More EGT Musings: ISRU Propellants

One of the ideas I had been thinking of blogging about was the thought of augmenting EGT asteroid deflection with in-situ derived propellants. The gravitation attraction force is usually the bottleneck in how fast you can do an asteroid deflection, but in some situations the propellant load might matter too.

What options are there for ISRU propellants in this case?

  • If the asteroid is a carbonaceous chondrite, water might be your best bet. There are some promising SEP technologies, like the ELF thrusters being developed by MSNW that can operate efficiently with water as the propellant. The challenge is that water is only present in some asteroids, might not be super easy to extract, and might require enough infrastructure to not be worth it on net.
  • The other big option is asteroid regolith. This could be charged up and run in a similar manner to an electrospray engine, or if it the dust is magnetically susceptible, it could be accelerated by something similar to a coil gun, mass driver, or linear accelerator. One of my employees used to work at a LASP lab running a dusty plasma accelerator. Basically they’d charge up small particles of dust, put them in a crazy electric field, and accelerate them to ~100km/s to smash into other dust particles to study micrometeorite formation processes.

What are some of the considerations for such an idea?

  • You are probably going to be very power limited. This both impacts what you can do as far as propellant extraction, and also limits the exhaust velocity/Isp that is optimal for an asteroidal ISRU-fed propulsion system. Just as ion engine systems operating in gravity wells typically tend to optimize to a lower Isp/higher thrust, the optimal deflection per unit time likely won’t come from the highest theoretical Isp.
  • On the other hand, the lower the exhaust velocity, the more material you have to handle to produce the “propellant”. So the optimal exhaust velocity is likely somewhere in the middle.
  • Also, if you’re extracting water, that’s likely more energy intensive than dust.

Without running the detailed numbers, my guess is you’d want a dust “electrospray” engine with an Isp in the 100-1000s range to optimize the balance between thrust per unit power and required extraction capabilities. For instance a 500s Isp is maybe 25% of the Isp of the Xenon Hall Effect Thrusters they’re thinking of using for ARM. That would imply getting somewhere between 16x the thrust per unit time as running the same amount of power through the HET.You’d need 16x the propellant mass flow rate, but if you’re gathering hundreds of tonnes of regolith, rock, and boulders, I would think that wouldn’t be that hard to get say ~125tonnes of regolith. One nice thing is that some of this material can be gathered while landing to gather the additional mass for the enhanced gravity tractor.

Food for thought?

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Jonathan Goff

Jonathan Goff

President/CEO at Altius Space Machines
Jonathan Goff is a space technologist, inventor, and serial space entrepreneur who created the Selenian Boondocks blog. Jon was a co-founder of Masten Space Systems, and is the founder and CEO of Altius Space Machines, a space robotics startup in Broomfield, CO. His family includes his wife, Tiffany, and five boys: Jarom (deceased), Jonathan, James, Peter, and Andrew. Jon has a BS in Manufacturing Engineering (1999) and an MS in Mechanical Engineering (2007) from Brigham Young University, and served an LDS proselytizing mission in Olongapo, Philippines from 2000-2002.
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2 Responses to More EGT Musings: ISRU Propellants

  1. John Schilling says:

    If you’re mass-rich and power-limited, you want an exhaust velocity that is comparable to the mission delta-V. For ARM, or planetary defense, or anything else we are going to realistically attempt in the near future, that means pushing asteroids around at a few hundred m/s, tops.

    So forget about electrosprays or high-end electrothermal systems. If you’ve got water, you want a high-pressure pump and a pipe with a converging nozzle at the end. Water plus dust, make sure the pump can handle mud – the oil industry will have you covered there. If it’s a dry rock, you want a mechanical pellet launcher approximating the performance of a rapid-fire BB gun.

    Actually, on reflection, I think the proper figure of merit may be for the kinetic energy applied to the “exhaust” to be approximately equal to the energy required to extract the propellant from the asteroid. If, e.g., you’ve got water available for the cost of melting ice, you’ll want an exhaust velocity of ~800 m/s, or an 80-second Isp. A bit sporty for a pump and nozzle, but there are industrial water jet cutting systems that can do that.

  2. Jonathan Goff Jonathan Goff says:

    John,
    Totally agree that’s the orthodox opinion I’ve heard on several occasions, but I think the problem is that while power is a real constraint, that material extraction and the associated hardware is also likely a big constraint. That is why I was suggesting something in the middle (ala Akin’s Law #8 of Spacecraft Engineering: In nature, the optimum is almost always in the middle somewhere. Distrust assertions that the optimum is at an extreme point.). Using the 1m/s number, you’d need to throw off around half the asteroid mass to get a 1m/s diversion. Using the 800m/s number, you’d still need to extract on the order of 1500 tonnes of water in order to divert a 100m asteroid by 1m/s. My guess is that once you factor in the scarcity of easily accessible “ISRU propellant mass”, the mass and power required to run it, and the mass and power of the ISRU-fed thruster system, that the optimal Isp is going to be high enough that you’re probably talking in the range I mentioned in the blog post (100-1000s Isp), most likely somewhere in the middle.

    But I haven’t run a deep enough analysis to know where in that range makes sense–note your second suggestion was right around the lower end of my guess range.

    ~Jon

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