The NASA Ames Feature Innovative Propulsion System Gets Ready to Help Study Moon Orbit for Artemis says:

CAPSTONE’s journey to the Moon will take about three months, starting with its launch to low-Earth orbit on a Rocket Lab Electron. Rocket Lab’s Photon spacecraft will take over next and conduct a series of orbit-raising maneuvers to prepare the CubeSat for its transfer path to the Moon. After separating from Photon, CAPSTONE will utilize an energy-efficient ballistic lunar transfer using its onboard propulsion system and enter into a near rectilinear halo orbit in the vicinity of and around the Moon. There, it will maintain the orbit to inform future spacecraft and demonstrate new technologies.

CAPSTONE’s propulsion system is designed and built by Stellar Exploration Inc. of San Luis Obispo, California. Initially funded by NASA’s Small Business Innovation Research program, the system is approximately 8-inches square by 4-inches deep. The system’s eight thrusters are fed hydrazine propellant from an unpressurized tank. CAPSTONE’s super small, high-performance thrusters integrate proven NASA technology with state-of-the-art industry fabrication techniques.

Question: What makes CAPSTONE-1's eight hydrazine thrusters so "super small, high-performance"? How to they work without any tank pressurization?

Do they have higher ISP than other hydrazine thrusters, or is it that they have similar performance to other high performance hydrazine thrusters but fit into a smaller volume? Without any pressurization in the tank, how does liquid hydrazine reliably get into to the thruster?

What is CAPSTONE? links to many resources, including this image:

The CAPSTONE mission is planned for launch in 2021. Rocket Lab’s Photon satellite bus will deliver CAPSTONE into a trajectory toward the Moon. Credits: NASA/Rocket Lab/Advanced Space/Tyvak Nano-Satellite Systems

The CAPSTONE mission is planned for launch in 2021. Rocket Lab’s Photon satellite bus will deliver CAPSTONE into a trajectory toward the Moon. Credits: NASA/Rocket Lab/Advanced Space/Tyvak Nano-Satellite Systems


1 Answer 1


An exhaustive 42 seconds of googling reveals this on Stellar Exploration's website:

As part of early R&D for a NASA funded deep space nano-satellite mission we have designed, built, and tested a miniaturized hypergolic bipropellant thruster. It provides 3 N of thrust with 285 sec specific impulse. The propellant is pressurized with gear pumps, allowing for precise propellant metering and low pressure tank design.

Presumably the tank is unpressurized relative to ambient pressure at the launch pad, i.e. pressurized to around 15 psia (absolute), enough to deliver propellant to the inlet of the gear pump, which then raises the pressure to something useful in a small thruster (most of Aerojet's small biprop thrusters, for example, want 100-400 psia).

Gear pumps are just a certain type of fluid pump. I assume these are electrically driven, since the mass flow rate required for 3N thrust isn't extremely high (on the order of a gram per second). This is quite clever; typically, very small rocket engines rely on tank pressurization because a turbopump is a big investment in both flown mass and development effort. A small electric motor does the job, and it should be practical to arrange for other electrical equipment (instruments, or even comms) to be idle while doing sustained thruster work. In exchange for the pump motor, you get to reduce the weight of your tankage.

The CAPSTONE article mentions hydrazine propellant while the Stellar site advertises hypergolic bipropellant. As JFL points out, the datasheet states they offer "monoprop alternatives (for less demanding applications)".

"Super small" is what you'd expect a 3N thruster to be. Aerojet's MR-111G thruster, for example, yields 4N from an 8 inch, 370 gram unit.

"High-performance" is marketing language and doesn't mean anything, except possibly "when you see the specs you probably won't think 'wow, that's terrible performance'" (unless you're David Hammen). 285 seconds specific impulse (for the biprop version) is comparable to Aerojet's smaller bipropellant thrusters, but their larger ones, with high-expansion ratio nozzles, get into the low 300s.

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    $\begingroup$ Regarding mono vs biprop, the datasheet (accessible through your link) mention "For missions with demanding propulsion requirements (>1 km/sec) [...] hypergolic bipropellant (hydrazine and NTO)" and "Monoprop alternatives (for less demanding applications)" $\endgroup$
    – JFL
    Commented Jul 6, 2021 at 7:37
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    $\begingroup$ Wow, that's terrible performance. $\endgroup$ Commented Jul 6, 2021 at 14:38
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    $\begingroup$ Shuttle APUs used gear pumps for their hydrazine fuel - driven by the APU's turbine through a gearbox. Startup flow provided by a pressurized tank. $\endgroup$ Commented Aug 9, 2021 at 13:38

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