The Science behind the technology

“Mars and back on one tank of fuel”

We’ve built a brand new kind of ion engine (that’s a kind of rocket), that has just broken the world record for specific impulse previously held by NASA’s HIPEP thruster.

This level of fuel efficiency is so good that one of these engines could send a probe to Mars and back on a single fuel rod. But it could also be great for keeping satellites in their proper position in orbit, or cheaply sending all the heavy equipment ahead of a manned mission somewhere.

How does it work?

The Neumann Drive uses solid fuel and electricity to produce thrust. It is a “wire-triggered pulsed cathodic arc system” and works kind of like an arc welder.

Arc welders have a cathode (the welding rod, charged negative) and an anode (the work piece, which has the anode-lead clamped to it). When the tip of the welding rod gets close enough to the work piece, an arc of electricity sparks between them. This happens because the electric field between the cathode and anode is strong enough to rip electrons off the air molecules between them and causes a giant “spark” to jump. The arc allows electrical current to flow through the cathode, which heats the material on the tip of the welding rod.

As the electrons jump off the end of the rod and enter the arc, they carry along with them some atoms from the rod in the form of plasma. In an Arc Welder, these iron and carbon atoms then get deposited on the work piece at high energy, creating a small melt pool and the desired weld. In the Neumann Drive, these atoms will be hurled off into space, producing thrust in the drive itself.

In our system the cathode is a cylindrical rod of conducting material that we choose to use as fuel (eg magnesium, vanadium etc). The cathode is charged negative with respect to the anode, and a charging voltage of between 80 and 250V is typically used. The anode is a hollow cylinder that is aligned coaxially with the cathode, but offset slightly forwards, as shown in the diagram below.

 

The cathode rod has a hole bored down the centre of it, which holds the insulated trigger pin. It needs to be insulated so that the arc can be triggered only at the right time. Our system uses an electrical flashover system to trigger the arc, which means that we pulse a high voltage signal from the trigger pin to the cathode, creating the conditions necessary for plasma formation to occur. There are other methods for triggering an arc (including lasers), but they are not as robust as this.

Once the arc has been triggered, plasma will be created in very small and very bright spots located on the cathode surface close to the trigger location; these plasma generation sites are called “cathode spots” for obvious reasons. The cathode spots erode material from the cathode, ionising and accelerating it into the vacuum chamber so that the plasma moves downstream at high velocity through the anode mouth.

This so-called “drifting plasma” is the exhaust of our rocket, and pushes the rest of the system forwards as it hurtles away. Higher exhaust velocities mean more efficient fuel use which is measured in specific impulse, often called “bounce per ounce.”

What fuels does it use?

We’ve tried out all kinds of materials as fuel. We’ll be posting a showcase about several of them over the coming few weeks, and I’ll add links to these posts as they appear. The list below contains a non-exhaustive list of the things that we’ve tried:

  • Molybdenum – our fastest fuel – best for sending people to Mars
  • Magnesium – our most efficient fuel – best for sending equipment on long missions
  • Aluminium – best for recycled space junk
  • Carbon – our most interesting fuel – reusing * ahem * waste-products from astronauts to get them to where they’re going
  • Titanium
  • Vanadium
  • Tin – a pretty poor fuel that we learned a lot from
  • Bismuth – our most useless fuel… just don’t bother

Footnotes and details

aligned coaxially: this means that the central axis of the fuel rod is aligned with the central axis of the cylinder

other trigger methods: Other triggering methods include mechanical methods (similar to the welding rod analogy above) or laser triggering (blast some material off the cathode surface, and use this to create a runaway discharge), but neither of these are as robust as an electrical system.

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