that used astrogator and apparently when you use bplane targeting you need a reasonable accurate initial condition. They used trajopt, but I was wondering if there was an alternative to the DOS program. I could probably just do it by hand, but what am I looking for, the angle between earth and Venus at the time of the flyby? From that I guess I could setup the bplane target. I went through the tutorial videos on AGI’s website and while they are pretty good, they don’t explain how to set up interplanetary trajectories or bplane gravity assists. (E.g in the mars probe tutorial they go from interstellar space to mars, not earth to mars; in the fast transfer video they go from one earth orbit to another; in the bplane video they use it to target a specific orbit around the moon, not perform a gravity assist).

• Is your question "How to set up an EEV maneuver in astrogator?" and you are looking for an exhaustive explanation, or do you have an idea how to do it and you just have a few specific questions on certain points? As it stands you post is quite long and asks readers to follow along an extensive train of thought. The more specifically you can identify exactly what question you want answered, the better. I can only find one question mark in your post, and that sentence is not really a question.
– uhoh
Oct 21, 2019 at 6:28
• I added a TLDR for a more specific question. Oct 21, 2019 at 12:18
• I've moved it to the top where I think it will be even more helpful. Thanks!
– uhoh
Oct 21, 2019 at 22:32

The New Horizons mission tutorial you refer to may come from our blog here. You'd have to do something similar. Astrogators Guild Blog

For multiple flyby missions, Marty Ozimek and Justin Atchinson at APL did a great job of describing a similar problem with multiple flybys here: APL Example

• About alternatives to trajopt, there are only a few. MAnE is one that I've used in the past, but it's not free. What you really need is a good first guess at the swingby conditions at the multiple planets. For new Horizons we had bplane parameters to target for at Jupiter. If you can get b-plane parameters and flyby times, you can do it like Ozimek and Atchinson did. If you don't have those, but only have the encounter times, you can still do it. Find me on twitter or linkedin, and contact me, and I can go into more details. Feb 5, 2020 at 7:23

The full answer is beyond the scope of StackExchange. A quick Astrogator tutorial for earth departure to Mars orbit can be found here. Ultimately you will need to use b-plane targeting if you want to get anywhere close to a gravity assist.

Even for just simple targetting you will need to know the rough departure asymptote (normally from a Pork Chop plot) before you can even begin to use the differential corrector in Astrogator. The differential corrector is not great for finding a solution from nothing, it needs a pretty good seed. Without a good initial condition you'll never hit another planet in 3D space. Also in Astrogator you normally need to do multiple propagation segments with multiple gravity fields (see tutorial).

I found two other resources which may be of help, although I've never used them: NASA Ames Trajectory Browser and Matlab Venus Porkchop Generator