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Redwire is the prime integrator on DARPA’s Otter program.
Forget Jeff Bezos’ and Elon Musk’s internet satellite rivalry in low Earth orbit. The next big thing in space is closer to home: very low Earth orbit.
One of the earliest tests of whether spacecraft can fly in this boundary region between space and the atmosphere, about 90-250 km (56-155 mi.) in altitude, will come in 2027 when DARPA plans to fly a vehicle in very low Earth orbit (VLEO) for at least a year.
- Spacecraft must withstand corrosive atomic oxygen
- Without propulsion, spacecraft in very low Earth orbit would deorbit rapidly
DARPA awarded the Redwire Corp. a contract in June as part of the agency’s Otter program to be the prime mission integrator for an air-breathing spacecraft that will fly in VLEO. The vehicle is to be based on Redwire’s Sabresat VLEO dart-shaped spacecraft, which is designed to slip through the outer layers of the atmosphere. An air-breathing propulsion system is slated to power it.
Orbiting closer to Earth has intelligence, surveillance and reconnaissance advantages.
“We’re going to be twice as close as any other satellite,” says Spence Wise, Redwire’s senior vice president of missions and platforms. “The imagery that we’re capable of generating is going to be meaningfully different than what can be collected at higher altitudes.”
Unlike typical spacecraft, which carry propellant onboard, the design of the vehicle’s electric propulsion system is expected to harvest ambient low-density air, ionize the gas and spit it out the back to create thrust. DARPA has set a threshold goal for the engine’s electric efficiency to be 30 millinewton (mN), or 0.0067 lb. thrust, per kilowatt, with an objective goal of 40 mN per kilowatt. The “inlet capture efficiency” is classified.
Spacecraft flying in VLEO need propulsion because the atmosphere is relatively thin but just thick enough to generate drag. Without propulsion, spacecraft in VLEO would deorbit rapidly.
DARPA announced in March and April that it has given separate electric propulsion development contracts to Electric Propulsion Laboratory (EPL) of Monument, Colorado, and Phase Four of Hawthorne, California. It is not clear if the agency will select one or both for demonstration.
Because conditions in VLEO are difficult to recreate within ground-test facilities, DARPA sees its Otter demonstration as a year-long “orbiting wind tunnel.” The agency notes the extreme conditions a propulsion system will face in VLEO, including the approximately 7.5-km/sec. (4.7 mi/sec.) flow of atmosphere into its inlet and atomic oxygen, which is corrosive.
“The biggest challenge in VLEO is drag,” says Sarah Popkin, DARPA Otter program manager. “DARPA’s Otter program is addressing that key challenge with a focus on enabling new, enduring propulsion technology. Beyond drag, there are still several known unknowns—and likely unknown unknowns—in VLEO that require unique technology solutions.”
On-orbit test data will be used to update models and simulations for designing VLEO satellites.
“There’s a lot that we don’t know about the VLEO environment because it sits in between two different regions that are well characterized and well understood: up to 90 km we have high-altitude balloons and aircraft that can characterize atmospheric density, and then we have plenty of spacecraft flying in [low Earth orbit],” says Jason Wallace, president of government programs for Phase Four.
DARPA is planning a workshop on Sept. 24 to examine ways to address other VLEO technology development gaps.
Phase Four is developing a radio-frequency thruster that is a derivative of its Maxwell electric propulsion system already used by multiple active satellites. Capella Space, an operator of satellite-based synthetic aperture radar, is one customer.
“Phase Four’s radio-frequency plasma thruster operates by using radio frequency to ionize propellant and heat the resulting plasma,” Wallace says. “The energetic plasma is directed away from the thruster using permanent magnetic fields, producing thrust without needing neutralizing cathodes.”
The atmosphere in VLEO includes atomic oxygen and nitrogen, whose molecules are close in weight, he adds.
“We can tune the performance of our thruster around the unique molecular weight,” Wallace notes. “We can actually get roughly the same performance on both.” Atomic-oxygen corrosion is avoided by constructing Phase Four’s electric propulsion system out of a ceramic material, he explains.
For its part, EPL likens its system to a “Mach 25 Free Molecular Flow Plasma Scramjet.” The company did not respond to requests for comment, but it stated in a March announcement that its air-breathing engines are “flow-through devices and thus incur no ram inlet drag penalty due to their over 90% open-area fraction to the orbital airflow.”
EPL says its system also uses materials that are “virtually immune from the long-term erosion effects of atomic oxygen.” The company notes that its flow-through, air-breathing engine allows for a spacecraft designed around a single- or dual-engine configuration with a lower drag coefficient, mass and cost.
The specific configuration of the Otter program spacecraft has not been disclosed, but the Sabresat upon which it is based is designed to carry a payload of about 200 kg (440 lb.). The spacecraft, which has aerodynamic solar panel fins to generate electricity, is modular and can be shortened or lengthened depending on its mission.
“Shorter buses are advantageous for getting on lower-cost launchers. Longer buses are advantageous for providing much more power and payload swap,” Wise points out.
Larger payloads could include Earth observation cameras, infrared sensors, communications or electronically scanned array radars, he says.
The SabreSat is intended for intelligence, communications, navigation and Earth science missions. Redwire says the spacecraft, which it also calls an “orbital drone,” can be produced in large volumes to populate large constellations. DARPA seeks propulsion for its VLEO Otter spacecraft that will allow it to “maneuver without regret,” the agency’s fiscal 2025 justification book states.
In May, Redwire revealed that it also is developing a VLEO spacecraft called the Phantom for the European Space Agency’s Skimsat demonstrator program.
Redwire is best known as a Tier 1 supplier of solar arrays, cameras, star trackers, radios and other subsystems for spacecraft manufacturers, as well as experimental payloads for the International Space Station. However, the company sees the opportunity provided by this new orbital regime as big enough to depart from its core business strategy and focus on becoming a prime integrator for VLEO spacecraft.
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