NASA's NASA CubeSat to Demonstrate Water-Fueled Moves in Space says:

A NASA CubeSat will launch into low-Earth orbit to demonstrate a new type of propulsion system. Carrying a pint of liquid water as fuel, the system will split the water into hydrogen and oxygen in space and burn them in a tiny rocket engine for thrust.

and later

PTD-1’s propulsion system will produce gas propellants – a mix of hydrogen and oxygen – from water, only when activated in orbit. The system applies an electric current through water to chemically separate water molecules into hydrogen and oxygen gases, in a process called electrolysis. The CubeSat’s solar arrays harness energy from the Sun to supply the electric power needed to operate the miniature electrolysis system.

These gases are more energetic fuels than water; burning hydrogen and oxygen gas in a rocket nozzle generates more thrust than using "unsplit" liquid water as propellant. This strikes a better balance between performance and safety for spacecraft propulsion, meaning CubeSats will get more bang for the buck.

"What’s new is that this system uses water as the fuel in an energetic way, with an inherently safe system," said Mayer. "This mission will show that we can use water electrolysis in a rocket engine in space – that’s pretty cool."

Wikipedia's Pathfinder Technology Demonstrator mentions a scheduled December 2020 launch and says:

HYDROS is a hybrid chemical/electrical technology to provide propulsion using water. It uses an electrolysis cell to split water propellant into gaseous hydrogen and oxygen that are stored under pressure in separate tanks. The system then burns the hydrogen and oxygen mix in a simple thruster nozzle to provide up to 1 Newton and a specific impulse of 258 seconds. This propulsion system is being developed by Tethers Unlimited, Inc.

Gunter's Space Page PTD 1 (Pathfinder Technology Demonstrator) lists it as `2021-006BX.

However it does not mention an engine that burns hydrogen and oxygen and derives thrust from the exhaust exiting through a nozzle, and instead says it uses electrospray thrusters for propulsion:

The primary goal of the first Pathfinder Technology Demonstrator spacecraft is to flight qualify and characterize a novel micro-electrospray thruster provided by Busek Space Propulsion and Systems.

Each PTD mission consists of a 6-unit (6U) CubeSat weighing approximately 11 kilograms and measuring 30 cm × 25 cm × 10 cm. Each PTD spacecraft will also be equipped with deployable solar arrays that provide an average of 44 watts of power while in orbit.


  1. Has Demonstrator-1 2021-006BX demonstrated any split-water derived propulsion?
  2. Has it demonstrated, or at least is the plan to demonstrate propulsion using combustion of hydrogen and oxygen burned in an engine with exhaust exiting through a nozzle i.e. a conventional "rocket engine"?
  • $\begingroup$ I really hope that "a mix of hydrogen and oxygen" is just bad writing on their part. What is the longterm storage safety of compressed, stoichiometric H2+O2 gas mixture? $\endgroup$ Jul 6, 2021 at 11:39
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    $\begingroup$ @PcMan luckily the following sentence elaborates: "... chemically separate water molecules into hydrogen and oxygen gases..." $\endgroup$
    – uhoh
    Jul 6, 2021 at 12:02
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    $\begingroup$ We can infer that some form of propulsion has been demostrated $\endgroup$ Jul 6, 2021 at 13:12
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    $\begingroup$ @BrendanLuke15 Definite 2 * 'reboost' actions there. But I calc less than 0.6m/s total delta-v.. At the stated 258s ISP, that's less than 1.2g of fuel burned! $\endgroup$ Jul 6, 2021 at 15:52
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    $\begingroup$ @BrendanLuke15 since there are no answers immediately forthcoming, that certainly seems like it could be the basis for a partial answer rather than just a comment. I assume you mean the circularization steps mid April and beginning of may? Add an approximate delta-v they represent, a burn time estimate (thrust and mass are probably available) and I think it will be a well-received answer. We'll have to hold off for additional confirmation from some press release I suppose before accepting, but it sure looks believable to me! (just don't send us off-site for the plot) $\endgroup$
    – uhoh
    Jul 8, 2021 at 22:04

2 Answers 2


Partial answer trying to coax information from historical TLEs:

Orbital Progression

Note: disregard (relatively) large change from the first datum (epoch 24-Jan-2021 15:54:35 UTC) as looking at the time of launch/liftoff (24-Jan-2021 15:00 UTC, Transporter-1) and webcast shows it hadn't deployed from the second stage yet.

Looking at the orbital period we can identify 5 clear orbit raising maneuvers:

Orbital Period progression w/ maneuvers identified

For a total delta-V of ~0.66 m/s, significantly less than the ~100 m/s it is theoretically capable of given it is "carrying a pint [473 ml] of liquid water as fuel" (first link in question + Isp spec from Tethers Unlimited datasheet).

It also appears that its mission could be over (first link in question):

Its flight demonstration, lasting four to six months, will verify propulsion performance through programmed changes in spacecraft velocity and altitude executed by the water-fueled thrusters.

It hasn't performed an orbit raising maneuver since the end of April 2021 (~3 month since launch).

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    $\begingroup$ Were any of these events energetic enough to verify that actual combustion occurred, not merely spraying of pressurized propellant gas out the nozzle? $\endgroup$ Jul 11, 2021 at 8:48
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    $\begingroup$ Ok.. By their data sheet one optimal thrust delivers about 1.75Ns total impulse (from 0.4g of fuel). This would give the craft (11kg) about 0.16m/s. So the observed 0.66m/s seems about right in total. Actually, it is very likely that the large shift on that last boost is the result of 2 or more consecutive thrusts. $\endgroup$ Jul 11, 2021 at 10:46
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    $\begingroup$ btw I've added a bounty for "a more detailed answer to this question" but adding updating or adding more detail to your existing answer would certainly count! $\endgroup$
    – uhoh
    Nov 20, 2023 at 17:17

Funnily enough the document(s) to answer your question was uploaded 3 days after the question was asked. They were able to get the system to work, with the electrolyzer generating hydrogen and oxygen for both cold and hot burns.

The first thrust event resulted in a slight decrease in the nozzle temperature, indicating an unsuccessful ignition due to the cold gas flow dropping in temperature as it expands out the nozzle. The large temperature rises in the later three thrust events indicated successful combustion.

Altitude change due to thruster firings (it's just a bunch of green lines don't worry you ain't missing anything

electrolyzer performance graph

Summary of on orbit thruster performance. Anyway, where does this text show up? In the HTML? Is it just for non-seeing users?

Another humorous side, the spacecraft didn't come with accelerometers that could measure the impulse of the thruster, so they had to the same thing Brendan did to derive the performance of the thrusters from the orbital position. Only the last burn identified by him was identified in the paper as a thruster burn. They were doing electrolysis checkouts in the months leading up to the burns however, so maybe venting that gas could've generated the other identified raises.

  • $\begingroup$ That's great, thanks! When you say "3 days after the question was asked" you mean July 2021, or when I added the bounty last week? $\endgroup$
    – uhoh
    Nov 22, 2023 at 15:37
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    $\begingroup$ @uhoh When you asked the question; the document was uploaded on the July 09, 2021, question asked on July 6th, 2021. $\endgroup$ Nov 23, 2023 at 3:16
  • $\begingroup$ This reminds me of Feynman's ARW 357. $\endgroup$
    – uhoh
    Nov 23, 2023 at 3:37

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