DARPA is on the verge of reaching a new low – an orbital one – as the Defense Department’s research arm moves its Very Low Earth Orbit (VLEO) Otter satellite program into the production phase. 

The Otter Program is moving along to phase 2 with the award of a $44 million contract to Florida-based Redwire, the company announced Wednesday. Redwire was awarded the prime contract for Otter last year to develop a VLEO satellite able to use air-breathing electric propulsion to stay in orbit without the need to rely on a fixed amount of fuel. 

VLEO, which spans altitudes between 90 and 450 kilometers (56-280 miles), is relatively uncrowded compared to low-Earth and geosynchronous orbit altitudes thanks largely to the fact that there’s a lot more atmospheric drag on objects at that height. That means a more frequent need to use propulsion systems to maintain orbit, giving satellite missions in VLEO limited lifespans. 

“Otter aims to enable extended satellite operations at VLEO altitudes by harvesting ambient low-density air, and ionizing and accelerating the resultant ion flow, thereby utilizing a virtually unlimited supply of propellant,” DARPA notes on the program’s web page.  

As The Register noted in an early 2024 story about DARPA seeking such VLEO systems prior to Redwire’s award, the idea is that air-breathing electric propulsion systems can suck up what little air exists at VLEO altitudes in order to turn it into plasma in an ionization chamber. The charged particles can then be shot out of the craft to act as a propellant without the need for onboard gas. Without the need for onboard fuel, such satellites can be built to be smaller and lighter, and they could theoretically last well beyond the lifespan of a device reliant on traditional propellants. 

Such systems are tricky to build, however, and if it’s successful, Redwire’s design would be the first air-breathing spacecraft to reach orbit. 

VLEO missions have advantages, because their closer proximity to ground targets can improve sensor perception and reduce latency while increasing their ability to revisit specific points on Earth, according to Redwire. VLEO operations are also ideal because spent satellites and debris at those lowest altitudes can lose orbit in mere hours or, higher up, a few days, rather than lingering in space for decades.

SabreSat prepares for flight – eventually

The design chosen for the Otter program is Redwire’s SabreSat, a surprisingly tiny (just 100 cm or 39 inches long) drone-like device that can hang out at altitudes of around 150 km (93 miles) for up to seven years, at least as designed. SabreSat is designed for a number of missions, including intelligence gathering, surveillance and reconnaissance, communications, navigation, and scientific research. 

That said, it’s not apparent that Redwire has actually built a functional SabreSat or VLEO air-breathing electric thruster yet. DARPA also mentions on the Otter program page that it’s difficult to recreate VLEO conditions in ground test facilities, so the eventual launch of a functional SabreSat will be the first opportunity the company has to field its design. 

Per DARPA, the Otter program will culminate in an “orbiting wind tunnel” demonstration lasting over a year that will test SabreSat’s electric propulsion technology. Ground tests of Redwire’s thruster and air inlet tech will also be conducted so that data can be compared to information gleaned from orbital operations. 

Along with developing SabreSat for DARPA’s Otter program, the company is also working with the European Space Agency to develop its own VLEO platform dubbed Phantom. Unlike the drone-style device being designed for DARPA, Phantom looks more like a traditional satellite, has a shorter lifespan (five years instead of seven), and lacks air-breathing capabilities. 

Redwire is also working with Deepsat to develop a constellation of VLEO Earth observation satellites. That contract was only awarded this past July, and it’s not clear what stage of development the project is in. 

It’s also not clear when DARPA will want to get Redwire’s Otter spacecraft in the air. According to solicitation documents from 2023, DARPA envisions that the program will run for four years. With Redwire receiving its initial contract in 2024, Otter might not sail the skies until 2027. That provisional timeline was defined two years ago, however, and air-breathing electric VLEO engines are sure to be tough to develop. 

Redwire didn’t respond to questions for this story. ®