Momentus Scaling Microwave Plasma Propulsion to 100 Ton Payloads

Momentus is a startup with $40 million in funding for development of microwave plasma propulsion.

The microwave plasma propulsion can have three times the efficiency of chemical rockets while having as much power as chemical rockets. Other hyper-efficient propulsion is far more limited in how much power they can use.

They will drastically reduce cost, increase safety, prolong commercial and governmental deep space missions, and provide booster stage propulsion for launch companies.

In 2022, cislunar and interplanetary rides with up to ten tons of payload will be possible with the Valoride™. This will enable the next phase of space exploration: water prospecting and delivery throughout the solar system, solar power stations, in-space manufacturing, and space tourism.

They have a plan a one hundred ton payload hyper-efficient orbit and beyond space tug. This would make large scale space operations a lot more efficient and capable.

15 thoughts on “Momentus Scaling Microwave Plasma Propulsion to 100 Ton Payloads”

  1. Let’s just delude ourselves, and assume you can get the same thrust level as a chemical rocket. Three times ISP means one third the fuel burn for the equivalent 6 month trip to Mars, not bad. Maybe that means no refueling is required in low earth orbit? Now consider Slough’s Plasma Magnet Drive, Zubrin’s Dipole-Drive, and Greason’s q-drive. All three related technologies use the Solar Wind to get to Mars in a week, instead of 6 months! And no additional propellant mass is required! That is a big deal! Because no one wants to spend half a year getting to Mars. But Megawatts of solar cells would be required to power the plasma sail concepts, so high specific efficiency “watts per kg” solar cells are needed. You would think Tesla would be doing R&D on these types of solar cells anyway!

  2. It’s actually a very low cost and very bright idea. You pressurize the tank before lift off, myth busters style, with an off-board electric heater and it performs well as a very cheap and safe booster; which it is: a booster-stage getting you off the pad.

  3. ISP of Microwaved H2O is 900 s-ish compared to usually <300s for hydrazine (most used chem prop. on sats/tugs because of its density, stability and simple plumbing due to hypergolic ignition). So yes, one could argue the use of three times less chem prop. I.e. 1000 m/s delta-V on a payload of ~10mton gives prop use of 1.201 ton compared to 3.385 ton. Now, the thrust levels will be quite low in initial apps because you need to generate the Kw-e’s. Besides that, the patent has expired 10 years ago so everyone who desires to do so can duplicate this tech.

  4. “The microwave plasma propulsion can have three times the efficiency of chemical rockets while having as much power as chemical rockets.”

    Uh, what do you mean by power here?

    Let’s take the humble AJ-10, aka the OMS engine. It has a specific impulse of 319 s (i.e., an exit velocity of 3128 m/s) and a thrust of 44 kN.

    Thrust = mass flow * exit velocity, so mass flow = 14.1 kg/s


    power = 0.5 * mass flow * exit velocity² = 69.0 MW

    Meanwhile, the super-duper science fiction version of the Momentus line says it’ll need solar power > 10 kW. So, unless that “>” sign is a massive understatement, the chemical rocket is almost 7000x more powerful.

    Let’s take a look at the “Vigoride Extended” specs, which advertise a wet mass of 450 kg and v = 5 km/s with 100 kg of payload. Let’s figure a structural mass fraction of 15% (because it’s electric propulsion, and that would be really freakin’ good with enough solar panels to do anything), so dry mass should be 68 kg. Here comes the rocket equation!

    5000 m/s = ve * ln((450+100)/(68+100))

    So ve = 4216 m/s = 430 s (in the more common formulation, where you’re doing impulse per weight instead of impulse per mass). That’s not quite as good as an RL10.

    Notice also that they go to great lengths to hide the actual thrust of this system by expressing everything in terms of total impulse. My guess is it might get a bit higher power-specific thrust than your garden-variety Hall thruster, but that’s not saying very much.

  5. There is another tradeoff to be made there.

    The efficiency of your microwave drive (or other electrical drive mechanisms) depends on the chemical makeup of your exhaust stream.

    Hydrogen is the best. You get the most exhaust velocity (= ISP) from the lightest atoms at a given temperature. Once you’ve combined the hydrogen with oxygen to get a chemical heat source, you’re now dealing with much heavier molecules that take a lot more heat to reach the same exhaust velocity.

    Indeed Hydrogen/Oxygen rockets usually run with a significant surplus of hydrogen. The lower temperature of your rich mixture is worth it because the free H gets such a high exhaust velocity.

    The momentus approach uses water, despite the heavier molecules, because the enormous ease of having a water tank instead of liquid Hydrogen and Oxygen means the overall system (might) work out lighter and cheaper.

  6. JP Aerospace have a hybrid electric rocket concept that does essentially that for their “Airship to Orbit” system. Not sure it’s really that effective for anything over ~700 seconds Isp though. The electric energy input becomes >75% by that point. When it’s at the 1,000 second level, most of the jet-power is from electrical and chemical adds very little to the mix.

  7. It’s a trade-off of thrust vs performance. A chemical rocket has a fixed performance (Isp/exhaust velocity). Electrical propulsion can feed less energy into a larger mass flow, and get more thrust at the expense of exhaust velocity. If you are in a hurry, that trade-off may make sense.

    Once you have gone electric, the choice comes down to electric power vs exhaust power efficiency, and power and thrust ranges you want to achieve. Ion and magnetoplasma (VASIMR) start demanding pretty big solar arrays or a nuclear source when pushing big payloads.

  8. In theory you could do substantially better by using a reasonably stable chemical propellant, and using the microwaves to increase the exhaust velocity. Sort of a microwave “afterburner”.

  9. Steam rocket yes, battery? No, Microwave or laser energy transfer sure, but the only way a battery will work is its a nuclear one

  10. How much thrust this thing is able to generate? I could not find the answer to this question on Momentus pages or anywhere else.

  11. Don’t need anything fancy. A thruster using an electric heated electrode and hydrogen can give you similar results.

  12. Ha!
    Never heard of them before. What a joke.

    TL;DR, they want to launch to orbit with electrically heated steam rocket using a battery.

  13. So… it uses an exhaust velocity that is higher than chemical rockets, but lower than most ion drives.

    Hence, more propellant efficient than chemical rockets because higher velocity means more impulse from the same mass of propellant.

    And more energy efficient than ion rockets, because lower velocity means less energy needed for a given impulse.

    Of course this means it is LESS propellant efficient than other ion drives, and LESS energy efficient than chemical rockets. But they forgot to include that information.

    It’s a compromise. Now it might be a useful compromise, but the writeup doesn’t admit it’s just a halfway point between the two extremes.

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