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April 17, 2012

Possible Breakthroughs to over 250,000 Specific Impulse Propulsion

Recently we have reviewed several space propulsion technologies which could breakthrough to specific impulse of over 250,000 seconds. One pound of fuel would provide over one pound of thrust for over 250,000 seconds. The best ion drives are currently at about 5000 seconds and an advanced VASIMIR propulsion system (which is still in development) could get to 20,000 ISP.

1. A light-weight propulsion system called HIIPER (Helicon Injected Inertial Plasma Electrostatic Rocket) employs one of the highest density plasma sources (Helicon source) for plasma production and one of the most erosion-resistant accelerators (Inertial Electrostatic Confinement (IEC)) for plasma acceleration.


HIIPER allows for improved variable specific impulses and high thrust to power ratio by decoupling the ionization (helicon) and acceleration (IEC) stages of the plasma thruster. While VASIMR uses decoupling with ICRH antenna heating, the IEC heating section allows unmatchable ion energies, power scaling and efficiency, with the added advantage of being simple and light-weight. The current 500-Watt HIIPER lab experiment is capable of specific impulses around 3,000 s, with a final multi-kilowatt device capable of around 276,000 s.

This is technology readiness 4 with components demonstrated in the lab, but it apprears they could rapidly advance to technology readiness 6 with a prototype demonstrated on the ground.




2. Last year, John Chapman of NASA proposed a pulsed laser system for megawatt class fusion propulsion. In Chapman’s aneutronic fusion reactor scheme, a commercially available benchtop laser starts the reaction. A beam with energy on the order of 2 x 10^18 watts per square centimeter, pulse frequencies up to 75 megahertz, and wavelengths between 1 and 10 micrometers is aimed at a two-layer, 20-centimeter-diameter target.

The incremental thrust from a laser triggered p-11B target, assuming about 100,000 Alphas from a single laser pulse has been estimated to yield a few pico-Newton impulse per laser pulse from a 10 micron square target area. High pulse rate laser systems coupled with multiple square centimeters of active target area could effectively augment the effective thrust level towards Newton magnitude levels, particularly in conjunction with increased alpha yields from optimized target designs. Recent advances in laser technology indicate possibility of higher laser quantum efficiencies (over 25%) and higher femtosecond pulse train rates (~75MhZ). Bremstrahlung radiation and non-productive plasma also result in losses as well as particle collisions with the structure represent additional power losses from the propellant exhaust stream. Power is also lost in transverse momentum resulting in exhaust stream spreading.

Future development and the availability of high efficiency short pulse laser systems may result in overall gains that may make the A-LIFT (Aneutronic Laser Induced Fusion Thruster) offering ISP ~900,000 approach an attractive alternative to previous fusion ~1-10 kW/kg or ionic (ISP range from 2000 – 100,000 propulsion for In-Space thrust applications.


This is technology readiness 3 or 4 with some lab demonstrations or proofs of concepts done although not proving out the full propulsion aspects (just benchtop lasers generating particles when they hit targets.) It seems that lasers are rapidly advancing and a concentrated program effort should yield good results for space propulsion and for power generation.

3. Fission fragment propulsion could achieve ISP of between 500,000 to 1 million. 10s To 100’s newtons of continuous thrust could be generated for years. The initial space based prototype for human travel to Mars would be large and there appear to be a number of manufacturing capabilities that would need to be developed which puts a timeframe of at least a couple of decades for this technology. The technology seems less flexible and less adaptable to more scaled down missions and other applications.




4. Vacuum propulsion would be a huge game changer if it proves out.

The near term focus of the laboratory work is on gathering performance data to support development of a Q-thruster engineering prototype targeting Reaction Control System (RCS) applications with force range of 0.1-1 N with corresponding input power range of 0.3-3 kW. Up first will be testing of a refurbished test article to duplicate historical performance on the high fidelity torsion pendulum (1-4 mN at 10 to 40 Watts). The team is maintaining a dialogue with the ISS national labs office for an on orbit DTO.

How would Q-thrusters revolutionize human exploration of the outer planets? Making minimal extrapolation of performance, assessments show that delivery of a 50 mT payload to Jovian orbit can be accomplished in 35 days with a 2 MW power source [specific force of thruster (N/kW) is based on potential measured thrust performance in lab, propulsion mass (Q-thrusters) would be additional 20 mT (10 kg/kW), and associate power system would be 20 mT (10 kg/kW)]. Q-thruster performance allows the use of nuclear reactor technology that would not require MHD conversion or other more complicated schemes to accomplish single digit specific mass performance usually required for standard electric propulsion systems to the outer solar system. In 70 days, the same system could reach the orbit of Saturn.


Thrust is realized via Lorentz reaction of electromagnetic forces coupled to the spacecraft frame

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