Harry White's and Paul March's PDF at NETS2012 about "Advanced Propulsion Physics: Harnessing the Quantum Vacuum"
Historical test results
have yielded thrust levels of between 1000-4000 micro-
Newtons, specific force performance of 0.1N/kW,
and an equivalent specific impulse of ~1x1012 seconds.
Where does that Isp come from?
See attached slide.
Thanks, but I'm not seeing that derivation unless your power source is pure matter/anti-matter with 100% conversion efficiency to usable power. ISTR you mentioning a spacecraft a while ago powered by an H2/O2 power-cell. By retaining the reactants, only the mass of the power output (by E=MC2) goes overboard.
If I've remembered that correctly, that seems to be completely the wrong way to analyse the situation. Instead, you are producing power by reacting H2 & O2. To calculate Isp correctly, the reaction product (water) should be sent overboard, and the thrust equated to the rate of consumption / disposal of hydrolox.
Try to remember that we are NOT talking about rockets in this example, which you are trying to do, though I tried to use a standard rocket parameter to bridge the gap between the two propulsion concepts and to demonstrate the performance enhancements that such a field propulsion device could bring to bear on the tyranny of the rocket equation. Instead we are talking about gravity/inertial (G/I) field propulsion systems that use the ambient G/I field to generate the Mach-Effect (M-E) momentum transfers from the vehicle to the field and thus to the rest of the universe that created this field in the first place. So the G/I field propulsion process does not require the expulsion of mass or E&M radiation away from the vehicle to generate the noted reactive forces, for it directly reacts with the G/I field instead just like a ship uses its propeller to interact with the ocean's water to generate thrust.
completely clear on the "propellantless" element of the topic title, but you're really missing the point here.
Isp relates the consumption of consumables against the amount of impulse generated. If you are producing electricity via H2
in a fuel cell, a kilogram of consumables will be converted to a certain amount of impulse through the thruster. That is clearly the basis on which Isp is calculated, and works irrespective of whether you throw the reactants overboard.
Again, I ask the simple question - given "specific force performance of 0.1N/kW", how much impulse could you generate from consuming 1kg of H2
I presume you'd need to start from the energy density of H2/O2, apply efficiency of the fuel cell and note losses in your electrical sub-system to calculate net energy at the thruster from 1kg of fuel. If you multiply this by "specific force performance of 0.1N/kW" shouldn't it give Isp in m/s? (Or divide by g to give it in seconds.)
For instance, if H2
has an energy density of 123 MJ/kg
, then H2
at stochiometric ratio has energy density of 13.66 MJ/kg.
Assuming 33% combined efficiency in fuel cell output and conditioning power for the MLT (a WAG!), the thruster will see 4.55 MJ/kg of fuel consumed.
4550 KJ/kg * 0.1 N/(kJ/s) = 455 N.s/kg (by mass) = 455 m/s. Divide by g to get a specific impulse of 46.5s (by weight), about 1/10th that of an RL-10.
Obviously, if you have better figures for H2
energy density (apologies for using Wiki figures), or efficiency of the fuel cell and electrical sub-system that would affect the final result.
Of course, you're not limited by chemical energy densities - power it from a solar cell and you can keep going for ever. However, with SEP having such high Isp it will compete quite well for Dawn-like solar powered missions.