[AR] Re: NASA test of quantum vacuum plasma thruster (was "Anyone heard of this?")
- From: "Troy Prideaux" <GEORDI@xxxxxxxxxxxxxxxx>
- To: <arocket@xxxxxxxxxxxxx>
- Date: Wed, 6 Aug 2014 14:47:14 +1000
As we've concluded here many times in the past, rockets don't care how fast
they're going with respect for the surroundings. More energy or more Isp per
given mass ratio = better performance as per the rocket equation. It does
matter a lot with air breathing propulsion, but not with rockets or for that
matter the technology of current discussion. Yes, higher Isp (or energy
intensive) propulsion systems do generally (*BUT NOT ALWAYS*) have higher dry
masses, but that's got nothing to do with surroundings, propulsive efficiencies
or vehicle velocities.
Troy
>> It would only come into its own where final velocities above about 50
>km/s
>> are needed, and even a Vasimr drive would need a mass ratio well above 10.
>
>Wait, VASIMR engines reach 50 km/s exhaust velocity. 1-1/e is about
>63% reaction mass.
>
>> It might be most useful in a nuclear-powered interstellar probe, at about
>1%
>> of the speed of light.
>>
>> That's if it works at all ..
>
>Right.
>
>Keith
>
>> -- Peter Fairbrother
>>
>>
>>
>>>
>>> For electric thrusters, a kg of reaction mass per second accelerated
>>> to 20 km/s (typical VASIMR) acquires a Ke of 1/2 mV^2. This takes 200
>>> MW (at 100% efficient).
>>>
>>> The force is 1kg x 20,000 m/s or 20,000 Newtons, 100 N/MW or 0.1 N/kW,
>>> which is 100,000 micronewtons per kW.
>>>
>>> Reducing this number is the VASIMR efficiency (~70%) and the reaction
>>> mass fraction, which for LEO to GEO and this exhaust velocity is
>>> around 20%. Earth to Mars is similar.
>>>
>>> It's still 1000 times more energy effective to use ion engines,
>>> assuming I have not made some boneheaded error.
>>>
>>> Keith Henson, L5 Society founder and EE (if you still need one)
>>>
>>>
>>
>>
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