Redwire brings together the best tools and modeling techniques to ensure sustainable and safe spacecraft operations in VLEO
Very low Earth orbit (VLEO), typically 300 kilometers or less above Earth, is becoming a crucial domain for the future of defense and intelligence operations and commercial Earth observation missions. Closer proximity to the ground lets users capture higher-resolution images of the Earth’s surface, and the shorter distance also provides clearer, more accurate data and lower latency than what is possible in Low Earth Orbit (LEO).
In this Q&A, Andrew Hermetet, principal engineer of Redwire’s Modeling and Simulation group, explains the challenges of atmospheric modeling in VLEO and how Redwire is developing a better predictive model to help counter the effects of changing weather in this much lower orbit that is key for customers to achieve their mission goals. Redwire designs spacecraft for VLEO, where atmospheric conditions vary significantly, so its digital engineering models aim to ensure satellite stability and longevity, with applications in both the defense and commercial sectors.
What is the challenge with operating in VLEO?
As spacecraft fly in this lower orbit, trace atmosphere becomes a larger and larger issue that you have to constantly combat; otherwise, your orbit decays very quickly and your satellite burns up in the upper atmosphere. If you don’t act proactively, your mission will be over in tens of orbits.
The atmosphere is incredibly variable at this altitude. In VLEO, temperatures can vary by 400 or 500 Kelvin (equivalent of 260 to 440 degrees Fahrenheit). Also, atmospheric density and pressure can move by two or three orders of magnitude. That’s quite a range that we have to operate in, so being able to shape and orient the spacecraft such that it can handle these challenges is the problem we’re addressing with simulation.
Also, wind speeds in VLEO can exceed 1,000 meters per second. Not accounting for wind could result in very inaccurate positioning. For instance, MIT Lincoln Laboratory’s Agile MicroSatellite Demonstrator team found that their spacecraft’s predicted position after one orbit could be off by more than 100 kilometers. If you’re planning on taking a picture of something with an intelligence, surveillance, and reconnaissance satellite, you’d better be where you think you’re going to be when you slew to snap the picture. Being able to model this and cope with these dynamics is important for the end customers of these spacecraft.
How is Redwire confident that its approach will build spacecraft that successfully operate in this challenging orbit?
We use a number of state-of-the-art simulation tools to virtually recreate the VLEO environment, including our mission analysis tool, ACORN 2.0. As I previously said, this is a hard thing to do on the ground. To validate the model, that is to know that it is giving us the right answer, we’ve recreated previous VLEO missions in our environment and predicted their trajectories. This practice is called blind correlation and it is a cornerstone to validating simulation models. We’ve had tremendous success in our approach so far, correlating against notable missions that have already flown in VLEO, like GOCE and GRIDSPHERE.
Our commercial and dual-use customers have been pleased with our correlation efforts and are all confidently progressing in their program development.
What options do you have for managing spacecraft orientation in VLEO given the extremes of wind and drag?
Superficially, you have two options. Option one is to keep the vehicle’s nose pointed where you think you’ll get the maximum sunlight on the solar panels. Due to crosswinds, you’ll often experience more drag doing that; you will then have to use more fuel to stay in orbit. Option two is to point the nose into the apparent wind to minimize that drag. That way you don’t get as blown off course and don’t have to use as much fuel. But now your thruster is not pointed in the right direction and you’re not getting as much sunlight, which powers the vehicle.
Then the question becomes, “Is there some optimal way, looking at all the trade-offs, to point the spacecraft where we can deal with a little bit of extra drag and a little bit of blowing off course but also keep our thruster mostly aligned and our solar panels almost completely illuminated?”
To answer that question, we must marry a lot of different applications from multiple disciplines: weather models, aerodynamics codes, and numerical optimization software. We then have to get these disparate applications to talk to each other and our mission analysis software ACORN. To date, we’ve done this very effectively; at Redwire, we call this digital engineering approach DEMSI [Digitally Engineered Mission System Integration].
How does Redwire define success in VLEO?
A lot of the missions that we’re flying have very ambitious goals for how long our customers want to remain in orbit at these low altitudes, ranging from a year to five years. If we’re able to come appreciably close to those goals that’s a programmatic measure of success. There’s a common joke in simulation that all models are wrong, but some models are useful. So usefully predicting when and where a VLEO spacecraft will be is my definition of success. And right now, we have the tools and techniques to do just that.
Andrew Hermetet is a career expert in computational fluid dynamics. Prior to joining Redwire, Hermetet worked at several laboratories and startups where he tackled diverse design problems ranging from the aeroelastic behavior of hypersonic vehicles to optimizing the aerodynamic and thermal performance of Formula 1 race cars. He now serves Redwire as a simulation subject matter expert and technical lead on several VLEO programs.
Read more about the challenges of building resilient systems in VLEO in a deep dive with Redwire’s Spence Wise and Juan Pablo Ramos here.
