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tahanson43206

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Registered: 2018-04-27

Posts: 19,263

Electrostatic Thruster vs Magnetic Thruster

Recently Void showed a link to research that appears to be about using magnetic force to accelerate atoms to produce thrust.

The use of electrostatic force to accelerate ions to produce thrust has been known for many decades.

The article cited by Void claims a 10:1 improvement in thrust using magnetic acceleration.

This topic is offered for NewMars members with an interest in physics to study and report on the differences between the two techniques.

Magnetic force has been used to accelerate suitable ions in scientific accelerators for many years.

There may be a correlation, and if there is it would be interesting to see explained in this topic.

(th)

tahanson43206

Moderator

Registered: 2018-04-27

Posts: 19,263

Re: Electrostatic Thruster vs Magnetic Thruster

This post is reserved for an index to posts that may be contributed by NewMars members over time.

Post #3 contains  text from Calliban about the benefits of the Neumann drive:
https://newmars.com/forums/viewtopic.ph … 86#p227686

(th)

Calliban

Member

From: Northern England, UK

Registered: 2019-08-18

Posts: 3,777

Re: Electrostatic Thruster vs Magnetic Thruster

Recent posts by Void prompted me to look into the Neumann drive.
https://neumannspace.com/neumann-drive/

This is a pulsed electrostatic thruster and appears to be similar to the arcjet.  ISP is estimated to be >1000s.  What distinguishes this drive is its propellant.  Any conductive metallic material can be cast into anode rods, which are vaporised by an electric arc between the rod and cathode.  The arc has a temperature around 100,000K, so exhaust velocity may be several times greater than a chemical rocket.  In an arc jet, the cathode is toroidal and composed of a high melting point metal like tungsten.

What distinguishes the Neumann drive is its propellant.  Metal oxides are abundant in the inner solar system and can be reduced to metallic propellant for casting into rods.  Iron oxide is likely to be the easiest to reduce and most abundant metal oxide.  Many condrite asteroids and lunar regolith even contain natural reduced iron in the form of iron nickel cobalt alloy.  So a ship propelled by a neumann drive is a mule that can eat mountain scrub.  In principle, the electric thruster that could use oxide dusts as well, especially as part of a metal-oxide cermet.  The problems with these propellants are high melting point, high breakdown voltage, poor electrical conductivity and the generation of oxygen ions, which attack metallic components.  This is one of the problems with using Martian CO2 as arcjet propellant.  As the neumann drive propellant is reduced metal, it can function as a simple arcjet, without oxygen ions attacking the cathode.

What I havn't been able to find yet is estimates of power consumption per newton of thrust.  This is important because the ship power supply must be scaled accordingly and more power means more dead weight.  Still, this would appear to be a good candidate propulsion system for the interplanetary ships that Robert D and Kbd512 have discussed.  Thrust will be low, so orbital capture at destination may need to be augmented by chemical rockets.  But in the absence of nuclear pulse propulsion, electric thrusters that run on metal propellant offer versatility whilst maintaining a good ISP.

A life limiting issue with all arcjets is gradual erosion of the cathode.  The anode rods experience the greatest heating due to higher current density.  But the cathode will gradually erode as ions collide with it and material is sputtered into space.  We could deal with this by making the cathode out of something easily replaceable, like pure iron or by coating the cathode with a replaceable sheath.  Maybe something that can be bolted and unbolted between missions.

Last edited by Calliban (2024-11-04 14:34:35)

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"Plan and prepare for every possibility, and you will never act. It is nobler to have courage as we stumble into half the things we fear than to analyse every possible obstacle and begin nothing. Great things are achieved by embracing great dangers."

kbd512

Administrator

Registered: 2015-01-02

Posts: 7,826

Re: Electrostatic Thruster vs Magnetic Thruster

Calliban,

I took a look at this the other day, but I also looked at fission fragment propulsion from dusty plasmas, and I think if the large ship already requires a nuclear power source for practical propulsion, then you're better off with a direct energy conversion to propulsive power.

Final Report: Concept Assessment of a Fission Fragment Rocket Engine (FFRE) Propelled Spacecraft

The Isp of these dusty plasma fission fragment devices ranges somewhere between 5,000s (gas injection "afterburner" for 4.5kN) and 1,000,000s (only 10s of Newtons of thrust).

NASA's AFFRE (a horseshoe-shaped reactor with two electromagnetic nozzles) has a 2,500MW reactor power output level and it generates 4,651N of thrust, with an Isp of 32,000s.  Mass flow is up to 4oz per hour for the Uranium powderized "fuel" and 140lbs per hour for the LH2 gas injected into the engine's "afterburner".  You could increase thrust and reduce Isp by a factor of 10 by injecting a denser gas than Hydrogen, which is probably what you should do to exit through the Van Allen Belts as quickly as possible.  For example, CO2 should do the trick, producing a near-impulsive burn capable of pushing a heavy transport ship onto an interplanetary transfer trajectory in a matter of hours, minimizing crew and vehicle radiation exposure.

Total mass for the engine is 198,000kg / 198t.  The engine itself will easily fit in the payload bay of a Starship.  Starship 3 has the payload performance to carry the engine to orbit.  107t is the mass of the engine itself, including electromagnetic nozzle and reactor shielding, and 91t is the moderator oil, so the engine could be delivered in pieces if desired or required.  Total mass for all components, including tankage for the LH2, was predicted to fall between 223t and 269t (mass margin growth allowance).  Delivering the engine to orbit using Starship flights is a fairly trivial matter, though it will likely come in several large pieces so that everything easily fits and doesn't push any limits.

As I understand it, the engine is non-nuclear (contains no fissile inventory, unlike what we normally think of as a prototypical "reactor core") prior to first ignition and has very few moving parts.  The nuclear fuel is a separately stored powder induced to fission in the core using a very thick / heavy moderator, so the fuel can be transferred and resupplied when desired, such that total nuclear / fissile inventory is not above what is required for a single flight.  This is for the second iteration of the engine design that doesn't use massive radiators.  Isp is far less spectacular than the first iteration with the gigantic radiators, well within the range of what VASIMR is capable of delivering, but the total mass / complexity / cooling requirements are much lower than other types of electric propulsion systems, while still incorporating the whiz-bang factor of superconducting electromagnets.

Even though we could feasibly reach Mars much faster, I say we forego faster transit times, as well as wear and tear on the engine and vehicle, in favor of greater total ship tonnage.  Adding lots of mass margin and artificial gravity confers crew safety all by itself, because it means the ship can be equipped to handle contingencies.  6 months is a standard deployment for US Navy ships, and subjecting a crew provided with artificial gravity and generous amenities is not an overly burdensome voyage.

I think the dusty plasma fission fragment rocket engine still qualifies as a form of electromagnetic propulsion that uses high velocity fission fragments and a superheated gas ejected out of an electromagnetic nozzle to either generate lots of thrust or very high Isp thrust.  The primary advantage, at least as I see it, is that the engine / reactor is a purpose-built propulsion system, rather than a nuclear reactor attached to a propulsion system.  The second design iteration, unless I'm mistaken, does not require the large high temperature radiators of the first.  Most of the generated thermal power is reflected back into the reaction chamber and the chamber is optimally shaped to avoid the thermal heating of the first.  The first iteration absorbs far more heat into the device due to its sub-optimal core geometry (trying to shape it like a conventional reactor core or rocket engine combustion chamber).

Calliban

Member

From: Northern England, UK

Registered: 2019-08-18

Posts: 3,777

Re: Electrostatic Thruster vs Magnetic Thruster

Direct fission based propulsion systems are the only ones that can generate the power needed to deliver high ISP and high thrust simultaneously.  ISP x thrust = power.  The sort of torchship drives that you see on the expanse, capable of exhaust velocity in the 100s km/s whilst accelerating at several gee, are only possible using fission or hybrid fission fusion propulsion.  That is because that combination of high thrust and high ISP requires GW to TW of power for a reasonable sized ship.  There is no way of doing that aside from direct conversion of nuclear energy into kinetic energy.  The fission fragment rocket is one such example.

What stands in their way is politics.  If Trump enters the white house with Elon as his trusted advisor, things like this might really be on the table.  If we could truly unleash the potential of fission for spacecraft propulsion, it would unlock the solar system.  We have known that all along on this board.  In a few hours, we may know whether such dreams will be possible or not.

Last edited by Calliban (2024-11-05 18:29:54)

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"Plan and prepare for every possibility, and you will never act. It is nobler to have courage as we stumble into half the things we fear than to analyse every possible obstacle and begin nothing. Great things are achieved by embracing great dangers."

kbd512

Administrator

Registered: 2015-01-02

Posts: 7,826

Re: Electrostatic Thruster vs Magnetic Thruster

Calliban,

It looks as though President Trump was reelected, so I'd like to see some serious / non-pie-in-the-sky hard science-based proposals we can feasibly undertake to open up the rest of the solar system for affordable routine transit.