What the heck is Alpha?
What Alpha am I talking about? A power source's ratio of mass to power. The first time I ran into this quantity was in a NASASpaceFlight discussion of VASIMR's 39 day trip to Mars.
Kirk Sorensen worked for NASA 10 years. He has Master's degrees in aerospace as well as nuclear engineering. He is co-founder of Flibe Energy, a company that hopes to build thorium nuclear reactors. I don't always agree with him but I believe he has some expertise in this field. For the time being I am talking his word for it.
So this Magic Alpha, .50 kg/kWe, what is that? Here's an attempt to portray it:
This Ford Focus has a 160 horsepower engine. One horsepower = ~750 watts, so the engine is about a 120,000 watt power source. Not only the engine but also gasoline and oxygen. Pictured with the Focus is Dominique who masses about 60 kilograms. If she were a power source replacing the engine, gasoline and oxygen, we'd have an alpha of .5 kg/kW.
An electric car like the Tesla uses a battery. But just as a gas engine must make periodic stops to gas up, a Tesla must be frequently recharged. Enroute to Mars there are no gas stations and no electric outlets.
Later in the same NASA Space Flight thread, Sorensen says:
I don't really care if Samim Anghaie is crook but his 0.5kg/kWe number is a fantasy. Why FCD builds his VASIMR sales case on that number when all other reputable electric propulsion researchers have rejected it (even though makes their thrusters look incredible too) is beyond my understanding.
Indeed. Such a power source would make Hall thrusters look great. So far as I know Franklin Chang Diaz and Samim Anghaie are the only folks whose schemes rely on such an alpha.
Thermal Watts vs Electric Watts
39 day VASIMR trips to Mars are mentioned on Page 42 of The Plundering of NASA by Rick Boozer. Boozer argues that SLS and Orion are pork barrel make work programs and that money could be better spent on SpaceX and other programs. In general I agree with Boozer but was disappointed to see his endorsement of VASIMR.
So I asked Boozer about the Magic Alpha. Boozer came back with Project Nerva, a nuclear thermal rocket. He wrote:
Project NERVA claimed up to 5 GW possible with total mass of 38,600 kg. That works out to .00008 kg per W or .008 per kW. According to that Sorenson is incorrect.
NERVA's output is thermal watts. Thermal and electric watts are two very different things.
A nuclear electric power plant must first convert thermal watts to electric watts. But that's not the only problem.
The plant must dump waste heat. Massive cooling towers have become an icon for nuclear energy. From Wikimedia:
Earthly nuclear power plants can use water to carry off waste heat. In space there are no neighboring streams a nuclear power plant can use. In fact vacuum is a great insulator. A nuclear electric power source would need massive radiators.
I mentioned to Boozer that thermal and electric power sources were very different things.
David, I just don't know where you are coming from. There you went earlier lecturing me about the difference between thermal and electrical energy which is something that I teach physics students all the time. If I wasn't competent in physics my students wouldn't be making the high grades they are and I couldn't have got my Master's in astrophysics.I was hoping Boozer would demonstrate Sorensen was wrong. Sadly, pointing to his students' good grades and his degree did absolutely nothing to demonstrate the plausibility of Diaz' Magic Alpha. I was convinced of one thing though: Rick Boozer isn't credible.
I did not bother reading past page 42 of The Plundering of NASA.
What's the best plausible Alpha?
I search space forums for discussions of low mass power sources. So far as I can tell, thin film photovoltaics show the best promise. Roll Out and Passively Deployed Array (RAPDAR) might deliver 250 watts/kg. RAPDAR's thin film solar cells use an Elastic Memory Composite (EMC) for support and structure. Rolled up and cooled, the EMC will fit in a small volume and thus can fit under a fairing. When the sun warms it, the EMC will expand to the shape it needs to be.
On a Nasa Space Flight thread space entrepreneur Jeff Greason opined:
While I won't speak to this specific design, more generally I am quite convinced that thin film solar approaches 1 kW/kg are definitely possible near term. However there is very little serious work going on, and packaging such systems for launch and deploying them without spoiling the mass is not at all trivial.
But do keep thinking -- it is not crazy, at least 1 kW/kg rather than two.
Thin film solar is extremely fragile, however, so the packaging is really challenging.That's the rub, packaging. How useful are acres of Saran Wrap® with no structure? There needs to be a supporting frame to keep the film spread. It also needs to be kept pointing towards the sun so the supporting frame needs to be attached to gimbals and motors. What is the Alpha including supporting structure, gimbals and motors?
How will we deploy acres of Saran Wrap® from a small volume that fits within a fairing?
However Greason's optimism is somewhat reassuring. Being a bonafide space-cadet, I cling to optimistic opinions as long as I can.
Why is Alpha such a big deal?
As mentioned at the beginning of this post, a great alpha would make ion thrusters a more formidable tool. Presently ion thrusters have great ISP but very slow acceleration. A big cut to parasitic mass would give ion thrusters better acceleration.
Good Alpha would also make ISRU more plausible. Readers of my blog know I'm gung-ho on use of extra-terrestrial propellent, either near earth carbonaceous asteroids or frozen volatiles in the lunar cold traps.
Let's say we do mine water in the moon's neighborhood and we want to crack it to hydrogen/oxygen bi-propellent. Cracking a mole of water (18 grams) takes 287000 joules. A tonne of water is 55555 moles. 55555 moles*287000 joules/mole =13166666667 joules. If we wanted to crack 10 tonnes of water per day, we'd need a 1.5 mega-watt power source. And that doesn't include refrigerating the cryogens.
Cracking water isn't the only ISRU electricity hog. Just about all extra-terrestrial mining and industry will need lots of juice.
In the 50's and 60's NASA and the military provided big incentives to miniaturize electronics as low mass and small volume circuitry is a pre-requisite for rockets and missiles. I believe miniaturizing a power source should be a top goal for NASA. If we hope to settle space, a better Alpha should be given a higher priority than Apollo redux.